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A fire fighting 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:
1) A large store of water in tanks, either underground or on top of the building, called fire storage tanks
2) A specialized pumping system,
3) A large network of pipes ending in either hydrants or sprinklers (nearly all buildings require both of these systems)
A sprinkler is a nozzle attached to a network of pipes, and installed just below the ceiling of a room. Every sprinkler has a small glass bulb with a liquid in it. This bulb normally blocks the flow of water. In a fire, the liquid in the bulb will become hot. It will then expand, and shatter the glass bulb, removing the obstacle and causing water to spray from the sprinkler. The main difference between a hydrant and a sprinkler is that a sprinkler will come on automatically in a fire. A fire hydrant has to be operated manually by trained firefighters - it cannot be operated by laymen. A sprinkler will usually be activated very quickly in a fire - possibly before the fire station has been informed of the fire - and therefore is very effective at putting out a fire in the early stages, before it grows into a large fire. For this reason, a sprinkler system is considered very good at putting out fires before they spread and become unmanageable.
An electric fire pump located in a fire fighting pump room.
FIRE STORAGE TANKS
The amount of water in the fire storage tanks is determined by the hazard level of the project under consideration. Most building codes have at least 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, fireworks factories). The relevant building code lists which type of structure falls in each category. The quantity of water to be stored is usually given in hours of pumping capacity. In system with a capacity of one hour, the tanks are made large enough to supply the fire with water for a period of one hour when the fire pumps are switched on. For example, building codes may require light hazard systems to have one hours capacity and high hazard 3 or 4 hours capacity.
The water is usually stored in concrete underground tanks. It is essential to ensure that this store of water always remains full, so it must have no outlets apart from the ones that lead to the fire pumps. These tanks are separate from the tanks used to supply water to occupants, which are usually called domestic water tanks. Designers will also try and ensure that the water in the fire tanks does not get stagnant and develop algae, which could clog the pipes and pumps, rendering the system useless in a fire.
FIRE PUMPING SYSTEM
Fire pumps are usually housed in a pump room very close to the fire tanks. The key thing is that the pumps should be located at a level just below the bottom of the fire tank, so that all the water in the tanks can flow into the pumps by gravity.The pumps are controlled by pressure sensors. When a fire fighter opens a hydrant, or when a sprinkler comes on, water gushes out of the system and the pressure drops. The pressure sensors will detect this drop and switch the fire pumps on. But the only way to switch off a fire pump is for a fire fighter to do this manually in the pump room. This is an international code of practice that is designed to avoid the pumps switching off due to any malfunction in the control system.
The capacity of the pumps is decided by considering a number of factors, some of which are:
the area covered by hydrants / standpipes and sprinklers
the number of hydrants and sprinklers
the assumed area of operation of the sprinklers
the type and layout of the building
THE DISTRIBUTION SYSTEM
The distribution system consists of steel or galvanised 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 fire fighting 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, usegas 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 fire fighting 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.
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 fireghting 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 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.
Ideally the bottom of the firefighting pump room should be about 1m 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.