Classic Template - Blue · 8 Dust Classification Based on K St Measured K St Classification 300...

1

Objectives

• OSHA - Enforcement

• Define the terminology

• Recognize a Dust Explosion

hazard

• Learn hazard management

methods

Enforcement

• OSHA Established an NEP for

Combustible Dust

• Started the Standard Process

– Holding Stakeholder Meetings

– Gathering Information

– Proposed Standard – Not sure when this will

be done (No specifics at this time)

2

Types of Industries Involved in Dust Incidents

Types of Dust Involved in incidents

Definitions and Terminology

• Combustible Dust

• Deflagration

• Minimum Ignition Energy

(MIE)

• Kst

3

Combustible Particulate Solids

Chunks

Flakes

Fibers

Chips

Fines

Dusts

“Combustible Dusts”

are a sub-category of

“Combustible Particulate Solids”

Combustible Dust

• “A combustible particulate solid that presents a

fire or deflagration hazard when suspended in

air or some other oxidizing medium over a range

of concentrations, regardless of particle size or

shape.” [NFPA 654-2006]

• Dusts are just a small fraction of combustible

particulate solid.

• This definition does NOT encompass all of the

hazardous particulates!

Combustible Dust

420

1.3 mm

A particle that will pass a #40 Sieve

A fiber particle

12

4

Particle Size of Common Materials

Deflagration

• Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium.

• This action releases large quantities of heat very rapidly.

• This in turn produces large pressures to develop that can lead to structural failure and cause employee injuries.

Deflagration

Dust cloud

5

Deflagration

Deflagration

Deflagration

6

Deflagration

Minimum Ignition Energy

• MIE is the minimum energy, discharged

into the dust cloud by the test apparatus

and sufficient to cause flame propagation.

The MIE is measured in units of joules (J)

or millijoules (mJ).


Increasing particle

size increases the MIE,

decreasing the particle

size decreases the

energy necessary to

initiate deflagration.

7


MIE varies with

concentration. MIE

is reported at the

concentration that

deflagrates with the

lowest MIE.

Kst

• Volume-Normalized Rate of Pressure Increase.

• KSt is also referred to as the “Deflagration Index” or

“Dust Constant”. It is calculated as the rate of

pressure increase multiplied by the cube root of the

test vessel volume. KSt = (dP/dtmax)(V)1/3

• KSt is proportional to the speed of the flame front

through the dust cloud.

• KSt is measured in units of bar-meter per second (bar-

m/sec). KSt is just a way of classifying dust to estimate

the anticipated behavior of the dust during

deflagration.

KSt

• Dusts with a KSt less than or equal to 200 bar-m/sec are classified as “ST-1” dusts.

• Dusts with a KSt between 200 and 300 bar-m sec are classified as “ST-2” dusts.

• Dusts with a KSt greater than 300 bar-m/sec are classified as “ST-3” dusts.

• These classifications have NOTHING to do with hazardous area classifications under the NEC.

8

Dust Classification Based on KSt

Measured KSt Classification

<=200 St-1 *

200 – 300 St-2

>300 St-3 **

• * THIS CLASSIFICATION KILLS THE MOST EMPLOYEES

• ** THIS CLASSIFICATION IS USUALLY MADE UP OF METALS – Al, Mg,

Be, etc.

Combustible dusts are classified by the

numerical value of KSt.

Recognition

•Review the materials in the facility that are utilized and/or manufactured. If any

the material is reduced to fines, dust, chips, chunks, flakes or fibers then research

the material further because it may be a combustible dust.

•Keep in mind, any material containing “carbon” and/or un-oxided metal will most

likely explode.

•Check your material safety data sheet (MSDSs) concerning any unusual hazards

such as explosibility.

•Review the online German database

http://www.dguv.de/bgia/en/gestis/expl/index.jsp. The database is operated by

BGIA - Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung. It

contains combustion and explosion characteristics of more 4000 dusts samples

from virtually all sectors of industry.

•If you are unable to locate any information about your material, then try using

Google.com to look for such hazards.

Recognition (cont.)

• Take a sample of the dust in your plant from the highest points when possible. Have the material tested for its KSt. This will at least give you an idea whether the material will explode.

• Many locations need to be considered in an assessment. One obvious place for a dust explosion to initiate is where dust is concentrated. Equipment such as dust collectors may contain a combustible mixture whenever the equipment is operating.

• Other locations to consider are those where dust can settle, both in occupied areas and in hidden, concealed spaces. A thorough analysis would include all possible scenarios in which dust can be disbursed, both in the normal process and potential failure modes.

• A hazard may exist without visible dust in the air. Meaning an initial explosion may occur in dust collector and create secondary explosion in areas with poor housekeeping. A sure sign of close encounters may be a plant history of fires.

• While evaluating the dust in your facility, keep in mind the potential ignition sources such as flames, sparks from welding/cutting operations, friction or impact sparks, electric sparks, electrostatic discharges and hot surfaces.

http://www.dguv.de/bgia/en/gestis/expl/index.jsp

9

Combustible Dust Explosion

Pentagon

Fire Triangle - Dust Explosion Pentagon

FIRE TRIANGLE:

DUST EXPLOSION

PENTAGON:

Combustible Dust Oxygen in Air

Ignition Source

Dispersion Confinement

Explosion

1. Combustible Dust 2. Oxygen in Air

5. Ignition Source

3. Dispersion 4. Confinement

Explosion

1. Combustible

Dust

2. Oxygen in Air

3. Dispersion

4. Confinement

5. Ignition Source

IMPORTANT

NO DUST EXPLOSION OCCURS

if one or more elements

are missing

Combustible Dust Explosion Pentagon:

Five Elements – ALL Necessary

10

Element 1: Combustible

Dust

Agricultural Products such as:

• Corn Starch, Dry Milk, Sugar,

Wood Flour, Powered Milk

Agricultural Dusts such as:

• Cocoa Powder, Hops (malted),

Rice Flour, Wheat grain dust

Carbonaceous Dusts such as:

• Petroleum Coke, Pine Soot,

Bituminous Coal, Wood

Charcoal.

Chemical Dusts such as:

• Lactose, Sulfur, Calcium

Acetate, Methyl-Cellulose

Plastic Dusts such as:

• Phenolic Resin,

(poly)Propylene, (poly)Vinyl

Chloride, Melamine Resin

Metal Dusts such as:

• Aluminum, Magnesium, Zinc,

Bronze

Explosion

Combustible Dust Particles

may

break

into

or


• Size – Pellets > 2mm diameter

– Granules 0.42mm – 2mm

– Dust Particles <0.42mm

11


• Hazard increases as particle

size decreases

– Larger surface area for combustion

– Fine particles may have a larger

role in dust cloud ignition and

explosion and propagation

Combustible Dust Particle Size

Distribution

Decreasing Particle Size Increases dP/dtmax

EckhoffEckhoff

Aluminum Dust

dP

/dtm

ax [b

ar/

sec]

Explosion

Element 2: Oxygen in Air

• The Oxygen content in air is all that is

necessary to support an explosion.

12

Explosion

• Dust needs to be dispersed in the air

Element 3: Dispersion

Combustible Particulate

Largest particles

Smallest particles

•Generally the finest (most hazardous) fraction of the accumulated fugitive

dust settles highest in the compartment of building.

•Every large compartment is a particulate separator, separating particles

by mass.

Concentration

• Fact: When particles are suspended, a

concentration gradient will develop where

concentration varies continuously from high

concentration to low concentration,

Concentration

Gradient

Ignitable

Non-ignitable

Dust

Cloud

13

What

bulb?

25 watts

6 feet

Glass

40 g/m3 concentration of

comb. dust suspended in air

Glass

Dust Combustibility

• 25 watt light bulb probably can not be seen

through six feet of a mixture of combustible dust

in air > Minimum Explosible Concentration

Dust Combustibility

0

2.5 ug/m315 mg/m

3

50 g/m3

3000 g/m3

Explosible

Range

Health

Hazard

Range

Concentration 3000X

Explosion

Element 4: Confinement

• Confinement can be provided by buildings,

process equipment, ducting, piping, and

dust collection equipment.

14

Explosion

Element 5: Ignition Source

Can be Mechanical

• Match/lighter

• Spark

Can be Electrical

• Static –• Human - 20-30 mj

• Grain – filling – 10-25 mj

• Lightning

• Generated

Explosion

All 5 Elements = EXPLOSION

The “Typical” Explosion

Event

• Dust explosions occur as a series of

deflagrations leading to a series of

explosions in stages.

– While a single explosion is possible it is the

exception rather than the rule.

– Most injuries are the result of the “secondary”

deflagrations rather than the initial event.

15

A Dust Explosion Event

Primary Deflagration inside process equipment

0 25 50 75 100 125 150 175 200 225 250 300 325

Time, msec.

(Timing of actual events may vary)


Shock wave caused

by primary deflagration

Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325


Shock waves reflected by surfaces within the building

cause accumulated dust to go into suspension

Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

16


Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Dust clouds thrown in the air by the shock waves


Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Primary deflagration breaks out of the equipment

enclosure – creating a source of ignition


Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Secondary deflagration ignited

17


Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Secondary deflagration is propagated

through the dust clouds

Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Secondary

deflagration bursts

from the building


Time, msec.

0 25 50 75 100 125 150 175 200 225 250 300 325

Collapsed building with remaining fires

18

Equipment

& Operations

Blenders/Mixers

• Heat Generation due to

– Rubbing of Solids

– Rubbing of internal parts

• Electrostatic Charging of the Solids

• Dust Formation inside of the equipment Source:

http://www.fedequip.com/abstract.asp?ItemNumber=1

7478&txtSearchType=0&txtPageNo=1&txtSearchCriter

ia=ribbon_mixer

Dryers

• Direct-Heat Dryers– Convective Drying

System

– Heat provided by heated air or gas

– Moisture is carried by drying medium

• Indirect–Heat Dryers– Heat transfer by

Conduction

– Steam for Jacketed Dryers

Source:www.barr-rosin.com/products/rotary-dryer.asp

19

Dust Collectors

• Presence of easily ignitable fine dust atmosphere and high turbulence

• Experienced many fires over the years due to broken bags.

• Ignition source is electrostatic spark discharges

• Another ignition source is entrance of hot, glowing particles into the baghouse from upstream equipment

Fabric Filters (Baghouses)

Dust Collector

• If burning material is introduced into the dust collector, a deflagration can result from the operation of the bag or filter element cleaning cycle.

• Dust collector system located inside a building

Note: A dust collector, by its very operation, maintains a cloud of finely divided particles suspended in air. If a source of ignition initiates the combustion of the dust cloud, the collector casing could cause a violent rupture. When dust particles are known to be combustible, precautions for an explosion must be taken and suitable protection provided to reduce the risk of personal injury.

Dust explosion in

equipment

With dispersal

and ignition of 2 kg dust

by the flame jet

Dust

Collector

Dust explosion in

equipment

20

Pneumatic conveying system

• Downstream equipment have high rate of risk for fires and explosion

– Static electricity is generated from particle to particle contact or from particle to duct wall contact.

– Heated particles which are created during grinding or drying may be carried into the pneumatic conveying system and fanned to a glow by high gas velocity.

– Tramp metal in the pneumatic system may also cause frictional heating.

– Charged powder may leak from joints to the atmosphere and electrostatic sparking can occur resulting in an explosion.

Figure source:www.flexicon.com/us/products/PneumaticConveyingSystems/index.asp?gclid=COa2kKWK

4o8CFQGzGgodikc9Dg

Size Reduction System

• Size reduction equipment is regarded as a possible ignition source because of friction and hot surfaces arising from grinding

• Entrance of metal into the equipment

• Too slow feed rate can increase the possibility of fire/explosion hazard

Bucket Elevators

• Belts Slipping

• Belts and lagging are not fire and oil resistant

• Belts and bearings are not outside the casing

• Lack of preventive maintenance

Head

pulley

Lagging

21

Silos and Hoppers

• No inter-silo Venting

• Silos and hoppers shall be located outside the buildings

with some exceptions

• Air cannons not to be used to break bridges in silos

• Detection of smoldering fires in silos and hoppers can be

achieved with methane and carbon monoxide detectors

• Pressure containment, inerting, and suppression

systems to protect against explosions

• Venting is the most widely used protection against

explosions

Explosion

Hazard

Management

Methods

• There are three ways of handling combustible dust

hazards:

– Prevention

• Explosion suppression

• Dilution with non-combustible dust

• Oxidant concentration reduction

– Mitigation• Controlling the explosion energy

• Venting through a listed dust retention and flame arresting

device

– Combination of Prevention and Mitigation

22

Processes

• Combustible particulate solids should not

pass through fans.

• All conveyance ducts shall be steel or

other conductive material.

• All bends smooth.

• Changes in diameter with 10o(max) taper

transitions.

Processes

This Not this

This Not this

Explosion Venting

• The vent opening must be sized to allow the expanding gases to be vented at a rapid rate so that the internal pressures developed by the explosion do not compromise the structural integrity of the protected equipment

• The volume of the equipment to be protected

• The maximum pressure during venting (Pred)

• The KSt of the dust (or fundamental burning velocity of a gas)

• The burst pressure of the explosion

23

Deflagration Relief Venting (Explosion Venting)

Dust Collector

Filter Media

Dusty Air

Into Collector

Return Air

Vent


Dust Collector

Filter Media

Dusty Air

Into Collector

Return Air

Vent


Dust Collector

Filter Media

Dusty Air

Into Collector

Return Air

Vent

24


Dust Collector

Filter Media

Dusty Air

Into Collector

Return Air

Vent


Dust Collector

Filter Media

Vent

Vent Duct

Wall

Deflagration Suppression

• Reaction times are generally in the 30 to

40 millisecond time domain.

• Pressure sensors are generally calibrated

to 0.5 psi above ambient.

• Extreme care should be used when

Deflagration Suppression is used on

vessels also equipped with Deflagration

Relief Vents.

25


• Must be designed, installed and

maintained per NFPA 69.

• Relies on high speed pressure sensors

and high rate discharge extinguishing units

to detect initial shock wave from the

deflagration and discharge suppressing

agent before the pressure has gotten large

enough to cause damage.


Specialized Agents

• Gaseous agents

• Dry chemicals

• Water spray or mist

• Others

Pressure Sensor / Automatic Dust Explosion

Suppression System

26

Detectors

Detection & Supression System


Dusty Air

Into Collector

Return Air

Pressure

Sensors

Control

Unit

HRD

Extinguishing

Unit

27


Dusty Air

Into Collector

Return Air

Pressure

Sensors

Control

Unit


Dusty Air

Into Collector

Return Air

Pressure

Sensors

Control

Unit

HRD

Extinguishing

Unit


Dusty Air

Into Collector

Return Air

Pressure

Sensors

Control

Unit

HRD

Extinguishing

Unit

28

Spark Detection and Extinguishment

• Spark detectors are sensitive infrared

sensors that detect burning material on a

conveyor or in a pneumatic conveyance

duct.

• Spark detection is usually used in

conjunction with automatic water-spray

extinguishment.


• When the spark enters the field of view of

the detector it sends a signal to the control

unit which energizes the valve.

• The valve allows a spray of water to

commence in the duct.

• The spark collides with the water spray

and is quenched.

Dust Collector

Filter MediaReturn Air


Fan

Control

Unit

Pump

Solenoid

Valve

Spark

Detectors

Vent

29

Dust Collector



Fan

Control

Unit

Pump

Solenoid

Valve

Spark

Detectors

Vent

Dust Collector



Fan

Control

Unit

Pump

Solenoid

Valve

Spark

Detectors

Vent

Dust Collector



Fan

Control

Unit

Pump

Solenoid

Valve

Spark

Detectors

Vent

30

Dust Collector



Fan

Control

Unit

Pump

Solenoid

Valve

Spark

Detectors

Vent

Deflagration Relief Venting – Flame Arrestor

• Deflagration through a listed dust retention and flame-arresting device

Cut-away of deflagration vent

• Burned and unburned dust is

retained by filter and flame arrestor

• Combustion gases are cooled

• No flame emerges from the system

• Nearby blast effects are reduced

• May be vented indoors

• Change burst disc and clean filter

after each event (Europe)

No Isolation Device

31

Isolation Devices for Ductwork

Ventex ValvePassive device for interrupting dust explosion

Bursting Disc /or

other Vent CoverDevice for interrupting dust explosion

Back Flash Dampers for DuctworkIsolation Devices

Chemical & Mechanical Isolation

Devices

32

Isolation Devices

• Airflow control valve, NFPA 654 (7.8)

• Diverter valves seal mechanically and close all

other directions from air or material leakage

Diverter

valve

Normal flow

of material

Positive shut

off flap

Flame

front

Hinged

device

Isolation Devices• Segregation of the hazards (isolate with a diverter)

Flame

front

diverter

Roof Line

Air

Isolation Devices

• Segregation of the hazard (isolate with a diverter)

33

Isolation Devices

• Segregation of the hazard (isolate with a barrier)

Automatic Fast Acting Isolation Valve Is Shown

Isolation Devices

• Segregation barrier – Chemical Suppressor

Isolation Devices

• Segregation of the hazard (chemical isolation)

Flame front is

halted by barrier

released from

chemical

suppressor

34

Without Return Air Abort

• As the flame and hot gases fill the space

beneath the ceiling/roof deck sprinkler

heads begin fusing.

• Often more heads fuse than the riser is

designed to support.

• The excessive demand deprives entire

facility of required delivered density and

fires are not controlled.

Dust Collection Without

Return Air Abort

Dust

Collector

Return Air Duct

Radiant flux

Dust Collection Return Air Abort

• The diversion of return air to

building exterior is usually

implemented with spark

detection and a fast-acting

abort gate.

35

Dust Collection Return Air Abort

Dust

Collector

Return Air Duct

Abort Gate

Return Air Abort

• Upon actuation of the return air spark

detection the automatic bag cleaning is

shut-down.

• DO NOT shut down the air movement as

this can cause a deflagration

• If fire expands the sprinklers in the D/C will

fuse and dissipate the heat generated by

the fire.

Return Air Abort

• In lieu of using an abort gate,

fast acting valves can be used

to prevent fire and smoke

from returning to the occupied

space.

36

Dust Collector

Filter Media

Dusty Air

Into Collector

Return Air

Spark Detectors

Control

Unit

NFPA 664-2007 Section A 8.2.2.6 Minimum Compliance Design

Vent

Valve or

Flame-front

Diverter

Chokes for Screw Conveyors

Isolation Devices

Rotary Valve

Isolation Device

Rotary

Air Lock

37

Explosion Management – Pressure

Containment

Vessel must contain the

calculated maximum explosion

pressure equal to

2/3 yield

strength

Allows vessel

re-use

2/3 catastrophic

failure pressure

Contains the

event, but the

vessel must be

scrapped

Wet Scrubber/Collectors

• Air is drawn into the

collector and is forced

to churn through a

torturous path,

through a partially

submerged

baffle. Dust is

separated by making

contact with water in

this section.

Process Equipment - Bucket Elevators

• Deflagration venting

• Inside bucket elevators must be provided with deflagration venting ducted to the outside or otherwise properly protected against explosion

• Elevators must be dust tight and noncombustible

• Inlet and discharge hoppers are to be accessible for cleaning and inspection deflagration

venting test at

mock-up lab

38

Bucket Elevators

• Power cutoff

• Is a device used to cut off

power when the drive motor

drops to <80% of normal

• Feed is to be stopped or

deflected when the power to

the motor is stopped

Bucket Elevators

• Belts

• Non-slip material and (lagging) is to be provided on the head pulley to minimize slippage

• Belts and lagging are to be fire and oil resistant

Head

pulley

Lagging

Bucket Elevators

• Belts

• Bearings are to be outside the casing

• Provide openings at the head and tail pulley for

– Cleanout

– Inspection

– Alignment

Dirty

Really dirty and

“partially

inside”

construction

39

Bucket Elevators

• Drive components are

to be engineered to:

– Handle full rated

capacity

– Start the unchoked

elevator under full load

Bucket Elevators

• Monitors are to be provided

• Monitor the head and tail pulleys for:– High bearing temperature

– Vibration

– Head pulley and belt alignment

• An alarm requiring corrective action to sound at the operator’s station

• Exception: Conveyors <500 ft/minor <3750 ft3/hour

Head

Tail

Bucket Elevators

• Emergency Controls

• Bins where material directly discharges from the elevators must be provided with:

– Automatic high-level shutdowns

– Visual indicators

– Audible alarms sounding at the operator control station

40

Explosion Management

• Oxidant Concentration

Reduction

• Outside the combustible range

through oxidant concentration

reduction, fuel enrichment or

both

• Detailed engineering

requirements

• Life safety hazard

• Metals problematic

Normal

Limiting

Oxygen

Concentration

for combustion

of many dusts

Explosion Prevention Techniques

• Elimination of Ignition Sources Involves:

– Control of Heat Sources -NFPA 654

– Control of Friction -NFPA 654

• Mechanical friction

• Friction sparks

– Control of Electrical Sparks -NFPA 497, NEC

– Control of Static Electricity -NFPA 77

Ignition Control

• Separate heating systems

from dusts

• Proper use of cartridge

activated tools

• Adequately maintain

equipment.

Hot

work

permit_________

_________

_________

_________

Kaboom!

41

Ignition Control

• Control mechanical sparks and friction;

• Use separator devices to remove foreign materials capable of igniting combustibles from process materials;

• Separate heated surfaces from dusts;

Magnetic

Core

Non - Magnetic

rotating drum

Ignition Control

• Use appropriate electrical

equipment and wiring

methods

• Control static electricity,

including bonding of

equipment to ground

• Control smoking, open

flames, and sparks

No!

42

Ignition Control

• Other ignition sources

– OSHA 29 CFR 1910.178 (c)

regulates powered industrial

trucks in dust areas

– Coal handling operations

must comply with OSHA 29

CFR 1910.269

Class I & II

Group D & G

Explosion Prevention Techniques - Control of Heat

Sources

• If the material is subjected to heat as part of the normal process (e.g. during drying), the temperature should be maintained below the self heating temperature (for solids)

• Preventing the overloading of processing plant (grinders, conveyors, etc.). Internal buildup will BOTH reduce heat loss from material AND increase operating temperature above “normal”. Consider the installation of overload protection devices on drive motors

• Isolation or shielding of hot surfaces

• Prevention/removal of dust accumulations on hot surfaces

• Use of approved electrical equipment (correct temperature rating)


Control of Friction

Mechanical equipment and component

could be a source of ignition due to:

– Mechanical impacts producing:

• Small flying fragments of hot/burning material

• A pair of hot spots where impacting bodies touch

– Mechanical friction caused by objects rubbing against each

otherand producing hot surfaces

– Thermitereaction caused by impacts involving aluminum

and rust

43


Control of Friction

• Prevent overheating due to misalignment, loose objects, belt-slip/rubbing etc. by regular inspection and maintenance of plant

• Prevent foreign material from entering the system when such foreign material presents an ignition hazard. Consider use of screens, electromagnets, pneumatic separators, etc.

• Floor sweepings should not be returned to any machine

• Impact sparks can occur when for example operators use, drop, orotherwise strike metal equipment with metal tools or objects. Minimize the likelihood of impact sparks through:

– Proper tool selection

– Techniques to prevent dropping tools e.g. wrist straps

– Operator training

• Hot work operations should be controlled by a hot work permit system in accordance with NFPA 51B, Standard for Fire Prevention During Welding, Cutting and Other Hot Work

– Formation of dust clouds should be prevented, and dust deposits should be removed

– A gas/vapor detector may be used to ensure flammable vapors/gases are not present

Explosion Prevention Techniques –

Control of Static Electricity

Contact (Frictional) Charging

Electrostatic charges are usually generated when any

two materials make and then break contact

The build up of the charge on electrically isolated

conductors and/or on insulating materials, can give

rise to electrostatic discharges

Depending on the energy of the discharge, a

flammable atmosphere can be ignited.

44

Electrical Area Classification of Dust

Locations

• The following factors determine the extent of Class II locations:– Combustible material involved

– Bulk density of the material

– Particle sizes of the material

– Density of the particles

– Process or storage pressure

– Size of the leak opening

– Quantity of the release

– Dust collection system

– Housekeeping

– Presence of any flammable or combustible

gas

Housekeeping

• Facilities shall be maintained dust free.

• No blow-down unless ALL electrical power

and processes have been shut-down.

• No welding, cutting or grinding unless

under a hot-work permit per NFPA 51B.

• Comfort heating equipment shall obtain

combustion air from a clean outside

source

Housekeeping – Rule of Thumb

• If you can write

your name in

the dust there

is probably

sufficient dust

present to

blow the

building away.

45

Housekeeping

•The majority of the property damage and

personnel injury is due to the fugitive dust

accumulations within the building or process

compartment.

•Control, limitation of elimination of

accumulated fugitive dust is CRITICAL

and the single most important criterion for a

safe workplace.

Housekeeping

• Include:– Overhead beams, joists, ducts,

– Tops of equipment, and other

surfaces

– Vertical surfaces if they are dusty

– Structural members

– Conduit and pipe racks

– Cable trays

– Floors

– Above ceiling tile

– Equipment (leaks around dust

collectors and ductwork.)

?

Housekeeping

• Some recommendations:– Minimize the escape of dust from

process equipment or ventilation

systems to minimize the time

needed for housekeeping

– Use dust collection systems

– Utilize surfaces that minimize

dust accumulation and facilitate

cleaning Spots are

not

raindrops

46

Housekeeping

• Develop and implement a hazardous dust plan– Inspection

– Testing

– Housekeeping

– Control program

• Program should be written with established frequency and methods

Housekeeping

• Use cleaning methods

that do not generate dust

clouds

• Only use vacuum

cleaners approved for

dust collection

HAZ LOC

Dust Layer Thickness Guidelines

1/8” in grain standard

Rule of thumb in NFPA 654

1/32” over 5% of area

Bar joist surface area ~ 5%

Max 20,000 SF

Idealized

47

Training

• They need to know

– The safe work practices applicable to their job tasks

– The overall plant programs for dust control and ignition source control

• Training must be

– Before they start work

– Periodically to refresh their knowledge

– When reassigned

– When hazards or processes change

Training

• Training needs were identified for:

– Employees

– Supervisors

– Managers

Electrical

48

Hazardous (classified) Locations

1910-307

Subpart S

Electrical – Design

Safety Standards

for Electrical Systems

29 CFR 1910.307(c) Electrical

Installations

• Equipment, wiring methods, and installations of equipment in hazardous (classified) locations shall be intrinsically safe, approved for the hazardous (classified) location, or safe for the hazardous (classified) location !

Electrical Equipment for

Hazardous Occupancies

• All electrical equipment must be “listed” for

use in the occupancy based upon the

Class, Division and Group classification.

• When all electrical equipment in the

occupancy is listed for use in that

occupancy the electrical system is not

deemed to be a likely igniter.

49

Class II Hazardous Occupancies

• Occupancies containing combustible dusts.

• Division 1: – Dusts suspended in the air under normal operating conditions.

– Production system upset, equipment failure or maintenance can produce BOTH dust suspension and ignition source.

– Where combustible dusts are electrically conductive.

• Division 2:– Compartments where ignitable dust suspensions do not normally

exist outside process equipment.

– Where dust accumulations are not normally sufficient to cause electrical equipment to overheat.

Class II Locations

Class II locations are those that are hazardous because of the presence of combustible dust. The following are Class II locations where the combustible dust atmospheres are present:

Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, and other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment.

Group F. Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles (see ASTM D 3175, Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts.

Group G. Atmospheres containing other combustible dusts, including flour, grain, wood flour, plastic and chemicals.

Group E Dust

• Could cause a short in the electrical equipment

• Electricity may find the path of least resistance through a dust layer, heating up the dust particles in it path and thus providing a source of ignition. The resulting electric arc could ignite a dust layer or dust cloud.

*NFPA 499 . . . Hazardous (Classified)

Locations (2008) Sec 4.4

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Class II, Division Locations

• Equipment must be marked to identify suitability

for use with Group E, F and/or G.

• Where suitable for ambient temperatures

exceeding 40º C (104º F) marked with both

– The maximum ambient temperature and

– The operating temperature or temperature range at

that ambient temperature.

Nationally Recognized Testing

Laboratories (NRTLs)

• NRTLs are third party organizations recognized by OSHA

• Provide product safety testing and certification services to manufacturers on a wide range of products

• Testing and certifications are based on product safety standards developed by U.S. based standards developing organizations

• Often issued by American National Standards Institute (ANSI)

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NFPA 654 Guide to Area Electrical

Classification – A.6.6.2Negligible

dust -color

discernable

Not much dust

being released

N/A Unclassified –

general purpose

Negligible

dust to <

1/32 inch

(paper clip)

Infrequent –

Episodic release,

not > 2-3 times

yearly

Clean up

during same

shift

Unclassified –

general purpose

Negligible

dust to <

1/32 inch

Continuous to

frequent –

Continuous with

< 1/32 inch per

24 hours or

Episodic release,

> 3 times yearly

Clean as

needed –

maintain

average <

1/64 inch

(puffy cloud

with each

step)

Unclassified – dust

tight NEMA 12

enclosures and

/sealed non- heat

producing

enclosures (For

existing plants – New

to be Class II, Div. 2)

NFPA 654 Guide to Area Electrical

Classification – A.6.6.2

1/32 to

1/8 inch

(Two

Quarters

stacked)

Infrequent Clean up during

same shift

Unclassified –

Dust tight NEMA 12

enclosures and /sealed

non- heat producing

enclosures

(For existing plants – New

to be Class II, Div. 2)

1/32 to

1/8 inch

Continuous

to frequent

Clean as needed –

maintain average <

1/16 inch

Class II, Division 2

>1/8 inch Infrequent Immediately shut

down and cleanClass II, Division 2

> 1/4 inch Continuous

to frequent

Clean frequently –

minimize

accumulation

Class II, Division 1

Electrical Classification and

Cleaning

• If Clean-up is constant and– Dust layer is not apparent;

– Surface color is discernible;

– e.g.: Storage area with bags,

drums, or closed hoppers;

– No dust around.

Classify as

unclassified

Electrical

Equipment

Metal Dust is always

Class II Div I

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