Worked example: an oversized pump throttled back, and what it costs
A pump with an impeller that is too big, held in check by a valve, uses about twice the power of the right impeller. Work through the numbers step by step.
The situation
The duty is 68 m3/h at 47 m head. The pump has a large impeller (call it A) that makes 76 m head at this flow, so the discharge valve is throttled to hold 68 m3/h. A smaller impeller (call it E) would meet the duty with no throttling. The figures below are an illustrative calculation, not a field measurement.
Power with each impeller
| Item | Right impeller (E) | Large impeller, throttled (A) |
|---|---|---|
| Flow | 68 m3/h | 68 m3/h |
| Head the pump makes | 47 m | 76 m |
| Pump efficiency at that point | 60% | 50% |
| Hydraulic power | 8.7 kW | 14.1 kW |
| Shaft power | 14.5 kW | 28.2 kW |
| Electrical input at 90% motor efficiency | 16.1 kW | 31.3 kW |
The cost of throttling
The difference is about 15 kW, running all the time. Over 8760 hours that is about 133,000 kWh a year. At an assumed tariff of Rs 4 per kWh, the extra cost is about Rs 5.3 lakh a year. In this example the yearly extra electricity cost is larger than the price of a new, correctly sized pump.
Why it is so costly
- The valve throws away 29 m of head, which is nearly 40% of what the pump makes.
- The large impeller runs at lower efficiency at this flow.
- The losses show up as heat, noise and vibration, which also shorten seal and bearing life.
How to use this on your own pump
- Find the duty flow and the head that is really needed.
- Read the head the pump makes at that flow from its curve, and compare it with the head the system needs. The difference is wasted at the valve.
- Estimate the power for both cases, using hydraulic power, pump efficiency and motor efficiency.
- Multiply the power difference by the yearly running hours and the tariff.
- Compare the result with the cost of a smaller impeller, a trim or a drive.