Worked problem

Fire Pump Duty: Flow, Pressure and Elevation

Build a simplified pressure budget for a 750 gpm demand with elevation, friction and suction pressure, and check it against a supplied fire pump curve.

Fire Protection · Last reviewed October 17, 2026

Given

A building system needs 750 gpm at 85 psi at the top of the riser. The riser top is 60 ft above the pump. At 750 gpm the friction loss in the piping is 14 psi and the backflow preventer and valves lose 7 psi. The water supply gives 40 psi at the pump suction at 750 gpm. All numbers are exercise values. The supplied pump curve (centrifugal fire pump rated 750 gpm at 100 psi) gives 118 psi at no flow (churn), 100 psi at 750 gpm and 70 psi at 1,125 gpm (150% of rated flow). The maximum static pressure at the suction is 55 psi.

Find

The net pressure the pump must add, whether the pump meets it, and the highest pressure the system sees.

Solution

Elevation: 60 ft × 0.433 psi/ft = 25.98 psi.
Pressure needed at the pump discharge = 85 + 25.98 + 14 + 7 = 131.98 psi.
The suction already supplies 40 psi, so the pump must add 131.98 − 40 = 92.0 psi at 750 gpm.
The pump curve gives 100 psi at 750 gpm. Margin = 100 − 92.0 = 8.0 psi (8.7%). The pump meets the duty.
Maximum pressure at no flow = churn + maximum suction pressure = 118 + 55 = 173 psi. Check that every component in the discharge piping is rated for it.

Answer

net pressure required: 92.0 psi at 750 gpm  |  pump supplies 100 psi  |  margin 8.0 psi  |  maximum system pressure 173 psi

Item	Pressure (psi)
Required at top of riser	85.0
Elevation, 60 ft	26.0
Friction in piping	14.0
Backflow preventer and valves	7.0
Total at the pump discharge	132.0
Less suction pressure	−40.0
Net pressure the pump must add	92.0

Where the duty point sits

The pump curve gives 100 psi at 750 gpm and the system needs 92 psi there, so 750 gpm can be delivered with 8 psi to spare. The operating point is where the pump curve meets the system demand curve, and the spare pressure is what lets the system draw more than 750 gpm if the demand grows. The pump should also still deliver at least 65% of its rated pressure at 150% of rated flow; the supplied curve gives 70 psi, which is 70% of the rated 100 psi.

Common mistakes

Adding the suction pressure instead of subtracting it. A positive suction pressure helps the pump.
Forgetting that fire pumps are checked at churn, rated flow and 150% flow, not at the rated flow only.
Treating the pump net pressure and the discharge pressure as the same. The net pressure is the discharge pressure minus the suction pressure.
Using the maximum suction pressure for the demand check, or the minimum suction pressure for the pressure rating check.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

Questions

Why check the maximum pressure?

At no flow the pump produces its churn pressure on top of the suction pressure. The pipes and fittings must withstand that sum.

Is 8 psi a good margin?

The margin here is an exercise result. Real projects follow the applicable standard and the authority having jurisdiction, and typically leave margin for future changes and pump wear.

What would a larger elevation do?

Each extra foot of height adds 0.433 psi. 100 ft instead of 60 ft would add 17.3 psi and the pump would no longer meet the duty.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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