The Hydreco Model 1919 gerotor pump has a displacement of 4.53 in3/rev. The manufacturer gives performance data up to a maximum speed of 3000 rpm. The maximum pressure curve is 2500 psi.
When driven at 1200 rpm, the volumetric efficiency drops from 92% at 500 psi to 78% at 2500 psi (Fig. 4.21a). At 1800 rpm, the volumetric efficiency drops from 93% at 500 psi to 84% at 2500 psi (Fig. 4.21b). Why is the volumetric efficiency higher at the higher speed?
This question can be answered by comparing performance at a specific pressure. In this case, the comparison will be made at 1500 psi. The leakage flow is
Leakage flow is primarily a function of pressure, with rotational speed being a much less significant factor. Suppose that leakage flow is the same for the performance test run at 1800 rpm, 1500 psi.
Measured volumetric efficiency was evp = 88.7%, slightly less than the efficiency calculated by assuming constant leakage flow. There is some increase in leakage at higher speeds due to the increased turbulence of the fluid. This explains why the volumetric efficiency is less than that calculated by assuming constant leakage flow as speed increases. It is generally good design practice to select a smaller pump and operate it at higher speed to achieve a higher volumetric efficiency.
A volumetric efficiency in the 85 to 90% range is achievable with a gerotor pump if pressures are below 1500 psi. Remember, however, that volumetric efficiency decreases as operating temperature increases. Viscosity of the fluid decreases, and more leakage occurs through the clearances in the pump.