Pumping Systems – Cavitation & Net Positive Suction Head “NPSH”

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Pumping Systems – Cavitation & Net Positive Suction Head “NPSH”

Pumping Systems – Cavitation & Net Positive Suction Head “NPSH”

The net positive suction head NPSH for a pump is concept for which no clear consistent definition is available. Consequently the expression cited vary from one text to another. Some sources express NPSH as a pressure head while others as an energy head, hence difference in the handling of the velocity head term.

The net positive suction head NPSH is defined as the minimum absolute pressure head required at the pump inlet (relative to the inlet) in excess of the vapor pressure of the fluid. By definition this is given by :

$$NPSHR = {{{P_s} - {P_v}} \over {\rho g}}$$

where :

Ps : Absolute pressure at the pump inlet (suction)

Pv : Vapor pressure of the fluid

The division by ρg is a conversion back to meters head of water.

The required net positive suction head, designated NPSHR, is a function of the pump design and is supplied by the pump manufacturer (as contour lines of equal NPSHR on the pump characteristic curves). It varies with the discharge.

Application of the energy equation (Bernoulli's equation) between the source and the inlet to the pump results in an alternative expression for NPSH :

$$NPSHA = {{{P_a} - {P_v}} \over {\rho g}} - {h_s} - {h_{ls}} - {{v_s^2} \over {2g}}$$

where :

hs : Elevation difference between the source and the pump inlet

hls : Head loss in the suction pipeline

Pa : Absolute pressure at the source (upstream of suction line)

vs : Flow velocity at the pump inlet

The velocity head term on the right side of the equation is required if the source represents a free water body such as a reservoir or stream.

This head, designated NPSHA, is the head available at the pump inlet and is a function of the system. It comprises three constant terms (the pressures and the elevation term) and the loss term and optional velocity head term which vary with Q2.

Cavitation will occur whenever NPSHR NPSHA. The discharge at which this occurs is best determined graphically by the superposition of the two curves as shown in the following figure.

Net Positive Suction Head

The discharge calculated here may be greater or less than that defined by the operating point of the pump. If greater, the pump will operate safely at the natural operating point without risk of cavitation. If less, the operating point of the pump must be altered to reduce the discharge below that indicated by the NPSH calculations. This can be achieved by :

  • partially closing a gate valve on the delivery line
  • reducing the pump speed
  • reducing the impeller diameter

Alternatively it might be possible to sufficiently alter the NPSHA by altering the pipe upstream of the pump to increase the supply pressure.

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