by Wallace Wittkoff
March 31, 2010

The root issue with rotary PD 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:

  • Viscosity changes
  • Differential pressure changes
  • Clearance allowances for temperature change
  • Wear (resulting in an increase in clearance)

Given these product/process variables, tight performance occurs when the pump maintains close to its theoretical displacement independently of changes to the above variables. The definition of a PD pump is a pump that transfers a set displacement per unit operation, such as revolution or stroke.

Tight versus loose pump performance is the extent to which, under a given range of conditions, the pump maintains high volumetric efficiency. High volumetric efficiency is the extent (ratio) in which the true displacement of the pump approximates its theoretical displacement for given process/product conditions. 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 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.

To understand slip for traditional PD pumps, see Figure 5. It illustrates the possible loose-performance range (the yellow area) of a typical PD pump when operating in variable conditions (changes in viscosity, back pressure, temperature and wear). This graph shows how flow for a given pump speed (A) can vary from the theoretical (intersection BA) to an extreme (intersection EA), which indicates no flow. This condition occurs in pumps with worn pumping elements, for example.

PICTURE

Figure 5

Even in non-extreme cases, such as when needing a flow rate of (B), the pump would need to be accelerated from (A) to (F) to achieve the flow (B). This can prove to be an automation challenge and result in a reduction of reliability. If the automation system does not have a way to compensate for loss of flow and the pump remains at the same speed (A), the flow rate (D) would be inadequate. An actual curve for such a pump with 0.153 gallons/revolutions can be seen in Figure 6.

PICTURE

Figure 6

Most users specifying pumps realize this and attempt to control the extreme variabilities of viscosity, pressure, temperature and wear simultaneously. 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 for 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).

Figure 5 illustrates a tight performance band, which is shown as the green performance band range superimposed on the same graph. Even with large variations in pumping conditions in its published performance limits, the maximum variation in flow versus pump speed would be between (B) and (C) instead of (B) and (E), illustrated by the yellow loose-performance band. An actual curve band for such a pump can be seen in Figure 7. Both pumps (Figures 6 and 7) have a theoretical displacement of 0.15 gallons/revolution, but the curve in Figure 6 shows how loose the pump's performance is at 250 rpm, producing as much as 28 gpm of slip while attempting to pump 38 gpm. The pump shown in Figure 7 has only 4 gpm of slip under the same conditions.

PICTURE

Figure 7

Today's 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 for compensating for performance. At times, however, these performance changes cannot be adequately or reliably compensated and may not produce optimal control.

Next month, we will look at the effects of pump component wear and a narrow versus wide performance band.

This article builds on the principles presented in the Hydraulic Institute's "PD Pump Fundamentals, Design and Applications (Part One)" (Pumps & Systems, February 2009, available on www.pump-zone.com).

Pumps and Systems, April 2010

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