Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2013

Health and Safety Executive

Technical input on ventilation effectiveness for area classification guidance EI15

RR993Research Report

MJ Ivings and A KelseyHealth and Safety LaboratoryHarpur HillBuxtonDerbyshire SK17 9JN

The Energy Institute’s guidance on hazardous area classification “Area classification code for installations handling flammable fluids”, commonly known as EI15 (and previously IP15), is used extensively in the UK and elsewhere by the petroleum industry and others, and is currently under revision. Ventilation is a key factor in area classification and it was recognised that the current Edition 3 of EI15 required improvement in this area in particular. The current approach, based on air changes per hour, is shown to be inappropriate since it is not related to the size of the release it is trying to dilute. The main aim of the work reported here is to provide an improved methodology for assessing the adequacy of the ventilation such that an enclosure containing secondary grade release sources can be properly classified as Zone 2 as defined by BS EN 60079-10-1:2009. It is shown that limiting the average concentration to 25% LFL in the enclosure generally achieves the ventilation objective for a Zone 2 area but the degree of confinement and congestion at the point of release must be assessed. Guidance on this assessment is provided including the use of a factor to quantify the efficiency of mixing.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

Technical input on ventilation effectiveness for area classification guidance EI15

HSE Books

Health and Safety Executive

© Crown copyright 2014

First published 2014

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Acknowledgements

I would like to thank Roger Santon for his input to this project which has been invaluable in ensuring that the outcome is fit for the intended purpose.

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CONTENTS

1 INTRODUCTION ......................................................................................... 1

2 VENTILATION ASSESSMENT ................................................................... 3 2.1 Introduction .............................................................................................. 3 2.2 Natural ventilation .................................................................................... 4 2.3 Summary ............................................................................................... 11

3 DEFINING ADEQUATE VENTILATION ................................................... 13 3.1 Introduction ............................................................................................ 13 3.2 Existing approaches .............................................................................. 13 3.3 The average gas concentration at the outlet .......................................... 17 3.4 Summary ............................................................................................... 21

4 VENTILATION CORRECTION FACTOR ................................................. 23 4.1 Introduction ............................................................................................ 23 4.2 The effects of confinement / congestion on gas dispersion ................... 23 4.3 Analysis of JIP data ............................................................................... 28 4.4 Summary ............................................................................................... 35

5 EXAMPLE CALCULATIONS .................................................................... 37 5.1 Natural ventilation rate calculations ....................................................... 37 5.2 Assessment of adequate ventilation ...................................................... 44

6 REFERENCES .......................................................................................... 49

ANNEX A EI15 FLUID CATEGORIES ............................................................. 50

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EXECUTIVE SUMMARY

Background

The Energy Institute’s guidance on hazardous area classification “Area classification code for installations handling flammable fluids”, commonly known as EI15 (and previously IP15), is used extensively in the UK and elsewhere by the petroleum industry. Following recent developments in the field of area classification, including the coming into force of the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR, 2002) and the joint industry project on area classification of flammable gases carried out by HSL (Ivings et al., 2008), the Energy Institute wish to update EI15.

The main aim of the work reported here is to provide a methodology for assessing the adequacy of the ventilation such that an enclosure containing secondary grade release sources can be properly classified as Zone 2 as defined by BS EN 60079-10-1:2009. This work builds on a previous project carried out by HSL for the Energy Institute which provided look-up tables of ventilation rates, for a range of different flammable releases, which could be used as the basis for defining adequate ventilation. The scope of this work is limited to the classification of Zone 2 areas and the categories of releases published in EI15.

Objectives

This project has three main objectives:

• To develop and describe practical approaches for determining natural ventilation rates of enclosures and buildings.

• To develop an approach for defining adequate ventilation in an enclosure such that an enclosure containing secondary grade release sources can be properly classified as Zone 2.

• To develop a methodology for taking account of congestion / confinement around the leak location suitable for use in area classification.

Main Findings

Assessing natural ventilation rates

A range of approaches of varying degrees of complexity are available for estimating the natural ventilation rates of enclosures.

For very simple enclosures with a small number of well-defined openings, simple analytical expressions can be derived that allow the wind-induced and buoyancy-induced ventilation rates to be calculated. As a first approximation, the larger of the two rates could then be used in an area classification assessment. Such an approach could easily be written into an area classification standard.

For most practical applications, a better approach is to use simple numerical techniques that solve a set of equations describing the flow rate through the openings in the enclosure. Two good examples of such an approach are included in the software programme Quadvent available from HSL (www.hsl.gov.uk) and the spreadsheet developed for ventilation design available from the Chartered Institute of Building Services Engineers, CIBSE (www.cibse.org/venttools).

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This iterative approach allows much greater flexibility in defining the number, type and location of the openings and automatically accounts for the combination of wind and buoyancy induced ventilation.

For complex spaces, experimental measurements may be necessary to determine the ventilation rate, but even this needs to be done carefully taking into account many factors including the weather conditions during the tests.

Clearly the amount of effort that is put into determining the ventilation rate should be in proportion to the degree of accuracy required for the ventilation rate estimate.

Defining adequate ventilation

Three key standards / guidance documents on area classification provide three different metrics and approaches to defining adequate ventilation and hence the conditions required for defining a space as Zone 2. The current edition of the international standard on area classification, BS EN 60079-10-1:2009, uses a hypothetical gas cloud volume, Vz, to differentiate between different levels of ventilation. The Institute of Gas Engineers and Managers guide IGEM/SR/25 uses the average gas concentration within the enclosure during a release. And the current version of EI15 uses a definition of adequate ventilation based on a requirement of 12 air changes per hour within the enclosure.

The main disadvantage of the approach currently used in EI15 is that the requirement is not related to the size of the release and therefore the same level of ventilation is required for a room containing potentially large releases as one that may contain very small releases. Also, a definition of adequate ventilation in terms of an air change rate will require huge volumes of air to be moved to achieve the criterion in large enclosures. However, in practice, the dispersion of releases in large enclosures will behave in a similar manner to a release outdoors and therefore the air change rate is not critical to the gas dispersion.

The concept of Zone 2NE is used in BS EN 60079-10-1:2009 and IGEM/SR/25 for cases where the ignition of a secondary release has a negligible effect (NE) and therefore no special protective measures are required. Quantitative criteria have been developed to define when this negligible effect zone can be used. The criteria are based on a requirement that the gas cloud volume, Vz, is less than 0.1 m3 and it has been shown that limiting the average gas concentration at the ventilation outlet to 10% of the lower flammability limit (LFL) together with other limitations generally leads to achieving the 0.1 m3 criterion.

In an area classified as Zone 2 specific measures are taken to prevent the ignition of the flammable gases/vapours. Therefore the aim of the ventilation in this case is not to dilute the released volume down to a specified ‘safe’ level as it is for Zone 2NE. Rather it is to limit the size of the flammable region to ensure that it stays within the zoned area (which may be the whole enclosure) and dilute short-duration releases once they have stopped, so that concentrations rapidly fall well below LFL. It therefore is not appropriate to define the adequacy of the ventilation based on some criterion for a ‘safe’ gas cloud volume, as it is for Zone 2NE, as it is accepted that from time to time there will be accumulations of flammable mixtures within the zone.

However, area classification is not designed to account for catastrophic failure, so some limit needs to be put on the size of release in an enclosure that area classification can be applied to. Such a limit is already provided in EI15 for outdoor releases, and is based on releases that lead to a hazard radius greater than 30 m. However, no such limit is currently applied for releases indoors.

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It is shown in the present report that limiting the average concentration to 25% LFL in the enclosure generally achieves the objective of the ventilation for a Zone 2 area as defined above. This criterion is also consistent with IGEM/SR/25. However, this only demonstrates that the overall level of ventilation is sufficient and it does not demonstrate that the local ventilation is adequate. It is the local ventilation effectiveness that will determine how the release disperses. It is therefore important that the 25% LFL criterion should be a minimum requirement as other factors need to be taken into account. If such a criterion is only just met then this is a good indication that further effort may be required to ensure that the ventilation local to the release point is indeed adequate.

While use of the 25%LFL criterion does achieve the above objectives, large gas clouds may still occur, particularly in large enclosures. It is to be noted that there is a requirement under DSEAR Reg 6(4)(d) to prevent the formation of an explosive atmosphere. As such, in those cases where an enclosure may become substantially filled with a large flammable gas cloud, appropriate measures should be taken to limit the potential leak size through, for example, use of alternative equipment or improved maintenance regimes.

In some cases only a small volume of the enclosure may need to be zoned, but in others the whole enclosure should be zoned. A simple equation has been described that allows the zone extent to be calculated directly for releases of flammable gas (EI15 Fluid Categories G(i) and G(ii), but not Categories A, B and C; EI15 fluid categories are detailed in Annex A).

Account for the effects of confinement and congestion

The build up of flammable gas/vapour following a release of a pressurised fluid can be strongly affected by local obstructions to the resulting jet and to the ventilation flow. Both have a very significant effect on the dispersion of the flammable gas and the resulting size of gas cloud. It is therefore very important that this effect is taken into account in an area classification methodology.

BS EN 60079-10-1:2009 uses an approach to account for “the efficiency of the ventilation” that scales the gas cloud volume Vz (which is then used to determine the zone) by a factor, f, between 1 and 5. However, in reality the gas cloud volume in confined or congested spaces can be over 100 times larger than the equivalent unobstructed case.

In addition to increasing the gas cloud volume, confinement and congestion are likely to increase the persistence time of a release and increase the chance that the gas cloud volume will extend beyond the zoned area predicted using BS EN 60079-10-1:2009. In such cases it may not be possible to describe the space as adequately ventilated based solely on the overall ventilation rate of the enclosure.

In the present report, it is shown how the gas cloud volume depends on the distribution of the ventilation within the enclosure, which has been characterised by a term introduced as the ‘efficiency of mixing’, ε. Note that this factor is not the same as the factor, f, used in BS EN 60079-10-1:2009, which simply scales the gas cloud volume. It is also shown how an initial assessment of the effects of congestion and confinement can be made using this efficiency of mixing parameter, which effectively reduces the ventilation rate. This approach is suitable if the average concentration at the outlet is between, roughly, 1% and 50% LFL. In other cases where the gas release is small relative to the enclosure volume, e.g. for large enclosures, then the gas cloud build up only depends weakly on the overall ventilation rate and the efficiency of mixing cannot be used to highlight cases where the (local) ventilation is not adequate.

Based on this approach, a pragmatic method to assess the adequacy of the ventilation for releases in confined or congested spaces is firstly to multiply the actual ventilation rate by the

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factor ε and assess whether the resulting ventilation rate still meets the 25% LFL ventilation criterion. For confinement/congestion levels that are moderate or high, suitable values for factor ε are ½ and 3

1 , respectively. If the 25% LFL criterion is met then this indicates that the global ventilation rate is sufficient. It then just remains to check that the local ventilation is adequate.

If the degree of confinement/congestion is particularly high then the ventilation rate scaled by ε is unlikely to indicate whether or not the local ventilation is adequate. Instead an assessment of the local ventilation effectiveness needs to be carried out, or the extent of the zone needs to be determined taking the obstructions into account. Carrying out smoke tests would be one way of assessing the local ventilation effectiveness. The degree to which the 25% LFL criterion is met, taking into account a suitable value for the efficiency of mixing, ε, provides a very useful indication of the amount of effort needed in these additional assessments.

To determine whether or not an area should be classed as congested or confined, it is useful to assess the number and size of obstructions within the distance in which it takes for an equivalent unobstructed release to dilute to below the flammability limit. A simple approach applicable to pressurised gas releases for calculating this distance is provided above but other more generic approaches can be used.

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1 INTRODUCTION

The control of sources of ignition by the use of specially protected equipment in areas where flammable gases or vapours may arise has been a fundamental safety measure for many years. Thus hazardous areas are classified into zones based on the frequency of the occurrence and the duration of an explosive gas atmosphere. The zones for gases, vapours or mists are defined in Schedule 2 of DSEAR (which substantially reproduces the provisions of Annex I of Council Directive 99/92/EC – commonly known as the ‘ATEX User Directive’) as:

Zone 0 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently.

Zone 1 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally.

Zone 2 – A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

EI15 defines three defined grades of flammable gas, vapour or liquid release. These are defined in terms of their frequency and duration and are:

Continuous grade release: A release that is continuous or nearly so, or that occurs frequently and for short periods.

Primary: A release that is likely to occur periodically or occasionally in normal operation i.e. a release which, in operating procedures, is anticipated to occur.

Secondary: A release that is unlikely to occur in normal operation and, in any event, will do so only infrequently and for short periods i.e. a release which, in operating procedures, is not anticipated to occur. Such releases may be of known size e.g. fracture of a drain, or unknown size e.g. corrosion hole.

These generally give rise to areas classified as Zones 0, 1, and 2 respectively.

Various standards, guides and codes of practice are available that provide practical advice on how to carry out Hazardous Area Classification (HAC). The Energy Institute’s guidance on HAC “Area classification code for installations handling flammable fluids”, commonly known as EI15 (and previously IP15), is used extensively in the UK and elsewhere by the petroleum industry. The Energy Institute now wish to update EI15 to take into account recent work in this field, in particular the joint industry project carried out at HSL (Ivings et al., 2008), and also to ensure that the revised EI15 is consistent with the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR, 2002) which were fully implemented in 2006. While the approach to HAC in open or outdoor spaces in EI15 is still seen as appropriate, improvements are sought on the approach to area classification for ventilated indoor spaces.

The main aim of this work is to provide a methodology for assessing the adequacy of the ventilation such that an enclosure containing secondary-grade release sources can be properly classified as Zone 2. This work continues on from a previous project carried out by HSL for the Energy Institute which provided look-up tables of ventilation rates, for a range of different flammable releases, which could be used as the basis for defining adequate ventilation. The

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scope of this work is limited to the classification of Zone 2 areas and the release scenarios published in EI15. Five fluid categories are introduced in EI15 for the purpose of area classification and these are listed in Table A.1 in the Annex A.

The project has three main objectives:

• To develop and describe practical approaches for determining natural ventilation rates of enclosures and buildings.

• To develop an approach for defining adequate ventilation in an enclosure such that an enclosure containing secondary-grade release sources can be properly classified as Zone 2.

• To develop a methodology for taking account of congestion / confinement around the leak location suitable for use in area classification.

A useful source of background information on area classification is provided by Santon et al (2012).

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2 VENTILATION ASSESSMENT

2.1 INTRODUCTION

The scope of this section is the calculation of ventilation flows that occur when there is no release occurring in an enclosure. Ventilation flows introduce fresh air into enclosures from outside, and remove air, possibly with contaminants, from within enclosures. The main focus is on natural ventilation. The possible uses of local exhaust ventilation and mechanical ventilation are discussed but not described in detail.

The Institution of Gas Engineers and Managers (IGEM) publishes a standard IGEM/SR/25 (IGEM, 2010) on hazardous area classification of natural gas installations. This only considers natural gas installations and the approach taken is to specify the area of ventilation openings required to achieve the necessary ventilation flow rates. The requirements for adequate ventilation are described in Appendix 7 (IGEM, 2010) and equations to calculate the size of openings needed to achieve ventilation flow rates due to buoyancy or wind are described. The buoyancy calculations in IGEM/SR/25 include the effect of the released natural gas on the ambient density in the enclosure, unlike the present work where ventilation rates without releases occurring are considered.

Gas dispersion in enclosures is affected by both the internal ventilation flow within the enclosure and the nature of the gas release. For small releases, the released gas is likely to be rapidly diluted and its dispersion behaviour may then resemble that of a passive tracer, where the ventilation flow dominates. However, for large high-pressure gas releases, the jet produced by the release and the resulting buoyant plume may dominate the pre-existing ventilation flow. Between these extremes the resulting flow and hence gas dispersion during a release will be affected by a combination of the release and ventilation flows.

EI15 states that the location of continuous or primary grade sources within enclosed areas is unacceptable and should be avoided1 (EI15 Table 6.1 note 1). Therefore, ventilation of enclosed spaces should mainly need to consider secondary grade releases, i.e. those that are unlikely to occur in normal operation and will, in any case, only occur infrequently and for short periods (EI15 1.6.4).

The ventilation flow rate for an enclosure (which has units of volume per unit time) is defined as the product of the air change rate and the enclosure volume. Therefore, for a given air change rate, the ventilation flow rate varies with the size of enclosure. A test method for determining the air change rate in an enclosure using tracer gases is described in ASTM E741-11 (ASTM, 2011).

If a gas release is taking place within an enclosure, the average concentration of gas leaving through the ventilation outlets is related to the ratio of the gas release rate and the ventilation flow rate.

For releases outdoors the ambient air flow will influence dispersion. External air flows may be affected by regions containing obstructions, with reduced flow rates through the obstructed region as compared to the ambient flow, or regions that are sheltered by structures, where the air flow recirculates. These types of flows may lead to the accumulation of released gas, but they are not ventilation flows.

1 However EI15 Section 6.4.3.1 states that local exhaust ventilation can be used as a means of controlling secondary and primary releases.

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Mechanical ventilation of enclosures typically uses fans to force air into an enclosure, where the resulting pressure in the enclosure is then greater than the ambient (i.e. positive), or to extract air from an enclosure, where the pressure in the enclosure is then less than the ambient (i.e. negative). Fans can be specified to provide a required ventilation rate and therefore in principle offer the ability to produce the ventilation flow rate necessary to control the accumulation of flammable gas clouds. In practice, the natural ventilation flows induced by wind and temperature gradient can affect the actual flow rate achieved in enclosures fitted with mechanical ventilation.

Where a process is known to release gases or vapours in a limited area, it may be possible to prevent their accumulation within the enclosure by using local exhaust ventilation (LEV). For example, the process may be performed under an extraction hood or in an extraction booth. Such controls are distinct from the general room ventilation that is the subject of the present report. The use of LEV is acceptable as a method of controlling both primary and secondary grade releases (see EI15 Section 6.4.3.1).

2.2 NATURAL VENTILATION

Natural ventilation of enclosures is driven primarily by two forces: the atmospheric wind outside the enclosure, which gives rise to a pressure distribution around the external faces of the enclosure, and buoyancy forces, which result from temperature differences between the inside and outside of the enclosure. Often, natural ventilation flows result from a combination of these two forces.

The flow rate due to natural ventilation through large openings with sharp edges (which have been designed for ventilation) can be calculated from the following equation:

( )ρ

pACpq ed

∆∆=

2sgn (2.1)

where:

q is the volumetric flow rate in m3s-1,

dC is the discharge coefficient, characteristic of large ventilation openings, inlet or outlet, and accounts for the turbulence and viscosity, typically 0.50 to 0.75, dimensionless,

eA is the effective equivalent area of the opening in m2,

p∆ is the pressure difference in Pa due to wind and temperature effects, and will determine the direction of flow. The magnitude, p∆ , and sign, ( )p∆sgn , of the pressure difference are used to calculate the volumetric flow rate,

ρ is the air density in kg m-3.

Natural ventilation will also take place by infiltration, through non-airtight doors and windows, or through cracks and gaps in the structure. These infiltration flows will occur even if there are no designed openings in the walls and/or roof, or if they are closed. In the present work, any ventilation flows that result from infiltration are ignored. The flow rates estimated by taking this approach are therefore likely to be less than the actual flow rate (i.e. conservative).

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There may be multiple inlets and outlets to an enclosure. To simplify the analysis of these, the equivalent opening area, eA , is used to reduce these multiple openings to single inlet and outlet openings. For parallel openings on the same face of an enclosure, the areas of openings are simply added together. For wind driven flows the openings on the upwind surface form an inlet with an equivalent area equal to the sum of their areas, while those on the downwind surface form an outlet. Similarly for buoyancy driven flows all the openings at low level form an inlet while those at high level form an outlet.

Openings in series are formed from upwind inlet to downwind outlet for wind driven flows, or from low-level inlet to high-level outlet for buoyancy driven flows. For openings in series, the reciprocal of the square of the equivalent area is equal to the sum of the reciprocal of the squares of the openings in series.

The flow rate due to natural ventilation may be modified by gas releases, both because of their mass and momentum, and due to changes in the composition (density) of air in an enclosure. The following equations do not consider these effects on ventilation flow rates, only the natural ventilation due to wind and temperature differences.

In the following sections, a methodology is presented to determine the pressure drop across the openings of an enclosure due to wind and temperature differences. This methodology is then demonstrated using a simple ventilation scenario, for which an analytical solution can be obtained. The assumptions made in adopting this methodology to the scenario are discussed. Following this, a more general, iterative, approach to the calculation of ventilation flows is described.

2.2.1 Wind-induced ventilation

The degree of natural ventilation due to the wind depends on the size and position of the openings relative to the wind direction, as well as on the shape of the building. Windward (upwind) openings normally act as the inlet openings and leeward (downwind) and roof openings as the outlet openings.

Measurements of the wind close to the enclosure location (usually presented in the form of a windrose) should be used to select suitable, conservative, values of wind speed and direction to use in the calculations of the wind-driven ventilation flow rate. In IGEM/SR/25, the mean wind velocity exceeded 80% of the time is used in the calculation of ventilation flow rate. Methods for estimating the wind speed as a function of height for different terrain types is provided in BS5925 (BSI, 1991).

Calculation of wind-driven ventilation requires the pressure difference across openings due to the wind to be calculated. For an individual opening the pressure difference, Δp, can be calculated from:

2U2

ρpCp =∆ (2.2)

Where pC is the dimensionless pressure coefficient characteristic of the opening and U is the undisturbed wind speed, in ms-1, at the specified reference height for the pressure coefficient.

For sharp-edged enclosure structures, the magnitude of the pressure on the surface of the structure is determined by the wind speed. However, the way this pressure is distributed over

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the building surface is largely independent of the wind speed. The distribution depends instead upon the shape of the structure and how the flow separates from the sharp edges. The pressure distribution can therefore be represented by maps of the coefficient of pressure, pC , over the surface of a building, which can be scaled by a wind velocity at a reference height to calculate the pressures at different locations. The reference height used with the coefficient of pressure is typically taken to be the height of the structure and the velocity used is the speed of undisturbed wind at that height.

The most reliable way to assess the pressure coefficient for a building is by CFD modelling or wind-tunnel testing. If information from these sources is not available, wind pressure coefficients for simple building geometries are available in BS5925:1991 Table 13 (BSI, 1991). A set of constants are also provided in Table 8 of BS5925 (BSI, 1991) that can be used to convert UK Meteorological measurements of wind speed at a height of 10 m in open countryside into the wind speed at any given height. This can be used to determine the undisturbed wind velocity at the reference height for a building, for input into the surface pressure calculation. The ratio of the mean wind speed exceeded for a stated percentage of the time, compared to the 50% mean wind speed can also be calculated from values in Table 9 of BS5925 (BSI, 1991). However, this information is only valid for the UK.

The pressure difference across openings due to wind is used in an iterative calculation of the ventilation flow, which is described later in Section 2.2.5. For a simple building, with only upwind and downwind openings, the driving force of wind-induced ventilation is the pressure difference between the upwind and downwind sides of the building. For the simplest case, where the openings are of equal size, or an equivalent effective area is used, half the pressure drop will occur at each opening. Half the value of the pressure drop, obtained from Equation 2.2, is then substituted into Equation 2.1, to calculate the flow rate. The flow rate due to wind through either opening is then given by:

2p

ed

CUACq

∆= (2.3)

where:

eA is the equivalent effective area of upwind or downwind openings in m2, defined

22

21

22

21

AAAAAe +

= (2.4)

and

1A is the area of the upwind opening in m2,

2A is the area of the downwind opening in m2.

The pressure coefficient for a building, pC∆ , is the difference in the pressure coefficients

across the surfaces of a building, downstream ,upstream , ppp CCC −=∆ .

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2.2.2 Buoyancy-induced ventilation

Buoyancy induced "stack effect" ventilation is accomplished by the movement of air due to the difference between indoor and outdoor temperatures. The driving force results from the difference in air density, due to the different temperatures.

The rate at which the air pressure changes with height depends upon the density of air. If the average indoor temperature is higher than the outdoor temperature, the indoor air will have a lower density. The pressure gradient will then be smaller indoors than out, which will produce a pressure difference between the interior and exterior of the enclosure.

If an enclosure has openings at different heights, and the air is heated within the enclosure, air will enter through the lower openings and leave through the upper level openings. The flow rate will increase as the magnitude of the temperature difference grows larger. Buoyancy-induced ventilation will typically be more effective when there are lower outdoor temperatures, when the difference between indoor and outdoor temperatures is greatest. As the outdoor temperature increases to match that indoors, the buoyancy-induced ventilation will become less effective. If the ambient outdoor temperature rises above the indoor temperature (for instance, in air-conditioned buildings) the flow reverses, with air and entering at high level and leaving through low-level openings.

If it is assumed that the air inside of the building is fully mixed, then the temperature does not vary with height. Applying the same assumption for the outdoor environment, the pressure difference across an opening can be written

( )gzppp inoutin ρρ −−−=∆ (2.5)

This equation is used to calculate pressure differences across openings due to temperature differences in an iterative calculation of the ventilation flow described in Section 2.2.5. Using the ideal gas law the pressure difference can be written

gzTTR

ppppinout

in

−−−=∆

11 (2.6)

where:

p∆ is the pressure difference across the opening in Pa,

p is the ambient pressure in Pa, measured at the specified pressure reference height,

inp is the pressure in the enclosure in Pa, measured at the specified pressure reference height,

R is the gas constant for air in J kg-1 K-1,

g is the acceleration due to gravity in m s-1,

z is the vertical distance from the specified pressure reference height to the midpoint of the opening in m,

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inρ is the indoor gas density in kg m-3,

outρ is the outdoor gas density in kg m-3,

inT is the indoor temperature in K,

outT is the outdoor temperature in K.

For the simple scenario where internal and external temperatures do not vary with height the airflow caused by the temperature difference between indoors and outdoors can be obtained from the following analytical equation:

( )gH

TTT

ACqin

outined

−= (2.7)

where:

H is the vertical distance between the midpoints of the lower and upper openings in m,

eA is the equivalent effective area to the lower and upper openings, in m2, used to calculate the ventilation flow rate, it is defined:

22

21

22

21

AAAAAe +

= (2.8)

and:

1A is the area of the lower opening in m2,

2A is the area of the upper opening in m2.

If the temperature inside the enclosure varies with height (but the outside temperature does not), then assuming that the inside temperature at the lower opening is the same as the outside temperature, outT , and the inside temperature at the upper opening is inT , the volume flow rate can be calculated from the average temperature inside the enclosure as follows:

( )( ) gH

TTTTACq

outin

outined +

−= (2.9)

2.2.3 Natural ventilation induced by wind and thermal buoyancy in combination

The two driving forces for natural ventilation (wind and thermal buoyancy) can occur separately but are likely to occur at the same time. The flow rates due to buoyancy and wind cannot simply be added, rather the pressure differences due to the different effects must be combined. Pressure differences due to thermal buoyancy will typically dominate on calm cold days with practically no wind, whereas those created by wind will typically dominate on windy warm

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days. Their forces can oppose or complement each other depending on the position of the inlet and outlet openings in relation to the wind direction. BS5925 (BSI, 1991) suggests that, in practice, either the buoyancy-driven flow or the wind-driven flow will dominate and that the larger of the two flow rates can be used to represent the combined ventilation rate.

2.2.4 Assumptions and limitations

The equations presented above (Equations 2.1 – 2.9) are all for a single zone, that is, an enclosure where any connections to other enclosures are small compared to the openings in the envelope of the enclosure. Also, there are either no internal barriers, or openings in any internal barriers are sufficiently large that there are no pressure differences within the enclosure (Etheridge, 2002). It is assumed that the pressure distribution inside an enclosure is uniform, the velocities within it are very small and the internal flow, including the presence of any objects does not interact with the envelope and external flow.

It is assumed that the flow inside and outside the enclosure are not coupled. This means, for example, that the outside conditions are assumed not to change in response to the flows induced by natural ventilation.

Furthermore, it is assumed that the pressure distribution across the external surface of the structure without openings is not modified by the presence of any openings. Both of these assumptions are valid only if the ventilation openings are small in comparison to the size of the structure. CIBSE Guide A (CIBSE, 2006) suggests that this ‘small opening’ behaviour can be assumed for openings occupying up to 30% of the external surface.

If the openings in an enclosure are large then there can be high velocities at some distance away from the openings. The flow rate through the openings may then be modified by the interaction of the flow with objects and blockages within the enclosure. The simplified model described above is no longer applicable in this case, and coupled flow solutions are necessary. This would normally require the use of CFD or physical experiments, although these would need to consider carefully the effects of scale, if wind tunnels were used.

Detailed information for all the parameters needed to perform ventilation calculations may often not be available. For instance, the position and sizes of ventilation openings may be known but the value of their discharge coefficient may be uncertain. Pressure coefficient characteristics for buildings are affected by their surroundings but generic values for isolated buildings may have to be used, unless there has been a relevant CFD or wind-tunnel study. The availability of information will limit the accuracy of ventilation calculations and this should be considered when using the results of calculations.

2.2.5 Iterative Calculation

The analytical solution for the simple cases described above cannot be extended to more complicated cases, involving additional openings on different surfaces or flows involving a combination of buoyancy and wind effects. Instead, for these more complex cases, the flow rate is found using an iterative calculation method, e.g. BS (1991), CIBSE (2005), Etheridge & Sandberg (1996).

Solutions are obtained by ensuring that mass is conserved across all the openings in an enclosure.

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0=∑ iiqρ (2.10) where

iq is the volumetric flow rate through opening i in m3s-1,

iρ is the density of the flow through opening i in kg m-3.

The flow rate through each opening, i , is calculated from:

( )i

iidii

pACpq

ρ∆

∆=2

sgn (2.11)

and the pressure difference across each opening, i , is calculated from:

2,00 5.0 UCgzpp outipii ρρ +∆−∆=∆ (2.12)

with

ref0 pppp in −−=∆ (2.13)

and

inout ρρρ −=∆ 0 (2.14)

where p is the ambient pressure and inp the pressure in the enclosure, both at a reference height, refp is a reference pressure used in the definition of pC , and inρ and outρ are the internal and external air densities, respectively.

Equation 2.12 can be rewritten to describe the pressure difference across each opening, i . For N openings there are 1+N equations, including the continuity equation (Eqn 2.10). An iterative solution method can then be used to find the value of 0p∆ , so that the flows into and out of an enclosure conserve mass. An iterative solution allows the combined effects of wind and buoyancy driven ventilation to be calculated directly.

This type of calculation can be performed using spreadsheets or simple software programs. An example of the calculations described in CIBSE (2005) is available as an Excel spreadsheet at www.cibse.org/venttool/. The model of gas cloud build up in ventilated enclosures, Quadvent, available from HSL, www.hsl.gov.uk, also includes the above iterative model. A screenshot of a calculation using the CIBSE spreadsheet combining wind and temperature driven flows is shown in Figure 1. The spreadsheet shows a design calculation for the areas necessary to give stated flow rates. The internal pressure is varied using a slider in the spreadsheet and the areas needed to give the required flow rates are calculated. However, for an existing structure, where the areas of the ventilation openings are already specified, the Excel “Goal Seek” Function could be used to find the internal pressure that enforces conservation of mass, to give the flow rates for the known areas.

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Figure 1 Screen shot from the CIBSE model

2.3 SUMMARY

The flow rate of air into and out-of enclosures due to natural ventilation can be predicted using a range of approaches, from simple analytical approaches to complex CFD simulations to tracer gas measurement, see Table 1.

For simple cases, an analytical approach can be used to determine the ventilation flow rate due to wind or thermal buoyancy, based on methods described in BS5925 (BSI, 1991). However, this approach is limited in terms of the possible location of openings, and it cannot account for combined wind and thermal-buoyancy effects. It is therefore probably too simple for most practical applications.

An alternative iterative solution method allows greater flexibility and can account for these more complex effects. This iterative method involves the solution of Equations (2.11) and (2.12) by varying the internal pressure until mass balance (2.10) is achieved. Simple software programs or spreadsheet have been written to solve this system of equations and are available, for example from HSL in the QUADVENT model (www.hsl.gov.uk) and CIBSE (www.cibse.org/venttools).

Multi-compartment buildings have not specifically studied. Models such as COMIS2 and CONTAM3 are available that include consideration of the additional parameters that need to be

2 http://epb.lbl.gov/comis/ (accessed on 11th May 2011) 3 http://www.bfrl.nist.gov/IAQanalysis/CONTAM/index.htm (accessed on 11th May 2011)

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taken into account for such situations; e.g. the pressure drop between internal spaces. A summary of other modelling techniques is given in Table 1.

Irrespective of which model is used, the quality of information used to describe the enclosure will influence the reliability of the results. In principle, it is possible to measure natural ventilation rates, but this is outside of the scope of the present work.

Table 1 Classification of approaches available for estimating ventilation rates in

naturally ventilated enclosures Approach Application Notes Single space (analytical)

Flow through the enclosure envelope

Uncoupled external and internal flow. Only describes a limited range of geometries. Equations for the effect of wind or buoyancy. Combination of effects of wind and buoyancy is approximate.

Single space (iterative)

Flow through the enclosure envelope

As above, but: Can be used for openings on multiple sides of structures and heights of openings. Effects of wind, buoyancy (and mechanical) can be combined

Multiple space Flow through the envelope and between internal spaces

As above, but also can include pressure drops between internal spaces, implemented in freely available software, e.g COMIS, CONTAM.

CFD External flow and pressure coefficients over structures. Coupled external/internal flows.

Few limits on geometry. Coupled flows where internal geometry affects envelope flow. Requires more information and greater effort than simpler modelling approaches

Physical modelling External flow and pressure coefficients over structures

Few limits on geometry. Production of physical models and performance of measurements requires information, equipment and effort.

On-site measurements, for example, tracer gas techniques

Building ventilation rates Specific to the structure of interest but will depend on the conditions on the day measurements are performed

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3 DEFINING ADEQUATE VENTILATION

3.1 INTRODUCTION

In this Section, different approaches are considered for providing a quantitative description of ‘adequate ventilation’. EI15 defines this as:

“the achievement of a uniform ventilation rate of at least 12 air changes/hr, with no stagnant area”

and also describes it in terms of its objective:

“The objective of adequate ventilation is to ensure that a building containing secondary grade release sources can be properly classified as Zone 2”

In an area classified as Zone 2, specific measures are taken to prevent the ignition of the flammable gases/vapours. Therefore the aim of the ventilation in this case is not to dilute the released volume down to a specified ‘safe’ level as it is for Zone 2NE. Rather it is to limit the size of the flammable region to ensure that it stays within the zoned area (which may be the whole enclosure) and dilute short-duration releases once they have stopped, so that concentrations rapidly fall well below LFL.

Previous work by Ivings et al. (2008) has shown that a definition of adequate ventilation in terms of an air change rate is of limited utility since it takes no account of the size of flammable gas release that it is trying to dilute, and it does not scale appropriately with the size of the enclosure.

The aim of the present work is therefore to develop an alternative quantitative definition of adequate ventilation that meets the above objective, i.e. it is appropriate for a Zone 2 area with specified secondary releases. Adequate ventilation is therefore a relative term and will depend on the size of the leak that the ventilation is being used to dilute.

Note that the scope of this work is restricted to secondary grade releases and therefore where the gas cloud volume Vz is referenced in this report, it is defined as the volume of gas with an average concentration of 50% LFL. Further background information on Vz and its definition can be found in Ivings et al (2008).

Before considering different approaches for defining adequate ventilation it is useful to look at a number of different approaches to hazardous area classification including approaches for defining adequate ventilation (i.e. the condition required for classifying an area as Zone 2). In Section 3.2 below the following approaches are therefore reviewed: EI15, BS EN 60079-10-1:2009, IGEM/SR/25, the joint industry project of Ivings et al (2008) and Quadvent.

3.2 EXISTING APPROACHES

3.2.1 EI15

As discussed above the quantitative criterion described in the current version of EI15 for adequate ventilation is a requirement of 12 air changes per hour in the enclosure. The main disadvantage of this approach is that the requirement is not related to the size of the release and therefore the same level of ventilation is required for a room containing potentially large

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releases as one that may contain very small releases. Also, a definition of adequate ventilation in terms of an air change rate will require huge volumes of air to be moved to achieve the criterion in large enclosures. However, in practice dispersion in large enclosures will behave in a similar manner to a release outdoors and therefore the air change rate is not critical to its dispersion.

The Energy Institute are currently revising EI15 and work carried out for them by HSL has suggested adopting an approach based on the average concentration at the outlet, see Kelsey and Ivings (2010).

3.2.2 BS EN 60079-10-1:2009

The international standard on area classification, BS EN 60079-10-1:2009 (Table B.1 in Appendix B), defines the conditions required for a Zone 2 classification. For secondary releases, it states that there is a requirement of a so-called Medium or High degree of ventilation4 (and no requirement on ‘availability of ventilation’). In simple terms a High degree of ventilation corresponds to a requirement that the gas cloud volume Vz is smaller than 0.1 m3 and is therefore used as the basis for a classification of Zone 2 NE (Negligible Extent). The definition of a Medium degree of ventilation is as follows:

B.5.3.4 Medium ventilation (VM)

If the ventilation is neither high (VH) nor low (VL) then it should be regarded as medium (VM). Normally, Vz will be less than or equal to V0 [the enclosure volume]. Ventilation regarded as medium should control the dispersion of the release of flammable vapour or gas. The time taken to disperse an explosive gas atmosphere after release has stopped should be such that the condition for either a Zone 1 or Zone 2 is met, depending on whether the grade of release is primary or secondary. The acceptable dispersion time depends on the expected frequency of release and the duration of each release. When the volume Vz is significantly less than the volume of the enclosed space, it may be acceptable to classify only part of the enclosed space as hazardous. In some cases, depending on the size of the enclosed space, the volume Vz can be similar to the enclosed volume. In this case, all of the enclosed space should be classified as hazardous.

In outdoor locations except where Vz is very small or where there are significant restrictions to air flow, the ventilation should be regarded as medium (VM).

The key element of this definition is that Vz will normally be smaller than the enclosure volume5. This means that (for secondary releases) the flammable volume Vz can approach the volume of the enclosure and then, by definition, the average concentration in the enclosure will approach 50% LFL. This observation is important to note as it differs somewhat from the Zone 2 definition used in IGEM/SR/25 as described below.

Critically, it is also to be recognised that the approach for estimating Vz detailed in BS EN 60079-10-1:2009 lacks a scientific basis and caution is required in its use.

5 The conditions in which it could be larger are not defined

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3.2.3 IGEM/SR/25 Ed. 2

IGEM/SR/25 introduces different categories of ventilation effectiveness that it uses as part of the process of defining the zone. The categories of ventilation and a brief description of their definitions are as follows:

“more than adequate” – the ventilation achieves an average concentration in the enclosure equal to or less than 10% LFL

“adequate” – the ventilation achieves an average concentration in the enclosure equal to or less than 25% LFL, but more than 10% LFL

“inadequate” – the ventilation achieves an average concentration in the enclosure of more than 25% LFL, but less than 50%LFL.

“poor” – the ventilation achieves an average concentration in the enclosure of more than 50% LFL.

These four categories generally correspond to classifications of Zone 2NE, Zone 2, Zone 1 and Zone 0 respectively (although other factors need to be taken into account). This approach therefore allows a secondary release to be classified as Zone 2 NE or Zone 0 or anything in between depending on the level of ventilation.

The terminology of “adequate ventilation” in IGEM/SR/25 is consistent with EI15 in that they both refer to the level of ventilation that is required for a classification of Zone 2.

Direct comparison can be made between the definition of adequate ventilation in IGEM/SR/25 and BS EN 60079-10-1:2009 as the latter defines adequate ventilation to include cases where Vz approaches the enclosure volume and hence the average concentration will approach 50% LFL, whereas in IGEM/SR/25 an average concentration of between 25% LFL and 50% LFL would not be sufficient for a classification of Zone 2, but instead would lead to a classification of Zone 1.

3.2.4 Joint Industry Project

A large joint industry project on area classification was undertaken at HSL between 2004 and 2008, which is documented in the report by Ivings et al. (2008). The project focused on developing an approach for defining the conditions in which a Zone 2 NE classification could be used for releases of flammable gas.

One of the early outcomes from this project was that it was demonstrated through experiments and modelling that the criterion for Zone 2NE that Vz should be less than 0.1 m3 used in BS EN 60079-10-1:2009 is reasonable.

It was then shown that, for a less than 10 barg release in an enclosure, if the average gas concentration at the ventilation outlets is less than 10% LFL, then in the vast majority of cases the Zone 2NE criterion, i.e. Vz < 0.1 m3, would be met. This therefore provided a simple means for classifying Zone 2 NE areas as the average gas concentration at the outlet is easy to compute based on the ventilation rate and mass leak rate.

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3.2.5 Quadvent

The international standard BS EN 60079-10-1:2009 states that alternative means can be used to calculate the gas cloud volume Vz. A simple alternative model, Quadvent, applicable to releases of pressurised gases has been developed by Webber et al (2011) based on ventilation theory and a simple integral jet model. The approach is applicable to releases of pressurised gas and generally leads to much smaller gas cloud volume predictions than the BS EN 60079-10-1:2009 standard. Quadvent has been validated against CFD simulations which themselves have been validated against an extensive programme of experiments.

A practical guide to using this approach is provided by Santon et al. (2012)

3.2.6 Discussion on existing approaches

The approaches for defining the conditions required for an area to defined as Zone 2 discussed above have many similarities but include some clear differences. The most significant difference between the three approaches is the parameter that is used to make the distinction: IGEM/SR/25 uses the average concentration in the enclosure, BS EN 60079-10-1:2009 uses the gas cloud volume Vz (and the standard provides a method for calculating it) and EI15 uses an air change rate.

Before looking at which of these approaches would be suitable for defining adequate ventilation in EI15, recall that in an area classified as Zone 2, specific measures are taken to prevent the ignition of the flammable gases/vapours. Therefore the aim of the ventilation in this case is not to dilute the released volume down to a specified ‘safe’ level as it is for Zone 2NE. Rather it is to limit the size of the flammable region to ensure that it stays within the zoned area (which may be the whole enclosure) and does not continue to grow. It therefore is not appropriate to define the adequacy of the ventilation based on some criterion for a ‘safe’ gas cloud volume, as it is for Zone 2NE, as it is accepted that from time to time there will be accumulations of flammable mixtures within the zone. It is for this reason that ignition sources are controlled within the zone.

One of the key advantages of using an approach based on the average concentration at the outlet is that it is simple to apply to the different categories of fluids used in EI15 for area classification. However, this makes the assumption that the release can be treated as a gas or vapour which is a significant assumption for the fluids that are stored as liquids, i.e. principally Fluid Category B. Assuming that all of the releases vaporise is likely to be a conservative assumption as some of the fluids will lead to a significant proportion of rainout. Further work is required to provide more detailed guidance on the area classification of liquids, including flashing liquid releases. A joint industry project is currently being undertaken by HSL on the area classification of flammable mists (Fluid Category C) and HSE are also commissioning HSL to provide an approach for classifying flashing releases (Fluid Category A, e.g. LPG and ammonia).

A further advantage of using the average concentration at the outlet to determine the adequacy of the ventilation is that it is straightforward to provide the required data in a look-up table format that is consistent with EI15 and its use of different fluid categories, hole sizes and release pressures.

In the next section the implications are assessed of using the average gas concentration at the outlet for providing a means of defining adequate ventilation.

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3.3 THE AVERAGE GAS CONCENTRATION AT THE OUTLET

The concept of using the average gas concentration at the outlet for defining conditions under which a classification of Zone 2NE is appropriate was introduced by Ivings et al (2008). This concept was then used by Kelsey and Ivings (2010) for defining adequate ventilation. These latter results will be analysed further below. Before doing this, two scenarios will be examined briefly, which provide relevant bounding cases.

An average gas concentration at the outlet of 10% LFL is used for the definition of Zone 2NE (along with other criteria, such as those limiting the mass release rate of flammable material) and therefore this implies that a higher average concentration could be acceptable for Zone 2. Clearly if the criteria for a classification of Zone 2NE are met then this demonstrates that the ventilation is adequate (or is ‘more than adequate’ using the IGE/SR/25 terminology).

An average gas concentration at the outlet of 50% LFL implies an average gas concentration within the room of 50% LFL. This effectively means that the Vz gas cloud volume has filled the room, although a much smaller fraction of the enclosure will be within the flammable range. This situation presents a far from negligible hazard and, if the gas cloud is ignited (which it should not, because it is a zoned area with suitable protective equipment), it could lead to a significant overpressure and thermal hazard.

A definition of adequate ventilation based on a defined percentage of LFL at the outlet would broadly meet the original objective for adequate ventilation. A criterion based on 10% LFL could be considered overly cautious as it is used as the basis of the definition for Zone 2NE. On the other hand, a value of 50% LFL implies the Vz gas cloud has filled the room. The value used by IGEM/SR/25 for defining adequate ventilation, 25% LFL, provides a more conservative approach than using 50% LFL and it is also consistent with IGEM/SR/25, in that it reduces the extent of the flammable zone within the enclosure which in turn will reduce the likelihood of ignition.

In the following Section the use of an average concentration of 25% LFL is considered as a definition of adequate ventilation.

3.3.1 An average concentration of 25% LFL

The hole sizes and pressures used by EI15 for the area classification of the five classes of fluid defined in EI15 are shown in Table 2 below. The table presents the release rate (kg/s) for each case and the ventilation rate (m3/s) required to dilute the release down to an average gas concentration of 25% LFL at the ventilation outlet. Note that this is independent of the enclosure volume and full vaporisation of the fluid has been assumed. Also in this table is the estimated gas cloud volume from each release.

The release rates in Table 2 were calculated using DNV PHAST and the ventilation rates calculated based on the values for physical parameters listed in Table C8 of EI15. These values came from Table 3 in the report by Kelsey and Ivings (2010), where more details can be found on how they were calculated. To calculate the gas cloud volumes, the same approach has been used as that in Kelsey and Ivings (2010). As DNV’s PHAST can’t model the effects of ventilation directly on the gas cloud volume these results have been scaled from the equivalent outdoor unobstructed case.

Quadvent predicts that the gas cloud volume resulting from a gas release in an enclosure with a background concentration of 25% LFL to be approximately 2.28 times larger than for the

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equivalent outdoor unobstructed release (see Table 3 below6). Therefore this same scaling factor has been applied to the PHAST predictions across all of the fluid categories to obtain the gas cloud volumes inside enclosures with a background concentration of 25% LFL. While this is a significant assumption, as the dispersion mechanisms leading to gas cloud build up are rather different for the different fluid categories, the results do provide a reasonable first estimate of the magnitude of the hazard posed by a release in an enclosure.

Note that some of the entries in Table 2 are in brackets as they are excluded from a classification of Zone 2 in EI157 for outdoor releases, due to the fact that the zone extents are over 30 m. EI15 states that “If a resulting hazard radius is greater than 30 m then the size of the release is generally larger than that considered for hazardous area classification purposes and consideration should be given to modifying the facility to minimise the size of the release.” Clearly if this is the case for releases outdoors a similar, if not more stringent, approach should be used indoors as this would present an even greater hazard. See below for further discussion on this issue. Note, however, that this exclusion of releases leading to a hazard radius greater than 30 m is specific to EI15. Zone extents from large natural gas vents far in excess of 100 m are not uncommon, as shown in IGEM/SR/25.

The ventilation rates in Table 2 that are required to give an average concentration at the outlet of 25% LFL could be used in EI15 as a minimum requirement for defining adequate ventilation. As discussed above this approach would fulfil the general requirements for adequate ventilation. To check further that this is reasonable, it is useful to assess the appropriateness of the definition by considering the gas cloud volume predictions also provided in Table 2.

These show that in most cases the gas cloud volume is relatively small, and that therefore the ventilation is sufficient to control the release. (Note that these gas cloud volumes are the gas cloud volume above the LFL not the Vz gas cloud volume. The Vz gas cloud volume will always be significantly larger than the gas cloud volume above the LFL – in the case of a free unobstructed gas jet it will be about 30 times larger). For example for hole sizes up to 2 mm diameter the flammable gas cloud volume is always less than about 1 m3.

However, there are some circumstances where larger gas cloud volumes are produced. Does this mean that the ventilation is not adequate in this case? For example consider the case that leads to the largest gas cloud volume: a release of G(ii) through a 10 mm hole at a pressure of 10 bar. This gives a gas cloud volume of 50 m3, which is clearly very large. Is it reasonable therefore to say that ventilation for this case is adequate? There are three important considerations.

Firstly, it should be considered whether or not it is appropriate to classify this as a secondary release, since the hole size and pressure are both very large. Perhaps guidance should instead say that instead of trying to ensure that the specified level of ventilation is achieved, steps should be taken to reduce the hole size, or otherwise mitigate the risk posed by such a release.

Secondly, if this case was classified as Zone 2, then the size of the zone can be made as large as necessary to ensure that flammable material does not leave the zone. The hazard radius at ground level for the equivalent outdoor release is 14 m (EI15 Table C9(b)). A method for defining zoning distances indoors has not been developed yet – see discussion later.

6 Note that these values differ from a similar Table presented by Kelsey and Ivings (2010) as a different version of the Quadvent model has been used. The main difference is the value of the entrainment coefficient which has been used, where in the present work it has been set to 0.05. 7 Note , however, that this exclusion is specific to EI15. Zone extents from large gas vents far in excess of 100m are not uncommon as shown in IGEM/SR/25.

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Finally, to meet the ventilation condition for this case would require a very high ventilation rate, i.e. 243 m3/s. For an enclosure volume of 100 m3, this would require a ventilation rate of 9,500 air changes per hour, which is clearly unrealistic. For a larger enclosure volume of 100,000 m3, this would still need a high air change rate, 9.5 air changes per hour, and so there are likely to be very few cases where this condition can be achieved. Even if the condition could be met, the flammable gas cloud volume would only fill 0.05% of the enclosure volume. The final case to consider is a very large enclosure volume of 1,000,000 m3. In this case the air change rate requirement would be equivalent to about 1 air change per hour. This may be a realistic air change rate for the enclosure volume, but the behaviour of the gas dispersion in this case will be almost indistinguishable from the equivalent release outdoors. Therefore, if a Zone 2 classification were acceptable for the equivalent outdoor case, then it would also be acceptable for this very large enclosure.

To conclude, the 25% LFL criterion is a useful and appropriate definition for adequate ventilation as long as the release location is not confined or congested (see the next Section). However, consideration could be given to excluding some combinations of hole size and pressure for releases indoors if it is thought that they are outside of the scope of a secondary release, in the same way that a release that gives a zoning distance greater than 30 m is excluded from a classification of Zone 2 for releases outdoors. While the gas cloud volumes for fluid categories A, B and C in Table 2 have only been estimated using a relatively simple approach, the results indicate that keeping the average concentration at the outlet to below 25% LFL still demonstrates that the release is being controlled.

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Table 2 Mass flow rates and ventilation flow rates for dilution to 25% LFL, releases calculated using DNV PHAST and gas cloud volumes calculated from Quadvent (for the gases) or scaled from Phast free jet calcs

Fluid category

Release pressure (bar(a))

Release flow rate (kg/s) Ventilation flow rate to dilute to 25% LFL (m3/s)

Gas cloud volume above LFL, (m3)

Release hole diameter Release hole diameter Release hole diameter 1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm

A 6.81 0.01 0.05 0.29 (1.17)1 1.14 4.56 28.5 (114.) 0.020 0.145 2.40 (21.9) 10 0.02 0.06 0.381 (1.50) 1.46 5.84 36.5 (146.) 0.027 0.203 3.37 (32.2) 50 0.04 0.14 0.88 (3.50) 3.40 13.6 85.1 (340.) 0.050 0.513 8.94 (92.6)

100 0.05 0.20 1.24 (4.97) 4.83 19.3 121. (483.) 0.078 0.648 11.70 (122.) B 5 0.01 0.05 0.30 (1.18) 0.99 3.94 24.6 (98.4) 0.074 0.559 12.49 (103.)

10 0.02 0.07 0.44 (1.76) 1.47 5.86 36.6 (147.) 0.111 0.933 19.96 (163.) 50 0.04 0.16 1.02 (4.09) 3.41 13.6 85.3 (341.) 0.121 0.978 18.66 (191.)

100 0.06 0.23 1.45 (5.81) 4.84 19.4 121. (484.) 0.116 1.014 18.52 (194.) C 5 0.01 0.05 0.31 (1.23) 0.94 3.74 23.4 (93.6) 0.110 0.684 8.48 (62.0)

10 0.02 0.07 0.46 (1.84) 1.40 5.62 35.1 (140.) 0.113 0.984 19.66 (161.) 50 0.04 0.17 1.07 (4.29) 3.27 13.1 81.8 (327.) 0.121 1.051 19.29 (190.)

100 0.06 0.24 1.52 (6.10) 4.65 18.6 116. (465.) 0.124 1.061 18.95 (196.) G(i) 5 0.001 0.002 0.015 0.06 0.07 0.27 1.70 6.79 0.00006 0.0006 0.007 0.059

10 0.001 0.005 0.032 0.13 0.14 0.56 3.48 13.9 0.0002 0.0018 0.028 0.223 50 0.007 0.027 0.170 0.68 0.75 2.99 18.7 74.7 0.0031 0.025 0.364 3.059

100 0.015 0.059 0.370 1.48 1.63 6.51 40.7 163. 0.0096 0.076 1.33 11.389 G(ii) 5 0.0004 0.0015 0.01 0.04 0.13 0.52 3.23 12.9 0.0005 0.004 0.040 0.445

10 0.001 0.003 0.02 0.07 0.26 1.05 6.57 26.3 0.0010 0.009 0.152 1.297 50 0.004 0.02 0.1 0.4 1.33 5.31 33.2 133. 0.0157 0.115 1.896 16.02

100 0.008 0.03 0.2 0.7 2.64 10.6 66.1 264. 0.0351 0.305 5.461 50.07

1. The saturated vapour pressure of Fluid category A is 6.8 bar(a) at 20 °C and this pressure is used to calculate the discharge rate.

2. Note that the cases in brackets are those for which a Zone 2 classification is inappropriate for outdoor releases (EI15) and therefore are by default also inappropriate for a classification of Zone 2 indoors. The data are therefore provided for information only.

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Table 3 Comparison of cloud volumes above LFL for zero background concentration

and ventilation dilution to 25% LFL using QUADVENT Fluid

category Release pressure (bar(a))

Gas cloud volume above LFL, zero background concentration

(m3)

Gas cloud volume above LFL, background concentration 25% LFL

(m3) Release hole diameter Release hole diameter

1 mm 2 mm 5 mm 10 mm 1 mm 2 mm 5 mm 10 mm G(i) 5 0.0001 0.0008 0.01 0.1 0.0002 0.002 0.03 0.24

10 0.0003 0.002 0.04 0.3 0.0007 0.005 0.08 0.67 50 0.003 0.03 0.4 3.2 0.007 0.06 0.91 7.30

100 0.009 0.07 1.13 9.0 0.02 0.16 2.57 20.6 G(ii) 5 0.0007 0.006 0.09 0.7 0.002 0.01 0.20 1.58

10 0.002 0.02 0.24 1.92 0.004 0.04 0.55 4.37 50 0.02 0.17 2.64 21.1 0.05 0.39 6.01 48.1

100 0.06 0.48 7.46 59.7 0.16 1.08 17.0 135.8

3.4 SUMMARY

Three key standards or guidance documents on area classification provide three different metrics and approaches to defining adequate ventilation and hence the conditions required for defining a space as Zone 2: The current edition of the international standard on area classification, BS EN 60079-10-1:2009 uses a hypothetical gas cloud volume, Vz, to differentiate between different levels of ventilation. The Institute of Gas Engineers and Managers guide IGEM/SR/25 uses the average gas concentration within the enclosure during a release. And the current version of EI15 uses a definition of adequate ventilation based on a requirement of 12 air changes per hour within the enclosure.

The main disadvantage of the approach currently used in EI15 is that the requirement is not related to the size of the release and therefore the same level of ventilation is required for a room containing potentially large releases as one that may contain very small releases. Also, a definition of adequate ventilation in terms of an air change rate will require huge flow rates of air to achieve the criterion in large enclosures. However, in practice the dispersion of releases in large enclosures will behave in a similar manner to a release outdoors and therefore the air change rate is not critical to the gas dispersion.

The concept of Zone 2NE is used in BS EN 60079-10-1:2009 and IGEM/SR/25 for cases where the ignition of a secondary release has a negligible effect (NE) and therefore no special protective measures are required. Quantitative criteria have previously been developed to define when this negligible effect zone can be used. The criteria are based on a requirement that the gas cloud volume, Vz, is less than 0.1 m3 and it has been shown that limiting the average gas concentration at the ventilation outlet to 10% LFL together with other limitations generally leads to achieving the 0.1 m3 criterion.

In an area classified as Zone 2, specific measures are taken to prevent the ignition of the flammable gases/vapours. Therefore the aim of the ventilation in this case is not to dilute the released volume down to a specified ‘safe’ level as it is for Zone 2NE. Rather it is to limit the size of the flammable region to ensure that it stays within the zoned area (which may be the whole enclosure) and does not continue to grow. It therefore is not appropriate to define the adequacy of the ventilation based on some criterion for a ‘safe’ gas cloud volume, as it is for

22

Zone 2NE, as it is accepted that from time to time there will be accumulations of flammable mixtures within the zone.

However, area classification is not designed to account for catastrophic failure, so some limit needs to be put on the size of release in an enclosure that area classification can be applied to. Such a limit is already provided in EI15 for outdoor releases, and is based on releases that lead to a hazard radius greater than 30 m.

It has been shown that limiting the average concentration to 25% LFL in the enclosure generally achieves the objective of the ventilation for a Zone 2 area as defined above. This criterion is also consistent with IGEM/SR/25. The ventilation rates provided in Table 2 provide the minimum level of ventilation required to ensure that the average concentration at the outlet stays below 25% LFL. However, this only demonstrates that the overall level of ventilation is sufficient and it does not demonstrate that the local ventilation is adequate. It is the local ventilation effectiveness that will determine how the release disperses. It is therefore important that the 25% LFL criterion should be a minimum requirement as other factors need to be taken into account. If such a criterion is only just met then this is a good indication that further effort may be required to ensure that the ventilation local to the release point is indeed adequate.

While use of the 25%LFL criterion does achieve the above objectives, large gas clouds may still occur, particularly in large enclosures. In these cases it may be advisable to try and reduce the potential leak size.

In some cases, depending on the size of the leak relative to the enclosure and taking into account the average concentration at the outlet, it may only be necessary to classify part of the enclosure as Zone 2. In others cases it may be more appropriate to zone the whole enclosure. A simple approach that can be used to define the zone extent is provided in the next Section.

23

4 VENTILATION CORRECTION FACTOR

4.1 INTRODUCTION

The joint industry project reported by Ivings et al (2008) has shown that the presence of obstacles and obstructions can have a significant effect on the dispersion of flammable gases. The resulting gas cloud volume for a confined or congested release of gas can be two to three orders of magnitude larger than an equivalent unobstructed case. While the gas cloud volume is not of direct interest itself in defining whether or not the ventilation is adequate8, the increase in its volume demonstrates that the local ventilation is becoming less effective at dispersing the gas. Confinement or congestion may have the effect that the average concentration in the enclosure increases, while, for a steady release, the average concentration at the ventilation outlet remains constant. For gas releases in large enclosures, or similarly for releases outdoors, the presence of congestion or confinement is likely to be a more important factor in determining the gas cloud volume than the ventilation rate (or wind speed) itself. It is therefore important that such factors are taken into account in a hazardous area classification methodology. It is critical that this effect is not underestimated as it could lead to the prediction of non-conservative gas cloud volumes.

The “efficiency of the ventilation” has been introduced in BS EN 60079-10-1:2009 as a means of accounting for the effects of local congestion and confinement on the ventilation flow. Hence the efficiency of ventilation, f, is defined as the efficiency9 of the ventilation in terms of its effectiveness in diluting the explosive gas atmosphere, with f ranging from f = 1 (ideal situation) to, typically f = 5 (impeded air flow). In practice this factor is used to scale the flammable gas cloud volume, Vz, such that it results in a gas cloud volume up to five times greater than an equivalent unimpeded case.

Two key limitations are apparent with the above approach of using a value of 1 to 5 for the efficiency of ventilation to scale the gas cloud volume. Firstly, in reality gas cloud volumes can be far greater than five times the size of the equivalent unimpeded case and, secondly, this approach does not take into account in any way the physics of gas dispersion or ventilation flows. The scaling of the gas cloud volume by the arbitrarily chosen value of 1 to 5 therefore is effectively applied as a safety factor, but there is no justification for its use and no guarantee that it is a conservative approach.

In this Section, we attempt to develop a more soundly-based methodology for taking into account the effects of congestion and confinement on gas dispersion which can be used for hazardous area classification.

4.2 THE EFFECTS OF CONFINEMENT / CONGESTION ON GAS DISPERSION

For a given leak, defined in terms of the gas pressure, hole size and its location and direction in an enclosure, the dispersion of the gas will be determined by the ventilation rate, the distribution of the ventilation and the interaction of the dispersing gas with obstacles and obstructions. Pressurised releases of gases tend to mix rapidly with the surrounding air due to the shear induced turbulence generated by the jet momentum. The presence of small-scale individual

8 This is because, for a Zone 2 area, we are not seeking to limit the gas cloud volume to any defined ‘safe’ level. In theory any size of gas cloud could be acceptable as long as it doesn’t leave the zone. 9 It should really be the inefficiency of the ventilation, as a large number indicates less well-mixed conditions

24

obstacles in the jet can reduce the momentum of the jet to a small degree but will also lead to additional turbulence being generated downstream of the obstacle. The overall effect on the gas cloud is that the volume may increase, but probably by only a small amount (Ivings et al., 2008). Larger arrays of obstacles on the other hand may have a more significant effect on the gas jet and will additionally limit the local ventilation flow. Large-scale obstructions, including walls and floors, can have a far more significant effect on the gas cloud volume when they are arranged in such a way that it leads to recirculating flows. These recirculation zones mean that instead of the jet entraining clean air and becoming diluted, it entrains further flammable gas and a significantly larger gas cloud volume results.

It is therefore the local background concentration of flammable gas, which itself is determined by the presence of large obstructions and obstacles, that is critical to the gas cloud build up. In such obstructed cases the local background concentration may be very much greater than the average concentration at the outlet. Unfortunately, while the latter is easy to calculate, the local background concentration is the more important factor in determining the gas cloud volume, but it is not possible to calculate it without carrying out detailed modelling.

When the ventilation flow is poorly distributed throughout the enclosure, or the release is obstructed leading to recirculation and gas cloud build up, the average concentration throughout the enclosure may be greater than the average concentration at the outlet. This departure from a well-mixed condition is therefore important in determining the size of the flammable volume and if we can make a realistic estimate of this effect it can be used to help determine a more realistic gas cloud volume.

The recent paper by Webber et al. (2011) describes simple ventilation theory that makes clear the distinction between the average concentration at the outlet and the average concentration within the enclosure. This is then used in a simple model for predicting the size of the flammable gas cloud volume Vz. The main relevant findings from this paper are summarised below.

4.2.1 Ventilation theory

Consider a room of volume V0 containing a source of hazardous gas that is released at a rate qs (volume per unit time). The rate of air inflow from ventilation in to the room is q0 and the total outflow is q1, as shown in Figure 2.

Assuming that the concentration (kg/m3) of gas at the outflow is C1, a balance of fluxes yields:

sqqq += 01 (4.1)

110 qCqCdt

dCV ss

b −= (4.2)

where Cs is the concentration at the source (which we assume to be pure flammable gas) and Cb is the background concentration.

The solution of these equations requires an assumption about how well-mixed the air is within the room away from the jet. If this is optimally efficient, then the background concentration will be essentially uniform, and we will have C1 = Cb. More generally

bCC ε=1 (4.3)

25

with a constant ε , which is a measure of the degree to which the air in the enclosure outside of the release zone is well mixed and a value of ε = 1 defines a well-mixed room volume. If the ventilation flux enters and leaves in a part of the room distant from the gas source, then we may expect ε < 1: the room is ventilated but the background mixing is such that the air is not having the optimal effect in diluting the jet.

While values of ε < 1 are of primary interest, values greater than one will occur if the gas is removed from the enclosure before it has time to become well mixed within the enclosure volume. Figure 3 below illustrates an example of this where the gas release is directed at the ventilation outlet.

Values much less than one will occur where the release is located in a poorly ventilated region, or there are significant recirculation regions around the release point. An extreme illustration of such a case is shown in Figure 4, where the ventilation short-circuits the enclosure and the release point is shielded from the ventilation flow (and in this extreme example you would expect 1<<ε ).

The above equations are solved in a straightforward manner in the steady state limit )( ∞→t to give

)( 01 qqq

Cqq

CCs

ss

ssb +

==εε

(4.4)

Figure 2 Volumetric fluxes in a ventilated room

26

q0

q1

qs

Figure 3 An illustrative example of a case with where efficiency of background mixing, ε > 1

q0

q1 qs

Figure 4 An illustrative example of a case with poor efficiency of background mixing, ε < 1

4.2.2 Quadvent

The flammable gas cloud volume Vz is defined as the gas cloud volume with an average molar concentration of xcrit, which for secondary gas releases is usually taken to be ½ LFL. Vz can be estimated using the Quadvent model (Webber et al., 2011) as follows:

critb

critbbcrit

b

s

bsZ

xxV

xxVxx

xrV

≥=

<

−

−

=

;

;,1

169

min

0

0

32/33

ρρ

απ

(4.5)

where rs is the pseudo source radius

−+= 9.15.010

as P

Prr , (4.6)

bρ and sρ are the background and source densities respectively, P and Pa are the stagnation and ambient pressures respectively, V0 is the enclosure volume, and α is the entrainment coefficient (recommended value 0.05).

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The background molar concentration of gas, xb, is calculated from the ratio of the gas release rate to the ventilation rate, accounting for the efficiency of background mixing:

1q

qx s

b ε= (4.7)

For any given size of hole, the Quadvent model demonstrates the following relationship between the hazardous volume and the room background concentration. For low background concentrations the dependence is weak: the hazardous volume depends essentially entirely on the properties of the jet, i.e. the behaviour is like that of a release outdoors. For higher background concentrations, the hazardous volume depends more strongly on the background concentration. In this case the hazardous volume depends both on the source and on the degree of ventilation, and may also be sensitive to the efficiency of background mixing, ε.

Following the Quadvent model approach, the axial distance, z, to some appropriate volumetric concentration x can be derived as an approximation to the zone extent

( )( )b

s

xxxrz

−−

=1

µ (4.8)

where

b

s

ρρ

αµ 2≡ (4.9)

This calculation only makes sense if the background concentration is less than the zone extent concentration, i.e. xb < x, otherwise the zone would extend throughout the enclosure. This equation is applicable to releases of flammable gas but not EI15 Fluid Categories A, B and C.

4.2.3 Summary

The above shows how we would expect the gas cloud volume to be affected by a departure from well-mixed conditions within the enclosure characterised by the efficiency of background mixing

bxx1=ε (4.10)

This factor effectively scales (down) the ventilation rate in the model of the gas cloud volume and we see that, as the average concentration at the outlet x1 approaches xcrit, then ε becomes an important factor. For example if the conditions in the enclosure lead to a situation in which ε = ½, i.e. less than optimal efficiency of background mixing, then this is equivalent to a halving of the ventilation rate. Or, as another way of looking at it, if you have poor mixing equivalent to ε = ½, then, according to the Quadvent model, you would need to have twice the level of ventilation to maintain the gas cloud volume to the same size as the equivalent well-mixed case.

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The above provides a quantitative description of how the efficiency of the background mixing affects the gas cloud volume which can be incorporated into an area classification methodology by scaling the ventilation rate. The way in which confinement and congestion affect gas cloud build up are not the same as the efficiency of the background mixing. The latter represents the non-uniformity of the background concentration, which is affected by the distribution of the ventilation flow through the space. Confinement and congestion on the other hand affect the jet’s self-dilution effect.

In the next Section we consider whether this efficiency of background mixing can be used to represent the effects of confinement and congestion. The modelling work that was carried out for the joint industry project on area classification (Ivings et al., 2008) allows us to compute directly the average concentration at the outlet and the average background concentration for a range of scenarios and hence provide realistic values of ε.

4.3 ANALYSIS OF JIP DATA

The joint industry project on area classification (Ivings et al, 2008) carried out about 50 simulations of methane gas dispersion in a ventilated enclosure. A wide range of release rates, pressures, hole sizes, ventilation rates, release locations and directions were considered. Most of the simulations, including all those used to validate the CFD model, were carried out in a 45 m3 enclosure that was 4 m wide, 4 m long and 2.92 m high. The majority of these simulations fell into three categories, based on the release location and direction. These simulations were carried out to assess the effect of differing levels of confinement on the gas dispersion. The three configurations are shown in Figure 5. The first configuration is essentially a free jet directed into the middle of the room. Configuration 2 consists of a jet running parallel to a side wall and the final configuration consists of a leak in a confined space located within a 50 cm wide gap between a large box and a wall. For each of these configurations a range of release rates and ventilation rates were modelled.

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Figure 5 Arrangements of the room, nozzle and box for Configurations 1, 2 and 3

Configuration 1

Configuration 2

Configuration 3

Fresh air drawn in through openings

Gas jet

Mixture of gas and air extracted

Mixture of gas and air extracted

Gas jet within cavity impinges onto the wall

Gas jet (close to far wall)

Fresh air drawn in through openings

Fresh air drawn in through openings

Mixture of gas and air extracted

30

4.3.1 Analysis of data for ε

For each simulation, the average gas concentration within the enclosure and the average gas concentration at the outlet10 have been calculated from the CFD simulations, as shown in Table 4. The ratio of these two values can then be calculated, using Equation 4.10, to give the factor ε. This analysis has been applied to the simulations where data exists for comparable cases (i.e. with the same leak and ventilation rates) across all three configurations. For example, the leak rates and ventilation rates are all the same for cases C1-10, C2-10 and C3-10 but the release location and direction are different.

The first thing to notice is that most values of ε are greater than one, which is generally due to the gas release exiting the enclosure before it has had time to become well-mixed within the enclosure. Considering again the cases C1-10, C2-10 and C3-10, as expected the gas cloud volume Vz is smallest for case C1-10 and largest for case C3-10. However, this does not correspond uniformly with a decreasing ε in each case. Actually ε is smallest for case C3-10, but the largest value of ε is for case C2-10. Examination of the geometry of the release location and direction, and the position of the vents for this case makes it clear why this is the case. The release is being directed towards the wall containing the ventilation outlets so that much of the room will not contain any flammable gas. While the side wall slows down the dilution of the gas cloud, leading to a larger Vz than case C1-10, the gas is effectively short circuiting the room.

In fact this example is fairly typical of the results as a whole. There is no clear correspondence between the value of ε and the size of the flammable cloud volume, nor is there a uniform decrease in ε as we move from the least constrained release, C1, through C2, to the most constrained releases, C3. The size of the gas cloud and the ratio of the average background concentration to the average outlet concentration, ε, depend on the details of each individual case.

Overall the results show a range of values of ε including many cases where ε is greater than 1. The smallest value of ε is 0.79 for case C3-9, which is a case where the gas source is in a confined space. In this case the gas cloud volume is 142 times larger than the equivalent unobstructed case (see the 4th column from the right which shows the ratio of the gas cloud volume to the equivalent free jet case (C1)). The cases considered in this analysis are rather limited as they are based on existing data. Ideally, to provide a better understanding of the effect ε on the gas cloud build up, a large number of CFD simulations would be undertaken specifically designed to extract the information required. The presented results for the C3 series of data do however indicate that in cases where ε is low11, (C3-9, C3-5, C3-10) larger gas cloud volumes are seen compared to the equivalent free jet case (C1-9, C1-5, C1-10). The values of ε obtained through this analysis should not be used to limit values of ε used in practical application of area classification.

A similar analysis to that described above has been carried out for three simulations carried out in a larger, 400 m3, enclosure. The details of the three cases can be found in Ivings et al. (2008), the results are shown in Table 5. The first case is a gas release into the centre of the enclosure which we take as the base case. The second two simulations consist of a release into a confined space in a corner of the enclosure. The second of these includes the effects of a stratified environment within the enclosure on the gas cloud build up. In this case the resulting stratification acts to reduce mixing in the enclosure and leads to a larger gas cloud volume.

10 Note that the average concentration at the outlet is taken from the CFD model rather than from the design conditions and therefore, as the CFD model solution isn’t steady, the CFD concentration may not be equal to the theoretical average concentration. 11 Excluding the cases where the gas cloud volume fills the enclosure even for the unobstructed case, C1-4 and C1-8

31

As in the previous analysis, the results show values of ε greater than unity. The reasons for this can clearly be seen by looking at the release location and direction, which in the first case is pointing towards the wall containing the ventilation outlet and in the other two cases the release location is in the corner of the room below the ventilation outlet. So while the confinement in the latter two cases leads to a larger gas cloud volume, most of the enclosure contains only fresh air thereby reducing the average concentration throughout the enclosure.

For the larger enclosure results, the gas cloud volume clearly become larger as ε decreases. For the third case, where a confined release location and thermal stratification act to increase the gas cloud volume, ε takes the lowest value of the three cases (a value of 1.11).

In summary, the above analysis has shown that there is no clear decrease in ε as the confinement of the release location is increased. This is not unexpected as the details of the release scenario are important in determining the average concentration in the enclosure and the gas cloud volume. However, it is clear that in many cases a lower value of ε does correspond with a larger gas cloud volume.

4.3.2 Analysis of data using Quadvent

A rather more pragmatic way of analysing the data to determine suitable values of ε for use in area classification is to calculate the increase in gas cloud volume that results from different levels of confinement using the CFD simulation results and then work out what value of ε in the Quadvent model is required to give that same level of increase in gas cloud volume. So if for example, the CFD results show that the gas cloud volume is five times greater for an obstructed release than for an unobstructed release, for a given release rate and ventilation rate, then the value of ε can be found that gives a five fold increase in the gas cloud volume using the Quadvent model. This should provide a realistic estimate of appropriate values for ε and therefore an approach for taking into account the effect of obstructions for area classification. Again this analysis makes the assumption that the efficiency of background mixing can be used to account for the effects of local confinement and congestion even though the physics of these two phenomena are different.

The results of this analysis are presented in Table 4 which shows the CFD model predictions for the gas cloud volume, Vz, and in the next column the ratio of this volume compared to the equivalent unobstructed case (Vz/Vzfree). For the unobstructed cases the Quadvent model predictions for the gas cloud volume Vz are also provided, where it can be seen that these are in reasonable agreement with the CFD model predictions. The next column then provides the Quadvent model result scaled up by the factor (Vz/Vzfree) calculated from the CFD model predictions. The final column in the table then provides the value of ε predicted by Quadvent to be needed to give the scaled Quadvent model volume from the previous column. The same analysis has also been carried out for the larger enclosure volume, see Table 5.

In Table 4 there are two cases (C1-4 and C1-8) where the gas volume Vz fills the enclosure volume and therefore these cases are omitted from the analysis.

For the 45 m3 size enclosure (Table 4) the results show values of ε ranging from about ½ to 1, that account for the effect of the confinement of the release location on the gas cloud volume. Therefore this indicates that, in these cases, incorporation of a value of ε of about ½ in an area classification methodology will take account of the effects of the confined release location.

For the larger enclosure volume (Table 5) much smaller values of ε are needed to account for the effects of the confined release location. The smallest value is about 10

1 . This is not

32

surprising when you consider that the average concentration at the outlet (4.4% LFL) in this case is well below the critical concentration (50% LFL) and we have shown above that the value of ε has a much more significant effect when the average concentration at the outlet approaches the critical concentration. This finding clearly highlights the fact that for large enclosures the average concentration at the outlet becomes far less important and the local ventilation effectiveness is the determining factor. For large enclosures the dispersion behaviour becomes more similar to that for an outdoor release. In both cases, there is a large volume of clean air that is potentially available to mix with the flammable gas, as long as the area local to the release can mix with it.

In summary, this analysis has shown that the efficiency of mixing, ε, can be used to indicate whether the gas cloud volume will be sensitive to local confinement and congestion. If the release rate is sufficiently large relative to the enclosure volume such that the average concentration at the outlet is of a similar order to 50%LFL, then using values of ε of ½ or 3

1 for releases in confined / congested spaces will indicate whether or not the overall ventilation rate is adequate. However, if the average concentration at the outlet is significantly smaller than 50% LFL, then high degrees of congestion and confinement can still lead to gas cloud build up significantly greater than the equivalent unobstructed case. For these highly congested cases it is not appropriate to consider the room ventilation rate, or the room ventilation rate modified by the efficiency of mixing, as the gas dispersion only depends weakly on it. Instead an assessment of the local ventilation effectiveness needs to be carried out or the extent of the zone needs to be determined appropriately (i.e. increased).

33

Table 4 Analysis of data on gas releases in a 45 m3 ventilated enclosure Test No. Gas

Release Rate

Ventilation Rate

pressure hole size Mean room conc., xb

Mean outlet conc., x1

ε (= x1/ xb)

CFD Vz CFD Vz / Vzfree

Quadvent Vz

Scaled up Quadvent

Vz

Scaling ε

(g/s) (hr-1) (barg) (mm2) (%LFL) (%LFL) (m3) (m3) (m3) C1-3 0.26 6 5 0.25 10.7 12.0 1.12 0.0015 1 0.00212 C1-4 0.47 2 10 0.25 55.5 59.3 1.07 45 1 45 C1-5 0.47 6 10 0.25 19.2 21.5 1.12 0.0082 1 0.0122 C1-6 0.47 12 10 0.25 9.4 10.6 1.12 0.0039 1 0.00478 C1-7 0.49 12 0.3 2.5 9.8 11.2 1.14 0.0075 1 0.0120 C1-8 0.86 2 1 2.5 95.8 100.4 1.05 45 1 45 C1-9 0.86 6 1 2.5 34.4 37.7 1.10 0.24 1 0.599 C1-10 0.86 12 1 2.5 17.0 19.8 1.16 0.015 1 0.0276 C2-3 0.26 6 5 0.25 10.6 11.9 1.13 0.0017 1.13 0.00240 0.88 C2-4 0.47 2 10 0.25 52.6 57.9 1.10 0.14 0.003 C2-5x 0.38 6 10 0.25 15.0 17.4 1.16 0.0084 1.02 0.0125 0.99 C2-6 0.47 12 10 0.25 8.5 10.2 1.20 0.0048 1.23 0.00588 0.80 C2-7 0.49 12 0.3 2.5 9.3 10.7 1.15 0.0122 1.63 0.0195 0.65 C2-8 0.86 2 1 2.5 94.9 101.9 1.07 33 0.73 C2-9 0.86 6 1 2.5 33.4 37.9 1.13 0.38 1.58 0.948 0.96 C2-10 0.86 12 1 2.5 16.2 20.7 1.28 0.033 2.20 0.0608 0.74 C3-3 0.26 6 5 0.25 10.8 12.0 1.12 0.0017 1.13 0.00240 0.88 C3-4 0.47 2 10 0.25 64.0 59.0 0.92 44 0.98 C3-5 0.47 6 10 0.25 25.2 22.5 0.89 0.17 20.7 0.254 0.54 C3-6 0.47 12 10 0.25 9.2 10.8 1.17 0.011 2.82 0.0135 0.48 C3-7 0.49 12 0.3 2.5 9.7 11.5 1.19 0.02 2.67 0.0319 0.50 C3-8 0.86 2 1 2.5 109.0 103.8 0.95 45 1.00 C3-9 0.86 6 1 2.5 41.6 33.0 0.79 34 142. 84.8 0.82 C3-10 0.86 12 1 2.5 18.4 19.5 1.06 0.89 59.3 1.64 0.46

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Table 5 Analysis of data on gas releases in a 400 m3 ventilated enclosure Test No. Gas

Release Rate

Ventilation Rate

pressure hole size Mean room conc., xb

Mean outlet conc., x1

ε (= x1/ xb)

CFD Vz CFD Vz / Vzfree

Quadvent Vz

Scaled up Quadvent

Vz

Scaling ε

(g/s) (ach) (barg) (mm2) (%LFL) (%LFL) (m3) (m3) (m3) Unobstruct

ed 0.86 6 1 2.5 2.3 4.2 1.78 0.00593 1 0.00839

Obstructed 0.86 6 1 2.5 2.9 4.2 1.44 0.026 4.38 0.036786 0.196 Non-

isothermal 0.86 6 1 2.5 2.7 3.0 1.11 1.0 168.6 1.41484

0.105

35

4.4 SUMMARY

The build up of flammable gas or vapour following a release of a pressurised fluid can be strongly affected by local obstructions to the resulting jet and to the ventilation flow. Both have a very significant effect on the dispersion of the flammable gas and the resulting size of gas cloud. It is therefore very important that this effect is taken into account in an area classification methodology.

BS EN 60079-10-1:2009 uses an approach to account for “the efficiency of the ventilation” that scales the gas cloud volume Vz (which is then used to determine the zone) by a factor, f, between 1 and 5. However, in reality, the gas cloud volume in confined or congested spaces can be over 100 times larger than the equivalent unobstructed case.

In addition to increasing the gas cloud volume, confinement and congestion are likely to increase the persistence time of a release and increase the chance that the gas cloud volume will extend beyond the zoned area predicted using BS EN 60079-10-1:2009. In such cases it may not be possible to describe the space as adequately ventilated based solely on the overall ventilation rate of the enclosure.

In this section, we have shown how the gas cloud volume depends on the distribution of the ventilation within the enclosure which has been described by the efficiency of mixing, ε. Note that this factor is not the same as the factor, f, used in BS EN 60079-10-1:2009 which simply scales the gas cloud volume. We have also shown how an initial assessment of the effects of congestion and confinement can be made by using the efficiency of mixing, which effectively scales the ventilation rate. This approach is suitable if the average concentration at the outlet is between, roughly, 1% and 50% LFL. In other cases where the gas release is small relative to the enclosure volume, e.g. for large enclosures, then the gas cloud build up only depends weakly on the overall ventilation rate and the efficiency of mixing cannot be used to highlight cases where the (local) ventilation is not adequate.

Based on this approach, a pragmatic approach to assessing the adequacy of the ventilation for releases in confined or congested locations is to firstly multiply the actual ventilation rate by the factor ε and assess whether the resulting ventilation rate still meets the 25% LFL ventilation criteria. For a moderate level of confinement/congestion a factor of ½ should be used and for slightly higher levels of confinement/congestion a value of 3

1 should be used. If this criterion is met then this indicates that the global ventilation rate is sufficient. It then just remains to check that the local ventilation is adequate.

If the degree of confinement/congestion is particularly high or the ventilation is poorly distributed within the enclosure then the ventilation rate scaled by ε is unlikely to indicate whether or not the local ventilation is adequate. The reason for this can be clearly seen by looking at the exaggerated case shown in Figure 4 where it can be seen that the room ventilation rate will have little effect on the gas dispersion. Judging whether or not the room ventilation rate scaled by ε is an appropriate way of assessing the adequacy of the ventilation for diluting a specific release can only be made by visual inspection. If it isn’t then in these cases an assessment of the local ventilation effectiveness needs to be carried out or the extent of the zone needs to be determined appropriately (i.e. increased). Carrying out smoke tests would be one such approach that could be used for assessing the local ventilation effectiveness. The degree to which the 25% LFL criterion is met, taking into account a suitable value for the efficiency of mixing, ε, provides a very useful indication of the amount of effort needed in these additional assessments.

36

To determine whether or not an area should be classed as congested or confined, it is useful to assess the number and size of obstructions within the distance in which it takes for an equivalent unobstructed release to dilute to below the flammability limit. A simple approach applicable to pressurised gas releases for calculating this distance is provided above but other more generic approaches can be used.

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5 EXAMPLE CALCULATIONS

5.1 NATURAL VENTILATION RATE CALCULATIONS

The calculation of natural ventilation rates is described in Section 2.2 and example calculations are presented in this section. The quantity of interest in these calculations is the predicted ventilation flow rate for an enclosure rather than the air change rate. The examples are based on measurements described in Ivings et al (2008) and additional information can be found in that report.

5.1.1 Brick kiosk

5.1.1.1 Description of kiosk

The enclosure was a brick built kiosk housing a low-pressure twin stream district gas regulator. There were no hot surfaces inside the building, Figure 6. The dimensions of the enclosure were 8.3 m by 5.09 m by 2.65 m.

Figure 6 Brick built Kiosk housing a low-pressure twin stream district gas regulator.

The provision for natural ventilation consisted of four airbricks on each of the two long sides of the building; two at high and two at low positions. The airbricks were terracotta with an overall size of 0.225 m by 0.225 m. Square holes in the bricks gave an equivalent open area of 0.0064 m2.

In addition to the planned ventilation openings there were double doors on both ends of the building and a single door on the long front side of the building. There were also numerous other unplanned openings, including gaps around the door and doorframe, and between the walls and the roof. Removal of equipment had resulted in a hole of approximately 0.20 m in the rear long side of the enclosure, Figure 7.

38

Figure 7 Photo taken from inside the building of the rear long side of the building.

5.1.1.2 Ventilation measurements

Measurements were made of the air change rate, with a tracer using the step down method. The wind velocity was recorded at a height of 4.7 m during the air change rate measurements. The observations are shown in Table 6.

Table 6 Measurements of air change rates and conditions

Test Air change rate (h-1)

Air flow rate (m3s-1)

Wind velocity (ms-1)

Temperature inside (ºC)

Temperature outside

(ºC)

1 4.5 0.14 2.8 18.4 17.5

2 3.7 0.12 2.2 19.4 17.1

5.1.1.3 Calculation of ventilation flow rates

In this example natural ventilation flow rates due to both wind and buoyancy were calculated for the planned openings. In addition the effect of the 0.20 m diameter unplanned opening on the ventilation due to wind was also calculated. The ventilation grills were on the long side of the enclosure so it was assumed that the wind was perpendicular to this face.

39

The air flow due to the wind through either the upwind or downwind planned ventilation openings can be calculated using Equation 2.3. The coefficient of discharge, dC , for planned ventilation openings is given a value of 0.61.

The ventilation area on the long faces of the kiosk, upwind and downwind, is the sum of the brick ventilation areas, A1 = A2 = Abricks= 4 × 0.0064 = 0.0256 m2. The equivalent area, Ae, can be calculated from Equation 2.4, giving Ae = 0.0256 m2.

The wind speed, U, is the undisturbed speed at the reference height used for the measurement of the pressure coefficient. The reference height for the pressure coefficients in BS5925, Table 13 (BSI, 1991) is the height of the roof of the structure which in this case is referencez = 2.65 m. The measurements of wind speed, measuredU , were made at a height, measuredz = 4.7 m, assuming the kiosk is in an urban area, the equivalent velocity at the building height can be calculated using information from BS5925 (BSI, 1991)

a

zzUU

=

measured

referencemeasured (5.1)

where a is a value for urban conditions, taking the value 0.25, BS5925, Table 8 (BSI, 1991).

The building pressure coefficient can be found from BS5925, Table 13 (BSI, 1991), for the geometry of the kiosk with long sides perpendicular to the wind the building pressure coefficient, pC∆ = 1.0.

The ventilation flow rates calculated using the wind velocity at the height of the kiosk are compared with observed flow rates in Table 7.

Table 7 Comparison of observed and calculated wind driven ventilation flow rates

Velocity (ms-1)

Flow rate (m3s-1)

Measured at 4.7 m Reference at 2.65 m Observed Calculated

2.8 2.4 0.14 0.03

2.2 1.9 0.12 0.02

A coefficient of discharge of 0.61 is appropriate for the unplanned opening of 0.2 m diameter as it is an opening with well-defined edges and area, not a crack or a gap. The effect on the area of the openings is to add an opening of 0.03 m2 to the downwind ventilation area, A2 = Abricks + A unplanned= 4 × 0.0064 + 0.0314 = 0.0570 m2. The equivalent area therefore becomes Ae = 0.0330 m2. Including the unplanned ventilation opening in the wind driven ventilation area increases the equivalent area, and hence the ventilation flow rate, by 29%.

The air flow due to buoyancy through either of the lower planned ventilation openings can be calculated using Equation 2.7. The coefficient of discharge, dC , for planned ventilation openings is again given a value of 0.61.

40

The distance between the centres of the lower and upper ventilation bricks, H, was estimated as being 2 m. The calculated ventilation flow rates due to buoyancy are compared to the observed flow rates in Table 8.

Table 8 Comparison of observed and calculated buoyancy driven ventilation flow rates

Temperature (ºC)

Flow rate (m3s-1)

Internal External Observed Calculated

18.4 17.5 0.14 0.004

19.4 17.1 0.12 0.006

The air flow rates calculated using the analytical solutions for the planned openings with the measured wind velocity referenced to the height of the kiosk are much lower than those measured using tracer tests, by a factor of five or six. Information was not available for the wind direction during the measurements but different wind directions would not have a large effect on flow rate and the rates calculated would be the largest for the planned openings. Consideration of one unplanned opening did increase the flow rate, but only by 30%. Ventilation flow rates due to buoyancy alone were also calculated but these were predicted to be an order of magnitude less than those due to the wind. Observations showed that there were significant unplanned openings due to cracks and gaps and these are probably the reason for the difference between measured and calculated ventilation flow rates.

5.1.2 GRP kiosk

5.1.2.1 Description of kiosk

The enclosure was a new GRP kiosk housing a low-pressure district gas regulator, Figure 8. There were no hot surfaces within the kiosk. The dimensions of the enclosure were 3.9 m × 3 m × 2.42 m.

41

Figure 8 GRP kiosk. Grilles can be seen at low level

At low level the provision for natural ventilation consisted of two grilles on each wall. Each grille had dimensions 0.20 m × 0.265 m with a free air space of 0.01258 m2. The ventilation area due to the grilles on each face, Agrille, was therefore 0.02516 m2. A continuous gap, 0.01 m wide, between the walls and the roof of the kiosk provided for high level ventilation. On the long faces of the kiosk the ventilation area provided by the gap, Agap, was 0.039 m2 ventilation area while on the short sides it was 0.03 m2.

5.1.2.2 Ventilation measurements

Measurements were made of the air change rate, with a tracer using the step down method. The wind velocity was recorded at a height of 4.7 m during the air change rate measurements. The observations are shown in Table 9.

Table 9 Measurements of air change rates and conditions

Test Air change rate (h-1)

Air flow rate (m3s-1)

Wind velocity (ms-1)

1 11.9 0.09 3.3

2 15.6 0.12 3.6

5.1.2.3 Calculation of ventilation flow rates

Only natural ventilation due to wind is calculated in this example but both analytical and iterative solutions are shown. The flow rate is calculated assuming the wind is perpendicular to the long dimension of the kiosk, ignoring the openings on the short side. The ventilation area on the short faces only differs due to the length of the gap between wall and roof of the kiosk.

There were no heat sources in the kiosk and, as with the brick kiosk, the temperature difference between the ambient and the interior of the kiosk was small, so ventilation due to buoyancy was not calculated.

42

The air flow through either the upwind or downwind openings can be calculated using Equation 2.3. The coefficient of discharge, dC , for planned ventilation openings is given a value of 0.61.

The ventilation area on the long faces of the kiosk, upwind and downwind, is the sum of the grille and gap ventilation areas, A1 = A2 = Agrille + Agap = 0.02516 + 0.039 = 0.06416 m2. The equivalent area, Ae, can be calculated from Equation 2.4, here Ae = 0.06416 m2.

The wind speed, U, is the undisturbed speed at the reference height used for the measurement of the pressure coefficient. The reference height for the pressure coefficients in BS5925, Table 13 (BSI, 1991) is the height of the roof of the structure, referencez = 2.42 m. The measurements of wind speed, measuredU , were made at a height, measuredz = 4.7 m, assuming the kiosk is in an urban area, the equivalent velocity at the building height can be calculated using Equation 5.1 where a = 0.25, BS5925, Table 8 (BSI, 1991).

The building pressure coefficient can be found from BS5925, Table 13 (BSI, 1991), for the geometry of the kiosk with long sides perpendicular to the wind the building pressure coefficient, pC∆ = 0.95.

The ventilation flow rates calculated using the measured wind velocity referenced to the height of the kiosk are shown in Table 10.

Table 10 Comparison of observed and estimated wind driven ventilation flow rates

Velocity (ms-1)

Flow rate (m3s-1)

Measured at 4.7 m Reference at 2.42 m

Measured Calculated (analytical)

Calculated (iterative)

3.3 2.8 0.09 0.08 0.11

3.6 3.0 0.12 0.08 0.12

An alternative to the calculation described above is to use the iterative approach described in Section 2.2.5. This could include both wind and buoyancy driven ventilation though here only wind driven ventilation is considered.

Using an iterative solution allows the flows through all the walls of the kiosk to be considered, previously only the upwind and downwind faces were considered and any contribution to the ventilation flow from the side walls were ignored.

The iterative calculation determines a reference pressure difference, 0p∆ , that ensures the ventilation flows, iq , determined by the pressure difference across each opening, ip∆ , obey the mass conservation (Equation 2.10). Therefore the pressure differences and flows across each surface must be calculated, see Table 11.

The building pressure coefficients were again taken from BS5925, Table 13 (BSI, 1991) using the values for the geometry of the kiosk with long sides perpendicular to the wind. The reference velocity for the pressure coefficients is the undisturbed velocity at the roof height, calculated as described above. For the measured velocity in the second test of 3.6 ms-1 this

43

gives an undisturbed velocity at the roof height of 3 ms-1. An air temperature of 20 ºC is assumed both inside and outside of the kiosk so the density of air is 1.2 kgm-3.

The openings are treated on a per surface, rather than a per opening, basis, hence there are four pressure differences and flow rates to solve for. If the individual openings were considered there would be eight pressure differences and flow rates to solve for, representing the grilles and gap on each surface. An iterative solution will work with any arrangement of openings with a continuity equation allowing a solution to be found. The ventilation area on the upwind and downwind surfaces is 0.064 m2, and the ventilation area on the sides is 0.056 m2, due to the smaller contribution to the area from the gap on the shorter sides. As before all openings are treated as planned ventilation openings with a coefficient of discharge of 0.61.

Tables 11 and 12 show the solutions of the flow rates for Tests 1 and 2 using an iterative technique. The predicted ventilation flow rates for Tests 1 and 2 are 0.11 m3s-1 and 0.12 m3s-1, calculated by adding either all the inflows or outflows. These can be compared to the observed flow rates of 0.09 m3s-1 and 0.12 m3s-1, see Table 10.

The flow rates were solved using the Goal Seek function in Excel 2000 (Microsoft, 1999). The goal used was to make the sum of the ventilation flow rates equal to zero by varying the value of the reference pressure difference 0p∆ .

The calculated air change rates using the analytical solution and the wind velocity at the reference height are lower but close to those observed, see Table 10. The ventilation flow rates predicted using the iterative solutions are higher than for the analytical solutions. For Test 1 the iterative method over-predicts the observed flow rate and the predicted and observed flow rates for Test 2 are same, see Table 10. The predicted pattern of the ventilation flow with the iterative solution is not the same as the analytical solution. In the analytical solution the flow through the enclosure is from the upwind to the downwind face. By contrast the iterative solution predicts most of the flow into the building from the upwind face with a low flow rate into the enclosure through the downwind face. The predicted flow out of the building is from the sides of the building. This could affect the build-up of gases within the enclosure, as released gas could be re-entrained into the enclosure from the downwind face.

Table 11 Iterative calculation of wind driven ventilation flow rate, Test 1

Surface 2,0 5.0 UCpp outipi ρ+∆=∆

( )i

iidii

pACpq

ρ∆

∆=2

sgn

ipC , outρ

(kgm-3) U

(ms-1) ip∆

(Pa) dC iA

(m2) iρ

(kgm-3) iq

(m3s-1)

Upwind 0.7 1.2 2.8 4.5 0.61 0.064 1.2 0.107

Side -0.6 1.2 2.8 -1.6 0.61 0.056 1.2 -0.056

Side -0.6 1.2 2.8 -1.6 0.61 0.056 1.2 -0.056

Downwind -0.25 1.2 2.8 0.02 0.61 0.064 1.2 0.007

=∆ 0p 1.19 =∑i

iq 0.001

44

Table 12 Iterative calculation of wind driven ventilation flow rate, Test 2

Surface 2,0 5.0 UCpp outipi ρ+∆=∆

( )i

iidii

pACpq

ρ∆

∆=2

sgn

ipC , outρ

(kgm-3) U

(ms-1) ip∆

(Pa) dC iA

(m2) iρ

(kgm-3) iq

(m3s-1)

Upwind 0.7 1.2 3 5.1 0.61 0.064 1.2 0.114

Side -0.6 1.2 3 -1.9 0.61 0.056 1.2 -0.060

Side -0.6 1.2 3 -1.9 0.61 0.056 1.2 -0.060

Downwind -0.25 1.2 3 0.02 0.61 0.064 1.2 0.007

=∆ 0p 1.37 =∑i

iq 0.001

5.2 ASSESSMENT OF ADEQUATE VENTILATION

Here we introduce a number of examples of an assessment of the adequacy of the ventilation for diluting a specified release using the approach described in Sections 3 and 4 above. In all of the cases it has been assumed that the ventilation rate is known, either through calculating the natural ventilation rate using one of the approaches described above or through knowledge of the mechanical ventilation rate.

In examples 1 to 4 it is assumed that the release is located in an unobstructed environment and the ventilation is evenly distributed within the space. In these cases therefore the efficiency of mixing can be taken to be ε = 1. In example 5 the effects of confinement / congestion on the assessment are considered.

5.2.1 Example 1

Consider an enclosure 10 m × 5 m × 3 m which gives an enclosure volume, V0, of 150 m3.

The ventilation rate, q1, is 0.5 m3/s.

The air change rate can then be calculated as

0

1

Vq

=ν (5.2)

giving =ν 12 air changes per hour (ach).

To determine whether this level of ventilation is ‘adequate’ for a classification of Zone 2, it is necessary to consider the leak size and the ventilation required to dilute the gas concentration to

45

25% LFL. This information was given in Table 2 and the pertinent data is copied in Table 13 below, which shows that 0.5 m3/s would only be sufficient to ensure the average concentration at the outlet is below 25% LFL in the five leak scenarios highlighted in red for gases G(i) and G(ii).

Table 13 Example 1: Ventilation requirement Fluid

category Release pressure (bar(a))

Ventilation flow rate to dilute to 25% LFL (m3/s)

Release hole diameter 1 mm 2 mm 5 mm 10 mm

A 6.8 1.14 4.56 28.5 (114.) 10 1.46 5.84 36.5 (146.) 50 3.40 13.6 85.1 (340.)

100 4.83 19.3 121. (483.) B 5 0.99 3.94 24.6 (98.4)

10 1.47 5.86 36.6 (147.) 50 3.41 13.6 85.3 (341.)

100 4.84 19.4 121. (484.) C 5 0.94 3.74 23.4 (93.6)

10 1.40 5.62 35.1 (140.) 50 3.27 13.1 81.8 (327.)

100 4.65 18.6 116. (465.) G(i) 5 0.07 0.27 1.70 6.79

10 0.14 0.56 3.48 13.9 50 0.75 2.99 18.7 74.7

100 1.63 6.51 40.7 163. G(ii) 5 0.13 0.52 3.23 12.9

10 0.26 1.05 6.57 26.3 50 1.33 5.31 33.2 133.

100 2.64 10.6 66.1 264.

5.2.2 Example 2

The second example is for a slightly larger enclosure 10 m × 15 m × 6 m which gives an enclosure volume, V0, of 900 m3.

The ventilation rate, q1, is again 0.5 m3/s which gives an air change rate of just 2 ach.

In this case, while the air change rate is different, the ventilation rate is exactly the same as for Example 1 and so the five leak scenarios previously highlighted in Table 13 remain the only cases where the ventilation rate of 0.5 m3/s is adequate. This example simply demonstrates that the adequacy of the ventilation depends on the ventilation rate and not the air change rate.

5.2.3 Example 3

Consider a larger still enclosure of 25 m × 12 m × 7 m, which gives an enclosure volume, V0, of 2,100 m3.

The ventilation rate, q1, this time is 7 m3/s which gives an air change rate of 12 ach.

46

This significantly higher ventilation rate means that the ventilation is adequate for diluting a wider range of leak scenarios. In this example the secondary releases that could be classified as Zone 2 are highlighted in Table 14 below.

Table 14 Example 3: Ventilation requirement Fluid

category Release pressure (bar(a))

Ventilation flow rate to dilute to 25% LFL (m3/s)

Release hole diameter 1 mm 2 mm 5 mm 10 mm

A 6.8 1.14 4.56 28.5 (114.) 10 1.46 5.84 36.5 (146.) 50 3.40 13.6 85.1 (340.)

100 4.83 19.3 121. (483.) B 5 0.99 3.94 24.6 (98.4)

10 1.47 5.86 36.6 (147.) 50 3.41 13.6 85.3 (341.)

100 4.84 19.4 121. (484.) C 5 0.94 3.74 23.4 (93.6)

10 1.40 5.62 35.1 (140.) 50 3.27 13.1 81.8 (327.)

100 4.65 18.6 116. (465.) G(i) 5 0.07 0.27 1.70 6.79

10 0.14 0.56 3.48 13.9 50 0.75 2.99 18.7 74.7

100 1.63 6.51 40.7 163. G(ii) 5 0.13 0.52 3.23 12.9

10 0.26 1.05 6.57 26.3 50 1.33 5.31 33.2 133.

100 2.64 10.6 66.1 264.

5.2.4 Example 4

Consider a larger still enclosure of 50 m × 50 m × 7 m, which gives an enclosure volume, V0, of 17,500 m3.

The ventilation rate, q1, this time is 30 m3/s, which gives an air change rate of 6 ach.

The higher ventilation rate in this case means that the ventilation is adequate for diluting a wide range of releases. In fact the ventilation is almost equivalent to that for the same release outdoors as there are only sixteen cases for which the ventilation is not adequate, but could be classified as Zone 2 outdoors.

47

Table 15 Example 4: Ventilation requirement Fluid

category Release pressure (bar(a))

Ventilation flow rate to dilute to 25% LFL (m3/s)

Release hole diameter 1 mm 2 mm 5 mm 10 mm

A 6.8 1.14 4.56 28.5 (114.) 10 1.46 5.84 36.5 (146.) 50 3.40 13.6 85.1 (340.)

100 4.83 19.3 121. (483.) B 5 0.99 3.94 24.6 (98.4)

10 1.47 5.86 36.6 (147.) 50 3.41 13.6 85.3 (341.)

100 4.84 19.4 121. (484.) C 5 0.94 3.74 23.4 (93.6)

10 1.40 5.62 35.1 (140.) 50 3.27 13.1 81.8 (327.)

100 4.65 18.6 116. (465.) G(i) 5 0.07 0.27 1.70 6.79

10 0.14 0.56 3.48 13.9 50 0.75 2.99 18.7 74.7

100 1.63 6.51 40.7 163. G(ii) 5 0.13 0.52 3.23 12.9

10 0.26 1.05 6.57 26.3 50 1.33 5.31 33.2 133.

100 2.64 10.6 66.1 264.

5.2.5 Example 5

In this case we an example is considered where the global ventilation rate isn’t a sufficient measure of the local ventilation effectiveness. Consider again the third example where the enclosure volume, V0, is 2,100 m3 and the ventilation rate, q1, is 7 m3/s, giving an air change rate of 12 ach.

If the release is in a confined space or the ventilation is poorly distributed then the average concentration in the room may be higher than the average concentration at the outlet. In such a case, the ventilation becomes less effective at diluting the release and a useful first assessment of this effect is to scale the actual ventilation rate down by a factor 2 (i.e. ε = ½) for a moderate level of confinement / congestion and by 3 (i.e. ε = 1/3) for a high level. Assuming a moderate level of confinement for this example means that the ventilation rate criterion is reduced from 7 m3/s to 3.5 m3/s. The result is shown in Table 16 below which indicates a significantly reduced number of cases (compared to Table 14) that can be described as having adequate ventilation.

However, the assessment of the effect of confinement / congestion should not end there as it has been shown that under certain conditions larger gas clouds can result if the local ventilation is poor. Say, for example, the G(ii) gas pressure is 10 bar and the hole size is 2 mm. This gives a ventilation requirement of 1.05 m3 /s which is well below 3.5 m3/s. Even in this case, evidence should be sought to demonstrate that the release is not located in a stagnant region and that any gas releases from that location will be mixed by the enclosure ventilation.

A useful way of assessing the extent of the confinement of the release location is to consider the axial distance to LFL of the equivalent unobstructed jet. This can be calculated in this case using Equation 4.8 where x is the LFL molar concentration. For this example, using ρb = 1.2

48

kg/m3, ρs = 0.16 kg/m3, α = 0.05, xb = 5.4% LEL (Equation 4.7), x = LEL = 0.04 (v/v), rs = 2.24 mm (Equation 4.6), P = 10 bar and Pa = 1.01 bar, which gives z = 1.4 m. This indicates that any large obstacles, or collection of smaller obstacles, within about 1.4 m are likely to have a significant effect on the gas dispersion and the effectiveness of the local ventilation will need to be considered.

Table 16 Example 5: Ventilation requirement Fluid

category Release pressure (bar(a))

Ventilation flow rate to dilute to 25% LFL (m3/s)

Release hole diameter 1 mm 2 mm 5 mm 10 mm

A 6.8 1.14 4.56 28.5 (114.) 10 1.46 5.84 36.5 (146.) 50 3.40 13.6 85.1 (340.)

100 4.83 19.3 121. (483.) B 5 0.99 3.94 24.6 (98.4)

10 1.47 5.86 36.6 (147.) 50 3.41 13.6 85.3 (341.)

100 4.84 19.4 121. (484.) C 5 0.94 3.74 23.4 (93.6)

10 1.40 5.62 35.1 (140.) 50 3.27 13.1 81.8 (327.)

100 4.65 18.6 116. (465.) G(i) 5 0.07 0.27 1.70 6.79

10 0.14 0.56 3.48 13.9 50 0.75 2.99 18.7 74.7

100 1.63 6.51 40.7 163. G(ii) 5 0.13 0.52 3.23 12.9

10 0.26 1.05 6.57 26.3 50 1.33 5.31 33.2 133.

100 2.64 10.6 66.1 264.

6 REFERENCES

ASTM (2011), ASTM E741 - 11 Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution, ASTM International, West Conshohocken, PA

BSI (1991), BS5925:1991 Code of practice for ventilation principles and designing for natural ventilation, British Standards Institution London

CIBSE (2005), Natural ventilation in non-domestic buildings, CIBSE Applications Manual AM10, CIBSE

CIBSE (2006), Environmental design CIBSE Guide A, CIBSE

EI15 (2005) Model code of safe practice Part 15: Area classification code for installations handling flammable fluids, 3rd Edition, Energy Institute, London.

Etheridge D. W. (2002), Non-dimensional methods for natural ventilation design, Building and Environment 37(11) 1057-1072

Etheridge D. W. and Sandberg M. (1996), Building ventilation: theory and measurement, John Wiley and Sons Ltd

IEC 60079-10-1:2008 Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres

BS EN 60079-10-1:2009 Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres

IGEM (2010), Hazardous area classification of Natural Gas installations, IGEM/SR/25 Edition 2, Communication 1748, IGEM

ISO21789:2009 Gas turbine applications - Safety

Ivings M., Lea C., Ledin H.S., Pritchard D., Santon R. and Saunders C.J., ‘Outstanding safety questions concerning the use of gas turbines for power generation – Executive report’, 2004, Health and Safety Laboratory Report CM/04/02

Ivings M.J., Clarke S., Gant S.E., Fletcher B., Heather A., Pocock D.J., Pritchard D.K., Santon R. and Saunders C.J., 2008, ‘Area Classification for secondary releases from low pressure natural gas systems’ Health and Safety Executive Research Report RR630 http://www.hse.gov.uk/research/rrhtm/rr630.htm

Kelsey A. and Ivings M.J. 2010 Indoor area classification calculations in support of EI15, Health and Safety Laboratory Report MSU/2010/19

Santon R., Ivings M.J., Webber D.M, and Kelsey A. ‘New Methods for Hazardous Area Classification for explosive gas atmospheres’ Hazards XXIII, 12-15 Nov 2012

Webber D.M., Ivings M.J., and Kelsey A. (2011) ‘Ventilation theory and dispersion modeling applied to hazardous area classification’ Journal of Loss Prevention in the Process Industries, vol 24, pp. 612-621

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50

Annex A EI15 Fluid Categories

Table A.1 Fluid Categories defined in EI15

Fluid category Description

A A flammable liquid that, on release, would vaporise rapidly and substantially. This category includes: (a) Any liquefied petroleum gas or lighter flammable liquid.(b) Any flammable liquid at a temperature sufficient to produce, on release, more than about

40% vol. vaporisation with no heat input other than from the surroundings.

B A flammable liquid, not in category A, but at a temperature sufficient for boiling to occur on release.

C A flammable liquid, not in categories A or B, but which can, on release, be at a temperature above its flash point, or form a flammable mist or spray.

G(i) A typical methane-rich natural gas.

G(ii) Refinery hydrogen.

Published by the Health and Safety Executive 01/14

Technical input on ventilation effectiveness for area classification guidance EI15

Health and Safety Executive

RR993

www.hse.gov.uk

The Energy Institute’s guidance on hazardous area classification “Area classification code for installations handling flammable fluids”, commonly known as EI15 (and previously IP15), is used extensively in the UK and elsewhere by the petroleum industry and others, and is currently under revision. Ventilation is a key factor in area classification and it was recognised that the current Edition 3 of EI15 required improvement in this area in particular. The current approach, based on air changes per hour, is shown to be inappropriate since it is not related to the size of the release it is trying to dilute. The main aim of the work reported here is to provide an improved methodology for assessing the adequacy of the ventilation such that an enclosure containing secondary grade release sources can be properly classified as Zone 2 as defined by BS EN 60079-10-1:2009.

It is shown that limiting the average concentration to 25% LFL in the enclosure generally achieves the ventilation objective for a Zone 2 area but the degree of confinement and congestion at the point of release must be assessed. Guidance on this assessment is provided including the use of a factor to quantify the efficiency of mixing.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.