Flashing and Cavitation in control valve

Let us understand the basic difference between vapor pressure and liquid pressure.

The vapor pressure of a liquid varies with temperature. As the temperature of liquid increases, its vapor pressure also increases. Conversely, vapor pressure decreases with temperature.

Under the condition of constant temperature, a change in pressure results in a transition from one phase to another. When local pressure decreases between the vapor pressure of the liquid, vaporization begins.  

FLASHING

In the process industry, if liquid local pressure does not recover above the liquid pressure, the liquid will remain in the vapor state. This process is called FLASHING.

When a liquid flowing through a valve and it encounters a restriction like a reduced port, it accelerates to a higher velocity to retain constant volumetric flow rate.

If the local pressure within the restricted flow area drops below the vapor pressure of the liquid which is a condition called vena contract, vaporization of fluid occurs ( vapor bubbles would form in the liquid). if the downstream pressure remains below the vapor pressure of the fluid, the process condition is called flashing.

 The outlet stream is going to be in a prevailing vapor phase.

CAVITATION

Cavitation is a phenomenon that occurs in control valves. This causes severe damage to control valves. Shortening their life span.

Cavitation occurs in control valves in liquid media applications. When speedily approaching liquid while passing through control valve vena contract, pressure drop takes place. This low pressure causes bubbles to form and then suddenly collapse.

The first liquid evaporates to vapor. In control valves, if pressure drops below the vapor pressure of the liquid, then bubbles will form. These bubbles interrupt the continuous flow.

As the pressure recovers, the bubbles collapse suddenly. In this transformation, the collapsing bubbles cause damage to the valve.

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If the vapor pressure of the fluid is below the upstream pressure, above the vena contract pressure and below the downstream pressure, vapor bubbles form as pressure drops. In this case, as pressure recovers the bubbles can suddenly collapse or implode a condition known as cavitation. Cavitation is often energetic and has great potential to damage valves and other piping.