Page 42 - Know and Understand Centrifugal Pumps

Cavitation

Inside the pump, the pressure decreases in the eye of the impeller
because the fluid velocity increases. For this reason the liquid can boil at
a lower pressure. For example, if the absolute pressure at the impeller
eye should fall to 1.0 psia, then water could boil or vaporize at about
100°F (see the Tables in Chapter 2 Properties ofwater I and 11).

Cavitation
- ~ ~-
Definition
Cavitation is the formation and subsequent collapse or implosion of
vapor bubbles in the pump. It occurs because the absolute pressure on
the liquid falls below the liquid’s vapor pressure.
When the vapor bubbles collapse with enough frequency, it sounds like
marbles and rocks are moving through the pump. If the vapor bubbles
collapse with enough energy, they can remove metal from the internal
casing wall, and leave indent marks appearing like blows from a large
ball pein hammer.
This book is dedicated to pumps but we should mention that cavitation
could occur in other parts of the pumping system. Under the correct
circumstances, valves and pipe elbows are also candidates to suffer
damage from cavitation.

The effects of vapor pressure on pump performance
___ ____ . ~-
When cavitation occurs in a pump, its efficiency is reduced. It can also
cause sudden surges in flow and pressure at the discharge nozzle. The
calculation of the NPSHr (the pump’s minimum required energy) and
the NPSHa (the system’s available energy), is based on an under-
standing of the liquid’s absolute vapor pressure.
The effects of cavitation are noise and vibration. If the pump operates
under cavitating conditions for enough time, the following can occur:
w Pitting marks on the impeller blades and on the internal volute
casing wall of the pump.
w Premature bearing failure.

w Shaft breakage and other fatigue failures in the pump.
w Premature mechanical seal failure.
These problems can be caused by:
w A reduction of pressure at the suction nozzle.