Cavitation and Its Impact on Industrial Equipment

Just as certain sound frequencies can have negative effects on the human body, certain frequencies can also cause severe damage to industrial equipment. When control valves are improperly selected, the risk of cavitation increases, leading to high noise and vibration levels that can rapidly damage the valve internals and downstream piping. Additionally, high noise levels often cause vibrations that can damage pipes, instruments, and other equipment.

As valves degrade over time and parts wear out, cavitation in the valve can lead to serious damage to the piping system. This damage is mainly caused by the vibration and noise energy produced by cavitation, which accelerates the corrosion process. The collapse of cavitation bubbles near the restriction and downstream can create high-amplitude vibrations, leading to significant damage. While this typically occurs in ball and rotary valves within the valve body, it can also happen in short, high recovery-type valves like V-ball valves, particularly on the downstream side of a butterfly valve’s piping.

When a valve is held in a particular position, cavitation is more likely to occur. This can lead to leaks at the welded repair joints in the valve’s piping, making the valve unsuitable for that segment of the pipeline.

Whether cavitation occurs inside the valve or downstream, equipment in the cavitation zone will suffer widespread damage. Components such as thin diaphragm sheets, springs, and small cross-section cantilever structures can experience failure due to high-amplitude vibrations, leading to oscillatory failures. Frequent failure points are found in instruments like pressure gauges, transmitters, thermocouple sheaths, flow meters, and sampling systems. Actuators, positioners, and limit switches that contain springs will experience accelerated wear, and mounting brackets, fasteners, and connectors may loosen and fail due to vibration.

Micrometric Corrosion

Micrometric corrosion, which occurs when wear surfaces are exposed to vibration, is common near cavitation zones. This can produce hard oxide particles that act as abrasives, accelerating wear between surfaces. Affected equipment includes isolation and check valves, as well as control valves, pumps, rotary screens, samplers, and any other rotating or sliding mechanisms.

Cracking and Corrosion of Valve and Pipe Components

High-amplitude vibrations can also cause cracking and corrosion of metal valve parts and pipe walls. The scattered metal particles or corrosive chemicals can contaminate the medium within the pipeline, which has a significant impact on sanitary-grade valves and high-purity pipeline media. This is a situation that must be avoided.

Cavitation Damage Prediction in Plug Valves

Predicting cavitation damage in plug valves is more complex than simply calculating flow restriction pressure drops. Experience has shown that it is possible for the pressure drop in the main liquid flow to reach the vapor pressure of the liquid, leading to local vaporization and the collapse of vapor bubbles. Some valve manufacturers predict the onset of cavitation damage by defining an initial damage pressure drop. One method for predicting cavitation damage is based on the fact that the collapse of steam bubbles causes cavitation and noise. Manufacturers have determined that if the calculated noise level is below the following limits, major cavitation damage can be avoided:

• Valves up to 3 inches: 80 decibels
• 4-6 inch valves: 85 decibels
• 8-14 inch valves: 90 decibels
• Valves 16 inches and larger: 95 decibels

Methods to Eliminate Cavitation Damage

To eliminate cavitation, special valve designs that use flow splitting and staged pressure drops are employed:

• Flow Splitting: This divides a large flow into several smaller flows within the valve’s flow path, with multiple small openings through which the flow passes. The size of cavitation bubbles is partially determined by the size of the openings. Smaller openings produce smaller bubbles, resulting in less noise and less damage.

• Staged Pressure Drop: This means the valve is designed with two or more sequential adjustment points so that instead of experiencing a single large pressure drop, the pressure drop is divided into several smaller steps. Smaller individual pressure drops prevent the fluid’s pressure from dropping below its vapor pressure, thereby eliminating cavitation.

By combining flow splitting and staged pressure drops in the same valve, improved cavitation resistance can be achieved. During valve modification, it is sometimes possible to eliminate cavitation problems by positioning the control valve where the pressure at the valve inlet is higher (such as further upstream or at a lower height).

Additionally, positioning the control valve where the liquid temperature (and thus vapor pressure) is lower (such as on the low-temperature side of a heat exchanger) can help prevent cavitation issues.

Conclusion

As shown, cavitation in valves not only degrades valve performance and causes valve damage, but also poses risks to downstream piping and equipment. Predicting cavitation and taking measures to eliminate it is the only way to avoid the costly valve consumption issues.