When the control valve is selected correctly, the risk of cavitation increases, which will result in higher noise and vibration levels, resulting in very rapid damage to the piping inside the valve and downstream. Additionally, high noise levels often cause vibration and can damage pipes, instruments and other equipment.
Over time, component degradation and cavitation from valves wreaked havoc on piping systems. This damage is mainly caused by vibration and noise energy that accelerate the corrosion process.
The formation and destruction of nearby and downstream bubble contractions caused by large-amplitude vibrations with high noise levels of cavitation reactions. Although this usually occurs in ball valves and rotary valves in the valve body. But it can actually happen in short, high-recovery V-shaped ball valves similar to wafer-style bodies, especially on pipes on the downstream side of butterfly valves.
When the valve is stressed at a certain position, cavitation is easy to occur, so leakage is easy to occur at the pipeline and the welding repair of the valve, and the valve is not suitable for this section of pipeline. Whether the cavitation occurs in or downstream of the valve, equipment in the cavitation zone can be damaged. Ultra-thin film, spring and small-section cantilever structure, large-amplitude vibration can trigger vibration failure.
Common points of failure are instrumentation such as pressure gauges, transmitters, thermometers, flow meters and sampling systems. Actuators, positioners, and limit switches that contain springs will experience accelerated wear, and mounting brackets, fasteners, and connectors will loosen and fail due to vibration.
Prediction of cavitation damage to plug valves is more complicated than simply calculating throttling pressure drop. Experience has shown that it is possible for the pressure in the main liquid flow to drop to the vapor pressure of the liquid before localized gasification and bubble collapse in this region.
Some valve manufacturers predict annual initial corrosion failures by defining the initial damage pressure drop. One valve manufacturer's initiation method for predicting cavitation damage is based on the fact that steam bubbles collapse, causing cavitation and noise. Manufacturers have determined that severe cavitation damage can be avoided if the calculated noise level is below the limit.
Special valve design to eliminate cavitation with split and stepped pressure drop:
"Valve diversion" is to divide the large flow into several small flows, and design the flow path of the valve so that the flow passes through several parallel small openings. The size of the cavitation bubbles is calculated from the openings through which the airflow passes. Smaller openings create small air bubbles, reducing noise and damage.
"Staging pressure drop" means that the valve is designed with two or more regulation points in series, so the entire pressure drop is not accomplished in one step.
Instead, several smaller steps are required. Less than a single pressure drop prevents the pressure in the systolic flow from reducing the vapor pressure of the liquid, thereby eliminating cavitation in the valve.
Combining flow splitting and pressure drop staging within the same valve can improve cavitation resistance in the following ways. During valve retrofits, positioning the control valve with a higher inlet pressure (eg, far upstream or lower altitude) can sometimes eliminate cavitation problems.
Additionally, locating the temperature of the liquid where the control valve is, so the vapor pressure is low (like the heat exchanger on the low temperature side) can help eliminate cavitation problems.