Selecting a Fire Pump From Its Curve, Not Its Nameplate
A fire pump is being added to an existing Ordinary Hazard Group 2 warehouse that is currently fed directly from the city main. At the calculated demand of 1,050 gpm, the required pressure at the pump discharge flange is 72 psi. The city supply measured at the pump suction is 35 psi static and 20 psi residual at 1,000 gpm; at 1,050 gpm the suction pressure is estimated to fall to 19 psi. No suction tank is proposed. The contractor submits a fire pump rated 1,000 gpm at 70 psi; the factory curve shows a shutoff pressure of 110 psi, 70 psi at 1,000 gpm, and 42 psi at 1,500 gpm. The building official has already told the contractor in writing that the pump must be judged at the actual demand flow and at the standard's off-design points, not at the rated point alone.
1. Determine the pump head required at 1,050 gpm and explain why the static suction pressure cannot be used.
Read the worked answer
Required pump head = required discharge pressure − suction pressure at the same flow.
- Required discharge at 1,050 gpm: 72 psi
- Suction at 1,050 gpm: 19 psi (not 35 psi static)
- Required pump head: 72 − 19 = 53 psi at 1,050 gpm
Using the 35 psi static reading would understate the required head by 16 psi and would make an inadequate pump look generous. Suction pressure falls as flow rises; the pump and the city main share the same demand, and the pump must make up the difference at the worst case, not at no-flow.
Also state the consequence: with a 19 psi suction and no tank, the pump's ability to deliver at 150% of rated flow depends entirely on the city main holding pressure at that higher flow. Ask for suction pressure at 150% of rated flow, not just at 1,050 gpm, before accepting a city-only suction design.
2. Test the proposed 1,000 gpm at 70 psi pump against the required duty and against the standard's off-design requirements, using the curve given. Does it comply?
Read the worked answer
Check three points.
- Duty point (1,050 gpm). Interpolating between 70 psi at 1,000 gpm and 42 psi at 1,500 gpm: 70 − (50 ÷ 500) × 28 ≈ 67 psi. Required pump head is 53 psi, so the duty point is satisfied with about 14 psi to spare.
- 150% of rated flow (1,500 gpm). The standard requires at least 65% of rated head: 0.65 × 70 = 45.5 psi. The curve gives 42 psi → fails.
- Shutoff (churn). The standard limits shutoff pressure to 140% of rated head: 1.40 × 70 = 98 psi. The curve shows 110 psi → fails.
Conclusion: reject the pump. It meets the calculated demand, which is exactly why the rated point alone is a trap - a pump that satisfies the hydraulic calc can still fail the acceptance criteria, and at churn it would push 110 psi into a system that may be designed and tested around a lower working pressure. Return it for a unit whose curve satisfies all three points, or a different rating entirely.
3. The contractor offers a 1,250 gpm pump rated at 60 psi, with shutoff 82 psi and 150% flow at 41 psi. Is this a better choice? What else must be verified before you accept it?
Read the worked answer
It passes the two off-design checks, but with thin margins that you must document.
- 65% of rated head: 0.65 × 60 = 39 psi at 1,875 gpm; curve gives 41 psi ✓
- 140% shutoff limit: 1.40 × 60 = 84 psi; curve shows 82 psi ✓
- At 1,050 gpm the curve delivers roughly 57–58 psi against the required 53 psi - only about 4 to 5 psi of margin
Still to verify: (1) the manufacturer's certified curve, not a catalog approximation, since the margin at 1,050 gpm is small; (2) suction pressure at 1,050 gpm and at 1,875 gpm - a city-only suction that holds 19 psi at 1,050 may not hold up at 1,875, and the pump must not cavitate during the 150% test; (3) that churn pressure plus city static does not exceed the rated working pressure of system components (often 175 psi) and that the hydrostatic test pressure is based on the real maximum; (4) driver sizing, controller rating, relief valve setting, and sensing line locations; (5) that the pump and its test header are shown on the working plans with the FDC and bypass arrangement.
A pump that passes on paper but has 4 psi of margin is a pump that fails the first time the city main has a bad day.