Cavitation in Control Valves

Cavitation in Control Valves

Fisher anti-cavitation trims for control valves help eliminate or control cavitation damage in clean and dirty service.

Control valve plug with cavitation damage

Understanding Cavitation in Control Valves

Cavitation is the formation and collapse of vapor bubbles (cavities) in the liquid  flowstreams caused by changes in pressure and velocity. There are four primary negative side effects of uncontrolled cavitation in control valves: high noise, excessive vibration, material damage, and deterioration of flow effectiveness. Physical damage to valve trim is usually characterized by a pitted, rough appearance.

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Principles of Control Valve Cavitation

Definition

Cavitation in control valves occurs only with liquid flows—gases cannot cavitate.

Vapor bubbles form if the liquid’s pressure falls to near the vapor pressure as the liquid passes through the control valve. Bubbles can suddenly collapse or implode as the pressure increases down line, producing cavitation.

Cavitation is one of the results of choked flow. It is the point at which increasing the pressure drop while maintaining a constant inlet pressure yields no further increase in flow rate.

Damage

Cavitation damage is characterized by a round, cinder-like appearance of the eroded surface. High-speed, destructive microjets and localized shock waves attack the material surface of the control valve during the vapor-to-liquid phase change.

Cavitation doesn’t always cause damage when it occurs.

The extent of cavitation damage is a function of numerous factors:

  • Intensity/magnitude of pressure drop
  • Materials of construction
  • Time of exposure
  • Quantity of flow
  • Valve/trim design
  • Leakage while closed
  • Fluid

Noise and Vibration

Cavitation can cause unacceptable noise and vibration. It is frequently described as sounding like gravel flowing through the control valve.

Although the noise associated with a cavitating liquid can be quite high, it is usually a secondary concern when compared to the material damage that can exist.

Vibration resulting from cavitation may be more prevalent than cavitation damage. It can affect the valve positioner, linkage, tubing, and adjacent pipe.

Damaging cavitation can take place without noise, and the level of noise and vibration from a control valve may not always coincide with the cavitation damage level.

Cavitation Control

Cavitation in control valves can be treated by several means. The first is to eliminate the cavitation and thus the damage by managing the pressure drop. To eliminate cavitation the total pressure drop across the control valve is split, using multi-stage trims, into smaller portions as shown in the image.

The second method does not eliminate the cavitation but rather minimizes or isolates the damage. The aim is to isolate the cavitation from control valve surfaces and to harden those surfaces that the cavitation does impact.

The third is to change the system in a manner to prevent the cause of the cavitation.

Emerson Testing

Emerson engineers conduct noise and vibration tests on cavitating applications. Transparent pipe allows visualization of the cavitation field. To arrange a cavitation demonstration, contact a local Emerson sales office.

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