What Causes Control Valve Cavitation?

Control valve cavitation is a common concern in liquid service, especially when a valve operates with a high pressure drop. It can cause excessive noise, vibration, trim damage and unstable valve performance.

Cavitation occurs when the pressure of a liquid falls sufficiently low as it passes through a control valve, causing vapor bubbles to form. If the pressure subsequently recovers, these bubbles can collapse violently and damage nearby valve surfaces.

Understanding what causes cavitation is important when selecting and sizing control valves for demanding process applications.

1. What Is Control Valve Cavitation?

Cavitation occurs when the local pressure of a liquid drops to or below its vapor pressure as the liquid passes through a restriction.

Inside a control valve, the highest flow velocity and lowest local pressure generally occur around the vena contracta. If the pressure at this point becomes low enough, vapor bubbles can form.

As the liquid moves downstream, its velocity decreases and pressure begins to recover. If the pressure rises above the liquid's vapor pressure, the vapor bubbles collapse.

This repeated formation and collapse of vapor bubbles is what creates cavitation.

Cavitation occurs only in liquid service. Gases and vapors do not undergo this type of liquid cavitation phenomenon.

2. What Causes Cavitation in a Control Valve?

Several process and valve conditions can contribute to cavitation.

High Pressure Drop

A large pressure drop across the valve increases the possibility that the local pressure at the vena contracta will fall below the liquid's vapor pressure.

This is one of the most important factors in cavitation assessment.

Applications with:

• High inlet pressure
• Low downstream pressure
• Large pressure differential
• High flow velocity

may require special attention during valve sizing and selection.

Low Downstream Pressure

The pressure downstream of the valve affects pressure recovery.

If the process conditions cause a significant pressure reduction through the valve, the local pressure can reach the vapor pressure of the liquid.

High Liquid Temperature

Vapor pressure depends strongly on liquid temperature.

As liquid temperature increases, its vapor pressure generally increases. This means that a liquid may become more susceptible to vapor formation at higher temperatures.

High Flow Velocity

High velocity through the valve increases the pressure reduction associated with the restriction.

Valve geometry, trim design and flow area all influence the resulting velocity and pressure profile.

3. Why Is Cavitation Harmful?

Uncontrolled cavitation can create several problems.

Noise

The rapid collapse of vapor bubbles can generate significant noise. The sound may resemble gravel or particles moving through the valve.

Vibration

Repeated bubble formation and collapse can create vibration in the valve and connected piping.

Severe vibration may affect valve components, actuator mounting and instrumentation.

Trim Damage

Cavitation damage commonly appears as pitting and rough surfaces on valve trim or other components exposed to bubble collapse.

If cavitation is severe, damage may extend beyond the valve into downstream piping.

Reduced Control Performance

Severe cavitation can interfere with stable flow control and may shorten the service life of the valve.

4. How to Check for Cavitation Risk

Cavitation should be evaluated during the control valve sizing process.

Important process information includes:

• Fluid type
• Flow rate
• Upstream pressure
• Downstream pressure
• Operating temperature
• Vapor pressure
• Fluid density
• Valve size
• Valve type
• Required pressure drop

Control valve sizing methods use pressure recovery and related fluid parameters to assess the potential for cavitation and choked flow.

Simply comparing the inlet and outlet pressure is not always sufficient because the pressure at the vena contracta can be substantially lower than the downstream pressure.

5. How Can Control Valve Cavitation Be Reduced?

The solution depends on the severity and operating conditions.

Reduce the Pressure Drop per Stage

Instead of allowing the entire pressure drop to occur in one severe restriction, specialized valve trim can distribute the pressure reduction across multiple stages.

This reduces the local pressure drop and helps control vapor formation.

Use Anti-Cavitation Trim

Specialized anti-cavitation trim can control the pressure and velocity profile inside the valve.

For example, Fisher Cavitrol III trim uses multiple flow restrictions to stage the pressure drop and reduce the damaging effects of cavitation.

For more demanding applications, specialized valves such as the Fisher CAV4 are designed for high-pressure-drop cavitating services.

Review Valve Sizing

An incorrectly sized valve may operate under conditions that increase velocity and pressure drop.

The valve should be sized according to the actual minimum, normal and maximum process conditions rather than simply matching the pipeline diameter.

Consider Valve Geometry

Valve body and trim geometry affect pressure recovery.

High-recovery valve designs can be more susceptible to cavitation under certain conditions because the pressure at the vena contracta may fall significantly.

6. Cavitation vs Choked Flow

Cavitation and choked flow are related, but they are not exactly the same thing.

Choked liquid flow occurs when increasing the pressure drop no longer produces a corresponding increase in flow rate.

Cavitation can occur as part of this high-pressure-drop condition when the local liquid pressure falls to the vapor pressure and vapor bubbles form.

Therefore, a valve operating near a choked-flow condition requires careful evaluation of cavitation risk.

7. Common Applications Where Cavitation May Occur

Cavitation is more likely to become a concern in applications involving high pressure drops, including:

• Boiler feedwater
• Pump discharge control
• High-pressure liquid letdown
• Oil and gas processing
• Petrochemical plants
• Chemical processing
• Power generation
• High-pressure water systems

Severe-service control valves are commonly used in applications involving high pressure drop, cavitation, erosion, vibration and other demanding conditions.

8. What Information Should Be Provided for Valve Selection?

If cavitation is suspected, provide the following information when requesting a control valve quotation:

• Fluid / liquid type
• Minimum, normal and maximum flow
• Inlet pressure
• Outlet pressure
• Operating temperature
• Vapor pressure
• Specific gravity or density
• Pipe size
• Valve size
• Required control range
• Existing valve model, if applicable

This information allows the supplier or valve engineer to evaluate the pressure profile and determine whether standard trim or an anti-cavitation solution is required.

Conclusion

Control valve cavitation is mainly associated with liquid service, high pressure drop and low local pressure.

The basic process is:

High flow velocity → Low pressure at vena contracta → Vapor bubbles form → Pressure recovers → Bubbles collapse → Noise, vibration and damage

Preventing severe cavitation starts with correct valve sizing and process evaluation. Depending on the application, solutions may include staged pressure reduction, specialized anti-cavitation trim, suitable valve geometry and appropriate materials.

We supply industrial control valves and valve positioners for demanding process applications, including solutions from Fisher, Samson, Masoneilan, Flowserve, Koso and other manufacturers.

If you are dealing with valve noise, vibration or repeated trim damage, provide the valve model and process conditions for a configuration review.