Element 2 – Principles of Fire and Explosion

1. Jan 08-E2-Q5-10
Question 5 With examples of actual incidents, describe the principles and effects of a vapour cloud
explosion. (10)

1. A vapour cloud explosion may be confined for example in a tank or vessel or unconfined. Its key
principles include the presence of flammable vapour at a concentration between the upper and
lower explosive limits and an ignition source that exceeds the minimum ignition energy.

2. Unconfined vapour clouds may travel a considerable distance before igniting or they may be
dispersed to a concentration below the lower explosive limit depending on conditions.

3. The effects of vapour cloud explosions include overpressure, thermal effects and the emission of
debris. In confined explosion cases, vessel or containment rupture may occur resulting in a rapid
release of liquefied gas.

4. In unconfined explosion cases, damage to people and property may be caused by the pressure
wave and thermal radiation. Suitable examples could have included
 Flixborough 1974,
 Abbeystead 1984,
 Grangemouth 1987
 Buncefield 2005.

2. Jul09-E2-Q2-10
Question 2 In 1974 a plant in Flixborough, United Kingdom, producing highly flammable
pressurised gas developed a leak on the fifth of six reactors. In order to continue production
the fifth reactor was removed from service and a bypass assembly then installed between the
fourth and sixth reactor. The bypass assembly subsequently ruptured and an unconfined
vapour cloud explosion followed.
(a) Describe the mechanism for an unconfined vapour cloud explosion. (3)
(b) Outline the technical failings that contributed to this major accident. (7)

The incident at Flixborough occurred following the uncontrolled release of a gas, cyclohexane, from a
temporary pipeline which had been used to bypass a reactor taken out of service for repair. On
release the gas formed a flammable mixture in air
within explosive limits and the large vapour cloud either immediately came across a source of ignition
or travelled some distance to find one where it was quickly detonated causing rapid flame
propagation.

In answering part (b), candidates should have described

1. The absence of a professionally qualified mechanical engineer on site to oversee the modification
between the fourth and fifth reactor.
2. The plant modification took place without a full assessment being made of the potential
consequences of its installation.
3. There was an absence of drawings for the proposed modification,
4. No reference was made to any recognised standard,
5. The constructed assembly was of unknown strength and
6. Flow and pressure calculations were undertaken on a straight bypass line and not a dog leg as
subsequently fitted.
7. Pneumatic tests that were carried out were for leak testing and not in order to assess the strength
of the assembly.
8. There was apparently no appreciation that the pressurised assembly would be subject to a turning
moment, imposing lateral shear forces on neither the bellows nor an appreciation of a hydraulic
thrust on them and consequently
9. N
10. No calculations were undertaken for the dog leg line or the bellows to withstand these strains.
11. No reference was made to the Designer’s guide so that bellows were used out of line without
adequate support, and insufficient support provided for the newly installed pipe work under
working conditions.
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Element 2 – Principles of Fire and Explosion

12. Finally and in short, those concerned with the design, construction and lay out of the modification
did not consider the potential for a major disaster to occur instantaneously .

Answers provided for this part of the question were particularly disappointing. Some candidates
described what happened rather than why it happened and some appeared unfamiliar with
Flixborough.

3. Jan10-E2-Q1-10
Question 1 In 1988 a disaster occurred on the Piper Alpha platform in the United Kingdom.
(a) Identify mechanical failures that contributed to the initial explosion. (4)
(b) Outline the systems failures associated with the permit-to-work system that contributed to the
initial explosion. (6)

Part (a) - Mechanical failures that contributed to the initial explosion at Piper Alpha
1. Removal of a safety pressure valve and its replacement with a blank flange assembly which
was incorrectly fitted and was not leak tight.
2. A running condensate pump tripped and a stand by pump, associated with the blanked flange
was operated.
3. Condensate and gas escaped from the blanked flange and the gas exploded.

Part (b) - Systems failures associated with the PTWS that contributed to the initial explosion:
1. Failures with the PTWS which was in operation for the removal of the pressure valve
including a failure in communication at the end of the working day with the suspended permit
returned to the control room but not displayed and a failure to check that the blanked valve
was leak tight at the time
2. Failure in communication at shift handover with the absence of any written procedures for this
event
3. The absence of formal training for those with the responsibility of issuing permits
4. Inadequate written procedures for the permit to work in that,
 There was no reference to locking off or tagging valves to prevent inadvertent operation
 No mention of the need to cross reference permits
 No place on the permit for the issuer to declare that the work had been left in a safe
condition
 No attention drawn to the potential dangers associated with the ‘suspension’ of permits
5. Finally inadequate monitoring and auditing by management of the permit to work procedure.

4. Jan10-E2-Q11-20
Question 11 In November 1984 the San Juanico gas storage facility near Mexico City exploded with
catastrophic consequences.
(a) Outline the circumstances of the disaster. (8)
(b) Outline how a Boiling Liquid Expanding Vapour Cloud Explosion (BLEVE) occurs with a gas
storage vessel constructed of metal. (8)
(c) Outline the immediate rescue and restoration measures that took place. (4)

A major fire and a series of catastrophic explosions occurred at the San Juanico LPG terminal near
Mexico City. As a result of the incident,
1. Some 500-550 people died and the terminal destroyed.
2. The terminal was supplied with LPG from three refineries on a daily basis.
3. On the day of the incident it was being supplied from a refinery some 400km away.
4. Two large spheres and 48 cylindrical vessels (torpedoes) were filled to 90% capacity and four
smaller spheres to 50% capacity.
5. A drop in pressure was noticed in the control room but the operators were unable to identify the
cause of the pressure drop.
6. An 8 inch pipe between a sphere and a series of cylinders had ruptured.
7. The gas accumulated around surrounding tanks and over an area measuring 150 metres by 200
metres.
8. The gas cloud drifted to a flare stack and exploded.
9. A number of ground fires from the original point of fracture were directed at adjacent cylinders. A
series of BLEVEs occurred involving four of the six spheres and a number of the cylindrical
vessels.

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10. Injuries sustained were caused by radiated heat and burning droplets of LPG.

In answer to part (b), candidates should have outlined that

1. Gases such as butane and propane are stored under pressure in metal storage vessels.
2. When the valve to the vessel is opened, the resulting drop in pressure restores the LPG to the
gaseous state.
3. If, however, the vessel is involved in a fire, the liquefied gas boils and the contents of the vessel
will revert to the gaseous phase with a resulting increase in pressure inside the vessel.
4. The pressure relief valve opens and vaporised liquid escapes and the liquid level drops.
5. As there is less and less liquid left to absorb the heat, the vessel above the liquid level absorbs
the heat and the metal starts to change and weaken and ultimately ruptures as a result of ductile
failure.
6. The remaining contents burst out and vaporise under atmospheric pressure sending debris from
the vessel into the surrounding area. The vapour cloud which includes gas droplets finds an
ignition source and explodes.

Following the explosion,

1. Some 4,000 emergency workers were drafted into the terminal.
2. The primary measures were concerned with the temporary evacuation and transport of the
injured.
3. Further measures were aimed at the prevention of an epidemic, the removal of debris and
identification of those who had been killed.
4. Fires were extinguished, temporary shelters provided and arrangements made for the mass
internment of those who had died.

This question was not popular and was answered by only a few candidates. While many were able to
give a reasonable description of the mechanics of a BLEVE it became obvious that they had little
knowledge of the circumstances of the Mexico City incident.

5. Jul10-E2-Q1-10
Question 1 (a) Outline the principles of a vapour cloud explosion AND give an example of a major
incident associated with a vapour cloud explosion. (8)
(b) Outline the effects of a vapour cloud explosion. (2)

Part (a)
1. A vapour cloud explosion may be confined for example in a tank or vessel or unconfined.
2. Its key principles include the presence of flammable vapour at a concentration between the
upper and lower explosive limits and an ignition source that exceeds the minimum ignition
energy.
3. Unconfined vapour clouds may travel a considerable distance before igniting or they may be
dispersed to a concentration below the lower explosive limit depending on conditions.

Part (b) - The effects of vapour cloud explosions include:

1. Overpressure, thermal effects and the emission of debris.
2. In confined explosion cases, vessel or containment rupture may occur resulting in a rapid
release of liquefied gas.
3. In unconfined explosion cases, damage to people and property may be caused by the
pressure wave and thermal radiation.
4. Suitable examples could have included Flixborough 1974, Abbeystead 1984, Grangemouth
1987 and Buncefield 2005.
5. Also, outline the effects of heat radiation and toxic fume emissions.

6. Jan11-E2-Q6-10
Question 6 A diesel engine is being used to power a machine in a potentially flammable atmosphere.
(a) Identify the sources of ignition from the diesel engine. (4)
(b) Outline the protection that should be applied to the engine to minimise the risk of an explosion. (6)

In the first part of the question, candidates were asked to identify possible sources of ignition from a
diesel engine.
1. They should have referred to flames or sparks from the exhaust and inlet systems,

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Element 2 – Principles of Fire and Explosion

2. sparks arising from the engine’s electrical system,

3. Sparks caused by the presence of static electricity, through over speeding or overloading of the
engine and
4. from friction or the surface temperature.
Answers were to a reasonable standard though there were some candidates with the misconception
that spark plugs were likely sources of ignition and that the fuel was highly flammable.

Part (b) required an outline of the protection that should be applied to the engine to minimise the risk
of explosion.
1. An initial protection measure would be the fitting of flame and spark arrestors to prevent flashback
to atmosphere if flammable vapour was drawn into the inlet system and
2. To prevent any flames from the exhaust system escaping into the air of the workplace.
3. The engine and exhaust system should be designed to ensure that their surface temperatures
were below the ignition temperature of the flammable substance and the fitting of a water jacket
on the exhaust system together with thermal sensors and air cooling for ‘hot spots’ would provide
additional protection.
4. Electrical equipment on the engine should be suitable for zones 1 and 2,
5. Mechanical alternatives, as for example the fuel gauge, should be fitted where possible and
electrical starters wired to a plug and socket in a safe area.
6. The use of electrically conducting materials for parts such as tyres and drive belts would reduce
static build up,
7. while fitting speed limiters would help prevent over speeding or over loading of the engine.
8. Finally it would be advisable to fit an air inlet shut-off valve to stop the engine if at any time vapour
ingestion does lead to overspeeding.

Answers to this part of the question were not to the same standard as part (a).

7. Jul11-E2-Q5-10
Question 5 Many household aerosol canisters contain propellants that are a mixture of highly
flammable materials such as iso-propane and iso-butane. These canisters are used for multiple
purposes including cleaning products, furniture polishes, cosmetics, deodorants, etc.

(a) Describe how a fire and explosion may occur when large quantities of these canisters are stored
in a commercial warehouse. (5)

(b) Outline the fire precautions that could be taken to limit the spread of any resulting fire and
explosion in such a warehouse. (5)

There are a number of factors which may cause fire and explosion where large quantities of aerosol
canisters containing highly flammable materials are stored in a warehouse.
1. The canisters may have suffered damage for example to the valve or body of the canister, leading
to loss of containment and the formation of flammable vapours
2. Which if between the upper and lower flammable limits can be ignited by a heat source, perhaps
a nearby fire or hot work and even by direct sunlight.
3. The canisters may then be propelled from their original position as projectiles as the radiant heat
from the initial fire causes boiling of the contents of adjacent canisters.
4. This causes the release and ignition of additional vapour sources resulting in multiple BLEVEs.

Answers to this part of the question were limited with few candidates able to describe adequately how
a fire and explosion might occur. While some mentioned that an ignition source was necessary, they
gave no indication where this might come from. Few discussed the possibility of projectiles or the
effect of radiant heat on the contents of adjacent canisters.

(b) Fire precautions that can be taken to limit the spread of any resulting fire and explosion in a similar
type of warehouse include
1. the segregation of the canisters from other flammable products;
2. the separation of high flammability canisters by physical barriers; using compartmentation within
the warehouse with fire resistant partitions;
3. the provision of containment cages to prevent projectiles; the installation of automatic
fire detection and alarm systems;

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Element 2 – Principles of Fire and Explosion

4. The use of automatic extinguishing systems and venting and explosion reliefs.

With this part of the question, candidates again did not take the time to consider exactly what was
required. It was concerned with precautions that might be taken to limit the spread of fire but some
wrote at length about fire drills, the appointment of fire marshals and emergency action procedures
without indicating how these would have limited the spread of fire. There was an original suggestion
made that the canisters should be fitted with pressure relief valves.

8. Jul11-E2-Q9-20, Jul12-E2-Q10-20& Jan09-E2-Q11-20

Question 9 In relation to dust explosions:
(a) outline the conditions that must be present for a primary dust explosion to occur; (4)
(b) outline the additional conditions necessary for secondary explosions to occur; (4)
(c) identify FOUR factors that may affect the reliability of a nitrogen inserting system used in a
powdered aluminium process; (4)
(d) identify the design features that would minimise the likelihood and effect of a dust
explosion. (8)

In order for a primary dust explosion to occur,

 the dust must be combustible,
 must be capable of becoming airborne and mixing with air and
 Its particle size and distribution must be capable of propagating flame.
 Additionally the concentration of dust must fall within the explosive limits,
 An ignition source of sufficient heat energy must come in contact with the dust and
 the atmosphere must contain sufficient oxygen to sustain combustion.

A recall of the dust explosion pentagon would have assisted candidates in answering this part of the
question.

(b) Additional conditions necessary for secondary explosions to occur include

 the dislodgement of accumulated dust from horizontal surfaces within the affected building by
the pressure wave and consequent air turbulence created by the primary explosion and
 the airborne suspension of combustible dust throughout the affected area which is ignited by
the original primary explosion ignition source or by the combustion of products from the
primary explosion or by any other ignition source with sufficient heat energy within the
affected area.

Reasonable answers were provided for this part of the question though some candidates failed to
refer to the dislodgement of accumulated dust or to identify possible ignition sources.

(c ) In identifying factors that might affect the reliability of a nitrogen inserting system, candidates
could have chosen from
 the location and number of sampling points;
 the type and calibration of the sensor;
 the possibility of contaminants in the system interfering with the readings;
 An inadequate supply of inserting gas;
 the number of locations where air might enter the plant or process;
 the safe means of shutdown if oxygen levels become too high; and
 the reliability of the electronic control system.

This part of the question produced some good answers though some candidates neglected to mention
the number and location of sampling points and the type and calibration of the sensor.

(d) In identifying design features that would minimise the likelihood and effect of a dust explosion,
candidates should have referred to;
 The initial design of the ducting and equipment to withstand the effects of an explosion;
 The importance of ensuring the ducting was dust tight;
 Providing local exhaust ventilation at points of transfer; installing explosion relief or
suppression systems together with systems for suppressing fire;
 Using screw conveyors instead of pneumatic systems for moving materials;

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 Providing a magnetic extraction system for removing metal from product fed to the plant;
 Interlocking equipment to prevent overfilling of vessels and over pressurisation;
 Using instrument systems with integral emergency shutdown;
 Using intrinsically safe electrical equipment and bonding all metal work to earth;
 Preventing dust build up in the plant by the use of sloping surfaces and introducing a
mechanized system for the humidification of the air.

Many of the above design features appeared in the answers provided with those less frequently
mentioned being the prevention of over-pressurisation and the introduction of systems with integral
emergency shutdown.

9. Jan12-E2-Q1-10
Question 1 Compartmentation in buildings consists of fire resisting doors, walls, floors, ceilings and
windows. Window frames are potentially a weak point in this fire resistance.
(a) Outline how window frames made with the following materials might behave in a fire:
(i) wood; (3)
(ii) metal; (3)
(iii) plastic. (3)
(b) Identify which of these window frame materials gives the best overall fire resistance. (1)

 As far as wooden frames are concerned, some varieties of timber are more fire resistant than
others, for example, those which have been treated. Thin sections promote fire, generating fumes
and smoke although the charred surfaces may act as an insulator providing protection for the
inner timber. Timber frames tend to maintain their shape and will have limited distortion with the
smoke seal and level of fire resistance being maintained.

 Metal window frames expand and distort leading to a reduction in fire resistance and the escape
of smoke with the possibility that the window unit as a whole will become nseated and fall out. The
metal conducts heat, will regain strength on cooling but may remain permanently deformed.

 Plastic window frames expand, distort and melt at relatively low temperatures producing toxic
fumes and smoke with molten plastic flaking off and falling to the area below the window. The
frame will suffer a complete loss of integrity leading to a reduction in fire resistance and the
escape of smoke with permanent deformation on cooling.

It was not sufficient to decide only whether the three materials burn or not. Answers needed to
indicate whether the material maintained its shape and integrity and continued to provide a barrier
against the spread of smoke and fire. Many did not. The provision of correct answers for part (a),
would inevitably have led candidates to the conclusion that wooden frames provide the best overall
fire resistance. Many
chose metal and some even plastic. There were a few who opted for wood and metal, probably to
cover all options, but this strategy did not gain them the mark.

10. Jan12-E2-Q3-10
Question 3 You have been asked to investigate a boiler explosion at one of your company’s sites.
The maintenance manager has drawn you a sketch showing the position of the water column isolating
valves at the time of the explosion (see below).

(a) Describe the chain of events that could have led to an explosion due to the water column valve
being closed. (6)
(b) Other than the safety feature on the water column, identify FOUR other safety features that you
would expect to find on the boiler. (4)

As for the events that could have led to an explosion,

1 Since a valve on the water column was found to be closed
2 The column would give a false reading.
3 The actual water level would fall as steam was drawn on demand,
4 The crown would become exposed, heat transfer in that area would be ineffective and
5 a localised heat build-up would take place.

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6 The crown would start to yield as a result of internal pressure and a ductile failure followed by a
rupture would occur.
7 There would be a loss of containment with an instantaneous change from water to steam and an
explosion as stored energy was released.
8 There was a lack of understanding of the possible sequence of events and of the failure modes
involved.

Candidates did not seem to appreciate the consequences of a reduced water level and wrote about
boiler tubes, reaction vessels and flammable vapours with the occasional suggestion that a BLEVE
had occurred.

Answers provided for part (b) of the question were to a better standard with many candidates
identifying safety features such as
 pressure and temperature indicators,
 a pressure relief valve,
 a low water level alarm,
 a bursting disc or fusible plug, and
 an automatic control system incorporating a fuel cut-off.

11. Jan12-E2-Q11-20
Question 11 A road tanker is to be filled with petroleum (gasoline) at an installation.
(a) Identify FOUR hazards associated with this operation. (4)
(b) Describe how a vapour cloud explosion could be generated in this situation. (4)
(c) Outline practical ways of minimising the risk of an electrostatic charge. (4)
(d) Outline control measures that could reduce risks with road tanker filling. (8)

In answering part (a) of the question candidates were expected to identify hazards such as
1 Over-filling, over-loading or spillage;
2 The presence of incompatible or contaminated products;
3 The presence of fumes and/or vapour together with an ignition source such as static electricity or
smoking resulting in a fire or explosion;
4 A vehicle collision or a vehicle driven away while the hoses are still attached; and
5 The risk of a fall from working at height.

For part (b), candidates should initially have referred to the fact that
1 The explosion could be confined in a tank or vessel or be unconfined in the open air if large
quantities of vapour were present.
2 It would be generated by the formation of a flammable mixture in air within the upper and lower
explosion limits, which would be ignited by a source with energy in excess of the minimum
required.
3 All three elements of the fire triangle would need to be present for the explosion to occur.

There were not many candidates who were able to describe a credible scenario which would result in
either a confined or unconfined vapour cloud explosion and there were but few references to the fire
triangle.

(c) Practical ways of minimising the risk of an electrostatic charge include the use of
1 Tankers and pipe work with high conductivity;
2 Reducing the velocity of the filling or draining operation by controlling the pump rate;
3 Reducing constrictions by the use of smoother pipes unrestricted by strainers;
4 Increasing the relaxation time by using longer pipe work;
5 Earthing the pipe work and tanker and bonding to the same electrical potential; and
6 Using anti-static clothing and footwear and anti-static additives in liquids.

Some candidates suggested ‘inerting’ although this does not prevent the charge forming but prevents
a possible fire by excluding oxygen. Earthing and bonding to the same electrical potential was hardly
ever mentioned.

For part (d), candidates were expected to outline control measures such as
1 The preparation of written procedures and instructions;

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2 The identification of specified entry and exit routes for tankers together with a high level of
communication and cooperation between the tanker driver and the site operator;
3 The use of pumps, hoses and other equipment which are suitable for purpose, properly installed
and regularly maintained;
4 Fitting devices such as a high level alarm to prevent overfilling;
5 The erection of barriers or the use of bollards and warning cones to prevent a collision between
vehicles and installing a system to prevent the tanker driving away if hoses are still connected to
it;
6 Ensuring tankers are positioned correctly with wheel chocks and hand brake applied;
7 Ensuring hoses are coupled without submitting them to abnormal stress and valves operated in
accordance with the laid down procedures;
8 Switching off electrical equipment which was not required for filling such as mobile phones and
bonding and equalising the potential between the tanker and the fixed installation;
9 Using bottom loading tankers in preference to those which are top loaded but if this is not
possible, ensuring that the top loaded tankers have a fill pipe which touches the bottom of the
tanker and that hand rails or fall arrest equipment is provided to prevent falls; and
10 Finally preparing procedures to be followed in the event of an emergency and ensuring that all
workers are fully conversant with them.

Many concentrated on control measures to reduce the risk of fire without considering the other
hazards that might exist.

12. Jul12-E2-Q2-10
Question 2 (a) Outline the principles of a Boiling Liquid Expanding Vapour Explosion (BLEVE) AND
give examples of actual incidents to support your answer. (8)
(b) Outline the effects of a BLEVE. (2)

This question was assessing IC2.1 Outline the properties of flammable and explosive materials and
the mechanisms by which they ignite.

Most candidates were not able to show that they did understand the principles of a BLEVE. Many
chose to answer in a general way and thought that a BLEVE was no more than an explosion in a
vessel. The expectation was that candidates could outline
1. The process by which flammable gases (mainly liquefied petroleum gas) that are stored as liquids
under pressure are subjected to an external heat source. This heat has two effects.
 Firstly, in the liquid phase the heat is absorbed and leads to the liquid phase boiling and an
increase in pressure in the vessel.
 Secondly, the gas phase absorbs less of the heat and energy is absorbed by the metal
structure leading to thinning of the material and eventual rupture. This loss of containment
causes a sudden drop in pressure inside the vessel leading to large quantities of liquid boiling
off. The expanding vapour is ignited by the external fire or heat source leading to the
explosion.

Suitable examples were

 the San Juanico disaster, Mexico City (1984),
 The Los Alfaques tanker explosion in Alcanar, near Tarragona, in Spain (1978) and
 Tanker explosion in a tunnel near Palermo, Italy (1996).
Other relevant examples were acceptable.

When answering part (b), many candidates referred to

 Flying debris
 The thermal radiation effects or the overpressure effects on the surrounding area. References
to fireballs, debris and missiles, thermal radiation and shockwave were needed to get the full
marks for part (b).

13. Jul08-E2-Q4-10
Question 4 The transfer of an extremely flammable liquid from a bulk storage tank to a road
tanker may generate static electricity. Outline the control measures which would reduce the
risk of ignition of the extremely flammable liquid vapour due to static electricity. (10)

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In answering this question, candidates should have addressed the main methods of avoiding the
build-up of static electricity. These would have included:
1. Controlling the pump rate and transfer speed;
2. Earthing and bonding the pipeline, the tanker and the storage tank to ensure all equipment was at
zero potential after allowing sufficient relaxation time;
3. The provision of an interlock between the earthing arrangements and the pump;
4. The avoidance of splash filling and the selection of conductive/smooth materials for the transfer
system;
5. Elimination of possible contamination arising for example from the presence of water in the
pipeline;
6. The use of a vapour return system; nitrogen blanketing of the road tanker barrel to minimise the
risk of explosion; the provision of instrumentation, for example, to detect earth leakage and the
use of anti static clothing and footwear.

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