11.3 Pump Performance Evaluation: Net/Gross Curves and the Complete Acceptance Criteria

Evaluate pump condition and delivered system demand

NFPA 25 (2023), 8.3.7, evaluates both the pump assembly and its connected supply. Net performance helps assess what the pump adds. Gross performance establishes what is available to meet the connected system demand. A pump can retain its net curve while a deteriorated supply makes the overall installation inadequate. Conversely, a high municipal pressure can hide a weak pump if only the discharge gauge is considered.

Qualified personnel interpret the results. Obtain the original manufacturer's curve, original unadjusted field curve, nameplate data, prior tests, and owner-provided system demand. Use comparable flows and apply speed and velocity-pressure adjustments where applicable. Preserve the unadjusted measurements as well as any corrected values so the conclusion can be checked independently.

The four acceptance conditions

Under 8.3.7.2.3, all four conditions must be satisfied: the pump meets the flow and pressure requirements of the most demanding supplied systems from owner-provided design information; it supplies 100% of rated flow; net pressure at each flow point is at least 95% of one of the permitted reference curves; and the water-supply requirements of 8.1.5 are met. The permitted curves are the original manufacturer's curve, original unadjusted field curve, or a curve generated from the fire pump nameplate.

The wording “one of” matters. Do not declare an otherwise acceptable test failed solely because it is below 95% of a higher original field curve if another permitted curve satisfies the requirement. Do not use that flexibility to ignore the other three conditions or fabricate an undocumented curve. A missing system-demand record also cannot be replaced by assuming that the largest hose outlet is the only demand.

Net pressure calculation

For a simplified horizontal-pump example with gauges at equivalent elevations and negligible velocity-head difference, net pressure equals discharge pressure minus suction pressure. A discharge of 138 psi and suction of 36 psi give 102 psi net. Those assumptions must be stated. Elevation and velocity differences can require corrections, and a vertical turbine arrangement requires appropriate reference to the pumping water level rather than blindly subtracting a fictitious suction gauge.

For a negative suction gauge reading, subtracting a negative number adds its magnitude: 90 − (−4) = 94 psi in the simplified example. That arithmetic does not establish that negative suction is permissible at the installation. Section 8.1.5 addresses supply requirements and qualifying existing arrangements. Observe the actual allowable condition and do not force flow into unsafe suction merely to obtain a plotted point.

Apply the 95% comparison

Assume an allowed reference curve shows 100 psi net at a particular measured flow. The 95% threshold is 0.95 × 100 = 95 psi. A valid comparable 96 psi result meets that element; 94 psi does not meet it against that curve. The percentage reduction from a reference pressure is (reference − measured) / reference × 100. Thus, 100 to 94 is a 6% reduction, not 94% degradation.

Compare at the same flow. If one result is at 1,000 gpm and another at 1,200 gpm, directly dividing the pressures can misrepresent condition because the pump curve slopes. Use the appropriate curve interpretation. Do not simply compare each year's maximum discharge pressure or choose the most favorable isolated point while ignoring the others.

Speed corrections explain, not excuse

For an applicable centrifugal-pump comparison, flow scales with speed and head with speed squared. To normalize a measured point to a reference speed, the approximate factors are reference speed/measured speed for flow and its square for head. For 1,770 rpm reference and 1,750 rpm measured, the flow factor is about 1.0114 and the head factor about 1.0230. Both coordinates move; pressure correction alone can put a point at the wrong flow.

Apply these relationships only within their valid use and the standard's procedure. NFPA 25 expressly prohibits increasing engine speed beyond rated pump speed as a method of meeting rated performance. A technician must correct the actual cause of unacceptable output, not overspeed the equipment or erase unfavorable raw readings.

Action on unacceptable results

Notify the owner in writing and investigate the cause. Failure to provide maximum system demand is an impairment. Degraded performance that still provides maximum demand is a noncritical deficiency under the stated provision; excessive vibration or worn/loose components have their own deficiency implications. Do not classify every 5% curve difference as the same critical deficiency regardless of demand.

Investigate instrument accuracy, supply condition, valve positions, speed, mechanical wear, and the applicable driver observations. Complete correction, retest as required, and notify the owner of the work completed. Keep the raw data, demand information, selected reference curve, adjustment method, and written conclusion together. A graph with no units or unidentified curve cannot support a reliable pass decision.

Source checked October 19, 2026: NFPA 25 (2023), 8.1.5, 8.3.7, and 8.4.