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What is Cavitation Phenomenon in Valve
Created on 03.20
What is Cavitation Phenomenon
Just as sound can have a negative impact on the human body, certain frequencies can wreak havoc on industrial equipment. When the control valve is properly selected, there is an increased risk of cavitation, which will lead to high noise and vibration levels that can cause very rapid damage to the valve's internal and downstream pipes. In addition, high noise levels often cause vibrations that can damage pipes, instruments, and other equipment.
With the passage of time, the valve degrades its components, and the cavitation generated by the valve causes serious damage to the piping system. This damage is mostly caused by vibration and noise energy, which accelerates the corrosion process. The formation and collapse of the constriction flow near and downstream of the bubble generated by the vibration of large amplitude of high noise level reflected by cavitation. While this usually occurs with ball valves and rotary valves in the valve body, it can actually occur on a short, high-recovery wafer-like body section of the V-shaped ball valve, specifically the downstream side of the valve piping of the butterfly valve. When the valve is stressed in a position, it is easy to produce cavitation, which is easy to leak in the piping and welding repair of the valve, and the valve is not suitable for this section of the pipeline.
Whether cavitation occurs inside or downstream of the valve, equipment in the cavitation zone can be extensively damaged. Ultra-thin sheet, spring and small section cantilever structure, large amplitude vibration can excite oscillation faults. Frequent fault points are found in instrumentation such as pressure gauges, transmitters, thermowells, flow meters, sampling systems. Actuators, positioners and limit switches containing springs will suffer accelerated wear and tear, and mounting brackets, fasteners and connectors will loosen and fail due to vibration.
Micro-corrosion, which occurs between wear surfaces exposed to vibration, is common in nearby cavitation valves. This creates hard oxides that act as abrasive wear between the wear surfaces. Affected equipment includes isolation and check valves, in addition to control valves, pumps, rotating screens, samplers, and any other rotating or sliding mechanisms.
High amplitude vibrations can also crack and corrode metal valve parts and pipe walls. Scattered metal particles or corrosive chemical materials can contaminate the media in the pipeline, which can have a significant impact on sanitary valve pipes and high-purity pipeline media. This is also not allowed.
The prediction of cavitation failure of plug valves is more complex and is not a simple calculation of the blocking pressure drop. Experience suggests that it is possible that the pressure in the main fluid stream drops to the point where the vapor pressure of the liquid is local vaporization and the collapse of the vapor bubble in the area. Some valve manufacturers predict early erosion failure by defining an initial damage pressure drop. A valve manufacturer's method of predicting cavitation damage is based on the fact that it is the collapse of vapor bubbles, causing cavitation and noise. The manufacturer has determined that significant cavitation damage can be avoided if the calculated noise level is below the limits listed below.
Valve size up to 3" - 80 dB
Valve size 4-6 inches - 85 dB
8-14 inch valve size - 90 dB
Valve size 16" and larger - 95 dB
Methods to eliminate cavitation damage
Special valves designed to eliminate cavitation use split flow and graded pressure drop:
"Valve shunt" is to divide a large flow into several small flows, and design the flow path of the valve to make the flow flow pass through several parallel small openings. Since the size of the cavitation bubble is calculated by the opening through which the flow passes. Smaller openings make small bubbles, resulting in less noise and less damage when
"Graded pressure drop" means that the valve is designed to have two or more adjustment points connected in series, so instead of the entire pressure drop in a single step, it takes several smaller steps. Less than an individual pressure drop prevents the pressure in the constriction from the vapor pressure of the liquid falling, thus eliminating valve cavitation.
The combination of shunt and pressure drop staging in the same valve allows for improved cavitation resistance by: During valve modification, the pressure at the inlet of the control valve is higher (e.g. on the farther upstream side, or at a lower altitude), which can sometimes eliminate cavitation.
In addition, positioning the control valve at the location of the liquid temperature and therefore the vapor pressure, low (such as the heat exchanger on the cryogenic side) can help eliminate cavitation problems.
The summary has shown that the cavitation phenomenon of valves is indeed not only about degrading performance and damaging valves. Downstream pipelines and equipment are also at risk. Predicting cavitation and taking steps to eliminate it is the only way to avoid the problem of expensive valve consumption.
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