11.1 Centrifugal Pumps, Pump Curves, and Pump Affinity Laws

Pumps, Curves, Affinity Laws, and NPSH

The operating point

A centrifugal pump operates where its pump curve intersects the system-resistance curve. The manufacturer curve for the installed model, impeller diameter, speed, and fluid provides flow versus head, efficiency, brake horsepower, and net positive suction head required (NPSHR). The field system curve changes with valve position, fouling, control-valve state, and piping configuration.

Measure suction and discharge pressure at appropriate locations and correct for gauge elevation, velocity-head differences when material, and fluid specific gravity. For water with gauges at the same elevation, pump head in feet is approximately differential psi × 2.31. Compare the calculated head and independently measured flow with the correct manufacturer curve.

Shutoff head

At zero flow, differential pressure can help identify speed or impeller trim. If suction is 35 psig and discharge is 75 psig at the same elevation, the difference is 40 psi, or 92.4 feet of water. Plot that value at zero flow on the correct curve.

A shutoff test is not routine valve manipulation. It must be authorized, permitted by the pump and manufacturer procedure, time-limited, and performed with required minimum-flow and equipment protections. Monitor pressure, sound, vibration, and temperature, and reopen the valve promptly. Never close a valve if doing so would damage a positive-displacement pump or violate the system procedure.

Speed affinity laws

For the same pump, impeller, fluid, and geometrically similar operating conditions:

• flow varies with speed;
• head varies with speed squared; and
• power varies with speed cubed.

If 400 GPM, 60 feet, and 7.5 BHP occur at 1,750 RPM, a 10% speed increase predicts 440 GPM, 72.6 feet, and about 10.0 BHP. The calculation is a prediction; the actual operating point moves along the system curve and must be measured. Check motor, drive, pressure, NPSH, and equipment limits before any authorized increase.

Affinity approximations for impeller diameter changes are less exact than speed laws because trim changes geometry and efficiency. Use the pump manufacturer's selection data and allowable trim range for final decisions. The CT may calculate a predicted trim but does not machine an impeller or approve a redesign.

Series and parallel pumps

Pumps in series add head at approximately the same flow. Pumps in parallel add flow at approximately the same head, but the actual combined point depends on both pump curves, the system curve, check valves, and control sequence. Verify rotation and confirm that each operating pump shares load as intended. A nonrunning parallel pump with a failed check valve can become an unintended bypass.

Cavitation and NPSH

Cavitation occurs when local absolute pressure falls low enough for vapor bubbles to form and then collapse in higher-pressure regions. Common signs include gravel-like sound, vibration, unstable pressure, reduced performance, and damage. The curve's NPSHR is a pump requirement at a stated flow; NPSHA is the system's available absolute suction head after elevation, vapor pressure, and suction losses. The required margin comes from the design or manufacturer—not a universal two-to-five-foot rule.

When cavitation is suspected, stop or stabilize the test under the authorized procedure and escalate. Investigate suction restriction, valve state, strainer condition, fluid temperature, system static pressure, pump speed, liquid level in an open system, and the accuracy and elevation of the readings. Never throttle a suction isolation valve as a balancing remedy. Do not automatically raise fill pressure or clean a strainer without identifying the condition and obtaining authorization.

Diagnostic sequence

Confirm system mode, valve lineup, fluid and temperature, instrument zero, gauge elevation, pump rotation, speed, and curve identity. Then compare flow, head, power or current, vibration, and NPSH evidence. One reading rarely proves a single cause. Record initial and final conditions, all authorized adjustments, and any limitation that prevents a valid curve comparison.

Curve-reading discipline

Confirm that axes and units match the test: feet versus psi, GPM versus another flow unit, and the correct speed and impeller curve. Interpolate only within published data. A point to the right of the curve, outside the preferred region, or above the horsepower line is not validated by affinity arithmetic alone. Manufacturer minimum-flow, maximum-speed, and operating-region limits still govern.

Power and current are valuable cross-checks. A predicted cubic power increase can expose an unsafe proposed speed change before it is made. After an authorized change, measure the actual point because system resistance, control valves, and pump efficiency determine the real outcome.