Cavitation occurs in fluid flow systems where local static pressures are below fluid vapor pressure.
Cavitation is a common problem in control pumps and valves - causing serious wear, tears and damage. Under wrong conditions, cavitation drastically reduces component life.
What is cavitation?
Cavitation can occur when the local static pressure in a fluid reaches a level below the vapor pressure of the liquid at its actual temperature. According to the Bernoulli Equation, this can happen when a fluid accelerates in a control valve or around a rotor. bomba.
Vaporization itself does not cause damage - damage happens when vapor almost immediately after evaporation collapses when speed decreases and pressure increases.
Avoiding cavitation
Cavitation can generally be avoided by
- Increasing the distance (pressure difference) between the real local static pressure in the fluid - and the fluid vapor pressure at actual temperature
This can be done by:
- reengineering components starting high speed speeds and low static pressures
- increasing the total or local static pressure in the system
- reducing fluid temperature
Component reengineering starting high speed speed and low static pressure
Cavitation and damage can be avoided using special components designed for real adverse conditions.
- conditions with large pressure drops can - with limitations - be treated by control valves of various stages
- Challenging conditions challenging with fluid temperatures near vaporization temperature can be treated with special bombs - working according to other principles besides centrifugal pumps
Increasing the total or local pressure in the system
By increasing the total or local pressure in the system, the distance between static pressure and vaporization pressure is increased and vaporization and cavitation can be avoided.
The ratio between static pressure and vaporization pressure - an indication of the possibility of vaporization is often expressed by the cavitation number.
Unfortunately it is not always possible to increase total static pressure due to systems classifications or other limitations. Local static pressure on components can be increased by lowering (raising) the component in the system. Control valves and pumps in general should be positioned in the lower part of a system to maximize the static gauge height.
This is a common solution for boiler feed pumps that receive hot condensate (water near 100 o C ) Condenses receptors in steam plants.
Reducing fluid temperature
Vaporization pressure depends on fluid temperature. The vapor pressure of the water - Our most common fluid - is indicated below:
| Temperature ( o C) |
Vapor pressure (kPa, 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 |
Observation! - Be aware that the evaporation pressure - and possible cavitation - increases dramatically with the water temperature.
Cavitation can be avoided by positioning components in the coldest part of the systems.
Example - It is common to locate bombs and modulating valves in heating systems on “cold” return lines before heat heaters and exchangers.
Source: https://www.engineeringtoolbox.com/cavitation-d_407.html
