Churn · rated · 150% flow · pump curve reference
Common Fire Pump Sizes (Quick Select)
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Estimated Pump Curve
| Operating Point | Flow (GPM) | Net PSI | Discharge PSI | Est. BHP |
|---|
Listed fire pump sizes per NFPA 20. Rated pressure varies by manufacturer and impeller selection. The pressures below are typical ranges — always verify from the specific pump curve.
| Rated Flow (GPM) | Typical Rated Pressure (PSI) | Common Applications |
|---|
This calculator provides a quick reference for estimating fire pump performance at three key operating points: churn (shutoff), rated flow, and 150% of rated flow. Enter the pump's rated flow and pressure, and the tool estimates the pump curve shape based on NFPA 20 performance requirements.
If suction pressure is known, the calculator also shows estimated discharge pressures at each operating point (suction + net pump pressure).
This tool estimates a simplified pump curve using three points and a quadratic fit. Real pump curves are determined by manufacturer testing and depend on impeller size, speed, casing geometry, and other factors. Always use the certified pump curve from the manufacturer for design-level calculations, pump selection, and acceptance testing.
NFPA 20 establishes minimum performance criteria for listed fire pumps at three operating points.
Fire pumps are selected based on the system demand (flow and pressure) determined by hydraulic calculations. The pump must provide the required net pressure at the system demand flow rate, with suction pressure accounted for. Designers should evaluate the full pump curve — not just the rated point — to ensure adequate pressure across all expected operating conditions.
Key considerations beyond basic pump sizing include driver type (electric vs. diesel), controller requirements, test header sizing, suction supply adequacy (NPSH), jockey pump sizing, and alarm/supervisory signal integration. Fire pump installations are governed by NFPA 20 and must be reviewed by the authority having jurisdiction.
Churn (or shutoff) pressure is the pressure a fire pump produces at zero flow — when the discharge valve is closed. NFPA 20 limits churn to no more than 140% of the pump's rated pressure. High churn pressure is important for system relief valve sizing and component pressure ratings.
The 150% flow point ensures the pump maintains usable pressure even when system demand exceeds the rated capacity. This can occur when more sprinklers operate than the design area assumed, or during hose stream demand. NFPA 20 requires at least 65% of rated pressure at 150% of rated flow.
Net pump pressure is the pressure the pump adds to the water — the difference between discharge and suction pressure. Discharge pressure is the total pressure at the pump outlet: suction pressure plus net pump pressure. Hydraulic calculations use net pump pressure; field gauges read discharge and suction pressures directly.
A pump curve plots net pressure (vertical axis) against flow rate (horizontal axis). The curve slopes downward from left to right — pressure decreases as flow increases. The three key points are churn (zero flow, maximum pressure), rated (100% flow at rated pressure), and the 150% point (150% flow at reduced pressure).
This calculator estimates a simplified curve. Actual acceptance testing per NFPA 25 requires field-measured data at churn, rated, and 150% flow using calibrated gauges and a flow meter. Compare field results to the manufacturer's certified curve, not to this estimate.
BHP = (GPM × PSI) / (1714 × efficiency). Pump efficiency varies across the curve — it's highest near the rated point and drops at both churn and overload conditions. The 150% flow BHP is critical because NFPA 20 requires the driver (motor or engine) to be sized to handle this point without overloading.
NFPA 20 requires electric motors for fire pumps to have a 1.15 service factor, meaning the motor can deliver 115% of its nameplate HP continuously. The motor must be sized so that the BHP at 150% flow, multiplied by the 1.15 SF, does not exceed the next standard motor size. Diesel engines do not use this service factor — they are rated directly at the 150% flow BHP.
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