Hazard Hazard IdentificationIdentification

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The Most Common Chemical Plant Accidents is Fire,

Explosion, Toxic Release

To prevent these accidents, engineers need to familiar with:

(1) Fire, explosion and toxicity properties of materials

(2) Nature of fire, explosion and toxic release process

(3) Procedures to reduce fire, explosion and toxic release hazards

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Fires

1.Pool Fire

Liquid spilled onto the ground spreads out to form a pool. Volatile liquid (e.g. petrol) evaporate to atmosphere and soon form flammable mixture with air. Upon ignition, a fire will burn over the pool. The heat vaporizes more petrol and air is drawn in round to the side to

support combustion. Danger to people is by direct thermal radiation and burn.

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2. Flash Fire

If spilled material relatively volatile (e.g. propane, butane, LPG) it would still form a pool but evaporation would be much more rapid.

If ignition did not take place immediately to form pool fire, then sizeable vapor cloud would form, drifted away by wind, to form cloud within

flammable range. If found source of ignition, flash fire will occur. People at risk from

thermal radiation effects. Usually unexpected event and short duration

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3.Torch Fire

High pressure release of gas from a vessel or pipeline ignites almost

immediately. This give rises to a giant burner of flame length tens of meters. Danger from thermal radiation and also impingement on adjacent

pressurized vessel, such as LPG vessel, heating the content followed by pressure build up causing ‘boiling liquid expanding vapor explosion’ (BLEVE).

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Distinction between Fires and Explosions

Major difference between fires and explosions is the rate of energy release.

Fires release energy slowly, while explosions release energy very rapidly (eg

automobile tire explosion).

For explosion to occurs, • Explosive mixtures • Initiation of reaction (ignition or detonation)• Rapidity of reaction• Rapid liberation of heat causes gas to expand and high pressure build up• Rapid expansion of gases to rapidly generate high pressure• …….Explosion!!!!!!!

Note: Rubber tire explosion and vessel rupture due to overpressure are examples of mechanical explosion, i.e. no explosive mixture involve.

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1. BLEVE (Boiling Liquid Expanding Vapor Explosion)

Flammable materials stored under pressure at ambient temp, e.g LPG

or ethylene oxide bullet tank. Fire could start from external spillage or

leak and the flames impinge on the side of the vessel.

The metal of the heated vessel at high temperature becomes weak

and finally ruptures allow the content to rapidly escape forming large

vapor cloud and entrained liquid.

Cloud then ignited by original fire. Casualty from blast effect is due to

thermal radiation or missiles.

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Examples:

a) Feyzin, France, 4/1/66. A leak on propane storage sphere

ignited and caused fire which burned fiercely around the vessel and led

to BLEVE. 18 death, 81 injuries.

b) Mexico City, Mexico, 19/11/84. A series of LPG explosions at

LPG gas distribution plant resulted in 542 killed and over 7000 injured.

LPG was stored in 6 spheres and 48 cylindrical bullets holding 4 million

gallons of LPG.

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Video on Bleve Explosion

http://www.youtube.com/watch?v=GWjxrAhpBQk

http://www.youtube.com/watch?v=wcmmLvAYqkI

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2. UVCE (Unconfined Vapor Cloud Explosion)

An explosion occuring in the open air which results from the

ignition of flammable gas. An unconfined vapor-cloud explosion

may result from the accidental release of a flammable liquid or

gas.

Example :

Flixborough, UK, 1/6/74. Plant producing caprolactam for nylon manufacture and part of the process involved reaction of cyclohexane with air. Massive failure of a temporary bypass pipeline cause 40 -50 ton of liquid cyclohexane to escape and formed a large vapor cloud. Subsequent explosion cause damage up to 3 miles away. 28 killed, 36 injured on site, 53 off-site.

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UVCE

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UVCE and Pool Fire

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3. Confined Vapor Explosion Explosion in a vessel or building Vapor cloud drifts/leaks into a building and ignites, the resulting fire

raises temperature and increases pressure by a factor of 8 to 10….sufficient to collapse wall or roof.

Example: Abbeystead disaster - Build up methane gas from earth within tunnels in water pumping station ignited, killing several of visitors.

4. Dust Explosions Inherent hazard whenever combustible solids of small particle size are

handled. Eg Coal, flour, wood dust, resin dust Example: Series of explosions in silos at New Orleans in 1977. 45

silos containing corn, wheat and soy beans involved…35 to 50 people killed.

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Damage bywood dust explosion

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Damage by resin dust explosion

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Toxic Release

Toxic chemicals can cause harm by inhalation, skin absorption, or

ingestion.

Short term and also long term effect from inhalation, absorption and

ingestion as well as identification, evaluation and control of toxicants are

covered under chapter: Toxicity and Industrial Hygiene.

Example:

a) Seveso, Italy, 10/7/76. Release of material containing dioxin to

atmosphere. Incident after a series of not following specified procedure.

About 2 kg of dioxin was discharge through relief valve to atmosphere.

Heavy rain washed toxic chemical, absorbed into soil. 600 people

evacuated, 2000 given blood test and many people suffered from skin

disease

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b) Manfredonia, Italy, 26/9/76. Aerosol mixture containing 10 tons of

K3AsO3 and H3AsO3 escaped from NH3 cooling column. Explosion

resulted in 60 tons of water, 10 tons of arsenic trioxide and 18 tons of

potassium oxide released into atmosphere. Contaminated 15 km2 of

cultivated land from 2 km2 area around the plant with arsenic. Fishing

prohibited and 30 people contaminated. Area declared safe in Jan 1977.

c) Bhopal, India, 3/12/85. Runaway reaction caused release of methyl

isocynate (MIC) and possibly hydrogen cyanide. 2500 fatalities and

200,000 injuries. Identified causes : Inadequate design pipe work,

inadequate procedures, inadequate emergency plan, inadequate job

supervision, inadequate maintenance of protective equipment,

inadequate management capabilities, and possibly sabotage.

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• Hazard Survey/Hazard Inventory - Identifies all stocks of hazardous material with details of conditions of storage and information on nature of hazard i.e toxic, flammable etc (conceptual design stage).

• Hazard Indices - Checklist method of hazard identification which provides a comparative ranking of the degree of hazard posed by a particular design conditions, i.e the Mond Index and the Dow Fire and Explosion Index (detailed design stage).

• Hazard and Operability Study (HAZOP) - A formal systematic method of identifying hazards and operability problems by used of guide words (detailed design stage).

• Failure Mode and Effects Analysis (FMEA) - Hazard identification method where all known failure modes of components or features of a system are considered in turn and undesired outcomes noted. If the chances of failures and the seriousness of the consequences are ranked to identify the most critical features it becomes Failure modes, Effects and Criticality Analysis (FMECA) (detailed design stage).

Hazard Identification