Q - We have a centrifugal pump directly driven by a 100 horsepower (HP) electric motor running at 3565 RPM. It used to pump water at 1.0 specific gravity, but now we want to use it at exactly the same flow rate and head, but with a different non- viscous liquid with 0.80 specific gravity.
I'm expecting that the electric motor power consumption should drop by 20% - or by a direct ratio of their specific gravities. Am I right? The pump has new wear parts and its running clearances are restored to factory original.
A - Theoretically, you are right that a 20% lighter specific gravity should result in 20% reduction in motor power consumption provided the pump operates at the same flow rate and head, and both liquids do not require any viscous efficiency correction.
But in real world situations the actual power reduction will be less than expected for two reasons:
Electric motor has different efficiencies at full and partial loads. The motor will operate at partial load, and is less efficient, when the pump is handling lighter specific gravity liquid. For example, a typical 100 HP motor will have a 1/2 point drop in efficiency between full load and 3/4 load.
A motor full load speed is lower than its synchronous speed due to slip. In the same example, the 100 HP may have a full load speed of 3565 RPM but at 3/4 load its speed may go up to 3575 RPM, resulting in the pump developing slightly higher flow rate and head at lighter specific gravity.
So instead of getting a 20% reduction in power usage, you should expect less than that.
R: 0210-ELMO C: design, operation F: electric motor amperes