The root issue with rotary positive-displacement pumps is that the flow performance on all pumps is to some degree affected by internal clearances that result in slip. The degree of slip changes with:
- Change in viscosities.
- Change in differential pressure.
- Clearance allowances for temperature change.
- Wear (resulting in an increase in clearance).
Given these product/process variables, tight performance is one in which the pump maintains close to its theoretical displacement independent of changes to the variables listed above. The very definition of a positive-displacement pump is a pump that transfers a set displacement per unit operation, such as revolution or stroke.
Tight vs. loose pump performance is the extent to which, under a given range of conditions, the pump maintains high volumetric efficiency. Pump slip is the difference between the theoretical displacement and the actual displacement. Therefore, the lower the pump slip in any condition, the tighter the pump’s performance would be under conditions of changing viscosity, pressure, temperature or wear.
Classifying a pump as simply positive displacement without quantifying the tightness of its performance band can greatly affect the desired results in an application. The extreme example is one in which, regardless of the pump speed, the slip is 100 percent. That is, all fluid that is pumped forward then flows (slips) back through the pump’s internal clearances to produce no net fluid transfer. While sounding dramatic, it is not uncommon that a pump reaches this point (total loss of flow) before it is taken out of service to be repaired or replaced.
Most users specifying pumps attempt to control the extreme variabilities of viscosity, pressure, temperature and wear all at the same time. In many applications, this variation is sufficient to produce a challenging operational scenario. In some cases, advanced automation can help, such as using flowmeters with speed/pressure control loops. However, there are cases in which the possible variation cannot be compensated without recalibration or retuning the processes. These methods can prove costly or unfeasible, and could also increase system complexity (thus reducing reliability).
Today’s more advanced pump manufacturers provide the tools that permit evaluating the possible slip for a given application. Curves are supplied that demonstrate how to down-rate the flow given changes in back pressure, viscosity or change of internal component clearance to handle certain temperature ranges. These tools are helpful to be able to compensate for the performance. At times, however, these performance changes can’t be adequately or reliably compensated and may not produce optimal control.
In the next installment, we will take a look at how a narrow vs. wide performance band can affect pump component wear. For more information, please visit www.pumpsg.com/PDF/whitepaper_PSG_PumpPerformance.pdf or www.pumpsg.com.