Net Positive Suction Head Available (NPSHA) of Pump

Net Positive Suction Head Available (NPSHA) is the head which is owned by the liquid at the pump suction side which is reduced with the saturated steam pressure of liquid in that place. In the case of pump that sucks liquid from the open place, then the amount of available Net Positive Suction Head Available (NPSHA) can be written as follows:

hsv = (Pa/γ) - (Pv/γ) – hs – hls

Where:
hsv        = NPSH available (m)
Pa        = atmospheric pressure (kg/m2)
Pv        = saturated steam pressure (kg/m2)
γ          = weight of liquid per unit volume (kg/m3)
hs         = static suction head (m), hs is positive (marked +) if the pump is located above the liquid surface, and negative (marked -) if below the liquid surface.
hls        = head loss in suction pipe (m).

If the liquid is sucked from closed tank, then the   Pa value states absolute pressure which acting on the surface of the liquid inside closed tank. Especially if the pressure above the liquid surface is equal to saturated steam pressure, then Pa = Pv.  In the case of a pump that sucks the liquid from the open, so the amount of Net Positive Suction Head Available (NPSHA) can be written as follows:

hsv = – hs - hls

The value of hs is negative (-) because the surface of the liquid inside the tank is higher than the suction side of pump. Installation of this kind pump is required to obtain a positive NPSHA value.
Figure 1: Position of pump is located above the surface of sucked fluid 
Figure 2: Position of pump is located under the surface of sucked fluid 

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