This blog outlines a process to determine if a fire pump is needed for the sprinkler system and explains how to select a fire pump that meets the required pressure and flow. The need for a fire pump should be decided ideally when project scope is being developed. For better understanding of the process, definitions have been provided for relevant terms about fire pumps.
1. Definitions:
- Fire Pump Unit: An assembled unit consisting of a fire pump, driver, controller, and accessories.
- Fire Pump: A pump that is a provider of liquid flow and pressure dedicated to a fire protection system.
- Rated Flow: The capacity of the pump at rated speed and rated pressure as marked on the manufacturer’s nameplate.
- Pressure Maintenance (Jockey or Make-Up) Pump: A pump designed to maintain the pressure on the fire protection system(s) between preset limits when the system is not flowing water.
- Discharge Pressure: The total pressure available at the fire pump discharge flange.
- Rated Pressure: The net pressure (differential pressure) at rated flow and rated speed as marked on the manufacturer’s nameplate.
- Pump curve details: A curve for pump plotted between pressure and flow (See Figure 1)
2. Determination Basis:
To determine if a fire pump is needed, the fire suppression system demand must be compared with the available water supply. If available supply cannot meet the system demand, a fire pump is required. Conversely, if the water supply can meet the system pressure and flow requirements, a fire pump is not needed. Always first try to increase pipe sizes, because adding a pump is an expensive option.
Water can be supplied from a variety of different sources including public water mains, dedicated fire protection fire mains, elevated tanks, etc. It is important to ascertain that if the water supply can accommodate the flow demands. We assume in this blog example that the water supply contains an adequate volume to meet flow and duration needs. Therefore, only pressure needs to be boosted by fire pump.
When connecting to a fire main, hydrant-flow tests can be used to analyse the supply data. Static and residual pressures and flow rate data need to be obtained to determine the available water supply which can then be compared to the sprinkler system demand flow and pressure requirements. It is most helpful to conduct the flow test as close to the fire suppression system riser as possible to decrease errors resulting from hydraulic-calculation estimates.
3. Steps to determine if a fire pump is needed:
a) Gather fire water-supply information.
b) Calculate required flow.
c) Calculate required pressure.
d) If supply cannot meet flow, provide fire water tank or reservoir.
e) If pressure cannot be met, provide a fire pump.
f) If supply can meet required pressure and flow, no fire water tank or fire pump is needed.
4. Steps to select a fire pump:
a) Gather fire water-supply information.
b) Calculate required flow.
c) Calculate required pressure.
d) Calculate pressure boost required.
e) Select pump where flow demand is between 100% and 150% of rated flow. Preferably between 115% and 135%.
f) Select pump with performance curve where pressure boost is sufficient at flow demand.
5. Example:
After calculation, it is found that sprinkler system demand is 650 GPM @ 95 PSI. From the flow test, it is seen that city water has a static pressure of 45 PSI, and a residual pressure of 30 PSI @1100 GPM (See Figure 1). A fire pump rated at 600 GPM @ 65 PSI is selected to boost pressure. Figure 2 includes a plot of the pump curve with churn pressure being 120% of the pump’s rated pressure, or 78 PSI, and 150% rated flow (900 GPM) at 51 PSI. For more details, see item 6 below. In figure 3 the pump curve and the city water supply curve are combined, and it shows that system demand is below combined supply curve. Therefore, the selection of pump is ok.
6. The Basis for picking the right type of fire pump:
Many fire pump manufacturers provide selection tools on their websites where required flow and pressure can be input, and their results show the pumps offered that can meet those requirements. NFPA 20 provides limits on the performance of pumps. This ensures that pump curves are not too steep, which would allow the pressure boost to drop quickly. At 150% of a pump’s rated capacity, a fire pump is not permitted to provide less than 65% of its rated total head (See Figure 1).
As pumps flow at rates beyond their rated flow, the pressure they can provide decreases. Some pumps have flatter curves where the pressure drops slowly as the flow increases, and others lose pressure more quickly. It is important to consider where on the pump curve the flow requirement is located. At the water-flow demand point, the pressure boost provided by the pump needs to be greater than the pressure required.
There are likely many pumps offered that meet the performance requirements, but knowing the pressure and flow requirements will allow the design to progress and help scope the general size, cost, and space needed for the installation. Many options, styles, and arrangements of fire pumps exist, but the fact remains that each must meet the specific performance required by the suppression system.
7. Conclusion:
It needs to be emphasized that fire pumps and associated components are costly to install, test, and maintain. PLC Fire Safety Engineering (PLC) has extensive knowledge and expertise to review and perform hydraulic calculations including sizing and selection of fire pumps. In the design optimization process for water-based suppression systems, including an increase of pipe sizes, selection of appropriate discharge devices, rearranging distribution system layouts, PLC engineers can help in reducing pump size, or possibly, eliminate the need altogether.
