1. Define required flow
State the demand basis: simultaneous fixtures, a process requirement, irrigation zone demand, refill time or measured draw. Separate average use from peak duty. A storage tank can change the instantaneous pump flow even when daily demand stays the same.
2. Establish static head
Measure vertical elevation from the relevant source water level or pump reference to the delivery point. Do not use sloping pipe length as elevation. Check minimum and maximum source levels where they change.
3. Add delivery pressure
Convert the required gauge pressure at the delivery point to head. Record the gauge elevation and whether the target is a minimum dynamic pressure or merely a static reading.
4. Calculate friction and minor losses
Use actual internal diameter, duty flow, length and a justified roughness or C value. Add valves, fittings, treatment devices and control components using loss coefficients, equivalent length or manufacturer data—without counting the same loss twice.
5. Build TDH
Add elevation, delivery pressure head and flow-dependent losses, then subtract available inlet pressure head. Calculate more than one operating case if levels, demands or valve positions change.
6. Read the pump curve
Find where the system requirement meets the manufacturer curve. Check efficiency, power, allowable operating region and shutoff behavior. A candidate that barely touches one duty point may not handle variation.
7. Check suction conditions
Estimate NPSHA with absolute pressure, water level, suction losses and vapor pressure. Compare it with NPSHR plus the application-specific margin required by the manufacturer and applicable guidance.
8. Check power and control
Use duty-point efficiency—not a marketing peak—to estimate shaft and electrical input. Confirm the complete power curve, motor selection, control strategy and minimum flow.
Common mistakes
Guessing friction, using nominal rather than internal diameter, treating static pressure as dynamic pressure, choosing a pump by pipe connection size, and adding an arbitrary large safety factor can all move operation away from a reliable region.
Worked example
Required flow is 120 L/min. Static lift is 18 m, delivery pressure is 250 kPa, pipe and equipment losses total 8.5 m, and flooded inlet pressure contributes 35 kPa. TDH is approximately 48.4 m. The preliminary duty point is therefore 120 L/min at 48.4 m—not simply “a 120 L/min pump.”
Sources
- U.S. Department of Energy — Pump Systems
- Hydraulic Institute — Combined Pump & System Curves
- Hydraulic Institute — Pump FAQs