A fire pump is the most expensive and complicated piece of equipment in a water-based fire protection system. Specifically designed, tested and listed for fire protection, fire pumps are used to bridge the gap between system demand and water supply pressure (measured in psi or bar). Fire sprinkler systems are usually the main application for a fire pump but they can also be found in standpipes, foam systems and other types of systems that require an increased hydraulic flow such as high-rise buildings.
When a fire alarm is activated or a fire department demand signal is sent to the fire pump, it will automatically switch on and begin pumping water. This is accomplished through a series of mechanical components including a pump, motor and driver. The fire pump is the most critical part of the fire safety system and must be properly sized to handle the demands of its specific application. If the pump is undersized it can cause spray patterns to not develop correctly, not provide enough pressure to hose valves at the top of the building or even burst open.
The NFPA 20 fire pump standard describes a number of requirements that must be met in order for a pump to meet the demands of a specific application. This includes the pump curve and power sizing. The curve is a graph that shows the head (pressure) against the flow rate and can be constantly rising or with a local maximum point at the end of the curve. In addition to the head/flow chart, NFPA 20 also defines a “primary rating point” and a “secondary rating point.” The primary rating point expresses a percentage of the system’s total Q (flow) and P (pressure) requirements and is used to select the fire pump model and horsepower. The secondary rating point is the percentage of the system’s total gallons per minute (gpm) requirement that must be achieved.
Depending on the type of fire pump and application, there are several different driver options available. Electric motors are the most common option and are outlined in NFPA 20 Chapter 9. These take electrical power from either a utility connection, generator or other approved source and turn a shaft that spins an impeller. When the shaft is spinning, it creates a vacuum and water is pulled into the pump.
The pumps are paired with a controller that is responsible for determining when the pump should start and stop. The fire pump controller can use a traditional bourdon-type pressure switch with mercury or an electronic pressure transducer to sense drops in pressure and send a signal to the motor to start and/or stop. These controllers may also be capable of remote monitoring and/or control through IIoT solutions. Keeping up with the various operating principles, types, drivers and applications of fire pumps is important to understanding how they work and how to size them for their application. Stay tuned for future articles where we will cover topics like mandatory alarms, starting method type and more.