The measurement of the power coming out of the pump divided by power going into the pump, or the ratio of water-power to shaft power. The shaft power is the actual power being delivered by the motor or any other driver being used. The power is measured on a pony brake or dynamometer and is not to be confused with the power going into the motor or driver which will be subject to friction losses.
Here are the formulas for the water power:
In USCS units the power in water horsepower is
In SI units the power in water kilowatts is
Pw = 9.8 x Q x H x specific gravity
- Q = Capacity in gpm or liters/second.
- H = Total head in feet or meters
- Specific gravity for water at 39°F (4°C) = 1.0
Some pump-driver units are constructed in such a way that it is impossible to get the actual power input to the pump. Canned and magnetic drive designs are a good example. In such a case only an overall efficiency can be calculated. If the driver is an electric motor it is called the “wire to liquid efficiency” because we use the power going into the motor instead of the power coming out.
If you wanted a high efficient pump you would run with a close clearance between the impeller outside diameter and the volute cut water to reduce the amount of internal recirculation, but if the clearance is too small you can experience the “vane passing syndrome” type of cavitation.
Open impeller to volute clearance and wear ring clearances in closed impeller designs have a major impact on pump efficiency. Here are some other factors that reduce efficiency:
- Surface roughness.
- Mechanical losses from packing, bearings, lip seals, mechanical seals.
- High suction specific speed impellers. Especially those over 11,000.
- Trimming the impeller causes a slippage between the impeller tip and the volute.
- Viscosity of the pumpage.
- Slurries. Large particles cannot pass through and will clog the impeller.
- Vortex volute pump designs.
- Double volute pump designs.