What is Cavitation how it works in a control valve?
Basically cavitation is a phenomenon that occurs during flow of a liquid in a closed channel such as a pipe. Normally when a liquid flows over any distance, there a drop in pressure – which is essential to maintain the flow.
At some point this pressure drops much below even atmospheric pressure – particularly in case of water flowing in a pipe. When this happens water in the pipeline tends to evaporate. This water vapor forms bubbles within water body and these bubbles keep on popping up as well as collapsing. Any solid surface facilitates this process. as such surface of the pipe suffers maximum variation in pressure.
This causes large vibrations and stresses on pipelines. For that reason it is very important to maintain good pressure in a pipeline carrying water over long distances.
Similar actions also happen in a centrifugal pump and cause not just deterioration in performance – delivery volume, pressure loss etc, it also causes severe erosion / wear out of pump and damages overall installation.
The difference is that with the cavitation phenomenon, the liquid pressure increases over its vapor pressure during pressure recovery and turns back into liquid state while during flashing the liquid pressure remains below the vapor pressure throughout. The key differences are:
Requirements for occurrence of cavitation
- The fluid at both the inlet and outlet must be in all-liquid condition.
- The liquid must be sub-cooled state at the inlet, because if the liquid will be in a saturated state, then any pressure drop across the valve will cause the presence of vapor downstream.
- The valve outlet pressure must be either at or above the vapor pressure of the liquid.
Requirements for occurrence of flashing
- The fluid at the inlet must be in all-liquid condition, but some vapor must be present at the valve outlet.
- The fluid at the inlet may be in either a saturated or a sub-cooled condition.
- The valve outlet pressure must be either at or below the vapor pressure of the liquid.
How to avoid Cavitation:If cavitation is ever encountered, consider the following corrective actions: 1. The first is to equip the control valve with special trim and ensure that the plug and seat are made of a hard facing material that can resist both the onset and effect of cavitation (e.g. stellite hard facing). 2. The second is to use a valve with a low recovery coefficient . 3. The third is to increase the downstream pressure by installing a flow restrictor if possible or reducing the pipe size of a short piece downstream.
Valve Selection for Cavitation Service:
Cavitation damage is characterized by a rough, cinder-like appearance of the eroded surface as shown figure. It is distinctly different from the smooth, polished appearance caused by the erosion of flashing. The previous section describes how cavitation occurs when the vena contracta pressure is less than Pv , and P2 is greater than Pv. Cavitation can be treated by several means.
The first method is to eliminate the cavitation and thus the damage by managing the pressure drop. If the pressure drop across the valve can be controlled such that the local pressure never drops below the vapor pressure, then no vapor bubbles will form. Without vapor bubbles to collapse, there is no cavitation. To eliminate cavitation the total pressure drop across the valve is split, using multiple-stage trims, into smaller portions. Each of these small drops keeps its vena contracta pressure above the vapor pressure so no vapor bubbles are formed.
The second method does not eliminate the cavitation but rather minimizes or isolates the damage much the same as with flashing solutions. This method aims to isolate the cavitation from valve surfaces and to harden those surfaces that the cavitation does impact.
A third method is to change the system in a manner to prevent the causes of cavitation. If the P2 can be raised enough so that the vena contracta pressure does not fall below the vapor pressure, that is the valve is no longer choked, then cavitation will be avoided. P2 can be raised by moving the valve to a location that has more static head on the downstream side. Applying an orifice plate or similar backpressure device can also raise P2 at the valve; the downside is the potential for the cavitation to transfer from the valve to the orifice plate.