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Principles of Fire Fighting

April 15, 2014

Fire-Fighting

The material in this post is extracted from Chapter 9 of the book Plant Design and Operations.

In Chapter 1 it was noted that risk can be divided into four components: hazard, consequence, likelihood and safeguards. It was also noted that it is generally best to reduce risk in that order, i.e., to start by removing hazards and only to rely on safeguards if no other action can be taken. Therefore, with regard to firefighting, it is suggested that the following sequence of actions should be followed wherever possible.

Remove the Fuel Source

The most effective way to extinguish a hydrocarbon fire is to stop feeding fuel to it. This is often done with isolation valves that are located at the perimeter of the facility. These valves either stop the flow of fuel to the fire or they direct the inventory of hydrocarbon to a safe location, as with emergency depressuring valves. The valves must be able to withstand the largest plausible fire radiation (this is often done by placing the valves behind an earthern wall or bund). If operators need to reach these valves during an emergency they should be provided with protected access and egress routes.

Local fires are also best extinguished by stopping the flow of fuel. For example, if a pump seal leaks and catches fire the best response is to close the suction and discharge valves on either side of the pump. If the pump is in critical service or at a critical location the valves should be capable of remote operation.

Probably the best, and most tragic, example of not stopping the flow of fuel to a fire occurred at the Piper Alpha platform in the North Sea in the year 1988. It is thought that seven men died in the initial explosion. Yet the final death toll was 167. This is surprising given that the inventory of oil and gas on a platform such as Piper is low — a fire should burn itself out in less than 15 minutes. Yet the fire continued to burn for hours — the reason being that adjacent platforms, which were feeding hydrocarbons to Piper (which was a hub platform) — did not stop feeding the raging fire because “no one told them to stop”.

Consequence and Likelihood

If the flow of fuel to a fire cannot be stopped, priority should be given to controlling the consequences of that fire. For example, fire water can be used to keep equipment and tanks in the area cool. This action does not stop the fire itself, but it does limit the overall damage.

The likelihood term is not relevant at this stage since the fire has already started.

Safeguards

The last line of defense in firefighting response is the use of safeguards such as emergency response systems and specialized PPE for the members of the Emergency Response Team.

Single Fire Concept

The fire water system and the fire fighting equipment are generally designed to handle just one major fire at a time. In other words, the design capacity of major fire fighting facili­ties is determined by the largest single fire contingency (this is analogous to the single event scenario concept used in relief valve design). Some fire fighting systems are sized to handle less significant contingen­cies. For instance, foam concentrate requirements are usually determined by a tank fire rather than by the worst contingency, which may be a fire in the process area.

Fire Zones

For all but the smallest facilities fire zones are used. They ensure that fire fighting systems are used only in those areas that actually have a problem. Offshore platforms, for example, are typically divided into about seven zones.

Figure 9.1 shows the use of fire zones. A ring main goes around the entire facility. It is filled with water whose pressure is maintained with a jockey pump. Connected to the ring main are multiple zones. The fire water headers in each zone are normally dry. In this example, there are two firewater pumps, each of which has sufficient capacity on its own to handle the design fire case. These pumps are placed in different locations at the facility so that, if one is destroyed, the other will provide a full flow of fire water. It is common for them to have different power supplies — in particular, one of them will be driven by a stand-alone diesel motor that operates independently of the facility’s utility systems.

If a fire occurs in one of the zones a fusible link will fail, causing the pressure control deluge valve (PCDV) to open and the main fire water pumps to start. Water will flow out of the sprinkler heads in that zone only. The PCDV can also be tripped manually.

Figure 9.1
Fire Protection System

Fire Ring Main

Once the fire has been brought under control the system is reset. If seawater is used as deluge water, then it is important to flush the zone headers and deluge nozzles with fresh water, otherwise corrosion products will build up.

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