How
How
How to
to
to Calculate
Calculate
Calculate Your
Your
Your System’s
System’s
System’s Net
Net
Net Positive
Positive
Positive Suction
Suction
Suction Head
Head
Head ––
– Available
Available
Available
(NPSHa)
(NPSHa)
(NPSHa)
For a given flow rate, there must be a minimum
fluid head pressure supplied to the pump to prevent
cavitation. This minimum head is shown on the
Performance Curve, labeled as NPSHr. The units
are feet WC (Water Column) absolute. The NPSHa
of your system must be greater than the NPSHr to
prevent cavitation and therefore increase efficiency
and the life of you pump. To calculate the NPSHa of
your system, use the following equation:
NPSHa
NPSHa
NPSHa =
=
= H
H
H
a
a
a
±
±
± H
H
H
zz
z
––
– H
H
H
fff
––
– H
H
H
vv
vp
p
p
Where:
Where:
Where:
H
H
Ha
a
a is the absolute pressure on the surface of the liquid in the supply tank. Typically, this is atmospheric
pressure for a vented supply tank, e.g. 34 feet at sea level.
H
H
Hzz
z is the vertical distance in feet between the surface of the liquid in the supply tank and the centerline of the
pump inlet. Value should be positive if the level is higher than the pump and negative if the level is lower
than the pump. Always be sure to use the lowest level the liquid can reach in the tank.
H
H
Hfff is the total of the friction losses in the suction piping.
H
H
Hvp
vp
vp is the absolute vapor pressure of the liquid at the pumping temperature.
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