11.2 Net positive suction head
The net positive suction head (NPSH) is an important concept in judging the suction behaviour of a centrifugal pump. A distinction needs to be made between the NPSH available to the pump and the NPSH required by the pump.
The NPSH available is the difference between the inlet head to the pump and the saturation pressure of the liquid being handled. As the available NPSH is reduced, cavitation commences, first at flowrates away from best efficiency and eventually over the complete operating range. This is a result of the local pressure falling to the vapour pressure of the liquid at the particular temperature condition. Operation in a cavitating mode for extended periods results in erosion damage, regardless of impeller material. Feed pumps with a high head per stage are most liable to cavitation damage because of the higher energy input to the fluid.
Cavitation performance breakdown has been shown to be directly proportional to (shaft speed)2 for a centrifugal impeller. It has also been shown that the head drop at which unacceptable cavitation damage in pump impellers occurs, increases with pump speed. From these findings, it is evident that the suction head levels needed for high speed pumps are significantly greater than those for lower speed machines.
The minimum NPSH required can be defined as that value of NPSH needed by the pump under long term load conditions to prevent the occurrence of harmful cavitation effects, in particular that required to avoid cavitation damage and ensure satisfactory operation.
Normal practice for determining the NPSH required to limit cavitation damage to an acceptable level is based on head drop tests. For high duty feed pumps, the assessment of visual test data is now also used as an additional means of assessing the adequacy of NPSH margins.
The NPSH head drop test can be included as a standard proof of performance test, with measurements being taken to establish the NPSH at which a 3% drop in generated head occurs. From data available, it is considered prudent to provide for feed pumps at least 3 x 3% head drop NPSH to avoid cavitation problems .
Visual cavitation tests are carried out on a special test facility, where, for example, a perspex window allows an uninterrupted view of the impeller eye, and enables visual observations to be made under stroboscopic lighting. This is an important tool, allowing the exact cavitation inception point to be determined, which effectively defines the zero cavitation damage line. Although observations of the pressure face of the blade are difficult, this problem can be overcome by using acoustic inception techniques to establish the onset of cavitation. When checking NPSH adequacy using the visual technique, current practice requires the available NPSH to have a margin over the visual appearance NPSH, which itself is associated with bubble cavities having an agreed maximum length.