How to Size a Fire Fighting Water Pump

A fire fighting water pump is used to increase the water flow and pressure in a sprinkler system or hose standpipe. It is important to understand the different operating principles, types and drivers along with how to size a pump based on the most demanding factor.

There is a trade off between volume and pressure. A nozzle must be matched to the pumps rated PSI and GPM to achieve the best performance.
1. Hose Size

The size of the fire fighting water pump’s hose dictates its flow rate capabilities. A larger hose has a higher flow rate and is the preferred choice for applications requiring substantial water transfer abilities.

It’s important to evaluate the hose size in conjunction with the pump’s capacity and pressure capabilities. Evaluate the distance from the pump to the hose and consider any elevation changes. Then, ensure that the hose diameter aligns with the pump’s discharge port for optimal performance.

Another consideration is the fire fighting water pump’s PSI and GPM – which stand for pounds per square inch and gallons per minute. The PSI measures how much force the water is coming out of the nozzle, and the GPM measures how many gallons of water are being pumped out each minute.
2. PSI & GPM

Fire fighting water pumps are designed to deliver a specific net pressure at their rated flow. This is a combination of the water supply pressure, plus the engine compression, friction loss and elevation change between the pump and the water supply.

Figuring this all out is a lot of math. Many fire departments calculate the friction losses for all of their nozzles and write them down on what is called a “pump chart.” This way, a pump operator can simply look at the charts and add the numbers. This saves time on the fireground where every second counts.

The pump must be sized correctly to avoid over pumping and also to meet the required pressures for proper operation of the fire system. This is the job of a professional fire safety engineer or Local Authority Having Jurisdiction (AHJ). The fire protection engineering process should take all of these factors into account in order to find that “sweet spot.” The pump that is too big wastes resources and the one that is too small may fail to provide enough pressure.
3. Power

The fire fighting water pump transforms a chosen potential energy source such as electric or diesel fuel into the kinetic energy of flowing water. This flow of water takes on some friction energy as it churns through the system piping. This friction loss needs to be factored into the calculations. Most departments calculate these numbers ahead of time and write them on a pump chart. This saves a lot of work on the fireground.

Fire pumps are installed to detect when a drop in pressure occurs in the fire sprinkler system and start boosting the pressure to ensure adequate water is available for the fire. They also need to perform under demanding performance requirements such as low NPSH curves (to avoid cavitation which causes decreasing head and flow). They must also undergo annual testing.
4. Horizontal Split Cases

Fire fighting water pumps are designed to perform a few specific tasks. They must be able to meet NFPA 25 requirements for hydraulic performance, but they also need to be able to keep working in the event of an emergency. That’s why most fire protection contractors favor split case horizontal fire pumps over other types.

They have a simpler design that makes them easier to work on than end suction pumps. And while a horizontal split case pump can’t be mounted on a concrete housekeeping pad, they are lighter and more compact than other fire pumps.

NFPA recommends routine maintenance for your fire pump, including adjusting the packing gland to maintain 1 drop per second (check with each manufacturer’s guidelines). This will prevent water from being lost through the seal and help extend the life of the pump.
5. Electric Motor-Powered Engines

The GPM and PSI of a fire fighting water pump affect how quickly it can move your hose line. Both should be considered when sizing your pump.

The NFPA requires annual inspection, testing, and maintenance (ITM) of the fire pump. Flow tests must be performed to ensure the pump is performing properly.

In addition to sizing and testing, the pump casing must be designed for easy disassembly and inspectability. This includes having the ability to remove the impeller without disturbing suction piping.

Ideally, the churn point (zero flow) and maximum demand points should not exceed 150% of the pump’s rated flow. This information can be found in the fire pump’s test data sheet or in its operation and maintenance manual. It is also commonly included on the pump nameplate.