What is valve cavitation
What is valve cavitation?
Just as there can be negative effects of sound on the human body, certain frequencies can play a serious damage to 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, resulting in very rapid damage to the valve’s internal and downstream piping. In addition, high noise levels usually cause vibration and may damage pipes, instruments and other equipment.
With the passage of time, the parts of the valve are degraded, and the cavitation caused by the valve is easy to cause serious damage to the pipeline system. This kind of damage is mostly caused by vibration and noise energy, which accelerates the corrosion process. Cavitation reflects the formation and collapse of the contraction flow near and downstream of the bubble caused by the large amplitude vibration of high noise level. Although this usually occurs in ball valves and rotary valves inside the valve body, it can actually occur in a short, high recovery V-type ball valve similar to the wafer body, especially in the downstream side of the valve. When the valve is forced to a certain position, it is easy to produce cavitation, so it is easy to leak in the piping and welding repair of the valve, and the valve is not suitable for this section of pipeline.
Whether cavitation occurs inside or downstream of the valve, the equipment in the cavitation zone will be widely damaged. Ultra thin film sheet, spring and cantilever structure with small cross section, large amplitude vibration can trigger oscillation fault. Frequent fault points are found in instruments, such as pressure gauges, transmitters, thermowells, flowmeters, sampling systems. Actuators, positioners and limit switches containing springs will suffer accelerated wear, mounting brackets, fasteners and connectors will be loose and fail due to vibration.
Fretting corrosion, which occurs between worn surfaces exposed to vibration, is common near cavitation valves. This produces hard oxides as abrasives to accelerate wear between worn surfaces. The equipment affected includes isolation and check valves, in addition to control valves, pumps, rotary screens, samplers and any other rotating or sliding mechanisms.
High amplitude vibration can also cause metal valve parts and pipe wall cracking and corrosion. Scattered metal particles or corrosive chemical materials may pollute the medium in the pipeline, which will have a significant impact on the sanitary valve pipeline and high-purity pipeline medium. This is not allowed.
The prediction of cavitation damage of plug valve is more complex than simply calculating the pressure drop. Experience has shown that it is possible that the pressure in the main stream drops before the vapor pressure of the liquid is local vaporization and bubble collapse in the region. Some valve manufacturers predict annual incipient damage by defining an incipient damage pressure drop. A valve manufacturer’s method of predicting the onset of cavitation damage is based on the fact that steam bubbles collapse, causing cavitation and noise. The manufacturer has determined that significant cavitation damage can be avoided if the calculated noise level is below the following limits.
- Valve sizes up to 3 inches – 80 dB
- 4-6 inch valve size – 85 dB
- 8-14 inch valve size – 90 dB
- 16 inch and larger valve sizes – 95 dB
Methods of eliminating cavitation damage
The special valve designed to eliminate cavitation adopts split flow and staged pressure drop
“Valve shunt” is to divide a large flow into a number of small flows. The flow path of the valve is designed to make the flow pass through a number of parallel small openings. Because 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 used.
“Staged pressure drop” means that the valve is designed to have two or more regulating points in series, so instead of the overall pressure drop in a single step, it takes several smaller steps. Less than the individual pressure drop can prevent the pressure in the contraction flow from decreasing the vapor pressure of the liquid, thus eliminating the cavitation phenomenon of the valve.
The combination of split flow and pressure drop staging in the same valve can achieve improved cavitation resistance in the following ways. In the process of valve modification, if the pressure at the inlet of the control valve is higher (such as far upstream side or at a lower height), cavitation can sometimes be eliminated.
In addition, the positioning of the control valve at the location of the liquid temperature, and therefore the vapor pressure, low (such as the low temperature side of the heat exchanger) can help eliminate cavitation problems.
It has been shown that the cavitation phenomenon of the valve is not only the degradation performance and the damage of the valve. Downstream piping and equipment are also at risk. Predicting cavitation and taking measures to eliminate it is the only way to avoid the problem of expensive valve consumption.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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