lecture 10 explosion

ECE 4353

Chemical Process Safety

Explosion

(Lecture 10)

Last Updated:1 December 2015

© LMS SEGi education group 2

LEARNING OBJECTIVES

Explain the government-industry’s responsibility for

health and safety

Evaluate the nature of hazards posed by materials

which are flammable, toxic and reactive

Identify and quantify common industrial methods to

control hazards.



Lecture 10

10.1 Explosions

10.2 Vessel rupture

10.3 Rapid phase transition

10.4 BLEVE

10.5 Vapor cloud explosion



Last Updated:1 December 2015 4

Major classifications of explosion

10.1 Explosion




• An EXPLOSION is defined as a phenomenon where a blast

(pressure or shock) wave is generated in air by a rapid release

of energy.

• The resulting blast wave is largely responsible for the damage

that was caused.

• The mechanism of propagation of an explosion into the

unburned material is characterized as a deflagration or a

detonation.

10.1 Explosion




Deflagration Detonation

Mechanism for

propagation of explosion

reaction

Heat and mass transfer Shock compression

healing

Rate of transfer of energy Slow (propagating rates

< speed of sound)

Very rapid (propagating

velocities > speed of

sound)

Overpressure profile Shock wave forms slowly

and develops a

significant distance from

centre of explosion

Shock front develops

rapidly close to source

of explosion

Example Combustion of

flammable vapor in a

pipeline

Most Vapor cloud

explosions

TNT explosion




• The amount of explosion overpressure is

determined by the flame speed of the explosion.

• Flame speed is a function of : turbulence within the vapor

cloud that is released and the level of fuel mixtures within the

combustible limits.

• Maximum flame velocities are obtained in mixtures containing

slightly more fuel than is required for stoichiometric

combustion.

• Turbulence is created by confinement and congestion.

• (Confinement and congestion are available on most offshore

production platforms)




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Physical Explosion is caused by a sudden release of

mechanical energy; not a chemical reaction.

A vessel rupture explosion occurs when a process

vessel containing a pressurized material fails

suddenly.

The failure can be due to mechanical failure,

corrosion, heat exposure, cyclical failure etc.

10.2 Vessel rupture




Factors in pressure vessel failures

Operation > maximum allowable working pressure (MAWP) ™

Improper sizing or pressure setting of PSV ™

Improper operation of relief devices due to faulty maintenance and

failure to test regularly. ™

Failure of the vessel due to fatigue from repeated pressurization,

general thinning from corrosion or erosion, stress corrosion

cracking, holes and leaks. ™

Failure to inspect frequently enough. ™

Improper repair of a leak or other defect involving welding that

embrittles and further weakens the vessel.

Overpressuring and failure of the vessel due to exothermic reaction

or polymerization. ™

Vessel exposure to fire




Pressure Vessel Laws

ASME Boiler and Pressure Vessel Code provides

rules for pressure vessel design, fabrication,

inspection, weld procedures, welder qualifications,

and pressure testing.

The first Boiler and Pressure Vessel Code (1914 Edition) was published in 1915 out of a need to protect the safety of the public. In the 19th century there were literally thousands of boiler explosions in the United States and Europe.




Pressure Vessel Laws according to ASME VIII

(covers vessels operating between 15 psi and 3000 psi)

Marking the ASME Code on the vessel with specified information eg.

manufacturer, serial number, the year built, MAWP and any special suitability

such as for low temperature and poisonous gases or liquids. ™

Vessel approved for installation with the submission of drawings,

specifications, welding details, calculations and having authorized inspection

Operating at pressures < MAWP

Periodic Testing and inspection ™ according to the NBIC Manual for Boiler

and Pressure Vessel Inspectors or American Petroleum Institute (API) 510

Repairing or altering only according to a plan approved by an

authorized inspector and conducted by test-qualified welders. The inspector

must be satisfied that the repairs are performed according to NBIC or API

510 and specify any necessary non-destructive and pressure testing.

Increasing the maximum allowable working pressure or temperature is

considered an alteration whether or not physical work is done.




Rapid phase transition

A rapid phase transition occurs when a material is

exposed to a heat source, causing a rapid phase

change and resulting change in material volume.

As the LNG storage temperature is around -161ᵒC

when it comes in contact with anything in

surrounding that is at the ambient temperature,

there is a fast heat transfer due to large

temperature difference leading to rapid

vaporization of LNG.

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A hot oil at a temperature of 250°C was pumped into a distillation column for processing. Initially, valves A and B were closed. Due to a previous maintenance operation, water was present in the blocked-off pipe section between valves A and B. Valve A was accidently opened during the operation, exposing the water to the high temperature oil. The water flashed explosively, resulting in extensive internal damage to the column.




BLEVE (boiling liquid expanding vapor explosion)

• BLEVE is an explosion caused by the rupture of a

vessel containing a pressurized liquid above its

boiling point.

• BLEVE is a physical reaction in which the material

rapidly, and instantaneously, converts from a

liquid to a gas. It is a change of state that yields

pressure.

• Not a chemical reaction

• The vessel failure causes sudden flashing of the

liquid into vapor, ejection of liquid and vessel

contents possibly a fire ball may result if the

material is combustible.

10.4 BLEVE




BLEVE is any sudden loss of

containment of a liquid above its

normal boiling point at the moment of

its failure.




Fire impinged on tank Liquid converts to vapor in tank




Liquid vaporizes causing pressure inside the tank to increase




Relief valve opens to vent the excess pressure




As pressure in the tank decreases, valve begins to close. With continued heating, pressure increases and relief valve re-opens. .





Where flames impinge on the tank, the liquid in the tank absorbs the heat allowing tank temp to remain at the same temp. With operation of the relief valve, the liquid level drops exposing the tank to the effects of heating. The tank metal begins to weaken, stretch and tear apart.





Tank containment gives way. Large contents of liquid and vapor are released in a powerful explosion.





The heat radiated is sufficient to ignite combustibles at a great distances





Tank sections containing rapidly igniting fuels can become flying missiles causing secondary fires and other damages





BLEVE




A container can fail due to damages caused by:

1. Damage by impact from an object

2. Vessel internal pressure

3. Vessel material brittleness

4. Impingement of fire

Facts about Bleve

BLEVE is independent of the cause of the container failure. For a BLEVE to occur, the container has to be under pressure, the pressure has to exceed the strength of the container, and the container has to be weakened in some way (impact, corrosion, fire).




A steam explosion is a violent boiling or flashing of water

into steam.

• Pressure vessels, eg. pressurized water (nuclear) reactors,

that operate above atmospheric pressure can also provide

the conditions for a steam explosion.

• The water changes from a liquid to a gas with extreme

speed and increases dramatically in volume.

• A steam explosion sprays steam and boiling-hot water

and the hot medium that heated it in all directions

BLEVEs occur with many types of liquefied gases, flammable and nonflammable.




If the material is flammable, a fireball may

follow it. The rapid explosion can also cause

projectile effects.




• A BLEVE or bursting pressure vessel can produce

fragments that fly away from the explosion

source.

• These primary fragments, which are part of the

original vessel, are hazardous and may result in

injuries to people and damage to structures

• For brittle vessels, or in the case of detonations,

severe fragmentation is possible.

• The velocity of fragments from a pipeline will be

greater than that from an isolated vessel at the

same conditions because of the replacement of

gas loss by flow from the intact pipe




A vapor cloud explosion (VCE) results from the

ignition of a flammable mixture of vapor, gas,

aerosol, or mist, in which flame speeds accelerate

to sufficiently high velocities to produce significant

overpressure.

VCEs are generally associated with the release of a

sufficient quantity of flammable gas or vaporizing

(flashing) liquid from a storage tank, process

or transport vessel, or piping system.





Conditions present before a VCE with damaging overpressure can occur:

1. The released material must be flammable or within

the flammable range for the material.

2. A cloud of sufficient size must be formed prior to

ignition.

3. The vapor cloud must sufficiently mix with air to

produce a sufficient mass in the flammable range.

4. An ignition source. Higher-energy ignition sources

can lead to a more severe explosion than do lower-

energy sources.

5. The speed of flame propagation as the vapor cloud

burns. Without this acceleration, only a flash fire will

result.





On the March 23, 2005, a hydrocarbon vapour

cloud explosion occurred at the ISOM

isomerization process unit at BP's Texas City

refinery in Texs City, Texas.

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Group Project – Case Study on Significant Disasters

Prepare a Case Study on Significant Disasters:

The following topics shall be discussed:

The Incident (15%)

The sequence of events leading to the incident (15%)

The After Effects of the incident (15%)

Recommendations about how safety should be improved (35%)

(provide an in-depth study based on your chemical process knowledge)

- Quality of Presentation (20%)

(flow of ideas, clarity etc. )

- NOTE : Video may be shown but marks are not allocated to the video




Group Project – Case Study on Significant Disasters

Topics

1. Piper Alpha disaster which killed 167 workers on 6 July 1988 off the coast of Aberdeen which is the world's deadliest ever oil rig accident.

2. Bhopal gas tragedy which was gas leak incident in India, 2-3 Dec 1984, considered the world's worst industrial disaster.

3. Flixborough explosion which happened in June 1974. It had a major impact on Chemical Engineering in UK.

4. Fukushima nuclear disaster - following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on 11 March 2011; the worst nuclear incident since Chernobyl.

lecture 10 explosion

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