Fire Pump Rated / Churn / Overload Curve Check (NFPA 20)
Whether a stationary fire pump sits inside the NFPA 20 three-point envelope.
Example
You enter
- Pump rated flow (gpm) 500
- Pump rated net pressure (psi) 100
- Measured churn / shutoff pressure (psi, 0 = not entered) 128
- Measured pressure at 150% flow (psi, 0 = not entered) 72
You get
- Churn limit (140% of rated) 140.0 psi
- Overload flow (GPM) 750
- Overload min (psi) 65
- Churn check PASS (margin 12.0% of rated)
- Overload check PASS (margin 7.0% of rated)
Details, formula, and sources
The churn (no-flow) ceiling at 140% of rated pressure, and the overload floor of 65% of rated pressure at 150% of rated flow, with a pass/fail and the margin at each end of the curve. A 500 gpm at 100 psi pump must hold under 140 psi at churn and over 65 psi at 750 gpm. A flat, high-shutoff curve fails churn and drives a pressure-relief-valve requirement; a weak curve fails the overload reserve the sprinkler demand needs. A design and acceptance-check aid, not a stamped fire-pump submittal; a qualified fire-protection engineer and the AHJ govern.
churn_limit = 1.40 x rated_psi; overload_flow = 1.50 x rated_gpm; overload_min = 0.65 x rated_psi; churn_ok = churn <= churn_limit; overload_ok = measured >= overload_min; margins as % of rated pressure.
NFPA 20 (Standard for the Installation of Stationary Pumps for Fire Protection), 2022, by name; the 140% churn ceiling and the 65%-of-rated-at-150%-flow overload point are the listed centrifugal fire pump curve limits.
NFPA 20 is viewable free of charge through NFPA's online free-access reader; the 1.40 / 1.50 / 0.65 envelope arithmetic is public.
Estimate. AHJ and licensed professional govern.
Field names used by the API: rated_gpm, rated_psi, churn_psi, overload_psi, churn_limit_psi, overload_flow_gpm, overload_min_psi, churn_margin_pct, overload_margin_pct
- Curve envelope churn <= 140% of rated pressure and >= 65% of rated pressure at 150% of rated flow, the two ends of the listed-pump acceptance curveNFPA 20 (2022)
- Field test points the churn and 150%-flow pressures come from the field acceptance test with a calibrated flow-measuring device; leave 0 to skip either checkNFPA 20 (2022)
- Relief valve separate a churn pressure over the system component rating requires a listed pressure-relief valve; NFPA 20 does not by itself waive thatNFPA 20 (2022)