How to Size a Fire Fighting Water Pump

A fire fighting water pump is a special type of water pump designed to provide the hydraulic pressure needed for various fire systems, including sprinklers and standpipes in a commercial building. Fire pumps are designed to meet a number of different requirements and must comply with stringent regulations. They also must be properly installed and maintained to ensure they work correctly when called upon to fight a fire.

A building owner, fire safety specialist and local authorities having jurisdiction should all be involved in the design process to ensure the proper size is chosen and that all required components are included. A hazard analysis should be performed to determine the most demanding area of the building that requires a fire pump. Using this information, a fire pump can be sized to meet that most demanding requirement. A sizing diagram is then used to create the required pressure/flow chart for the system.

The most demanding factor typically determines the fire pump size, but there are a few other factors that may come into play. For example, if the building has a sprinkler demand that is higher than the building’s normal water pressure, then the sprinkler system will require a larger fire pump to operate at peak performance. This is known as the high-pressure sizing scenario.

Similarly, if the fire sprinkler system is designed to operate with a lower flow rate than the normal operating range, then that will also influence the fire pump size. Whether the fire sprinklers or fire standpipe system will be operating under lower flow conditions should be determined by a hazard analysis of the building.

One of the most common problems with fire pumps is cavitation. Cavitation occurs when vapor pockets form in the pump, which causes them to implode and damage the pump impeller. The resulting damage can be severe and lead to a costly repair or replacement. To prevent cavitation, the NPSH (non-pressure drop head) of the fire pump must be greater than the vapor pressure of the water in it.

Another problem with fire pumps is excessive cycling, which can occur when the water supply pressure drops in a sprinkler or standpipe system, or when the nozzles of a firefighting water system are opened and closed. To minimize this issue, a pressure maintenance pump, commonly referred to as a Jockey pump, can be installed in the system to keep the pressure up in non-fire conditions and prevent unnecessary cycles of the main fire pump.

A fire fighting water pump can be either centrifugal or positive displacement and is driven by an electrical motor. The motor takes electrical power from the utility connection, generator or other approved power source and turns a shaft connected to the pump’s impeller. The most common fire pump driver type is the electrical motor, outlined in NFPA 20: Standard for Installation of Stationary Pumps for Fire Protection. Other types include diesel engine and steam turbine systems. Each type of driver must be inspected, serviced and kept in good working condition to make sure it will function as intended during a fire.






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