Pump Sizing 101: A Practical Engineer's Guide

Getting a pump wrong is expensive. Oversized pumps run off the curve, cavitate, and eat seals; undersized pumps starve the process and trip on overload. This guide walks through the numbers an engineer actually uses on the job.

1. Start with the duty point

Every pump sizing problem is really two numbers: flow and total dynamic head (TDH). Everything else — power, motor, NPSH — falls out of those.

• Flow (Q) is dictated by the process, not the pump. If the plant needs 100 GPM, that's Q. Add a design margin (10–20%) only where the process genuinely swings.
• TDH is the total energy the pump must add per pound of fluid, expressed in feet of the fluid being pumped.

TDH = (P_discharge − P_suction) × 2.31 / SG   +   static head   +   friction losses

The 2.31 converts psi to feet of water; divide by specific gravity for other fluids.

2. Hydraulic, brake, and motor power

Once you have Q and TDH:

• Hydraulic HP = Q × H × SG / 3960 — the theoretical minimum.
• Brake HP = Hydraulic HP ÷ pump efficiency. This is the shaft power the driver must deliver.
• Motor kW = Brake HP × 0.7457 ÷ motor efficiency.

Realistic pump efficiencies:

3. Don't forget NPSH

More pumps die from cavitation than from wear. Compute NPSHa — the head available at the suction — and confirm it exceeds NPSHr from the manufacturer's curve by at least 3 ft or 20%, whichever is larger.

Common NPSH killers:

• Hot fluid (vapor pressure rises fast).
• Long suction runs with elbows.
• Suction lifts on volatile fluids.

4. Motor sizing

Always round motor selection up to the next standard NEMA/IEC frame — 7.5, 10, 15, 20 HP. Then check that the motor runs between 75% and 100% of full load; below 50% and power factor collapses.

5. Common mistakes

1. Using drum pressure as discharge pressure. You need drum + safety-valve margin + line friction to the point of use.
2. Ignoring fluid temperature. Hot water is lighter and higher-vapor-pressure — both hurt sizing.
3. Trusting manufacturer efficiency at duty point. Curves peak at BEP; if the duty is far from BEP, efficiency drops fast.
4. Forgetting minimum flow. Every centrifugal pump has a minimum stable flow — usually 25–30% of BEP.

Ready to try it?

Run the numbers with the Pump Sizing Calculator or, for boiler service specifically, the Boiler Feed Pump Calculator. If you're unsure about suction geometry, start with the NPSH Available Calculator.