Fire Explosion Hazards

91
S AFETY AFETY E E NGINEERING NGINEERING H H ANDBOOK ANDBOOK S S ERIES ERIES FIRE AND EXPLOSION FIRE AND EXPLOSION HAZARDS ANALYSIS HAZARDS ANALYSIS Publication Publication 2010 2010 P AGE AGE 1

description

Fire and Explosion Hazards Analysis

Transcript of Fire Explosion Hazards

Page 1: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

FIRE AND EXPLOSIONFIRE AND EXPLOSION HAZARDS ANALYSISHAZARDS ANALYSIS

PublicationPublication20102010

PPA G EA G E 11

Page 2: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

AACKNOWLEDGMENTSCKNOWLEDGMENTSThe Fire and Explosion Hazards Analysis textbook is a result of the collaborative efforts of the author with other professional representatives in the field of industrial safety engineering. They contributed substantially to the development of this manual by providing technical information and review. Although they are too numerous to name individually, I wish gratefully acknowledge their valuable contributions.

About the AuthorJoão Luís Santos is a professional safety engineer, responsible for executing risk, safey and fire risk analysis services provided to diversified industry sectors, including oil and gas industry. The author holds a Bachelor (B. Eng.) degree in Chemical Engineering and Licentiate (Lic. Eng.) degree in Chemical Engineering from School of Engineering of Polytechnic Institute of Oporto (Portugal), and a Master (M. Sc.) degree in Environmental Engineering from Faculty of Engineering of the University of Oporto (Portugal). Also, he has an Advanced Diploma in Safety and Occupational Health from the Institute for Welding and Quality (ISQ) and he is licensed and certified by ACT (National Examination Board in Occupational Safety and Health, Work Conditions National Authority).

NoticeNo warranty, guarantee, or representation, expressed or implied, is made by or on behalf of any public or private company, as well on behalf of any public or private institution, as to the absolute correctness or sufficiency of any representation contained in this document. The author assumes no responsibility in connection therewith, nor can it be assumed that all acceptable safety measures are contained in this or associated documents, or that other or additional measures may not be required under particular or exceptional conditions or circumstances. The mention of a brand name product or company does not constitute endorsement by the author. This report was prepared as an account of work sponsored by Risiko Technik Gruppe (RTG). Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Risiko Technik Gruppe, any agency thereof, or any of their contractors or subcontractors.

This textbook is available to the public from the sponsor agency:Risiko Technik Gruppe Office of Scientific and Technical Information can be requested to:E-Mail: [email protected]

This textbook is also available to the public from the author:E-Mail: [email protected]

PPA G EA G E 22

Page 3: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

TT A B L EA B L E O FO F C C O N T E N T SO N T E N T SUnderstanding Fire and Explosion Hazards..........................................................................................................................7

Introduction....................................................................................................................................................................7Hydrocarbon Fuels and Their Properties........................................................................................................................8

Flammability Limits...................................................................................................................................................8Auto-ignition Temperature........................................................................................................................................8Minimum Ignition Energy.........................................................................................................................................9Other Relevant Hydrocarbon Considerations.........................................................................................................10

The Combustion Process...............................................................................................................................................10Sources of Oxygen...................................................................................................................................................12Ignition Sources.......................................................................................................................................................13Carbanionic Radical Mechanism in Hot Surface Combustions..............................................................................15

Smoke Production and Properties.................................................................................................................................18Size Distribution......................................................................................................................................................19Smoke Properties.....................................................................................................................................................20

Dust Explosion..............................................................................................................................................................22Hazards Associated With Combustible Dusts ........................................................................................................22Risk Assessment for Dust Explosion........................................................................................................................25

Hazardous Area Classification......................................................................................................................................28Class I Hazardous Areas..........................................................................................................................................28Class II Hazardous Areas.........................................................................................................................................29Class III Hazardous Areas........................................................................................................................................30Area Classification Assessment...............................................................................................................................30Protection Methods and Hazard Reduction............................................................................................................30

Bibliography and References.........................................................................................................................................31Fire Hazard Analysis Techniques........................................................................................................................................33

Introduction..................................................................................................................................................................33Performing a Fire Hazard Analysis................................................................................................................................34

Developing Fire Scenarios and Design Fire Scenarios.............................................................................................34Quantification of Design Fire Scenarios.......................................................................................................................37

Design Fire Curves...................................................................................................................................................37Prediction of Fire Effects..........................................................................................................................................39

Prediction of Hazards....................................................................................................................................................41Simple Fire Hazard Calculations.............................................................................................................................41Simple Analytical Solution Techniques...................................................................................................................44Computer Models....................................................................................................................................................45Analyzing the Impact of Exposure...........................................................................................................................47Accounting for Uncertainty....................................................................................................................................47Final Review............................................................................................................................................................47

Bibliography and References.........................................................................................................................................48Understanding Liquid Natural Gas Fire Hazards................................................................................................................49

Introduction..................................................................................................................................................................49Hazards of Cryogenic Exposure on Liquified Natural Gas............................................................................................52

Hazards of Cryogenic Exposure...............................................................................................................................53Hazards to Structures and Equipment.....................................................................................................................53Hazards of Fire Exposure.........................................................................................................................................53Risk-based Protection of Onshore Facilities............................................................................................................53Protection of Offshore Facilities..............................................................................................................................54

Thermal Radiation from Pool Fires...............................................................................................................................55Hazardous Liquids...................................................................................................................................................56Hazardous Gases......................................................................................................................................................58Determining the Acceptable Separation Distance (ASD)......................................................................................59Estimation of Thermal Radiation (Analytical Model)............................................................................................59

Bibliography and References.........................................................................................................................................61Risk Analysis and Safety Implications of Liquified Natural Gas (LNG) Spills Over Water...............................................63

PPA G EA G E 33

Page 4: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Introduction..................................................................................................................................................................63Risk Assessment of Liquified Natural Gas Spills Over Water.......................................................................................64

Risk Analysis Elements of a Potential Spill..............................................................................................................64Spill Risk Assessment and Management Process....................................................................................................65The Elements of an Spill Over Water......................................................................................................................65Risk Reduction Strategy: Prevention and Mitigation..............................................................................................67

Spill Consequence Analysis...........................................................................................................................................67Asphyxiation Potential and Impacts........................................................................................................................67Spill Dispersion and Thermal Hazards....................................................................................................................68The Simplified Simulation Model and Theory........................................................................................................69

Bibliography and References.........................................................................................................................................74A Fire Risk Assessment Tool to Evaluate Fire Safety in Industrial Facilities and Large Spaces.........................................76

Introduction..................................................................................................................................................................76The Fire Development Models......................................................................................................................................76

Suppression Effectiveness........................................................................................................................................78Fire Department Response and Effectiveness..........................................................................................................78Occupant Response and Evacuation.......................................................................................................................78

Life Hazard Model.........................................................................................................................................................79Expected Number of Deaths...................................................................................................................................80Economic Consequences.........................................................................................................................................81

Hazard Analysis Procedure...........................................................................................................................................81Bibliography and References.........................................................................................................................................83

Human Resistance Against Thermal Effects, Explosion Effects and Obscuration of Vision..............................................84Introduction..................................................................................................................................................................84Thermal Effects.............................................................................................................................................................85

Physiological Effects.................................................................................................................................................85Pathological Effects..................................................................................................................................................86

Effects of Explosions.....................................................................................................................................................88Toxic Effects..................................................................................................................................................................89

Effects of Carbon Monoxide....................................................................................................................................90Effects of Carbon Dioxide........................................................................................................................................90Effects of Oxygen Depletion....................................................................................................................................91Overall Smoke Effects.............................................................................................................................................91Effects of Other Gases.............................................................................................................................................92

Obscuration of Vision....................................................................................................................................................93Bibliography and References.........................................................................................................................................94

PPA G EA G E 44

Page 5: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Chapter 1Chapter 1Understanding Fire and Explosion HazardsUnderstanding Fire and Explosion Hazards

efINTRODUCTIONINTRODUCTION

The potential for air and hydrocarbons to mix is an inherent risk in oil and gas operations. As a result, the industry continues to experience serious oilfield fires and explosions that injure and kill workers. Industry Recommended Practice IRP 18 standard is aimed at prevention of fire and explosion incidents. The single most significant observation from the case studies was the overall lack of awareness of the fire and explosion hazards. Committee members from Industry Recommended Practice were consistently surprised that the workers involved did not recognize and respond to obvious warning signs. The evidence suggests that those involved in the planning and execution of oilfield operations must have a better understanding of fire and explosion hazards to reduce the potential for similar events. The fire triangle shoen in Figure 1.01 is used to illustrate the three critical components required for combustion to occur. It is widely understood that to eliminate the potential of a fire or explosion, one of the three sides of the fire triangle must be eliminated. Given the nature of well drilling and completion operations, this is not as simple as it seems:(1) There is always potential for flammable and combustible substances to be present. More importantly, their

properties can vary based on operating conditions.(2) There is a wide range of oil and gas operations with an equally wide range of circumstances where air (oxygen) can

be combined with fuels. The accidental release of hydrocarbons into a work area is an ongoing concern. The planned or accidental entry of air into a closed system also requires attention.

(3) There are a wide range of ignition sources. Some ignition sources, such as static electricity and adiabatic compression, are not well understood and are even more difficult to identify and control.

The ability to develop effective solutions for improving industry safety depends on developing a better understanding of these elements. A modified fire triangle with expanded parameter lists is shown in Figure 1.01. The total avoidance of fire or explosion hazards is not always possible. However, there are steps that can be introduced to decrease the overall risk of a fire or explosion. Based on the results of the investigative work completed on Industry Recommended Practice (IRP 18), the first step is increasing the understanding of the hazards. To prevent future incidents, an accurate assessment of the operational threats and the required “barriers” is necessary for each dimension of the fire triangle. It is important to know the properties of the air-fuel mixture as well as the range of potential ignition sources. With this knowledge it is possible to determine the best steps to take to insure that the safest procedures are in place. Having all three parts of the fire triangle does not guarantee that an explosion will occur. The complex mechanics involved in combustion never guarantees that the same result happens every time. The probability of an explosion occurring in certain situations can be very high; however it is never absolutely certain. Operations cannot be considered safe based on the fact that there have been no previous incidents. Any time air and fuel are permitted to mix in flammable proportions it should be assumed that the potential for ignition exists.

HYDROCARBON FUELS AND THEIR PROPERTIESHYDROCARBON FUELS AND THEIR PROPERTIESThree important properties that must be understood are the flammability limits, the minimum auto-ignition temperature, and minimum ignition energy of a fuel.

FF LA M MA B I L I T YLA M MA B I L I T Y L L I M I TSI M I TS

The upper (rich) and lower (lean) flammability limits define the range of concentrations of a gas or vapour in air that can be ignited and sustain combustion. Any composition outside of these limits cannot be ignited. The lower flammability limit decreases slightly as pressure is increased. However, the upper flammability limit can increase substantially as pressure increases. While these trends are consistent for all hydrocarbons, each fuel has a different flammable range. Other factors that widen the flammability range include:(1) Increased temperatures widen the flammable range.(2) Flammability limits are widened by the increased energy of the ignition source.(3) If pure oxygen, instead of air, mixes with a hydrocarbon, the flammability limits may be widened.

Moisture and other contaminants will also affect the flammability range.

PPA G EA G E 55

Page 6: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

AA U TOU TO -- IG N I T I O NIG N I T I O N T T E MP E RAT U R EE MP E RAT U R E

A fuel and air mixture can ignite without the introduction of an ignition source. The minimum auto-ignition temperature is the lowest temperature at which the fuel vapours spontaneously ignite. Hydrocarbons that have been heated can ignite if they are exposed to air. Methane has the highest auto-ignition temperature. As the number of carbon atoms present in the hydrocarbon increases, the auto-ignition temperature decreases. In other words, heavier hydrocarbons tend to auto-ignite before lighter hydrocarbons. Increased pressures can also reduce the auto-ignition temperature.

Figure 1.01 – The expanded fire triangle.

PPA G EA G E 66

Heat / Ignition Sources:1. Hot work.2. Electric arcs and sparks.3. Static electricity.4. Hot surfaces.5. Friction and mechanical sparks.6. Chemical action and sparks.7. Spontaneous combustion.8. Hypergols.9. Pyrophors (i.e. iron sulphide).10. Pressure and compression ignition (dieseling).11. Sudden decompression.12. Catalytic reactions.

Oxygen Sources:1. Planned Introduction of AirAir-based operations.Air purging.

2. Unplanned Introduction of AirUnderbalanced operations.Swabbing and other operations that create a vacuum.Pockets of air created during the installation and servicing of equipment.Oxidized (weathered).

3. HydrocarbonsOxidizers.Chemical reactions.On-site generated nitrogen.

4. Release of Hydrocarbons into Air

Gases:Natural gas.Hydrogen sulphide.Liquified petroleum gases (LPG).Are the other relevant gases(i.e. hydrogen, acetylene).

Liquids and Vapours:Crude oil and condensate.Natural Gas Liquified (NGL) liquids.Hydrocarbon based fluids.Gasoline, diesel and other fuels.Methanol.

Chemicals:Solvents and cleaning agents.Special compounded hydraulic fluids and lubricants.Chemicals used for well servicing and stimulations.

Solids:Lubricants.Sealants.Packings, “O” rings, diaphragms and valve seats.Paints and coatings.

Energy(Ignition)

Oxygen Source(Air)

Fuel Source(Hydrocarbons)

Page 7: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

MM I N I MU MI N I MU M I I G N I T IO NG N I T IO N E E N ER G YN ER G Y

The minimum amount of energy supplied that is neededfor combustion is the minimum ignition energy. Every different fuel and air mixture will have different minimum ignition energies. Factors that affect the minimum ignition energy are:(1) The temperature;(2) The total energy supplied;(3) The rate at which energy is supplied, or time period over which it is delivered;(4) The area over which energy is delivered.

The minimum ignition energy values are usually given as the energy required to ignite the most reactive mixture of fuel and air. A flammable mixture that is close to either the upper or lower limits may require a higher amount of energy than the minimum ignition energy to ignite.

OO T H E RT H E R R R E L EVA N TE L EVA N T H H YD R O CA R B O NYD R O CA R B O N C C O N S ID E RAT I O NSO N S ID E RAT I O NS

With the exception of a few reactive or unstable substances, liquids do not ignite. It is in fact the vapours given off from the surface of the liquids that ignite. Liquids will give off vapours at some temperature. The ability to give off vapours and the rate at which this occurs defines the volatility of the liquid. The flash point should only be used as an approximate reference. The liquid may behave differently in the field than in laboratory tests performed to determine flash points. It is possible for an explosive atmosphere to exist even if the temperature of the environment is below the flash point of the liquid. The auto-ignition temperature for the liquid vapours is similar to that of gases. A very fine mist from a hydrocarbon liquid may act as a pure gaseous substance. These aerosols may become an explosive mixture at temperatures that are far below the liquid’s flash point. The droplets have to become vapourized but because of the small volume of the drops the energy required to do this is lowered significantly. The chemicals and hydrocarbon based liquids typically used by the oil and gas industry also have the potential for creating explosive mixtures including:(1) Chemicals used for well servicing and stimulations;(2) Solvents and cleaning agents;(3) Special compounded hydraulic fluids and lubricants.

In unique circumstances, some “solids” used by the oil and gas industry may create explosive mixtures. As the solid is heated it can undergo pyrolysis, a chemical degrading that occurs resulting in a release of vapours. The vapours released have the ability to form an explosive atmosphere and can ignite. These can include:(1) Lubricants;(2) Sealants;(3) Packings, “O” rings, diaphragms and valve seats;(4) Paints and coatings.

THE COMBUSTION PROCESSTHE COMBUSTION PROCESSCombustion is the rapid exothermic oxidation of combustible elements in fuel. Incineration is complete combustion. Pyrolysis is the destructive distillation, reduction or thermal cracking and condensation of organic matter under heat and pressure in the absence of oxygen. Partial pyrolysis, or more commonly called “starved-air combustion” is incomplete combustion and occurs when insufficient oxygen is provided to satisfy the combustion requirements. Wet oxidation is a form of incomplete combustion that occurs under high temperatures and pressures. Oxygen requirements for a complete combustion of a material may be determied from knowledge of its constituents, assuming that carbon and hydrogen are oxidized to the ultimate end products: carbon dioxide(CO2) and water (H2O). The formula becomes,

CxOyHwNz + (x + 0.25∙w – 0.50∙y)∙O2 x∙CO2 + 0.5∙w∙H2O + 0.5∙z∙N2 [1.01]

The theoretical quantity of air required will be 4.35 times the calculated quantity of oxygen because air is composed of 23% oxygen on a weight basis. To ensure complete combustion, excess air amountin to about 50% of the theoretical amount will be required.

PPA G EA G E 77

Page 8: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Adiabatic Flame TemperatureThe adiabatic flame temperature and product species are calculated by thermodynamics principles and thermochemical codes, i.e. PEP, NASA-Lewis, Edwards. For actual methane flame products the chemical equilibrium is given by,

CH4 + 2∙O2 0.42∙CO2 + 1.46∙H2O + 0.58∙CO + 0.27∙H2 + 0.35∙OH + … [1.02]

For actual ethane flame products the chemical equilibrium is given by,

C2H6 + 3.5∙O2 0.81∙CO2 + 2.12∙H2O + 0.19∙CO + 0.42∙H2 + 0.59∙OH + … [1.03]

Adiabatic implies that the enthalpy variation (∆H) between reagents and products is null. Therefore, for a reacting system,

∑ ni⋅ H ireactants=∑ n i⋅ H iproducts [1.04]

where

H i= H fi∫T 0

T f

c pi⋅dT [1.05]

Substituting Equation [1.05] into Equation [1.04] and taking the reactants at initial temperature (T0),

∑ ni⋅ H fi∫T 0

T f

c pi⋅dT products=∑ ni⋅ H fi reactants [1.06]

The final temperature (Tf) depends on the enthalpy variation (∆Hf) of both reactants and products: the number of moles (ni) of both reactants and products, and the heat capacities (cpi) of products only. Table 1.01 present some adiabatic flame temperatures for some combustible gas.

Table 1.01 – Typical adiabatic flame temperatures (Kelvin degrees) and flame speed for combustible gases.

CombustibleCombustible ∆∆HHff Moles ofMoles of AdiabaticAdiabatic Flame SpeedFlame Speed

GasGas (Kcal(Kcal∙∙molmol−−11)) Oxygen (OOxygen (O22)) Temperature (TTemperature (Tff)) (cm(cm∙∙ss−−11))

Carbon Monoxide −26.42 0.5 2,977 29

Methane −17.90 2.0 3,054 43

Hydrogen 0.00 0.5 3,080 170

Ethane −20.20 3.5 3,086 44

Propane −24.80 5.0 3,096 46

Butane −29.80 6.5 3,101 45

Benzene 19.80 7.5 3,136 48

Acetylene 54.20 2.5 3,342 144

Products and oxygen (O2) are very similar for both fuels; the main variations being in the number of moles (ni) and enthalpy (∆Hf) of the fuel. Therefore, the adiabatic flame temperature (Tf) values depend primarily on enthalpy of the fuel (reactant). Alkanes have very similar heats of formation, their adiabatic flame temperatures are constant.

Flame SpeedThe flame speed is controled by the chemical kinetics of combustion. Ideal flames stoichiometric for methane is given by,

PPA G EA G E 88

Page 9: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

CH4 + 2∙O2 CO2 + H2O [1.07]

Ideal flames stoichiometric for ethane is given by,

C2H6 + 3.5∙O2 2∙CO2 + 3∙H2O [1.08]

A general solution of the energy and species conservation equations leads to an equation which correlates flame speed with thermal diffusivity dependency,

S L= ⋅⋅c p⋅

12=⋅

12

[1.09]

Thus, the flame speed is dependent on the thermal diffusivity, the reaction rate and the density of the reacting gases. The pressure dependence is,

S L~Pn−212 [1.10]

The pressure dependence of the mass burning rate is,

mb=⋅S L~Pn2 [1.11]

The temperature dependence of flame speed is given by,

S L~10−ER⋅T f

12

[1.12]

From Table 1.01 we can see that flame speed does not necessarily correlate with enthalpy (∆Hf) or with adiabatic flame temperature (Tf). Typical flame speeds for hydrocarbons are between 40 cm∙s−1 and 50 cm∙s−1.

DetonationA detonation is chemical driven shock wave in a mixture or between exothermic material. Detonation temperatures are similar to combustion temperatures: ~ 2,500K in air or greater than 3,000K in pure oxygen. Detonation velocity is primarily a function of product conditions,

D=2

1⋅2⋅R2⋅T 2 [1.13]

Typical detonation velocities are around 103 m∙s−1; detonation pressures are between 20 atm and 40 atm. Detonation pressure and detonation velocity are dependent on final products characteristics: molecular weight and temperature.

Table 1.02 – Differences between typical magnitude ratios detonation and deflagration.

Magnitude RatiosMagnitude Ratios DetonationDetonation DeflagrationDeflagration

Burned Gas Velocity / Speed of Sound 5 – 10 0.0001 – 0.03

Burned Gas Velocity / Initial Gas Velocity 0.4 – 0.7 4 – 106

Final Pressure / Initial Pressure 13 – 55 0.98 – 0.976

Final Temperature / Initial Temperature 8 – 21 4 – 16

Final Density / Initial Density 1.4 – 2.6 0.06 – 0.25

PPA G EA G E 99

Page 10: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

For methane, the maximum detonation is accomplished by using the following chemical stoichiometric,

CH4 + O2 0.09∙CO2 + 0.86∙H2O + 0.90∙CO + 1.05∙H2 + … [1.14]

In Table 1.02 is shown typical magnitude ratios for detonation and deflagration.

SS O U RC ESO U RC ES O FO F O OX YG E NX YG E N

It is widely understood that air (which contains approximately 21% oxygen by volume) when mixed in the correct proportions with a hydrocarbon it forms an explosive mixture. This can occur when air is deliberately used, or when it is inadvertently trapped in piping, vessels, and wellbores. There are several ways that oxygen can form explosive mixtures with hydrocarbons:(1) Planned air introduction during air-based operations or when piping and vessels are purged with air.(2) Introduction of air during underbalanced operations or operations that create a vacuum (i.e. swabbing).(3) Pockets of air created during the installation and servicing of equipment.(4) The use of oxidizing chemicals (i.e. Persulphates).

A number of potential sources of oxygen are not well understood or obvious and deserve further explanation.

Oxidized (Weathered) HydrocarbonsLiquid hydrocarbons in the presence of air may oxidize forming oxidized hydrocarbon products such as hydroperoxides, aldehydes, and ketones. These compounds can decompose during a small or sudden change in operating parameters such as pressure or temperature, releasing significant amounts of energy which may lead to explosions. Reaction rates will increase at the higher pressures and temperatures encountered subsurface. The behaviour of oxidized hydrocarbons and the associated relevant mechanisms are not well documented or understood. Further research is needed to establish basic understanding of the various relevant mechanisms associated with oxidized hydrocarbons.

On-site Generated NitrogenNitrogen can be generated on-site for well drilling and other applications such as purging using a nitrogen production unit. The purity of the nitrogen depends on the flow rate, temperature and pressure of the compressed air feed. Although some of the modern commercial membrane separation units provide de-oxygenated air containing small amounts of oxygen (3% to 10% by volume), these levels may pose an explosion risk at subsurface conditions. Therefore, application specific screening must be carried out to establish the maximum allowable oxygen concentration for safe operation.

II G N I T I O NG N I T I O N S S O U RC ESO U RC ES

Once the fuel and oxygen are present, an ignition source is needed to complete the fire triangle (see Figure 1.01). Hydrocarbons can be ignited in two ways. The first is if the temperature is raised above the auto-ignition temperature then ignition will result. An example of this is the compression ignition of a diesel engine. Forced ignition (i.e. external or piloted) is the most common form of accidental ignition. An external ignition source is classified as anything that can deliver enough energy in the form of heat to ignite a substance. This category includes such sources as open flames, electric arcs and sparks and mechanical sparks.

Identifying Ignition SourcesMany of the fires and explosions reviewed were attributed to ignition sources that are difficult to identify as shown in Figure 1.02. While some ignition sources are well understood and readily identified, others deserve further discussion:(1) Static electricity is the electrical charging of materials through physical contact and separation and the positive and

negative electrical charges formed during this process. A common source of static electricity is the movement and transport of nonconductive liquids. When liquids are filtered, sprayed, pumped, mixed, or flow through pipes, static electricity can be generated. This type of “internal” static charge cannot be eliminated by bonding or grounding. If there is sufficient potential difference between the surface charge and the metal shell, and an object is lowered into the tank or well, a static arc may occur. This is of particular concern if there is a vapour space above the surface of a liquid. For example, the static arc created by when well servicing tools contact the fluid in a well has ignited this type of air-vapour mixture.

(2) Electric arcs and sparks. Sparks are the discharge of electrons that may or may not expend all of the energy in a single discharge. An arc is a continuous stream of electrons bridging a gap between two conductive surfaces in close

PPA G EA G E 1010

Page 11: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

proximity. The size or intensity of arcs and sparks depends on the resistance of the substance between the points of discharge. Once the voltage is high enough to overcome the dielectric strength of the air, the air will ionize allowing a conductive path for electricity to flow. Due to the high resistivity of air there will generally be enough energy dissipated to ignite a flammable vapour. The current or amount of electricity that is flowing will dictate the temperature of the arc. The higher the current, the higher the temperature. Even arcs with lower currents generate enough heat energy that the likelihood of ignition is high. Some common examples of arcs and sparks as an ignition source are sparking of electric motors and generators or from other electrical rotating equipment, arcing between contacts (i.e. switches and relays), arcs due to broken and inadequate or failed insulation, lightning strikes, discharge of a charged capacitor through a gas, poor contacts between conductors (i.e. such as poorly fitted light bulbs and their sockets), arcs intentionally created during electric welding. Many of these ignition sources can be created during hot work, which is defined as an operation that can produce enough heat from flame, spark or other source of ignition, with sufficient energy to ignite flammable vapours, gases, or dust. Welding, cutting, grinding, brazing, flaming, chipping, air gouging, riveting, drilling, and soldering are all forms of hot work that can create sparks or high temperatures.

(3) Mechanical sparks occur when there is excessive friction between metals or extremely hard substances. As the two substances rub against each other small particles are torn off of the surfaces. The tearing of these small particles is due to the large amount of friction. For the metal to spark it must satisfy three conditions:(1) the energy, which supplies the tearing off of the particles, must also be sufficient to heat the metal to high temperatures (if it is a softer metal then it will usually deform before it will spark); (2) for a metal to spark it must be able to oxidize and burn easily, generally the temperature at which a metal sparks is the same as its burning temperature; (3) the specific heat of a metal is the last factor.A(ametal that has a low specific heat will reach a higher temperature for the same amount of energy input).

(4) Hot surfaces. Surfaces that exceed the minimum autoignition temperature of a hydrocarbon have the potential to ignite hydrocarbon vapours. Experience shows that hot surfaces must exceed the minimum auto-ignition temperature by a substantial margin to cause ignition.

(5) Pyrophoric iron sulphides. Pyrophoric iron sulphides form when iron is exposed to hydrogen sulphide, or any other compound that contains sulphur, in an oxygen deficient atmosphere. They are frequently found in vessels, storage tanks, and sour gas pipelines. Pyrophoric iron sulphides present a hazard when equipment and tanks are opened for cleaning, inspection, and maintenance. As the iron sulphides compounds dry out and come in contact with air, they react with the oxygen and spontaneously ignite.

(6) Pressure (compression ignition) can occur when gases are compressed. Heat is generated, or more accurately, energy is transferred. If the rate of heat generation within a system exceeds the rate of heat loss (energy transfer) to the surroundings, the temperature of the system will rise. If the rate of compression is rapid enough such that the heat loss may be considered negligible, resulting in “adiabatic compression”, the temperature rise will depend on the compression ratio.

(7) Sudden decompression of air-hydrocarbon mixtures, particularly air-liquid hydrocarbon mixtures, is not well understood. Some of the compounds present are highly unstable especially when subjected to sudden pressure and temperature changes. Decomposition of such products can yield significant energy rapidly and may provide an ignition source for the air-hydrocarbon mixture. In addition, during sudden decompression of air-hydrocarbon mixtures, the release of dissolved gases within the liquid hydrocarbons may atomize the liquid hydrocarbons thus enhancing their reactivity.

Natural suppressants are frequently used in oilfield operations that affect the probability of ignition. These include:(1) Water (including formation water);(2) Inert Gases;(3) Thickening agents;(4) Salts;(5) Detergents.

These suppressants may unknowingly aid in the prevention of fire and explosions during drilling and completion operations. This may explain why fire and explosions do not occur more frequently.

PPA G EA G E 1111

Page 12: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Figure 1.02 – Common causes of types of ignition sources identified in oil and gas industry.

CCA R BA N I O N ICA R BA N I O N IC R R A D I CA LA D I CA L M M EC HA N I S MEC HA N I S M I NI N H H O TO T S S U R FA C EU R FA C E C C O M BU S T I O NSO M BU S T I O NS

Hot surface oxidations are commonly thought to involve initial free radical hydrogen atom abstraction. Evidence implicates initial Lewis base deprotonation by atomic oxygen radical anions (O•−) to form negatively charged carbanions. Subsequent rate determining electron transfers generate free radicals which only then give rise to combustion. Correlations regarding ignition temperatures and hydrocarbon oxidation product identity are consistent with carbanionic but not free radical effects. Highly polarized surfaces (e.g. quartz and corroded surfaces), and addition of polar compounds to fuel and air mixtures facilitate ignitions. For seemingly uniformly hot surfaces there are transient widely disparate high temperature incandescent “red spot” zones due to flameless oxidations induced by concentrated electrostatic negative charges at surface defects. Isotope, ignition temperatures and combustion trends are consistent with Seebeck effects: ease of electron migration in unevenly heated areas. There are preliminary though not yet verified indications that electrostatic charges on hot surfaces may facilitate combustion. Implications would then involve fire mitigation and enhancement, and industrial manufacture of many important organic combustion products. In selective (flameless) oxidations of hydrocarbons, C−H bonds rupture with insertion or addition of oxygen species to form combustion products. In total oxidation (with or without flame), the hydrocarbon is completely oxidized to carbon dioxide (CO2) and water (H2O). Fuel flammability indices include flash points (the minimum temperatures for liquid fuels to sustain sufficient vapor concentrations in air to produce ignition when an open flame is passed over the surface), and hot surface ignition, or autoignition temperature (with no open flame for a surface in contact with a given fuel causing “self-sustained combustion”). Currently accepted hydrocarbon (RC) oxidation mechanisms involve free radical attack by oxygen species such as the atomic oxygen radical anion (O•−) on C−H bonds, to yield propagating alkyl free radicals and hydroxide ion,

RC−H + O•− RC• + OH− [1.15] The following sequence of events to explain hydrocarbon oxidation on hot surfaces is speculated:(1) The fuel drop vaporizes, cooling the immediate impact area of the hot surface. Fuel concentrations within the cloud

are initially above the upper flammability limit.(2) Within the fuel cloud there is initial flameless oxidization, forming carbon monoxide (CO), carbon dioxide (CO2),

and water (H2O) and further reducing oxygen (O2) levels. Heating causes thermal eddies and convection currents, effecting isolated contiguous hotter and cooler surface areas. Within hotter areas oxidation rates are enhanced to

PPA G EA G E 1212

10 %

10 %

12 %

8 %

Open Flame Welding-ArcStatic Electricity

Pyrophoric Iron Sulphide

Hot Surfaces

22 %

22 %

Adiabatic CompressionFrictional and Mechanical Sparks

8 %

Electrical Arc and Sparks

8 %Vehicle Ignition

Page 13: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

the extent that there can be momentary attainment of an incandescent “red spot”, flickering and moving around as a result of convection currents.

(3) For “red spots” at the periphery of the fuel cloud, where oxygen levels are higher in contiguous cooler areas (but still hot enough to sustain ignition), the fuel and air mixture may be lean enough to afford ribbons of blue “cool flame” along this periphery, which suddenly transform to a sheet of white hot flame over the entire surface.

Combustion occurs prior to flaming combustion, with carbon monoxide (CO) and carbon dioxide (CO2) being prominent products. The transient incandescent “red spots” do not persist long enough for true temperature registration, but would be hot enough for carbon monoxide (CO) ignition. Also, the range of lower to upper flammability limit is much greater for carbon monoxide (the flammable limit region ranges from 12.5 % to 74 %, ideal for a fuel rich environment) than for alkane fuels (e.g. hexane, with very narrow flammability limit band ranges of 1.1 % to 7 %). Thus, autoignition temperature (AIT) determinations may pertain not to the substrate being measured, but rather to decomposition products such as carbon monoxide. This suggests that the first pre-flame phase is a relatively low energy selective process. It is believed that adsorbed oxygen radical anions thermally react as strong Lewis bases with the hydrocarbon C−H bonds to form carbanionic incipient intermediates. These then undergo electron transfer to form corresponding hydrocarbon free radicals, and proceed through previously documented mechanistic pathways to form selective and total oxidations products. As oxidation ensues with rising temperatures, the rate of formation of the free radical population also increases. When the rate of free radical propagation exceeds the rate of flameless free radical depopulation processes, runaway flame chemistry ensues, and positively charged cationic) reactions tend to proceed with much less energy reqirements than is the case for ionic reactions in the gas phase. However, pre-flame combustion reactions take place by interaction with oxygen species adssorbed onto the solid phase surface of a hot target, and gas phase arguments are not applicable.

Polarity in Ambient Air EffectsIn studies of mixtures of hydrocarbon gases with halons, hydrogen bromide, and water (all highly efficient polar fire extinguishing agents which appreciably polarize ambient air in the vicinity of hot surfaces), it was found that instead of inhibiting ignition, these actually were ignition promoters! These results provide further argument for a polar rather than a free radical pathway in facilitating ionic ignition initiation processes in surface ignitions:(1) Seebeck Effects Favoring Polar Combustion Mechanisms – In heated metal surfaces with varying surface

temperatures there are electrical charge effects with polarizing influences in these different zones. Seebeck effects induce charge transfers in heated metals, the polarity and magnitude of these depending on the metal. Many “n-type” metals exhibit a negative Seebeck effect, with a diffusion of electron density toward the cooler side, and a corresponding partially negatively charged character at the cooler side. For positive “p-type” metals (such as tungsten, molybdenum, copper, gold and silver) there is a reversed electron diffusion from cooler to hotter metal surfaces, and a corresponding positively charged polarization at the cooler end (see Figure 1.03). In the context of Figure 1.03, the “hot” end in the system is the driving heat source, and the “cool” end is the combustion surface on which the actual “hot surface combustion” takes place. Thus, as temperatures rise on the cooler surfaces during combustion for the “n-type” metals, this surface becomes less negatively charged as there is a relative migration of electron density back to the hotter end. Combustion is dependent on negative charge character, and therefore combustion tendencies on such “n-type” metals will require hotter and hotter temperatures. Likewise, for the “p-type” metals, the negative charge character at the combustion surface will increase during combustion, thus requiring lower temperatures for ignition to take place. Thus, “n-type” metals would be expected to have higher ignition temperature, and “p-type” metals would have lower ignition temperatures. Metals with the highest positive coefficients have lower ignition temperatures, and metals with higher negative values have higher ignition temperatures. The “n-type” nickel species appears to be anomalous, in terms of its relatively low ignition temperature. However, the nickel surface (unlike all the other metals discussed here) becomes irreversibly oxidized at temperatures below typical autoignition temperatures (AIT) values. Therefore, metal oxide coatings provide lower ignition temperatures than do non-corroded metals.

(2) Oxyanionic and Corroded Surface Effects – As noted above, there is an apparent negative electrostatic character to be associated with facilitation of combustion. Although it is non-metallic with no Seebeck effect, and there are typically no significant surface defects, quartz is a highly efficient ignition source, with low ignition temperature. This may be due to the strong polarization of adsorbed oxygen molecules, due to the very high negative charge of the oxygen species which comprise the entire surface of the silicate matrix. Similar considerations can be made for other oxidized surfaces, such as nickel oxide and iron rust, also excellent ignition sources. There have also been findings of “p-type” character of oxides of metals such as nickel, for which the metal itself possesses “n-type”

PPA G EA G E 1313

Page 14: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

character. This effect would serve also to reduce ignition temperatures.(3) Electrostatic Field Effects Arising from Surface Microdefects – The magnitude of Seebeck coefficient charge effects

(typically microvolts∙°C−1 change in temperature) may appear to be very small, but microscopically all surfaces are entirely covered by considerable defects in terms of terraces, steps, ledges, and kinks and similar elevated features. Increased catalytic activity and electrical effects at the tips or edges of such features have also been observed. In this connection it can be noted that electrostatic field effects totally accumulate at the top most tips of these types of surface features. Since the area comprising these tips is exceedingly miniscule in comparison to the total surface area, there is accordingly a very drastic increase in density of these localized charges. An inference can then be drawn from this effect in terms of surface microdefects at a hot surface (with a hotter heat source driving a heat flow and Seebeck electron drift to or from this surface). On microscopic examination such surfaces exhibit an uneven landscape characterized by valleys with surrounding steppes, shelves, cliffs, spires and crags.

Figure 1.03 – Polarization by Seebeck effect for negative “n-type” metals and positive “p-type” coefficient metals.

The Carbanionic Radical MechanismA currently accepted mechanism for hot surface hydrocarbon oxidations involves a preliminary attack in which a C–H bond is severed by an attacking highly reactive species such as a hot surface energized adsorbed atomic oxygen radical anion, producing two fragments. In one of these fragments, the hydrogen is transferred to the attacking species; in the other fragment is the hydrocarbon radical (less the hydrogen) remaining after the C–H bond rupture (see Equation [1.15]). The atomic oxygen radical anion has three non-bonding electron pairs, and a seventh lone (free radical) electron. The three electron pairs impart strong Lewis basicity, in addition to its free radical character. In the commonly accepted theory, the oxygen radical anion (O•−) attacks via its lone radical electron in a high energy rate determining reaction pathway to afford energetic propagating free radicals. These further engage in free radical propagations to yield a variety of selective oxidation products. Accompanying energy releases can result in flame oxidation. From evidence gathered, the much lower energy initial step involves attack by the very highly basic oxygen radical anion (O•−), to form an alkyl carbanion (RC–) and a hydroxyl free radical (OH•),

RC−H + O•− RC– + OH• [1.16]

A high energy rate determining electron transfer from the carbanion to the hydroxyl free radical then results in an alkyl free radical (RC•) and hydroxide anion (OH–),

RC– + OH• RC• + OH− [1.17]

The same products arise, but through a much lower energy ionic pathway before proceeding into a higher energy free radical mechanism, and the same isotope effects apply for both free radical and ionic pathways. In the solvated state, hydrocarbon pKA values are about 62, but when unsolvated and at elevated temperatures these are much stronger Brønsted acids with much lower pKA values, and in the presence of a strong base such as the unsolvated atomic oxygen radical anion adsorbed on a hot surface, an acid-base interaction would be expected to occur.

PPA G EA G E 1414

e- e--

h++h+

"Hot

Sid

e"

"n-type" metal surfaceIncrease of electron density

"Col

d Si

de"

"p-type" metal surfaceIncrease of electron density

Page 15: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

SMOKE PRODUCTION AND PROPERTIESSMOKE PRODUCTION AND PROPERTIESThe term “smoke” is defined in this chapter as the smoke aerosol or condensed phase component of the products of combustion. This differs from the American Society for Testing and Materials (ASTM) definition of smoke, which includes the evolved gases as well. Smoke aerosols vary widely in appearance and structure, from light colored, for droplets produced during smoldering combustion and fuel pyrolysis, to black, for solid, carbonaceous particulate or soot produced during flaming combustion. A large fraction of the radiant energy emitted from a fire results from the black body emission from the soot in the flame. The effects of the smoke produced by a fire depend on the amount of smoke produced and on the properties of the smoke. The following section presents experimental results on smoke emission for a variety of materials. The smoke emission, together with the flow pattern, determines the smoke concentration as smoke moves throughout a building or a contained space. The most basic physical property of smoke is the size distribution of its particles.The smoke properties are primarily determined by the smoke concentration and the particle size distribution. Smoke emission is one of the basic elements for characterizing a fire environment. The combustion conditions under which smoke is produced (i.e. flaming, pyrolysis, and smoldering) affect the amount and character of the smoke. The smoke emission from a flame represents a balance between growth processes in the fuel-rich portion of the flame and burnout with oxygen. While it is not possible at the present time to predict the smoke emission as a function of fuel chemistry and combustion conditions, it is known that an aromatic polymer, such as polystyrene, produces more smoke than hydrocarbons with single carbon-carbon bonds, such as polypropylene. The smoke produced in flaming combustion tends to have a large content of elemental (graphitic) carbon. Pyrolysis occurs at a fuel surface as a result of an elevated temperature; this may be due to a radiant flux heating the surface. The temperature of a pyrolyzing sample (between 600K to 900K), is much less than the gas phase flame temperature (between 1,200K to 1,700K). The vapor evolving from the surface may include fuel monomer, partially oxidized products, and polymer chains. As the vapor rises, the low vapor pressure constituents can condense, forming smoke droplets appearing as light-colored smoke. Smoldering combustion also produces smoke droplets, but in this case the combustion is self-sustaining, whereas pyrolysis requires an external heat source. While most materials can be pyrolyzed, only a few materials, including cellulosic materials (wood, paper, cardboard) and flexible polyurethane foam, are able to smolder. The temperature during smoldering is typically 600K to 1,100K. In Table 1.03 the smoke conversion factor (ε) is given for a variety of materials commonly found in buildings. The quantity ε is defined as the mass of smoke produced by mass of fuel burned.

Table 1.03 – Smoke production for wood and plastics.

Type of MaterialType of Material Smoke ConversionSmoke ConversionFactor (Factor (εε))

CombustionCombustionConditionsConditions

Fuel AreaFuel Area(m(m22))

Diugla Fir 0.03 – 0.17 Pyrolysis 0.005

Douglas Fir 0.01 – 0.25 Flaming 0.005

Polyvinylchloride 0.03 – 0.12 Pyrolysis 0.005

Polyvinylchloride 0.12 Flaming 0.005

Polyurethane (flexible) 0.07 – 0.15 Pyrolysis 0.005

Polyurethane (flexible) 0.01 – 0.035 Flaming 0.005

Polyurethane (rigid) 0.06 – 0.19 Pyrolysis 0.005

Polyurethane (rigid) 0.09 Flaming 0.005

Polypropylene 0.12 Pyrolysis 0.005

Polypropylene 16 Flaming 0.005

Cellulosic Insulation 0.01 – 0.12 Smoldering 0.02

The references cited in Table 1.03 should be consulted regarding the detailed description of the combustion conditions. In many instances, the conversion factor (ε) was measured for a range of radiant fluxes, oxygen concentrations, sample orientations, and ambient temperatures. It is seen in Table 1.03 that s has a greater range for flaming combustion, with values in the range 0.001 to 0.17, compared to pyrolysis and smoldering, with values in the range 0.01 to 0.17. The following factors should be taken into account when using this table for smoke emission estimates:

PPA G EA G E 1515

Page 16: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

(1) Most of the measurements reported in Table 1.03 were made on small-scale samples.(2) Most experiments were for free burning at ambient conditions (reduced ventilation can strongly affect the smoke

production).(3) In transport, the smoke may coagulate, partially evaporate, and deposit on surfaces through diffusion and

sedimentation. Also, additional smoke may be formed through condensation.

SS I Z EI Z E D D I S T R I BU T IO NI S T R I BU T IO N

Smoke particle size distribution, together with the amount of smoke produced, primarily determines the properties of the

smoke. A widely used representation of the size distribution is the geometric number distribution, N

log d versus

Log (d), where “d” represents the particle diameter. The quantity ∆N represents the number of particles per cubic centimeter (cm3), with diameter between Log (d) and Log (d) + ∆Log (d). For many applications, the most important characteristics of a size distribution are the average particle size and the width of the distribution. A widely used measure of the average size is the geometric mean number diameter (dgn), defined by,

log d gn=∑i=1

n N i⋅log d iN [1.18]

where N is the total number concentration (total number of particles), Ni is the number concentration in the ith interval, and log is the logarithmic function to the base 10. The corresponding measure of the width of the size distribution is the geometric standard deviation (σg),

log g=[∑i=1

n log d i−log d gn2⋅N i

N ]12

[1.19]

A perfectly monodisperse distribution would correspond to geometric standard deviation unity (σg= 1). The parameters dgn and σg are useful because actual size distributions are observed to be approximately log-normal, which is the same as a normal or Gaussian distribution, except that Log (d) is normally distributed instead of particle diameter (d). An important characteristic of the log-normal distribution is that 68.3 percent of the total particles are in the size range Log dgn + Log σg. To correlate the smoke volume-particle size distribution, the geometric mean volume diameter (dgv) is a convenient measure of average particle size,

log d gv=∑i=1

n

V i⋅log d i

V T

[1.20]

where VT is the total volume concentration of the smoke aerosol. For a log-normal distribution, there is the following relationship between the geometric mean volume diameter (dgv) and the geometric mean number diameter (dgn),

log d gv=log d gn6.9⋅log g 2 [1.21]

Some devices, such as an ionization-type smoke detector, have an output depending primarily on geometric mean number diameter (dgn), while others, such as light-scattering-type detectors, have an output depending more on geometric mean volume diameter (dgv). More than one instrument is necessary for a complete characterization of the smoke size distribution, because it is typically quite wide. Smoke measurements pose special problems because of the high concentration, wide particle size range, and sometimes high temperature. In selecting an instrument it is important to make the following considerations:(1) Will the instrument respond to the smoke of interest? For example, the piezoelectric mass monitor does not respond

well to soot.(2) Will dilution of the smoke be required?(3) Is the measurement size range of the instrument adequate?

PPA G EA G E 1616

Page 17: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

(4) Is a mass or number distribution measurement appropriate?(5) What is the particle size resolution needed?(6) Is real-time measurement capability needed?(7) Will the instrument perform at the temperature of the smoke environment?

Smoke aerosols are dynamic with respect to their particle size distribution function. Smoke particles or droplets undergoing Brownian motion collide and stick together. The result of this behavior is that, in a fixed volume of smokeladen gas, the number of particles decreases while the total mass of the aerosol remains unchanged. This process is known as coagulation. The fundamental parameter for describing coagulation is the coagulation coefficient (Γ) the rate constant for the coagulation equation,

dNdt

=−⋅N 2[1.22]

Integrating Equation [1.22] yields,

N=N 0

1⋅N 0⋅t[1.23]

For smoke produced from incense sticks, coagulation coefficient (Γ) was found to be about 4∙10−10 cm3∙s−1 and about 1∙10−9 cm3∙s−1 for smoke produced from flaming α-cellulose. The coagulation process has a more pronounced effect on the number distribution than the mass distribution as small particles collide to form larger particles.

SS MO KEMO KE P P RO P E RT I E SRO P E RT I E S

The smoke properties of primary interest to the fire community are light extinction, visibility, and detection. The most widely measured smoke property is the light extinction coefficient. The physical basis for light extinction measurements is Bouguer’s law, which relates the intensity (Iλ

0) of the incident monochromatic light of wavelength (λ) and the intensity of the light (Iλ) transmitted through pathlength (L) of the smoke,

I

I0 =e−K⋅L

[1.24]

where K is the light extinction coefficient. When Equation [1.24] is expressed in terms of base 10,

I

I0 =10−D⋅L

[1.25]

The quantity D is defined as the optical density per meter,

D= K2.3 [1.26]

The extinction coefficient (K) is an extensive property and can be expressed as the product of an extinction coefficient per unit mass (Km) and mass concentration of the smoke aerosol (m),

K=K m⋅m [1.27]

The specific extinction coefficient (Km) depends on the size distribution and optical properties of the smoke through the relation,

PPA G EA G E 1717

Page 18: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

K m=3

2⋅⋅m⋅∫

d min

d max 1d⋅⋅m⋅d

⋅Qext⋅ d

, nr⋅d [1.28]

The single particle extinction efficiency (Qext) is a function of the ratio of particle diameter to wavelength of light and of the complex refractive index of the particle (nr). The quantity prepresents the particle density. Mulholland has described the general design of a light extinction instrument that satisfies Bouguer’s law. Two key features are the use of monochromatic light and the elimination of forward scattered light at the detector. The specific optical density (Ds) is measured in a standard laboratory smoke test for assessing the amount of visible smoke produced in a fire. The dimensionless quantity Ds is defined by,

D s=D⋅V c

A[1.29]

where Vc is the volume of the chamber, and A is the area of the sample. This is a convenient quantity to measure if the decomposed area is well defined. Since specific optical density (Ds) depends on the sample thickness, the same thickness should be used for relative rating of materials tested. If the mass loss of the sample is measured, then the mass optical density (Dm) is the appropriate measure of visible smoke,

D s=D⋅V c

M[1.30]

This technique requires an accurate measurement of the mass loss of the sample (∆M) in addition to a light extinction measurement. The extinction coefficient, in turn, is related to visibility through the smoke, as discussed below.

VisibilityVisibility of exit signs, doors, and windows can be of great importance to an individual attempting to survive a fire. To see an object requires a certain level of contrast between the object and its background. For an isolated object surrounded by a uniform, extended background, contrast, can be defined as,

C= BB0

−1 [1.31]

where B is the brightness or luminance of the object, and B0 is the luminance of the background. For daylight conditions, with a black object being viewed against a white background, a value of C = –0.02 is often used as the contiast threshold at which an object can be discerned against the background. The visibility of the object (S) is the distance at which the contrast is reduced to –0.02. Most visibility measurements through smoke have relied on test subjects to determine the distance at which the object was no longer visible rather than the actual measurement of contrast (C) with a photometer. Visibility depends on many factors, including the scattering and the absorption coefficient of the smoke, the illumination, whether the sign is light-emitting or light-reflecting, and the wavelength of the light.

DetectionIn addition to their utility for estimating visibility, light extinction measurements are also widely used in characterizing smoke detector performance. The electrical output of a detector (P) from a lightscattering or ionization-type smoke detector can be represented as an integrated product of the size distribution function and the basic response of the detector, R(d).

P=∫d min

d max

R d ⋅ Nd

⋅d [1.32]

The ionization-type smoke detector is more sensitive to smoke particles smaller than about 0.3 µm, and the light-scattering type more sensitive to particles larger than 0.3 µm. The basic principle of ionization detectors is the

PPA G EA G E 1818

Page 19: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

interception of gaseous ions by smoke particles, reducing the ion current in the detector until a preset alarm point is reached. The detector response function is approximately proportional to the product of the number concentration and particle diameter. For one detector the response function is given by,

Rd =c⋅d [1.33]

where c has a value of 7 in units of µV per particle concentration per µm(µV∙cm3∙µm−1). Such detectors tend to be most sensitive to high concentrations of small particles, such as those produced by flaming paper and wood fires, and least sensitive to the low concentration of large smoke droplets produced in smoldering fires. Light-scattering smoke detectors have a high sensitivity to smoke particles with diameters approximately equal to the wavelength of light (λ), and low sensitivity to particles much smaller than the wavelength (λ). The response function, R(d), depends on the wavelength of the light source in the smoke detector, the scattering angle, and the scattering volume. For smoke particles with diameter greater than about 0.3 µm, the output of several light-scattering smoke detectors was found to be approximately proportional to the mass concentration of the smoke. Light-scattering detectors complement ionization detectors in that they have high sensitivity to smoldering fires and low sensitivity to low-smoking flaming fires, such as paper and wood fires. The purpose of smoke detectors is to give the occupants of a given space or room adequate warning to escape a developing fire.

DUST EXPLOSIONDUST EXPLOSIONDust is readily ignited and can burn fiercely or explode. Spontaneous combustion can occur, so dust should be kept under control. Environments should be regularly cleaned to avoid the risk of fire. A wise solution can prevent a loss of lives, money and downtime. Combustible particulate solids are defined as “any combustible solid material, composed of distinct particles or pieces, regardless of size, shape, or chemical composition”. Combustible dust can also be defined as “a combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape”. Why the distinction between a combustible particulate solid and combustible dust? Even though a combustible particulate solid might not ignite readily or be capable of being suspended in air in its particulate form (the pieces could be too large), the material can break down during such activities as shipping, handling, conveying, mixing, and pulverizing. It will then become an immediate hazard in its dust form if a source of ignition is available. Pneumatic conveying equipment and dust control exhaust systems that transport combustible particulate solids need to be protected from fire and dust explosions. Combustible particulate solids that have settled onto surfaces such as floors, platforms, suspended ceilings and building structural members as well as inside pipes and ducts can burn if exposed to a source of ignition. If combustible particulate solids are thrown into the workplace air during cleaning or by excessive drafts in the plant, the resulting combustible dust can present a fire or deflagration hazard if exposed to an ignition source. If the concentration of suspended combustible solids is above the minimum explosion concentration (MEC) and the source of ignition produces energy above the minimum ignition energy (MIE), the dust will ignite. If the combustible dust is in a confined space such as in a room, silo, bin, filter-receiver, dust collect or or cyclone, the burning dust can produce enough pressure for a deflagration to occur.

HH A ZA R DSA ZA R DS A A S S O C I AT E DS S O C I AT E D W W I T HI T H C C O M BU S T I B L EO M BU S T I B L E D D U S TSU S TS Industrial dust explosions have been a risk for as long as man has been processing, storing and transporting materials. The amount of heat generated during an explosion results in extremely high pressures damaging process equipment, halting production, and placing personnel at risk of serious injury or worse. And if the explosion is allowed to propagate, subsequent explosions can occur, often with catastrophic results. Preventing the buildup of dust is one of the key means for controlling fire and explosion hazards. The principal engineering control technology for control of dust is exhaust ventilation. The primary work practice control is good housekeeping. In February 1999, a deadly fire and explosion occurred in a foundry in Massachusetts. The Occupational Safety Health Administration (OSHA) and state and local officials conducted a joint investigation of this incident. The joint investigation report indicated that a fire initiated in a shell molding machine from an unknown source and then extended into the ventilation system ducts by feeding on heavy deposits of phenol formaldehyde resin dust. A small primary deflagration occurred within the ductwork, dislodging dust that had settled on the exterior of the ducts. The ensuing dust cloud provided fuel for a secondary explosion which was powerful enough to lift the roof and cause wall failures. Causal factors listed in the joint investigation report included inadequacies mainly in the following area of housekeeping to control dust accumulations.In January 2003, devastating fires and explosions destroyed a North Carolina pharmaceutical plant that manufactured

PPA G EA G E 1919

Page 20: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

rubber drug-delivery components. Six employees were killed and 38 people, including two firefighters, were injured. The United States Chemical Safety and Hazard Investigation Board (CSB), an independent Federal agency charged with investigating chemical incidents, issued a final report concluding that an accumulation of a combustible polyethylene dust above the suspended ceilings fueled the explosion. The Chemical Safety and Hazard Investigation Board (CSB) was unable to determine what ignited the initial fire or how the dust was dispersed to create the explosive cloud in the hidden ceiling space. The explosion severely damaged the plant and caused minor damage to nearby businesses, a home, and a school.In February 2003, a Kentucky acoustics insulation manufacturing plant was the site of another fatal dust explosion: 7 killed, 37 injured. The Chemical Safety and Hazard Investigation Board (CSB) also investigated this incident. Their report cited the likely ignition scenario as a small fire extending from an unattended oven which ignited a dust cloud created by nearby line cleaning. This was followed by a deadly cascade of dust explosions throughout the plant. Finely dispersed airborne metallic dust can also be explosive when confined in a vessel or building. In October 2003, an Indiana plant where auto wheels were machined experienced an incident: 1 killed, 1 injure. A story similar to the previously discussed organic dust incidents: aluminium dust was involved in a primary explosion near a chip melting furnace, followed by a secondary blast in dust collection equipment. In the late 1970s a series of devastating grain dust explosions in grain elevators left 59 people dead and 49 injured. In response to these catastrophic events, Occupational Safety Health Administration (OSHA) issued a “Grain Elevator Industry Hazard Alert” to provide employers, employees, and other officials with information on the safety and health hazards associated with the storage and distribution of grain. The lessons learned in the grain industry can be applied to other industries producing, generating, or using combustible dust. The elements needed for a fire are the following:(1) Combustible dust (fuel);(2) Ignition source (heat);(3) Oxygen in air (oxidizer). Additional elements needed for a combustible dust explosion are: (1) Dispersion of dust particles in sufficient quantity and concentration;(2) Confinement of the dust cloud.

The addition of the latter two elements to the fire triangle creates what is known as the “explosion pentagon”, as shown in Figure 1.04. If a dust cloud (diffused fuel) is ignited within a confined or semi-confined vessel, area, or building, it burns very rapidly and may explode. The safety of employees is threatened by the ensuing fires, additional explosions, flying debris, and collapsing building components. An initial (primary) explosion in processing equipment or in an area were fugitive dust has accumulated may shake loose more accumulated dust, or damage a containment system (such as a duct, vessel, or collector). As a result, if ignited, the additional dust dispersed into the air may cause one or more secondary explosions. These can be far more destructive than a primary explosion due to the increased quantity and concentration of dispersed combustible dust. If one of the elements of the explosion pentagon is missing, a catastrophic explosion can not occur. Two of the elements in the explosion pentagon are difficult to eliminate: oxygen (within air), and confinement of the dust cloud (within processes or buildings). However, the other three elements of the pentagon can be controlled to a significant extent.Everyone wants to assure that their plant is safe to work in. But how do you begin to evaluate the safety conditions in your plant if it is processing combustible particulate solids? A combustible dust explosion hazard may exist in a variety of industries, including: food (e.g. candy, starch, flour, feed), plastics, wood, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals (e.g. aluminium, chromium, iron, magnesium, and zinc), and fossil fuel power generation. The vast majority of natural and synthetic organic materials, as well as some metals, can form combustible dust, any industrial process that reduces a combustible material and some normally non-combustible materials to a finely divided state presents a potential for a serious fire or explosion. Facilities should carefully identify the following in order to assess their potential for dust explosions:(1) Materials that can be combustible when finely divided;(2) Processes which use, consume, or produce combustible dusts;(3) Open areas where combustible dusts may build up;(4) Hidden areas where combustible dusts may accumulate;(5) Means by which dust may be dispersed in the air;(6) Potential ignition sources.

The primary factor in an assessment of these hazards is whether the dust is in fact combustible. Any “material that will

PPA G EA G E 2020

Page 21: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

burn in air” in a solid form can be explosive when in a finely divided form. Combustible dust is defined as “any finely divided solid material that is 420 microns or smaller in diameter and presents a fire or explosion hazard when dispersed and ignited in air”.

Figure 1.04 – Dust fire and explosion pentagon.

Different dusts of the same chemical material will have different ignitability and explosibility characteristics, depending upon many variables such as particle size, shape, and moisture content. Additionally, these variables can change while the material is passing through process equipment. For this reason, published tables of dust explosibility data may be of limited practical value. In some cases, dusts will be combustible even if the particle size is larger than that specified in the definition, especially if the material is fibrous. Dust collection is best accomplished at the source, at the point of operation of the equipment, if feasible. For many pieces of equipment, well-designed ducts and vacuum hoods can collect most of the dust generated before it even reaches the operator. Very fine dust that manages to escape point-of-source collection can be captured from above by general exhaust points located along the ceiling. These control technologies are effective for most equipment, excepting machines that commonly produce the very finest dust or large quantities of dust. The amount of dust accumulation necessary to cause an explosive concentration can vary greatly. This is because there are so many variables: the particle size of the dust, the method of dispersion, ventilation system modes, air currents, physical barriers, and the volume of the area in which the dust cloud exists or may exist. As a result, simple rules of thumb regarding accumulation (such as writing in the dust or visibility in a dust cloud) can be subjective and misleading. The hazard analysis should be tailored to the specific circumstances in each facility and the full range of variables affecting the hazard. Many locations need to be considered in an assessment. One obvious place for a dust explosion to initiate is where dust is concentrated. In equipment such as dust collectors, a combustible mixture could be present whenever the equipment is operating. Other locations to consider are those where dust can settle, both in occupied areas and in hidden concealed spaces. A thorough analysis will consider all possible scenarios in which dust can be disbursed, both in the normal process and potential failure modes. The following are some recommendations for the control of dusts to prevent explosions:(1) Minimize the escape of dust from process equipment or ventilation systems; (2) Use dust collection systems and filters; (3) Utilize surfaces that minimize dust accumulation and facilitate cleaning; (4) Provide access to all hidden areas to permit inspection; (5) Inspect for dust residues in open and hidden areas, at regular intervals; (6) Clean dust residues at regular intervals; (7) Use cleaning methods that do not generate dust clouds, if ignition sources are present; (8) Only use vacuum cleaners approved for dust collection;

PPA G EA G E 2121

Fire and Explosion

Confinement ofDust Cloud

Energy(Ignition)

Dispersion ofDust

Particulates

CombustibleDust

Oxygen Source(Air)

Page 22: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

(9) Locate relief valves away from dust hazard areas; (10)Develop and implement a hazardous dust inspection, testing, housekeeping, and control program (preferably in

writing with established frequency and methods).

Good housekeeping extends to periodic hand cleaning of your entire facility, as some dust will escape from even the best exhaust system and will eventually accumulate on rafters and other out-of-the-way spots. Also, it is extremely important to inspect and clean your exhaust ventilation system on a regular basis to maintain maximum efficiency. Workers are the first line of defence in preventing and mitigating fires and explosions. If the people closest to the source of the hazard are trained to recognize and prevent hazards associated with combustible dust in the plant, they can be instrumental in recognizing unsafe conditions, taking preventative action, and alerting management. While standards require training for certain employees, all employees should be trained in safe work practices applicable to their job tasks, as well as on the overall plant programs for dust control and ignition source control. They should be trained before they start work, periodically to refresh their knowledge, when reassigned, and when hazards or processes change. A qualified team of managers should be responsible for conducting a facility analysis (or for having one done by qualified outside persons) prior to the introduction of a hazard and for developing a prevention and protection scheme tailored to their operation. Supervisors and managers should be aware of and support the plant dust and ignition control programs. Their training should include identifying how they can encourage the reporting of unsafe practices and facilitate abatement actions. Inspection and maintenance have various benefits. These benefits can include: (1) Less process downtime. (2) Greater plant morale and productivity. (3) Reduced maintenance costs. (4) More effective dust control for a heal thier environment and improved product quality. (5) Good public relations.

RR IS KIS K A A S S E S S ME N TS S E S S ME N T F O RF O R D D U S TU S T E E XP L O S IO NXP L O S IO N

Some of the most destructive explosions have been caused by dust. There is more explosive energy in the dust from grains such as wheat, barley and corn, than in an equal amount of TNT. While fires are far more common, explosions are far more costly in terms of loss of life, injury and property damage. Dust explosions have been quite common in the past. For example, there were 645 explosions in the coal mines of England from 1835 to 1850, in which coal dust was the major contributing factor. There were 1,085 dust explosions resulting in 351 fatalities, in the United States of America from 1900 to 1956. Also in the United States of America, among the 15,000 grain handling facilities from 1958 to 1977, there were 220 dust explosions resulting in 48 deaths and 500 injuries. The potential for a dust explosion has now become a well recognised hazard. Any combustible solid material which can be dispersed in air as a dust cloud, is capable of causing a dust explosion. Explosions on record have originated from dusts from the following sources:(1) Agriculture – grain dust, flour, sugar, milk powder, wool, paper, and wood.(2) Metals – aluminium, magnesium, zinc.(3) Mining – coal, combustible sulphide ores.(4) Chemical industry – sulphur, most plastics, pesticides, pharmaceutical's including aspirin and vitamin C.

The Bureau of Mines of the United States Department of the Interior developed an arbitrary scale (see Table 1.04) based on tests using small amounts of dusts, as a guide to the degree of hazard of each type of dust. There are two terms used on the scale, the ignition sensitivity (how easy to ignite), and explosion severity (how big a bang).

Table 1.04 – Explosion hazard scale for dust.

ExplosionExplosion Ignition SensitivityIgnition Sensitivity Explosion SeverityExplosion Severity

Weak < 0.2 < 0.5

Moderate 0.2 – 1.0 0.5 – 1.0

Strong 1.0 – 1.5 1.0 – 2.0

Severe > 5.0 > 2.0

The higher the number on each scale the greater the hazard that each dust represents (Table 1.05). A dust explosion is the very rapid combustion of a dust cloud, to produce a flame and a pressure front. The flame front frequently causes loss of life, while the pressure front will often cause extensive damage to buildings. With some dusts, there is sometimes

PPA G EA G E 2222

Page 23: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

only a flame front and the pressure front is only minimal; with others, the flame spreads with the effect of an explosion. An explosion occurs as the flame generates heat and combustion products, and expansion from both these sources causes an immediate pressure rise which must then move out as a pressure wave and impact on the surroundings. Pressures of up to 700 kPa to 800 kPa can be generated, and if it occurs in a confined space such as a building, the effects can be devastating, since most buildings can only withstand 2 kPa at most.

Table 1.05 – Explosion potential of some dusts.

Type of DustType of Dust Ignition SensitivityIgnition Sensitivity Explosion SeverityExplosion Severity

Aluminium 7.3 > 10.2

Aspirin 2.4 > 4.3

Coal 2.2 1.8

Coffee Bean 0.1 0.1

Cotton < 0.1 < 0.1

Egg White < 0.1 0.2

Flour 2.1 1.8

Grain Dust 2.8 3.3

Magnesium 3.0 7.4

Milk (powdered) 1.6 0.9

Polyethylene 24.0 2.2

Polystyrene 6.0 2.0

Rubber 4.6 1.6

Sugar 4.0 2.4

Sulfur 20.2 1.9

Vitamin C 1.0 2.2

Explosion will produce blast wave, load noise, fly debris and vibration. All these will effect to the human and environment surrounding the explosion event. The main factor which govern the magnitude peak overpressure in a blast wave from the detonation in free air are as following: (1) distance of the wave from the center of explosion (D), (2) the weight of the charge (W), (3) the explosion parameters of the charge. With the assumption, it is pessimistic to express the relationship between the weight of the explosive charge (W), to the shockwaves effect at a given distance (D). A common empirical formula which had been widely used to estimate the blast effect of the explosive as follow,

D=Z⋅W 0.33 [1.34]

where Z is the scale factor, D is the distance from the centre of explosion (m), W is the weight of charge in TNT equivalent mass (kg). Where weight of charge (W) in TNT equivalent mass (kg) could be calculated by the following equation,

W =M⋅Co [1.35]

where M is the weight of explosive (kg), and Co is conversion factor for a particular explosive or dust. Since Z is the scale factor for the overpressure value (P), the overpressure value can be estimated by using this formula,

P=101.3⋅103⋅P s [1.36]

PPA G EA G E 2323

Page 24: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

where Ps is the scale overpressure, and P is the overpressure in Pascal (Pa). The explosion overpressures depend on the peak overpressure that reaches the person. Direct exposure to high overpressure levels may be fatal. The fatality is a result of the explosion even though the overpressure that caused the structure collapse would not directly result in a fatality if the person were in a open area. Two blast wave effects to human were estimated there are: fatality and eardrum rupture. The probit equation for these two effects are as follow in Table 1.06.

Table 1.06 – Explosion hazard scale for dust.

EffectsEffects Probit EquationProbit Equation

Human Fatality Y = −77.1 + 6.91∙Log(P)

Ear Drum Rupture Y = −15.6 + 1.93∙Log(P)

Where P is the overpressure value in unit N∙m2. The load noise is evaluated by use the following equation,

LP=20⋅log P20⋅10−6 [1.37]

Contributing FactorsDust clouds may be ignited by the effects of mechanical friction such as overheated bearings, motors overheating from air cooling vents being clogged with dust, particles of steel or stone caught up in grinding machinery producing sparks, overheated dust coated light bulbs, static electricity, electrical arcing, welding sparks and naked flames. As a general rule, dusts require 20 to 50 times more energy from an ignition source compared with a flammable vapour, or they need direct contact with surface temperatures ranging from 300°C to 600°C. The finer the dust the greater the hazard. Not only can it be more easily blown into the air, it will stay suspended in air much longer. It has a greater surface area per unit volume so that it can burn all the more rapidly, increasing the intensity of the flame front and the violence of the explosion. Dusts like flammable vapours, have lower and upper explosive limits. The lower limit is the concentration of dust in air to just sustain the flame front. The lower flammability limit ranges from about 10 gm∙m−3 to 40 gm∙m−3

depending on the type of dust. At these concentration it will be quite visible to the naked eye as a fog or cloud. The upper limit is usually difficult to measure since there appears to be no clear cut-off point. Instead it may, or may not, ignite at a given concentration. If it does ignite, it tends to leave behind increasing amounts of charred residue. Concentrations of dust which are potentially explosive are intolerable for people to remain in and are not likely to be found in the open, however they can be found around machinery used for crushing, grinding, sanding, milling, filtering, blending, shredding, spray drying, or conveying bulk quantities of solid materials. Why so dangerous? Dust explosions are dangerous, because they can set off a chain reaction. The initial explosion is usually small and localised, however it is often sufficient to disturb surrounding dust deposited on floors, roofs, beams, and machinery to form a second much larger cloud, which in turn can lead to a far more devastating explosion. Further explosions can follow in other parts of the building or even neighbouring buildings. These explosions may occur seconds or even minutes apart, and have been described by those who have survived a dust explosion, as sounding like “rolling thunder”. A fire can then follow from scattered burning particles, or from other small dust accumulations that have been ignited.

Incidents Involving DustThe initial explosion need not necessarily be caused by dust. It may only be a small gas or vapour explosion, or even a mechanical or man-made disturbance sufficient to create a dust cloud. One incident on record involved an explosion of an air compressor in a wool combers store. It did not hurt anybody, however it triggered a dust cloud, which ignited and the resultant flame shot right across the room with 18 workers being burned, one fatally. In another, a fitter died after being enveloped in flame after walking across a 6 metre grating platform. The vibrations from his footsteps created a cloud of polymethylmethacrylate (a plastic!) dust, which dropped onto a shrink wrapping machine fuelled by an open propane gas flame.On the 15th March, 1987, in Harbin, China, an explosion of flax dust triggered a chain of explosionsthat destroyed the whole 13,000 m2 linen plant, causing 58 deaths and 177 injuries. The strength and time of each explosion was detected at the nearby earthquake monitoring station.On the 2nd March, 1982, in the United Kingdom, an articulated lorry carrying 19 tonnes of powdered resin crashed into a roadside cottage spilling part of its load. A dust cloud was thrown up, which ignited and exploded, setting fire to the entire load. Noxious fumes from the fire affected the driver, occupants of the cottage, workmen at the nearby building

PPA G EA G E 2424

Page 25: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

site and some fishermen in a boat about 1.5 kilometres out to sea.In the United States of Amrica on 23rd December, 1977, at Westwego, Louisiana, a series of explosions destroyed 37 grain silos completely and damaged many others, causing 35 deaths and USD100,000,000 worth of damage. The facility consisted of 73 silos with a capacity of 150,000 m3. It was one of 5 grain storage explosions that occurred over an 8 day period, and was attributed to unusually dry weather conditions.On the 6th February, 1979, in Bremen, Germany, the greater part of the 40,000 square metre Roland Mill complex was destroyed by a series of explosions. The complex included a seven story flour store, six story mill, other silos and administration building. A pressure wave struck a loaded truck, throwing it against a wall spreading its load into the air and believed to have caused an open air explosion. No traces were found of seven of the fourteen killed in the fire, believed to be cremated in parts of the fire where all traces of combustibles were consumed; an estimated 1,000°C for several hours.

HAZARDOUS AREA CLASSIFICATIONHAZARDOUS AREA CLASSIFICATIONThe concept of assessing and limiting the risk associated with installing electrical devices in areas where potentially explosive atmospheres may be present is referred to as area classification. Hazardous area classification assessment is a probability analysis and risk assessment evaluation of a manufacturing or process area processing a potentially flammable atmosphere that focuses exclusively on minimizing or eliminating electrical energy as a potential ignition source. Hazardous area classification is not intended to be a secondary line of defense against poor process design, poor facility and equipment maintenance, fault equipment operation, or catastrophic vapor releases. Hazardous areas are divided into three distinct classes that totally depend on the material type that is encountered in the process.

CC L A S SL A S S I H I H A ZA R DO U SA ZA R DO U S A A R EA SR EA S

These are locations where flammable gases or vapors are or may be present in the air in quantities sufficientto produce an explosive or ignitable mixture. In Class I areas that utilize the division concept methodology,two distinct divisions are predicted on the operational interpretation of normal versus abnormal and frequentversu infrequent. Division I locations where ignitable concentrations of flammable gases r vapors can existsre due to:(1) Under normal operating conditions.(2) Frequently because of maintenance or repair.(3) Frequent leakage.(4) Below grade where adequate ventilation does not exist.(5) When releases from faulty process equipment operations result in the simiultaneous failure of equipment and

machinery.

Division II locations here ignitable concentrations of flammable gases or vapors can exist are:(1) Failure of closed containment systems.(2) Abnormal operation or failure of processing and ventilation equipment.(3) Area is adjacent to a Division I location.

In Class I hazardous areas that utilize the division concept methodolog, four distinct groups are based solely on the liquid or gas ease of ignitability and its corresponding range of flammability or explosivity (ER, range of explosivity in volume percent):(1) Group A – Atmospheres that contain acetylene (2.5% ≤ ER ≤ 100%).(2) Group B – Fammable gas (e.g. hydrogen) or vapor atmospheres having either a maximum experimental safe gap

(MESG) less than orequal to 0.4 mm or a minimum ignition current (MIC) ratio less than or equal to 0.40 mm (4.0% ≤ ER ≤ 75.0%).

(3) Group C – Fammable gas (e.g. ethylene) or vapor atmospheres having either a maximum experimental safe gap (MESG) greater than 0.45 mmm and less than orequal to 0.75 mm or a minimum ignition current (MIC) ratio greater than 0.40 mm and less than or equal to 0.80 mm (2.7% ≤ ER ≤ 36.0%).

(4) Group D – Fammable gas (eg. propane) or vapor atmospheres having either a maximum experimental safe gap (MESG) greater than 0.75 mmm or an minimum ignition current (MIC) ratio greater than 0.80 mm (2.1% ≤ ER ≤ 9.5%).

The explosive ranges are base on normal atmospheric pressure and temperature. As the mixture temperature increases,

PPA G EA G E 2525

Page 26: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

the flammable range shifts downward. As the mixture temperature decreases, the flammable range shifts upward. It can be easily determined from examining the above statements that the mixture volatility is much greater for Group A mixtures compared to Group D. Classes of combustible liquids include Class II which is any liquid with a flash point greater than 100°F (37.8°C) and less than 140°F (60°C). Class III liquids are liquids with a flash point greater than 140°F (60°C). Class III liquids are further divided as either Class III-A liquids or Class III-B liquids. Class III-A liquids have flash point greater than 140°F ()60°C and less than 200°F (93.3°C). Class III-B liquids have a flash point greater than 200°F (93.3°C).

CC L A S SL A S S II H II H A Z A R D O U SA Z A R D O U S A A RE A SRE A S

These are hazardous locations because combustibledust is present. Combustible dust is defined as any solid material 420 microns or less in diameter that present a fie or an explosion hazard when dispersed in air. Like Class I areas, Class II areas are also divided into two distinct divisions that again depend on operational interpretation of normal versus abnormal conditions. Division I is a location where combustible dust is present in the air:Under normal operating conditions, in quantities sufficient to produce an explosive or ignitable mixture.(1) The dust is electrically conductive. Dusts are considered to be electrically conductive if the electrical resistivity of

the solid material from which the dust is formed has a value of less than 105 Ω∙cm.(2) Releases from faulty operation of process equipment result in the simultaneous failure of the electrical equipment,

causing the electrical equipment to become a source of ignition.

Division II is a location where combustible dust is:(1) Present in the air only under abnormal operating conditions in quantities sufficient to produce an explosive or

ignitable mixture.(2) Accumulations are normally insufficient to interfere with the normal operation of the electrical equipment or other

apparatus, but combustible dust could be in suspensions in the air due to infrequent process equipment malfunctions.

(3) Accumulations on, in, or in the vicinity of the electrical equipment could be sufficient to interfere with the safe dissipation of heat from electrical equipment, or could be ignitable by abnormal operation or electrical equipment failure.

In Class II areas three distinct goups are based primarily on the physical characteristics of the dust:(1) Group E – Atmospheres that contain combustible metal dusts, including aluminium, magnesium, and their

commercial alloys, or other combusible dusts whose particle size, abrasiveness and conductivity present similar hazards in the use of electrical equipment.

(2) Group F – Atmospheres that contain combustible carbonaceous dusts that have more than 8% total entrapped volatiles or that have been sensitized by other materials so that they present an explosion hazard. Representative combustible dusts that fall into this grouping are coal, carbon black, charcoal and coke.

(3) Group G – Atmospheres containing other combustible dusts, including flour, grain, wood flur, plastic and chemicals.

Explosion severity is a measure of the damage potential of the energy release by a dust explosion. The United States of America Bureau of Mines (USBM) has defined the equation for calculating explosion severity (Sexp) as,

Sexp=Pmax⋅P State1

Pmax⋅P State 2[1.01]

where Pmax is the maximum explosion pressure (bar), and P is the maximum rate of pressure rise (bar∙s–1); subscript «State 1» refers to the values used for Pittsburgh seam coal (Pmax is 8.1 bar, P is 214 bar∙s–1), and subscript «State 2» refers to the values for the specific dust in question. Ignition sensitivity is a measure of the ease by which a cloud of combustible dust can be ignited. The United States Bureau of Mines has defined the equation for calculating ignition sensitivity (Isen) as,

I sens=T c⋅E⋅M c[State1 ]

T c⋅E⋅M c [State2 ][1.02]

where Tc is the minimum ignition temperature (°C), E is the minimum ignition energy (mJ, milijoule), and Mc is the

PPA G EA G E 2626

Page 27: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

minimum explosion concentration (g∙m–3 or gpcm); subscript «State 1» refers to the values used for Pittsburgh seam coal (Tc is 591°C, E is 160 mJ, Mc is 70 g∙m–3), and subscript «State 2» refers to the values for the specific dust in question. Dusts that have ignition sensitivities equal to or greater than 0.2 or explosion severities equal to or graeter than 0.5 are considered to have enough volatility to warrant locations processing these dusts to be classified. The material published by United States Bureau of Mines (USBM) is no longer in print and copies are hard to find.

CC LA S SLA S S III H III H A ZA R DO U SA ZA R DO U S A A R EA SR EA S

These are hazardous locations because easily ignitable fibers and flyings are present. In Class III areas, there are no goupings as in Class I and Class II areas. There are, however, divisions that are based on how the material is processed. Division I is a location where easily ignitable fibers producing combustible flyings are handled, manufactured or used. Division II is a location where easily ignitable fibers are stored or handled other than in the manufacturing process.

AA R EAR EA C C LA S S I F I CAT I O NLA S S I F I CAT I O N A A S S ES S M E NTS S ES S M E NT

A risk assessment methodology must be developed prior to beginning the actual area classification assessment itself. This methodology sets the ground rules by which the assessment is conducted. Once the risk assessment methodology is developed, then the actual process of classifying the area is ready to begin. A typical assssment study will include seven basic steps:(1) Step I – Obtain the required documentation that was determined from the assessment methodology. Provide a

lower level view of the process for equipment identification and process arrangements.(2) Step II – Field-survey the area in question to determine if the plot plans are accurate and verify location of all point

sourcesof emissions.(3) Step III – Determine the classified area extent that surrounds each point source will play in the overall composite

area classification diagram. The extent of classification diagrams should come from NFPA 497 (National Fire Protection Association) for petrochemical applications, API RP500 (American Petroleum Institute) for petroleum refinery applications or gas dispersion modelling software tools. Gas and vapor dispersion modeling software should be utilized when one out of these three scenarios exists: Extreme process conditions are encountered such as large flowrates (> 250 gpm), pressures (> 275 psig), and liquids with a vapor pressure above 70 psia at operating temperature. Combustibe liquids are heated to temperatures above 100°F (37.8°C) of their respective flash points. The stream composition is a complex mixture of hydrocarbons.

(4) Step IV – Develop the composite area classification plan drawing that embellishes the contribution of all point sources.

(5) Step V – Develop elevation drawingsto provide clarity where here are emission sources located in multilevel process structures. A lan view will be required for each level in the process structre.

(6) Step VI – Conduct the compliance audit.(7) Step VII – Create a detailed assessment report that documents the following informaton: the rationale used to

classify the areas; the critical process material information usuall obtained from the material safety data sheets (MSDS); a detailed listing of all point sources of emissions that appear on the drawings; special out-of-the-ordinary exceptions that were taken when classifying a particular location; the results or findings obtained from the compliance audit; a vapor dispersion modeling graph. All area classification documentation should be placed under the protection of the facilities management of change process control. As modifications are made to the facility, these documents should be reviewed to verify the impact of these modifications.

PP RO TE C T IO NRO TE C T IO N M M ET H O D SET H O D S A N DA N D H H A Z A R DA Z A R D R R ED U C T IO NED U C T IO N

Hazard reduction is where a facility reduces the probability or risk of significant property damage and loss of life as the result of an explosion or a fire. It helps ensure that installing equipment or machinery in a hazardous location does not significantly raise the risk or probability of an explosion or fire. This is the point where steps are taken to provide compliance with the area classification assessment. Mitigation options are discussed and corresponding action items are carried out. In Class I areas it is important to follow the key protection methods:(1) Physically isolate the hazard by placing or relocating the arc-producing electrical devices to a nonhazardous area;

this is an attractive option when approved equipment for the classified area is not readily or commercially available.(2) Confining explosion is the most common and widely accepted protection method. It deploys the use of devices that

are vendor-certified, through listing or labeling, as explosion-proof. Explosion-proof means that the device enclosure is designed and tested in a manner that guarantees if a flammable vapor enters the enclosure and is ignited by an electrical arc or a hot surface within the enclosure, the resulting explosion is contained within the enclosure. The electrical apparatus contained within the enclosure should still be operational.

PPA G EA G E 2727

Page 28: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

(3) Energy limiting is known as an intrinsic safety measure, that prevents ignition by limiting the released energy resulting from wiring and component failures or faults. Underwriters Laboratory (UL) listed intrinsically safe electrical devices are incapapble of releasing enough energy under normal or abnormal conditions to cause ignition of a specific hazardous atmosphere in its most easily ignitable concentrations.

(4) Hermetically sealed types of protection ensure that arc or heat-producing devices are sealed against the intrusion of the hazardous vapor.

(5) Pressurization is the process of supplying an enclosure with a protective gas with or without continuous flow to prevent the entrance of a flammable vapor, combustible dust or ignitable fiber.

(6) Purging is the process of supplying an enclosure with a protective gas at a sufficient flow and positive pressure to reduce the concentration of any flammable vapor initially present to a safe level.

In Class II areas it is important to follow the key protection methods:(1) Physically isolate the hazard in the same manner as for Class I areas.(2) Utilization of dust ignition-proof equipment requires that the enclosure is dust-tight, and the enclosure is

constructed so that heat generated inside will not ignite a dust layer on or a combustible cloud surrounding the enclosure.

(3) Purging may be used as long as the NFPA 496 requirements are followed.(4) Energy limiting is at the same level of protection as in Class I areas.

In Class III areas are employed the same methods that were utilized for Class II areas. The basic requirement is to make use of dust-tight enclosures for all normal arc-producing electrical devices and electrostatic producing devices.

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESAmyotte, P., Kahn, F., and Dastidar, A., 2003. Reduce Dust Explosions the Inherently Safer Way, Chemical Engineering Progress, vol. 99, no. 10, October 2003, pp. 36-43.Bartknecht, W., 1989. Dust Explosions: Course, Prevention, and Protection, Springer-Verlag.Bielanski, A., Haber, J., 1991. Oxygen in Catalysis, Marcel Dekker, Inc., New York, 46.Bielanski, A., Haber, J., 1991. Oxygen in Catalysis, Marcel Dekker, New York, 54.Bowen, J. E., An Overview: Grain Dust Explosions. Fire Engineering, 136(1983):22,23,25,27.Britton, L. G., Cashdollar, K. L, Fenlon, W., Frurip, D., Going, J., Harrison, B. K., Niemeier, J., and Ural, E. A., The Role of ASTM E27 Methods in Hazard Assessment Part II: Flammability and Ignitability, Process Safety Progress, v. 24, pp. 12-28, 2005.Carey, F. A., 1992. Organic Chemistry, 2nd edition, McGraw-Hill, Inc., New York, p. 4559.Cashdollar, K. L., Overview of Dust Explosibility Characteristics, Journal of Loss Prevention in the Process Industries, v. 13, pp. 183-199, 2000.Chemical Safety and Hazard Investigation Board (CSB). CSB Investigators Find Likely Source of Dust Explosion at Indiana Automotive Plant. CSB News Release, Washington, DC, November 5, 2003.Chemical Safety and Hazard Investigation Board (CSB). Investigation Report: CTA Acoustics, Inc. Combustible Dust Fire and Explosions. CSB, Washington, DC, September 2004.Chemical Safety and Hazard Investigation Board (CSB). Investigation Report: West Pharmaceutical Services, Inc. Dust Explosion. CSB, Washington, DC, September 2004.Cross, J., and Farrer, D., Dust Explosions, New York: Plenum Press, 1982.Eckhoff, Rolf K., 2003. Dust Explosions in the Process Industries, 3rd Edition, Gulf Professional Publishing.Finnerty, A. E., 1976. The Physical and Chemical Mechanisms Behind Fire-Safe Fuels, BRL-1947, Ballistic Research Labs, Abderdeen Proving Ground, MD.Fire Protection Handbook, Ed., McKinnon GP, 14th ed., 1976, NFPA, Boston, Mass.FM Global, Prevention and Mitigation of Combustible Dust Explosions and Fire, Data Sheet No. 7-76, January 2005.Frank, Walter. Dust Explosion Prevention and the Critical Importance of Housekeeping, Process Safety Progress, vol. 23, no. 3, September 2004, pp. 175-184.Hatwig, M., and Steen, H. (eds.), Handbook of Explosion Prevention and Protection, Wiley-VCH, 2004.Hodnett, B. K., 2000. Heterogeneous Catalytic Oxidation, John Wiley & Sons, New York.March, J., 1992. Advanced Organic Chemistry, 4th edition, McGraw-Hill Co., New York, NY, 188 , 248-254, 176, 326-327.McCracken, D. J., 1970. Hydrocarbon Combustion and Physical Properties, Ballistic Research Laboratory Report No.

PPA G EA G E 2828

Page 29: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

1496, Aberdeen Proving Ground, Aberdeen, MD.National Fire Protection Association (NFPA). Industrial Fire Hazards Handbook, 3rd Edition. NFPA, Inc., Quincy, MA, 1990.Nelson P., Dust Explosions: The Forgotten Killer. Australian Safety News, 65(1994):51-55.NFPA 325, Fire Hazard Properties, Flammable Liquids, Gases and Volatile Solids, National Fire Protection Association. Assn. Boston, 1994, pp. 4-5.Occupational Safety and Health Administration (OSHA), the Massachusetts Office of the State Fire Marshall, and the Springfield Arson and Bomb Squad. Joint Foundry Explosion Investigation Team Report. OSHA, Springfield, MA, (No date).Smyth, K.C., Bryner, N. P., 1990. Short-Duration Autoignition Temperature Measurements for Hydrocarbon Fuels, NISTIR 4469, National Institution of Standards and Technology (NIST), Gaithersburg, MD, December, 1990, p.36.

PPA G EA G E 2929

Page 30: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Chapter 2Chapter 2Fire Hazard Analysis TechniquesFire Hazard Analysis Techniques

efINTRODUCTIONINTRODUCTION

Quantitative fire hazard analysis is becoming the fundamental tool of modern fire safety engineering practice and is the enabling technology for the transition to performance-based codes and standards. The tools and techniques described in this chapter provide an introduction to this topic and the motivation for fire protection engineers to learn more aboutthe proper application of this technology. Predicted fire hazards are a function of the design fire scenarios analyzed. Therefore, when performing a fire hazard analysis, it is important to select design fire scenarios thatare challenging enough to represent a realistic “worst case”, but not so challenging that the likelihood of occurrence is tooremote. There are many fire hazard calculations that can be performed with a hand calculator, a simple spreadsheet, or a computer program. In some cases, these simple methods would not be sufficient, for example, in cases where compartment geometry is complex, where it is desired to optimize cost or benefit, or where predicted hazard values are very close to acceptable limits. However, even in these types of cases, simple methods can be used for initial predictions or as a reality check of results from more complex models. In any engineering analysis, it is incumbent on the user to understand the application and limitation of any methods used. This chapter outline a number of simple fire hazard calculation methods, but the applicability and limitations of the methods were not included. Users are referred to the documents in reference or in the text for information regarding the applications and limitations of any of the methods included in this chapter. Available methods to estimate the potential impact of fire can be divided into two categories: risk-based and hazard-based. Both types of methods estimate the potential consequences of possible events. Risk-based methods also analyze the likelihood of scenarios occurring, whereas hazard-based methods do not. The goal of a fire hazards analysis (FHA) is to determine the expected outcome of a specific set of conditions called a fire scenario. The scenario includes details of the space dimensions, contents, and materials of construction; arrangement of rooms in the building; sources of combustion air; position of doors; numbers, locations, and characteristics of occupants; and any other details that have an effect on the outcome of interest. This outcome determination can be made by expert judgment, by probabilistic methods using data from past incidents, or by deterministic means such as fire models. Fire models include empirical correlations, computer programs, full-scale and reduced-scale models, and other physical models. The trend today is to use models whenever possible, supplemented if necessary by expert judgment. Although probabilistic methods are widely used in risk analysis, they find little direct application in modern hazard analyses. Typically, when the potential impact of fire is estimated, a hazard basis is used. When probabilities or frequencies are considered, it is usually in the context of determining whether or not a scenario is sufficiently likely to warrant further analysis. Hazard analysis can be used for one of two purposes. One is to determine the hazards that are present in an existing or planned facility. The other use is for design, where trial design strategies are evaluated to determine whether they achieve a set of fire safety goals. Hazard analysis can be thought of as a component of risk analysis. That is, a risk analysis is a set of hazard analyses that have been weighted by their likelihood of occurrence. The total risk is then the sum of all of the weighted hazard values. In the insurance and industrial sectors, risk assessments generally target monetary losses, since these dictate insurance rates or provide the incentive for expenditures on protection. In the nuclear power industry, probabilistic risk assessment has been the basis for safety regulation. Here the risk of a release of radioactive material to the environment is commonly examined, ranging from a leak of contaminated water to a core meltdown. Available fire hazard calculation methods range from relatively simple equations that can be performed with a hand calculator to complex methods that require powerful computers, and many methods that fall between.vailable methods to estimate the potential impact of fire can be divided into two categories: risk-based and hazard-based. Both types of methods estimate the potential consequences of possible events. Risk-based methods also analyze the likelihood of scenarios occurring, whereas hazard-based methods do not.

PPA G EA G E 3030

Page 31: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

PERFORMING A FIRE HAZARD ANALYSISPERFORMING A FIRE HAZARD ANALYSISPerforming an fire hazard analysis (FHA) is a fairly straightforward engineering analysis. The steps include the following:(1) Selecting a target outcome.(2) Determining the scenario(s) of concern that could result in that outcome.(3) Selecting an appropriate method(s) for prediction of growth rate of fire effects.(4) Calculating the time needed for occupants to move to a safe place.(5) Analyzing the impact of exposure of occupants or property to the effects of the fire.(6) Examining the uncertainty in the hazard analysis.(7) Documentation of the fire hazard analysis process, including the basis for selection of models and input data.

Fire hazard analysis can also be used as part of the performance-based design process.Selecting a Target OutcomeThe target outcome most often specified is avoidance of occupant fatalities in a building. Another might be to ensure that fire fighters are provided with protected areas from which to fight fires in high-rise buildings. The objectives for such fire hazard analyses (FHA) include the following:(1) Minimizing the potential for the occurrence of fire.(2) No release of radiological or other hazardous material to threaten health, safety, or the environment.(3) An acceptable degree of life safety to be provided for contractor personnel and no undue hazards to the public from

fire.(4) Critical process control or safety systems are not damaged by fire.(5) Vital programs are not delayed by fire (mission continuity).(6) Property damage does not exceed acceptable levels (e.g. 150 million dollars per incident).

An insurance company might want to limit the maximum probable loss to that on which the insurance rate paid by the customer is based, a manufacturer might want to avoid failures to meet orders to avoid erosion of its customer base, and some businesses might want to guard their public image of providing safe and comfortable accommodations. Any combination of these outcomes could be selected as appropriate for an fire hazard analysis (FHA).

DD E V E L O P I N GE V E L O P I N G F F I R EI R E S S C E NA R IO SC E NA R IO S A N DA N D D D ES I G NES I G N F F I R EI R E S S C E NA R IO SC E NA R IO S

Determining the fire source is one of the most important parts of performing a fire hazard analysis. To determine the fire source, a design fire scenario must be developed. A fire scenario is a set of conditions that defines the development of fire and the spread of combustion products. Fire scenarios comprise three sets of features: building characteristics, occupant characteristics, equipment characteristics, material properties and characteristics, environmental characteristics, and fire characteristics. Building characteristics describe the building features that could affect fire development and the spread of combustion products. Occupant characteristics describe the state(s) of occupants at the time of the fire. Equipment characteristics describe the working features of the equipment and machinery that could be affected in a fire development and combustion situation. Materials properties and characteristics describe the physical and chemical state(s) of all materials (i.e. liquid, solid, gas, or vapor) including the potential contribution for a combustion and fire scenario. Environmental characteristics (internal and external) describe the sorrounding features that could affect the fire development and the spread of combustion products. Fire characteristics describe the ignition and growth of the fire. A design fire scenario is a set of conditions that defines the critical factors for determining the outcomes for trial fire protection designs of new buildings ad facilities or modifications to existing facilitiess. Design fire scenarios are the fire scenarios that are selected to analyze a trial design. They are generally a subset of the fire scenarios. The design fire scenario is based on a fire that has a reasonable likelihood of developing from a series of events. Fire scenarios need to be based on reality and should be developed accordingly. For example, the occupancy, the purpose for which the design is being developed, the fuel load, potential changes in the property, the presence of sprinklers and fire detection, the presence of alarm and notification systems, and smoke management should be considered. Design fire scenarios differ by occupancy and should be based on reasonably expected fires and worst-case fires. Although this chapter deals with hazard-based approaches, some risk must be included in the analysis when developing design fire scenarios. For instance, if a fire may be technically plausible but is extremely unlikely, that scenario may not be necessary to include in the design fire scenarios.

PPA G EA G E 3131

Page 32: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Determining the Scenario(s) of ConcernRecords of past fires, either for the specific building or for similar buildings or class of occupancy, can be of substantial help in identifying conditions to be avoided. Statistical data from National Fire Protection Agency (NFPA) or from the National Fire Incident Reporting System (NFIRS) on ignition sources, first items ignited, locations of origin, and the like can provide valuable insight into the important factors contributing to fires in the occupancy of interest. Anecdotal accounts of individual incidents are interesting but might not represent the major part of the problem to be analyzed. Murphy’s law, “if anything can go wrong, it will”, applies to major fire disasters; that is, significant fires seem to involve a series of failures that set the stage for the event. Therefore, it is important to examine the consequences of things not going according to plan. In fire hazard analyses (FHA), one part of the analysis is to assume both that automatic systems fail and that the fire department does not respond. This is used to determine a worst-case loss and to establish the real value of these systems. A common fire that starts while either the fire alarm system or the sprinkler system (in turn) is rendered ineffective. Given the normal high reliability of these systems, it is not required for the performance objectives to be met fully under these conditions, but stakeholders should feel that the resulting losses are not catastrophic or otherwise unacceptably severe. In a risk assessment, the consequences of such failures would be weighted by the probability of failure and added into the total risk. In a hazard analysis, the objective is hazard avoidance, so the contribution of low probability events is more subjective. Scenarios must be translated into design fires for fire growth analysis and occupant evacuation calculation.

Bounding ConditionsDuring development of the fire scenarios and design fire scenarios, the allowable future changes in the facility must also be considered. The extent of the changes that are considered by the design become bounding conditions for the analysis and subsequent use of the facility. One can expect that a design fire scenario is not exactly what will happen and that the building or facility as originally designed and anticipated will not remain exactly as analyzed. Therefore, as one develops design fire scenario and one calculates the expected fire response, some amount of change in those scenarios must also be considered. When conducting a hazard analysis, it is important to consider the types of changes that may occur. If the hazard analysis only considered a specific set of initial conditions, then it would be necessary to revise the fire hazard analysis any time changes were made in the future. Bounding conditions must be clearly identified because changes in the building may occur. Other situations that might occur on a more general basis, for any occupancy, include the response of a fire department and cutbacks in fire department funding or unwanted alarms causing deactivation of a system. Some of these bounding assumptions can be addressed specifically; for instance, maximum fuel load or occupant characteristics.

Implied RiskAlthough this chapter addresses fire hazard analysis, there is some implied risk in any such analysis. The primary risk factors involved are included in the design fire development. The design fires should include such accidents as gasoline tanker trucks crashing into the side of the facility or bombs ignited at the base of the building. There is always the risk that these events could happen, but the engineer must evaluate the likelihood of these events. For example, buildings are typically not designed to survive the impact and ensuing fire of a missile strike. If this were to occur, achievement of the design goals and objectives might not be expected. Similarly, it is conceivable that simultaneous fires could occur, although prescriptive building codes such as the National Fire Protection Agency NFPA 101 standard explicitly exclude such an event. These might be limitations described in the fire strategy report to clarify what is covered and what is not. When proposing to exclude a scenario from further consideration, it is important to ensure that stakeholders understand the implications of excluding the scenario. For example, if the fire scenario associated with a gasoline tanker truck crashing into the side of the facility is dismissed, and the facility is located to a major oil refinery, stakeholders would need to understand and accept that if a gasoline tanker truck did crash into the side of the facility, goals and objectives might not be met.

Data SourcesIn developing design fire scenarios, it is useful to have data on which to base future quantification. Members of the National Fire Protection Agency Life Safety Code Technical Committees developed the design fire scenarios based on statistical analyses prepared by the National Fire Protection Agency Fire Analysis and Research Division and also on past fires that have occurred in different occupancy types. Other sources addressing typical fires in occupancies include Factory Mutual (FM) data, state or local jurisdiction data for various occupancies, the National Fire Incident Reporting System (NFIRS), or past fire history published in the National Fire Protection Agency Journal. Other possibilities include fire test results, many of which can be found on the National Institute of Standards and Technology Fire

PPA G EA G E 3232

Page 33: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

(NISTF) internet site, manufacturers’ data regarding specific fire performance of materials, or listings of materials by recognized test labs. It can be reasonably expected that the amount of data to develop a design fire will not be sufficient to exactly predict what will happen in all cases.

Developing a ScenarioThe first step is to investigate potential fires that might occur so that the design fire scenarios can be chosen. If the purpose of the scenario is to perform an egress analysis, the worst-case fire may be all that is necessary for evaluation. To quantify the fire, users might look at the fuel load and estimate the rate of burning, they might look at a set of fire examples and extrapolate, they might look at fast or ultrafast fires and assume the fire peaks at the estimated sprinkler (or other automatic fire fighting mechanism) response time, they might assume the rate of the development of fire and so not have the fire peak at the estimated sprinkler response time, or they might specify sprinklers inside the facility and limit the fire size. The user would likely try a combination of these factors to see the effects. Once the fire scenario is developed, smoke-filling calculations can be performed to determine the clear height of a smoke layer over time. Those calculations would be compared to the timed evacuation analysis. Both calculations would likely start without suppression or smoke control to see whether the evacuation can occur without those two systems. If so, the analysis is simplified. Finally, the user would identify bounding conditions via a sensitivity analysis. For instance, is the size of the facility important? How about the materials of the surroundings and the facility? Has any of the existing fuel been removed or isolated from the facility? If smoke control is necessary to make the design work, that smoke control needs to be identified as a critical system. Similarly, the occupant load, the exit sizes, the number of disabled persons, and the availability of an alarm system as well as its audibility must all be considered in the sensitivity analyses. Once all of these factors have been considered and dealt with, the hazard-based analysis is complete. The documentation of the analysis is the next important part and cannot be omitted from any fire hazard analysis. The assumptions, bounding conditions, scenarios considered, and limitations should be identified to the auhority has jurisdiction (AHJ), the owner, and other interested parties.

Figure 1.01 – The timeline of response to a fire.

PPA G EA G E 3333

Fire

Spr

inkl

er a

nd A

utom

atc

Fire

Figh

ting

Equi

pmen

t Act

ivat

es

Smok

e A

larm

Act

ivat

es

10Time (minutes)

Consequences (Severity)

Fire Fighting BrigadesResponse to Fire and Setup Equipment

20

Detection of Fire

Reporting Fire

Fire Conditions Established

SafetyMargin

Without sprinklers FLASHOVER can occur in as little as 3 minutes

Page 34: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

QUANTIFICATION OF DESIGN FIRE SCENARIOSQUANTIFICATION OF DESIGN FIRE SCENARIOSQuantification of design fire scenarios involves two steps. The first step is to develop the design fire curve for the design fire scenario or portion of the design fire scenario of interest. The design fire curve represents the heat release rate over time for the fire in question. Once the design fire curve is estimated, the second step, predicting the fire effects, is then possible. The purpose of the design fire is similar to the assumed loading in a structural analysis; that is, to answer the question of whether the design will perform as intended under the assumed challenge. Keeping in mind that the greatest challenge is not necessarily the largest fire (especially in a sprinklered facility), it is helpful to think of design fires in terms of their growth. Figure 1.01 represents the timeline response for a given fire after combustion.

DD E S IG NE S IG N F F I R EI R E C C U RV ESU RV ES

The design fire curve is a description of the intensity (heat release rate) of a fire as a function of time. The design fire curve can be divided into four phases: ignition, growth, steady-burning, and decay. Because there is not a single framework for developing the entire design fire curve, each step is typically developed separately and then brought together as a single curve. It is not always necessary to quantify each phase of a design fire curve, depending on the goals of the analysis. For example, to predict when a fire detection or suppression system would activate, it might only be necessary to quantify the growth phase. For sizing a smoke control system, only the maximum heat release rate might be needed. A structural analysis might need the peak burning rate and the duration of peak burning. Performing an evacuation analysis might require quantification of the growth and fully developed stages. The design fire curve starts at ignition. A simple approach to developing a design fire curve is to assume that an ignition source of sufficient intensity is available to instantaneously ignite the initial fuel package to establish burning. However, if the heat transfer to a combustible object or the temperature of the object is known, calculations can be performed to predict whether the object will ignite. Calculations to determine whether ignition occurs depend on the state of the fuel: solid, liquid, or gas. Ignition can be divided into two categories: piloted and nonpiloted. In the case of piloted ignition, a “pilot” such as a spark or flame initiates flaming. For nonpiloted ignition, flaming occurs spontaneously as a result of heating in the absence of flame or spark. Except for piloted ignition of gases and liquids that are at a temperature above their flashpoint, all materials must first be heated before ignition can take place. With the exception of smoldering combustion, for a solid to ignite it must first be heated sufficiently to release flammable vapors. Flammable vapors can be given off either by pyrolysis or by melting and subsequent vaporization. Pyrolysis occurs when a material is heated and decomposes, releasing vapors known as pyrolyzates. Unlike melting and vaporization, in which no molecular changes occur, the vapors given off are different from the material that was originally heated. The process of pyrolysis can be viewed as “thermal cracking,” in which larger molecules are broken into smaller molecules. Piloted ignition occurs if the concentration of pyrolysis gases is above the lower flammable limit and a “pilot” is present. For nonpiloted ignition to occur, the pyrolysis gases must be at a concentration above the lower flammable limit and they must be above their autoignition temperature. Because of this, it requires less energy for piloted ignition to occur than for nonpiloted ignition. Methods of predicting ignition of solid materials exposed to thermal radiation differ depending on whether a solid is thermally thin or thermally thick. A thermally thick material is one in which a temperature rise will not be perceived on the unexposed surface when the material is heated. Wood is a typical example of a thermally thick material, whereas sheet metal is a good example of a thermally thin material. An engineering guide published by the Society of Fire Protection Engineers (SFPE) focusing on piloted ignition contains six methods for predicting the piloted ignition of solid materials under radiant exposure. For thermally thin materials, the method of Mikkola and Wichman can be used to calculate the time to ignition (tig, in seconds),

t ig=⋅L⋅c⋅[ T ig−T 0 ˙qext− ˙qcrit ] [2.01]

where Tig is the ignition temperature (°C), T0 is the initial temperature (°C), ρ is the density of the material (kg∙m−3), c is the specific heat of the material (kJ∙kg−1∙°C–1), L is the thickness of the material (m), ˙qext is the external heat flux

(kW∙m−2), and ˙qcrit is the critical heat flux for ignition (kW∙m−2). For thermally thick materials, the following methods can be used. Mikkola and Wichman method,

t ig=4⋅k⋅⋅c⋅[ T ig−T 0

2

˙qext− ˙qcrit 2 ] [2.02]

PPA G EA G E 3434

Page 35: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

where k is thermal conductivity (W∙m−1·K−1). The Tewarson method,

t ig=4⋅[ r

2

˙qext− ˙qmin2 ] [2.03]

where Θr is thermal response parameter (kW·sec½∙m−2), and ˙qmin is minimum heat flux for ignition (kW∙m−2). The Quintiere and Harkleroad methd,

t ig= ˙qmin

b⋅ ˙qext 2

[2.04]

for t ≤ tm, where b is a constant related to k∙ρ∙c (sec–½) and tm is characteristic time to reach thermal equilibrium (in seconds). The Janssens method,

t ig=0.563˙ k⋅⋅c

h ig2 ⋅ ˙qext

˙qcrit−1

−1.83

[2.05]

where hig is heat transfer coefficient at ignition, which incorporates both the radiative and convective components (W∙m−1·°C−1). And finally, theToal, Silcock, and Shields method,

t ig=n

˙qext− ˙qcrit n [2.06]

where φn is flux time product, and n is flux time product index is greater than or equal to 1. For a liquid to ignite, it must be at a temperature that is equal to or greater than its flashpoint. National Fire Protection Agency Fire NFPA 30 standard, “Flammable and Combustible Liquids Code”, defines flashpoint as the minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture with air, near the surface of the liquid or within the vessel used. A number of test methods can be used to measure the flashpoint of a liquid. Flashpoint is not a physical property and is instead a model of physical phenomena associated with vaporization of a sufficient quantity of fuel to establish a gaseous mixture that is at the lower flammable limit at a distance above the fuel surface and therefore can change with the test method employed. Ignition of a liquid at its flashpoint is analogous to piloted ignition of a solid, in that for ignition to occur, a pilot must be present. The analogy for nonpiloted ignition of liquids would be ignition at the auto-ignition temperature. Values for flashpoints and autoignition temperatures for some common materials can be found in National Fire Protection Agency Fire NFPA 497 standard: “Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas”. For ignition of a flammable gas to occur, it must be mixed with a sufficient quantity of oxygen for a reaction to take place. Concentrations where this occurs are represented by a flammability range, which corresponds to gas and air concentrations that are at or above the lower flammable limit and not exceeding the upper flammable limit. Flammability limits for a variety of gases can be found in National Fire Protection Agency Fire NFPA 497 standard. For mixtures of flammable gases, Le Chatelier’s principle can be used to determine the lower flammable limit. Le Chatelier’s law states that,

LFLm=100

∑i Pi

Li [2.07]

where LFLm is the lower flammability limit of the mixture, Pi is the volume fraction of gas, and Li is the lower flammable limit of gas. Following ignition, a fire might grow as it develops on the first item ignited or spreads to additional items. To determine whether spread would occur to adjacent items, the problem can be approached from the perspective of whether or not these items would ignite. For growth involving a single item, the fire could spread to unignited portions

PPA G EA G E 3535

Page 36: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

of the item. This could either lead to the entire item burning, or earlier ignited portions might burn out before the fire spreads to involve the entire item, such that the entire item is never fully involved.

PP RE D I C T IO NRE D I C T IO N O FO F F F I R EI R E E E F F E C TSF F E C TS

The primary importance of the appropriate selection of the design fire’s growth is in obtaining a realistic prediction of detector and sprinkler activation, time to start of evacuation, and time to initial exposure of occupants. In 1972, Heskestad first proposed that for the early fire growth period the assumption that fires grow according to a power law relation works well and is supported by experimental data. He suggested fires of the form,

Q=⋅t n [2.08]

where Q is he rate of heat release (kW), α is the fire intensity coefficient (kW∙s−1), t is the time (in seconds), and n is a integer number (n = 1, 2, 3). Later, it was shown that for most flaming fires, except flammable liquids and some others, the value of n is equal to 2, the so-called t-squared growth rate. A set of specific t-squared fires labeled slow, medium, and fast, with fire intensity coefficients such that the fires reached 1,000 Btu∙s−1 (1,055 kW) in 600, 300, and 150 seconds, respectively, were proposed for design of fire detection systems. Later, these specific growth curves and a fourth called “ultrafast”, which reaches 1,055 kW in 75 seconds, gained favor in general fire protection applications. This set of t-squared growth curves is shown in Figure 2.02. The slow curve is appropriate for fires involving thick, solid objects (e.g. solid wood table, bedroom dresser, or cabinet). The medium growth curve is typical of solid fuels of lower density (e.g. upholstered furniture and mattresses). Fast fires are thin, combustible items (e.g. paper, cardboard boxes, draperies). Ultrafast fires are some flammable liquids, some older types of upholstered furniture and mattresses, or materials containing other highly volatile fuels. These t-squared curves represent fire growth starting with a reasonably large, flaming ignition source. With small sources, there is an incubation period before established flaming, which can influence the response of smoke detectors. During this incubation period, the fire may not significantly grow in size, although smoke would still be produced in quantities potentially sufficient to activate smoke detectors.

Figure 2.02 – Set of t-squared growth curves for fire.This specific set of fire growth curves has been incorporated into several design methods, such as that for the design of fire detection systems in National Fire Protection Agency Fire NFPA 72 standard, “National Fire Alarm Code”. They are also referenced as appropriate design fires in several international methods for performing alternative design analyses in Australia and Japan and in a product fire risk analysis method published in this country. Although in the Australian

PPA G EA G E 3636

1,000

6,000

Time from Ignition (seconds)

Hea

t Rel

ease

Rat

e (k

W)

Ultrafast Fire Rate Slow Fire Rate

SlowMediumFast

Page 37: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

methodology the selection of growth curve is related to the fuel load (mass of combustible material per unit floor area), this is not justified, since the growth rate is related to the form, arrangement, and type of material and not simply its quantity. Consider 22 lb (9.98 kg) of wood arranged in a solid cube, as sticks arranged in a crib, and as a layer of sawdust. These three arrangements would have significantly different growth rates although representing identical fuel loads.

Steady BurningWhere a fire scenario involves a fire in an enclosure, fire growth might continue until all the combustible items within the space are involved. Once this occurs, the rate of burning is influenced by one of two factors: (1) the available ventilation or (2) the available fuel. Calculation of fire temperatures within the space is easily accomplished by use of simple algebraic equations. Although computer models are frequently used in hazard analyses, they are generally no more accurate (and indeed may be less accurate) than simple hand calculations for prediction of temperature and burning rate during fully developed burning. For example, for post-flashover fires, hand calculation methods are generally used to estimate compartment temperatures. Society of Fire Protection Engineers’ engineering guide on fire exposures of structural elements provides calculation methods for predicting fire temperatures and burning rates in fully developed compartment fires. Some of these methods are based on an assumption of ventilation limited burning, and others model fuel-controlled conditions. For most cases, the method developed by Law was found to provide bounding predictions when the ψ factor was not used and the predicted burning duration was increased by a factor of 1.4. Law’s method is as follows,

T=T gm⋅1−e−0.05⋅ [2.09]

T gm=6,000⋅[ 1−e− 0.1⋅A

A0⋅ H 0

AA0⋅H 0

] [2.10]

m f =0.18⋅A0⋅H 0⋅WD ⋅1−e

−0.036⋅AA0⋅H 0 [2.11]

where T is the bounding temperature of compartment, Tgm is the maximum compartment temperature (°C), A is the surface area of interior of enclosure (m2), A0 is the area of ventilation opening (m2), H0 is the height of ventilation opening (m), W is the width of wall containing ventilation opening (m), D is the depth of compartment (m), and mf is the mass burning rate of fuel (kg∙s−1)and. Thus, the factor ψ can be determined by,

=M f

A⋅A0[2.12]

where Mf is the mass of fuel (kg). Law reports that the correlation for predicting burning rate is valid for the condition,

m f

A0⋅H 0 ⋅ DW

1260 [2.13]

In some cases, it may only be desired to predict whether flashover is possible for a given fire scenario involving a fire in an enclosure.

Decay of FireAll fires eventually decrease in size. A fire can decay for one of three reasons: consumption of available fuel, oxygen depletion, or suppression. Because the hazards posed during the decay phase are typically insignificant in comparison to the hazards posed during the fully developed phase, decay is typically omitted from analysis. An exception is in

PPA G EA G E 3737

Page 38: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

calculations involving structural fire resistance of concrete or insulated steel. Where test data are available, they might include decay. If decay occurs due to the exhaustion of fuel, the expected temperature change as fuel is depleted. For fires with a predicted duration of less than 60 minutes, a decay rate of 10°C∙min−1 can be used. In other case, a decay rate of 7°C∙min−1 can be used. Decay could also occur in the event that a sprinkler system is present and activated. A simple assumption is that the fire immediately goes out, but this is not conservative. A National Institute of Standards and Technology (NIST) study documents a (conservative) exponential diminution in burning rate under the application of water from a sprinkler. Since the combustion efficiency is affected by the application of water, the use of values for soot and gas yields appropriate for post-flashover burning would represent the conservative approach in the absence of experimental data.

Prediction of FlashoverFlashover occurs when a fire grows to such a size that it involves all combustible items within an enclosed space. Although occurrence of flashover is not a hazard in itself, flashover would affect the occurrence of other hazards. Several correlations are available to predict the minimum heat release rate necessary for flashover to occur in a enclosed space. The time at which flashover occurs can be estimated by determining when the fire is predicted to reach this minimum size. The following methods can be used to predict the minimum heat release rate necessary for flashover.The method of Babrauskas is given by following the equation,

Q=750⋅A0⋅H 0 [2.14]

where Q is the minimum heat release rate required for flashover (kW), A0 is the area of opening into compartment (m2), and H0 is the height of opening into compartment (m). The method of McCaffrey, Quintiere, and Harkleroad, is given by following set of equations,

Q=610⋅hk⋅AT⋅A0⋅ H 012 [2.15]

hk=k

[2.16]

where k is the thermal conductivity of compartment surface (kW∙m−1·K−1), δ is the thickness of compartment surface (m), and AT is the total area of compartment surfaces (m2). The method of Thomas is given by following the equation,

Q=7.8⋅AT378⋅A0⋅H 0 [2.17]

where the variables are as defined above.

PREDICTION OF HAZARDSPREDICTION OF HAZARDSFire is a dynamic process of interacting physics and chemistry, so predicting what is likely to happen under a given set of circumstances is daunting. The simplest predictive methods are algebraic equations. Computer models are used to automate fire hazard calculations and are particularly useful where many repeated calculations must be performed.

SS I MP L EI MP L E F F I R EI R E H H A ZA R DA ZA R D C CA LC U L AT IO NSA LC U L AT IO NS

Once the design fire curve has been developed, it is then possible to predict the hazards that would result. The types of hazards that might be of interest include the following:(1) Radiant heat flux, which affects the potential for ignition of materials and damage to structures or thermal injury to

people.(2) Smoke production, which dictates the volume of smoke produced.(3) Fire plume and ceiling jet temperatures and velocities, which could cause weakening of exposed structural elements.(4) Species production, which affects the rate at which an untenable environment could be created.(5) Depth of upper layer, which can be used as a surrogate for an untenable environment.

As was the case with the stages of design fire curves, it is not always necessary to quantify all of the hazards that result

PPA G EA G E 3838

Page 39: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

from a design fire scenario. The hazards that are quantified are a function of the goals of the analysis. For example, if the purpose of the analysis is to determine whether a thermally activated detection or suppression system activates, only the plume and ceiling jet temperatures and velocities might be determined. For analysis of a smoke control system, only the smoke production rate might be determined. A structural analysis might only require calculation of the heat transfer to the structure. An evacuation analysis might require quantification of all of the hazards listed.

Radiant Heat FluxRadiant heat flux is a measure of the rate of radiative heat transfer per unit area. An example of radiant heat transfer is the heating that can be felt from exposure to the sun on a hot day, although the intensity of thermal radiation in sunlight is too small to be of concern from a fire standpoint. The radiant heat flux from a single burning item can be predicted as a function of the distance from the item. For radiant heat fluxes resulting from fire gases, such as in a compartment fire, the radiant heat flux can be calculated if the gas temperature and the temperature of the target object are known by applying the following equation,

Qr=⋅⋅T gas4 −T targ

4 [2.18]

where Qr is the rate of radiant heat transfer (kW), ε is the emissivity of gas (0 ≤ ε ≤ 1), and σ is the Stephan-Boltzmann constant (5.67 × 10–11 kW∙m−2·K−4), Tgas is the temperature of gas (K) and Ttarg is the temperature of target (K). The equation is only applicable for instantaneous calculations, as the temperature of the target will rise as a function of the thermal radiation that it receives.

Smoke ProductionWhen calculating smoke production rates, smoke is usually defined as the products of combustion and the air entrained into the fire plume. Therefore, the amount of smoke produced is a function of the height above the fire.

Fire Plumes and Ceiling Jet Temperatures and VelocitiesA fire will produce a plume of hot gas that will rise and contact the ceiling of a given space (for a compartment or enclosed space), forming a ceiling jet. Similarly, the temperature and velocity of a ceiling jet can be calculated in accordance with the following equations,

rH

≤0.18

T=16.9⋅

˙Q

23

H53

[2.19]

rH

0.18

T=5.38⋅ Q

r 23

H

[2.20]

rH

≤0.15

U=0.96⋅ QH

13

[2.21]

PPA G EA G E 3939

Page 40: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

rH

0.15

U=0.195⋅˙

Q13⋅H

12

rH

56

[2.22]

where ∆T is the temperature rise over ambient (°C), U is the ceiling jet velocity (m∙s−1), H is the height above fire (m), r is the horizontal distance from fire centerline (m), and Q is the total heat release rate (kW). When using these equations, it must be cautioned that they are only valid for horizontal, unobstructed ceilings where there is no smoke layer present. In cases where a layer forms, higher temperature rises can be expected.

Species ProductionFires can create a number of products of combustion that can be toxic or corrosive, including carbon dioxide (CO2), water vapor (H2O), carbon monoxide (CO), and many others that vary with the fuel and burning conditions. Species production rates can be calculated from the following equation,

G j= y j⋅QH c

[2.23]

where G j is the smoke production rate of species j (kg∙s−1), yj is the yield fraction of species j, and ∆Hc is the heat of combustion of fuel (kJ∙kg−1). Yield fractions for several fuels are available in the Society of Fire Protection Engineers (SFPE) “ Handbook of Fire Protection Engineering”.Depth of Upper LayerAs smoke is produced in a compartment, it forms a layer that descends as a function of time. This is analogous to filling a bowl of water. However, it should be noted that the mass production rate of smoke is not constant, since as the layer descends, the smoke production rate decreases due to the reduced vertical distance available to entrain air into the plume.

ToxicityToxic gases produced by a fire can incapacitate or kill people who are exposed to them. A commonly used approach to determine whether the fire-induced environment is potentially harmful to people exposed is the fractional effective dose (FED) model developed by Purser. This can be expressed as follows,

F INC= F COF COF HCN ⋅V HYFF LOX=F IDC [2.24]

where FINC is the fraction of an incapacitating dose of all asphyxiating gases, FCO is the fraction of an incapacitating dose of CO, FHCN is the fraction of an incapacitating dose of HCN, FIRR is the fraction of irritant dose, VHYP is the multiplication factor for carbon doixide (CO2) induced hyperventilation, FLOX is the fraction of an incapacitating dose of low-oxygen hypoxia, FIDC is the fraction of an incapacitating dose of CO2. Purser gives the following equations for calculation of the individual fractional effective doses,

F CO=8.2925×10−4⋅∣CO∣1.036

30[2.25]

where |CO| is the concentration of carbon monoxide (CO), expressed in parts per million.

F HCN = 1220

⋅10∣CN∣

43 [2.26]

where |CN| is the concentration of HCN in parts per million added to the concentration of other nitriles minus the concentration of NO2, FIRR is the fraction of the incapacitating dose from all incapacitating products (HCl, HBr, etc.).

PPA G EA G E 4040

Page 41: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

V HYF=10∣CO2∣

5 [2.27]

where |CO2| is the concentration of carbon dioxide in percent.

F LOX=1

10[8.13−0.54⋅20.9−∣O 2∣][2.28]

where |O2| is the concentration of oxygen in percent.

F IDC=1

106.1623−0.5189⋅∣CO2∣[2.29]

where |CO2| is the concentration of carbon dioxide (CO2) in percent. It should be noted that the equations for fractional effective dose (FED) and the components of fractional effective dose are based on a one minute exposure. For exposures to constant concentrations of fire products, the fractional effective dose (FED) can be determined by multiplying the value determined using theprevious equations by the exposure time in minutes. For exposures where the concentrations vary with time, the total fractional effective dose (FED) can be calculated by discretizing the exposure, i.e. determining the average exposure at each one minute interval and summing the fractional effective dose (FED) determined for each one minute interval. It should be noted when applying the previous correlations that some populations are more susceptible than others to fire products (e.g. asthmatics, the old, and the young). Additionally, no single fractional effective dose (FED) value for design has been widely agreed on even for “average” populations.

SS I MP L EI MP L E A A N A LY T IC A LN A LY T IC A L S S O LU T I O NO LU T I O N T T E C H N I Q U ESE C H N I Q U ES

Simple computer programs and spreadsheets can be used to perform simple fire hazard calculations. In the case of the equations listed previously or referenced in other chapters, this is a relatively straightforward task. However, many fires and fire effects are not steady state. An example is smoke filling within an enclosure. The smoke production rate is a function of the smoke layer height, so the rate of smoke layer descent is not constant. In such instances, spreadsheets can be used to develop solutions to differential equations for which developing an exact solutionis nontrivial. For a differential equation of the following form,

dydt

= f y , t [2.30]

where the initial value y(t = 0) is y0. The Euler method is a numerical technique for solving differential equations of this form, and can be stated as,

yn1= ynh⋅f yn ,t n [2.31]

where yn is the value of equation y at time step n, yn+1 is the value of equation y at time step (n+1), and h is the time step size. This process can be iterated over the desired length of time to obtain the desired solution. Since the Euler method determines the value of equation y at time step (n+1) based on the value at time step n and the slope of the tangent to y at time step n, errors can be introduced based on the nonlinearity of equation y. There are methods available to reduce this error, such as the improved Euler method. However, another method of reducing the error is to reduce the size of the time step, recognizing that as the size of the time step approaches zero, the difference between the predicted value of y and the actual value of y also approaches zero. The computational power offered by modern computers allows very small time steps to be used and to still get a solution rapidly. It should be noted that the default for many spreadsheets is to not permit iterative calculations. The spreadsheet views the “circular reference” as an error. Spreadsheets for which this is the case would need to be configured to allow iteration. The spreadsheet’s user’s manual or help function can be consulted for assistance.

Example of ApplicationThermal detector response can be used to illustrate application of the Euler method to a fire protection problem. This

PPA G EA G E 4141

Page 42: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

example uses an algorithm similar to that used by the computer fire model DETACT QS19 to predict the time to activation of a thermal detector for a heat release rate that follows a power law curve. Two calculations will be performed. First, the instantaneous ceiling jet velocities are calculated in accordance with Equation [2.19] through Equation [2.22]. A quasi-steady assumption is made, which means that transport delays from the fire to the detector are ignored. Then, based on the temperature and velocity of the ceiling jet and the thermal response characteristics of the sensor (response time index), the temperature change at the detector will be calculated. The change in temperature of the detector can be expressed as,

dT d

dt=U g⋅T g−T d

d[2.32]

where Td is the temperature of detector (°C), Ug is the ceiling jet velocity at detector (m∙s−1), Tg is the ceiling jet temperature at detector (°C), t is time (in seconds), τδ is the detector response time index (m½∙s½). A Euler solution to this expression can be expressed as,

T d ,n1T d , n t⋅[ U g⋅T g−T d , nd ] [2.33]

where Td,n+1 is the temperature of detector at time step (n+1), Td,n is the temperature of detector at time step n, ∆t is the size of time step (in seconds). Equation [2.33] could easily be programmed into a spreadsheet or simple computer program, along with the ceiling jet temperature and velocity correlations expressed in Equation [2.19] through Equation [2.22], to calculate Ug and Tg. It is also necessary to include a method of calculating the heat release rate at each time step.

CC O M P U T E RO M P U T E R M M O D EL SO D EL S

The key to determining which are appropriate to a given situation and which are not is a thorough understanding of the assumptions and limitations of the individual model or calculation and how these relate to the situation being analyzed. Single-space models are appropriate where the conditions of interest are limited to a single, enclosed space. Where the area of interest involves more than one space, and especially where the area of interest extends beyond a single floor, multiple-compartment models should be used. This is because the interconnected spaces interact to influence fire development and flows. Many single-compartment models assume that the lower layer remains at ambient conditions (e.g. ASET). Since there is little mixing between layers in a enclosed space (unless there are mechanical systems), these models are appropriate. However, significant mixing can occur in doorways, so multiple-compartment models should allow the lower layer to be contaminated by energy and mass. The model should include the limitation of burning by available oxygen. This is straightforward to implement (based on the oxygen consumption principle) and is crucial to obtaining an accurate prediction for ventilation-controlled burning. For multiple-compartment models, it is equally important for the model to track unburned fuel and allow it to burn when it encounters sufficient oxygen and temperature. Without these features, the model concentrates the combustion in the room of origin, overpredicting conditions there and underpredicting conditions in other spaces. Heat transfer calculations take up a lot of computer time, so many models take a shortcut. The most common is the use of a constant “heat loss fraction”, which is user-selectable (e.g. ASET or CCFM). The problem is that heat loss can vary during the course of the fire. Another problem can occur in tall spaces, for example, atria. The major source of gas expansion and energy and mass dilution is entrainment of ambient air into the fire plume. It can be argued that in a very tall plume, this entrainment is constrained. However, most models do not include this constraint, which can lead to an underestimate of the temperature and smoke density and an overestimate of the layer volume and filling rate (in enclosed spaces), the combination of which may give predictions of available safe egress times that are either greater or less than the correct value. In the model CFAST, this constraint is implemented by stopping entrainment when the plume temperature drops to within 1°C of the temperature just outside the plume, where buoyancy ceases.

DocumentationOnly models that are rigorously documented should be allowed in any application involving public health, safety, or welfare, such as in code enforcement or litigation. This means that the model should be supplied with a technical reference guide that includes a detailed description of the included physics and chemistry, with proper literature referencesa a listing of all assumptions and limitations of the model, and estimates of the accuracy of the resulting

PPA G EA G E 4242

Page 43: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

predictions, based on comparisons to experimental data. Public exposure and review of the exact basis for a model’s calculations, internal constants, and assumptions are necessary for it to have credibility in a regulatory application (e.g for any model used in a regulatory application should comply with ASTM E1472-05.25 guide). Although it may not be necessary for the full source code to be available, the method of implementing key calculations in the code and details of the numerical solver used should be included. This documentation should be freely available to any user of the model, and a copy should be supplied with the analysis as an important supporting document.

Input DataEven if the model is correct, the results can be seriously in error if the data that are input to the model do not represent the condition being analyzed. The fire hazard analysis (FHA) should include a listing of all data values used, their source (i.e. what apparatus or test method was employed and what organization ran the test and published the data), and some discussion of the uncertainty of the data and its result on the conclusions. The National Institute of Standards and Technology’s (NIST) website contains a section of well-documented data for use in calculations, called FASTDATA.

Figure 2.03 – Reduced walking speeds resulting from crowded conditions and smoke density.

Egress ModelsThe prediction of the time needed by the building occupants to evacuate to a safe area can be performed and compared to the predicted available safe egress time. Whether the evacuation calculation is done by model or hand calculation, it must account for several crucial factors. First, unless the occupants see the actual fire, time is required for detection and notification before the evacuation process can begin. Next, unless the information is compelling (such as seeing the actual fire), it takes time for people to decide to take action. The action they choose may or may not be evacuation. Finally, the movement begins. All of these factors require time, and that is the critical factor. No matter how the calculation is done, all of the factors must be included in the analysis to obtain a complete picture. An excellent discussion of this topic is found in Society of Fire Protection Engineers' Engineering Guide “Human Behavior in Fire”. The process of emergency evacuation of people follows the general concepts of traffic flow. A number of models perform such calculations and may be appropriate for use in certain occupancies. Most of these models do not account for behavior and the interaction of people (providing assistance) during the event. The literature reports incidents of providing assistance to disabled persons, again especially in office settings. If such behavior is expected, it should be included, as it can result in significant delays in evacuating a building. Crowded conditions, as well as smoke density, can result in reduced walking speeds. A person’s walking speed decreases in dense smoke until he or she moves as if

PPA G EA G E 4343

Nonirritating Smoke ZoneIrritating Smoke ZoneT

rans

ient

Zon

e

0.25

Extiction Coefficient, α (1/m)

Walking Speed of Blindfold Person

Wal

king

Spe

ed (

m/s

)

0.50 0.75 1.00

0.5

1.0

1.5

1.25

Page 44: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

blindfolded (see Figure 2.03). Care should be exercised in using models relative to how they select the path (usually the shortest path) that the person travels. Some models are optimization calculations that give the best possible performance.

AA NA LYZ I N GNA LYZ I N G T H ET H E I I M PA C TM PA C T O FO F E E XP O S U R EXP O S U R E

In most cases, the exposure will be to people, and the methods used to assess the impacts of exposure of people to heat and combustion gases involve the application of combustion toxicology models. The HAZARD I software package contains the only toxicological computer model, called TENAB, that is based on research at National Institute of Standards and Technology on lethality to rats and by Purser on incapacitation of monkeys. TENAB model accounts for the variation in exposure to combustion products as people move through a building, by reading the concentrations from the fire model in the occupied space during the time the person is in that space. If the person moves into a space with a lower concentration of carbon monoxide, the accumulated dose can decrease. Details such as these ensure that the results are reasonable. Assessing the impact of exposure to sensitive equipment is more difficult, since little data exist in the literature on the effects of smoke exposure on such equipment. Of particular importance here is the existence of acid gases in smoke, which are corrosive and especially harmful to electronics. Fuels containing chlorine (e.g. polyvinyl chlorides) have been studied. However, unless the equipment is close to the fire, acid gases, especially HCl, deposit on the walls and lower the concentration to which the equipment may be exposed. CFAST in the HAZARD I package contains a routine that models this process and the associated diminution of HCl concentration.

AA C C O U N T I NGC C O U N T I NG FO RFO R U U NC E RTA I N TYNC E RTA I N TY

Uncertainty analysis refers to dealing with the unknowns and variation inherent in any prediction. In the calculations, this uncertainty is derived from assumptions in the models and from the representativeness of the input data. In evacuation calculations, there is the added variability of any population of real people. In building designs and codes, the classic method of treating uncertainty is with safety factors. A sufficient safety factor is applied such that, if all of the uncertainty resulted in error in the same direction, the result would still provide an acceptable solution. In the prediction of fire development and filling time, the intent is to select design fires that provide a worst likely scenario. Thus a safety factor is not needed here, unless assumptions or data are used to which the predicted result is very sensitive. The fire hazard analysis (FHA) report should include a discussion of uncertainty. This discussion should address the representativeness of the data used and the sensitivity of the results to data and assumptions made. If the sensitivity is not readily apparent, a sensitivity analysis (i.e. varying the data to the limits and seeing whether the conclusions change) should be performed. This is also a good time to justify the appropriateness of the model or calculation method.

FF I N A LI N A L R R E V I EWE V I EW

If a model or calculation produces a result that seems counterintuitive, there is probably something wrong. Cases have been seen in which the model clearly produced a wrong answer (e.g. the temperature predicted approached the surface temperature of the sun), and there have been others in which it initially looked wrong but was not (e.g. a dropping temperature in a space adjacent to a room with a growing fire was caused by cold air from outdoors being drawn in an open door). Conversely, if the result is consistent with logic, sense, and experience, it is probably correct. This is also a good time to consider whether the analysis addressed all of the important scenarios and likely events. Were all the assumptions justified and were uncertainties addressed sufficiently to provide a comfort level similar to that obtained when the plan review shows that all code requirements have been met?

PPA G EA G E 4444

Page 45: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESASTM E1472, Standard Guide for Documenting Computer Software for Fire Models, American Society for Testing and Materials, W. Conshohocken, PA, 2005.Cooper, L. Y., and Forney, G. P., 1990. The Consolidated Compartment Fire Model (CCFM) Computer Code Application CCFM. VENTS Part 1: Physical Basis, NIST IR 4342, National Institute of Standards and Technology, Gaithersburg, MD.Cooper, L. Y., and Stroup, D. W., 1982. Calculating Available Safe Egress Time (ASET): A Computer User’s Guide, NBSIR 82- 2578, National Bureau of Standards, Gaithersburg, MD.Department of Energy General Memorandum, United States of America, Department of Energy, Germantown, MD, November 7, 1991.Drysdale, D., An Introduction to Fire Dynamics, 2nd ed., Wiley, Chichester, UK, 1998.Engineering Guide – Fire Exposures to Structural Elements, Society of Fire Protection Engineers (SFPE), Bethesda, MD, 2004.Engineering Guide – Human Behavior in Fire, Society of Fire Protection Engineers, Bethesda, MD, 2003.Engineering Guide – Piloted Ignition of Solid Materials Under Radiant Exposure, Society of Fire Protection Engineers, Bethesda, MD, 2002.Gann, R., et al., 2001. International Study of the Sublethal Effects of Fire Smoke on Survivability and Health (SEFS): Phase I Final Report, NIST Technical Note 1439, National Institute of Standards and Technology, Gaithersburg, MD.Heskestad, G., 1972. Similarity Relations for the Initial Convective Flow Generated by Fire, FactoryMutual Report 72-WA/HT-17, Factory Mutual Research Corporation, Norwood, MA.Heskestad, G., and Delichatsios, M. A., 1977. Environments of Fire Detectors – Phase 1: Effect of Fire Size, Ceiling Height, and Material, Vol. 2, Analysis, NBS-GCR-77-95, National Bureau of Standards, Gaithersburg, MD.Hurley, M., 2005. Evaluation of Models of Fully Developed Post-Flashover Compartment Fires, Journal of Fire Protection Engineering, Vol. 15, No. 3, pp. 173–197.National Fire Protection Agency, NFPA 10, Life Safety Code.National Fire Protection Agency, NFPA 30, Flammable and Combustible Liquids Code.National Fire Protection Agency, NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.National Fire Protection Agency, NFPA 72, National Fire Alarm Code.Peacock, R. D., Forney, G. P., Reneke, P., and Jones, W. W., 1993. CFAST, The Consolidated Model of Fire Growth and Smoke Transport, NIST Technical Note 1299, National Institute of Standards and Technology, Gaithersburg, MD.Purser, D., 2002. Toxicity Assessment of Combustion Products, SFPE Handbook of Fire Protection Engineering, 3rd ed., P. J. DiNenno et al. (eds.), National Fire Protection Association, Quincy, MA.Schifiliti, R. P., Meacham, B. J., and Custer, R. L. P., 2002. Design of Detection Systems, SFPE Handbook of Fire Protection Engineering, National Fire Protection Association, Quincy, MA,2002.Society of Fire Protection Engineers, Engineering Guide to Performance-Based Fire Protection, National Fire Protection Association, Quincy, MA, 2006.Tewarson, A., 2002. Generation of Heat and Chemical Compounds in Fires, SFPE Handbook of Fire Protection Engineering, 3rd ed., P. J. DiNenno et al. (eds.), National Fire Protection Association, Quincy, MA.Vol. 146, No. 12, 1993.Walton, W., and Thomas, P., 2002. Estimating Temperatures in Compartment Fires, SFPE Handbook of Fire Protection Engineering, 3rd ed., P. J. DiNenno et al. (eds.), National Fire Protection Association, Quincy, MA.

PPA G EA G E 4545

Page 46: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Chapter 3Chapter 3Understanding Liquid Natural Gas Fire HazardsUnderstanding Liquid Natural Gas Fire Hazards

efINTRODUCTIONINTRODUCTION

Potential hazards resulting from intentional or accidental spilling of large quantities of liquified natural gas (LNG) include thermal radiation from vapor cloud fires (also referred to as flash fires) and pool fires. There is general agreement among liquified natural gas experts regarding the following aspects of potential liquified natural gas fire and explosion hazards:(1) Vapors from large, un-ignited spills of liquified natural gas (LNG) cannot travel far into developed areas without

finding an ignition source, igniting, and burning back to the source.(2) Once delayed ignition of the vapor cloud occurs, and provided that the cloud is unconfined and rich in methane,

the liquified natural gas (LNG) vapors will burn in the form of a vapor cloud fire.(3) A vapor cloud traversing over commercial or residential terrain will almost certainly encounter an ignition source

early in its downwind drift and the resulting vapor cloud fire will burn back to the source.(4) The vapor cloud fire will burn back to the source and cause a pool fire at the source if the release is a continuous

release and the release duration is longer than the time it takes the cloud to find an ignition source.(5) If the vapor cloud is confined or the vapors contain large amounts of heavier hydrocarbons (C2+), then the flame

can accelerate and result in an explosion. The magnitude of the explosion and explosion damage will depend on several factors including the amount of vapors above the lower flammable limit, the presence of obstacles and degree of confinement, the composition of the vapor cloud, and the strength of the ignition source.

(6) If immediate ignition occurs, a pool fire will result. The extent of the pool spreading (diameter) and flame height will depend on several factors including the flow rate of liquified natural gas (LNG), the spill surface type (water or land), the spill surface geometry, spill surface roughness, release composition, release temperature, ambient wind speed, ambient temperature, and ambient relative humidity.

(7) If the liquid pool is unconfined and the inventory of liquified natural gas (LNG) is large, the fire will continue to burn until all the fuel is exhausted by the pool fire. It is not practical or even possible to extinguish large liquified natural gas (LNG) pool fires resulting from large spills of liquified natural gas unless the flow of liquified natural gas feeding the pool can be stopped.

The maximum vapor cloud fire hazard area from large liquified natural gas (LNG) spills is typically estimated by calculating a downwind dispersion distance to the lower flammable limit (LFL) and a cross-wind dispersion distance to half of the lower flammable limit (LFL) at low wind speed and stable atmospheric conditions. This maximum fire hazard zone is very unlikely to be experienced in any situation where the cloud drifts over populated areas. As indicated in point three above, the cloud will soon encounter an ignition source and burn back to the source well before the maximum hazard area is reached. Only the outdoor population present within the flammable boundaries of the vapor cloud is assumed to be injured due to (a) short exposure to very high thermal radiation fluxes from the vapor cloud fire, (b) direct flame contact, (c) secondary fires of clothing, and (d) inhalation of hot combustion products. It is assumed that people inside buildings at the time of the flash fire will not be injured. It is also assumed that people inside buildings which are ignited by flash fire or a pool fire will be able to escape from the burning structure without direct thermal impact injuries. This is because the flash fire will ignite buildings from the outside and it will take some time for the fires to penetrate inside. Published thermal radiation damage criteria often associate a level of damage with a heat flux value or an integrated heat flux value for short duration events such as fireballs. Typical values used and their observed effects are provided by American Institute of Chemical Engineers Center for Chemical Process Safety (CCPS), as shown in Table 3.01. There is general disagreement among liquified natural gas experts pertaining to the extent of thermal radiation hazard zones resulting from large liquified natural gas (LNG) pool fires due not only to uncertainties regarding flame emissive power but also the limiting thermal radiation impact criteria.

Flame Emissive PowerSome experts argue that very large liquified natural gas (LNG) pool fires such as those resulting from a terrorist attack on an liquified natural gas tanker will produce sooty flames and the flame emissive power is expected to be much less than 220 kW∙m−2. The main argument is that the pool center will be starved from oxygen. An opposite view, which is more likely to be the correct one, is that the fuel that does not burn at the center of the pool due to oxygen starvation will rise due to thermal buoyancy and burn at a higher elevation as it contacts oxygen there. As a result, the flames will

PPA G EA G E 4646

Page 47: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

be taller and the associated thermal radiation hazard footprints may be higher. In addition, in large scale pool fires field data such as the Montoir field trials, emissive power values of approximately 300 kW∙m−2 were reported. In general, the thermal radiation flux experienced by an observer exposed to a fire decreases in proportion to the square of distance from the fire source.

Table 3.01 – Thermal radiation flux.

Thermal Radiation FluxThermal Radiation Flux(kW(kW∙∙mm−−22))

Observed EffectObserved Effect

37.5 Sufficient to cause damage to process equipment.

25.0The minimum energy required to ignite wood at indefinitely long exposure (nonpiloted).

12.5The minimum energy required for piloted ignition of wood, and melting of plastic tubing. This value is typically used as a fatality number.

9.5Sufficient to cause pain in 8 seconds and second degree burns in 20 seconds.

4.0Sufficient to cause pain to personnel if unable to reach cover within 20 seconds. However, blistering of skin (second degree burns) is likely; 0% lethality.

1.6 Will cause no discomfort for long exposure.

Limiting Thermal Radiation Damage CriteriaSome experts argue that a 20 second 5 kW∙m−2 limiting thermal radiation exposure criterion is sufficient to establish safe separation distances for the general public. The main argument here is that a typical person will sense pain quickly and can run away fast enough and take shelter. This criterion is adopted for example by National Fire Protection Association NFPA-59 without reference to exposure duration. An opposite view argues that these criteria cannot be applied to sensitive population or critical areas and infrastructures. Elderly and the very young for example, constitute sensitive populations that may not be able to take cover within 20 seconds when outdoors. “Critical areas” include unshielded areas of critical importance where people without protective clothing can be expected or required at all time including during emergencies. “Critical infrastructure” includes buildings or places that are difficult to evacuate on short notice such as sport stadiums, hospitals, schools, play grounds, theaters, and so an. As a result a lower criteria is adopted by European standard EN-14737 (1.5 kW∙m−2), the United States Department of Housing and Urban Development (450 BTU∙ft−2∙hr−1 or 1.4 kW∙m−2), American Petroleum Institute standard API-5218 (1.58 kW∙m−2), and the Society of Fire Protection Engineers (SFPE Handbook) recommends a level of (2.5 kW∙m−2) as a public tolerance limit. We must recognize that in the specific case of liquified natural gas terminals large quantities of liquified natural gas (LNG) will be stored in bulk storage tanks and frequently arriving by ship. Under the right scenario, loss of containment can yield very large pool fires and the extent of the potential hazard zones must be accurately determined in order to establish aprudent estimate of a safe separation distance. We must also recognize that there are some uncertainties associated with the application of several of the models used to establish safe thermal radiation separation zones. For example, the flame height correlations have not been validated against pool fires that are several hundred meters in diameter. There are two practical approaches to addressing the issues of thermal radiation damage criteria, assuming we can all agree on what to use as a reasonable value of flame emissive power:(1) Be prudent and conservative. Set the value low enough such that anyone that is continuously exposed will not

suffer irreversible injuries.(2) Evaluate the risk accurately. Consider both the exposure duration and the exposure flux (dosage), and consider the

demographics of the current and projected population density nearby the proposed facility to be sited, i.e. what fraction of the people will be outdoor, what fraction is sensitive, where the critical locations are, etc. This approach will require a risk tolerability criterion that is acceptable to the community tolerating the risk in lieu of some economic benefit.

The National Fire Protection Association standard NFPA-59 thermal radiation criteria should not be confused with or considered as a risk acceptability criteria. Hazards are just one aspect of risk. Other important aspects of risk

PPA G EA G E 4747

Page 48: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

management include operational, economic, social, political, and environmental factors as well as the probability of the occurrence of the hazard itself. The 5 kW∙m−2 limiting criterion does not adequately represent the risks presented by an liquified natural gas facility to sensitive population and critical areas or buildings. Dosage must be considered as mentioned in second item above. The most widely recognized and used methods for establishing the impact of thermal radiation on people are those developed by TNO in the Green Book. These methods are referred to as thermal radiation probits or vulnerability models.

Thermal Radiation Damage ProbitsProbits are used to relate level of injury and exposure duration to a hazardous event of a given intensity. Hazardous events of interest in consequence modeling include dispersion leading to exposure to toxic chemicals, fires leading to exposure to thermal radiation, and explosions leading to exposure to overpressure and flying fragments. The method of probit analysis was first introduced between 1940 and 1950. A probit (probability unit, Y) is a normally distributed random variable with a mean (µ) of 5 and a standard deviation (σ) of 1. The mortality response (percent fatality) is expressed as,

P= 12⋅

⋅∫−∞

Y −5

10− u2

2 ⋅du= 121

2⋅erf Y −5

2 [3.01]

For mortality response to a toxic exposure of concentration (C) and duration (t), the function Y is given by,

Y =AB⋅ln C N⋅t [3.02]

If concentration (C) varies with time (t), then function Y can be expressed as,

Y =AB⋅ln ∫0

t

C N⋅dt [3.03]

Here, the integral containing concentration represents a dose factor. Probit analysis can also be applied to thermal radiation hazards,

Y =AB⋅ln I43⋅t [3.04]

Where, parameter A and parameter B are the probit parameters established from measurements or critically evaluated scientific data, I is the radiation intensity in W∙m−2, and t is the exposure time in seconds. The TNO Green book provides probit functions for first and second degree burns as well as lethality from exposure to heat radiation within the infra-red part of the spectrum. The last probit function reported in Table 3.02 accounts for clothing protective influence on fatality for humans. It assumes that 20% of the body area remains unprotected for an average population. As a result, the fatality for protected bodies is about 14% of the fatality for unprotected bodies.

Table 3.02 – Heat radiation probit parameters (taken from TNO Green Book).

DamageDamage Probit FuncionProbit Funcion

First degree burns Y = −39.83 + 3.02∙ln(I4/3∙t)

Second degree burns Y = −43.14 + 3.02∙ln(I4/3∙t)

Fatality (unprotected) Y = −36.38 + 2.56∙ln(I4/3∙t)

Fatality (protected) Y = −37.23 + 2.56∙ln(I4/3∙t)

The influence of running away from a location with high heat radiation to a location where the level of heat radiation is safe (approximately 1 kW∙m−2) can also be used for the assessment of injury and fatality from heat radiation. TNO considers 1 kW∙m−2 as the maximum heat-flux the skin can absorb during a long time without feeling pain. The probits presented in Table 3.02 can be modified to take that into account by replacing the exposure time (t) by an effective

PPA G EA G E 4848

Page 49: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

exposure time (teff),

t eff =t r0.6⋅xu⋅[1−1u

x⋅t v−

53 ] [3.05]

Where, tr is the reaction time and is about 5 seconds (see Table 3.03), x is the distance to 1 kW∙m−2, u is the run velocity in m∙s−1, and tv is the time to reach 1 kW∙m−2. Note that the probit equations shown in Table 3.02 should not be extrapolated to values less than 1 kW∙m−2.

Table 3.03 – Heat radiation probit parameters (taken from TNO Green Book).

IntensityIntensity(kW(kW∙m∙m−2−2))

Time to ReactTime to React(s)(s)

22 0.2

18 1.5

11 3.5

8 5.5

5 9.0

2.5 25.0

HAZARDS OF CRYOGENIC EXPOSURE ON LIQUIFIED NATURAL GASHAZARDS OF CRYOGENIC EXPOSURE ON LIQUIFIED NATURAL GASThe cryogenic nature of liquified natural gas (LNG) facilities poses a potential risk to low temperature exposure of personel, structural steel, equipment, and instrumentation, control and power cabling. The probability of cryogenic exposure from containment loss of liquified natural gas (LNG) is inherently greater than the probability of a fire. Many precautions are taken to eliminate ignition sources in liquified natural gas (LNG) facilities. For the most part, onshore liquified natural gas (LNG) falities have generous spacing between equipment; so significant cost savings in fire and cryogenic protection can be achieved without compromising safety. Generous spacing helps by locating some potential liquified natural gas leak sources away from process equipment and liquified natural gas (LNG) storage tanks. In addition, relocating personnel to safe area is usually not an issue. The decision to provide thermal protection becomes an asset protection and capital investment question for onshore facilities. Offshore liquified natural gas (LNG) facilities have close spacing due to the high cost of building offshore. Accordingly, fire and cryogenic protection must be applied to a much higher proportion of equipment and structural steel. Egress and relocation to safe refuge areas are also significant factors. If the structure of the offshore platform is compromised, it would have to be abandoned using egress chutes, davit boats, freefall boats, life rafts, and so on. Two philosophies can be applied to fire and cryogenic protection. One is ti protect all structural steel and equipment that could be eposed to fire and cryogenic temperatures. A second philosophy only protects structural steel and equipment where failure could ecalate the incident.Liquified natural gas (LNG) is a safe and practical way to transport natural gas (NG) by sea from remote locations to user distribution systems. Liquified natural gas (LNG) is also an effective means for storing natural gas at peak-shaving plants during low demand periods. As with any hydrocarbon processing facility, fire prevention and protection are important considerations in liquified natural gas (LNG) facilities. Because of its cryogenic nature (atmospheric boiling point approximately –162.2°C) liquified natural gas (LNG) also poses exposure to employees, facility structure and equipment. The design and operation of liquified natural gas (LNG) terminals minimize ignition sources; thus cryogenic exposure is more likely than a fire incident. This s particularly true in the high pressure processing areas where the fluid inventory is lower but where the higher pressure creates greater potential for cryogenic exposure. Cryogenic exposure can cause freeze burns to employees and embrittlement to carbon steel, thus possibly resulting in structural failure. Protection from cryogenic exposure, as well as from fire exposure, is needed. Protective measures should be chosen that are effective for both fire and cryogenic exposure. But, remember the following: protective measures add cost, and should only be applied to chose parts of facilities where the possibility of harm exists. Consequence modeling can be used to predict the extent of potential fire and cryogenic exposure so that protection can be applied where necessary.

PPA G EA G E 4949

Page 50: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

HH A ZA R DSA ZA R DS O FO F C C RYO G E N I CRYO G E N I C E E X P O S U R EX P O S U R E

Exposure of personnel to liquified natural gas (LNG) and cld gas can cause severe cryogenic burns, similar to frost bite or thermal burns. Contact with non-insulated and een insulated parts of equipment or vessels containing cryogenic fluids can also result in frost bite. Unprotected skinmay stick to low twmperature surfaces and flesh may be torn upon removal. These hazards should be conrolled by separation, guarding, insulation and personnel protective equipment such as gloves , safety glasses, and face shields. Inhaling cold vapor can damage lungs and may trigger an asthma attack in susceptible indivduals. Asphyxiation is a seriou hazard because vaporized liquified natural gas (LNG) is usually odorless. Air contains 21% oxygen (O2). If the oxygen content falls below 18%, adverse effects such as loss of mental alertness and performance may result. At 6% to 10% of oxygen content or less, exertion is impossible; collapse and unconsciousness occurs. At 6% of oygen content or below, death would occur in six to eight minutes. Personnel working in the vicinity of an liquified natural gas release can quickly be enveloped by cold hydrocarbon vapors resulting in oxygen deficient zones. The expansion ratio of liquified natural gas (LNG) is approximately 600 to 1. therefore, the release of 1 m3 of liquified natural gas (LNG) will produce 600 m3 of 100% natural gas (NG) in a short period of time.

HH A ZA R DSA ZA R DS TOTO S S T RU C T U R E ST RU C T U R E S A N DA N D E E Q U I P M E N TQ U I P M E N T

Carbon and steel, which is widely used in process plant structures and in the hulls of liquified natural gas (LNG) carriers, loses its ductility and becomes brittle when exposed to liquified natural gas (LNG) or cold natural gas (NG): the AISI 4130 steel loses half of its impact resistance at –60°F (–51.1°C); other carbon and steel structures become brittle at temperatutes of –20°F (–28.9°C). Liquified natural gas (LNG) has a boiling point of –260°F (200 R). Since the beginning of liquified natural gas (LNG) tanker trade 1969, there have benn eight marine liquified natural gas incidents resulting in spillage with some hull damage due to cold fracture. However, no cargo fires have occurred. Direct contact of liquified natural gas (LNG) with structural steel can rapidly cool the steel to below embrittlement temperature. When combined with suggested failure criria for structural steel sections due to embrittlement, these high heat transfer fluxes predict steel section failre in as little as one to five seconds. Vapor heat transfer due to contact with cold natural gas (NG) velocities is predicted to be much slower. The cooling rate of structural steel depends on the amount of liquified natural gas (LNG) available for chilling per surface area, i.e. the liquified natural gas liquid flux in the jet. The liquified natural gas liquid flux is controlled by the flowrate and the location of the steel relative to the liquified natural gas (LNG) release origin. Because cooling rates are so rapid, early leak detection, system isolation and shutdown have little effect on managing cryogenic liquified natural gas (LNG) hazards in the immediate release area. By the time the detection and shutdown system has activated, the cryogenic damage is complete within liquified natural gas (LNG) exposure hazard envelope. Thus, cryogenic protection requires changing position, changing aterials of construction, or adding protection such as cryogenic insulation or shielding. Rapid detection and process isolation will serve to limit the total volume of liquified natural gas (LNG) release and mitigate the potential for liquified natural gas (LNG) to spread over an even greater area, thereby exposing even more equipment and structures to cryogenic conditions. Polymeric materials, such as plastics and elastomers, are also subject to rapid brittle fracture on exposure toliquified natural gas (LNG), thus compromising some equipment components and electrical insulation. In the United States of America, NFPA 59A (National Fire Protection Agency standard) is one of the key design documents for the design of liquified natural gas (LNG) facilities. In European Union, EN 1473 (European Union standard)is normally used. Both standards, NFPA 59A and EN 1473, require that equipment controls and structures whose failure would result in incident escalation must be protected from cryogenic embrittlement.

HH A ZA R DSA ZA R DS O FO F F F I R EI R E E E X P O S U R EX P O S U R E

In contrast to cryogenic hazards, fire hazards associatd with vaporization of liquified natural gas (LNG) releases can be substantially reduced by rapid detection of releases, followed by shutdown and isolation of equipment. Experience has shown that fire impinging upon structural steel takes a few minutes of exposure to threaten the steel's integrity. Heating rate would be more rapid for direct impingement of jet fires. The heat flux associated with large pool fires would be approximately 120kW∙m−2 for fires larger than the object exposed and approximately 85 kW∙m−2 for pool fires comparable in size to the exposed object. The heat flux associated with jet fires would be approximately 250 kW∙m−2

maximum. Due to the rapid detection and shutdown system, large jet fires are limited to the high pressure pumps, vaporizers and export gas pipelinesections of an liquified natural gas (LNG) receiving, storage and regasification terminal. Low pressure liquified natural gas releases from isolated lower pressure sections of the terminal are expected to cause local pool fire hazards if ignited. Pool fires can be controlled with sloping, curbingand trenching. Liquified natural gas (LNG) releases that discharge at a pressure less than 4 barg are assumed to form liquid pools rather than jets.

PPA G EA G E 5050

Page 51: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

RR I S KI S K-- BA S EDBA S ED P P RO TE C T IO NRO TE C T IO N O FO F O O NS H O R ENS H O R E F FA C I L I T I E SA C I L I T I E S

Ordinary structural steel can fail rapidly when exposed to liquified natural gas (LNG). Ther are dozens of onshore liquified natural gas (LNG) facilities around the world. Because most onshore liquified natural gas (LNG) facilities are comparatively open and eqipment is not congested, risk to life is low. In those cases, risk management becomes primarily a matter of asset protection.one risk-based philosophy that minimizes initial capital cost is to protect all assets that could be exposed to cryogenic fluids or fire, whose failure could lead to escalation of the incident. Because of wide spacing, and because many assets can fail without causing escalation, only some assets will require protection. Of course, loss of even a part of a facility can cause extended lost production with a major impacton the overall costs of the facility. The choice is a business decision.

Coatings and InsulationFire approved insulation with a cementitious insulation can also provides economical protection of structural steel against short-term cryogenic exposure. Unfortunately, not all potential insulation products have been tested for their ability to withsta cryogenic exposure as well as fire exposure. Industry testing has been conducted on intumescent and subliming fireproofing coatings. These materials in conjunction with a cryogenic insulating coating can provide good protection from both cryogenic and jet-fire exposure. But these fireproofing materials are more expensive to apply than most cementitious systems. There are no current industry standard tests for the performance of the coating materials to resist the effects of liquified natural gas (LNG) cryogenic exposure and then a subsequent fire. Current testing data available is generally the result of manufacturers' independent research, and ongoing construction projec testing.

Liquified Natural Gas (LNG) PoolsIf pooled liquified natural gas (LNG)does not ignite, then the bases of columns and equipment supports could be exposed and then fail. A spill containment system consisting of curbing, sloped paving and troughs should be provided undeer all liquified natural gas (LNG) lines and equipment in the plant. The containment systems limit the area that can be affected by an liquified natural gas (LNG) spill and the exposure duration. Limiting exposure duration keeps insulation requirements from becoming too thick and impractical. The containment area layout should consider the potential exposure areas that could result from a pressurized liquified natural gas release. When designing the liquified natural gas spill containment system, consider the Leiden frost effect which leads to higher liquid velocities from creating a vapor film between the solid-spill containment system and boiling liquified natural gas (LNG). These higher liquid velocities, when compared to flowing water, could cause splashing around obstructions and overshoot the sloped trough at turns and changes in elevation. Where structural steel and critical equipment supports are within the curbing and drainage paths, they should be supported on a suitable concrete base that prevents exposure of the steel to the pooling, splashing or draining liquid. Protection of instrument and electrical cabling is normally not done because these systems are designed to be fail-safe. However, specific review of the potential exposure to the shutdown and blowdown controls should be conducted. Direct exposure from cryogenic spray to shutdown and blowdown valves or actuators could result in the failure to isolate or deinventory the process. In general, the probability of the cryogenic spray impinging on the specific equipment should be considered for the overall probability of the equipment's failure on demand to evaluate the need for additional protective measures.

PP RO TE C T IO NRO TE C T IO N O FO F O O F F S H O R EF F S H O R E F FAC I L I T I ESAC I L I T I ES

Although less common and more expensive than onshore liquified natural gas (LNG) facilities, liquified natural gas facilities can be located offshore when there are no suitable onshore sites. Offshore, because of close spacing, protection of all assets that could be damaged by exposure to cryogenic fluids or fire is recommended. Because of weight restrictions , a lightweight ablative layering system can provides both fire and cryogenic protection. Insulation suitable for offshore structural steel, decking and equipment must be resistant to salt water as well as to cryogenic and fire exposure. An epoxy-based system can provide both cryogenic and jet fire protection; it can also serve as coating to inhibit corrosion effects. When compared with onshore liquified natural gas (LNG) facilities, the sigificantly smaller areas associated with an offshore facility increase the potential for liquified natural gas release incidents to impair occupant evacuation and can escalate damage to the facilities. One method to reduce potential exposures (both cryogenic and pressurized fire), is to provide flange guards on specific flange connections. Such flange guards serve to reduce the potential spray area, and to prevent well-formed jets from occurring.

Valvescryogenic spray exposure on shutdown and blowdown valves can cause failure of the actuators prior to the valve moving to the safe position. This is acknowledged as a low probability even since it would require direct spraying onto an

PPA G EA G E 5151

Page 52: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

actuator to induce failure. However, the probability and consequence of the event should be reviewed for critical valves, and protection provided if necessary. In general, fire exposures to such valves is less likely to prevent the valve from moving to the safe position because the embrittlement effets from the cryogenic liquified natural gas (LNG) spray can induce a more rapid failure.

Cloud EffectWhen liquified natural gas (LNG) vaporizes it crates a condensation cloud in the air around the natural gas (NG) cloud. This cloud is often mistaken for natural gas itself, but is merely condensed water vapor resulting from the cryogenic release. Creating large fog clouds during liquified natural gas (LNG) releases can impair the employees' visual abilities. They may not be able to see the pooled liquified natural gas on the deck if it is obscured by a condensation fog cloud. Based on this, all portions of the process unit should be reviewed to assure that the employees have access to more than one evacuation route to the temporary refuge no matter where an incident may occur. In addition, the emergency response team members should be specifically trained on liquified natural gas (LNG) release characteristics so that they are properly prepared to respond.

THERMAL RADIATION FROM POOL FIRESTHERMAL RADIATION FROM POOL FIRESIndustrial fires can be intense emitters of heat, smoke, and other combustion products. This is particularly true if the fuel is a petroleum based substance, with a high heat of combustion and sooting potential. The radiant energy flux can be sufficiently high to threaten both the structural integrity of neighboring buildings, and the physical safety of fire fighters, plant personnel, and potentially people beyond the boundaries of the facility. The United States Department of Housing and Urban Development (HUD) has established thermal radiation flux levels of 31.5 kW∙m−2 (10,000 BTU∙ft−2∙hr−1) for buildings and 1.4 kW∙m−2 (450 BTU∙ft−2∙hr−1) for people as guides in determining an “Acceptable Separation Distance” (ASD) between a fire consuming combustible liquids or gases and nearby structures and people. The calculation procedure for determining acceptable separation distance (ASD) is set forth in a 1982 Housing and Urban Development (HUD) Guidebook entitled, “Urban Development Siting with Respect to Hazardous Commercial and Industrial Facilities”. Much of the theoretical development for this guidebook is contained in a 1975 Housing and Urban Development (HUD) Guidebook entitled, “Safety Considerations in Siting Housing Projects”. In the quarter century since that report was released, the field of fire science has grown rapidly, leading to improved methods of measurement and prediction of fire behavior. A review by the Building and Fire Research Laboratory at the National Institute of Standards and Technology (NIST) of the 1975 Housing and Urban Development (HUD) guidelines for thermal radiation flux has revealed that for certain fire scenarios the methodology can produce estimates of radiation flux that are up to an order of magnitude larger than those actually measured in field experiments. The source of this discrepancy is the assumption that the fire is unobscured by smoke, that is, a person watching the fire from a distance sees the entire extent of the combustion region. In reality, large fires of most combustible liquids and gases generate an appreciable amount of smoke. Depending on the fuel and the size of the fire, up to 20 % of the fuel mass is converted to smoke particulate in the combustion process. This smoke shields much of the luminous flame region from the viewer, and it is this luminous flame region that is the source of most of the thermal radiation. This shielding effect is most pronounced for fires that are tens or hundreds of meters in diameter because of the decreased efficiency of combustion at these scales. A schematic diagram is shown in Figure 3.01. The analysis of hazardous liquid fires is relatively independent of the type of liquid; burning rates and heat release rates do not vary significantly from fuel to fuel, nor does the nature of the fire. However, hazardous gases stored under pressure, especially liquified natural gas (LNG) and liquified petroleum gas (LPG), are not as predictable. There are a number of references to fires involving liquified natural gas (LNG) and liquified petroleum gas (LPG) in which a cloud of combustible gas ignited to form fireballs on the order of 100 meters in diameter. The radiation from fires fueled by gases leaking from storage tanks can cause a boiling liquid expanding vapor explosion (BLEVE) within a tank that not only produces a tremendous amount of thermal radiation, but also often causes parts of the tank to be thrown tens or hundreds of meters. In particular, liquified petroleum gas (LPG) is so volatile that it is more likely to vaporize than form a liquid pool, thus much of the research into large liquid fuel fires may not be applicable to liquified petroleum gas (LPG) fires. Predicting the thermal radiative flux from a fire of leaking combustible gases is more complicated than that of a liquid fuel fire because there are a number of potential fire scenarios to consider. With a liquid fuel fire, the dynamics of the fire is more understood and predictable than with a gaseous fuel. Rather than develop a separate methodology for estimating thermal radiation for each potential gaseous fire scenario, it is preferable to employ a simple procedure which encompasses a wide variety of scenarios, removes most of the geometrical parameters from the calculation, and remains conservative. Such a method is known as the “point source” radiation model. All that it requires is an estimate of the total heat release rate of the fire, and the fraction of

PPA G EA G E 5252

Page 53: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

that energy that is emitted as thermal radiation. These data are available based on far-field radiometer measurements. The point source method is accurate in the far-field, but is considered overly conservative within a few fire diameters because it assumes that all of the radiative energy from the fire is emitted at a single point rather than distributed over an idealized shape (usually a cone or cylinder) meant to represent the fire.

Figure 3.01 – Schematic diagram of a large liquid fuel fire.

For liquid fuel fires, however, the point source model may be too conservative because these fires are more predictable and there is much more experimental data available to validate a more detailed model. A popular method of estimating radiation flux from large liquid pool fires is the “solid flame” radiation model. In this case, the fire is idealized as a solid vertical cylinder emitting thermal radiation from its sides. This model is relatively simple, but it does require estimates of the diameter and height of the cylinder, plus an estimate of the emissive power. Determining the height and width of the idealized cylinder is discussed in the next textlines.

HH A Z A R D O U SA Z A R D O U S L L IQ U ID SIQ U ID S

In the solid flame radiation model, the thermal radiation flux q from a fire to a nearby object is given by the expression,

q=F⋅⋅ f⋅E f [3.06]

where F is a geometric view factor that defines the fraction of energy radiated by the fire that is intercepted by the receiving object, τ is the atmospheric transmissivity to thermal radiation, mainly a function of humidity and distance between the radiation source and the receiver, εf is the effective emissivity of the flame, generally expressed as,

f =1−e−⋅D [3.07]

where κ is an attenuation coefficient and D is the width of the fire, and Ef is the total emissive power of the flame at the flame surface. For fires greater than a few meters in diameter, the effective emissivity (εf) of the flame can be taken as one. Also, to be on the conservative side, the transmissivity is taken as one. What remains to be computed are the view

PPA G EA G E 5353

Luminous Band

VisibleFlameHeight

AirEntrainment

Fire Base Diameter

Zone 1

Zone 2"AnchoredPulsatingFlame"

Zone 2"Intermittancy

Region"Flame NotContinuous

Large scale eddiesentrained from the

atmosphere

Page 54: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

factor F and the emissive power of the flame (Ef). Computation methods of the past, considered the view factor and the emissive power independently, which for some fire scenarios led to estimates of radiative emission from the fire in excess of the total energy of the fire. What was missing from these analyses was an overall accounting of energy. This problem has been remedied in the last few decades because it is now generally recognized by the field of fire protection engineering that a fire’s total heat release rate (HRR) is the best measure of its potential to do harm. Moreover, the heat release rate (HRR) of a fire is easier to estimate than its temperature or physical size because the heat release rate (HRR) is proportional to its rate of fuel consumption, a quantity that is relatively easy to measure. The Building and Fire Research Laboratory (BFRL) has performed on the order of one hundred large scale fire experiments in the past two decades with a variety of combustible liquids and gases, and one of the more reliable measurements is that of the mass burning rate from which the fire’s total heat release rate (HRR) can be estimated. A fraction of the fire’s total heat release rate (HRR) is emitted in the form of thermal radiation. For fires up to roughly four meters in diameter, the ratio of the rate of energy radiated to the surroundings to the total heat release rate (HRR) of the fire (χr) is between 0.30 and 0.40, and this value decreases with increasing fire diameter due to smoke obscuration. Much of the thermal radiation from a large, sooty fire is emitted from the luminous “wall” of flame encircling the base of the fire. The flames above this luminous wall are obscured from view by the smoke formed due to incomplete combustion. Air is entrained into the fire at its base, and soot quickly forms in the combustion process, creating a thermal barrier higher up in the fire that traps radiant energy from escaping the fire’s interior. An idealized picture of a fire used in most analyses of thermal radiation is one in which the fire is assumed to be cylindrical in shape with a height (H) and diameter (D) with a total heat release rate (HRR) of radiated energy. More generally, the fire can be assumed to be of arbitrary shape with a perimeter length (P). The radiated energy from the fire can be expressed as,

r⋅Q=P⋅H⋅E f [3.08]

Radiometer measurements from large fire experiments suggest that total heat release rate (HRR) of the fire (χr) decreases with increasing fire diameter (D) according to,

r=r ,max⋅e−⋅D [3.09]

where χr,max is 0.35 and κ is 0.05 m−1. These values are based on a curve fit to experimental data involving a range of different combustible liquids. The total heat release rate (HRR) of the fire Q can be expressed as the product of the heat release rate per unit area q f and the area (A) of the base of the fire,

Q= q f⋅A [3.10]

For a given fuel, the heat release rate per unit area q f is relatively constant because the fuel mass burning rate per unit area is relatively constant. The two remaining parameters in Equation [3.09] are the emissive power (E f) and the height (H) of the idealized cylinder. The reported values of the emissive power for flammable liquids and gases vary widely from source to source. The variation in reported values of emissive power has to do with the definition of the height of the idealized cylinder that represents the fire. When viewed from a distance, the actual fire appears smokey, with occasional bursts of luminous flame emerging from the smoke. The flame height of the actual fire is defined as the vertical extent of the combustion region, and it can be thought of as the maximum height above the ground at which these luminous bursts can be seen. Taking an idealized cylindrical fire with height equal to the flame height of the actual fire, on average about 20% of the surface area of the cylindrical fire consists of visible flames and 80% is smoke. Most of the visible flame is at the base of the fire, although periodically luminous flames burst through the smokey plume higher up. The reported values of emissive power are most often average values for the entire flame height, and will be significantly less than the emissive power of the luminous flames. If the relatively low average emissive power is applied to the surface area of the idealized cylinder whose height is equal to the flame height of the actual fire, then the estimate of the radiative flux at distances greater than a few fire diameters away will be accurate. However, at closer distances the radiative flux estimates will typically be under-estimated because the radiant energy is assumed to be distributed over the entire height of the fire, rather than concentrated near the base as it is in reality. For example, for fires larger than 30 meters in diameter, the average emissive power reported by many researchers is less than 31.5 kW∙m−2 (10,000 BTU∙ft−2∙hr−1), the threshold value used by Department of Housing and Urban Development (HUD) for determining the acceptable separation distance (ASD) for buildings and combustible structures. Both the method of Shokri and Beyler, and the method of Mudan and Croce use an emissive power averaged over the flame height of the fire. Both

PPA G EA G E 5454

Page 55: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

correlations fall below 31.5 kW∙m−2 for fires larger than 30 meters in diameter. These correlations can wrongly be interpreted to mean that buildings can be built right next to sites of potentially large fires simply because the predicted flux would never exceed 31.5 kW∙m−2 regardless of its distance from the fire. The height of the luminous flame zone (H) can be found from Equation [3.08]. To simplify the analysis, the fire area is assumed circular,

P=⋅D [3.11]

and

Q=⋅ D2

2

⋅q f [3.12]

but this is not a restrictive assumption. Substituting expressions for χr and Q into Equation [3.08] yields an expression for the height (H),

H=r ,max⋅e−⋅D⋅D⋅q f

4⋅E f[3.13]

Height (H) is plotted as a function of diameter (D). In these cases, all that is required to obtain the height of the luminous zone is an estimate of the heat release rate per unit area. What remains of calculating the thermal radiation flux using Equation [3.06] is determining the view factor (F) from the fire to a target. The view factor calculation can be simplified by assuming the fire is surrounded by a vertical wall of height (H) emitting radiation energy at a rate of emissive power (Ef), and that the wall is composed of circular or linear elements for which analytical recipes of the view factor are available. The presence of man-made or natural thermal barriers can be incorporated into the view factor calculation. Although the methodology presented here is designed to be conservative, it is not conservative in one regard. Because the radiative energy output is concentrated near the base of the fire rather than distributed over the entire height of the fire, the effectiveness of a thermal barrier in blocking thermal radiation might be exaggerated. Recent measurements of a 20 meters diameter crude oil fire showed that 85% of the radiant energy of the fire was emitted at heights lower than 20 meters. The remaining 15% of the radiant energy was emitted mainly by hot black smoke at higher levels, and by occasional luminous bursts of flame. The heat release rate (HRR) per unit area q f for crude oil is approximately 2,000 kW∙m−2.

HH A Z A R D O U SA Z A R D O U S G G A S ESA S ES

Fire scenarios involving combustible gases vary widely, from a pool fire of a liquified gas, like liquified natural gas (LNG) or liquified petroleum gas (LPG), to a flare formed by burning vapors escaping a storage tank, to a fireball following the release of a large amount of gas that subsequently ignites. Because it is difficult to predict the structure of the fire, it is important to employ a methodology for predicting the thermal radiation flux from the fire. The simplest method of calculating the thermal radiation, known as the “point source” model, is to estimate the heat release rate of the fire, assume a fraction of the total energy is released in the form of thermal radiation, and then divide this radiated energy over the surface area of a sphere whose radius is the distance from the center of the fire to the target,

q=R⋅Q

4⋅⋅r2 [3.14]

Essentially, this method assumes that all of the thermal radiation emanates from a point. For targets greater than several fire diameters away, this assumption is reasonably good. However, at closer distances, the assumption is not valid, but it is conservative because it assumes all of the energy is concentrated at a point rather than spread over the height and width of the fire, as was assumed by the “solid flame” model above. Equation [3.14] requires two pieces of information: the radiative fraction (χr) and the total heat release rate (HRR). Because gaseous fires are often in the form of flares, it is not appropriate to assume that radiative fraction (χr) decreases with fire diameter as in the case of liquid fires above. Flares are substantially more luminous than liquid pool fires because the oxygen is better able to penetrate the combustion region and thus the combustion is more efficient and less smoke is formed in the process. A conservative estimate of radiative fraction (χr) is 0.20, appropriate for a wide range of gaseous fuels. The estimate of the heat release

PPA G EA G E 5555

Page 56: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

rate Q is not as easy as it is for liquids because more often than not there is no fire “diameter” because there is no liquid pool, even for liquified gaseous fuels like liquified natural gas (LNG) or liquified petroleum gas (LPG). It is more appropriate in this case to estimate a mass burning rate m and then multiply this by a heat of combustion,

Q=m⋅ H c [3.15]

Because of the uncertainty inherent in predicting the hazard associated with pressurized storage of gases, the consideration of thermal barriers as a means of lessening the radiation flux to distant targets is difficult. Liquified gases may form a pool that erupts in fire, or the gases may vaporize so quickly that a fireball or turbulent jet fire forms. In the former case, a wall surrounding the fire may block a substantial fraction of the radiation energy, whereas in the latter case, a wall will do little to lessen the impact of thermal radiation on surrounding targets. Consequently, consideration should not be given to thermal barriers when assessing the thermal radiation hazard from fires of pressurized storage tanks or pipelines of combustible gases.

DD E T ER M I N I N GE T ER M I N I N G T H ET H E A A C C EP TA B L EC C EP TA B L E S S EPA R AT IO NEPA R AT IO N D D IS TA NC EIS TA NC E (ASD) (ASD)If the fuel is liquid at atmospheric temperature and pressure, if the fire is roughly circular around its base, and if there are no obstructions to be considered, simplified charts can be used to determine the acceptable separation distance (ASD). The acceptable separation distance (ASD) values presented in the simplified charts are based on the assumption that the perimeter of the fire is a circle. If this is not the case, an equivalent fire diameter needs to be calculated. If the ratio of the longest dimension to the shortest is less than 2.5, then an equivalent cylinder of diameter,

D= 4⋅A

[3.16]

can be assumed. Otherwise, a combination of either vertical circular arcs or vertical flat plates can be used as surrogates for the actual fire shape. Once the equivalent fire diameter has been determined, the acceptable separation distance (ASD) for people and buildings can be obtained from acceptable separation distance (ASD) chart. This chart displays the distance from a fire at which the radiative flux is expected to be 31.5 kW∙m−2 (10,000 BTU∙ft−2∙hr−1) and 1.4 kW∙m−2 (450 BTU∙ft−2∙hr−1), respectively. Note that the acceptable separation distance (ASD) is measured from the leading edge of the fire. A useful feature of the chart is that it shows that there are maximum separation distances from a fire beyond which the thermal radiation flux impinging on a structure or person is less than the acceptable separation distance (ASD) threshold values regardless of the fire size. Another useful application of screen acceptable separation distance (ASD) is in cases where the fuel spill is unconfined. The 1975 Housing and Urban Development (HUD) guidelines and the SFPE Handbook discuss methods of estimating the diameter of an unconfined spill fire. The simplest method of obtaining a spill diameter is,

D=10⋅V [3.17]

where D is in meters and V is in cubic meters. The equivalent formula in english units is,

D=2⋅V [3.18]

where D is in feet and V is in gallons. Equation [3.17] asserts that the liquid will continue to spread until it is about 1 cm in depth. However, given the wide variety of potential spill scenarios and the number of assumptions that have to be made in order to apply the correlations, it is preferable to apply the screen acceptable separation distance (ASD) distances in cases of unconfined spills rather than relying on the spill diameter correlations.

EE S T I MAT I O NS T I MAT I O N O FO F T T HE R MA LHE R MA L R R A D I AT IO NA D I AT IO N (A (A NA LYT I CA LNA LYT I CA L M M O DE LO DE L ))Thermal radiation plays a very important role in pool fires, because the burning rate of large pool fires is greatly influenced by radiative heat transfer. The problem of thermal radiation hazards also depends on the characteristics of radiative heat transfer. A simple analytical model which is called one mesh model is introduced to estimate thermal radiation from large pool fires. The flame shape of pool fires is assumed to be a cylindrical one for simplicity as shown in Figure 3.01. Properties such as temperature of the flame (Tf) and absorption coefficient (k) are considered to be uniform within the flame model. Heat generation due to the combustion of fuel is assumed to take place within the flame model.

PPA G EA G E 5656

Page 57: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Therefore, the heat balance equation for this flame model are made up of heat generated by combustion of fuel (Qf), heat carried with entrained air (Qa), radiative heat from the surrounding air (Qr), heat of fuel vaporization (Qv), heat carried away from the top of the flame by flown (Qg), and heat loss by radiation to the surroundings (QL). Then the heat balance equation is,

Qf + Qa + Qr =Qv + Qg + QL [3.19]

where Qf is,

Q f =⋅m f⋅ H C [3.20]

where η is the combustion efficiency, mf is the fuel burning rate(kg∙s−1), and ∆Hc is the heat of combustion (W∙s∙kg−1). Qa is given by,

Qa=ma⋅cpa⋅T a [3.21]

where ma is the mass velocity of entrained air (kg∙s−1), cpa is the specific heat of air (W∙s∙kg−1∙K−1), and Ta is air temperature (K). Qr is given by,

Qr=m f⋅[ H f T b−T a⋅c f ] [3.22]

where ∆Hf is the heat of vaporization (W∙s∙kg−1), Tb is the boiling point of fuel(K), and cf is the specific heat of fuel (W∙s∙kg−1∙K−1). Qg is given by,

Q g=m f ma⋅cp g⋅T m [3.23]

where cpg is the specific heat of combustion gas (W∙s∙kg−1∙K−1), and Tm is mean temperature of of the flame model. Total heat loss by radiation to the surroundings of the flame model (QL) is,

QL=4⋅1− s⋅⋅k m⋅T m4⋅V [3.24]

where σ is Stefan-Boltzmann constant (5.67∙10−8 W∙m−2∙K−4), V is flame volume and αs is a self-absorption factor. Approximate value of as for various shapes of gas mass can be calculated easily using the following equation cosisting of km, V and a surface area of the flame model (S).

s=1−1−10−c⋅km⋅L

k m⋅L [3.25]

where c is a factor, which is given by Hottel at al., for various gas shapes. L is the mean beam path length and can be expressed as,

L=4⋅VS [3.26]

Radiation from the entire flame model is assumed to be emitted uniformly to the surroundings from the point which located on the center line of flame of height (H). Therefore the radiative energy per unit area (qx) at the distance L=5∙H is,

q X=1−s⋅QL

4⋅⋅ L2H 2[3.27]

Qr is radiative heat from the surrounding air,

PPA G EA G E 5757

Page 58: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Qr=⋅A⋅S⋅T a4 [3.28]

εa is an emissivity of air and set to unity. Qr of Equation [3.22] is also expressed as,

Qr=⋅base⋅AS⋅T base4 As⋅hs⋅T m−T b [3.29]

where εbase is an emissivity at the bottom of the flame model,

base=1−100.814⋅0.85⋅k⋅H [3.30]

Tbase are the temperature at the flame model bottom and hs is a convective heat transfer coefficient between flame and fuel surface. The flame height (H) and the fire diameter (D) correlation and experimental data is given by Moorhouse (1982) model,

HD

=6.2⋅F0.254[3.31]

or by Cox and Chitty (1985) model,

HD

=430.4⋅F [3.32]

and by Thomas (1965) model,

HD

=55⋅F23 [3.33]

being F the dimensionless burning rate or combustion Froude number,

F=m f

a⋅g⋅D[3.34]

where ρa is specific mass for combustible material (kg∙m−3), and g is the gravity constant (m3∙kg−1∙s−2).

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESCPR-16E, 1992. Methods for the Determination of Possible Damage to People and Objects Resulting from Releases of Hazardous Materials, First Edition, published by TNO, 1992.D. C. Hamilton and W. R. Morgan. Radiant Interchange Configuration Factors. NACA Technical Note 2836, National Advisory Committee for Aeronautics, Washington, D.C., 1952.D. J. Finney. Probit Analysis. Cambridge University Press, 1977.Delichatsios, M. A., “Air Entrainment into Buoyant Jet Diffusion Flames and Pool Fires”, Combustion and Flame, Volume 70, Pages 33-46, 1987.Delichatsios, M. A., 1987. Air Entrainment into Buoyant Jet Diffusion Flames and Pool Fires, Combustion and Flame, Volume 70, Pages 33-46, 1987.Department of Housing and Urban Development. Safety Considerations in Siting Housing Projects, 1975. HUD Report 0050137.Finney, D. J., Probit Analysis . Cambridge University Press, 1977.Fire Protection Handbook. National Fire Protection Association, Quincy, Massachusetts, 18th edition, 1997.G. Opschoor, R. O. M. Van Loo, and H. J. Pasman. Methods for Calculation of Damage Resulting from Physical Effects of the Accidental Release of Dangerous Materials, in international conference on hazard identification and risk analysis,

PPA G EA G E 5858

Page 59: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

human factors and human reliability in process safety, pages 21–32. Center for Chemical Process Safety, AIChE, 1992.G. Opschoor, R. O. M. Van Loo, and H. J. Pasman. Methods for Calculation of Damage Resulting from Physical Effects of the Accidental Release of Dangerous Materials. In International conference on hazard identification and risk analysis, human factors and human reliability in process safety, pages 21–32. Center for Chemical Process Safety, AIChE, 1992.Guidelines for Chemical Process Quantitative Risk Analysis, Second Edition, page 269, American Institute of Chemical Engineers Center for Chemical Process Safety (CCPS), 2000.Guidelines for Chemical Process Quantitative Risk Analysis, Second Edition, page 269, American Institute of Chemical Engineers Center for Chemical Process Safety (CCPS), 2000.H. Koseki and T. Yumoto. Air Entrainment and Thermal Radiation from Heptane Pool Fires. Fire Technology, 24, February 1988.Hottel, H. C. and Sarofim A. F., Radiative Transfer, McGraw-Hill, 1967.J. C. Yang, A. Hamins, and T. Kashiwagi. Estimate of the Effect of Scale on Radiative Heat Loss Fraction and Combustion Efficiency. Combustion Science and Technology, 96:183–188, 1994.K. Cassidy and M. F. Pantony, 1988. Major Industrial Risks – A Technical and Predictive Basis for On and Off Site Emergency Planning in the Context of United Kingdom Legislation, Symposium Series No. 110, pages 75-95, Institute of Chemical Engineers, Hemisphere Publishing Corporation.K. S. Mudan and P. A. Croce. SFPE Handbook, chapter Fire Hazard Calculations for Large Open Hydrocarbon Fires. National Fire Protection Association, Quincy, Massachusetts, 2nd edition, 1995.N. Takahashi, H. Koseki, and T. Hirano. Temporal and Spatial Characteristics of Radiation from Large Pool Fires. Bulletin of Japanese Association of Fire Science and Engineering, 49(1):27–33, 1999.Nedelka, D., Moorhouse, J., and Tucker, R., 1989. The Montoir 35 Meters Diameter LNG Pool Fire Experiments, Paper 3, Session III, International Conference on Liquefied Natural Gas: LNG 9 Proceedings, Nice, France, 1989.Society of Fire Protection Engineers, Bethesda, Maryland. Engineering Guide for Assessing Flame Radiation to External Targets from Pool Fires, June 1999.V. Babrauskas. SFPE Handbook, chapter “Burning Rates”. National Fire Protection Association, Quincy, Massachusetts, 2nd edition, 1995.

PPA G EA G E 5959

Page 60: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Chapter 4Chapter 4Risk Analysis and Safety Implications ofRisk Analysis and Safety Implications of

Liquified Natural Gas (LNG) Spills Over WaterLiquified Natural Gas (LNG) Spills Over Wateref

INTRODUCTIONINTRODUCTIONMany studies have been conducted to assess the consequences and risks of LNG spills from both storage terminals and liquified natural gas tankers. However, while recognized standards exist for the systematic safety analysis of potential spills or releases from liquified natural gas (LNG) storage terminals and facilities on land, no equivalent set of standards exists for the evaluation of the safety or consequences from LNG tanker spills over water. Due to limited experience and experimental testing associated with large-scale spills over water, most studies use simplifying assumptions to calculate and predict the hazards of a large liquified natural gas (LNG) spill. The range of assumptions and estimates for many complicated spill scenarios can lead to significant variability in estimating the probability, hazards, consequences, and overall risks of large liquified natural gas (LNG) spills over water. Natural gas liquefaction dates back to the 19th century, when british chemist and physicist Michael Faraday experimented with liquefying different types of gases, including natural gas. A prototype liquified natural gas (LNG) plant was first built in West Virginia in 1912, and the first commercial liquefaction plant was built in Cleveland, Ohio, in 1941. The Cleveland plant liquefied natural gas and stored the liquified natural gas (LNG) in tanks, which was vaporized later for use during heavy demand periods. Natural gas continues to be liquefied and stored for use during peak demands, with almost 100 liquified natural gas (LNG) peaking facilities in the United States. Typical properties of liquified natural gas (LNG) are:(1) Liquified natural gas (LNG) is simply natural gas that has been cooled to its liquid state at atmospheric pressure:

−260°F (−162.2°C) and 14.7 psia. Currently, imported liquified natural gas (LNG) is commonly 95% to 97% methane, with the remainder a combination of ethane, propane, and other heavier gases.

(2) Liquified natural gas (LNG) is transported at ambient pressures.(3) Liquefying natural gas vapor, which reduces the gas into a practical size for transportation and storage, reduces the

volume that the gas occupies more than 600 times.(4) Liquified natural gas (LNG) is considered a flammable liquid.(5) Liquified natural gas (LNG) vapor is colorless, odorless, and non-toxic.(6) Liquified natural gas (LNG) vapor typically appears as a visible white cloud, because its cold temperature condenses

water vapor present in the atmosphere.(7) The lower and upper flammability limits of methane are 5.5% and 14% by volume at a temperature of 25°C.

Table 4.01 – Flammability limits of the selected fuel compounds (at 25°C).

FuelFuel Lower Flammability LimitLower Flammability Limit(% by volume in air)(% by volume in air)

Lower Flammability LimitLower Flammability Limit(% by volume in air)(% by volume in air)

Methane 5.5 14.0

Butane 1.6 8.4

Propane 2.1 9.6

Ethanol 3.3 19.0

Gasoline (100 octane) 1.4 7.8

Isopropyl alcohol 2.0 12.7

Ethyl ether 1.9 36.0

Xylene 0.9 7.0

Toluene 1.0 7.1

Hydrogen 4.0 75.0

Acetylene 2.5 85.0

PPA G EA G E 6060

Page 61: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

The increasing demand for natural gas will significantly increase the number and frequency of liquified natural gas tanker deliveries to ports across the world. Because of the increasing number of shipments, concerns about the potential for an accidental spill or release of liquified natural gas (LNG) have increased. In addition, since the incidents surrounding September 11 (2001), concerns have increased over the impact that an attack on hazardous or flammable cargoes, such as those carried by liquified natural gas ships, could have on public safety and property. The risks and hazards from an liquified natural gas (LNG) spill will vary depending on the size of the spill, environmental conditions, and the site at which the spill occurs. Hazards can include cryogenic burns to the ship’s crew and people nearby or potential damage to the liquified natural gas ship from contact with the cryogenic liquified natural gas (LNG). Vaporization of the liquid liquified natural gas (LNG) can occur once a spill occurs and subsequent ignition of the vapor cloud could cause fires and overpressures that could injure people or cause damage to the tanker’s structure, other liquified natural gas (LNG) tanks, or nearby structures. Therefore, methods to ensure the safety, security, and reliability of current or future liquified natural gas terminals and liquified natural gas shipments are important from both public safety and property perspectives, as well as from a regional, energy reliability standpoint. Methods to reduce the risks and hazards from a potential liquified natural gas (LNG) spill must be considered on a site-specific basis and will vary, depending on factors such as location, geography, operational considerations, and weather conditions. Table 4.01 lists the flammability limits for several compounds.

RISK ASSESSMENT OF LIQUIFIED NATURAL GAS SPILLS OVER WATERRISK ASSESSMENT OF LIQUIFIED NATURAL GAS SPILLS OVER WATERHigh consequence operations such as the transportation, off-loading, and storage of liquified natural gas (LNG) imply potential risks to people and property. Risk is defined as the potential for suffering harm or loss and is often quantified as the product of the probability of occurrence of a threatening event times the system vulnerability to that event and the consequences of that event. Thus,

R=PH⋅PS⋅S [4.01]

where PH is the exposure to the potential occuring threat (the probability of an accidental or intentional threat, hazard or harm), PS is the probability that preventive or mitigating measures fail (system failure), and S is the consequences usually expressed in fatalities or costs. Effectively evaluating the risks of a large liquified natural gas (LNG) spill over water requires that the potential hazards (results of events that are harmful to the public and property) and consequences be considered in conjunction with the probability of an event, plus the effectiveness of physical and operational measures of liquified natural gas (LNG) transportation to prevent or mitigate a threatening event. For example, safety equipment, operational considerations and requirements, and risk management planning can work together to reduce the risks of an liquified natural gas (LNG) spill by reducing both the probability of an event that could breach the liquified natural gas tanker and by reducing the consequences of a spill. Because of the difficulty in assessing the effectiveness of safety measures and operational safety and security strategies, many studies assume the probability of an event and a containment’s vulnerability to be one; therefore, the concentration is on calculating expected consequences. This often provides worst-case results with low probability and very high uncertainty, which can inappropriately drive operational decisions and system designs. Therefore, for high consequence and low probability events, a performance-based approach is often used for developing risk management strategies that will reduce the hazards and risks to both public safety and property.

RR IS KIS K A A N A LY S ISN A LY S IS E E L E ME N T SL E ME N T S O FO F AA P P O TE N T IA LO TE N T IA L S S P I L LP I L L

The risk analysis approach of a potential liquified natural gas (LNG) spill should include:(1) Uncertainty – Assessment of the accuracy of the assumptions used and the probable ranges.(2) Comprehensiveness – Do the failure modes considered account for all major avenues of loss? Understanding the full

range of consequences associated with a catastrophe can require considerable effort. Completeness is important to properly support risk assessment and risk management. Two important variables are “directness of effect” and “latency”. For example, if an explosion breaches an liquified natural gas (LNG) cargo tank on a ship, that is a direct effect. Conversely, if a resulting explosion damages an liquified natural gas (LNG) terminal – hampering future liquified natural gas (LNG) deliveries for extended periods – that is an indirect or latent effect. Latency refers to when the effects are felt. Immediate effects occur simultaneously with the threat; whereas latent effects occur after an interval, the length of which might vary from system to system. It should be emphasized that indirect and latent effects sometimes dominate other consequences.

(3) Evaluation of risk reduction measures – One way to reduce risk is to remove or block the threat (i.e. prevent the

PPA G EA G E 6161

Page 62: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

disaster from occurring in the first place). For example, reinforce ships against collisions or reduce ship speeds in a harbor to reduce the chance of a spill.

(4) Threat as a moving target – Many avenues to failure – mechanical, environmental insult, operator error – are amenable to analysis and can be confidently predicted to occur with some probability in the future. Other types of threats can be constantly changing and difficult to assess accurately, requiring more robust approaches for prevention or mitigation and frequent re-evaluations of new threats.

SS P I L LP I L L R R IS KIS K A A S S E S S ME N TS S E S S ME N T A N DA N D M M A N AG EM E N TA N AG EM E N T P P R O C ES SR O C ES S

The risk analysis, in turn, helps support a program for managing risks of liquified natural gas (LNG) deliveries to terminals for site-specific locations and conditions. The risk assessment and management process includes:(1) Evaluating the potential for an event that could cause a breach or loss of liquified natural gas (LNG);(2) Establishing the potential damage to a the containment system from these events and the potential spills that could

occur;(3) Estimating the volume and rate of a potential liquified natural gas (LNG) spill based on the dimensions and location

of the breach, properties and characteristics of the liquified natural gas (LNG), structure construction and design, and environmental conditions (e.g. wind, waves, currents, and other factors);

(4) Estimating the dispersion, volatilization, and potential hazards of a spill based on physical and environmental conditions;

(5) When necessary, identifying prevention and mitigation approaches and strategies to meet risk management goals.

If risks, costs, or operational impacts are deemed to be too high, the overall process cycles back through the evaluation to identify alternative approaches for improving system performance. Safeguards could include a range of risk management options: improvements in system protection, modification of existing operational and safety and security management procedures, improvements in emergency response coordination, or changes in support operations or services. The risks are then re-evaluated according to the new approaches to determine if they meet identified risk management goals. If not, then the evaluations can be repeated with additional provisions or changes until the risk management goals are reached. The potential alternatives, changes, and upgrades can be compared through the process to identify appropriate and effective approaches for improving overall system safety and security. Deciding on the sufficiency of protection measures to meet risk management goals is often aided by a benefit-cost evaluation. In most locations and most operations, some level of risk is common and, therefore, a “residual” risk often remains. For example, certain levels of safety equipment are standard features in automobiles, such as seat belts, air bags, and antilock brakes. While they might be effective safety measures, they do not provide total protection in all automobile accident scenarios. Therefore, the public does have some level of risk associated with driving. While many potential safeguards might be identified for a given location, the level of risk reduction and risk management required to be protective of public safety and property for liquified natural gas (LNG) transportation and storage will vary based on site-specific conditions. The risk management goals for a given location should be determined in cooperation with all stakeholders. Stakeholders include the general public, public safety officials and elected officials, facility operators, port and transportation safety and security officials, underwriters, utility representatives, regulatory agencies, and ship management companies.

TT H EH E E E LE M E N TSLE M E N TS O FO F A NA N S S P I L LP I L L O O V E RV E R W WATE RATE R

Quantifying the size and likelihood of spills from different events drives the spill and dispersion analysis. Following a tank or containment breach or other spill event, depending on the size and location, liquified natural gas (LNG) can be expected to spill onto or into the liquified natural gas (LNG) structure itself, escape through a breach onto the water surface, or both. Depending on whether there is early or late ignition, liquified natural gas (LNG) dispersion can occur through either volatilization of the liquified natural gas (LNG) into the air and transport as a vapor cloud or transport as a liquid on the surface of the water. Several variables must be addressed in developing an assessment of an liquified natural gas (LNG) spill and its general dispersion, including potential ignition sources and ignition times. These factors determine whether the liquified natural gas (LNG) disperses without a fire, burns as a pool fire, or burns as a vapor fire. Assumptions made in addressing or analyzing these variables can have a significant impact on estimates of the potential hazards associated with an liquified natural gas (LNG) spill. The consequences or hazards from an liquified natural gas (LNG) spill include a wide range of potential events.

AsphyxiationMethane is considered a simple asphyxiant, but has low toxicity to humans. In a large-scale liquified natural gas (LNG) release, the cryogenically cooled liquid liquified natural gas (LNG) would begin to vaporize upon release from the breach

PPA G EA G E 6262

Page 63: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

of an liquified natural gas cargo tank or container. If the vaporizing liquified natural gas (LNG) does not ignite, the potential exists that the liquified natural gas (LNG) vapor concentrations in the air might be high enough to present an asphyxiation hazard to the ship crew, pilot boat crews, emergency response personnel, or others that might be exposed to an expanding liquified natural gas (LNG) vaporization plume. Although oxygen deficiency from vaporization of an liquified natural gas (LNG) spill should be considered in evaluating potential consequences, this should not be a major issue because flammability limits and fire concerns will probably be the dominant effects in most locations.

Cryogenic Burns and Structural DamageThe very low temperature of liquified natural gas (LNG) suggests that a breach of an liquified natural gas container that could cause the loss of a large volume of liquid liquified natural gas (LNG) might have negative impacts on people and property near the spill, including crewmembers or emergency personnel. If liquified natural gas (LNG) liquid contacts the skin, it can cause cryogenic burns. Potential degradation of the structural integrity of an structure could occur, because liquified natural gas (LNG) can have a very damaging impact on the integrity of many steels and common structural connections, such as welds. Both the structure itself and other cargo tanks could be damaged from a large spill.

Combustion and Thermal DamageIn general, combustion resulting from industrial incidents such as an liquified natural gas (LNG) spill can result in thermal and pressure loading. Thermal loads are very dependent on the rate of energy conversion (“heat release rate”). Pressure loads are very dependent on the power density; that is, the heat release rate per unit volume. Thus, how combustion occurs is as important to the consequences of a spill as is the energy available. Table 4.2 shows the general type of thermal radiation damage from a fire. These levels are often used to establish fire hazard areas. Generally, combustion of liquified natural gas (LNG) vapor is controlled by two limiting factors:(1) Whether the liquified natural gas (LNG) vapor does not have enough time to mix with the air (called non-pre-

mixed combustion);(2) Whether the ignition occurs after the fuel has time to mix with the surrounding air (appropriately called “pre-mixed

combustion”). Therefore, ignition time is important in spill scenarios to assess appropriately the type and extent of thermal radiation from an liquified natural gas (LNG) spill and fire.

Table 4.02 – Approximate thermal radiation damage levels.

Incident Heat FluxIncident Heat Flux(kW(kW∙m∙m−2−2))

Type of DamageType of Damage

35.0 – 37.5Damage to process equipment including steel tanks, chemical process equipment, or machinery.

25.0Minimum energy to ignite wood at indefinitely long exposure without a flame.

18.0 – 20.0 Exposed plastic cable insulation degrades.

12.5 – 15.0 Minimum energy to ignite wood with a flame; melts plastic tubing.

5.0Permissible level for emergency operations lasting several minutes with appropriate clothing.

As noted in Table 4.02, combustion and thermal damage from a fire can have severe consequences and should be carefully and thoroughly analyzed.

FireballsTwo types of combustion modes might produce damaging pressure: “deflagration” and “detonation”. Deflagration is a rapid combustion that progresses through an unburned fuel-air mixture at subsonic velocities; whereas, detonation is an extremely rapid combustion that progresses through an unburned fuel-air mixture at supersonic velocities. For low reactivity fuels such as natural gas, combustion will usually progress at low velocities and will not generate significant overpressure under normal conditions. Ignition of a vapor cloud will cause the vapor to burn back to the spill source. This is generally referred to as a “fireball”, which, by its nature, generates relatively low pressures, thus having a low potential for pressure damage to structures.

PPA G EA G E 6363

Page 64: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Air ExplosionsCertain conditions, however, might cause an increase in burn rate that does result in overpressure. If the fuel-air cloud is confined (e.g. trapped between ship hulls), is very turbulent as it progresses through or around obstacles, or encounters a high-pressure ignition source, a rapid acceleration in burn rate might occur. The potential for damaging overpressures from such events could occur under some limited spill and dispersion scenarios, specifically in confined areas. However, effects will be localized near the spill source and are not expected to cause extensive structural damage.

Rapid Phase Transitions (RPT)Rapid Phase Transitions occur when the temperature difference between a hot liquid and a cold liquid is sufficient to drive the cold liquid rapidly to its superheat limit, resulting in spontaneous and explosive boiling of the cold liquid. When a cryogenic liquid such as liquified natural gas (LNG) is suddenly heated by contacting a warm liquid such as water, explosive boiling of the liquified natural gas (LNG) can occur, resulting in localized overpressure releases. Energy releases equivalent to several kilograms of high explosive have been observed. The impacts of this phenomenon will be localized near the spill source and should not cause extensive structural damage.

RR IS KIS K R R ED U C T IO NED U C T IO N S S TR ATE G YTR ATE G Y: P: P RE V E NT I O NRE V E NT I O N A N DA N D M M I T IG AT I O NI T IG AT I O N

Many factors can impact risks to public safety and property from an liquified natural gas (LNG) spill: design, materials selection, manufacturing methods, inspection and testing, assembly techniques, worker training, and safety operations, among others. Other significant factors include terminal location and design, port handling elements (e.g. tugboats and firefighting equipment), communications systems, and emergency response capabilities. It is important to realize that a decision involving large capital expense can have long-lasting effects (e.g. liquified natural gas terminal site selection). For this reason, it is imperative to consider carefully all risk management decisions in order that residual or future risks can be managed to an acceptable level. In general, risk can be managed by prevention or mitigation. Prevention seeks to avoid an accident or attack; mitigation reduces the effects of an accident or attack. While the prevention and mitigation strategies identified are possible, many might not be cost-effective or even practical in certain locations or applications. Risk management should be based on developing or combining approaches that can be effectively and efficiently implemented to reduce hazards to acceptable levels in a cost-effective manner.

SPILL CONSEQUENCE ANALYSISSPILL CONSEQUENCE ANALYSISThe consequences or potential hazards to the public of a large liquified natural gas (LNG) spill over water will depend on:(1) Potential damage to an liquified natural gas (LNG) from either an accidental or intentional breach and the size,

location, release rate and volume of liquified natural gas (LNG) spilled;(2) Environmental conditions such as wind, tides and currents, and waves that could influence the spread or

orientation of a potential liquified natural gas (LNG) spill over water;(3) Potential hazards resulting from an liquified natural gas (LNG) spill over water, such as cryogenic damage or thermal

damage to the vessel or other structure, which might lead to cascading failures of additional structures and systems or several damage to the liquified natural gas (LNG) container;

(4) The location and magnitude of a potential liquified natural gas (LNG) spill where the hazards from a spill, such as fire and thermal radiation, might impact or damage other critical infrastructures or facilities such as bridges, tunnels, petrochemical or power plants, government buildings or military facilities, national icons, or population or business centers;

(5) Potential impact on the regional natural gas supplies from the damage of an liquified natural gas (LNG) vessel, unloading terminal, or loss of use of a waterway or harbor due to the immediate or latent affects of a spill.

AA S P H Y X IAT I O NS P H Y X IAT I O N P P O TE N T IA LO TE N T IA L A N DA N D I I MPAC TSMPAC TS

Methane, an ingredient of liquified natural gas (LNG), is considered a simple asphyxiant; but it has low toxicity to humans. In a large-scale liquified natural gas release, the cryogenically cooled liquid liquified natural gas would begin to vaporize upon its release due to the breach of an liquified natural gas container. If the vaporizing liquified natural gas (LNG) does not ignite, the potential exists that the liquified natural gas vapor concentrations in the air might be high enough to present an asphyxiation hazard to the ship’s crew, pilot boat crews, emergency response personnel, or others that might encounter an expanding liquified natural gas (LNG) vaporization plume. To date, experimental data show that vaporization from an liquified natural gas (LNG) spill tends to spread essentially in a cigar-shaped, disk pattern due to the high-density characteristics of liquified natural gas. The vapor cloud spreads out in a mostly broad, flat

PPA G EA G E 6464

Page 65: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

configuration, generally with a plume of 10 feet to 30 feet in height. This is much different from the traditional Gaussian-type distributions, most often assumed for atmospheric dispersion of many common pollutants. Beard (1982) described a study of the effects of hypoxia on the cognitive abilities of 100 test subjects in a low-pressure chamber. The threshold for reduced mental performance occurred at an oxygen partial pressure of 85 torr for three of the test subjects. This is equivalent to an oxygen concentration of 11.1% at sea level. Approximately 75% of the test subjects showed reduced mental performance at 65 torr oxygen pressure, which is equivalent to 8.5% oxygen at sea level. These data were most likely obtained on a cohort of physically fit, medically qualified individuals. ANSI Z88.2-1992 standardprovides the data in Table 4.03 for inhalation of air that is deficient in oxygen (ANSI 1992).

Table 4.03 – Response of a person to inhalation of a atmosphere deficient in oxygen.

Oxygen (%)Oxygen (%)at Sea Levelat Sea Level

Oxygen Partial PressureOxygen Partial Pressure(mmHg)(mmHg) Physiological EffectsPhysiological Effects

20.9 159 Normal.

19.0 144 Some adverse physiological effects, but they are unnoticeable.

16.0 121 Impaired thinking and attention. Reduced coordination.

14.0 106Abnormal fatigue upon exertion. Emotionally upset. Faulty coordination. Poor judgment.

12.5 95Very poor judgment and coordination. Impaired respiration that might cause permanent heart damage. Nausea and vomiting.

< 10.0 < 76Inability to perform vigorous movement. Loss of consciousness. Convulsions. Death.

ANSI Z88.2-1992 standard requires air-supplying respirators for workers who enter an atmosphere having less than 16% oxygen at sea level. The ANSI standard assumes that nearly all workers will be able to escape from an atmosphere having 16% oxygen, even if it requires a moderate amount of exercise, such as climbing a ladder. When oxygen concentrations are less than 19.5% oxygen at sea level, ANSI Z88.2-1992 standard requires workers to use air-supplying respirators that have an emergency air supply for escape purposes. It assumes that some workers will be injured or debilitated by a 12.5% oxygen atmosphere, to the point at which they could not escape. ANSI’s recommendations are intended to protect nearly all workers; and it assumes that workers are medically qualified and fit for duty. Workers are, on average, more fit than the general population. To summarize, any reduction in oxygen concentrations will carry some risk to the population, because there will always be sensitive individuals. These probably include people with pulmonary or heart disease. On the basis of the references that were reviewed, it appears that minimal permanent injuries or deaths should occur in a physically fit and medically qualified population from a transient release of methane, if oxygen concentrations do not drop below 12.5% at sea level. If concentrations do not drop below 14% oxygen at sea level, the frequency of permanent injuries or deaths in the general population should be minimal as well. Of greater issue will be the potential for a fire from ignition of an liquified natural gas (LNG) cloud.

SS P I L LP I L L D D I S P E R S IO NI S P E R S IO N A N DA N D T T H E RM A LH E RM A L H H A Z A R D SA Z A R D S

If ignition occurs immediately upon spillage, then non-pre-mixed combustion occurs. In industrial spills, non-pre-mixed combustion is referred to as a fire, and the fuel-air mixing rate is controlled by flow turbulence. In laboratory settings, non-pre-mixed combustion is referred to as a diffusion flame, because mixing is controlled by diffusive processes. Specifically for liquified natural gas (LNG) spills, the fire would be referred to as a “spill” or “pool” fire, as the liquid spilling from the ship results in a quasi-steady-state fire. The hazard from this type of combustion is thermal, primarily driven by radiating heat flux. Other types of non-pre-mixed combustion, including jet and spray flames, are not relevant to liquified natural gas (LNG) spills, due to liquified natural gas’ low storage pressure and low boiling point. If mixing occurs before ignition, then the resulting combustion is pre-mixed. In industrial accident settings, two forms of pre-mixed combustion can occur, depending upon the strength of the ignition source and geometric factors. The two forms are termed deflagration and detonation. Deflagration is the most likely mode to occur. Because the fuel is pre-mixed with air, the flames spread at a rate relative to the chemical mixture (flame speed) and the rate at which turbulent mixing can enhance the flame area. Deflagrations differ in their consequences, depending on whether they occur in confined or unconfined volumes. In large open areas, the hot combustion products are buoyant and will entrain the air into the fuel mixture. The result is known as a fireball. In enclosed volumes, the combustion will result in pressure

PPA G EA G E 6565

Page 66: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

generation due to confinement of the volume expansion of the hot gases. The result is usually the failure of the enclosure. These events are loosely termed explosions. Propane leaks in houses are a typical example. If ignition occurs sometime during mixing, not before mixing takes place and not at the end when the fuel is completely mixed, then a mixture of combustion modes will result. Generally, a pre-mixed combustion event will occur first, followed by a non-pre-mixed combustion event; and pre-mixed combustion occurs faster than most mixing events. Thus, upon ignition, a pre-mixed flame will propagate from the ignition source to the spill location. This phenomenon is known as a flashback. It can generate high pressures or result in a slow burn or fireball. The flame will anchor on the spill source and a fire will result at the spill source for the duration of the spill.

TT HEHE S S I MP L I F I E DI MP L I F I E D S S I MU LAT I O NI MU LAT I O N M M O DE LO DE L A N DA N D T T HE O RYHE O RY

The distance and thermal damage to structures from a range of different spills iss calculated based on the following selection of nominal spill conditions: (1) spill calculations drainage from a non-pressurized tank with a single hole, and (2) the diameter of the spill was determined by assuming a steady state where the mass coming in is balanced by the mass going out. Note that, for all calculations in this example, a tank with volume of 25,000 m3 could be expected to spill approximately 12,500 m3. An initial liquid height in the tank above the breach of 15 meters and a density of 450 kg∙m−3 for liquified natural gas (LNG) can be used. First apply the continuity equation,

dmdt

=⋅A⋅vin−⋅A⋅v out [4.01]

If (ρ∙A∙v)in is null,

dmdt

=−⋅A⋅v out [4.02]

where ρ is the specific mass (kg∙m−3), A is the area (m2), and v is the velocity (m∙s−1). Mass can be express as ρ∙V, and the volume (V) becomes,

V =At⋅h [4.03]

where At is the cross-sectional area (m2) of a cylinder (e.g tank or circular container), and h is the height (m). Substituting into Equation [4.02],

d ⋅At⋅hdt

=−⋅A⋅v out [4.04]

The velocity of the fluid coming out of the tank can be expressed as a function of height through invoking Bernoulli’s equation,

12⋅⋅v t

2 p t⋅g⋅ht=12⋅⋅v0

2 p0⋅g⋅h0 [4.04]

⋅g⋅h t=12⋅⋅v0

2[4.05]

and

v0=2⋅g⋅ht [4.06]

Multiply by a discharge coefficient (Cd) to account for resistance of the hole,

v0=Cd⋅2⋅g⋅ht [4.06]

PPA G EA G E 6666

Page 67: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Substitute Equation [4.06] into Equation [4.04] and integrate with initial condition, t = 0 and h=hi,

t= 2g⋅ At

Cd⋅A0 ⋅hi− h [4.07]

Then the height of liquid throughout time can be determined. Total time to drain is given by the following expression,

t= 2g⋅ At

Cd⋅A0 ⋅h i [4.08]

The flow rate will be greatest at the beginning of the spill, due to the hydrostatic head having a maximum. The flow rate has a linear dependence on time, so an average flow rate was determined by dividing the maximum flow rate by 2. The maximum flow rate can be found by substituting Equation [4.06] into Equation [4.02], and using m = ρ∙V to express in terms of volume per time. Then,

dVdt avg

=− A⋅v2

out=−Cd⋅A0

2 ⋅2⋅g⋅hi [4.09]

Equation [4.09] was used for the calculations to determine the average flow rate out of the tank. The diameter of the spill was determined by assuming a steady state where the mass coming in is balanced by the mass going out, due to the heat flux from the heating of the water below and from the fire above, denoted by vtotal.Thus,

⋅A⋅v in=⋅A⋅vout [4.10]

dVdt avg

=− A⋅v out=⋅D 2

4⋅v total [4.11]

and

D= 4⋅v total

⋅ dVdt avg

[4.12]

Equation [4.12] was used to determine the diameter of the spill.

Distance to a Specified Radiative Flux LevelA right cylinder, solid flame model was used to model the pool fire. The effect of wind on the flame was considered negligible. The Moorehouse correlation for liquified natural gas (LNG) was used to calculate flame height (H), found in SFPE Handbook (Fire Protection Engineering, 2nd ed., 1995). The term u is a non-dimensional wind velocity taken to be 1 for low wind speeds,

H=6.2⋅D⋅ ma⋅ g⋅D

0.254

⋅u−0.044[4.13]

The radiative flux incident (q) upon an object can be determined by,

q=E p⋅⋅F [4.14]

where Ep is the average emissive power (kW∙m−2), τ is the transmissivity, and F is the view factor. In order to determine distance to a specified radiative flux incident we need the non-dimensional distance from the flame axis as a function of view factor and fire height-to-radius ratio. Because radiative flux incident (q), average emissive power (Ep), and

PPA G EA G E 6767

Page 68: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

transmissivity (τ) are specified, the view factor (F) can be determined by Equation [4.14], and height-to-radius ratio from Equation [4.13]. Then the thermal hazard distance can be determined.

Consideration of Mass Fires and Pool FiresAll of the liquified natural gas (LNG) fire studies reviewed assume that a single, coherent pool fire can be maintained for very large pool diameters (greater than 100meters). This might be unlikely due to the inability of air to get into the interior of the fire and support combustion. At some very large size, the flame envelope would break up into multiple flamelets. The heights of these flamelets are much less than the fuel bed diameter (Zukoski, Corlett, Cox and Chitty). The break up into flamelets would result in a much shorter flame height than that assumed by the reviewed studies, which are applying height correlations far out of the diameter range for which they were developed. It is expected that

theLD (height-to-pool diameter) would probably be much smaller than that predicted by existing correlations. The

correlations predict anLD ratio between one and two, while a more realistic ratio for a mass fire would be under 0.5.

The view factor is very sensitive to flame height at distances not close to the fire (greater than one pool diameter). View factors are used to determine how much radiative flux an object receives. Thus, if a more realistic flame height is used, lower than that which is typically calculated, then the amount of heat flux that an object receives would be less, thereby decreasing the thermal hazard zone. The zone could be decreased by a factor of two to three, depending upon the damaging heat flux levels of interest. Various correlations for flame height have been developed for a range of pool diameters up to 30 meters. The height-to-pool diameter correlations are typically expressed in terms of a non-dimensional heat release rate (Q). As pool diameter increases, the non-dimensional heat release rate (Q) decreases

because it is proportional to1

Dand Zukoski states that there are different transition regions that occur. For very

large pool fires, the flame breaks up into a number of independent flamelets as non-dimensional heat release rate (Q) decreases, and the flame height depends on the diameter and the heat release rate. For high non-dimensional heat release rate (Q), the height of the flamelets appears to become roughly independent of the source-diameter and depends only on the local heat release rate per unit area (or fuel flow per unit area). This assumptions is based upon pool fire tests where fuel vaporization is not affected by a substrate such as water. It is unknown what the limiting diameter for break up is for liquified natural gas (LNG) pool fires on water.

Liquified Natural Gas DispersionIn most of the scenarios identified, the thermal hazards from a spill are expected to manifest as a pool fire, based on the high probability that an ignition source will be available from most of the events identified. In some instances, such as an intentional spill without a tank breach, an immediate ignition source might not be available and the spilled liquified natural gas (LNG) could, therefore, disperse as a vapor cloud. For large spills, the vapor cloud could extend to as much as 1,600 m or more, depending on spill location and site atmospheric conditions. In congested or highly populated areas, an ignition source would be likely, as opposed to remote areas, in which an ignition source might be less likely. If ignited close to the spill, the thermal loading from the vapor cloud ignition might not be significantly different from a pool fire, because the ignited vapor cloud would probably burn back to the source of liquid liquified natural gas (LNG) and transition into a pool fire. If the cloud is ignited at a significant distance from the spill, the thermal hazard zones can be extended significantly. The thermal radiation from the ignition of a vapor cloud can be very high within the ignited cloud and, therefore, particularly hazardous to people. Experimental data and analytical estimates for vapor spreading suggest that a large vapor plume could extend to large distances, depending on atmospheric conditions. Therefore, while the impact from a vapor cloud dispersion and ignition from a large spill can potentially extend beyond 1,600 meters, the area of high impact might be reduced. This suggests that liquified natural gas (LNG) vapor dispersion analysis should be conducted using site-specific atmospheric conditions, location topography, and operations to adequately assess the potential areas and levels of hazards to public safety and property, and consideration of risk mitigation measures, such as development of approaches and procedures to ignite a dispersion cloud quickly if conditions exist that the cloud would impact critical areas.

Fireballs Resulting From an Liquified Natural Gas SpillA fireball will result from an liquified natural gas (LNG) spill only if some mixing of the fuel and air occurs prior to ignition. Thus, if ignition occurs immediately upon release, no fireball will result. For a fireball to occur there must be fuel release, spread, vaporization, and ignition after significant premixing. If all these events have occurred, a fireball is

PPA G EA G E 6868

Page 69: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

the most benign form of combustion that can result. The hazards are principally short-time thermal damage high in the air and away from structures and people. The duration of the fireball from combusting clouds is given as,

t=4.6⋅M 0.2 [4.15]

in which the fuel mass (M) is in metric tons and the time (t) is given in seconds. Similarly, the maximum radius of the fireball is given as,

R=23⋅M 0.35 [4.16]

in which the fuel mass (M) is in metric tons and the radius (R) is given in meters. Unlike a pool fire, however, the fireball is of short duration (in the order of seconds to tens of seconds), depending upon the mass of fuel in the air. The fireball will entrain and burn all flammable vapors and provide an ignition source to the underlying liquid spill. The overall threat from a fireball is typically not of primary concern if a long duration pool fire follows it.

Thermal Damage on StructuresThe potential for damage to structures from an liquified natural gas (LNG) spill and fire needs to be considered to determine the overall risk. The potential for fire damage from spills can be relatively extensive. The thermal radiation that will damage structures is approximately 37 kW∙m−2 for durations of more than 10 minutes. Damage can be expected to the liquified natural gas (LNG) structure and nearby steel structures, because steel strengths are reduced to 60% and 75% range of their room temperature values at 800K. Further reduction in strength will result for temperatures above 800K. Steel will melt at approximately 1,800K and is generally considered to have no strength at half the melt temperature, or 900K. The calculations suggest that these temperatures could exist at a spill from an liquified natural gas (LNG) tank from 30 minutes to an hour and, therefore, potentially damage nearby steel and other structures. Of even greater importance is the possibility that a large spill could cause a cascading set of liquified natural gas (LNG) tank failures. In this instance, significant long-term fire damage could result to a nearby steel structure, unloading terminal, or unloading platform. Positive operational and risk management measures can be taken to try to prevent these types of issues.

Analysis of Fire Damage to InsulationThe insulation used in liquified natural gas (LNG) structures and containers varies considerably, from rigid foams to bulk zeolite-type materials. The susceptibility of these insulation materials to either burning or thermal degradation also varies considerably. Many liquified natural gas (LNG) structures use foam insulation materials that include polystyrene, polyurethane, phenolic resin, and hybrid foam systems. These foams are considered combustible to slightly combustible; meaning, they will burn when exposed to an open flame, as might occur in a breach with a resulting fire. Of greater importance, though, is that these foams will begin to decompose at temperatures of about 550K. Because an liquified natural gas fire can be expected to burn at temperatures of approximately 3,000°F, thermal loading on the liquified natural gas (LNG) tank from an engulfing fire, if sufficient in duration, could lead to heat transfer through the structure, decomposition of the foam, and an increase in the liquified natural gas (LNG) volatilization rate in an impacted tank or other structure. This could lead to rupture or collapse of the tank and structure, additional damage to other structures, and greater hazards to both the public and property. Foam used to insulate liquified natural gas (LNG) is enclosed within a steel weather cover, or within the inner hull of the liquified natural gas (LNG) tanker. Extensive burning of the foam is not expected, given the lack of sufficient air to support combustion in these regions, even in cases with limited damage to the hull or weather cover. Based on the foam being located within an enclosure, thermal decomposition of the liquified natural gas foam insulation is more likely. Heat transfer will result in thermal decomposition of the foam insulation, the products of which will burn if vented to the air, or cause an increase in the pressure in the region between the steel and the inner container. Accidental spills with general pool fire diameters of 200 meters might be possible. The flame height for such a spill might approach 150 m, high enough to engulf the top of an liquified natural gas (LNG) tanker. For this size of fire, at least some portions of adjacent liquified natural gas (LNG) tanks would probably be exposed to the fire. A fire from a spill could last from five to twenty minutes.

Liquified Natural Gas – Air CombustionTwo types of combustion modes might produce damaging pressure, deflagration, and detonation. Deflagration is a rapid combustion that progresses through unburned fuel-air mixture at subsonic velocities, whereas detonation is an extremely rapid combustion that progresses through an unburned fuel-air mixture at supersonic velocities. In order for deflagration

PPA G EA G E 6969

Page 70: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

to occur, the fuel-air concentration must be above the minimum flammable limit (lean limit) and below the maximum flammable limit (rich limit). For liquified natural gas (LNG), these limits are 3.8% to 17% fuel by volume. If the fuel concentration is within these limits and encounters an ignition source, it will ignite and burn. Because of the moderate flammability range, the amount of time lapse between dispersal and ignition is limited. For low reactivity fuels such as natural gas, combustion will usually progress at low velocities and not generate overpressure. Certain conditions, however, might cause an increase in burn rate that does result in overpressure. If the fuel-air cloud is confined, is very turbulent, or progresses through obstacles, a rapid acceleration in burn rate might occur (Benedick et al., 1987). In extreme cases, the burn rate might increase to supersonic velocities. This is known as deflagration-to-detonation transition (DDT). Under specialized conditions, pre-mixed combustion can result in a detonation. This mode is not common and is generally considered to be very unlikely (but not impossible) to occur in most industrial accident situations, such as an liquified natural gas (LNG) spill. Detonations have the highest power density of any combustion mode and, thus, result in the highest pressures and most damage. In a detonation, the combustion front typically travels at Mach 5 and, for hydrocarbons, has a peak pressure about 15 times the initial pressure. A detonation can be directly initiated in a fuel and air mixture from high initiation pressures or, under very limited circumstances, it can transition from a deflagration to a detonation (called DDT, or deflagration to detonation transition in the pre-mixed combustion literature) under conditions involving confinement. In industrial accidents, detonations are also sometimes called “unconfined vapor cloud explosions”. In military literature, gas phase detonations are termed fuel-air explosions (FAE). Detonation is the most violent form of fuel-air combustion. For detonation to occur, the fuelair mixture must be within the minimum and maximum detonation limits. These limits are much narrower than flammability limits. To ignite a fuel-air mixture within the limits of detonation, shock initiation is necessary. Shock initiation can be produced by “igniting” the fuel-air cloud with an explosion or by the deflagration-to-detonation transition involving confinement. For low reactivity fuels, the initiation energies are quite large and unlikely to occur in an accidental breach, but might be possible in an intentional breach or tank rupture scenario.

Table 4.04 – Properties of common hydrocarbon fuels.

FuelFuel Lower FlammableLower Flammable Limit (Vol., %)Limit (Vol., %)

Upper FlammableUpper Flammable Limit (Vol., %)Limit (Vol., %)

Heat of CombustionHeat of Combustion(kJ(kJ∙g∙g−1−1))

Ignition TemperatureIgnition Temperature((°°C)C)

Boiling PointBoiling Point((°°C)C)

Methane 5.5 14.0 55.5 650 −161

Ethane 3.0 12.5 51.9 472 −89

Ethylene 2.7 36.0 50.3 490 −104

Acetylene 2.5 82.0 49.9 305 −84

Propane 2.2 9.5 50.3 450 −42

Propylene 2.4 10.1 48.9 455 −48

Propyne 2.1 12.5 48.3 NA −23

Octane 1.0 6.5 47.9 NA 126

Spilled liquified natural gas (LNG) could become trapped between the inner and outer hulls which, if ignited, could lead to an explosion. In general, large releases will involve sufficient liquified natural gas (LNG) for this space to be fuel rich. Of greater concern are small leaks where a flammable mixture could develop. Another potential for an explosion is if liquified natural gas (LNG) is spilled without an ignition source, such as an intentional spill from premature offloading of liquified natural gas. In this scenario, there could be extensive volumes of liquified natural gas (LNG) that can be spilled either onto the ship or onto the water surface without and ignition source. These type of approaches have been considered and used and are very sensitive to environmental and meteorological conditions (Tieszen, 1991). Therefore, the potential for this type of event exists, but actually getting an explosion can be difficult. Table 4.04 provides some physical and chemical properties of hydrocarbon fuels. Further, all fuels become less able to detonate if they are not perfectly mixed to stoichiometric proportions. For many sources, refined liquified natural gas (LNG) has a high percentage of methane at the wellhead compared to natural gas. The level of refinement of natural gas stored as liquified natural gas can have an effect on detonation sensitivity, with a less processed product being more sensitive to detonation.

PPA G EA G E 7070

Page 71: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Magnitude of Liquified Natural Gas – Air Explosion OverpressureIn order to estimate the overpressure at a given distance from a fuel-air explosion, several parameters must be defined. First, the mass of fuel within the flammability limits must be determined. To find the energy released, the mass of fuel within flammability limits is then multiplied by the heat of combustion. Finally, the velocity of combustion, or flame Mach number (Mf), must be estimated. For explosively initiated detonations, a value of 5.2 should be used for flame Mach number. Once the total energy release and combustion velocity are known, the scaled overpressure versus scaled distance curve can be used to estimate an overpressure at a specific distance. Most structures are significantly less resistant to internal blasts than they are to external blasts. If natural gas finds it way into a structure and then ignites, severe structural damage can occur. This is a potential concern to the liquified natural gas tank if the spilled liquified natural gas (LNG) is somehow trapped, as well as for nearby structures where the liquified natural gas (LNG) might settle and ignite. While detonations are unlikely, some type of overpressure events could occur on a liquified natural gas structure with a large liquified natural gas (LNG) spill and provisions to prevent these types of events should be considered.

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESAnderson R. P., and Armstrong D. R., 1972. Experimental Study of Vapor Explosions, 3rd international conference on liquefied natural gas, Washington, DC.American National Standard for Respiratory Protection, ANSI Z88.2-1992. American National Standards Institute, New York, 1992.Barry, Thomas (20021). Risk-Informed Performance-Based Industrial Fire Protection, Tennesse Valley Publishing, p. 50.Beard, R., 1982. Inorganic Compounds of Oxygen, Nitrogen, and Carbon, in Patty’s Industrial Hygiene and Toxicology, Volume 2C: Toxicology, George D. Clayton and Florence E Clayton, editors, Wiley Interscience, New York.Benedick, W. B., Tieszen, S. R., and Sherman, M. P., 1987. Flame Acceleration and Transition to Detonation in Channels. Sandia National Laboratories Report SAND-87-1444C.Berthoud, G. (2000) Vapor Explosions, Annu. Rev. Fluid Mech., 32, 673-611.Blackmore, D. R., Eyre, J. A. and Summers, G. G., 1982. Dispersion and Combustion Behavior of Gas Clouds Resulting from Large Spillages of LNG and LPG on to the Sea , Trans. I. Mar. E. (TM) 94, paper 29.Blevins, R. D., 1984. Applied Fluid Dynamics Handbook, Van Nostrand Reinhold Company, Inc., pp. 136-141.Boyle, G. J. and Kneebone A., 1973. Laboratory Investigations Into the Characteristics of LNG Spills on Water. Evaporation, Spreading and Vapor Dispersion, Shell Research Ltd., Thornton Research Centre, Cester, England, Report 6-32, March.Bradley, T.M. et al., 2001. Flame Acceleration Due to Flame-Induced Instabilities in Large-Scale Explosions, Comb. and Flame, 124, 551-559.Brandeis, J. and Ermak, D. L., 1983. Numerical Simulation of Liquefied Fuel Spills: II. Instantaneous and Continuous LNG Spills on an Unconfined Water Surface, Int. J. for Num Meth. In Fluids, Vol. 3, 347-361.Chan, S. T. et al., 1984. Numerical Simulations of Atmospheric Releases of Heavy Gases Over Variable Terrain, Air Pollution Modeling and its Applications III, 295-328.Chan, S. T., September, 1997. A Three-Dimensional Model for Simulating Atmospheric Dispersion of Heavy-Gases Over Complex Terrain, UCRL-JC-127475, Lawrence Livermore National Laboratory.Drake, et al., 1975. Transient Boiling of Liquefied Cryogens on a Water Surface: II. Light Hydrocarbon Mixtures, Int. J. Heat Mass Transfer, Vol. 18, 1369-1375.Hirst, W. J. S. and Eyre, J. A., 1983. Maplin Sands Experiments 1980: Combustion of Large LNG and Refrigerated Liquid Propane Spills on the Sea, Heavy Gas Risk Assessment, 211-224.Jazayeri, B., 1975. Impact Cryogenic Vapor Explosions. M.S. Thesis, MIT, Cambridge, Massachusetts.Lee, J. H. S. and Moen I. O., 1980. The Mechanism of Transition from Deflagration to Detonation in Vapor Cloud Explosions, Prog. Energy Combust. Sci., Vol. 6, 359-389.Mizner, G. A., and J. A. Eyre, 1983. Radiation from Liquefied Gas Fires on Water. Combustion Science and Technology, Volume 35, pp. 33-57.Moen, I. O., 1993. Transition to Detonation in Fuel-Air Explosive Clouds, J. Haz. Mat., 33, 159-192.Napier D. H. and Roochland, D. R., 1984. Ignition Characteristics of Rapid Phase Transition Explosions, Combustion Institute Canadian Section 1984 Spring Technical Meeting.Porteous, W. M. and Blander, M., 1975. Limits of Superheat and Explosive Boiling of Light Hydrocarbons, Halocarbons and Hydrocarbon Mixtures, AIChE Journal, 31(3), 560-566.Pritchard, M. J., and T. M. Binding, 1992. FIRE2: A New Approach for Predicting Thermal Radiation Levels from

PPA G EA G E 7171

Page 72: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Hydrocarbon Pool Fires. IChemE Symposium Series, No. 130, pp. 491-505.Tieszen, 1991. Gaseous Hydrocarbon-Air Detonations, Combustion & Flames, Vol 84, pg 376-390, 1991.Zukoski, E. E., 1995). Properties of Fire Plumes, in Combustion Fundamentals of Fire (ed. G. Cox), pp. 101-219. Academic Press, London.

PPA G EA G E 7272

Page 73: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Chapter 5Chapter 5A Fire Risk Assessment Tool to Evaluate FireA Fire Risk Assessment Tool to Evaluate Fire

Safety in Industrial Facilities and Large SpacesSafety in Industrial Facilities and Large Spacesef

INTRODUCTIONINTRODUCTIONA computer model for evaluating fire protection systems in industrial buildings is presented. The model has been developed as a tool to assist fire protection engineers, building officials, fire service personnel and researchers in performing fire safety engineering calculations, and can be used to conduct hazard and risk analyses, as well as to evaluate whether a selected design satisfies established fire safety objectives. While the model is primarily designed for use in warehouses and aircraft hangars, it can be modified for application to other industrial buildings. This chapter describes the framework for the model, along with its capabilities and flexibility. Individual models used to perform calculations are discussed, particularly those that calculate fire development and life hazard. Methods used by the model to conduct risk assessments are also briefly described. The model has been designed as a tool that can be used to support performance-based fire protection engineering design. The individual models included in the overall system are based on accepted fire protection engineering practice and were chosen to give an appropriate level of sophistication, and still result in a system that can be run in a reasonable length of time on a desktop personal computer. This would allow for use of the system by all of the intended parties, and facilitate the examination of multiple fire scenarios and fire protection engineering options. The model allows the user to perform a number of fire protection engineering calculations in order to evaluate fire protection systems in industrial buildings. At startup, the model provides several calculation options, which allow the user to:(1) Use standard engineering correlations;(2) Run individual models;(3) Conduct a hazard analysis;(4) Conduct a risk analysis.

The standard engineering correlations model is a collection of relatively simple equations that can be used to quickly perform simple fire protection engineering calculations. The model contains procedures for calculations in the general areas of fire development, plume dynamics, smoke movement, egress, fire severity and ignition of adjacent objects. The model can also be used to evaluate whether a fire protection system for a building will satisfy specific fire safety objectives. This can be done using individual models, or through a hazard or risk analysis. For example, individual models can be used to evaluate single components of a fire safety design, such as the time of activation of heat detectors or sprinklers, the time to flashover and the time of failure of construction elements.

THE FIRE DEVELOPMENT MODELSTHE FIRE DEVELOPMENT MODELSModels are currently available for the following fire scenarios:(1) Liquid pool fires;(2) Storage rack fires;(3) t2 fires, i.e. the heat release rate is assumed to be proportional to the square of the elapsed time, which is often used

to simulate fires). Each of the fire development models calculates the quantities, which characterize the fire (heat release rate, temperature and thermal radiation heat fluxes) as functions of time. Currently, the equations used are standard engineering correlations, such as those found in the SFPE Handbook of Fire Protection Engineering. The heat release rate at any time is calculated by assuming that the pool fire can be described as an ultrafast t2 fire, using the following equation,

Q t =⋅t 2 [5.01]

where Q is the heat release rate of the fire at any time (kW), α is the fire growth coefficient (0.1876 kW∙s−2), and t is the time (s). This heat release rate (Q) is limited to the maximum heat release rate possible based on either the amount of fuel in the compartment or the oxygen that can be supplied to the fire from the compartment and through ventilation

PPA G EA G E 7373

Page 74: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

openings. The duration of a confined pool fire is calculated using the volume of fuel burned, the dike area and the burning rate of the fuel, which are specified by the user. The model assumes that the diameter of a pool fire increases linearly with time to the maximum diameter (Dmax) entered by the user. This maximum diameter of the pool fire is assumed to be its equilibrium diameter (the spill rate equals the burning rate). The time required to reach this maximum diameter (tmax) is given by the following equation,

tmax=0.564⋅[ Dmax

g⋅v f⋅Dmax 13 ] [5.02]

where vf is the burning (regression) rate of the fuel (m∙s−1), and g is the acceleration due to gravity (m2∙s−1). The thermal radiation heat fluxes from the pool fire to a point located 1 meter from the ground at various distances are calculated using the solid flame model of Mudan and Croce. A height of 1 meter was chosen so as to be representative of the mid-section of a person. The following equation is used to calculate thermal radiation heat fluxes at a distance from the fire,

q t =qe⋅F t ⋅⋅ [5.03]

where q is the emissive power of the pool fire (kW∙m−2), F(t) is the view factor from the pool fire to the point, calculated using the height and diameter of the flame (0 ≤ F ≤ 1), τ is the atmospheric transmissivity (it can be assumed to be 1.0 because of the relatively short distances considered), and ς is a safety factor. The emissive power of the flame is calculated using the following equation,

qe=E f⋅e−⋅DE s⋅1−e−⋅D [5.04]

where Ef is the maximum emissive power of the visible portions of the fire (140 kW∙m−2), Es is the maximum emissive power of the smoky portions of the fire (20 kW∙m−2), κ is an experimentally determined parameter (0.12 m−1), and D is the pool diameter (m). The fire development model provides the heat fluxes from the pool fire to the life hazard model to calculate the probability of death from exposure to high heat fluxes. The fire development model also supplies the building element failure model with information to calculate the convective and radiative heat fluxes from the fire to the boundaries of the compartment. The ceiling im pingement gas temperature (Tce) and the effective plume temperature (Tep) used to evaluate the time to failure of ceilings and walls of the compartment, respectively, are given by the following equations,

T cet =T amb0.22⋅[ kW⋅Qt 23

H53 ] [5.05]

where Tce is ceiling impingement gas temperature (K), Tamb is the ambient temperature (°C), kw is a factor to take into account the effect of the compartment walls on the temperature of the hot plume gases (kw = 1 if no walls are nearby; kw = 2 if the fire is close to one wall – default value; kw = 4 if the fire is in a corner), Q is the heat release rate of the fire at any time (W), and H is the distance or height between the top of the fuel and the ceiling (m).

T ep4 t =T f

4 t ⋅ H f t H c T ce

4 t ⋅1−H f t

H c [5.06]

where Hf(t) is the height of the flame given in Equation [5.07] below from the correlation (m), Hc is the height of the compartment (m), Tf is flame temperature (K), and Tep is effective plume temperature (K).

H f t =0.011⋅k w⋅Q t 25 [5.07]

PPA G EA G E 7474

Page 75: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Equation [5.05] is derived from empirical data and is not valid when the flames are either very close to the ceiling or very far away from the ceiling; therefore, it cannot predict temperatures greater than 825°C accurately. In addition, the temperature at the centreline of the visible portion of the flame (Tf) is evaluated to be 980°C. This temperature value is calculated based on the thermal radiation heat flux of 140 kW∙m−2 assumed earlier for the visible portion of the flame, an emissivity of 1 and a view factor of 1. The assumed value of 140 kW∙m−2 and calculated temperature of 980°C are valid for pool fires only. For other types of fires, other values need to be used.

SS U P P RE S S I O NU P P RE S S I O N E E F F EC T I V E NE S SF F EC T I V E NE S S

The suppression effectiveness can be calculated by the effect of candidate automatic suppression systems on fires in the building or compartment. The model requires the user to input a suppression effectiveness value (η) from 0 to 1, which quantifies the ability of the automatic suppression devices to control the fire scenarios being considered. This value is then used to modify the fire heat release rate, diameter, thermal radiation heat fluxes and plume temperature. The heat release rate curve, Q(t), from the fire development model or another source is input to the suppression effectiveness. The suppression effectiveness value is used to produce a modified heat release rate curve, Qm(t). If the suppression system effectiveness is 1, the fire is controlled so that the heat release rate remains at its value at the time of automatic suppression system activation (i.e. Qm = Qact). If the effectiveness is 0 (or near zero), the original heat release rate curve (Qo) will not be modified (i.e. Qm = Qo). If the effectiveness is between 0 and 1, the modified heat release rate will be calculated at each time step using the following equation,

Qm t =1−⋅[Qot −Qact t ]Qact t [5.08]

In the case where heat release rate values decay, any value of Q(t), which is below Qact, is not modified in any way. This helps ensure that the suppression effectiveness does not increase the value of Q(t) in this situation. Heat fluxes and temperatures calculated using Equation [4.03] through Equation [4.07] are then corrected using the modified heat release rate data.

FF I R EI R E D D EPA RT M E N TEPA RT M E N T R R E S P O N S EE S P O N S E A N DA N D E E F F E C T IV E N ES SF F E C T IV E N ES S

The fire department response is used to determine the expected fire department response and intervention times, which are calculated using the times estimated for notification, dispatch and preparation, travel and set-up. These calculations are based on factors such as the fire scenarios selected by the user and activation times for the detectors in the building. The presence of fire alarms in the building, their direct connection to the fire department (some buildings may be linked to a dispatch station through an alarm that will directly notify the fire department in case of fire), the occupant response to fire cues or other warning signals, the location of the building relative to the fire department, preplanning, traffic volume, and street arrangement should also be considered for calculations. Once the fire department activities begin, the effectiveness of these activities is estimated according to information on the fire development at the time suppression commences and the resources (e.g. equipment, water and human resources) available to the fire department. Factors such as the nature of the fire department (e.g. professional, volunteer or a combination of both), firefighter experience and training are also considered in this calculation.

OO C C U PA N TC C U PA N T R R E S P O N S EE S P O N S E A N DA N D E E VAC UAT IO NVAC UAT IO N

The occupant response and evacuation information are used to track the movement of occupants in the building during the selected fire scenarios, based on the occupant characteristics entered by the user. The characteristics include the location of the occupant and their age. Calculations take into account the processes of perception (occupants become aware of fire by means of direct perception of fire cues, warning by alarm or others, etc.), interpretation (occupants make a decision to respond), and action (e.g. occupants call the fire department, pull the alarm, begin to evacuate, etc.). This is different from most occupant evacuation models, which assume that the occupants respond immediately to a fire alarm, or cue, which is not the case in occupant evacuation field studies or in real life.

PPA G EA G E 7575

Page 76: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

LIFE HAZARD MODELLIFE HAZARD MODELThe subject of life safety in fire has been studied by a number of researchers and the information has been produced in the SFPE Handbook (National Fire Protection Association), International Organization for Standardization (ISO) and National Fire Protection Association (NFPA). National Fire Protection Association / SFPE provides technical guidance on the subject of life safety. International Organization for Standardization (ISO) has published a technical specification document and a standard (ISO/TS 13571 and ISO/DIS 13344), to provide the necessary requirements and the technical guidance to evaluate life safety in fire. The National Fire Protection Association life safety code handbook also provides details on the life safety requirements and guidance to achieve these requirements. Life hazard model calculates the time-dependent probability of death for occupants in a compartment due to the effects of being exposed to high heat fluxes and hot and/or toxic gases. The life hazard model uses input from other models that describe the heat fluxes (fire development and smoke movement models) in the compartment, and the temperature and chemical composition of hot gases (smoke movement model). The time-dependent probability of death from exposure to high thermal radiation heat fluxes (PTR) at a given location in the compartment, is calculated using the sum of the heat fluxes from the fire (calculated by fire development model) and from the hot smoke layer (calculated by the smoke movement model). The revised vulnerability model of Tsao and Perry is used to calculate the probability of death from the heat flux data. This m odel uses the following probit equation,

Y =−12.82.56⋅ln I dose [5.09]

where Y is the probit function, and Idose is the thermal dose in (kW∙m−2)4/3∙s−1 units. The thermal dose (Idose) is calculated using the following equation,

I doset =∫0

t

qt 43⋅dt [5.10]

where q is the incident heat flux (kW∙m−2), and t is the exposure duration (s). For a square wave heat flux (i.e. a constant value), Equation [5.10] reduces to the following equation,

I doset =qt 43⋅t [5.11]

The probit function (Y) is then used to determine the probability of death due to thermal radiation heat fluxes,

PTR=1

2⋅⋅∫

−∞

Y −5

e− 2

2 ⋅d [5.12]

Life hazard model only considers the toxic effects of carbon dioxide (CO2) and carbon monoxide (CO), because in most practical fire situations, the effects of carbon monoxide (CO) are the most important. Carbon dioxide (CO2) will affect the rate of breathing and hence will affect the intake of carbon monoxide (CO). The fractional incapacitating dose due to carbon monoxide (FIDCO) is calculated carbon monoxide (CO) using the following equation and the concentration of carbon monoxide (CO) at a specified height in the compartment of interest,

FIDCO t =∫0

t

8.2925⋅10−4⋅C COt

1.036

30⋅dt [5.13]

where FIDCO(t) is the fractional incapacitating dose of carbon monoxide (CO), and CCO(t) is the concentration of carbon monoxide (CO) at a given time (t) in parts per million (ppm). The fractional incapacitating dose (FID) is defined such that the dose will be lethal when FID is equal to 1. The default height for these calculations is 1.5 meters. While it can be argued that all individuals can crawl under a smoke layer at this height, this height was chosen so as to be conservative. The user can also specify other heights for this calculation, depending on the occupancy. The concentration of carbon monoxide (CO) is used to calculate a factor (FCO2) which is used to increase the fractional incapacitating dose of carbon monoxide (FIDCO) to incorporate the increase in the breathing rate due to carbon dioxide

PPA G EA G E 7676

Page 77: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

(CO2),

F CO2t =1

6.8⋅e0.2496⋅CCO2 t 1.9086

[5.14]

where FCO2(t) is the multiplication factor for carbon dioxide-induced hyperventilation, and CCO2(t) is the percentage (%) of carbon dioxide (CO2) by weight in the compartment of interest. The total fractional incapacitating dose (FIDTG) for toxic gases is calculated using the following equation,

FIDTG t =FIDCO t ⋅FCO2 t [5.15]

This total fractional incapacitating dose (FIDTG) is then used as the probability of death due to breathing toxic gases (i.e. PTG = FIDTG). The life hazard model also considers the probability of death due to breathing or being exposed to hot gases (PHG). This probability is equal to the fractional incapacitating dose for exposure to hot gases (FIDHG), calculated using the following equation,

FIDHG t =∫0

t 160

⋅e−[5.1849−0.0273⋅T S t ]⋅dt [5.16]

where TS(t) is the temperature of the hot gases at a height of 1.5 meter in the compartment of interest (°C). Equation [5.16] is based on data from the literature for human tolerance times in experimental exposures to dry and humid air at elevated temperatures. An fractional incapacitating dose for exposure to hot gases (FIDHG) of 1.0 is said to represent the point where a person would become incapacitated by the exposure to the hot gases because of heat stroke, skin burns and respiratory tract burns. The total probability of death, PD(t), at a given location in the compartment is calculated using the union of the individual probabilities of death from being exposed to high thermal radiation heat fluxes, and breathing hot or toxic gases,

PD t =PTR t ∪PTG t ∪PHG t [5.17]

In order to calculate the total probability of death in any compartment using the life hazard model, the compartment is first divided into a number of rings from the fire. Equation [5.17] is used to calculate the total probability of death within each of the rings. The total probability of death for the compartment is then calculated using a weighted sum of the probabilities of death in each of the rings,

PC ,it =∑i

P D ,it ⋅Ai

AC[5.18]

where PC,i(t) is the total probability of death for compartment C, PD,i(t) is the probability of death for ring i, Ai is the area contained inside ring i, and AC is the total area of compartment C.

EE XP EC T E DXP EC T E D N N U M B ERU M B ER O FO F D D E AT H SE AT H S

The expected number of deaths model calculates the number of occupants expected to die in each compartment with time. This calculation is based on the residual population in each compartment computed by the occupant evacuation model and the probability of death in that compartment computed by the life hazard model. At each time step, the expected number of deaths are computed by multiplying the probability of death at that time with the residual live population at that time,

END t=∑C

PC ,it ⋅POPR ,C t [5.19]

where END(t) is the expected number of deaths in the building at time t, PC,i(t) is the probability of death for compartment C at time t, and POPR,C (t) is the residual live population in compartment C at time t.

PPA G EA G E 7777

Page 78: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

EE C O N O M I CC O N O M I C C C O N S E Q U E N C E SO N S E Q U E N C E S

The economic model for consequences calculates the costs of the building, fire protection systems, and contents based on information provided by the user. The sensitivity of the different parts of the building and contents to heat, smoke and water are also input by the user. Damage to the building and contents are then estimated for the fire scenarios selected by the user based on information from the fire development and smoke movement models and the sensitivities of the building and contents. These damage estimates can then be used along with the cost information to estimate the value of the property loss to the building and its contents. More detail on the model can be in found in references.

HAZARD ANALYSIS PROCEDUREHAZARD ANALYSIS PROCEDUREHazard analysis calculates the consequences of a specific fire scenario beginning in a specific compartment in the building or facility. The results of hazard analysis are the expected number of deaths, the expected cost of property and equipment damage and the expected interruption of business operations (downtime). The steps involved in hazard analysis for a multi-compartment building or facility are shown in Table 5.01 along with the models involved and their outputs. The user first specifies the compartment of fire origin and the fire scenarios that would occur in each compartment. In order to make hazard analysis calculations manageable, we should consider the fire spread to adjacent compartments.

Table 5.01 – Hazard analysis steps and outputs.

PhasePhase OutputOutput

Project efinition Building and occupant characteristics.

Fire development Heat release rates, temperatures and heat fluxes.

Fire detectionDetector activation time, sprinkler activation time, and times to fire cues.

Suppression effectiviness(sprinkler operation)

Modified heat release rates, modified temperatures and modified heat fluxes .

Fire department response Fire department response time.

Occupant responseProbability of occupants commencing evacuation. Time and probability of fire department notification.

Fire department effectiveness(operations)

Modified heat release rates, modified temperatures and modified heat fluxes.

Building element faiure Time to failure of building elements in compartment of fire origin.

Smoke movementHot gas layer temperatures and height, and concentrations of carbon monoxide (CO) and carbon dioxide (CO2).

Fire spreadTime of fire spread to adjacent compartments. Heat release rates, temperatures and heat fluxes in adjacent compartments.

Occupant evacuation Residual population with time.

Life hazard Probability of death with time.

Expected number of deaths Expected number of deaths.

Economics Expected property losses.

Downtime Expected interruption to operations.

PPA G EA G E 7878

Page 79: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Fire DevelopmentThe fire is estimated to reach flashover prediction at 221 seconds, and the maximum heat release rate of 20 MW at 330 seconds, when the fire runs out of fuel.

Detection and SuppressionThe description presented here is an example for guidance. The fire location can be assumed to be located in the middle of the space and centered between a set of sprinklers (if we assumed they exist). The sprinklers act as detectors and are assumed to have a response time index (RTI) of 80 m½∙s½, and an activation temperature of 85°C. The sprinklers are calculated to activate at 46 seconds. Although the sprinklers activate, they are assumed to have no effect on a fire of this magnitude, because of the nature of the fuel. There is a series of sprinklers located along the dome-shaped roof in the large space. Activation times are calculated by combining the time at which the smoke interface falls below the roller door and the time for the sprinkler to activate. This activation time is based on the heat release rate in the compartment of fire origin and an assumed fire area equivalent to the door width by 0.5 m. The height of the sprinklers is defined by the arc length along the dome, continuing down to the floor. The lowest sprinkler is considered to be located at 9.4 m along the wall, and the highest at 31.9 m along the wall. The lowest sprinkler in the hangar is predicted to activate at 108 seconds, and the highest is predicted to activate at 300 seconds.

Fire Department Response and EffectivenessThe fire department is notified by an automatic alarm that sounds with the activation of the sprinklers. The fire department is located 1 km away from fire location. The fire department is predicted to intervene within 10 minutes of being notified. The fire department response is being calculated the dispatch, preparation, and travel time to be 10 s, 46 s, and 109s, respectively. When these times are added to the notification time of 46 s, determined by the sprinkler activation, it is predicted that the fire department will respond at 211 seconds. The fire department effectiveness is not considered because the predicted intervention time (646 seconds) is longer than the times at which flashover and the peak heat release rate are reached (221 s and 330 s, respectively). In addition, the fire in the compartment of fire origin starts to decay after 330 seconds because all fuel is consumed.

Occupant ResponseAll occupants in the compartment of fire origin are predicted to take action in approximately 135 seconds, and all occupants in the remaining compartments are predicted to take action in approximately 260 seconds. The time the fire department would be notified by an occupant is compared with the time that the fire department is notified due to the activation of the sprinkler. Notification by an occupant is predicted to occur at 67 seconds. As it was calculated earlier that the fire department is automatically notified when the sprinkler system activates at 46 seconds.

Building Element FailureUsing the time and temperature data calculated by the fire development model, the 150 mm thick concrete walls of the building are predicted to fail after 155 minutes. The failure time is based on the absence of fire department action because the fire runs out of fuel and starts to decay after 330 seconds.

Smoke Movement AnalysisThe ventilation system is assumed closed which represents the worst-case scenario. Since kerosene (assume fuel type) is assumed as the fuel source, the smoke movement analysis predicts no carbon monoxide (CO) production. Predictions of carbon dioxide (CO2) concentrations in the building indicate a gradual increase, reaching 15% after 330 seconds. Carbon dioxide (CO2) concentrations in the large space and the main space reach much lower concentrations of 1% and 4%, respectively. The temperature increases rapidly for 320 s up to about 830°C. Temperatures in the large space, and the main space are much lower because of the larger room volumes. The hot gas layer in the building descends from 4.0 m (the ceiling), at ambient temperature, to 0.5 m above the floor at 320 seconds. The hot gas layer falls to a height of 1.5 m above the floor at about 130 s, which is assumed to represent untenable conditions for occupants in the building. The hot gas layers in the large space and the main space do not drop below a height of 1.5 m above the floor.

Occupant EvacuationThe occupant load for each compartment is selected based on the National Building Code of Canada, which defines the maximum number of persons per unit area. The total number of occupants in the modelled portion of the building is 169. Table 5.02 shows the distribution of this occupant load and the exit distance for each compartment. The occupant evacuation analysis assumes that all occupants can exit the building with the same travel speed, because the majority of

PPA G EA G E 7979

Page 80: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

occupants are trained military personnel.

Table 5.02 – Compartment characteristics.

CompartmentCompartment Number of OccupantsNumber of Occupants Exit Distance (m)Exit Distance (m)

Escape systems storage 2 15

Welding facility 9 11

Industrial facility 10 23

Main workshop 61 30

Large hangar 76 34

It is predicted that all of the occupants in the compartment of fire origin will evacuate in approximately 140 seconds. All the other occupants are predicted to evacuate the building within approximately 275 seconds.

Life Hazard AnalysisThe life hazard model uses the fire development and the smoke movement data to determine the probability of death. The cumulative probability of death for occupants in the compartment of fire origin reaches 100% at 168 seconds, and the predicted probability of death for all other occupants is 0%.

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESFeng, P., Hadjisophocleous, G. V., and Torvi, D. A., 2000. Equations and Theory of the Simple Correlation Model of FIERAsystem, IRC Internal Report No. 779, Institute for Research in Construction, National Research Council Canada, Ottawa, ON.ISO DIS 13344, Estimation of the Lethal Toxic Potency of Fire Effluents, International Organization for Standardization, ISO/TC 92/SC 3, Draft Edition, 2004.ISO TS 13571, Life-Threatening Components of Fire – Guidelines for the Estimation of Time Available for Escape Using Fire Data, International Organization for Standardization, First Edition, 2002.Mudan, K. S. and Croce, P. A., 1995. Fire Hazard Calculations for Large Open Hydrocarbon Pool Fires, SFPE Handbook of Fire Protection Engineering, Second Edition, National Fire Protection Association, Quincy, MA pp. 3-197–3-240.Purser, D. A., 1995. Toxicity Assessment of Combustion Products, SFPE Handbook of Fire Protection Engineering, Second Edition, National Fire Protection Association, Quincy, MA, pp. 2-85 - 2-146.Ron Coté, Editor, 2000. Life Safety Code Handbook, 8th edition, National Fire Protection Association, Quincy, MA, 2000.SFPE Handbook of Fire Protection Engineering, Second Edition, National Fire Protection Association, Quincy, MA, 1995. Torvi, D. A., Hadjisophocleous, G. V., and Hum, J., 2000. A New Method for Estimating the Effects of Thermal Radiation from Fires on Building Occupants, HTD-Vol. 366-5, Proceedings of the ASME Heat Transfer Division, Orlando, Florida, pp. 65-72, December.Torvi, D. A., Raboud, D. W., and Hadjisophocleous, G.V., 1999. FIERAsystem Theory Report: Life Hazard Model, IRC Internal Report No. 781, Institute for Research in Construction, National Research Council of Canada, Ottawa, ON.Watts, J. M. and Chapman, R. E., 2002. Engineering Economics, SFPE Handbook of Fire Protection Engineering, 3rd

edition, National Fire Protection Association, Quincy, MA, pp. 5-93 - 5-104.Yung, D., Hadjisophocleous, G. V., Proulx, G. and Kyle, B. R., 1996. Cost-Effective Fire-Safety Upgrade Options for a Canadian Government Office Building, Proceedings, International Conference on Performance-Based Codes and Design Methods, Ottawa, ON, pp. 269–280.

PPA G EA G E 8080

Page 81: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

Chapter 6Chapter 6Human Resistance Against Thermal Effects,Human Resistance Against Thermal Effects,

Explosion Effects and Obscuration of VisionExplosion Effects and Obscuration of Visionef

INTRODUCTIONINTRODUCTIONA study of environmental impact on humans was carried by DNV Technica and Scandpower on behalf of Statoil in September 1993. The main objective of the study was to carry out a state of the art study on environmental impacts on humans and provide a consistent set of criteria for use in fatality assessments in offshore and onshore risk analyses. Impact criteria for the following loads were established:(1) High air temperature.(2) Thermal incident fluxes.(3) Explosion loads.(4) Toxic gases.(5) Obscuration of vision.

It should be noted that the different effects should be seen in light of each other in order to identify the most critical one. However, fatal situations are often a result of a combination of the above mentioned parameters, together with panic among personnel. Probit functions, table values and charts which can be used to calculate the fatality rate for given loads and exposure times are presented in the following. It is important to use the results from the probit functions, table values and charts as guidance in the fatality assessment, rather than absolute values. In the fatality assessment load and exposure time are important parameters. Consequence calculations should form the basis for the assessment describing loads as a function of distance and exposure time taken into account shielding effects. Possibilities for personnel to escape from the accident venue, effect of protective measures as clothes and smoke masks are important aspects to address in the fatality assessment. In general offshore personnel will have less possibilities to escape from a large accident compared to onshore personnel. However, in general onshore personnel are lightly clothed compared to offshore personnel, making them more vulnerable to radiation in the immediate vicinity of the accident. In the following we should have present some definitions and terms used in this chapter.

Definitions and TermsLDxx, is the time related dose (heat radiation over time) which would be lethal to a given percent (xx) of the population. The thermal dose is defined by the following equation,

LD xx=I n⋅t [6.01]

where I is the incident flux (kW∙m−2), t is the exposure time (seconds) and n is a constant equal to four-thirds.LCxx is the time related dose (concentration over time) which would be lethal to a given percent (xx) of the population. Toxic dose is defined by the following equation,

LC xx=C n⋅t [6.02]

where C is the concentration in parts per million (ppm), t is the exposure time in minutes and n is a constant.Probit, is the range of susceptibility in a population to a harmful consequence can be expressed mathe matically using a criterion in the form of an equation which expresses the percentage of a defined population which will suffer a defined level of harm (normally death) when it is exposed to a specified dangerous load. This is a “Probit” equation which has the form,

P r=ab⋅ln I n⋅t [6.03]

where Pr is the probit (or the probability measure); a, b and n are constants. I is the radiation intensity given in kW∙m−2

and t is the exposure time in seconds.

PPA G EA G E 8181

Page 82: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

THERMAL EFFECTSTHERMAL EFFECTSThe main effects of high air temperature or incident heat fluxes is of physiological and pathological art. The impact criteria contained in this text relate to impact from short and long duration of high air temperature which may cause heat stress resulting in fatal outcome and of thermal radiation which may cause first, second, third degree burns or fatal outcome. Inside living quarters, control rooms or other compartments where personnel should be safe in a fire situation, the air temperature may become too high leading to physiological effects on humans such as difficulties with breath resulting in incapacitation, high pulse or core temperature leading to collapse. In most cases the air temperature inside the enclosures will not be sufficiently high for that pathological effects such as skin burns to be dominant. However, during escape or at the evacuation stations personnel may be directly exposed to the fire and ther mal radiation may be more critical than the air temperature and pathological effects will be dominant. Type of fire, the distance from the fire and the timeof exposure are very important parameters in the assessment of fatalities. On an offshore platform it is believed that personnel will be exposed to a fire for a longer time due to short distances and more time is needed to evacuate the platform than on an onshore installation. However, in general offshore personnel are more protectively clothed than onshore personnel, making them more resistant against thermal radiation. The majorities of the data are given for lightly clothed personnel which is representative for onshore personnel. However, some data are also presented for well clothed personnel which is representative for offshore situations.

PP HY S IO L O G IC A LHY S IO L O G IC A L E E F F EC T SF F EC T S

Most physiological effects of thermal radiation onto man involve voluntary exposures which are relatively lengthy, i.e. at least several minutes. However, inside living quarters, control rooms or other types of compartments exposed to fire where personnel may stay for a period of time, they will be exposed to low thermal radiation levels and instead high air temperature may become the most critical parameter. Personal trapped inside a helicopter due to a fire following a helicopter crash may be on example of a fire where high temperature and not heat radiation becomes critical. Table 6.01 indicates some physiological effects of elevated temperature levels on the human individual based on full-scale fire tests.

Table 6.01 – Elevated temperature response on human individuals.

Temperature (Temperature (°°C)C) Physiological ResponsePhysiological Response

127 Difficult breathing.

140 5-min tolerance limit.

149 Mouth breathing difficult, temperature limit for escape.

160 Rapid, unbearable pain with dry skin.

182 Irreversible injury in 30 seconds.

203 Respiratory systemes tolerance time less than four minutes with wet skin.

Elevated temperatures have influence on the pulse rate. The pulse rate climbs steadily with time and air temperature. The pulse jumps from normal 84 to 120 beats a minute when the air temperature increases to 100°C. It further increases to 150 beats per minute after 10 minutes at an air temperature of 113°C. In general the maximum air temperature that can be tolerated by the human respiratory tract is approximately 203°C. Above air temperatures of 150°C, the impact is dominated by pain from skin burns, which occur in less than 5 minutes. Between air temperatures of 70°C and 150°C, the impact is dominated by difficulties to breath. It is believed that below 70°C the situation inside a compartment will not be fatal, but may of course lead to an uncomfortable situation for personnel. No probit function has been developed on this matter, hence special assessment must be made to calculate the fatality rate among trapped personnel inside compartments if the temperature in side rises to between 70°C and 150°C. The average time to incapacitation (tinc) has been proposed as follows for temperatures between 70°C and 150°C.

t inc=5.33⋅108⋅ 1T 3.66 [6.04]

where tinc is the exposure time (minutes) until incapacitation, and T is the temperature (°C). With temperatures of 70°C and 150°C inside a compartment, time to incapacitation may be 94 minutes and 6 minutes respectively based on the above presented equation and curve.

PPA G EA G E 8282

Page 83: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

PPAT HO L O G IC A LAT HO L O G IC A L E E F F E C TSF F E C TS

Pathological effects on humans are relevant to address in the immediate vicinity of the accident, on unshielded escape ways and evacuation stations and inside enclosures if radiation becomes a dominant factor (above 150°C). Pathological effects covered in this text are:(1) Pain.(2) First degree burns.(3) Second degree burns.(4) Third degree burns.(5) Fatal burns.

Thermal doses required to reach second degree burns and third degree burns are approximately the same doses as 1% fatality and 50% fatality respectively to averagely dressed exposers. The severity of an injury from heat is determined by the depth of skin to which a temperature difference of 9K has occurred. The following burn types are reached for different depths of skin:(1) First degree burns (< 0.12 mm).(2) Secon d degree burns (< 2 mm).(3) Third degree burns (> 2 mm).

Thickness of skin varies from more than 5 mm on the back to only 0.5 mm on the eyelids, but on average is between 1mm and 2 mm. In the assessment of fatality rates on an offshore or onshore installation it is important to take into account the following factors:(1) Information prior to fire (alarms).(2) Development of accidents.(3) Personnel reaction time.(4) Emergency procedures.(5) Escape time.(6) Shielding effects.(7) Radiation levels as a function of time.(8) Total exposure time.(9) Other critical aspects like visibility, toxic gases, explosion loads etc.

In Table 6.02 ranges of thermal doses required to give pain and burns are given. For a given radiation level or a given exposure time, time or necessary radiation level to pain, first, second or third degree can be calculated by use of the thermal doses presented in Table 6.02 and Equation [6.01].

Table 6.02 – Ranges of thermal doses required to give pain, burns and fatal outcome.

EffectEffect Thermal DoseThermal Dose[(kW[(kW∙m∙m−−22))4/34/3∙∙s]s]

Pain 85 – 129

Significant injury level 600 – 800

First degree burns250 – 350210 – 700

Second degree burns(1% lethality level for average clothing)

900 – 1,300500 – 3,000

Third degree burns(50% lethality for average clothing)

2,000 – 3,000> 3,000

The fatality rate when personnel is exposed to thermal radiation over a given period of time can be calculated by use of probit functions. Several probit functions have been developed based on experiments carried out on animals and humans. The most known probit functions are the Eisenberg function for naked skin and the TNO function for naked skin. The Eisenberg probit function is based on experiments carried out at nuclear explosions. The TNO model is based on the Eisenberg probit function adjusted for experiments carried out at hydrocarbon fires. Compared to the probit

PPA G EA G E 8383

Page 84: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

function from Eisenberg, the TNO model for naked human skin comes up with higher fatality rate. The thermal dose required for a given lethality level is in general lower for hydrocarbon fires than for nuclear explosions, because radiation from hydrocarbon fires is long waved penetrating deeper into the skin compared to the radiation from nuclear explosions which is short waved. It is believed that the TNO model is more suitable for use in the estimation of fatality levels than the Eisenberg model in typical offshore and onshore risk analyses where personnel are directly exposed to the fire, because the TNO model is based on hydrocarbon fires. However, the calculated fatality rates should be used as guidance in the fatality assessment more than as absolute values. The TNO model for naked human skin is as follows,

P r=−12.82.56⋅I43⋅t [6.05]

The calculated fatality rates for different thermal incident fluxes and exposure times by use of the TNO probit function presented above are shown in Table 6.03.

Table 6.03 – Fatality rate as a function of radiationlevel and exposure time for naked human skin.

Exposure TimeExposure Time(s)(s)

Fatality Rate (%)Fatality Rate (%)

10 kW10 kW∙m∙m−2−2 20 kW20 kW∙m∙m−2−2 30 kW30 kW∙m∙m−2−2

10 0 5 39

20 1 53 93

30 11 87 100

40 31 97 100

50 53 99 100

60 71 100 100

If the probit function is not directly used in the fatality assessment, it is recommended to use the following radiation levels for lightly clothed personnel as 100% fatality limit in the below given exposure time intervals:(1) 16 kW∙m−2 – Exposure time less than 0.5 minute.(2) 10 kW∙m−2 – Exposure time from 0.5 minute to 1 minute.(3) 4 kW∙m−2 – Exposure time from 1 minute to 2 minutes.(4) 2 kW∙m−2 – Exposure time from 2 minutes to 10 minutes.

The critical radiation levels are based on the TNO probit function assuming that the 50% fatality limit represents the lethal dose for an average person and that incapacitation occurs close to the lethal dose, i.e. 75% of the LD50 is set as the incapacitation dose here. This corresponds to 81% of the lethal incident radiation flux. For clothed personnel the Neisser curve, is recommended to use assuming that the 50% fatality limit represents the lethal dose for an average person and that incapacitation occurs close to the lethal dose, i.e. 75% of the LD50 is set as the incapacitation dose. This corresponds to 81% of the lethal incident radiation flux. It is recommended to use the following radiation levels for clothed personnel as 100% fatality limit in the below given exposure time intervals:(1) 25 kW∙m−2 – Exposure time less than 0.5 minute.(2) 13 kW∙m−2 – Exposure time from 0.5 to 1 minute.(3) 8 kW∙m−2 – Exposure time from 1 minute to 2 minutes.(4) 4 kW∙m−2 – Exposure time from 2 minutes to 10 minutes.

The approach assumes a constant heat load over the exposure period. In reality, most fires will initially expand and then decay with time, and thus the radiation received at any given point will also be a function of time. A full integration of the dose received may be performed if greater detail is required.

PPA G EA G E 8484

Page 85: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

EFFECTS OF EXPLOSIONSEFFECTS OF EXPLOSIONSPeople can survive fairly strong blast waves and in accidental explosions there are very few cases in which the blast effect has killed people directly. Typical in juries following an explosion are caused by:(1) Burn.(2) Hitting fragments.(3) Buildings or other structures falling down or being disintegrated.(4) Persons falling or "flying" and subsequently hitting a solid object (i.e. whole body displacement).

Important parameters for determining the effects and the risk from an explosion are:(1) Maximum overpressure.(2) Time to reach the maximum overpressure.(3) Indoor or outdoor exposure of people.(4) Possibility at flying fragments.(5) Designed pressure sustainability of building.

In a risk analysis the most important effects are:(1) Flying fragments hitting personal.(2) To hole body displacement resulting in impact damage.(3) Damage due to impact coused by collapsed structures.

Overpressure EffectsIf the long axis of body is parallel to blast winds and the subject is facing any direction the acceptable overpressure will increase. If the thorax is near a reflecting surface that is perpendicular to the blast winds the acceptable overpressure will decrease.

Fragments EffectsFlying fragments from an explosion are more dangerous than the bare overpressure. Fragments may be debris from demolished buildings caused by the explosion or loose equipment in the building. Fragments from glass breakage is a very common type of serious and extreme dangerous type of fragments, possibility for glass fragments must be determined during an analysis of explosion effects. The pressure needed for breakage of conventional glass is:(1) 1% level glass breakage (Ppeak = 1.7 kPa).(2) 90% level glass breakage (Ppeak = 6.2 kPa).

Table 6.04 shows the expected effects of flying missiles from an explosion.

Table 6.04 – Injuries from missiles from an explosion.

InjuryInjury Peak OverpressurePeak Overpressure(kPa(kPa))

Impact VelocityImpact Velocity(m(m∙s∙s−1−1))

ImpulseImpulse(N(N∙s∙m∙s∙m−2−2))

Skin laceration threshold 7 – 15 15 512

Serious wound threshold 15 – 20 30 1,024

Serious wounds near (50% probability)

25 – 35 55 1,877

Serious wounds near (100% probability)

50 – 55 90 3,071

Hole Body DisplacementExplosion effects also involve whole-body displacements and subsequent impact. During the whole-body displacement, blast overpressure and impulses interact with the body in such a manner that it is essentially picked up and translated. The head is the most vulnerable part of the body for injuries from whole-body displacement. The whole-body displacement (accelerations) is a function of the size, shape and mass of the person and the blast forces. At least 50% of the people being picked up and translated with a speed more than 0.6 m∙s−1 will suffer minor injuries. One percent of those with a speed of about 4 m∙s−1 will suffer in juries like ruptured organs and bone fractures. If thrown against a solid

PPA G EA G E 8585

Page 86: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

wall about 40% will suffer major injuries. Table 6.05 shows the expected effects from hole body displacement.

Table 6.05 – Criteria for tertial damage involving totalbody impact.

Total Body Impact ToleranceTotal Body Impact Tolerance Related Impact VelocityRelated Impact Velocity(m(m∙s∙s−1−1))

Most "Safe" 3.05

Lethality threshold 6.40

Lethality (50%) 16.46

Lethality near (100%) 42.06

TOXIC EFFECTSTOXIC EFFECTSEffect of toxic gases can be divided into two categories:(1) Local irritant which may cause incapacitation mainly by effects on the eyes and the upper respiratory tract which

may impair escape capability and sometimes cause delayed death due to lung damage.(2) Systematically acting agents which cause damage to the body via the blood and distribution in the body, so called

narcotic gases.

The main toxic gases of fire effluents are carbon monoxide (CO), carbon dioxide(CO2), hydrogen sulphide (H2S), nitrogen oxides (NOx), ammonia (NH3), sulphur dioxide (SO2) and hydrogen fluoride (HF). Carbon monoxide (CO) and carbon dioxide(CO2) are classified as narcotic gases, while the other are classified as irritants. Although carbon monoxide (CO) is not the most toxic of the above mentioned gases, it is present in relatively high concentrations in smoke, and so its effects are usually dominant. There is a lot of uncertainties in the calculation of amount of smoke produced in a fire situation and amount of toxic gases in the smoke. This depends on type of burning fuel and ventilation conditions. The proportion of toxic gases in smoke depends on the chemical structure of the burning materials and the degree of ventilation to the fire. The differences between different hydrocarbons are quite small, and ventilation has the main effect. Fires in which the ventilation is restricted occurs only for fires in modules or compartments. These fires will either be fuel controlled or ventilation controlled. In general, reduced ventilation greatly increases the ratio of carbon monoxide (CO), while the oxygen (O2) and carbon dioxide(CO2) remain more or less unaffected. Typical gas concentrations close to the fire are given in Table 6.06.

Table 6.06 – Initial gas concentration in smoke.

GasGas

Concentration in Smoke (%)Concentration in Smoke (%)

Well Ventillated FireWell Ventillated Fire Under Ventillated FireUnder Ventillated Fire

Gas FireGas Fire Liquid FireLiquid Fire Gas FireGas Fire Liquid FireLiquid Fire

Carbon monoxide (CO) 0.04 0.08 3.0 3.1

Carbon dioxide (CO2) 10.9 11.8 8.2 9.2

Oxygen (O2) 0.0 0.0 0.0 0.0

On an onshore installation the possibilities to escape from the accident are greater than on an offshore installation. Based on this offshore personnel will be exposed to toxic gases over a longer time period leading to in general lower acceptable concentrations than on an onshore installation. The consequences of inhalation of toxic chemicals can only be derived from animal experiments. The uncertainties in translating animal data to data relevant for humans are large and therefore “safety factors” are included in the modelling. In general animals have a higher adsorption rate and humans have a higher respiratory rate in accident situations.

PPA G EA G E 8686

Page 87: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

EE F F E C TSF F E C TS O FO F C CA R B O NA R B O N M M O N OX I D EO N OX I D E

Extensive investigations examining human fire fatalities have shown carbon monoxide to be the primary toxicant in many deaths due to smoke inhalation. The toxicity of carbon monoxide is due to the formation of blood carboxyhemoglobin, which results in a reduced ability of the blood to transport oxygen to critical body organs referred to as anaemic anoxia. There exist further evidence that relatively low levels of carboxyhemoglobin saturation may have adverse effects on reaction time which is important to escape from a fire. The toxicity of carbon monoxide may be modified by heat stresses. Experiments on test animals under heat stress showed that blood carboxhemoglobin concentrations at the time of death were much lower than in animals not stressed by heat. The following physiological effects on human individuals from carbon monoxide is given below:(1) 1,500 ppm – Headache after 15 minutes, collapse after 30 minutes, death after 1 hour.(2) 2,000 ppm – Headache after 10 minutes, collapse after 20 minutes, death after 45 minutes.(3) 3,000 ppm – Maximum “safe” exposure for 5 minutes, danger of collapse in 10 minutes.(4) 6,000 ppm – Headache and dizziness in 1 minute to 2 minutes, danger of death in 10 to 15 minutes.(5) 12,800 ppm – Immediate effect, unconscious after 2 to 3 breaths, danger of death in 1 to 3 minutes.

The above presented effects of carbon monoxide (CO) indicates that with several thousand parts per million (ppm) of carbon monoxide (CO) in the atmosphere will cause very critical situations on an offshore installation or an onshore installation. Several probit functions have been developed based on experiments data from animals. However, the following probit function is recommended to use in the fatality assessment,

P r=−37.983.7⋅ln C CO⋅t [6.06]

where CCO is the carbon monoxide (CO) concentration, and t is time exposure elapsed (s). In Table 6.07 the lethality levels for different carbon monoxide (CO) concentrations and exposure times by use of the probit equation (Equation [6.06]) are presented. In this table also the necessary carbon monoxide (CO) concentrations and exposure time for a 50% lethality level are presented.

Table 6.07 – Lethality level for different carbon monoxide concentrations and exposure times.

Fatality Rate (%) AfterFatality Rate (%) After10 Minutes Exposure10 Minutes Exposure

Lethality LevelLethality Level(50% deaths)(50% deaths)

2,000 ppm2,000 ppm 6,000 ppm6,000 ppm 10,000 ppm10,000 ppm ConcentrationConcentration(ppm)(ppm)

Exposure TimeExposure Time(min)(min)

0.0 1.5 35.0

2,000 54

4,000 27

6,000 18

8,000 13

10,000 11

Based on a 50% lethality level it can be concluded that the probit function is more or less consistent with the previous presented threshold limits.

EE F F E C TSF F E C TS O FO F C CA R B O NA R B O N D D I OX I D EI OX I D E

While carbon dioxide is not particular toxic at levels normally observed in fires, moderate concentrations do stimulate the rate of breathing. This condition may contribute to the overall hazard of a fire gas environment by causing accelerated uptake of toxicants and irritants. The rate and depth of breathing are increased 50% by 20,000 ppm of carbon dioxide (CO2) and doubled by 30,000 ppm carbon dioxide in air. At 50,000 ppm, breathing becomes laboured and difficult for some individuals, although this concentration of carbon dioxide has been inhaled for up to one hour without serious after effects. Table 6.08 illustrates carbon dioxide responses. No probit functions have been found in the literature describing the lethality level of different carbon dioxide (CO2) concentrations and exposure time. Based on this the following 100% fatal limits of carbon dioxide (CO2) are recommended to use for different exposure times:(1) 150,000 ppm of CO2 – Exposure time less than 5 minutes.

PPA G EA G E 8787

Page 88: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

(2) 120,000 ppm of CO2 – Exposure time between 5 minutes and 30 minutes.(3) 100,000 ppm of CO2 – Exposure time greater than 30 minutes.

Table 6.08 – Carbon dioxide responses.

Concentration ofConcentration ofCarbon Dioxide (ppm)Carbon Dioxide (ppm) ResponseResponse

100,000 Approaches threshold of unconsciousness in 30 minutes.

120,000 Threshold of unconsciousness reached in 5 minutes.

150,000 Exposure limit 1 minutes.

200,000 Unconsciousness occurs in less than 1 minute.

EE F F E C TSF F E C TS O FO F O OX YG E NX YG E N D D EP L E T IO NEP L E T IO N

Oxygen constitutes 21% by volume of clean air. Decreases in oxygen concentration down to about 15% are counteracted by the body increasing the flow of blood to the brain, and only minor effects on motor coordination are apparent. Oxygen concentrations below 15% by volume produce oxygen starvation effects such as increased breathing, faulty judgement and rapid onset of fatigue. Oxygen concentrations below 10% cause rapid loss of judgement and comprehension followed by loss of consciousness, leading to death within a few minutes. This is taken to be the limiting oxygen concentration for escape lasting a few seconds. If escape is not possible within few seconds, incapacitation and death is assumed to occur. Oxygen concentrations of 10% and 15% require a clean air content in the mixing gas of 47% and 71% respectively. These would be achieved when the gas is diluted to 52% and 29% respectively of its concentration. A gas concentration of 52% would cause death unless escape is possible in a few seconds. Table 6.09 indicates the responses of human individuals to different reduced levels of oxygen in air.

Table 6.09 – Human responses due to reduced levels of oxygen in air.

Concentration ofConcentration ofOxygen in Air (%)Oxygen in Air (%) ResponseResponse

11 Headache, dizziness, early fatigue, tolerance time 30 minutes.

9Shortness of breath, quickened pulse, slight cyanosis, nausea, tolerance time 5 minutes.

7Above symptoms becomes serious, stupor sets in, unconsciousness occurs tolerance time 3 minutes.

6 Heart contractions stop 6 to 8 minutes after respiration stops.

3 – 2 Death occurs within 45 seconds.

No probit functions are found in the literature describing the lethality level for personnel when exposed to different concentrations of oxygen in the air and exposure time. Based on this the following fatal limits of oxygen (O2) depletion are recommended to use for different exposure times:(1) 10% of O2 – Exposure time less than 5 minutes.(2) 15% of O2 – Exposure time greater than 5 minutes.

OOV E RA L LV E RA L L S S MO KEMO KE E E F F E C TSF F E C TS

The combined effects of carbon monoxide (CO), carbon dioxide(CO2) and oxygen depletion are the main causes of fatalities in smoke. The criteria for them are compared in Table 6.10. For the under ventilated fires, carbon monoxide (CO) has the main effect, which depends strongly on exposure time. For well-ventilated fires, carbon monoxide (CO) production is much reduced and oxygen depletion appear to have main effect. Based on this the following concentrations of smoke may cause very critical situations (nearly 100% fatality rate) among exposed personnel after few seconds:(1) 52% of smoke in well-ventilated gas fuelled fires.

PPA G EA G E 8888

Page 89: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

(2) 48% of smoke in well-ventilated liquid fuelled fires.(3) 19% in under-ventilated gas fuelled fires.(4) 18% in under-ventilated liquid fuelled fires.

Table 6.10 – Smoke concentration to prevent escape in few minutes.

GasGas

Smoke Concentration to Prevent Escape (%)Smoke Concentration to Prevent Escape (%)

Well Ventillated FireWell Ventillated Fire Under Ventillated FireUnder Ventillated Fire

Gas FireGas Fire Liquid FireLiquid Fire Gas FireGas Fire Liquid FireLiquid Fire

Carbon monoxide (CO) − − 33 32

Carbon dioxide (CO2) 92 85 − −

Oxygen (O2) 56 56 56 56

Combined effects 52 48 19 18

The combined effects of carbon monoxide (CO), carbon dioxide(CO2) and oxygen depletion are a difficult task and the above pre sented values should be used as guidance only to identify the problem.

EE F F EC T SF F EC T S O FO F O O TH E RTH E R G GA S E SA S E S

Table 6.11 illustrates the effects likely to be experienced by humans exposed to various concentrations of hydrogen sulphide (H2S).

Table 6.11 – Effects on people exposed to hydrogen sulphide.

Concentration ofConcentration ofHydrogen Sulphide (ppm)Hydrogen Sulphide (ppm) EffectEffect

20 – 30 Conjunctivitis.

50 Objection to light after 4 hours exposure. Lacrimation.

150 – 200 Objection to light, irritation of mucous membranes, headache.

200 – 400 Slight symptoms of poisoning after several hours.

250 – 600 Pulmonary edema and bronchial pneumonia after prolonged exposure.

500 – 1,000 Painful eye irritation, vomiting.

1,000 Immediate acute poisoning.

1,000 – 2,000 Lethal after 30 to 60 minutes.

> 2,000 Acute lethal poisoning.

Several probit functions have been developed based on experiments data from animals. However, the following probit function is recommended to use in the fatality assessment,

P r=−31.423.008⋅ln C1.43⋅t [6.07]

where C is the hydrogen sulphide concentration, and t is time of exposure. The probit function is to some degree more conservative than the values presented in Table 6.11. The toxicological effects of nitrogen oxides (NOx), ammonia (NH3), sulphur dioxide (SO2) and hydrogen fluoride (HF) are given in Table 6.12. In Table 6.13 predicted lethal concentrations for humans and published values are given.

PPA G EA G E 8989

Page 90: Fire Explosion Hazards

FF I R EI R E A N DA N D E E X P L O S I O NX P L O S I O N H H A Z A R D SA Z A R D S A A N A L Y S I SN A L Y S I S

Table 6.12 – Toxicological effects of nitrogen oxides, ammonia, sulphur dioxide and hydrogen fluoride.

ToxicantToxicant Toxicological EffectsToxicological Effects

Nitrogen oxides Strong pulmonary irritant capable of causing immediate death as well as delayed injury.

Ammonia Pungent, unbearable odour; irritant to eyes and nose.

Sulphur dioxide A strong irritant, intolerable well below lethal concentrations.

Hydrogen fluoride Respiratory irritants.

Table 6.13 – Carbon dioxide responses.

ToxicantToxicantHuman LCHuman LC5050 (ppm) (ppm)

predicted from metabolic ratepredicted from metabolic rate Human LethalHuman LethalConcentrations (ppm)Concentrations (ppm)

5 minutes5 minutes 30 minutes30 minutes

Ammonia 55,000 − 2,000

Sulphur dioxide 17,000 8,000 600 – 800

Hydrogen fluoride 44,000 4,600 −

Nitrogen oxides 410 180 250

Several probit functions have been developed for ammonia (NH3), sulphur dioxide (SO2) and hydrogen fluoride (HF). Below probit function for each of these gases are presented to use in the fatality assessment. The probit function for ammonia (NH3) is as given,

P r=−9.820.71⋅ln C2⋅t [6.08]

where C is the ammonia concentration (ppm) and t is the exposure time (minutes); LC50 = 15,240 ppm for 5 minutes exposure. The probit function for sulphur dioxide (SO2) is the following,

P r=−16.672.10⋅ln C⋅t [6.09]

where C is the sulphur dioxide concentration (ppm) and t is the exposure time (minutes); LC50 = 3,765 ppm for 5 minutes exposure. The probit function for hydrogen fluoride (HF) is given by,

P r=−48.334.853⋅ln C⋅t [6.10]

where C is the hydrogen fluoride concentration (ppm) and t is the exposure time (minutes); LC50 = 11,845 ppm for 5 minutes exposure. No probit model is found in the literature for nitrogen oxides (NOx).

OBSCURATION OF VISIONOBSCURATION OF VISIONThe absence of vision may delay or prevent escape from fires and cause people to be exposed to the fire gases for an unacceptable long period of time. While the exposure to high concentrations of toxic and hot gases usually will be significant only in the vicinity of the fire, the effect of reduced visibility may also be significant far away from the fire source. For example, in multi-compartment buildings, the smoke blocking effect may be significant in rooms far away from the room of fire origin. Moreover, the smoke blocking effect is reported to be the first condition becoming critical of the three hazardous conditions of fires (i.e. heat stresses, obscuration of vision, toxic effects). The hazard of smoke is characterized by three factors. The first threat is reduced visibility due to soot. The second is that hot smoke can cause pain and injuries, and the third is that a concentration of toxic and irritating components can lead to incapacitation or death. The relative order of these factors can be found by comparison of threshold values with actual exposure in a fire scenario. A visibility of 4 meters to 5 meters is about the threshold of diminished performance, and this is the smoke

PPA G EA G E 9090

Page 91: Fire Explosion Hazards

SS A F E T YA F E T Y E E N G I N E E R I N GN G I N E E R I N G H H A N D B O O KA N D B O O K S S E R I E SE R I E S

level that one should have in mind when designing smoke ventilation systems. A visibility of less than one arm length will be of no help at all when escaping from a fire environment. Important factors to consider in a risk analysis with regard to obscuration of vision (and time to escape) are:(1) Exposure to smoke.(2) Arrangement of escapeways (layout, sign, illumination, railing, etc.).(3) Taining of personnel.(4) Familiarization with the installation.

BIBLIOGRAPHY AND REFERENCESBIBLIOGRAPHY AND REFERENCESAmerican National Standard for Respiratory Protection, ANSI Z88.2-1992. American National Standards Institute, New York, 1992.Department of Housing and Urban Development. Safety Considerations in Siting Housing Projects, 1975. HUD Report 0050137.Finney, D. J., Probit Analysis . Cambridge University Press, 1977.Fire Protection Handbook. National Fire Protection Association, Quincy, Massachusetts, 18th edition, 1997.G. Opschoor, R. O. M. Van Loo, and H. J. Pasman. Methods for Calculation of Damage Resulting from Physical Effects of the Accidental Release of Dangerous Materials, in International Conference on Hazard Identification and Risk Analysis, Human Factors and Human Reliability in process safety, pages 21–32. Center for Chemical Process Safety, AIChE, 1992.Fire Protection Handbook, Ed., McKinnon GP, 14th ed., 1976, NFPA, Boston, Mass.McCracken, D. J., 1970. Hydrocarbon Combustion and Physical Properties, Ballistic Research Laboratory Report No. 1496, Aberdeen Proving Ground, Aberdeen, MD.National Fire Protection Association (NFPA). Industrial Fire Hazards Handbook, 3rd Edition. NFPA, Inc., Quincy, MA, 1990.NFPA 325, Fire Hazard Properties, Flammable Liquids, Gases and Volatile Solids, National Fire Protection Association. Assn. Boston, 1994, pp. 4-5.

PPA G EA G E 9191