Fire Pump Acceptance Test with Conflicting Gauge Readings
A new electric fire pump is rated 2,839 L/min (750 gpm) at 620 kPa (90 psi). The suction comes from a 200 mm city main through a 150 mm backflow preventer. At churn, the discharge gauge reads 690 kPa and the suction gauge 210 kPa. At 150% of rated flow, the discharge gauge reads 480 kPa and the suction gauge 120 kPa. The contractor's acceptance report lists churn at 111% of rated and 150% flow at 77% of rated, and recommends acceptance. The system's calculated demand is 2,650 L/min at 540 kPa net. The AHJ notes that the gauges are at different elevations and asks for a proper evaluation before the pump is accepted.
1. Evaluate the pump acceptance data against NFPA 20 criteria. Does the pump pass?
Read the worked answer
Do not accept the contractor's percentages. Pump performance is based on net pressure: net = discharge pressure − suction pressure. Rated net = 620 kPa.
Churn: 690 − 210 = 480 kPa net = 480/620 = 77% of rated. That is not 111%; the contractor divided discharge pressure by rated pressure and ignored suction. NFPA 20 limits shutoff pressure to not more than 140% of rated, so 77% is not a high-limit failure, but it is below rated pressure and inconsistent with a normal centrifugal pump curve. It suggests a gauge/calibration error, a wrong pump curve, or a suction reading taken at a different elevation.
At 150% flow: 480 − 120 = 360 kPa net = 360/620 = 58% of rated. NFPA 20 requires not less than 65% of rated net pressure at 150% of rated flow. 65% of 620 = 403 kPa. The pump is 43 kPa short, so it fails the acceptance criterion on the data as presented.
The system demand is 2,650 L/min at 540 kPa net. That is below rated flow, so the pump may still meet the system demand, but acceptance testing is not based only on the demand point. A pump that cannot meet the 150% criterion may be unstable or unable to supply hose streams.
2. What steps should be taken before the pump is rejected or accepted, and what could explain the low net pressure?
Read the worked answer
Before rejecting the pump, verify the test itself: calibrate gauges, confirm gauge elevations and correct for elevation (about 9.81 kPa per metre), confirm suction pressure is measured at the pump flange, confirm speed is rated, and confirm flow is measured accurately. Recalculate net pressure after corrections.
If the corrected data still fail, check impeller trim, the published pump curve, suction piping, and driver speed. Possible causes include an impeller trimmed too small, the wrong pump selected, suction starvation or air entrainment, a closed valve, or an inaccurate flow meter. Also verify that the relief valve is not bypassing and that the sensing lines are not plugged.
If the pump truly fails, options include replacing the impeller or pump, selecting a different pump, or providing an engineering analysis accepted by the AHJ. Do not simply adjust the relief valve or reduce the system demand. Re-run the full acceptance test at churn, 100%, and 150% flow and record corrected net pressures.