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Pump Efficiency

Erhart Diagram Explained for Centrifugal Pumps

The Erhart diagram is a practical engineering aid used for estimating achievable pump efficiency during preliminary centrifugal pump calculations.

Erhart efficiency diagram

Erhart efficiency diagram

The smaller correction is usually applied to large pumps, while a larger correction is used for smaller pumps.

Typical use cases

  • early-stage pump sizing,
  • sanity checking expected efficiency,
  • preliminary hydraulic calculations,
  • educational pump design work,
  • estimating hydraulic efficiency before vendor curves are available.

Limitations

  • It is not a guaranteed performance value.
  • It should not be used as a contractual efficiency.
  • It does not replace pump test data.
  • It should be verified against real pump curves whenever possible.

Use the calculator

PumpToolkit includes a beta estimator based on this methodology.

Open Erhart Efficiency Estimator

Small vs medium vs large pump selection

The Erhart diagram is strongly connected to pump specific speed and hydraulic sizing philosophy. For the same duty point, a designer can often choose:

  • Small high-speed pump – higher rpm, smaller impeller diameter, compact construction, usually higher specific speed.
  • Medium balanced design – compromise between size, efficiency, NPSHr and mechanical robustness.
  • Large low-speed pump – bigger impeller diameter, lower rpm, often lower vibration and lower NPSHr.

Typical engineering tendencies

Design tendency Advantages Disadvantages
Small / high-speed pump Compact footprint, lower CAPEX, smaller motor Higher NPSHr, potentially lower lifetime, higher vibration/noise
Medium balanced pump Usually best compromise for industrial service No extreme optimization in any direction
Large / low-speed pump Lower NPSHr, often smoother operation, good reliability Larger footprint, heavier, more expensive construction

In practice, pump manufacturers usually try to stay near the best-efficiency region of the Erhart diagram while also balancing:

  • NPSH margin
  • Mechanical reliability
  • Initial cost
  • Maintenance accessibility
  • Motor availability
  • Project standards and customer preferences

Very high specific speed designs are not automatically “better”. A slightly larger and slower pump is often preferred in demanding refinery, chemical or continuous-process applications because reliability may outweigh pure efficiency optimization.