Firefighting System

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 a large store of water that is directed to the fire via pumps and a distribution system of pipes ending in hydrants or sprinklers. A hydrant is a vertical pipe that has an outlet to which a hosereel can be attached when water is to be sprayed on a fire.

The quantum of water is determined by the hazard level of the project under consideration. There are three levels, namely, Light Hazard (such as schools, residential buildings and offices), Ordinary Hazard (such as most factories and warehouses), and High Hazard (places which store or use flammable materials like foam factories, aircraft hangars, paint factories, firework factories). The National Building Code of India(NBC) lists out which structures fall in each category. The quantum of water to be stored is usually given in hours of pumping capacity, i.e. the fire pumps will finish pumping the water stored in say one hour. Light Hazard projects are required to have at least one hour’s capacity and High Hazard 3 or 4 hours capacity. The water is stored in underground tanks.

The pumps are sized by the area to be covered by the system; the more the hydrants, the larger the pumps. One must have standby pumps; if the power supply in the area is bad, one should provide a diesel pump in addition to the main and standby electric pumps. These will be housed in a pump room near the firefighting tanks, along with the electrical panels that control and power the pumps. The electric pumps must be connected to a Diesel Generator (DG) set. The pumps are switched on whenever one opens a hydrant (or a sprinkler comes on). When a hydrant is opened, the pressure in the line falls, and a pressure sensor near the pump switches the pumps on. The pressure in all firefighting pipes is normally maintained at 7 Kg/cm . A small pump, called a Jockey Pump, keeps switching on automatically to maintain this pressure and compensate for any small leakages in the system. The pumps will therefore start automatically when one opens a hydrant, but they should be stopped manually by switching them off in the pump room after one has ensured that the fire is completely extinguished (this is an international code of practice).

The distribution system consists of MS or GI pipes that are welded together. They are of course painted red. When running in external soil, they are wrapped with a special coating that prevents corrosion and protects the pipe. 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.

The system ends with either a hydrant or a sprinkler. Sprinklers are considered one of the best and most reliable forms of fire protection and are traditionally used to cover areas where flammable items are kept. However nowadays they are also used to protect office areas. The logic is that they will stop a fire quicker than other systems, so a small area in an office may be ruined by the water, but that will have prevented the loss of a larger area of the office due to the spread of fire. Each sprinkler has a small glass bulb with a liquid in it. In a fire, the liquid will expand and shatter the glass bulb at a pre-defined temperature. This will allow water to start flowing through the sprinkler, which sprays it in a radial pattern. Bulbs with different temperature ratings are available. They are signified by the colour of the liquid in the bulb. Remember that sprinklers are to be placed as close to the soffit of the roof as possible, as hot air gathers there. In a steel structure they may be pointed upwards in order to protect the structure too.

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.

When you design a firefighting system, remember the following:
• 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. The connection between the two tanks is through the suction header, a large diameter pipe that connects the various 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 300mm above the tank bottom without a sump, then a 300mm high pool of water will remain in the tank, meaning that the entire volume of the tank water will not be useable, to which the Fire Officer will object.
• Ideally the bottom of the firefighting pump room should be about 1m below the bottom of the tank. This arrangement gives positive suction to 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.
• Create a special shaft for wet risers next to each staircase. About 800 x 1500mm should suffice. It is better to provide this on the main landing rather than the mid landing, as the hoses will reach further onto the floor.
• As per the Indian NBC 2005, all office areas are to be sprinklered. If the space between the false ceiling and the slab above exceeds 800mm in height, a second layer of sprinklers should be provided within the false ceiling so that fire does not spread there.

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