Cavitation may occur in fluid flow systems where the local static pressure is below the vapor pressure
Cavitation is a common problem in pumps and control valves - Causing serious wear and tear and damage. Under the wrong conditions, cavitation Reduces the components life time dramatically.
Cavitation may occur when the local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature.
According to the Bernoulli Equation this may Happen when the fluid accelerates in a control valve or around a pump impeller.
The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately collapses after evaporation when the velocity is Decreased and Increased pressure.
Cavitation can in general be avoided by
increasing the distance between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature
reengineering components initiating high speed low velocities and static pressures
increasing the total or local static pressure in the system
reducing the temperature of the fluid
Reengineering of Components Initiating High Speed Velocity and Low Static Pressure
Cavitation and damage can be avoided by using special components designed for the actual rough conditions.
conditions as huge pressure drops can - with limitations - be handled by the Multi Stage Control Valves
challenging pumping conditions - with fluid Temperatures close to the vaporization temperature - can be handled with
special pumps - after working principles of centrifugal pumps than
Increasing the total or Local Pressure in the System
By increasing the total or local pressure in the system the distance between the static pressure and the pressure is Increased vaporization and vaporization and cavitation can be avoided.
The ratio between the static pressure and the vaporization pressure - an indication of the possibility of vaporization, is Often Expressed by the Cavitation Number.
Unfortunately it may not always be possible to increase is due to the total static pressure systems Classifications or other limitations. Local static pressure in components may be Increased by lowering the component in the system. Control valves and pumps in general should be positioned in the and lowest part of the systems to maximize static head.
This is a common solution for boiler feeding pumps receiving hot condensate (water close to 100 ° C) from condensate receivers.
Reducing the Temperature of the Fluid
The vaporization pressure of fluid temperature depends. Vapor pressure of Water, our most common fluid, is Indicated below :
Temperature ( ° C)
|
Vapor Pressure (kN / m 2 )
|
0
|
0.6
|
5
|
0.9
|
10
|
1.2
|
15
|
1.7
|
20
|
2.3
|
25
|
3.2
|
30
|
4.3
|
35
|
5.6
|
40
|
7.7
|
45
|
9.6
|
50
|
12.5
|
55
|
15.7
|
60
|
20
|
65
|
25
|
70
|
32.1
|
75
|
38.6
|
80
|
47.5
|
85
|
57.8
|
90
|
70
|
95
|
84.5
|
100
|
101.33
|
Note ! - The possibility of evaporation and cavitation increases dramatically with the water temperature.
Cavitation can also be avoided by locating components to the coldest part of a system. It is common to locate pumps in heating systems in the "cold" return lines.
This is the same for control valves. If it is possible control valves should be located on the cold sides of the heat exchangers.
