As a pump wears in service, internal leakage increases and, therefore, the percentage of flow available to do useful work (volumetric efficiency) decreases.

If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.

In a condition-based maintenance environment, the decision to change-out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.

Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. It is calculated by dividing the pump’s actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.

Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when stating volumetric efficiency.

For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS.

When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant.

To understand why this is so, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump’s displacement.

**Craig Cook**