Fire involving LNG & LPG cargo - various fire fighting agents & safety aspects
Natural gas contains numerous component gases but by far the greater percentage is methane (CH4), which represents between 60 and 95 per cent of the total volume. This fact is important when considering the safety aspects for fire-fighters tackling an LNG fire.
During the initial period of vaporisation of the gas, ignition may be accompanied by a flash of varying proportions.However, because the velocity of propagation of a flame is lower in methane than in other hydro-carbon gases, it is unlikely that future ignition will have flash effect. The fire-fighting plan should be well thought out in advance and a concentrated effort made rather than ‘hit and run’ tactics, as these will only consume the vessel’s extinguishing facilities without extinguishing the fire. Before attempting to tackle a large fire, you should seriously consider allowing the fire to burn itself out.
Should an attempt to extinguish the fire be made, extensive use of ‘dry powder’ should be employed from as many dispensers as can be brought to bear. Fire-fighters should be well protected against heat radiation and possible flash burns, and approach the fire from an upwind direction. Power dispensers should sweep the entire area of the fire, but direct pressure of powder jets on to the surface of the liquid should be avoided. Should dry powder guns be used, fire-fighters should be well practised in their use and be prepared for some kick-back effect.They should also be made aware that there is no cooling effect from the use of dry powder, and that re-ignition after a fire has been extinguished is a distinct possibility.
In the initial stages it is always preferable to isolate the fire by shutting off the source of fuel.This may not, however, always be possible. A final warning when tackling an LNG fire is that water should not be used directly, as this will accelerate vaporisation of the liquid.This is not to say that surrounding bulkheads and decks cannot be cooled down with water sprays, provided that water running off is not allowed to mix with burning LNG.
Dry powder :
Dry powder is provided both in large fixed installations and portable extinguishers. Any part of the deck can be reached by at least two hoses from the fixed installations.
Water extinguishing :
Water is not a suitable medium for fighting an LNG fire directly as it will cause a massive expansion of the fire, through an increase in the rate of vaporisation of the liquid to gaseous state. Water is however essential as a cooling medium for the area surrounding an LNG fire and to protect personnel who may need to approach the site. Water is also essential for protecting steel work from the effects of extreme cold in the event of a liquid spill.
A CO2 extinguisher system is available for cargo compressor rooms, electric motor rooms, inert gas dryer room and on some ships cargo control room. Ships plans should be consulted for what is applicable to the concerned vessel.
How to tackle LNG fire ?
The rapid vaporisation of any exposed LNG prevents any ignition of the liquid itself and an LNG fire is thus a cold vapour fire.
Ignition of a flammable mixture of natural gas vapour requires a spark of similar ignition energy as would ignite other hydrocarbon vapours. The auto-ignition temperature of methane in air (650°C) is higher than other hydrocarbons.
Electrostatic ignition of LNG is not a hazard during normal operations. This is because the permanent, positive pressure in LNG tanks maintained by gas boil-off prevents air entering these spaces to form flammable mixtures in tanks or lines.
The velocity of propagation of a flame is lower in methane than nearly all other hydrocarbons. Unless ignition occurs during the initial rapid vaporisation period, it is most unlikely that any flash will accompany an ignition. The term ‘lazy flame’ has been aptly used to describe the spreading characteristics of an LNG fire.
Burning of LNG vapours produces a similar flame size and heat radiation to other hydrocarbon fires, but little smoke is produced.
From a fire fighting viewpoint, LNG/cold vapour fires have the characteristics of both liquid and gaseous hydrocarbon fires.
The procedure for fighting these fires is:
The following fire fighting agents may be used:
Water spray systems
It is a requirement that a series of water spray nozzles are located at each tank liquid and vapour dome, at the midships manifold, on the compressor house, on the forward bulkhead of the accommodation block and around the midships cargo control room if applicable. The water for the operation of these nozzles is fed from a pump and line system independent from, but crossconnected with, the ship's fire main. In addition to the above system, the sides of the accommodation block may be protected by spray nozzles supplied with water from the fire main via isolating valves.
Water should NOT be used to extinguish LNG fires as it increases the vaporisation rate and hence the burning rate. However a water spray or fog should be used to protect personnel and to cool areas adjacent to the fire. The qualities that make water unsuitable for fighting LNG fires make it an ideal medium for spraying LNG spillages to increase evaporation rate and prevent re-ignition, provided that the LNG is not actually burning.
Care is necessary to avoid water running off adjacent structures and aggravating burning LNG, or splashing into spill trays which may contain LNG, thus causing it to overflow onto unprotected steelwork. Spill trays and areas under manifolds are in any case floodable with water to protect hull steelwork from damage due to exposure to the intense cold of LNG.
Water jets can be used to deflect burning jets from impinging on other tanks. Care should be taken to avoid extinguishing the fire with the consequent danger of re-ignition of large volumes of flammable gas.
Dry chemical powder
Dry chemical fixed installations are provided on Gas Carriers. Manufacturer’s instructions should be referred to for details of operation and maintenance procedures. Whenever a dry powder hose has been in use, it should be blown clear with nitrogen to prevent any possibility of blockage. The extinguishing power of dry chemical powders depends on the chemical reaction of the small particles when exposed to flame. They are flame inhibiting agents and have been widely proven in LNG fire tests.
The maximum possible rate of application of dry powder is desirable. As many high velocity jets as possible should be brought to bear at once, preferably in a down wind direction. Jets should be aimed with the objective of reducing boil-off rate by sweeping over the whole fire area and on no account must the surface of an LNG pool be agitated. Possible re-ignition must be guarded against.
Correct use of dry chemical powder equipment is essential if reserves are not to be wasted and the fire is to be successfully extinguished. Extinction with dry powder is obtained by maximising the rate of application and minimising any agitation of pools of LNG. This may be achieved by coordinating a simultaneous attack with all available applicators. A first-aid shot with only one hose or monitor may be warranted with small fires, but continuous individual efforts can never be as successful as a simultaneous attack with as many applicators as possible being brought to bear.
Operators must be adequately protected and positioned to obtain down wind line-of-sight application, with the powder jet slightly depressed below the horizontal. Powder jets should be swept rapidly back and forth over the entire fire area. The direct impact of powder jets on pool surfaces or leaks should be avoided. Where possible, powder should be aimed at vertical surfaces immediately behind the seat of the fire.
The high discharge rate hoses are as much as one man can handle and the reaction force and consequences of wasting or misdirecting powder requires that great care be taken in their use. If the above techniques are adopted, tests have shown that LNG fires can be readily extinguished. In fact extinction has often proved unexpectedly easy.
Re-ignition of LNG and vapour by burning paintwork, or other sources, must be expected. Water sprays should be activated as soon as possible to cool steel work and speed vaporisation.
Gas smothering systems
Although CO2 and nitrogen smothering systems are not suitable for use in exposed open air applications, they are otherwise the most efficient agent for fighting liquid and vapour fires. By diffusing in a burning mixture, they lower the oxygen content and render the mixture inert. If the flames can be separated from the liquid, the boil-off rate will also be reduced.
Nitrogen is more effective and less dangerous to personnel than CO2 but CO2 is more easily stored.
How to tackle LPG fires ?
The highest priority of action must be given to stopping the gas flow to limit the amount of flammable material available, and contain the fire in as small an area as possible. This may happen automatically with the operation of the Emergency Shut Down System. Fire fighters must wear protective clothing and self-contained compressed air breathing apparatus. Tackling the fire requires the use of two media, water and dry powder.
Large quantities of water spray are to be used:
The normal extinguishing medium for LPG fires is dry powder, which is propelled by nitrogen. The Master is to ensure that all Officers are familiar with the operation of this equipment, and the technique to be used in fighting a LPG fire.
The best results are achieved by applying dry powder at a maximum rate by using as many guns as possible from upwind. The guns must sweep rapidly backwards and forwards over the fire area. If a liquid spillage is involved, the surface of the spillage must not be disturbed by direct impact. Dry powder guns discharge at not less than 4 kilos per second. The initial recoil and subsequent force exerted by discharge means that in order to avoid the wastage of dry powder, a second person may be needed to help the operator maintain control of the gun.
If it is judged preferable to allow a flame to burn from a controlled leak, such as a pipe fracture, water spray is to be used to contain the fire without extinguishing the flame.
Either CO2 or Halon are fitted to the Cargo Control Room, Compressor Room and Motor Room on LPG ships.
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