3.1.4 Pump performance curve

Pump performance curve

Pump performance curve is not only one curve, it consist of multiple curves giving comprehensive information about that specific pump performance. The hydraulic performance of pumps for liquids, and especially for centrifugal pumps, is usually shown in the form of a curve indicating pump capacity in terms of flow versus head. The main interest is thus, the relationship between the volume of fluid transported per unit of time, the so called volume flow, and the pressure increase of the pump. The different main categories of pumps (positive displacement pumps, rotodynamic pumps and other pumps) have, in this respect, drastically different characteristics, which is shown in figure 3.5.

Figure 3.14 Different pump performance curves

The volume flow in positive displacement pumps is virtually independent of the pressure rise, i.e. the performance curve is almost a vertical line. The performance curve for the centrifugal pump shows increasing pressure rise with decreasing volume flow. Liquid ring pumps have a curve lying between these two groups of pumps, see the special section for pump curves for the category of “Other pumps”. The volume flow in a pump is determined by the relationship of the specified pump pressure increase to the counter-pressure, i.e. the resistance in the system, which is required in order to drive forward the volume flow.

If the liquid pressure somewhere in a pump is reduced to the vapour pressure of the liquid, the liquid evaporates. So called cavitation occurs when the vapour bubbles return to liquid form in areas of higher pressure, this phenomenon causes great local pressure rises. Cavitation is not desirable since it reduces both pump performance and the operating life of the pump. The concept NPSH is employed to describe the risk of cavitation and comprises the necessary pressure head measured above the vapour pressure of the liquid such that the effects of cavitation are limited. For further details see Section 3.5.