The head and flow rate determine the performance of a pump, which is graphically shown in Figure 5 as the performance curve or pump characteristic curve. The figure shows a typical curve of a centrifugal pump where the head gradually decreases with increasing flow.As the resistance of a system increases, the head will also increase. This in turn causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing to the pump to burn out.
Electrical Energy Equipment: Pumps and Pumping Systems
Figure 5. Performance Curve of a Pump
Pump operating point
The rate of flow at a certain head is called the duty point. The pump performance curve is
made up of many duty points. The pump operating point is determined by the intersection of the system curve and the pump curve as shown in Figure 6.
Figure 6. Pump Operating Point (US DOE, 2001)
Pump suction performance (NPSH)
Cavitation or vaporization is the formation of bubbles inside the pump. This may occur when at the fluid’s local static pressure becomes lower than the liquid’s vapor pressure (at the actual temperature). A possible cause is when the fluid accelerates in a control valve or around a pump impeller.
Vaporization itself does not cause any damage. However, when the velocity is decreased and pressure increased, the vapor will evaporate and collapse.
This has three undesirable effects:
- Erosion of vane surfaces, especially when pumping water-based liquids
- Increase of noise and vibration, resulting in shorter seal and bearing life
- Partially choking of the impeller passages, which reduces the pump performance and can lead to loss of total head in extreme cases.
The Net Positive Suction Head Available (NPSHA) indicates how much the pump suction exceeds the liquid vapor pressure, and is a characteristic of the system design. The NPSH Required (NPSHR) is the pump suction needed to avoid cavitation, and is a characteristic of the pump design.
