Course Notes - 1B

International Diploma in Occupational Safety and Health Unit 1B

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International Diploma - Course Contents Unit 1 Principles of Health and Safety Management

Element 1A – Fundamentals of Health and Safety Management Preventing Accidents

1A1

International Occupational Health and Safety Framework

1A2

Health and Safety Management Systems

1A3

Risk Assessment

1A4

Safe Workplace

1A5

Control of Contractors

1A6

Safe Systems of Work

1A7

Personal Protective Equipment

1A8

Human Failure

1A9

Role of the Health and Safety Practitioner

1A10

Protecting the Environment

1A11

Engaging the Workforce

1A12

Element 1B – Applied Health and Safety Management Fire

1B1

Explosion

1B2

Major Accident Hazards

1B3

Emergency Control

1B4

Work Equipment

1B5

Occupational Transport and Driving

1B6

Mechanical Handling and Lifting Operations

1B7

Electrical Safety

1B8

Pressure Systems

1B9

Carriage of Dangerous Goods

1B10

Construction and Demolition

1B11

Working at Height

1B12

Working in Confines Spaces

1B13

Lone Working

1B14

Violence against Employees

1B15

Reporting and Investigating Incidents

1B16

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BSC International Diploma | Unit 1 Element 1B: Applied Health and Safety Management

C O N T E N T S Study Unit 1B1

Title

Page

Fire

FIRE PRINCIPLES ............................................................................................................................................... 3 PHYSICS OF FIRE – FIRE TRIANGLE ............................................................................................................................. 3 FIRE PROPAGATION (FIRE SPREAD) .............................................................................................................................. 4 IGNITION TEMPERATURES ......................................................................................................................................... 5 FIRE CLASSIFICATIONS ............................................................................................................................................ 7 FIRE RISK ASSESSMENT ..................................................................................................................................... 8 FIRE HAZARDS AND ASSESSMENT OF RISK ..................................................................................................................... 8 SYSTEMS FOR THE PREVENTION, DETECTION AND CONTROL OF FIRE ........................................................... 11 MEANS OF DETECTION AND RAISING THE ALARM ........................................................................................................... 11 DESIGN AND APPLICATION OF FIRE DETECTION AND ALARM SYSTEMS .................................................................................. 11 PRINCIPAL COMPONENTS OF SYSTEMS – DETECTION AND SIGNALLING ................................................................................. 11 MANUAL AND AUTOMATIC ALARM SYSTEMS .................................................................................................................. 14 WHEN AND HOW TO FIGHT A FIRE ............................................................................................................................ 15 HOW TO ESTABLISH MEANS OF ESCAPE FROM FIRE ........................................................................................ 19 GENERAL PRINCIPLES ............................................................................................................................................ 19 PROTECTION OF ESCAPE ROUTES .............................................................................................................................. 19 EMERGENCY LIGHTING........................................................................................................................................... 20 SIGNS/ NOTICES.................................................................................................................................................. 20 DISABLED PERSONS .............................................................................................................................................. 20 FIRE EMERGENCY ARRANGEMENTS ................................................................................................................. 22 GENERAL EVACUATION PROCEDURES .......................................................................................................................... 22 FUNCTIONS OF FIRE WARDENS AND FIRE MARSHALS ...................................................................................................... 22 POST-EVACUATION DEBRIEF .................................................................................................................................... 23 PRINCIPLES OF FIRE SAFETY TRAINING ......................................................................................................... 24 EXTINGUISHER TRAINING ....................................................................................................................................... 24 EVACUATION PROCEDURE ....................................................................................................................................... 24

BSC International Diploma – Element 1B | Applied Health and Safety Management

BSC International Diploma | Unit 1 Element 1B: Applied Health and Safety Management Study Unit 1B1 | Fire Learning Outcomes When you have worked through this Study Unit, you will be able to:

1.B.1.1 Explain the principles of fire 1.B.1.2 Describe the process and main stages of a fire risk assessment 1.B.1.3 Advise employers on systems for the prevention, detection and control of fire 1.B.1.4 Advise employers on how to establish means of escape from fire 1.B.1.5 Explain the principles of fire emergency procedures 1.B.1.6 Outline the principles of fire safety training

Unit 1:

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Fire Principles Physics of Fire – Fire Triangle What do we need to start a fire? 

First, we need a combustible substance or fuel (wood, paper, plastics, etc.).



Secondly, we require oxygen (usually from air).



Then we have to apply an ignition source.



After which, if the conditions are right, the substance will catch fire, i.e. heat and light will be evolved, accompanied by volumes of smoke and gases which will rise away from the fire.

The first three factors above form the basis of the Fire Triangle, and all must be present to produce and sustain a fire.

The Triangle of

Combustion

Fuel Fuel takes the form of vapours and/or gases emitted by liquids and solids which themselves do not burn. The vapours and gases mix with air, oxygen or other oxidising agents, e.g. chlorine. If they come within the flammable range they may be ignited. Dusts of combustible solids will also ignite under certain conditions.

Air/Oxygen Although under certain unusual circumstances it is possible to produce combustion-like chemical reactions with materials such as chlorine or sulphur, it is safe to say that nearly all combustion requires the presence of oxygen. The higher the concentration of oxygen in an atmosphere, the more rapidly will burning proceed.

Ignition It is often easily overlooked that to start a fire it may be necessary to heat to a sufficient degree only a very small quantity of fuel and oxygen mixture. Then, since fires are by definition exothermic, the very small fire started by a tiny heat source supplies to its surroundings more heat than it absorbs, thus enabling it to ignite more fuel and oxygen mixture, and so on, until very quickly there is more heat available than is needed to propagate a large fire. The heat may be provided by various sources of ignition, for example: 

Open flames - matches, welding torches.



Electrical sparking sources.

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Overheating of surfaces due to excessive temperatures. 

Hot surfaces – dryers.



Spontaneous ignition.



Sparks from grinding or tools.



Static electricity.



Friction.

Fire Propagation (fire spread) Once a fire has started, there are four methods by which it can spread – convection, conduction, radiation and direct burning.

Radiation Radiation is the general term for the process by which energy is lost from a source without direct contact. Heat radiation, therefore, refers to the process whereby the heat given off by hot objects passes through air and through certain types of transparent material (such as glass). This radiant heat can, in itself, be sufficient to act as a source of ignition. For example, radiators are an obvious source of heat, and clothes that are left to dry too close to them may catch fire. Similarly, light bulbs give out heat (and in the case of certain types of spot lights, a large amount of heat) and any fabrics or flammable materials that are too close may start to burn. The intensity of radiant heat diminishes with the distance from its source. However, depending on the temperature of the source, heat transference may take place over quite large distances. For example, a fire burning on one side of a street may be sufficient to cause materials on the other side of the street to combust.

Convection Convection is the process whereby heat moves through a gas or liquid. When a gas or liquid – for example, air or water – is heated, it expands and becomes less dense. As a result, it rises and cooler air or water is drawn in to replace it, creating a current. Convection currents created in the air by fire are a major means of fire spread. They may carry burning materials through the air and into contact with other combustible materials, and also, depending upon the intensity of the fire and the heat generated, create strong, localised winds which may fan the flames and cause flare ups.

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Convection Currents

Conduction Heat may be transmitted along certain materials, known as conductors, without those materials themselves actually burning. This is particularly the case with metals. Thus, the heat generated by a fire (or any other process producing heat) may be transferred to a separate location where it can act as a source of ignition. This has considerable implications for many buildings, where there is widespread use of metal within both the structure of the building (for example, steel girders) and the services that run through it, such as pipes and various types of ducting.

Conduction

Ignition Temperatures Flash Point Flash point is the lowest temperature at which sufficient vapour is given off to flash, i.e. ignite momentarily, when a source of ignition is applied. Substances which have a flash point below ambient temperature will pose a hazard as they will be producing a flammable vapour. Liquids which produce flammable atmospheres have their flash point measured in a standard test. The usual test utilises a small container about half full of liquid which is slowly warmed up, the atmosphere above the liquid level being tested at regular intervals with an igniter. When the flash occurs the temperature is noted. The test is carried out in an Abel apparatus. In the UK Liquids with a flash point lower than 0ºC and a boiling point lower than or equal to 35ºC are classified as extremely flammable. Those with a flash point below 21ºC but which

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are not extremely flammable are classified as highly flammable. Those with a flash point between 21ºC and 55ºC are classified flammable. Some common flash points are given in the following table. Some Common Solvents and their Flash Points FLASH POINT (ABEL CLOSED CUP) (°°C)

SOLVENT Butanone (methyl ethyl ketone, MEK) Carbon disulphide (CS2) Diesel oil Ethyl ethanoate (ethyl acetate) Ethoxyethane (diethyl ether) Methylated spirit Methylbenzene (toluene) Petrol Phenylethene (styrene) Propanone (acetone)

−7 −30 (Auto ign temp 102°C) +40 (approx.) −4 −40 10 4 −40 (approx.) 32 −17

Note that: 

Petrol is far more dangerous than diesel oil if it is spilled because the vapours from petrol will ignite from any fortuitous ignition source, whereas those from diesel will not. Contemplate such a spillage into the bilges of a boat and you can see why the insurance premiums are higher if the boat has a petrol engine.



Substances with very low flash points are very volatile. Ether, acetone and carbon disulphide are all notoriously dangerous.

Flash point tests may be applied to any liquid and not just hydrocarbons.

Fire Point Fire point is defined as the lowest temperature at which the application of an ignition source will lead to continuing burning. The temperature is usually just above the flash point and, when the vapours are ignited, the heat of the flash raises the temperature of the liquid surface to a point where sufficient vapour is given off to sustain combustion.

Auto Ignition Temperature (AIT) This is the lowest temperature at which the substance will ignite without the application of an ignition source. Different substances have different AITs. The material can be a solid, liquid or gas, and once ignition has taken place, the material will sustain the self-ignition in the absence of spark or flame. The value is influenced by the material's size, the shape of the heated surface and, in the case of a solid, the rate of heating and other factors. A chemical reaction can supply the heat to raise a substance above its auto ignition temperature. Haystacks have been known to ignite due to bacteriological action causing internal heat rises. This phenomenon is termed “spontaneous combustion”. On a more practical note, the relatively low AIT of diesel, which is, surprisingly, lower than petrol, means that diesel engines do not have to have a spark plug. The action of compressing

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the fuel/air mixture in the cylinders of the engine is enough to raise it above the AIT and cause ignition.

Fire Classifications Fires are commonly classified into five categories according to the fuel type. The classification also serves as the basis for identifying the means of extinguishing different types of fire. The fire categories are: 

Class A These are fires involving solid materials, normally of an organic nature, such as paper, wood, coal and natural fibres. These fires usually produce burning embers.



Class B These are fires involving flammable liquids or liquefied solids, such as petrol, oil, grease, fats and paint.



Class C These are fires involving gases or liquefied gases, such as methane, propane, and mains gas.



Class D These are fires where the fuel is a metal such as aluminium, sodium, potassium or magnesium.



Class F These are fires fuelled by cooking fats, such as in the case of chip pan fires.

Fires are generally classified within Classes A to D, with Class F and electrical fires added solely for the purposes of fire extinguisher selection.

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Fire Risk Assessment Fire Hazards and Assessment of Risk Background Primary responsibility for workplace fire safety is placed on employers and those in control of workplaces. They must assess and provide the measures necessary to prevent or control the risks from fire and, in particular, must ensure the following points: 

That the workplace is equipped with appropriate fire-fighting equipment, fire detectors and alarms and that any non-automatic fire-fighting equipment is easily accessible, simple to use and indicated by signs.



That appropriate measures are taken for fire-fighting, the nomination and training of employees to implement those measures, and the arranging of contacts with external emergency services.



That emergency routes are kept clear, and comply with specific criteria relating to routes, doors and signs.



That there is a suitable system of maintenance for fire precautions in relation to workplace procedures in general and to specific equipment and devices, which must be kept in good working order and repair.

The fire risk assessment is intended to serve as the basis for planning, and maintaining, all aspects of fire precaution in the workplace under a specific person charged with the responsibility of fire prevention.

Format for Risk Assessment A fire risk assessment should follow the recognised format for other risk assessments, i.e.: 

Identify the hazards – what are the likely sources of ignition including any processes carried out (e.g. hot work). In some workplaces, there may be other sources of oxygen (other than in the air). This may be in the form of bottled medical or industrial oxygen, or oxidising agents. What fuels are present, and how are they arranged and stored?



Identify who is at risk – in the event of a fire happening, the main risk is from the rapid spread of smoke and gases from the combustion process. It is necessary to determine whether there are adequate means of escape. This will necessitate making an assessment of the likely speed and spread of smoke, heat and gases; the number of people (including visitors) there are likely to be in the premises; how awareness is raised in case of a fire; and whether they can make a quick and simple escape.



Evaluate the risk – what are the chances of fire happening (likelihood) and if it does happen, what is the likely outcome (severity)? Are there works activities being carried out that increase the likelihood of fire? When carrying out this evaluation, it is necessary to consider the fire precautions that are already in place and determine if any increased fire precautions are necessary. These changes will then need to be implemented.



Record the findings – if there are five or more employees, then the significant findings should be recorded, including any persons found to be at particular risk.



Reviewing the risk assessment – to be carried out, for example, if there is significant change, after an incident or as part of an on-going health and safety plan.

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The risk assessment must be suitable and sufficient and allow suitable additional control measures to be introduced, where necessary.

Likely Sources of Ignition We considered likely sources of ignition earlier in this Study Unit. To recap here, sources of ignition include: 

Open flames - matches, welding torches.



Electrical sparking sources.



Overheating of surfaces due to excessive temperatures.



Hot surfaces – dryers.



Spontaneous ignition.



Sparks from grinding or tools.



Static electricity.



Friction.

Fuels That May Be Present Fuel consists of flammable materials that cover all states of matter. They include: 

Solids such as: wood, plastics, paper and wrapping and packaging materials, soft furnishings and fabrics and even metals, for example, magnesium.



Liquids such as: petroleum and its derivatives, paints, solvents, oils, etc.



Gases such as: hydrogen, LPG, methane and oxygen. Although oxygen is not strictly a flammable gas, oxygen enrichment can increase the flammability of a fuel.

Work Activities That Could Increase Risk Various activities in the workplace, both those directly related to the work being conducted and non-related activities can increase the risk of fire. Particular people may be at higher risk due to their activities at work, including, for instance, people who work: 

Near a furnace, for instance, or any sort of incinerator.



In the nuclear industry.



In the kitchens of a restaurant.



With chemicals.



With portable appliances.

Site Plan The location where workers carry out their activities may also affect their likelihood of being at risk. The risk assessment should include a detailed plan of the site, with all principal sources of ignition clearly marked. The plan should show all electrical appliances, heating plant, site of hazardous processes, location of the electric mains switches and the main gas control valves. It should also show waste disposal areas and the location of fire extinguishers and all site accommodation. It should also include the names and positions of responsible persons and their duties.

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In premises where much of the work is carried out within a single area, it may be adequate to carry out the assessment of a building as a single unit. However, in most cases, it will be necessary to subdivide the building into discrete areas or rooms. It is particularly important not to lose sight of the effects that adjacent work or storage areas, or some normally inaccessible areas, may have on the rest of the building – for example, in respect of roof voids, boiler rooms or fuel storage.

Fire Precautions in Place As part of the risk assessment, it is important to consider whether existing fire safety measures are suitable and sufficient. This would include considering: 

Control of ignition sources and sources of fuel.



Existence of fire detection and warning.



Means of escape.



Means of fighting fire.



Maintenance and testing of fire precautions.



Fire safety training of employees.

Changes Once the existing fire precautions have been evaluated, the risk assessment must consider whether any increased fire precautions are necessary, as well as how to go about implementing any changes. Improvements made will need to be maintained to ensure the ongoing safety of workers.

Recording and Review The result of the assessment, as well as actions to be taken, should be recorded. Similarly, the assessment should be revised if there are any significant changes or after a suitable period of time. Various aspects of the work should be considered, as all may increase the risk of fire, for example: 

General working policies: for instance, no smoking.



Specific working practices: for instance, the removal of waste on a more frequent basis, reducing the use of flammable substances where alternatives are available, using fixed electrical installations (as opposed to portable appliances), or use of flame-resistant material on scaffolding.



The physical condition of the premises: for instance, the sealing of any gaps around the pipe-work running between rooms.



Movement of people and transport.

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Systems for the Prevention, Detection and Control of Fire Means of Detection and Raising the Alarm In order to determine the most suitable type of detection/alarm system for a particular building, it is necessary to determine the type of occupancy and escape strategy. Where the escape strategy is based on simultaneous evacuation then activation of a manual call point or detector should cause all fire alarm sounders to operate. Where the escape strategy is based on phased evacuation, however, a staged alarm system might be more appropriate (i.e. one tone for alert, another for evacuate). This system is used extensively in large organisations, college and university campuses for example, where full evacuation is often not necessary. All buildings should have provision for detecting fire. Consideration has to be taken of whether the chosen method of alarm can be heard in all parts of the building (the toilets, for example). In some circumstances a rotary gong would be sufficient, whilst other buildings would need electrically operated fire alarms. The fire-warning signal must be distinct from other signals in use.

Design and Application of Fire Detection and Alarm Systems All workplaces must have arrangements for: 

Sounding an alarm in the event of fire.



Evacuating staff to safe fire assembly points using means of escape routes.



Fighting the fire.

Regular tests of an alarm system serve to check the circuits and to familiarise staff with the call note. Fitting fire doors to a building cuts down the distances over which call bells are heard, and may mean further bells should be fitted or noise levels raised. A fire alarm can be raised automatically by a detection system or manually by a person in the affected building. The means for this will generally be either manual or manual/electric, not forgetting that an alarm can always be raised by shouting.

Principal Components of Systems – Detection and Signalling A fire detector identifies one or more physical changes in the protected environment indicative of the development of a fire condition. Usually mounted on ceilings or in air ducts, detectors are activated in the main by smoke or heat/light radiation. Such conditions can be readily identified: 

After ignition has occurred and the invisible products of combustion are released.



When visible smoke is produced.



When the fire produces flame and a degree of illumination.



When the temperature in the vicinity of the fire rises rapidly or reaches a predetermined value.

The types of detector designed to operate at one of these particular stages are: 

Ionisation smoke detectors.



Optical detectors.

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

Radiation detectors.



Heat detectors.

The most common types of detector system in use at present are those actuated by smoke and those actuated by heat. The final choice is based on the risk to be protected and the individual circumstances of each case (see the following table).

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Automatic Fire Detectors Type

Suitability

Smoke Ionisation Sensitive in the early stages of a fire when smoke particles are small. Sensitivity tends to drop as particles grow in size.

Areas having a controlled environment, i.e. free from airborne dust, etc., and generally housing complex equipment of a high intrinsic value, e.g. computer installations.

Optical Most effective in situations where the protected risk is likely to give rise to dense smoke (i.e. large particles).

Normally used as point detectors but have been developed to form zone sampling systems by monitoring air samples drawn through tubes.

Radiation Infra-red Rapid detection because of almost instantaneous transmission of radiation to the detector head. This is dependent, however, on the detector having a clear ‘view’ of all parts of the protected area.

Warehouses or storage areas, etc. Detectors are available which can scan large open areas and will respond only to the distinctive flame flicker. Can be used to detect certain chemical fires. The ultra-violet detector tends to be used mainly for specialised purposes.

Ultra-violet As for infra-red. Heat Fusible alloys Alloys will need replacing each time detector operates.

Areas of general risk where vapour and particles are normally present. Cost is relatively low compared to other types of detectors.

Expansion of metal, air and liquid Generally self-resetting.

Both ‘fixed-temp’ and ‘rate-of-rise’ are equally efficient but ‘fixed-temperature’ types are preferred in areas where a rapid rise in temperature is a likely result of the normal work processes.

Electrical effect Not widely installed. Some specialist use. Note for all types of heat detectors May be used as point or line detectors and are designed to operate at a pre-selected temperature (‘fixed-temperature’ type) or on a rapid rise in temperature (‘rate-of-rise’ type) or both. With all heat detectors (particularly fixedtemp types), ‘thermal lag’ needs to be considered when choosing the operating temperature.

‘Rate-of-rise’ types will compensate for gradual rises in ambient temperature and are more efficient than the ‘fixed-temperature’ type in low-temperature situations. (‘Rate-of-rise’ detectors generally incorporate a fixedtemperature device.)

Not all detectors will be equally sensitive in every possible situation. In some cases a combination of different detectors may be required. Smoke and heat detectors are suitable for

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most buildings. Radiation detectors are particularly useful for high-roofed buildings, e.g. warehouses, and situations in which clean-burning flammable liquids are kept. Laser infra-red beam detectors appear to have advantages where there are tall compartments or long cable tunnels, for example. Such generalisations should be considered in conjunction with the nature of the risk to be protected in order to establish: 

The reliability required. A more robust detector is necessary in an industrial setting than is required for hotel purposes. Dusty or damp atmospheres will affect some detectors more than others.



The sensitivity required. It would obviously be undesirable to install a smoke detector set at high sensitivity in a normally crowded hotel bar (or similar conditions).



The location of detectors. The detectors should be located so they are in the best possible position to perform their function.

All alarm systems must be maintained and tested regularly, and the results recorded. Any faults discovered must be rectified and the system rechecked. All staff must know how to raise the alarm and what to do when the fire alarm sounds. Staff with hearing or other physical disabilities must be accommodated within an evacuation plan (e.g. people in wheelchairs cannot use stairs if a lift is inactivated). Emergency lights or vibrating devices may be used in addition to bells or sirens.

Manual and Automatic Alarm Systems Manual Systems Manual systems are suitable for small workplaces. The purely manual means for raising an alarm involve the use of the following basic devices: 

Rotary gongs that are sounded by turning a handle around the rim of the gong.



Hand strikers, e.g. iron triangles suspended from a wall accompanied by a metal bar that is used to strike the triangle.



Hand bells.



Whistles.



Air-horns.

These devices are normally found on the walls of corridors, entrance halls and staircase landings, in a situation where they are readily available to anyone who may need to raise an alarm. While they give an alarm over a limited area, operation of one of them is rarely adequate to give a general alarm throughout the premises. As a person is required to operate them, a continuous alarm cannot be guaranteed for as long as may be necessary.

Manual/Electric Systems These are systems which, although set in motion manually, operate as part of an electrical alarm circuit. The call points in a manual/electric system are invariably small wall-mounted boxes which are designed to operate either: 

Automatically, when the glass front is broken.



When the glass front is broken and the button pressed in.

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Most available models are designed to operate immediately the glass front is broken. In order to raise an alarm, it is possible to use facilities that may already be installed in a building for other purposes, e.g. a telephone or public address system. With automatic telephone systems, arrangements can be made for a particular dialling code to be reserved for reporting a fire to a person responsible for calling the emergency services and sounding the general alarm. Alternatively, it can be arranged that use of the code automatically sounds the general alarm.

Automatic Systems An automatic fire alarm system may be designed to respond to heat, smoke and the products of combustion and flames. The system will give warning of a fire, and some more elaborate designs do incorporate a facility for additional functions, such as activating water sprinkler systems, closing down ventilation, or air conditioning plant or activating automatic door releases.

When and How to Fight a Fire Extinguishing a fire is based on removing one or more sides of the fire triangle: •

Removing the Fuel Extinction by this process is known as starvation. This can be achieved by taking the fuel away from the fire, taking the fire away from the fuel and/or reducing the quantity or bulk of fuel available. Thus, materials may be moved away from the fire (to a distance sufficient to ensure that they will not be ignited by any continuing radiant heat) or a gas supply may be turned off.

•

Removing the Oxygen Extinction by this process is known as smothering. This can be achieved by either allowing the fire to consume all the available oxygen, whilst preventing the inward flow of any more oxygen, or adding an inert gas to the mixture. The most usual method of smothering is by use of a blanket of foam or a fire blanket.

•

Removing the Heat Extinction by this process is known as cooling. Cooling with water is the most common means of fighting a fire and this has a dual effect in terms of: absorbing heat and thereby reducing the heat input into the fire, and reducing the oxygen input through the blanketing effect of the steam produced.

Portable Fire-Fighting Equipment The main types of portable fire-fighting equipment are fire extinguishers. These are appliances designed to be carried to the point of the fire and operated by hand. They contain an extinguishing media that is expelled by internal pressure on operating the release mechanism and can be directed by means of a horn or tube onto the fire. The pressure may be by compression within the extinguisher or may be the result of a chemical reaction or release of gas from a cartridge, triggered by the operation of the extinguisher. The range of fire extinguishers (their size, colour, method of operation and claims for performance) is so great as to produce a considerable degree of confusion. The equipment that the average person will grab in the event of fire should be, according to the experts, suitably located and suitable for the risk. The problems arise when more than one type

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of risk may be encountered and the person, who is operating under pressure, is faced with a choice of extinguisher. It could well be that the wrong choice could render the efforts wasted or even expose the person to danger! Firstly, we will identify the nature of the risk and the choices of agent which are available. The Nature of Risk FIRE CLASS

DESCRIPTION

EXAMPLES

EXTINGUISHING AGENTS

A

Solid - carbonaceous-based

Wood, paper, fibres, rubber

Water, also foam, dry powders and CO2

B

Flammable liquids

Those miscible with water

Alcohol, acetone, methyl acetate

Dry powders, special foam, vaporising liquids, water, CO2

Liquefiable solids (greases, etc.)

Those immiscible with water

Petrol, diesel, oil, fats and waxes

Dry powder, foam, vaporising liquids, CO2

C

Gases and liquid gases

Natural gas, liquefied petroleum gases, e.g. butane, propane

Dry powder, carbon dioxide and foam

D

Flammable metals

Potassium, sodium, magnesium, titanium

Inert dry powder with special applicator, dry sand

F

Cooking oils and fats

Deep fat fryers

Wet chemical

Gas fires can be difficult to deal with. Whilst dry powder and carbon dioxide may be used to knock the flame down there is a risk of a build up of gas if it cannot be turned off. In some situations, it may be preferable to allow the fire to continue and to call the emergency services. You will notice that electrical fires are not listed; this is because electricity is not a fuel, it will not burn. However, it can cause fires and it can be present in fires so we have to take it into consideration when fighting fires.

Identification of Fire Extinguishers In order to identify the different types of fire extinguisher in some countries a colour coded systems is used, this indicates the type of extinguishant inside. Where a colour code systems is not in use it is essential that the extinguisher contents are easily identifiable

The following table shows typical colour coding for the common types of extinguishant. Colour Coding for Fire Extinguishants Fire extinguisher content

Colour of body or label/band

Water

Red

Foam

Cream

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Dry Powder

Blue

Carbon Dioxide

Black

Wet Chemical

Yellow

It should be noted that across EU member states new fire extinguishers will be red in colour but with an identifying band.

Siting of Extinguishers The correct type of extinguisher should be available for the risk it is going to protect against. The fire-fighting equipment should be sited in an easily seen and reached position, usually by an escape route. The location should be marked and should not be further than 30 metres from an alternative equipment location. The location should be: 

Conspicuous.



Readily visible on escape routes.



Properly mounted.



Accessible (less than 30 metres from any other fire extinguisher).



Near to the specific fire hazards

Other Types of Fire-Fighting Equipment Fire Blankets These are portable fire-fighting devices designed to smother a fire. There are a variety of fire blankets suitable for different types of fire – light-weight ones suitable for class A and B fires, and heavy duty blankets for industrial use, including those that can be used for class D fires. They are especially useful in a kitchen for extinguishing chip pan fires and other types of small fat and oil fires (class F). When using a fire blanket, the corners need to be turned towards you so that you do not get burnt as the blanket is laid over the fire. It should be kept in place until all the heat has been removed. Hose Reels These are very effective as a first line of attack against class A fires. Reels should be located near exits, stairways or lobbies and arranged so that no part of the building is beyond the reach of the jet (6 metres). If the hose reel is fitted into a recessed installation the doors, whether glazed or not, should bear the words “FIRE HOSE REEL” in red letters at least 50 millimetres high on a white background. The hose has a shut-off nozzle and the supply is via a control valve at the connection to the main, which must be opened before the reel is pulled out. Some reels operate this valve automatically as the hose is rolled out. Reel installations have a number of advantages: 

Only the required length of hose has to be run out.



The hose is light and only one person is required to operate it.

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

The lack of back pressure from the nozzle makes it easy for persons of limited strength to handle it.



The control of the water at the nozzle of a hose reel will limit water damage.

Automatic Sprinklers There are several different types of sprinkler system, but essentially they all involve fixed pipe work in the ceiling of each part of the protected building. The pipe work is connected via control valves to a water supply and sprinklers are spaced at intervals along the pipe work so that the discharge patterns overlap and leave no part unprotected. They are activated by automatic fire detectors. The quantity of water discharged is designed to at least control any fire in the protected area, if not to extinguish it. Drenchers These are designed to provide a coverage of water over areas of a building or structure which could be damaged by radiant heat from a fire in close proximity. Normally adequate spacing limits the radiation hazard and, therefore, only vulnerable areas need be covered, such as unprotected doors and windows. Hydrants and Foam Inlets These are provided on the outside of buildings to allow the emergency services easy access to a supply of water or foam close to a potential fire hazard, with the type of extinguishing agent being appropriate to the type of hazard.

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How to Establish Means of Escape from Fire General Principles An integral part of any fire safety programme is the consideration and implementation of suitable means of escape. The “Means” will vary dependant on a number of factors which may differ from premises to premises. A standard factor in all premises however is the time it will take for people to safely evacuate from a burning building and to reach a place of safety; which in turn is largely dependant on: 

The number and types of people needing to escape.



Widths and capacity of escape routes and stairways.



The time available for people to escape to a place of safety.



The distances that have to be travelled to reach that place in that time.

Travel distances are provided as guidance for various levels of risk and are useful in helping to determine if for example, the numbers of fire exits are sufficient or the layout of the workplace is suitable. Suggested travel distances are: Escape Routes

Suggested Range of travel distance

Where more than one escape route is provided

45m in normal fire-risk areas

Where only a single escape route is provided

18m in normal fire-risk areas

If the fire risk is more or less than the guidelines shown then the travel distances would need to be adjusted. When assessing travel distances you must consider the distance to be travelled, allowing for walking around furniture etc and should be measured from the furthest point to the nearest place of reasonable safety which is: 

a protected stairway enclosure



a separate fire compartment from which there is a final exit to a place of safety



the nearest available fire exit

Protection of Escape Routes A protected route may consist of two interlinked structural components: (a)

A protected corridor (horizontal movement).

(b)

A protected stairway (vertical movement).

The principle to follow is that once people enter either (a) or (b) they should normally be able to proceed to a place of safety without leaving the protected route. The protection required at (a) and (b) is generally achieved by ensuring that the protected area has at least 30 minutes’ fire resistance.

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Emergency Lighting Emergency (or safety) lighting should be provided where failure of the normal system would cause problems, e.g. in buildings used after dark, or darkened, e.g. cinemas, hospitals, and sections of buildings used for means of escape. The purposes of emergency lighting are: 

To identify the escape route.



To provide illumination along the route.



To ensure that alarm call points and fire-fighting equipment can be easily located.

Emergency escape lighting should be provided in those parts of buildings where there is underground or windowless accommodation, core stairways or extensive internal corridors. Generally the need for such lighting will arise more frequently in shops than in factories and offices because of the greater likelihood of people in the building being unfamiliar with the means of escape. Emergency lighting is only designed to ensure that people can find their way out of a building, and so the light requirement is much lower than for normal use. The level of emergency illumination should be related to the level of normal illumination to avoid panic whilst eyes adapt to the reduced light.

Signs/ Notices It is commonly expected that emergency signs are square or rectangular with a green background and white symbols. The ‘running man’ sign (see the figure that follows) usually appear over all fire escape exits, and the ‘door and arrow’ sign is used to mark escape routes. To ensure that speakers of all languages are able to understand the signage it is advisable, where possible, not to use words.

Emergency Exit/Escape Route Signs

Disabled Persons Provision needs to be made for means of escape for disabled persons to ensure the safe evacuation of disabled persons from the workplace in a fire situation,. This may include: 

Means of communication with people with a hearing disability must be considered. Alarms using sound cannot be relied in such a case. Such means could include:

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−

Face-to-face communication.

−

Equipment such as: induction loops, textphone, TV subtexting, amplifiers, flashing lights.



When people may be asleep, special consideration must be given as to how they would be woken in case of emergency.



Floor surfaces should be even, non-slippery and free from obstacles to aid visually impaired people and wheelchair-users.



Sufficient space and time should be allowed for wheelchair-users when planning escape routes, including width of corridors and doors.



Means of communication with visually impaired people must be considered. This could include: taped messages, tactile surfaces and audible instructions.



Unlike normal passenger lifts, which are usually made inaccessible in the event of fire, it is essential that a lift that may be used to evacuate disabled people can still be operated safely if there is a fire in the building, as this may be the disabled person’s main means of escape.



Refuges: −

As disabled people will often need others to assist them to evacuate from a place of work, a refuge provides a temporary, safe area for disabled people to wait. The area should be both separated from the fire by fire-resisting construction, and provide access via a safe route to a storey exit.

−

As wheelchairs may well be used by disabled workers, the refuge should be big enough to allow wheelchair use and manoeuvrability within the space without difficulty. It is also essential that the location of any wheelchair spaces within refuges does not affect the means of escape for other people.

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Fire Emergency Arrangements General Evacuation Procedures With any building in which a great number of people are present the procedure must not allow panic as it will slow evacuation and may cost lives. The evacuation must be orderly and handled in such a manner that the escape facilities, e.g. capacity of escape routes, can handle the numbers of people using them. Typical duties of staff members in fire situations would include: 

Members of staff detailed to shepherd the public to the escape route.



Senior members of staff search all the floors, WCs, etc.



Members of staff keep exits open and clear.



A responsible member should meet the brigade on arrival and inform them of all relevant details.



On leaving the premises ensure all doors and windows are closed.

As soon as the premises have been evacuated and all personnel are assembled in their prearranged safe areas, fire wardens or other appointed persons must account for people – often done by carrying out a roll call on the basis of a continually updated register (the compilation of which will be a line management function). The roll must include staff, visitors, contractors, etc. If anyone is not accounted for, the Fire Officer in charge must be notified as soon as the Fire Services arrive. In the case of premises where there is random public access (shops, etc.), it may be necessary for the Fire Services to undertake a search.

Functions of Fire Wardens and Fire Marshals Whatever the number of employees, it is vital that responsibility for action in the event of fire is assigned to specific persons. All premises should have designated and trained fire marshals or fire wardens who are responsible for the following actions: 

Ensuring all members of staff (and other persons on the premises, including the general public in the case of shops and public buildings) leave by the designated escape route.



Searching all areas, including toilets, to ensure that the area is clear.



Ensuring that fire escape routes are kept open and clear at all times.



Ensuring all doors and windows are closed on leaving the area.



Conducting the roll call at the assembly area.



Meeting the emergency services on arrival and informing them of all relevant details.

In addition, premises with a large number of occupants or where there is a high fire risk may have trained personnel who will carry out first-aid and fire-fighting. They will also have a nominated senior person – for example, a safety officer or senior manager – who is responsible for all aspects of fire safety including training, overseeing of fire contracts (such as for equipment maintenance) and record-keeping.

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Deputies should be nominated to take over all these responsibilities when the marshal/warden or senior person is absent.

Post-Evacuation Debrief After any ‘real’ or pre-planned fire evacuation, it is essential that the management review the event to learn lessons for the future. For this to happen effectively, fire marshals, wardens and any others involved in the event should be de-briefed. The event should then be analysed for strengths and weaknesses, and actions for improvement identified. Feedback on the event and the identified improvements must be provided to all involved personnel. These de-briefs should be recorded to demonstrate best practise – and may be a requirement of the Fire Certificate.

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Principles of Fire Safety Training Extinguisher Training The ability to carry out fire-fighting with portable extinguishers may not only control the rate at which a fire spreads, thereby giving those precious few moments which mean the difference between a person escaping or becoming a victim, but often reduces fire damage to a lower level than would have been the case if the fire had proceeded without being checked. It is, therefore, very important that all personnel are familiar with the available fire-fighting equipment and are able to use it correctly. The following points form a general scheme for training in the use of fire-fighting equipment: 

General understanding of how extinguishers and other appliances operate.



The importance of using the correct extinguisher for different classes of fire (which should not be a problem as only the correct type of extinguisher should be available for use in any particular situation). Staff should be aware that using the wrong type of extinguishing agent on a fire may increase its intensity.



Recognition of whether the extinguisher has to be used in the upright position or in the upside-down position.



Practice in the use of different extinguishers. This can be done with or without a practice fire, although dealing with a live fire is obviously the better method. Opportunities for training may arise during the inspection process – when appliances are being tested or discharged. (Training in using carbon dioxide extinguishers is particularly important as they can be frightening to use – they start off with a bang and the horn gets freezing and can cause the skin to adhere to it, pulling the skin off if you try to remove your hand. Users should angle the CO2 horn into position before they activate the fire extinguisher. They should ensure that they do not touch or hold the horn during or immediately after operating it.)



When to and when not to tackle a fire. If the fire is small and has not involved the building structure, then portable extinguishers can generally be used. It must be understood that extinguishers can only provide a ‘first-aid’ treatment and evacuating the building must take precedence over fighting a fire if the conditions demand it. A means of escape must always be maintained.



When to leave a fire that has not been extinguished. As a general rule, once two extinguishers have been discharged, the fire requires the Fire Service. When leaving an unextinguished fire, all doors and windows should be closed to help contain the fire.

Evacuation Procedure Training in fire safety procedures is necessary for everybody. It is essential that key personnel who are responsible for implementing safety procedures are given adequate training in order to perform their duties. The skill is in identifying the level of training required. This can range from induction training for new employees, to more specialised training for members of staff with specific safety responsibilities; for example, fire wardens and people dealing with specialist processes or equipment. Training must be monitored and updated as necessary (e.g. when circumstances change in premises or staffing) and should be modified to suit individual requirements. Keeping records of the training given to individuals will not only ensure that all employees are made aware of their responsibilities, it also provides proof of provision of adequate training.

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The records will also be invaluable in dealing with investigations of any incidents or accidents. Carrying out and recording the results of regular risk assessments can also help identify training needs. Training should be specific to the particular premises and all staff should receive training at sufficiently regular intervals to ensure that existing members of staff are reminded of the action to take, and that new staff are made aware of the fire routine for the premises. Training should be given at least once in each period of 12 months; but in some circumstances where there is high turnover of staff, or where there is a high fire risk, or material changes which significantly affect the fire safety arrangements, training may have to be more frequent. Instruction and training should be based on written procedures and should be appropriate to the duties and responsibilities of the staff. It is particularly important that all staff (including those casually employed) should be shown the means of escape and told about the fire routine as soon as possible after they start work. It is also necessary to ensure that occasional workers, those on shift duties and others who work in the premises are similarly instructed. Special consideration should be given to any employees with language difficulties or with any disabilities that may impede their understanding of the information. Instruction should be given by a competent person and the following topics, where appropriate, should be covered in each training session with practical exercises where possible: 

Action to take on discovering a fire.



How to raise the alarm and the procedures this sets in motion.



Action to be taken upon hearing the fire alarm.



Procedures for alerting members of the public including, where appropriate, directing them to exits.



Arrangements for calling the emergency services.



Evacuation procedure for the premises to an assembly point at a place of safety.



Location and use of fire-fighting equipment.



Location of escape routes, including those not in regular use.



How to open all escape doors.



The importance of keeping fire doors closed.



How to stop machines and processes and isolate power supplies where appropriate.



The reason for not using lifts (other than those specifically provided or adapted for use by people with disabilities in accordance with BS 5588: Part 8).



The importance of general fire precautions and good housekeeping.

A fire drill should be carried out at least once and preferably twice a year simulating conditions in which one or more of the escape routes from the building is obstructed. During the drills the fire alarm should be operated by a member of staff who is told of the supposed outbreak, and thereafter the fire routine should be rehearsed as fully as circumstances allow. The training and instruction given should be recorded in a log or other suitable record, which should be available for inspection. The following are examples of matters that should be included in a fire record-keeping book: 

Date of the instruction or exercise.

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

Its duration.



Name of the person giving the instruction.



Names of the persons receiving the instruction.

On all premises, one person should be responsible for organising fire instruction and training, and in larger premises a person or persons should be nominated to co-ordinate the actions of the occupants in the event of a fire. Printed notices should be displayed at conspicuous positions in the building stating in concise terms the action to be taken upon discovering a fire or on hearing the fire alarm. The notices should be permanently fixed in position and suitably protected to prevent loss or defacement. Written instructions may be supplemented by advice in pictogram form.

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BSC International Diploma | Unit 1 Element 1B: Applied Health and Safety Management

C O N T E N T S Study Unit 1B2

Title

Page

Explosion

TYPES AND PRINCIPLES OF EXPLOSION ............................................................................................................ 3 VAPOUR PHASE EXPLOSIONS ..................................................................................................................................... 3 DEFLAGRATIONS .................................................................................................................................................... 3 DETONATIONS....................................................................................................................................................... 3 BOILING LIQUID EXPANDING VAPOUR EXPLOSION (BLEVE) ................................................................................................ 4 CONFINED/UNCONFINED VAPOUR CLOUD EXPLOSION (CVCE/UVCE) .................................................................................... 5 DUST EXPLOSIONS ................................................................................................................................................. 6 PRIMARY AND SECONDARY EXPLOSION ......................................................................................................................... 6 ASSESSING AND CONTROLLING EXPLOSION RISKS .......................................................................................... 8 RISK ASSESSMENT PRINCIPLES ................................................................................................................................... 8 RISK CONTROL PRINCIPLES – HIERARCHY OF CONTROL ..................................................................................................... 9 EQUIPMENT FOR USE IN POTENTIALLY EXPLOSIVE ATMOSPHERES........................................................................................ 10 CLASSIFYING EXPLOSIVE ATMOSPHERES ...................................................................................................................... 11

BSC International Diploma – Element 1B | Applied Health and Safety Management

BSC International Diploma | Unit 1 Element 1B: Applied Health and Safety Management Study Unit 1B2 | Explosion Learning Outcomes When you have worked through this Study Unit, you will be able to:

1.B.2.1 Explain the principles of explosion 1.B.2.2 Outline the principles of risk assessment and risk control in relation to explosion

Unit 1:

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Types and Principles of Explosion Vapour Phase Explosions An explosion is a sudden and violent release of energy, causing a pressure blast wave. Usually it is the result, not the cause, of a sudden release of gas under high pressure, but the presence of a gas is not necessary for an explosion. An explosion may occur from a physical or mechanical change or from a chemical reaction. An explosion can occur without fire, such as the failure through over-pressure of a steam boiler or an air receiver. In discussing the explosion of a flammable mixture, we must distinguish between detonation and deflagration. If a mixture detonates, the reaction zone propagates at supersonic velocity and the principal heating mechanism of the mixture is shock compression. In a deflagration, the combustion process is the same as in the normal burning of a gas mixture; the combustion zone propagates at subsonic velocity and the pressure build-up is slow. Whether detonation or deflagration occurs in a gas-air mixture depends on various factors, including the concentration of the mixture and the source of ignition. Unless confined or ignited by a high-intensity source (a detonator), most materials will not detonate. However, the pressure wave (blast wave) caused by a deflagration can still cause considerable damage. Certain materials, such as acetylene, can decompose explosively in the absence of oxygen and are thus particularly hazardous.

Deflagrations A deflagration is the very rapid auto-combustion of particles of explosive as a surface phenomenon. It may be initiated by contact with a flame or spark, and may be caused by impact or friction. Deflagration is a characteristic of low explosives. Exothermic reactions can lead to high temperatures and, in the case of large fires, to extensive loss of property and severe damage from radiant energy. However, in many plant accidents it is the sudden generation of pressure that leads to severe damage, injury and deaths. So it can be said that pressure blows up plants, not temperature. Of course, temperature and pressure are closely related, but it is the pressure effect that concerns us here. Deflagration is a reaction which propagates to the unreacted material at a speed less than the speed of sound in the unreacted substance. The conditions for a deflagration to occur are that the gas mixture is within the flammable range and that there is a source of ignition or that the mixture is heated to its auto-ignition temperature. In the absence of explosion relief, the deflagration explosion of a hydrocarbon-air mixture is easily capable of bursting a vessel if it is operating near its design pressure when the deflagration takes place. For reactions operating at or near atmospheric pressure, such as many drying and solids processing operations, it may be practical to construct facilities that will withstand the maximum explosion pressure of most dust-air and flammable gas-air mixtures.

Detonations A detonation is the extremely rapid, self-propagating decomposition of an explosive accompanied by a high pressure-temperature wave that can move at 1,000-9,000 m/s. It may be initiated by mechanical impact, friction or heat. Detonation is a characteristic of high explosives, which vary considerably in their sensitivity to shock. A detonation is a reaction that propagates to unreacted material at a speed greater than the speed of sound in the unreacted material; it is accompanied by a shock wave and extremely

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high pressures for a very short time. It is debatable whether the flammable range is the same as the detonable range. Detonation limits are normally reported to be within the flammable limits, but it is widely held that separate detonation limits do not exist. Detonation of a gas-air mixture may occur by direct initiation of detonation by a powerful ignition source, or by transition from deflagration. This transition is much more likely to occur in pipelines than in vessels. Two useful rules are: 

Almost any flammable gas mixture is detonable if initiated with a sufficiently energetic source.



Detonation of a gas-air mixture is possible in pipelines but is unlikely in vessels.

With a flammable mixture of gases burning in a pipe with one end closed, a series of pressure waves travelling at the speed of sound moves through the unburnt gas. Later waves travelling through the unburnt gas, which has been heated by compression from the earlier waves, speed up because of the higher temperature and overtake the first wave, and a shock wave develops. Flame then follows the shock wave and catches up with it, forming a detonation wave. A stable detonation wave may develop, which moves with supersonic speed relative to the unburnt mixture, and peak incident (side-on) pressures can be of the order of 30 times the initial absolute pressure.

Boiling Liquid Expanding Vapour Explosion (BLEVE) A BLEVE involves a sudden release of vapour, containing liquid droplets, owing to the failure of a storage vessel. This occurs when a pressure vessel containing liquid is heated so that the metal loses strength and ruptures, typically, as a result of exposure to fire. The failure is usually in the metal in contact with the vapour phase; the metal in this area heats to a higher temperature because there is no liquid heat sink to keep its temperature from rising rapidly, as there is where metal contacts a liquid phase. A BLEVE can occur with both flammable and non-flammable materials (e.g. water). The initial explosion may generate a blast wave and missiles. If the material is flammable, it may cause a fire or may form a vapour cloud that then gives rise to a secondary explosion and fireball. The best-known type of BLEVE involves LPG (liquefied petroleum gas). Once a fire impinges on the shell above the liquid level, the vessel usually fails within 10 to 20 minutes. In the case of a BLEVE involving a flammable material, the major consequences are, in decreasing order of importance: 

Thermal radiation from the resultant fireball.



Fragments produced when the vessel fails.



Blast wave produced by the expanding vapour liquid.

For example, a BLEVE of a propane sphere of diameter 15 m could cause damage as far away as 4,500 m, and radiation damage and fragmentation damage would each extend to about 1,000 m. Significant damage to equipment and buildings from radiation is possible from a BLEVE. Wooden structures may be ignited if the radiant heat density at the structure's location exceeds the threshold value for ignition of wood. Severe damage from fragmentation can be expected in the area where 50% or more of the fragments may fall (typically about 100 m from the vessel). As mentioned above, in a fire, a tank containing liquid is most vulnerable in the area controlling the vapour, because very little heat can be absorbed by the vapour, so the metal

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here will heat up rapidly and weaken. The metal contacting the liquid will heat up much more slowly. There is, therefore, a dilemma in that a BLEVE may occur sooner in a partly full vessel than a full one, but a full vessel will contain more fuel for the resulting fireball and fire than will a partly empty vessel.

Confined/Unconfined Vapour Cloud Explosion (CVCE/UVCE) Confined If a flammable vapour cloud is ignited in a container, e.g. a process vessel, or in a building - so that it is confined - pressure can build up until the containing walls rupture. This is a Confined Vapour Cloud Explosion (CVCE), as with natural gas explosions in buildings. A relatively small amount of flammable material, a few kilograms, can lead to an explosion when released into the confined space of a building. CVCEs can cause considerable damage, e.g. peak over-pressures of up to 8 bars can be experienced in a fully confined explosion, and much higher in the unlikely event of a detonation, but in general they have insufficient energy to produce more than localised effects as far as off-site damage is concerned (e.g. broken windows). Much of the major damage found within the confines of the cloud following a UVCE (see next section) probably results from local CVCEs. If the results of a CVCE affect nearby plant or equipment, serious secondary explosions can follow. For personnel close to the blast, missiles and flash-burns can result in serious or fatal injuries. For instance, fatalities have occurred from explosions during hot work on inadequately cleaned/purged 45-gallon drums that had contained flammable residues, or in natural gas or LPG explosions following leaks into rooms. Unconfined An Unconfined Vapour Cloud Explosion (UVCE) results from the release of a considerable quantity of flammable gas or vapour into the atmosphere, and its subsequent ignition. Such an explosion can cause extensive damage, such as occurred at Flixborough in the UK. When a large amount of volatile material is released rapidly into the atmosphere, a vapour cloud forms and disperses. If the cloud is ignited before it is diluted below its lower flammable limit, an unconfined (or uncontrolled) vapour cloud explosion will occur. This is one of the most serious hazards in the process industries. Both shock waves and thermal radiation will result from the explosion; the shock waves will usually produce the greater damage. The energy of the blast wave is generally only a small fraction of the energy available from the combustion of all the material that constitutes the cloud. The ratio of the actual energy released to that available is often called the explosion efficiency. Unconfined vapour clouds can deflagrate or detonate, but a deflagration is much more likely. A detonation is more destructive, but a deflagration can produce a damaging pressure wave. Deflagration occurs when the advancing flame front travels subsonically, in most cases