Fire pump selection is of great importance to fully ensure the fire protection standard. The need for a fire pump should be determined early – ideally, a comprehensive project should be developed first. In today’s article, we will explain the process of determining whether a fire pump is necessary and how to select a fire pump that meets the required pressure and flow.
When is a Fire Pump Needed?
A fire pump is required to provide the flow and pressure demands to fire suppression systems when the water supply is insufficient. To determine whether a fire pump is needed, the fire suppression system requirements should be compared with the available water source. If the water source cannot meet the demand, a fire pump is required. Conversely, if the water source can meet the pressure and flow requirements, a fire pump is not needed.
Water can come from various sources, including public water mains, dedicated fire protection water systems, elevated tanks, etc. Each scenario requires a slightly different approach, but it is important to first determine whether the water source can meet the flow requirement.
When connecting to a fire main, hydrant flow tests can be used to analyze the source. Static and residual pressures, along with flow data, determine whether the supply system can meet the required water demand. To minimize errors from hydraulic calculation estimates, it is ideal to keep these test data as close to the fire suppression system riser as possible.
When determining the worst-case scenario, consider flow demands from all fire suppression systems. In this example, the worst-case flow demand is an automatic sprinkler system in an office building.
Sprinkler demands are calculated according to NFPA 13 based on factors such as design area, density, hose stream allowance, excess, roof slope / pre-action system.
Example: 5-story office building with light hazard
Sprinkler system: 0.1 gpm / ft² × 1500 ft² = 150 gpm
Sprinkler system + 30% allowance: 150 × 1.3 = 195 gpm
Hose: 100 gpm
Total: 295 gpm
The calculation shows a flow demand of 295 gpm. If the connected fire main cannot provide this flow, a water tank and pump should be provided to supply the sprinkler system. With known flow requirements, the next step is to calculate the pressure needed to operate the sprinkler system.
First, consider the pressure required to meet the system demand and for the most remote sprinkler; then add all various pressure losses back to the source. The minimum pressure is defined by NFPA 13, typically 7 psi.
Sprinkler system pressure can be estimated at ~20 psi. This is a good guide when supplying simple sprinkler systems. As sprinkler piping becomes more complex due to unusual floor layouts, obstacles, and uneven ceiling heights, the number increases accordingly.
Role of the Sprinkler Riser
Friction losses in the pipe from the sprinkler riser to the water source can be estimated using the Hazen-Williams formula defined in NFPA 13. Hazen-Williams calculates friction loss per foot; this value can be multiplied by the total pipe length to determine the lost pressure.
Height is an important factor for pressure loss, as overcoming gravity requires a significant amount of pressure: 0.433 psi per foot can be assumed for the required height. Important features such as backflow preventers and strainers should also be considered, as pressure is lost as water passes through these devices.
Fire Pump Selection
Fire pumps should be selected based on nominal flow and pressure capacities. In our example, the required flow is 295 gpm. Fire pumps should operate at 150% of their nominal flow. Therefore, there is no need to select a pump rated exactly for the flow demand; this would result in an oversized pump. For example, if the flow demand is 295 gpm, a 200 gpm nominal fire pump can technically provide this flow.
Many fire pump manufacturers provide selection tools on their websites where required flow and pressure can be entered, showing pumps that meet these requirements. NFPA sets limits for pump performance, ensuring pump curves are not too steep and that pressure drops gradually.
As pumps operate beyond their nominal flow, the pressure they can provide decreases. Some pumps have flat curves where pressure slowly decreases as flow increases; others lose pressure faster. It is important to consider where the flow demand falls on the pump curve. At the flow demand point, the pressure increase provided by the pump must be higher than the required pressure.
For final verification, place the required flow and pressure point on the manufacturer’s pump curve. This point must be below the pump curve for the pump to meet the system requirements. There are likely many pumps that meet performance requirements, but having pressure and flow requirements ensures the required flow and pressure point is satisfied.
How to Determine if a Fire Pump is Needed:
- Gather information about the fire water supply.
- Calculate the required flow.
- Calculate the required pressure.
- If the supply cannot meet the demand, provide a fire water tank and fire pump.
- If pressure cannot be met, obtain a fire pump.
- If the supply can meet the required pressure and flow, no fire water tank or pump is needed.
How to Select a Fire Pump:
- Gather information about the fire water supply.
- Calculate the required flow.
- Calculate the required pressure. Calculate the required pressure increase.
- Select a pump where the flow demand is 100–150% of nominal flow, preferably 115–135%.
- Select a pump with a performance curve that provides sufficient pressure increase at the flow demand.