What is valve cavitation? How to eliminate it?

What is valve cavitation? How to eliminate it?

Tianjin Tanggu Water-Seal Valve Co., Ltd

Tianjin，CHINA

19th，June，2023

Just as sound can have a negative effect on the human body, certain frequencies can play 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, resulting in very rapid damage to the internal and downstream pipes of the valve.

In addition, high noise levels usually cause vibration that may damage pipes, instruments and other equipment Valve with the passage of time, the degradation of components, valve cavitation caused by the pipeline system prone to serious damage. This damage is mostly caused by vibration noise energy, accelerated corrosion process and cavitation reflected by the high noise level of large amplitude vibration generated by the formation and collapse of steam bubbles near and downstream of the shrinkage.

Although this usually occurs in ball valves and rotary valves in the body, it can actually occur in a short, high recovery similar to the wafer body part of the V-ball valve, especially butterfly valves on the downstream side of the valve when the valve is stressed in one position prone to cavitation phenomenon, which is prone to leakage in the valve piping and welding repair, the valve is not suitable for this section of the line.

Regardless of whether cavitation occurs inside the valve or downstream of the valve, equipment in the cavitation area will be subject to extensive damage to ultra-thin films, springs and small section cantilever structures, large amplitude vibrations can trigger oscillations. Frequent failure points are found in instruments such as pressure gauges, transmitters, thermocouple sleeves, flowmeters, sampling systems Actuators, positioners and limit switches containing springs will suffer accelerated wear, and mounting brackets, fasteners and connectors will loosen 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. Affected equipment includes isolation and check valves, in addition to control valves,pumps, rotating screens, samplers and any other rotating or sliding mechanism.

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 hygienic valve piping and high purity piping media. This is also not allowed.

The prediction of cavitation failure of plug valves is more complex and is not simply calculated choke pressure drop. Experience suggests that it is possible that the pressure in the main stream drops to the vapor pressure of the liquid before the local vaporization of the area and the collapse of the steam bubble. Some valve manufacturers predict premature eclipse failure by defining an initial damage pressure drop. A valve manufacturer’s method of starting with predicting cavitation damage is based on the fact that steam bubbles collapse, causing cavitation and noise. It has been determined that significant cavitation damage will be avoided if the calculated noise level is below the limits listed below.

Valve size up to 3 inches – 80 dB

Valve size of 4-6 inches – 85 dB

Valve size 8-14 inches – 90 dB

Valve sizes of 16 inches and larger – 95 dB

Methods for eliminating cavitation damage

Special valve design to eliminate cavitation uses split flow and graded pressure drop:
“Valve diversion” is to divide a large flow into several small flows, and the flow path of the valve is designed so that the flow flows through a number of parallel small openings. Since the portion of the size of the cavitation bubble is calculated through the opening through which the flow passes. The smaller opening enables small bubbles, resulting in less noise and less damage when it comes to damage.

“Graded pressure drop” means that the valve is designed to have two or more adjustment points in series, so instead of the entire pressure drop in a single step, it takes several smaller steps. Less than the individual pressure drop can prevent the pressure in the shrinkage from falling vapor pressure of the liquid, thus eliminating the phenomenon of cavitation in the valve.

The combination of diverting and pressure drop staging in the same valve allows for improved cavitation resistance by. During valve modification, positioning the control valve and the pressure at the inlet of the valve is higher (e.g. farther upstream side, or at a lower height), sometimes eliminating cavitation problems.

In addition, positioning the control valve at the location of the liquid temperature, and therefore the low vapor pressure (such as the low temperature side heat exchanger) can help eliminate cavitation problems.

The summary has shown that the cavitation phenomenon of valves is indeed not only about degradation performance and damage to 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 expenses.