A fire pump is not selected from a catalogue. Selection starts from a single point produced by the system's hydraulic calculation and is completed by six checks.
1. Set the duty point
The hydraulic calculation gives two numbers: the required flow and required pressure at the system entry point. Hose reel and hydrant flows operating at the same time are added. Available pressure on the suction side is deducted, giving the pressure the pump must generate.
2. Check three points on the curve
| Point | Check |
|---|---|
| Churn (shut-off) | Must not exceed the pressure rating of the system and its components |
| Rated flow | Must meet the required pressure |
| 150% flow | Must hold the minimum pressure the standard requires |
A duty point chosen very close to the right-hand end of the curve leaves no room for future demand growth; chosen very close to the left it raises the churn pressure. The middle region is preferred.
3. Choose the pump type
- Horizontal split case: high flow and easy maintenance; the most common industrial choice.
- End suction: a compact solution at small and medium capacities.
- Vertical turbine: where the water source sits below the pump axis (a well or open basin); it removes the suction lift problem.
- Vertical inline: where space is tight; maintenance access must be assessed.
4. Check NPSH
NPSHa (what the installation provides) must exceed NPSHr (what the pump requires), with a suitable safety margin between them. In calculating NPSHa, watch: tank water level (take the lowest), suction line friction loss, liquid vapour pressure and elevation difference. Without a margin, partial cavitation occurs: the pump runs but the impeller erodes and performance falls over time.
Positive suction is preferred. A water source above the pump axis removes both the NPSH problem and the priming problem. Standards impose additional conditions on suction lift arrangements.
5. Choose the driver
| Criterion | Electric | Diesel |
|---|---|---|
| Power independence | Depends on the mains or a generator | Independent (own fuel and batteries) |
| Maintenance burden | Low | High (weekly run, fuel, batteries) |
| Room requirements | Simple | Combustion air, exhaust, fuel tank, temperature |
| Capital cost | Lower | Higher |
| Typical use | Where the supply is reliable | Where the mains is unreliable, or at critical facilities |
Where redundancy is required, the common solution is one electric plus one diesel pump, removing both the single point of failure and the energy dependency.
6. Size the jockey pump
The jockey makes up small leaks and stops the main pump running unnecessarily. Two rules: its pressure setting sits above the main pump's, and its flow must not be large enough to delay the main pump. A jockey running continuously means the system leaks — and that is a corrosion warning.
7. Choose the controller
- A controller listed or approved for fire pump service.
- Automatic start, manual stop.
- Phase loss, phase reversal and undervoltage monitoring.
- Alarm outputs transmitted to a continuously monitored point.
- For diesel, two battery sets each capable of starting the engine.
- A running hour meter.
8. Design the pump room
- Separated by fire-resisting construction and accessible from outside.
- Floor drainage for test and gland water.
- Combustion air and exhaust routing for a diesel.
- Temperature control (against freezing and overheating).
- Maintenance access: room to withdraw the impeller and remove the motor.
- Test line and flow meter, with the discharge resolved.
- System schematic, valve plan and contact list displayed.
9. What is measured at acceptance?
"The pump runs" is not enough. Measure: churn pressure, pressure at rated flow, pressure at 150% flow, suction pressure, motor current and speed, and automatic start behaviour. The measured curve is compared against both the manufacturer's curve and the design requirement.
The five most common selection mistakes
- Hose reel and hydrant flow not added to the total demand.
- Churn pressure exceeding the system pressure rating.
- No NPSH margin left.
- An incorrect jockey pump setting.
- An undersized suction line.
Frequently Asked Questions
How is the pump duty point set?
Add the hose reel and hydrant flows that operate simultaneously to the flow and pressure from the hydraulic calculation, then deduct the available suction pressure to find the pressure the pump must generate.
Which three points on the curve are checked?
Churn pressure (must not exceed the system rating), pressure at rated flow, and the minimum pressure the standard requires at 150% flow.
Why is an NPSH margin needed?
Without a margin, partial cavitation occurs: the pump runs but the impeller erodes and performance falls over time. NPSHa must exceed NPSHr with a suitable margin.
Electric or diesel?
Where the mains supply is reliable, electric is enough. Where power independence is critical, diesel; where redundancy is required, the common solution is one electric plus one diesel.
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Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 20 (2025) · NFPA 25 · BS EN 12845:2015+A1:2019 · FM Global Data Sheets. This guide is a general road map; the binding text is the chosen standard itself and the approval of the authority having jurisdiction.