NET POSITIVE SUCTION HEAD:

In the diagram, H_es is the pressure head acting on the liquid surface at the suction inlet. The vertical distance of the pump centre H_s from the surface of the liquid is the head available at the pump. If the pump is below the liquid level, then H_s will be positive, if it is above the liquid level H_fs will be negative. Frictional resistance in the suction pipe resulting in a loss of pressure head H_{f ,} H_u is the head loss due to the velocity of the liquid, which is normally negligible except for very high velocities.

Every liquid has a pressure at which it will vaporise and this pressure varies with temperature. If the combination of pressure and temperature within the suction pipe is such that vaporisation occurs,

the efficiency of the pump deteriorates and a condition can be reached where the pump will cease to function. The vapour H_vap is thus usually shown as a suction head loss.

The Summation (H_es ± H_s - H_fs - H_v - H_vap) is known as the available NPSH (net positive suction head).

In application, this available NPSH must always be greater than the required NPSH to effect the designed delivery rate. The required NPSH is generally available from the pump manufacturer, whereas available NPSH may be calculated from the details of suction piping.

SUCTION LIFT AND DISCHARGE HEAD :

If the suction liquid level is below the centre line of the pump, it is generally called a "Static suction lift". This is the difference in elevation between the centre line of the pump and the level of the liquid in the suction well.

In practice, the suction lift will exceed 7 m only under very favourable conditions. This is because of friction losses in the suction pipe and limitation of pump design. Any increase in temperature of liquid above 15°C will have a detrimental effect on the vapour pressure, e.g. at 50°C water will boil at an abs. pressure of 0.14 bar, so that the lift reduces to 10.2 (1.013-0.14) = 9 m, drastically reducing the available NPSH. It follows that for water temperature above about 75°C, the suction head must be positive or if this is impossible, the suction pipe must be short, straight, free from interference and the speed of flow must be low, less than 1 m/sec.

Discharge head is the sum of static discharge head, friction head for the capacity considered, the exit losses at the end of the discharge line and the pressure head acting on the free surface at the terminal point. There will also be a velocity head loss but except in high velocities, this can be neglected.

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