Installation and Operation Manual
SA-07-11-87 Rev A
10
January 2019
Net Positive Suction Head
Liquids cannot be towed or dragged down a pipe,
they can only be pushed along a pipe. The push
the liquid needs to flow into the pump can only be
supplied by some external energy source. This
source can be the absolute pressure on the
surface of the liquid in the vessel and the head of
the liquid over the center line of the pump.
A system that has insufficient NPSH available will
cause the pump to cavitate. The general effects
of cavitation are noise, erosion of metal surfaces
and vibration of the system. The latter situation
will result in the magnetic coupling overheating
and the product-lubricated bearings will wear
rapidly. Cavitation starts when the pressure at the
pump suction falls near to the value of the vapor
pressure of the pump liquid and this varies with
temperature.
At the time the pump is selected, the NPSH
available will have been calculated as per the
following method: -
NSPH available = Ps-Vp+Hs+Vs-Hf
Where:
Ps = Absolute pressure in suction vessel
Vp = Vapour pressure of liquid
Hs = Total static head
Hf = Friction losses in suction line
Vs = Velocity Head =
G
V
2
2
A seal
Figure 4. NPSH Definition
In the equation listed, all parameters must be
converted to meters head of liquid before the
liquid calculation is carried out. The pump should
be selected so that the NPSH available is a
minimum of 0.5m (1.6 ft) greater than the NPSH
required by the pump.
No subsequent alteration should be made to the
pipework system without considering the effect
that it will have on the NPSH available.
Figure 3. Suction Piping
