HERO - Control Valve Flashing - H003

Control Valve Flashing

Reduce maintenance costs with Fisher control valves that minimize the effects of metal erosion damage caused by flashing applications.

Control valve plug with flashing damage X1552 small

Understanding Flashing in Control Valves

Flashing occurs when liquid flows through a control valve and  vaporizes, remaining a vapor. Flashing shares some common features with choked flow and cavitation in that the process begins with vaporization of the liquid in the vicinity of the vena contracta. For a control valve application to be considered a flashing application, the downstream pressure must be less than the vapor pressure (P< PV). Flashing is a concern because it limits flow through the control valve and the liquid-vapor mixture is highly erosive. Metal erosion caused by flashing is smooth and shiny in appearance. Since flashing cannot be prevented by the valve, it’s best to select a valve with proper geometry and materials to avoid or minimize damage.

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Principles of Control Valve Flashing

Damage Potential

Flashing is a direct result of application conditions and fluid properties. This means there is no way for any control valve to prevent flashing. Since flashing cannot be prevented by the valve, the best solution is to select a valve with the proper geometry and materials to avoid or minimize damage.

There is no erosion coefficient, no industry standards, and no scientific means for predicting the intensity of flashing damage. However, Emerson understands that flashing damage potential is a function of many factors which include flow rate, fluid velocity, pressure drop, corrosive nature of fluid, body geometry, and valve body and trim material.

How to Minimize Flashing Damage

In general, erosion is minimized by:

  • Preventing or reducing the particle (liquid droplets in this case) impact with the valve surfaces
  • Making those surfaces as hard as possible
  • Lowering the velocity of the erosive flow

Emerson engineers worldwide are trained to recognize flashing problems and address them in the most effective manner.

Control Valves for Flashing Applications

Selecting a valve with as few fluid directional changes as possible provides the least number of particle impacts. Sliding-stem angle valves are traditional solutions which provide such a flow path. Some rotary valves, such as eccentric rotary plug, and segmented ball valves, also offer straight-through flow paths.

Valves with expanded flow areas downstream of the throttling point are beneficial because the erosive velocity is reduced. For those areas where the fluid must impact the valve surfaces, at the seating surfaces for example, choose materials that are as hard as possible. Generally the harder the material the longer it will resist erosion.

Publications and Videos

Learn more about valve cavitation and some of the technical solutions using Fisher products.

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