Fire Pump Capacity for a New Standpipe Wing
A 750 gpm, 100 psi rated electric fire pump serves an existing wet system. Recent test data: churn 122 psi, 100% 100 psi, 150% 68 psi. A new four-story wing adds a standpipe; the hydraulic calculation shows a combined sprinkler/standpipe demand of 900 gpm at 95 psi at the pump discharge. The city main gives 45 psi static and 20 psi residual at 1,000 gpm at the pump suction flange, and the suction piping loses 3 psi at 900 gpm. The owner proposes to keep the pump and add a pressure-reducing valve at the new standpipe to "make the numbers work."
1. Evaluate the pump curve against the new demand and determine whether the existing pump is adequate.
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The pump test points are churn 122 psi, rated 750 gpm at 100 psi, and 150% (1,125 gpm) at 68 psi. Interpolating between rated and 150% for 900 gpm:
P ≈ 100 − [(900 − 750)/(1,125 − 750)] × (100 − 68) P ≈ 100 − (150/375) × 32 ≈ 100 − 12.8 = 87.2 psi
The demand is 95 psi at 900 gpm, so the pump is short by about 8 psi. The pump is not adequate for the calculated demand. The test points themselves are acceptable under NFPA 20 tolerances: churn is 122% of rated (not over 140%), and the 150% point is 68% of rated (not below 65%). That means the pump is healthy but undersized for the new system, not that the test failed.
Interpolation is only an estimate; the actual curve may be flatter or steeper. A field flow test at the new demand is needed before a final decision.
2. Check the suction condition at the new demand and at 150% flow. What concerns arise?
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Using a straight line between 45 psi static at 0 gpm and 20 psi residual at 1,000 gpm:
- At 900 gpm, city residual ≈ 45 − (900/1,000) × 25 = 22.5 psi. Subtract 3 psi suction friction: about 19.5 psi at the pump suction flange.
- At 1,125 gpm (150%), city residual ≈ 45 − 28.1 = 16.9 psi. Suction friction will be higher than 3 psi, so the flange pressure may fall to roughly 12 psi or lower.
The concern is not only whether the pump can physically operate; it is whether the suction pressure remains above the minimum allowed by NFPA 20 and the AHJ, with margin for a degraded city main. A pump that is already short at 900 gpm will be driven further out on its curve during a real fire, and low suction can cause cavitation and loss of capacity. The water supply and suction piping must be analyzed with the pump, not separately.
3. Evaluate the proposed pressure-reducing valve and recommend alternatives.
Read the worked answer
A pressure-reducing valve lowers pressure downstream; it cannot increase the pump discharge pressure or the pump's capacity. Adding one at the new standpipe would add loss and would not close an 8 psi deficit. It might also create a false sense of compliance if the valve is set to a lower outlet pressure.
The real options are:
- Replace or upsize the pump so the combined demand is met on the actual curve.
- Reduce the demand by revising the standpipe/sprinkler design, if the code permits.
- Improve the water supply or add storage and a booster.
- Re-evaluate the hydraulic calculation for errors or unnecessary margins.
If a PRV is still needed to protect the standpipe outlets from overpressure, it should be selected for the actual flows and pressures after the pump issue is resolved. Do not use it as a substitute for adequate supply.