Cavitation
The phenomenon of cavitation occurs when the absolute pressure at the pump suction
approaches the vapour pressure of water at the operating temperature. If the suction pressure
becomes lower than the vapour pressure, vapour pockets (in the form of bubbles) will form along
the pump impeller blade passages. This means that there is a change of phase from liquid to gas.
When these bubbles moves to the pressure side of the impeller, they collapse and implode on
the blade surfaces. Such implosion causes erosion on the surface and create a pressure wave (or
hammering noise) which can cause a noisy operation.
Such phenomenon is more critical with hot water because at higher temperatures, the vapour
pressure increases and the tendency of achieving this pressure is easier.
Therefore, it is important that a calculation of the pump inlet suction pressure to be made of the
designed system and check it with the pump performance limits.
The amount of this suction head pressure in excess of the vapour pressure required to prevent
formation of the vapour bubbles is known as the Net Positive Suction Head Required
(NPSHR).
This NPSHR is a characteristic of a pump and is available from the manufacturer pump curves.
Note: The NPSHR can also be defined as the frictional resistance from the pump inlet to the first
impeller.
In order to prevent cavitation, the actual system net positive suction head must be equal to or
greater than the NPSHR value. This actual head pressure is also called as the Net Positive
Suction Head Available (NPSHA).
NPSHR ≤ NPSHA
How can we calculate NPSHA?
There are two ways to do this:
1. For open systems, use the following equation:
NPSHA = P
a
+ P
s
- P
vp
- P
f
where P
a
is the atmospheric pressure
P
s
is the static head of water level above the pump impeller
P
vp
is the vapour pressure of water at the operating temperature
P
f
is the friction loss of the suction pipe, fittings and valves.
Generally, if the water tank is elevated compared with the mini-chiller unit, there will be no
problem with cavitation, unless the friction loss along the suction line is very high. See
following example.
Section 5 Page 82
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