Page 46 - Know and Understand Centrifugal Pumps

Cavitation

If your pump is in cavitation, you’ll have one or more of the following:

w Problems with pump packings.
w Problems with mechanical seals.
w Problems with alignment.
w Problems with the bearings.
w Problems with impellers, casings, and wear bands.

w Problems with pump efficiency.
Problems with leaks and fugitive emissions.
And these problems won’t go away until you resolve cavitation at its
source.
There are five recog.nized types of cavitation:
w Vaporization cavitation, also called inadequate NPSHa cavitation
w Internal re-circulation cavitation.

Vane passing syndrome cavitation.
w Air aspiration cavitation.
w Turbulence cavitation.
Let’s investigate each of these, their causes and resolutions:

Vaporization cavitation
Vaporization cavitation represents about 70% of all cavitation.
Sometimes it’s called ‘classic Cavitation’. At what temperature does
water boil? Well, this depends on the pressure. Water will boil if the
temperature is high enough. Water will boil if the pressure is low
enough.

According to Bernoulli’s Law, when velocity goes up, pressure goes
down. This was explained in Chapter 1. A centrifugal pump works by
acceleration and imparting velocity to the liquid in the eye of the
impeller. Under the right conditions, the liquid can boil or vaporize in
the eye of the impeller. When this happens we say that the pump is
suffering from vaporization cavitation.
This type of cavitation is also called inadequate NPSHa cavitation. To
prevent this type of cavitation, the NPSHa in the system (the available
energy in the system), must be higher than the NPSHr of the pump
(the pump’s minimum energy requirement).
A good suggestion to prevent vaporization cavitation is:
NPSHa > NPSHr + 3 ft or more safety margin