3.3.4 Positive displacement pump curves

Positive displacement pump curves

Positive displacement pump curves is displaying the pump performance, presented as a curve or in the form of a table. Performance curves cannot be of the same type as used for centrifugal pumps, since the power requirement is difficult to read off figure 3.34a.

The type of diagram shown in figure 3.34b is therefore necessary. Values for various viscosity, for example, may also be shown.

Internal leakage (slip) is represented on the performance curves as the difference between volume rate of flow at zero pressure increase, figure 3.103. Conversely, it is possible to predict a pump performance curve if the volumetric
efficiency is known, using the following equation:

η_v= 1 – s (Equ. 3.34a)

where s = the relative internal leakage (slip).

NOTE: For higher pressure increases the compressibility of the liquid must be taken into account.

performance curves positive displacement curve — Figure 3.34 (a and b)

Figure 3.34 Performance curves for displacement pumps. The type of curves used for turbine pumps, figure a can not be used since it is difficult to read off power requirements. The curves shown in figure b are therefore normally preferred.

displacement pump curve internal leakage — Figure 3_34c

Figure 3.34c Using the performance curve to determine displacement pump internal leakage.

For displacement pumps with integral relief valve (safety valve, reduction valve) the performance curve is modified when the pressure reaches the operating range of the valve figure 3.34d.

positive displacement curve with safety valve — Figure 3.34d

Figure 3.34d Performance curve for displacement pump fitted with safety valve.

The effect of viscosity on displacement pump performance curves can be shown as in figure 3.34e for example.

viscosity effect on positive displacement gear pump — Figure 3.34e

Figure 3.34e The effect of viscosity on volume flow rates for a small gear pump at two different speeds. Note that for the particular positive clearances chosen the volume flow rate is reduced to zero at low viscosity.

When choosing various speeds with regard to volume flow rates and pressure increase, a different diagram is used. Figures 3.34f and 3.34g show examples of performance curves for a lobe rotor pump pumping water and viscous fluid respectively.

performance curve for lobe rotor pump pumping water — Figure 3.34f

Figure 3.34f Example of performance curves for lobe rotor pump working in water.

performance curve for lobe rotor pump pumping oil — Figure 3.34g

Figure 3.34g Performance curves for same pump as in figure 3.34f but working with fluid of viscosity 100 cSt.