3.61 Net Positive Suction Head

Net Positive Suction Head

In connection with cavitation the concept Net Positive Suction Head (NPSH) is often encountered, the definition is as follows:

NPSH – the difference between the total pressure on the pump suction side p_os and the vapor pressure p_v expressed in metres liquid column.

From this definition and by substituting for p_os the following equation can be deducted:

The NPSH value is entirely determined by the parameters related to the plant system and totally independent of the pump construction. The terms system NPSH or available Net Positive Suction Head (NPSH_a) are often used by way of clarification.

If the static suction head (h_s) in a system is increased for example, the available NPSH will be reduced. At the same time p_min will reduce according to the equation 3.6b, which thereby increases the risk of cavitation. Cavitation begins when p_min= p_v. The following then applies:

i.e. the NPSH value is related to the pump characteristics for this condition only the onset of cavitation. The NPSH value at the onset of cavitation is called the required Net Positive Suction Head (NPSH_r) and is a measure of the pump sensitivity to  cavitation.

Since NPSH_r is determined by the pump design, pump manufacturers specify the NPSH value for various volume flow rates, together with other pump data, figure 3.61a.

Figure 3.61a Example of pump performance at constant speed.

To prevent doubt as to the meaning of the expression onset of cavitation, criteria are given in centrifugal pump testing standards which enable the determination of NPSH_r . As shown in figure 3.61b the NPSH_r is the NPSH value for which the delivery head H at a given volume rate of flow is reduced by a certain amount ΔH due to cavitation. According to the international standards, ΔH is 3% of H.

Figure 3.61b Experimental determination of required NPSH (NPSH_r).

The complete prevention of cavitation in a pump would involve considerable cost. The conditions which in practice are considered to be cavitation-free operation (ΔH<3% of
H) can be expressed:

NPSH_a > NPSH_r         (Equ. 3.61d)

The same conditions can also be expressed as:

p_min > p_v 

The term p_min representing the minimum pressure in any of the functional areas of the pump (neglecting local pressure drop in clearances, at sharp edges and in crevices etc).

NPSH_r  is dependent upon pump speed. For a given pump the NPSH_r  is proportional to the square of the speed.

This relationship applies to small changes in speed assuming similar velocity vectors i.e. when the volume flow rate changes according to:

Figure 3.61c NPSHr for a change in pump speed.

Permissible suction head

The only parameter concerning cavitation which the purchaser of a pump can normally influence is the static suction head h_s. To ensure cavitation-free operation the pump should be installed so that:

which is the same condition as expressed in equation 3.61d.

When both sides of the equation are equal the suction head (h_s) obtained is at a maximum. The first term on the right-hand side of the equation p_a /ρg reflects the influence of pressure on the lower fluid surface. If the pump is sucking from an open vessel then this pressure is  equal to atmospheric pressure.