2.3 Fire and explosion prevention and protection Flashcards
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
automatic detection and suppression systems
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
are those that are designed into the physical structure of the building, such as load bearing components, compartmentation, fire resistant materials, etc.
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
E30 (FD30S) or E60 (FD60S) providing 30 and 60 minutes protection respectively. Fire doors are available providing between 20 and 120 minutes of protection.
Whatever the type of building, typical situations that may assist the spread of fire and smoke include: 7
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.
Control measures
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
Mitigation measures
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.
HSE advice on safe working with HFL’s uses the acronym VICES as an aide-memoire for basic precautions
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.
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
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.
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
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.
Many factors will influence the overall reliability of an inerting system, including: 9
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.
Explosion relief methods
Explosion relief vents Explosion panels Bursting discsSuppressionInterconnected plant
A petrol storage tank in a bund containing three other similar
tanks is overfilled during a transfer operation resulting in a large
spill of petrol into the bund. The petrol vapour is ignited from a
remote source causing a catastrophic explosion and fire involving
the whole installation.
(a) Identify the design and construction
measures that could have prevented such
an event.
4 marks
(b) Outline the range of measures that would
be necessary to mitigate the effects of such
an event.
6 marks
(a) Design and construction measures that could have prevented the incident
include:
adequate segregation between adjacent tanks and the use of separate
bunding
an interlocked pumping system with high level alarms with at least double
redundancy of the alarms
level detection and vapour detector monitoring systems fitted in the bunds
together with foam vapour suppression
the provision of remote shut down systems and a good standard of earthing
and bonding.
(b) Measures that would mitigate the effects of this event include:
provision of adequate fixed foam installations to spray the surface of pools in
the bunded areas and spray deluge systems
installation of foam monitors capable of reaching the tops of tanks
erection of radiation walls between adjacent bunds to prevent other tanks
being heated
an adequate supply of fire fighting water
the redundancy of water pressurisation
the installation of remotely operated pumps and valves to empty affected
tanks easy routes of access for fire fighters
the provision of drainage interceptors to minimise the environmental effects
of fire water run-off
regular draining and cleaning to remove firewater from bunds
the preparation of an emergency plan to deal with such an event should it
occur the provision of training for operatives both in the content of the plan
and the actions expected of them.
Employees in a vehicle maintenance workshop undertake spray-
painting of vehicles using a solvent based paint that has a low
flash point.
Assuming that a risk assessment has been carried out, outline
the practical measures to control the risk of fire and explosion
associated with the paint spraying activity.
20 marks
Practical control measures would involve ensuring that the workshop as a whole
was constructed of fire resistant material or that the spray painting activity was
carried out in a dedicated fire resistant booth.
It might be possible to replace the paint in use with one that was less flammable
but whether or not this could be done, the quantities of paint and solvent in the
workshop should be reduced to a minimum and kept in non-spill containers
provided with lids.
A separate external fire resistant and ventilated storeroom for the bulk of the
paint and solvents should be provided at a safe distance from the workshop.
The workshop should be ventilated at both high and low level with local exhaust
ventilation being provided for the spraying activity.
All electrical equipment should be to the standard appropriate for the conditions
that might prevail such as flameproof or intrinsically safe, earthed to avoid the
possibility of electrostatic ignition and where possible the employees should
wear anti-static footwear and clothing.
Procedures would have to be put in place for the containment and clean-up of
spillages and for the cleaning of the spray guns with a fire resistant container
provided for waste rags.
Fire fighting equipment would have to be provided together with adequate fire
escape routes and emergency procedures drawn up and communicated to the
workforce who would also need instruction and training on the risks associated
with the work activity and the precautions that should be observed.
Outline the precautions that could be taken to prevent failure of
steel, concrete and wood in the event of a fire.
8 marks
Steel - might be protected by the use of sprayed concrete or fibre board
of an appropriate rating.
Concrete - the selection of the type of mix to improve fire resistance and
increasing the thickness of concrete between the external face and
reinforcement bars would provide some protection.
Wood - failure of wood may be prevented by selecting fire resistant
timber, increasing the thickness used to allow for a charring outer layer or
impregnating it with fire retardant.
Other general precautions include:
the provision of means such as compartmentalisation to prevent heat
transfer through long steel members in the event of fire
automatic means of applying water to structural members, should fire
occur
the use of fire resistant surface claddings or flammable sacrificial
cladding.
A diesel engine is being used to power a machine in a potentially
explosive atmosphere.
Outline the protection that should be applied to the diesel engine to
minimise the risk of explosion.
6 marks
Protection that should be applied to the diesel engine to minimise the risk of
explosion include:
fitting flame and spark arrestors to prevent flashback to atmosphere, if
flammable vapour was drawn into the inlet system, and to prevent any flames
from the exhaust system escaping into the outside atmosphere
designing the engine and exhaust system to ensure that their surface
temperatures were below the ignition temperature of any flammable
substances
the use of a water jacket on the exhaust system, which would provide
additional protection, as would the use of thermal sensors together with air
cooling for hotspots
use of electrical equipment suitable for zones 1 and 2, with mechanical
alternatives (e.g. fuel gauge) fitted where possible
engine cooling fans made from plastic
care taken to ensure that there’s adequate clearance between fixed and
rotating parts of the engine
a gas detecting device fitted, which should be linked to an automatic cut-off.
