Pumps and energy
Hydraulic, shaft and electrical power of a pump
Quick answer
Three power numbers explain every pump energy calculation: the power in the liquid, the power on the shaft and the power drawn from the supply. Efficiency links them.
The power chain
Electrical power goes into the motor. The motor turns the shaft, and the shaft drives the impeller. The impeller gives energy to the liquid. Each step loses some energy, so each number is a little larger than the one after it.
| Power | What it is | How to get it |
|---|---|---|
| Hydraulic power (kW) | The useful power in the liquid leaving the pump | Flow in m3/s x total head in m x density in kg/m3 x 9.81, divided by 1000 |
| Shaft power (kW) | The power the motor delivers to the pump shaft | Hydraulic power divided by pump efficiency |
| Electrical input power (kW) | The power drawn from the supply | Shaft power divided by motor efficiency |
A worked example with water
A pump delivers 68 m3/h against a total head of 47 m, with a pump efficiency of 60% and a motor efficiency of 90%.
| Step | Calculation | Result |
|---|---|---|
| Flow in m3/s | 68 divided by 3600 | 0.0189 m3/s |
| Hydraulic power | 0.0189 x 47 x 1000 x 9.81 divided by 1000 | 8.7 kW |
| Shaft power | 8.7 divided by 0.60 | 14.5 kW |
| Electrical input | 14.5 divided by 0.90 | 16.1 kW |
Where the energy goes
- Only 8.7 kW of the 16.1 kW drawn becomes useful work in the liquid, which is about 54% overall.
- The rest is lost as heat in the motor, as friction and recirculation inside the pump, and as bearing and seal losses.
Checking a pump in the field
- Read flow and head from instruments, or from the pump curve at the measured pressures.
- Read the motor input power from the drive or a meter.
- Compare the hydraulic power with the input. If the overall efficiency is far below the curve value, look for wear, recirculation, partly blocked suction, or a wrong operating point.
Takeaway. Hydraulic power is the useful part. Everything above it is the cost of losses, so increase the efficiency or reduce the head and flow you do not need.
Put it to work in the tool