C - Hazard Scenarios

8/20/2019 C - Hazard Scenarios

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C – Hazard and Risk Scenario Analysis

Let’s talk about an LPG pipe…

Hazard and Risk Scenarios

Scenario

Impact

Dispersion

Release

Failure

Consequence


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Mechanical Failure

• Failure occurs when the internal conditionsovercome the Mechanical Integrity of thesystem.

• Mechanical failure may cause:– Falling loads

– Projectiles

– Release of hazardous materials

• Mechanical failure may contribute directly toinjuries and property damage or it may triggera more serious secondary incident.

Machinery Failure

Defective Equipment

• Damaged equipment or in a state ofdisrepair is not reliable and is subject tofailure.

• Defective equipment must be closelymonitored and scheduled for repair orreplacement.

• Contingency plan to deal with earlyfailure.

• What is the justification and basis fordecision to continue to operate?


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Hydrotest FailureHydrostatic pressuretesting is preferred topneumatic pressuretestingGreater energy stored in acompressed gas

Immediate cause: quality ofmaterials of construction

Hazardous Material Release (1)

Release may occur due to ……

• Mechanical failure of pressureenvelope.– Seals and Packings, Hoses, Flanges,

Valves

– Sample connections, small boreconnections

– Corrosion under insulation

– Leaks on elbows and return bends


Release may occur due to ……• Internal upset causing overpressure or

high temp.– Overpressure lifting pressure relief valves

– Internal explosion due to violent chemicalreaction.

– Rupture of furnace tube or heat exchangertube.

– Failure of a process vessel due to thermalshock.


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• Human Error resulting in the directrelease of material to environment.– Operator inadvertently leaving a valve

open

– Overfilling a vessel or KO drum.

– Opening a maintenance connectionduring operation.


Release influenced by:– System pressure

– Hole size

– Ratio of liquid to vapor

Modeling is used to understand effects of

a release.– Experience and knowledge of models isrequired

Atmospheric Dispersion


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Dispersion Effects

Consequences

DispersesNo

DispersesNoNo

Yes

Fire at YNoat X

YesRelease

Explosion at YYesFlammable

Fire at XNo

Yes

Explosion at XYes

Explosion onIgnitionIgnition at YWind to YIgnition at X

DispersesNo

DispersesNoNo

Yes

Fire at YNoat X

YesRelease

Explosion at YYesFlammable

Fire at XNo

Yes

Explosion at XYes

Explosion onIgnitionIgnition at YWind to YIgnition at X

Event Tree describes the possible scenario consequences

Consequence Modelling

• Used to determine– Distances to Lower Flammable Limit

– Extent of vapor cloud

– Distance to toxic concentration level


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Consequences: Types of Fires

• Flash fire – The combustion of aflammable gas or vapor and airmixture in which the flamepropagates through that mixture ina manner such that negligible or nodamaging overpressure is generated.


• Fireball - Afuel rich cloudburns from theoutside in.Moderateoverpressure.High radiation

heat release.Typically 10secondsduration.


• Jet fire - Combustion of gaseous fuelunder pressure at point of release.


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Hickson & Welsh – New Office


• Pool Fire - A flammable liquid surfaceburning with a stationary flame.


• BLEVE - Boiling Liquid Expanding VaporExplosion.

Failure of containment due to high localized heatflux. Vapour Cloud Explosion with shrapnel!


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Offshore Platform Fire

Consequences

Feyzin - 1966

Case History - CH2


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Case History 2-Feyzin

1966 Elf Oil Refinery at Feyzin, France.

• Located close to residential community.

• 160 meters (525ft) from public highway

• LPG Tank farm contained 8 spheres.

The Incident

What Happened

• While attempting to drain the water a massivehydrocarbon release occurred through open draincircuit.

• Several minutes later a local vehicle enteredroadway adjacent to refinery. (ignition source)

• Vapor cloud explosion occurred.• Ground pool fire beneath leaking sphere caused it

to BLEVE 90 minutes after the initial leakage.• The other 5 spheres subsequently BLEVE’d


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Incident Details

Schematic

1. Downstream valveOPENED FULLY

2. Upstream valveCRACKED OPEN

3. Upstream valveunable to closeafter ice plugremoved

4. Unable to closedownstream valvedue to freezing

Massive vapour cloudforms – ignited by

passing vehicle

Massive vapour cloudforms – ignited by

passing vehicle

Incident Details – Cont’d

• Valves could only be operated by specialvalve wrench. (security feature)

• Valve manifold was not freeze protected.

• Shutoff valve was inaccessible duringemergency.

• Operator was not adequately trained.

Major Lessons Learned• Design must recognize extreme

weather exposures andpotential for human failure.

• Personnel must be trained tohandle unexpected situations inthe field.

• As the LPG expanded acrossthe valves it cooled enough tofreeze the water.

E n g i n e e r i n g S y s t e m s T r a i n i n g / C o m p e t e n c e

P r o c e s s S a f e t y I n f o r m a t i o n


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Explosions

• Deflagration is combustion wave propagating ata velocity less than the speed of sound.

• Detonation is propagation of a combustion-driven shock wave at or above the speed ofsound.

DEFLAGRATION

DETONATION

IGNITION SOURCE

IGNITION SOURCE

FLAME TRAVEL

F LA ME TR AV EL S HO CK WA VE

PRESSURE WAVE

HEATEDCOMPRESSED GAS

Vapour Cloud Explosion• High energy release associated with rapid

burning of a flammable vapor/ air mixture.• Physical confinement or obstacles required to

produce overpressure.– Only the quantity of material within the cloud in a

confined area is involved in the explosion.

• Wind

– May cause a vapor cloud to drift into a confinedarea.– It also disperses the cloud and reduces the

flammable mass.

• Usually causes failures which lead to secondaryfires

Texas City, 2005


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Texas City, 2005

Internal (Mechanical) Explosions

• Failure of containment system due tooverpressure or sudden impulse.

• Vessel burst pressure is the meanbetween maximum event (explosion)pressure and the yield stress of thevessel.

• Missiles may be projected severalhundred meters.

Grangemouth 1987


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Secondary Fires

• Caused by initial blast (damage),prolonged exposure to fire.

• Difficult to predict.

• Sometimes delayed.

• Often causes more damage than initialincident.

• May weaken pressure envelope andcontribute to a subsequent explosion.

Dust Explosions

• Dust explosions result from the rapidcombustion of airborne particles.

• Conditions required for dust explosioninclude:– dust must be within explosible range

– dust must be airborne

– dust must have small particle size anddistribution

– must be in contact with high energy ignitionsource

Toxicity• Measure of harm from direct exposure to certain

chemical substances.

• Type of exposure includes oral, skin contact andrespiratory.

• Several scales used to denote probability ofdeath for a given exposure.

• ERPG typically used for extreme toxic hazards.

• Life health risk is a function of type of contact,toxicity and duration of exposure.

• Reaction of individuals to a specific toxin varies.

• Toxicity data do not reflect chronic healtheffects.


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Toxicity

Gas Cloud Plume

Toxic Gas Plumes can travel a long way.

Toxicity Limits

IDLH (30 min) HYDROGEN SULFIDE 100 PPM

AMMONIA 300 PPM

CHLORINE 10 PPM

LC1 (30 min) HYDROGEN SULFIDE 255 PPM

AMMONIA 6100 PPMCHLORINE 70 PPM

ERPG (1hr) and TLV are also used.

China, December 23rd 2003, 243 fatalities


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Smoke

• The term smoke is frequently used to mean themixture of pyrolysis products and air.

• Smoke contains gases, solid particles, anddroplets of liquid.

• It is toxic because contains noxious andhazardous substances.

• It interacts with light to obscure vision

• About 75% to 80% of fire victims die as aresult of exposure to smoke and toxic gases oroxygen depletion.

Nitrogen

• Commonly used as an inerting gas in cases wherethe presence of flammable vapors or explosivedusts cannot cannot be practically maintainedoutside the flammable limits.

• In enclosed spaces, the displacement of oxygencan cause suffocation due to oxygen deficiency.

• Where inert gas is used and people are exposed,care must be taken to insure adequate fresh airventilation.

• Purged electrical enclosures are difficult tomaintain and should be used only as a last resort.

Consequence Impacts

Impacts–Fire - burns

–Explosion – vulnerabilitydue to overpressure

–Toxic cloud – acute healthimpacts

–Environmental damage

–Plant damage, secondaryincidents

Receptors–People

–Environment

–Plant


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Scenario Impacts

• Thermal radiationEffects - BTU/hr/sq.ft . (kW/m2)

12,000 (37.5) - Sufficient to cause damage to process

equipment.

8,000 (25.0) - Minimum energy required to ignite wood at

indefinitely long exposures.

4,000 (12.5) - Minimum energy required for piloted

ignition of wood, melt plastic tubing

1,250 (4.0) - Sufficient to cause pain if duration is longer

than 20 seconds; however, blistering of the

skin is unlikely.

500 (1.6) - Will cause no discomfort for long

exposures.

Scenario Impacts

Explosion overpressure– Direct Impact or

– Damage to structure making the peopleinside vulnerable

Damage to buildings

– Windows broken at less than 0.07 bar (1 psi)– Traditional load bearing construction begins

to fail in the 0.07 – 0.2 bar (1 to 3 psi) range

– Blast resistant construction is oftendesigned for 0.3 – 0.5 bar (5 to 7 psi)

Environmental Impacts

C - Hazard Scenarios

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