Control Valve Flashing vs Cavitation: What Is the Difference?

Flashing and cavitation are two common fluid phenomena associated with pressure drops across control valves. Both can occur when a liquid passes through a valve and its local pressure falls to or below the liquid's vapor pressure.

Because both conditions involve vapor bubbles, they are sometimes confused with each other.

The key difference is what happens to those bubbles after the liquid leaves the valve.

In flashing, the vapor bubbles remain in the downstream flow because the downstream pressure stays below the liquid's vapor pressure.

In cavitation, the pressure recovers above the vapor pressure and the vapor bubbles collapse.

1. What Is Flashing?

Flashing occurs when a liquid vaporizes as it passes through a control valve and remains partially vaporized downstream.

The process begins when the local pressure falls to or below the liquid's vapor pressure.

If the downstream pressure remains below the vapor pressure, the vapor bubbles do not collapse.

Instead, the liquid-vapor mixture continues downstream.

A control valve application is generally considered flashing when:

P₂ < Pᵥ

where:

• P₂ = downstream pressure
• Pᵥ = liquid vapor pressure

Flashing can cause significant erosion because the high-velocity liquid-vapor mixture can continuously impact valve and piping surfaces.

2. What Is Cavitation?

Cavitation begins in a similar way.

As liquid passes through the restriction inside a control valve, velocity increases and local pressure decreases.

If the pressure at the vena contracta reaches the liquid's vapor pressure, vapor bubbles can form.

The difference occurs downstream.

If the liquid pressure recovers above the vapor pressure, the bubbles collapse.

This collapse releases energy and can produce:

• Noise
• Vibration
• Pitting
• Trim damage
• Reduced valve performance

Cavitation damage is often characterized by a rough, pitted or irregular surface.

3. Flashing vs Cavitation

The distinction depends primarily on the relationship between downstream pressure and vapor pressure.

4. Why Does Flashing Cause Erosion?

When flashing occurs, the liquid changes into a liquid-vapor mixture.

The mixture can reach high velocity through and downstream of the valve.

Unlike cavitation, the vapor bubbles do not repeatedly collapse against the valve surface because the downstream pressure remains below the vapor pressure.

The main concern is therefore continuous erosion from the high-velocity two-phase flow.

Flashing damage can have a relatively smooth and polished appearance, which differs from the rough pitting commonly associated with cavitation.

5. Why Does Cavitation Cause Pitting?

In cavitating service, vapor bubbles form when the local pressure becomes sufficiently low.

As pressure recovers downstream, the bubbles collapse.

If collapse occurs close to a metal surface, the energy released during bubble collapse can cause localized damage.

Repeated collapse can produce a rough, pitted surface on valve trim.

In severe cases, the damage can extend into downstream piping.

6. How Do You Identify Flashing or Cavitation?

A few practical signs can help identify the problem.

Possible Signs of Cavitation

• Loud noise
• Strong vibration
• Pitted valve trim
• Repeated trim failure
• Unstable control
• Damage near the pressure recovery area

Possible Signs of Flashing

• Continuous erosion
• Smooth or polished damaged surfaces
• Two-phase flow downstream
• High downstream velocity
• Erosion of valve outlet and downstream piping

However, visual inspection alone is not enough to determine the exact flow condition.

Process pressure, temperature, flow rate and liquid vapor pressure should be reviewed.

7. How Can Cavitation Be Controlled?

Because cavitation is related to pressure recovery and bubble collapse, the objective is to reduce or control the conditions that allow damaging bubble collapse.

Possible approaches include:

Staged Pressure Reduction

Specialized trim can divide the pressure drop into multiple stages.

Anti-Cavitation Trim

Cages and flow passages can be designed to control velocity and pressure recovery.

Proper Valve Sizing

Correct sizing helps avoid unnecessarily high velocity and excessive pressure drop.

Appropriate Valve Selection

Valve body and trim geometry can have a major effect on pressure recovery and cavitation risk.

For example, Fisher Cavitrol III trim is designed to stage pressure reduction in cavitating applications.

8. How Can Flashing Damage Be Reduced?

Flashing cannot simply be eliminated by changing the control valve because it is determined by the process conditions.

If the downstream pressure remains below the liquid vapor pressure, flashing can occur.

The objective is therefore to manage the resulting erosion.

Common approaches include:

• Selecting erosion-resistant materials
• Reducing local fluid velocity
• Using suitable valve geometry
• Protecting vulnerable valve surfaces
• Increasing downstream flow area
• Selecting appropriate severe-service valve designs

Emerson notes that flashing is a system condition and recommends selecting valve geometry and materials that minimize erosion rather than attempting to prevent the flashing itself.

9. Can the Same Valve Be Used for Flashing and Cavitation?

Not necessarily.

The two conditions create different damage mechanisms, so the valve should be selected according to the actual process condition.

For cavitation, the design objective is generally to control pressure drop and prevent or reduce damaging bubble collapse.

For flashing, the design objective is more focused on managing high-velocity two-phase flow and minimizing erosion.

Severe-service control valves can be designed for specific combinations of cavitation, flashing, erosion, high pressure drop and high velocity.

10. What Process Data Is Needed?

When evaluating flashing or cavitation, provide:

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

For an existing valve, also provide the valve model and trim information if available.

This allows the valve supplier to evaluate the actual pressure conditions instead of diagnosing the problem based only on noise or visible damage.

Conclusion

Flashing and cavitation start from a similar condition: the liquid pressure falls sufficiently low for vapor bubbles to form.

The main difference is what happens afterward:

Flashing:

Vapor forms → Downstream pressure remains below vapor pressure → Vapor remains in the flow

Cavitation:

Vapor forms → Downstream pressure recovers → Vapor bubbles collapse

This difference leads to different damage mechanisms.

Flashing is mainly associated with erosion from high-velocity liquid-vapor flow, while cavitation can cause severe pitting, noise and vibration from collapsing vapor bubbles.

Correct valve sizing, accurate process data and appropriate valve or trim selection are important for both conditions.

For severe-service applications, we can supply control valves and specialized valve solutions from Fisher, Flowserve, Masoneilan, Samson, Koso and other manufacturers.

If you are experiencing valve erosion, noise or vibration, provide the process conditions and existing valve model for a technical review.