How to check a fire pump curve against demand and water supply
A quick screening check tells you whether a candidate pump plus your city supply can meet the sprinkler and hose demand, and whether the pump's curve has the usual shape limits. It needs a water flow test (static, residual and flow) and three points from the pump curve.
The method
- Supply at demand flow: P = Ps − (Ps − Pr) × (Q / Qt)1.85, where Ps is static pressure, Pr is residual pressure at test flow Qt.
- Net pressure the pump must add: demand pressure − supply at demand flow. This screening version ignores suction and discharge piping losses; add them for a real check.
- Pump net pressure at demand flow: fit a smooth curve through churn (0 flow), rated (100%) and 150% points and read it at the demand flow.
- Curve shape checks commonly applied to fire pumps: churn pressure not more than 140% of rated pressure, and at least 65% of rated pressure at 150% of rated flow.
Confirm these limits, suction pressure floors and maximum system pressures against the NFPA 20 edition your authority having jurisdiction adopted and the pump manufacturer's certified curve.
Free calculator
Worked example
Flow test 65 psi static, 50 psi residual at 1,200 gpm. Demand 700 gpm at 95 psi. Supply at 700 gpm = 65 − 15 × (700/1,200)1.85 = 59.5 psi, so the pump must add 35.5 psi. A 750 gpm, 50 psi pump with 60 psi churn and 36 psi at 150% gives about 51.4 psi at 700 gpm, a 15.9 psi margin before piping losses. Churn is 120% of rated (limit 140%) and 150% flow gives 72% of rated (minimum 65%), so both shape checks pass.
For suction and discharge piping losses, suction velocity, suction pressure at 150% flow and tank sizing, use the Fire Pump & Water Storage Tank Sizing Workbook.