Fire Pump Field Acceptance: 150% Flow Shortfall
A new 750 gpm, 100 psi rated electric fire pump is being field acceptance tested for a 12-story sprinklered building. The contractor's test report shows: churn 135 psi; 100% flow (750 gpm) 100 psi; 150% flow (1,125 gpm) 62 psi; suction pressure 20 psi at all points. The pump is driven by a 150 hp motor through a controller with a variable-speed pressure-maintaining feature set to 110 psi. The contractor says the 150% point is 'close enough' because the suction pressure was low and the pump did not cavitate. The building has a 25,000-gallon suction tank and a city suction supply. The acceptance test was witnessed by the owner's representative but not by the AHJ.
1. Evaluate the pump's performance against NFPA 20 acceptance criteria.
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Compare the field curve to the rated point: 750 gpm at 100 psi. Churn is acceptable if it does not exceed 140% of rated pressure, or 140 psi; 135 psi passes. At 100% flow, the pressure should not be less than 95% of rated, or 95 psi; 100 psi passes. At 150% flow, the pressure should not be less than 65% of rated, or 65 psi; 62 psi fails. The suction pressure must be handled correctly: fire pump pressure is net pressure, so subtract suction from discharge. If 62 psi is already net, it is still below 65 psi; if 62 psi is the discharge gauge reading, net pressure is only 42 psi and the failure is worse. The variable-speed feature does not excuse the 150% point, which is a fixed-speed acceptance criterion. The pump should not be accepted on the 'close enough' argument.
2. What are the likely causes and what should be done before acceptance?
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Likely causes include a trimmed impeller, low pump speed, a wrong pump curve, a restricted suction line, vortexing or air entrainment in the 25,000-gallon tank, a misreading flow meter, or a test valve not fully open. The contractor should verify the pump nameplate and curve, measure speed, check suction conditions and tank level, calibrate the flow meter, and repeat the test at the correct suction pressure. If the pump truly cannot make 65 psi at 150% flow, the impeller or pump may need replacement or the design may need a different pump. A suction supply problem must be corrected before the pump is blamed. Re-test after corrections and compare the full curve, not just one point.
3. What documentation and witnessing are required for NYC acceptance?
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NYC acceptance requires a field test witnessed by the AHJ, DOB or FDNY as applicable, using calibrated instruments and a written report. The report should include the manufacturer's certified curve, nameplate data, suction and discharge pressures, flow readings, speed, voltage, and the controller settings. The contractor, owner's representative, and witness should sign it. The variable-speed pressure-maintaining setting should be recorded and tested for its intended function, but it does not replace the fixed-speed curve test. Before sign-off, correct the 150% deficiency, repeat the test, and provide the as-built settings and maintenance instructions. The owner should not accept the pump on an unwitnessed report.