NPSH | Available and Required NPSH | Net positive suction head in pumping system
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Available & Required NPSH(Net positive suction head) in pumping system :
The value, by which the pressure in the pump suction exceeds the liquid vapour pressure, is expressed as a head of liquid and referred to as Net Positive Suction Head Available (NPSHA). This is a characteristic of the system design. The value of NPSH needed at the pump suction to prevent the pump from cavitating is known as Net positive suction head required (NPSHR). This is a characteristic of the pump design.
The three undesirable effects of cavitation described above begin at different values of NPSHA and generally there will be cavitation erosion before there is a noticeable loss of pump head. However for a consistent approach, manufacturers and industry standards, usually define the onset of cavitation as the value of NPSHR when there is a head drop of 3% compared with the head with cavitation free performance. At this point cavitation is present and prolonged operation at this point will usually lead to damage. It is usual therefore to apply a margin by which NPSHA should exceed NPSHR.
As would be expected, the NPSHR increases as the flow through the pump increases, In addition, as flow increases in the suction pipe work, friction losses also increase, giving a lower NPSHA at the pump suction, both of which give a greater chance that cavitation will occur. NPSHR also varies approximately with the square of speed in the same way as pump head and conversion of NPSHR from one speed to another can be made using the following equations.
Q α N NPSHR α N2
It should be noted however that at very low speeds there is a minimum NPSHR allow, NPSHR does not tend to zero at zero speed It is therefore essential to carefully consider NPSH in variable speed pumping.