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A firefighting system is probably the most important of the building services, as its aim is to protect human life and property, strictly in that order.
It consists of three basic parts:-
A large store of water in tanks, either underground or on top of the building, called fire storage tanks A specialized pumping system, A large network of pipes ending in either hydrants or sprinklers (nearly all buildings require both of these systems).
Underground tanks: water must flow from the municipal supply first to the firefighting tanks and then to the domestic water tanks. This is to prevent stagnation in the water. The overflow from the firefighting to the domestic tanks must be at the top, so that the firefighting tanks remain full at all times. Normally, the firefighting water should be segregated into two tanks, so that if one is cleaned there is some water in the other tank should a fire occur.
It is also possible to have a system in which the firefighting and the domestic water are in a common tank. In this case, the outlets to the fire pumps are located at the bottom of the tank and the outlets to the domestic pumps must be located at a sufficient height from the tank floor to ensure that the full quantity of water required for freighting purposes is never drained away by the domestic pumps. The connection between the two tanks is through the suction header, a large diameter pipe that connects the all the fire pumps in the pump room. Therefore there is no need to provide any sleeve in the common wall between the two firefighting tanks.
The connection from each tank to the suction header should be placed in a sump; if the connection is placed say 300 mm above the tank bottom without a sump, then a 300 mm high pool of water will remain in the tank, meaning that the entire volume of the tank water will not be usable.
Ideally the bottom of the firefighting pump room should be about 1 m below the bottom of the tank. This arrangement ensures positive suction for the pumps, meaning that they will always have some water in them.
All pump rooms should without fail have an arrangement for floor drainage; pumps always leak. The best way to do this is to slope the floor towards a sump, and install a de-watering pump if the water cannot flow out by gravity.
In cases where there is an extreme shortage of space, one may use submersible pumps for firefighting. This will eliminate the need for a firefighting pump room.
THE DISTRIBUTION SYSTEM
The distribution system consists of steel or galvanized steel pipes that are painted red. These can be welded together to make secure joints, or attached with special clamps. When running underground, they are wrapped with a special coating that prevents corrosion and protects the pipe.
There are basically two types of distribution systems
Automatic Wet systems are networks of pipes filled with water connected to the pumps and storage tanks, as described so far.
Automatic Dry systems are networks of pipes filled with pressurized air instead of water. When a fire fighter opens a hydrant, the pressurized air will first rush out. The pressure sensors in the pump room will detect a drop in pressure, and start the water pumps, which will pump water to the system, reaching the hydrant that the fire fighter is holding after a gap of some seconds. This is done wherever there is a risk of the fire pipes freezing if filled with water, which would make them useless in a fire.
Some building codes also allow manual distribution systems that are not connected to fire pumps and fire tanks. These systems have an inlet for fire engines to pump water into the system. Once the fire engines are pumping water into the distribution system, fire fighters can then open hydrants at the right locations and start to direct water to the fire. The inlet that allows water from the fire engine into the distribution system is called a Siamese connection.
In high-rise buildings it is mandatory that each staircase have a wet riser, a vertical firefighting pipe with a hydrant at every floor. It is important that the distribution system be designed with a ring main, a primary loop that is connected to the pumps so that there are two routes for water to flow in case one side gets blocked.
In more complex and dangerous installations, high and medium velocity water-spray systems and foam systems (for hazardous chemicals) are used. The foam acts like an insulating blanket over the top of a burning liquid, cutting off its oxygen. Special areas such as server rooms, the contents of which would be damaged by water, use gas suppression systems. In these an inert gas is pumped into the room to cut off the oxygen supply of the fire.