This article helps you understand the term NPSH, how it is calculated and its importance when selecting a centrifugal pump. This basic knowledge of NPSH will help you go a long way in identifying potential problems in your pump even before they occur.

1. Understanding Net Positive
Suction Head Of Centrifugal
Pump

2. What is NPSH ?
● To avoid cavitation, one must ensure that the pressure of the
fluid flowing through the centrifugal pump at all points
inside the pump remains above the saturation pressure.
● The quantity that is used to determine whether the pressure
of the fluid that is being pumped into the pump is adequate
for avoiding cavitation is termed as NPSH: Net Positive
Suction Head.

4. Centrifugal Pumps & Its Working
● A machine responsible for imparting energy to a fluid for two
purposes:
○ for increasing the pressure of the fluid or
○ moving it along the pipeline.
● This task is accomplished through the action of blades.
Continued On Next Slide

5. Centrifugal Pumps & Its Working
● As fluid enters the vane of the centrifugal pump, energy is
supplied in the form of velocity. When the velocity is
gradually reduced, the energy gets converted into pressure.
● A pump will not be able to render velocity to a liquid that is
outside the pump. Therefore, the liquid must enter from the
opening of the impeller and pass through the vanes before
the process of transfer of energy begins.
Continued On Next Slide

6. What is NPSH(A): Net Positive Suction Head Available
● The energy that is available to the liquid at the eye to
perform the energy transformation process and assure that
the liquid remains in a liquid state.
● NPSH(A) is the difference between the pressure at the
suction of the pump and the saturation pressure of the fluid
that is being pumped.
NPSH(A) = P(suction) – P(saturation)

7. ● When these pumps are designed, they have inbuilt physical
and hydraulic characteristics that determine:
○ the amount of energy that will be required to push the
liquid into the impeller,
○ energy needed to ensure that the liquid remains a liquid
throughout its path through the impeller.

8. Factors that determine amount of energy needed by pump
● Some of the factors that also help determine the amount of
energy needed by the pump are:
○ the nature of the pump eye,
○ the structure of the blades of the impeller,
○ their diameter,
○ the speed at which they operate

9. NPSH (R): Net Positive Suction Head Required
● The pump can only perform its functions properly when the
energy available is equal to or greater than the energy
required.
NPSH(A) ≥ NPSH(R)
● And if we consider a margin of safety, then this rule can be
best stated as:
NPSH(A) > NPSH(R)

10. ● To avoid cavitation, the minimum net positive suction head
necessary is known as the Net Positive Suction Head
Required.
● It is important to get the NPSH(R) value from your pump
manufacturer and insure that your NPSH(A) will be enough
to cover those needs.

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