Q: What value should I use for volumetric efficiency and friction torque when specifying a pump?

A: Every progressing cavity pump is different, even if it has the same nominal size and is constructed of the same materials as another. As a result, there is no way to determine the exact volumetric efficiency and friction torque for any given pump under downhole conditions. Moreover, these values can change over the pump’s life. Most notably, these will change over the early period when the pump is first in a well. The stator elastomer will swell (or in some cases shrink) when exposed to well fluid and put under operating pressure. If the stator swells, the friction torque and volumetric efficiency will normally both increase. In addition, changes in viscosity and pump speed will also affect the volumetric efficiency. In determining the friction torque and volumetric efficiency to use, you need to look at the pump’s bench test results and then use field experience—how have the torques and efficiencies of this type of pump in this field under operating conditions been different from their bench test results?

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