3.62 Pump suction head
Pump suction head
Permissible pump suction head or static suction head (hs) is the only parameter concerning cavitation which the purchaser of a pump can normally influence. 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 (hs) obtained is at a maximum. The first term on the right-hand side of the equation pa /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.
Figure 3.62a Lower fluid surfaces, open and closed vessels. Positive suction head (lift)
Atmospheric pressure varies, for example, due to the height above sea-level. This is illustrated in table 3.62a, the height of the barometer at sea-level is assumed to be 760 mm Hg. Atmospheric pressure also varies according to climatic conditions.
Table 3.62a The variations of atmospheric pressure due to height above sea-level (760 mm Hg at sea-level).
At sea-level the atmospheric pressure does not normally fall below 720 mm Hg (960 mbar, 9,8 m H20). In the case of closed vessels the pressure within the vessel should be used in equation 3.62a.
Figure 3.62b Vapour pressure for water.
The vapour pressure of a liquid is dependent upon temperature. Figure 3.62b, for example, shows the vapour pressure for water at various temperatures. When pumping warm water for example the term pv /ρg can be so great that the static suction head, according to equation 3.62a, can be negative. This means that the pump must be placed below the level of the lower fluid surface if the pump is to operate cavitation-free. Vapour pressure for various temperatures for a large selection of fluids appears in Chapter 7 Liquid properties Section 7.8>>>.
The third term NPSHr is obtained for a given pump speed by referring to the pump manufacturers data sheet, i.e. normally a curve as shown in figure 3.62c.
The losses in the suction pipe should be kept as low as is practically possible, since they have the effect of reducing the permissible suction head for cavitation-free operation. The suction pipe should be of large diameter, short and without unnecessary bends, valves etc. Suction pipe losses and NPSHr increase with increased volume flow through the pump. It is therefore important to know the largest volume flow rate at which the pump is to operate cavitation-free.
The static suction head is measured from the lower fluid surface to the upper reference plane according to figure 3.62d.
Figure 3.62d Upper reference plane for determining the geodetic suction head.
If the pump is located below the lower fluid surface the static suction head (lift) becomes negative and is called the supply head, figure 3.62e.
Figure 3.62e Supply head (negative suction head)