Fires involving Flammable liquids, liquefiable solids, grease, oil-based paints etc are categorized as “B” CLASS FIRES. The flammability of any liquid is dependent on its FLASHPOINT. Almost all petroleum products in liquid form come under this category.
The petroleum act 1934 defines PETROLEUM means any liquid hydrocarbon or mixture of hydro-carbon and any inflammable mixture (liquid, viscous or solid) containing any liquid hydrocarbon. The important property of flammable liquid is the FLASHPOINT (Flash-point of any petroleum means the lowest temperature at which it yields a vapour which will give a momentary flash when ignited). The less the flash point the more the risk a particular liquid can get ignited. The below categorization will help in dealing with liquid fires
All FLAMMABLE LIQUIDS are categorized into three classes based on FLASHPOINT, they are
CLASS – A: All liquid hydrocarbons having a flash-point below twenty-three degrees(<23°C) centigrade. Examples Petrol, Acetone, Methyl ethyl ketone, Ethyl acetate, methanol, ethanol etc
CLASS – B: All liquid hydrocarbons having a flash-point equal to above twenty-three degrees(≥23°C) centigrade and less than sixty-five degrees centigrade (<65°C). Examples White spirit (MTO), Isobutanol etc.
CLASS – C: All liquid hydrocarbons having a flash-point equal to above twenty-three degrees(≥65°C) centigrade and less than sixty-five degrees centigrade (<93°C), Example Lubricants, Lubricating oils.
TYPES OF LIQUID FIRES
The size, shape and appearance of the fire play a vital role in tackling B CLASS fires along with other properties. Understanding different types of liquid fires and their characteristics can affect the approach towards firefighting, they are classified into
- 1. POOL FIRES
- 2. SPILL FIRES
- 3. SPREADING FIRES
- 4. RUNNING FIRES
POOL FIRES: Pool fires as the name indicates gets accumulated at a place forming confined pools, these confined pools can vary in depth and area, with some pool fires can have pools over a large area, the dyke portions / bunded places of the storage tanks where flammable/combustible liquids collect can form into pool fires. The pool fires depending on the depth of the liquid accumulation will continue to fire for a considerable time. These fires are prone to give high levels of heat output which requires firefighters to make more effort in cooling to prevent any reignition after the fire gets extinguished. The pool fires require FOAM application where both heat resistance and fuel tolerance with fast-flowing characteristics is required. (IMP: water should not be used as a substitute as a cooling agent as this will break down the foam blanket and allowing reignition – burn back)
SPILL FIRES: Spill fires happen anywhere where the spillage happens due to a ruptured fuel container/drum, these spillages happens in unconfined areas where there is unevenness of the area. The average burn time of these spill fires is very less compared to pool fires as these do not have that much depth. In spill fires the burning time of fuel and subsequent cessation of fire depends on the intermittent leakage of fuel from the ruptured container, the leakage may create new localized surface pool fires which can restart the fire or spread to other areas. The presence of spray discharge increases the risk of fire for flammable liquids by providing a readily ignitable cloud above the spillage
SPREADING FIRES: In spreading fires the liquid fuel is continuously supplemented by the fuel from the ruptured tank in the form of spray, jet or stream. The continuous supply of fuel to the burning liquid will result in the fuel spreading to other areas in the vicinity and flowing into drainage systems and floor openings which can lead to burning fuel falling to basement areas if present.
The important thing is to prevent the contact of fresh fuel ejecting out from reaching the flames of the already burning fuel, this can be done by exploring all possibilities of cutting down fuel to arrest leakage, for example, fuel from pipeline gaskets/ broken pipelines/ valves calls for shutting down the main valve in the stream to stop fuel. Water displacement is considered in a ruptured pipeline where the possibility of closing any valves will not help, this would help water to take the place of fuel to the point above the outlet level, due to density difference so that water would flow instead of fuel.
Two situations are encountered in these spreading fires with
- Fuels having high flash point: Here the burn back rate of the flames from the leaking fuel mayor can be less than the rate at which the fuel is discharged from the container leak. The fire can be extinguished with a foam blanket.
- Fuels having low flash point: Here the burn back rate of the flames from the leaking fuel exceeds the rate at which the fuel is discharged out of the container leak. Here dry powder along with foam to be used to counter the discharging fuel which is also on fire.
The presence of spray discharge increases the risk of fire for flammable liquids by providing a readily ignitable cloud above the spillage
RUNNING FIRES: These situations happen when the burning liquid is flowing on the slope. This situation is very dangerous as the fire envelopes the materials and objects in the path of the flow. Rapid knockdown by the use of film-forming foams along with using Fluro-protein or Alcohol resistant foam to lay a band at the lower end of the flow path, so that the foam blanket is formed on the top.
Ideally, a combination of all the fires do happen at a single instance, which requires a careful analysis of the entire situation, for example, THREE DIMENSIONAL FIRES ( A three-dimensional fire is a liquid-fuel fire in which the fuel is being discharged from an elevated or pressurized source, creating a pool of fuel on a lower surface) In three-dimensional fires the spill fires are first tackled then the spreading fires are then tackled using a dry chemical agent
CONCLUSION
- Foam extinguishers work best for liquid fires where blanketing is necessary to prevent burn-back.
- Dry chemical powder with high flow rates works best for pressurized liquid fires and three-dimensional fires.
- For liquids with flashpoints higher than ambient temperature, use of water also is very advantageous due to the continuous availability and high heat removing potential
Article by PJ Mohan
Sr.Faculty, NIFS
1 comment:
Very informative... thank you sir
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