How Flashing Takes Place in a Control Valve

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When a control valve is used for liquid flow application, a phenomenon known as flashing can occur under certain flow conditions. Just like in the process of cavitation if the liquid velocity through the valve is high enough, the pressure in the liquid may drop to a level where the fluid may start to bubble or flash. Just as in the case of Cavitation, we can model the control valve as an orifice plate and then use the changes in pressure and velocity to explain flashing. The schematic below shows the changes in fluid pressure and velocity during flashing:

Flashing in a Control Valve

As shown in above, when the downstream pressure of a control valve falls below the liquid vapour pressure, part of the liquid is vaporized and remains as vapour downstream of the valve. Flow downstream of the control valve is part liquid and part vapour. The resulting flow is choked and therefore a decrease in downstream pressure does not increase the flow rate.

Flashing flow may cause mechanical difficulties, like erosion and vibration but unlike cavitation, the reason is the high velocity of the two-phase flow stream. High velocity is due to larger vapour volume compared with liquid. High velocity can be very erosive and it is recommended that more resistant materials be used where there is a possibility of flashing in the valve.

Controlling Flashing Damage in Control Valves
The strategies below are used to minimize flashing damage in control valve applications:

• Using hardface trim materials such as Stellite or Tungsten
• Using more erosion resistant body materials
• Increasing the size of valve thus reducing velocity
• Using angle valve i.e. flow over plug