c02_principles_of_fire_and_explosion_COMPLETE

Question 1

Flammability category and criteria Highest

Answer 1

CHIP Extremely Flammable: Flash point < 0°C and initial boiling point ≤ 35°C CLP Category 1: Flash point < 23°C and initial boiling point ≤ 35°C

Question 2

Flammability category and criteria Medium

Answer 2

CHIP Highly Flammable: Flash point < 21°C and not classified as Extremely Flammable CLP Category 2: Flash point < 23°C and initial boiling point > 35°C

Question 3

Flammability category and criteria Lowest

Answer 3

CHIP Flammable: Flash point ≥ 21°C and ≤ 55°C CLP Flash point ≥ 23°C and ≤ 60°C

Question 4

Flash point (FP): is defined as:

Answer 4

… the lowest temperature, corrected to a pressure of 101.325 kPa (atmospheric pressure at sea level), at which a liquid evolves vapours, under the conditions defined in the test method, in such an amount that a flammable vapour/air mixture is produced in the test vessel.

Question 5

The key points to note about the flash point are: 6

Answer 5

 It relates to volatile liquids.  It determines the flammability hazard classification.  It is established through various approved test methods.  It is the lowest temperature (at standard pressure) at which a liquid gives off enough vapour to form an ignitable mixture with air.  The presence of an ignition source is necessary to cause a flash.  The vapour will only burn briefly. For continuous combustion to be sustained a higher temperature (the fire point) is required.

Question 6

Fire point:

Answer 6

The fire point is the lowest temperature at which a vapour above a liquid will continue to burn once ignited; the fire point temperature is higher than the flash point.

Question 7

Vapour density:

Answer 7

The density of a gas or vapour is the ratio of the vapour’s mass divided by its volume, expressed in mg/m3

Question 8

Relative density:

Answer 8

The ratio of the density of a substance to the density of a standard substance under specified conditions. For liquids the comparator is water at 4oC, and for gases and vapours the comparator is air. Relative density can therefore be considered to be the measure of the density of a gas or vapour relative to air. It is calculated by dividing the Molecular Weight of the gas by that of air (approximately 28.90). Note: Air is approximately 21% Oxygen, 78% Nitrogen, 1% other gases – the molecular weight of Oxygen (O2) is 32 and Nitrogen (N 2 ) is 28.

Question 9

Flammability limits:

Answer 9

Vapour-air mixtures will ignite and burn only over a well-specified range of compositions. The mixture will not burn when the composition is lower than the lower flammable limit (LFL); the mixture is too lean for combustion. The mixture is also not combustible when the composition is too rich, that is, when it is above the upper flammable limit (UFL). A mixture is flammable only when the composition is between the LFL and the UFL. Lower explosive limit (LEL) and upper explosive limit (UEL) are used interchangeably with LFL and UFL. For example, the lower and upper flammability limits of toluene are approximately 1.3% and 7% respectively.  If the concentration of toluene vapour in air is less than 1.3% then combustion will not occur as the mixture of flammable vapour in air is too lean.  if the concentration of toluene vapour in air is greater than about 7% then combustion again will not occur; the mixture is to too rich.

Question 10

Stoichiometry:

Answer 10

The branch of chemistry and chemical engineering that deals with the quantities of substances that enter into, and are produced by, chemical reactions. Stoichiometry provides the quantitative relationship between reactants and products in a chemical reaction.

Question 11

The combustion equation was explained earlier. The process of combustion can be broken down into the following stages of growth and decay:

Answer 11

(1) Induction (2) Ignition (3) Growth (4) Steady state (5) Decay

Question 12

An explosion is

Answer 12

a rapid expansion of gases resulting in a rapidly moving pressure or shock wave. The expansion can be mechanical (by means of a sudden rupture of a pressurised vessel), or it can be the result of a rapid chemical reaction (such as combustion). Explosion damage is caused by the pressure or shock wave.

Question 13

Mechanical explosion:

Answer 13

An explosion resulting from the sudden failure of a vessel containing high-pressure, non-reactive gas.

Question 14

Deflagration:

Answer 14

An explosion in which the reaction front moves at a speed less than the speed of sound (sub-sonic) in the unreacted medium.

Question 15

Detonation:

Answer 15

An explosion in which the reaction front moves at a speed greater than the speed of sound (supersonic) in the unreacted medium.

Question 16

Confined explosion:

Answer 16

An explosion which occurs within a vessel or a building. These are most common and usually result in injury to the building inhabitants and extensive damage.

Question 17

Unconfined explosion:

Answer 17

Unconfined explosions occur in the open. This type of explosion is usually the result of a flammable gas release. The gas is dispersed and mixed with air until it comes in contact with an ignition source. Unconfined explosions are rarer than confined explosions because the explosive material is generally diluted below the lower flammable limit (LFL) by wind dispersion. These explosions are destructive because large quantities of gas and large areas are frequently involved.

Question 18

Boiling-liquid expanding-vapour cloud explosion (BLEVE):

Answer 18

A BLEVE occurs if a vessel that contains a liquid at a temperature above its atmospheric pressure boiling point ruptures. The subsequent BLEVE is the explosive vapourisation of a large fraction of the vessel contents, possibly followed by combustion or explosion of the vapourised cloud if it is combustible. BLEVE’s occur when an external fire heats the contents of a tank of volatile material. As the tank contents heat, the vapour pressure of the liquid within the tank increases and the tank’s structural integrity is reduced because of the heating. If the tank ruptures, the hot liquid volatilises explosively.

Question 19

Dust explosion:

Answer 19

This explosion results from the rapid combustion of fine solid particles. Many solid materials (including common metals such as iron and aluminium) become flammable when reduced to a fine powder.

Question 20

Shock wave:

Answer 20

An abrupt pressure wave moving through a gas. A shock wave in open air is followed by a strong wind; the combination of shock wave and wind is called a blast wave. The pressure increase in the shock wave is so rapid that the process is mostly adiabatic. (Adiabatic processes occur without input or release of heat within a system).

Question 21

Overpressure:

Answer 21

The pressure on an object as a result of an impacting shock wave.

Question 22

Critical temperature:

Answer 22

The temperature above which a gas cannot be liquefied, regardless of the pressure applied.

Question 23

Maximum explosion pressure (MEP):

Answer 23

The explosion pressure is the peak value of the time dependent pressure measured in a closed container (European standard test container) upon deflagration of an explosive mixture of defined composition. The maximum explosion pressure is the maximum value of the explosion pressure determined by varying the composition of the mixture.

Question 24

Rate of pressure rise:

Answer 24

The increase in pressure divided by the time interval necessary for that increase to occur. It is determined under the same test conditions as the maximum explosion pressure.

Question 25

A vapour cloud explosion may be confined (for example, in a tank or vessel) or unconfined (in the open air). The key requirements for a VCE to occur are: 2

Answer 25

 the presence of flammable vapour at a concentration between the upper and lower explosive limits  an ignition source that exceeds the minimum ignition energy.

Question 26

Confined vapour cloud explosions (CVCE)

Answer 26

An explosion can occur when a gas or vapour cloud is ignited within a confined volume such as a building or vessel. As the flame propagates through the vapour cloud it produces hot combustion products. The confinement prevents expansion of these combustion products and, as a consequence, the pressure increases. In general, this continues until the confining structure fails, in some cases catastrophically.

Question 27

Unconfined vapour cloud explosion (UVCE) UVCE’s occur in the following sequence:

Answer 27

 A large quantity of flammable vapour is suddenly released, usually as a consequence of a vessel rupture. (This is likely to be a minimum of 5 tonnes – for a flammable mix to occur in the open air).  The vapour disperses around the site and mixes with air.  The vapour cloud is ignited.

Question 28

There are two known mechanisms for generating an explosion in a relatively unconfined vapour cloud:

Answer 28

 Deflagration, where the flame accelerates to high speed, which requires a mechanism for generating the flame acceleration (turbulence).  Detonation which, if sustained, can be much more damaging. Detonation may arise from the coalescence of a strong shock wave and a fast-moving chemically reacting front, which together can propagate faster than the speed of sound and produce over-pressures at the wave front in excess of 10 bar.

Question 29

A BLEVE (ble-vee) occurs when

Answer 29

a tank containing a liquid, held above its atmospheric pressure boiling point ruptures, resulting in the explosive vapourisation of a large portion of the tank contents.

Question 30

Boiling-liquid expanding-vapour explosion (BLEVE) The process is: 10

Answer 30

 Fire impinges upon the walls of the LPG tank or cylinder.  Fire heats the walls of the tank.  The liquid within the tank helps to cool the tank metal.  As the temperature increases the tank contents boil and vapourise causing an increase in pressure.  The pressure relief valve operates letting off excess pressure (this vapour may be ignited by the fire and flare off).  Once the pressure is relieved the valve closes.  This cycle will be repeated.  The liquid levels within the tank fall.  The tank metal above the liquid level is not cooled, as it heats it expands and weakens until it eventually ruptures.  Upon rupture the tank contents are explosively vapourised.

Question 31

Precautions for controlling vapour phase explosions include: 7

Answer 31

 Segregation / separation of plant – reduces likelihood of ignition and minimises consequences of an explosion.  Hazardous area classification (HAC) and specification of appropriate electrical equipment (removal of potential ignition sources).  Designing plant to withstand pressure, or release it in a controlled manner via pressure relief valves, bursting discs, etc.  Regular inspection, testing and maintenance of plant to reduce the likelihood of loss of containment.  Reducing the inventory – UCVE’s in particular require a significant volume of vapour to create an explosive mix in the open air.  Use of inerting (replacing / reducing quantity of oxygen in air by replacing it with an inert gas) in fully enclosed systems to prevent the creation of an explosive atmosphere.  Monitoring of flammable vapour levels, linked to automatic ventilation or inerting systems.

Question 32

the dust explosion pentagon

Answer 32

For a dust explosion to occur the three sides of the fire triangle must come together: (1) Combustible dust (fuel) (2) Ignition source (heat) (3) Oxygen in air (oxidizer) The following additional conditions will also need to be met: (4) Dispersion of dust particles in sufficient quantity and concentration (5) Confinement of the dust cloud.

Question 33

For many organic materials the lower explosive limit is in the range

Answer 33

10 - 50g/m3dust cloud of this concentration resembles a very dense fog. Upper explosive limits are difficult to measure accurately, and have little practical importance, but are usually considered to be approximately 10 times the lower limit.

Question 34

The most violent explosions usually result from dust/air mixtures that are fuel rich. Other factors which can strongly affect the force of the explosion and the explosive limits include the shape and size of the dust particles, and the moisture content. Only weak explosions are likely where

Answer 34

the mean particle size of the dust exceeds 200 microns, or the moisture content exceeds 16%.

Question 35

Timber

Answer 35

 Timber burns though the burning or charring rate is predictable and varies slightly with the type of timber and not with the severity of the fire.  The timber behind the charring plane is largely unaffected and able to perform structurally as intended.

Question 36

Stone

Answer 36

Building stone is typically granite, limestone or sandstone.  Granite contains free quartz which expands very rapidly at 575oC completely shattering the rock.  Limestone (calcium carbonate) decomposes into free lime and CO 2 at approx. 800oC. The interior is protected by the outer skin.  Sandstone will shrink and crack in a fire at temperatures between granite and limestone.

Question 37

Bricks

Answer 37

 Traditionally in the UK bricks were made of fired clay. Concrete bricks and sand-lime bricks (calcium silicate) are now also popular.  Brick performs better than stone in a fire.

Question 38

Concrete

Answer 38

Concrete consists of aggregates, cement and water mixed to form a mouldable material which sets hard with high compressive strength and durability. It is non-combustible with a slow rate of heat transfer and can achieve fire rating requirements of 1 to 4 hours. Concretes are termed normal weight concrete (NWC) or lightweight concrete (LWC), depending on the density of the aggregates used. High strength concrete (HSC) uses a low water/cement ratio through the addition of fly ash, slag or superplasticisers, or additives such as silica fume to enhance its strength. Temperatures up to 95oC (200oF) have little effect on concrete. Above this threshold cement paste shrinks due to dehydration and aggregates expand due to temperature rise. For normal weight concrete the aggregate expansion exceeds the paste shrinkage resulting in an overall expansion of concrete.

Question 39

Concrete blocks

Answer 39

Blocks made with limestone aggregates have higher fire resistance. The fire resistance is greatly improved if plastered both sides with a lightweight gypsum plaster.

Question 40

Reinforced or pre-stressed concrete

Answer 40

 Fire resistance depends on the mass of concrete around the steel reinforcement.  Critical temperature (loss of 50% of cold strength) for mild steel is 550oC and 400oC for high tensile steel.  Reinforced concrete will deflect considerably under load but is unlikely to collapse suddenly.

Question 41

Structural Steel

Answer 41

 Structural steel loses 2/3 of its strength at 593oC and will sag and twist in the direction of, and in proportion to, the load applied.  Steel joists expand on heating (a 10m joist will expand 600mm at 500oC), which may cause load bearing support walls to collapse.

Question 42

Other metals

Answer 42

 Lead flashings and plumbing melt at 327oC.  Aluminium alloy cladding: stability affected at 100oC to 225oC; high expansion rate; high thermal conductivity; melts at 660oC.

Question 43

Glass

Answer 43

Non-combustible. May be a weak point in a fire barrier as it will break at high temperatures.

Question 44

Plasterboard

Answer 44

 Consists of a core of set gypsum or anhydrite plaster bonded to external facings of heavy paper.  Gypsum is non-combustible. Will retard fire spread until paper burns and gypsum core breaks up.

Question 45

Asbestos sheets and boards

Answer 45

 Asbestos cement sheets contain up to 15% asbestos. Shatter in the early stages of fire.  Asbestos insulating / wall boards are 80% asbestos 20% lime-silica bonding agent. They are non-combustible but will contract and bow away from heat source.

Question 46

Plastics

Answer 46

The term covers a broad range of chemicals. Generally plastics are combustible and the combustion products are likely to be toxic.

Question 47

Fire resistance is determined in accordance with performance over three criteria:

Answer 47

 Resistance to collapse (Ioad-bearing capacity) - which applies to loadbearing elements only, denoted R in the European classification of the resistance to fire performance (BS EN 13501).  Resistance to fire penetration (integrity) - denoted E  Resistance to the transfer of excessive heat (insulation) - denoted I

Question 48

Classification of linings

Answer 48

Small rooms of area not more than: (a) 4 m2 in residential accommodation (b) 30m2 in non-residential accommodation 3 Other rooms (including garages) Circulation spaces within dwellings 1 Other circulation spaces, including the common areas of blocks of flats 0

Question 49

Fire load density refers to

Answer 49

the quantity of fuel per unit area. It is normally expressed in terms of MJ/m2 or kg/m2 of wood equivalent.

Question 50

Fire prevention strategies are supported by measures to protect people, the building, and its contents. Approaches to fire and explosion protection include both active and passive measures. Active protection measures include

Answer 50

automatic detection and suppression systems

Question 51

Fire prevention strategies are supported by measures to protect people, the building, and its contents. Approaches to fire and explosion protection include both active and passive measures. Passive measures

Answer 51

are those that are designed into the physical structure of the building, such as load bearing components, compartmentation, fire resistant materials, etc.

Question 52

All fire-resisting doors are rated by their performance when tested to an appropriate British or European standard. The level of protection provided by the door is measured, primarily by determining the time taken for a fire to breach the integrity of the door assembly, together with its resistance to the passage of hot gases and flame. The principal categories are

Answer 52

E30 (FD30S) or E60 (FD60S) providing 30 and 60 minutes protection respectively. Fire doors are available providing between 20 and 120 minutes of protection.

Question 53

Whatever the type of building, typical situations that may assist the spread of fire and smoke include: 7

Answer 53

 Vertical shafts - examples: lifts, open stairways, dumb-waiters or holes for moving stock around.  Damaged or insufficient cavity barriers in modular construction.  False ceilings, especially if they are not fire-stopped above walls.  Voids behind wall panelling.  Large roof cavities, particularly in cold stores.  Unsealed holes in walls and ceilings where pipe work, cables or other services have been installed.  Doors, particularly to stairways, which are ill-fitting or routinely left open.

Question 54

Control measures

Answer 54

Reducing the quantity of dangerous substances to a minimum Measures to avoid or minimise the release of a dangerous substance Controlling the release of a dangerous substance at source Preventing the formation of an explosive atmosphere Avoiding ignition sources Avoiding adverse conditions that could result in harmful physical effects arising from a dangerous substance

Question 55

Mitigation measures

Answer 55

Minimising the numbers of people potentially affected Preventing fire and explosion from spreading to other vulnerable equipment or into the workroom For plant and equipment processing highly flammable solids and dusts this might include:  rotary valves  explosion suppression barriers  fast acting valves, chokes and baffles.For interconnected plant and equipment processing flammable gases and vapours it might include:  flame arresters  fast acting valves  suppression barriers.

Question 56

HSE advice on safe working with HFL’s uses the acronym VICES as an aide-memoire for basic precautions

Answer 56

Ventilation Prevents volatile vapours forming a flammable or explosive mix with air. Ignition Ignition sources should be removed or controlled so that, should a flammable mix of vapour and air occur, it cannot be ignited. Containment Suitable storage of flammable liquids to contain spills. Exchange Swapping flammable materials for less flammable materials. Separation Storage of flammables away from other stored materials and processes.

Question 57

Hazardous places are classified in terms of zones, on the basis of the frequency and duration of the occurrence of an explosive atmosphere. Zones 0, 1, 2

Answer 57

Zone 0 A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is present continuously or for long periods or frequently. Zone 1 A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is likely to occur in normal operation occasionally. Zone 2 A place in which an explosive atmosphere consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Question 58

Hazardous places are classified in terms of zones, on the basis of the frequency and duration of the occurrence of an explosive atmosphere. Zones 20, 21, 22

Answer 58

Zone 20 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently. Zone 21 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally. Zone 22 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only.

Question 59

Many factors will influence the overall reliability of an inerting system, including: 9

Answer 59

 the location and number of atmospheric sampling points  the type of sensor head  the frequency of calibration of the sensor  contaminants in the system that interfere with sensor readings  provision of safe means of control or shutdown, if the oxygen concentration exceeds a predetermined level  adequate supplies of inert gas for all foreseeable needs  the number of locations where air may enter the plant  the safety margin allowed when setting control levels for oxygen  the reliability of any electronic control system.

Question 60

Explosion relief methods

Answer 60

Explosion relief vents Explosion panels Bursting discsSuppressionInterconnected plant