- Cavitation is a condition in which the internal pressure of the liquid eventually drops below the vapour pressure, leading to the creation of vapor bubbles.
- After this, the pressure begins to increase above the vapor pressure once again, and during this pressure recovery stage, cavitation occurs and the bubbles burst.
- Flashing is a situation in liquid flow where the pressure drops below the vapor pressure and stays below it (similar to cavitation). Following that, two phases—liquid and vapor—flow downstream without the bubbles collapsing.
- Under specific circumstances, two-phase phenomena including cavitation and flashing can be seen in the liquid flow via the control valve.
- Cavitation and flashing have a common first stage in which the liquid pressure falls below the vapour pressure, resulting in vapour bubbles.
- This is the same as the well-known phenomenon of boiling. But in control valves, boiling is achieved by a pressure reduction during the throttling operation, not by an increase in temperature.
- Flashing and cavitation have different second stages from one another. In cavitation, as pressure rises above the vapour pressure, vapour bubbles rapidly burst and produce pressure shocks.
- At worst, pressure shocks could harm the valve’s mechanical components.
- Because the downstream pressure is lower than the vapour pressure in flashing conditions, the pressure does not rise above the vapour pressure.
- The fluid on the valve outlet side is partially vapour and partially liquid because vapour bubbles do not collapse.
Many factors affect the ability to predict cavitation damage.
- Because it depends on so many variables, such as pressure drop, flow medium, valve type, and materials, predicting cavitation damage is challenging.
- Incipient or partial cavitation, which should not be confused with incipient damage, is the beginning of cavitation. However, damage cannot be predicted using the concept of emerging cavitation.
- To avoid cavitation and lessen noise, all rotary control valves have been equipped with Q-Trim technology. The rotary valve performance is improved by Q-Trim, which is also appropriate for harsh service.
Flashing can produce comparable issues as erosion.
- Due to its high output velocity, flashing flow may result in issues similar to erosion.
- Flashing, in contrast to cavitation, cannot be avoided through valve selection because it is solely a product of process circumstances.
- Q valves are therefore rarely used in flashing services. An affordable alternative to using line-size globe valves is Standard Finetrol in the flow-to-close direction.
- Case-hardened trim and seat ensure that the valve can withstand even intense flashing conditions; Finetrol has been successfully used for flashing applications.
- In the middle of the pipe, the streamlining plug and seat direct the flow, and an expander after the valve ensures there is adequate room for two-phase flow.