Fire Fighting System
When a big fire has already started, there is no turning back. The fire has to be put off.
Wet Riser System
The wet riser system described below is a typical installation in a high-rise building. It is used to supply water from the wet riser water tank, through wet riser pipes, distributed to each floor, and ending at the canvas hoses and hose reels.
The pipes supplying water to the hoses are pressurized all the time. Three pumps supply the water from the tank to the hoses. The pumps are the duty pump, the stand-by pump, and the jockey pump. Pressure switches along the pipe control the starting of each pump.
The wet riser canvas hoses are located at each lift lobby. Each canvas hose has a diameter of 65 mm and a length of 30 m. When the hoses are in use, the pressure at each floor is controlled by landing valves that return excess pressurized water back to the wet riser tank through drain pipes.
The hose reels are also located at each lift lobby. Each hose reel has a diameter of 25 mm and a length of 30 m. Pressure reducers are installed at the end of each wet riser pipe to supply a lower pressured water to hose reels.
When the valves of the hoses are opened, pressurized water will shoot out through the nozzles of the hoses. The pressure in the piping will drop. The pressure switches at the pump room will sense the drop in pressure and the pumps will start to pressurize the pipe again. A typical system for a high-rise building is described below. The pressures are monitored at the pump room to control the pumps.
Jockey Pump
The first pump to start will be the jockey pump. The controlling pressure switch is set to start the pump at a pressure of 150 psi and stop it when the pressure reaches 230 psi.
If a small leak exist in the wet riser piping - either on the wet riser or hose reel - the pump will start in order to compensate for the leak.
Duty Pump
If the pressure drops below 125 psi, this means a wet riser landing valve or a hose reel gate valve has been opened. The pressure switch that senses this set pressure activates and starts the duty pump.
Stand-by Pump
If for any reason the pressure continues to drop below 125 psi, it means either the duty pump has not started or is not available (under repair...etc.). The stand-by pump then takes over the function that is not performed by the duty pump. The stand-by pump is set to cut-in at 95 psi.
Once the water flow is not needed any more the pressure builds up in the piping network. The duty or stand-by pumps stops after the cutout pressure is reached at 220 psi.
The jockey pump continues to run until the system pressure reaches its cutout pressure of 230 psi.
For a low building, the system will be less complicated. However, the purpose of supplying water to the fire fighting hoses is maintained.
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