A fire fighting water pump is a large machine designed to provide a boost of pressure and flow to help support sprinklers, standpipes and other fire systems in buildings that do not have enough natural gravity or available water supply to generate the required hydraulic pressure. Typical examples include high-rise buildings, where the available city supply cannot generate enough pressure to overcome the height of the structure. Fire pumps are also used to support foam systems, which require much higher water pressures than typical sprinkler or standpipe systems.
The fire pump’s size is typically dictated by the most hydraulically demanding area of the building, which in high-rise buildings may be the automatic fire standpipe system demand, usually 500 gallons per minute (gpm) at 100 pounds per square inch (psi) at the highest point in the building. For low-rise buildings, it is generally the ordinary group 1 hazard demand.
While there are many ways to calculate a fire pump size, the basic calculations are as follows: GPM (gallons per minute) and PSI (pounds per square inch). The higher the gpm, the more water the pump will produce, and the higher the PSI, the more pressure the pump will deliver.
There are several different types of fire pumps, including horizontal split case, inline and vertical turbine pumps. The choice of a particular fire fighting water pump will be determined by a number of factors, including cost and application requirements. A fire fighting water pump must meet NFPA 20, UL and FM requirements, including having a certified factory test curve.
The driver that powers the fire pump is another critical component. There are three drivers outlined in NFPA 20: electrical motor, diesel engine and steam turbine systems. Electric motors are the most common type of fire pump driver, and they take electrical power provided by a utility connection or generator to turn an impeller that spins a shaft connected to the pump.
During a normal operating cycle, a fire pump will operate in a steady state, maintaining its rated pressure (Psi) and rated flow at the discharge flange of the pump. It will only operate in an emergency mode when the demand of the fire protection system exceeds what is available from the natural water source.
When an alarm is activated, the pump will start up and increase its speed to produce a burst of water at the discharge nozzle. As the water is pumped out of the pump, friction loss slows the hoseline and reduces the available pressure and flow. This is why it is important to know the gpm and psi requirements for your equipment.
Most fire departments figure friction loss for their hoselines and appliances, and they write the figures on what is called a “pump chart.” This way, when an apparatus goes into service, the firefighter knows exactly how to calculate the psi and flow rate. This saves valuable time on the scene of a fire, when every second counts.