Different Problems in control valve – cavitation, flashing

There are different problems associated with the control valve which affects the flow control.

High-pressure drop:

The control valve is an obstruction to the flow as a simple orifice plate. The flows through an orifice will accelerate due to its smaller cross-section.  After leaving the orifice, the flow will gradually return to initial speed and pressure. However, in the actual conditions, each flow through a sudden change in cross section will experience turbulence/absorption which absorbs some of the energy possessed by the flow. Generally, this energy is obtained by decreasing fluid pressure. Some flow configurations produce a low-pressure drop high pressure often causes several valve problems such as cavitation, choked flow, high noise and vibration.

Cavitation:

Cavitation occurs in liquid fluids that flow under the same pressure conditions or under vapour pressure at that temperature. If a fluid before entering a valve is at a pressure of 2 bar, and when breaking through the valve produces a high-pressure drop. Because the fluid path in the valve is quite complex (for example for globe valves), the pressure drop that occurs is also quite high and causes the fluid pressure value to be below its vapour value as a result of cavitation.

Related image

Air bubbles produced by cavitation always cause damage because they move at high speed. The damage will be even greater if the bubble experiences an impulsion (opposite of the explosion) on the valve wall.

Cavitation can be prevented by using components that can prevent the high-pressure drop. One type of component is a backpressure device that functions to localize bubbles so as not to collect and force implantation to occur in capillary pipes/holes such as cavitation control images (a) process, (b) equipment.

Flashing:

In fluid flow, when the fluid pressure is equal to or less than the vapour pressure, there is a steam bubble in the area. If the recovery pressure is above the vapour pressure, cavitation will occur. But if the recovery pressure is still below the vapour pressure, the vapour bubble will persist not to experience impulsion. This condition is known as flashing. Damage caused by flashing is known as erosion, as shown in the picture. Unlike cavitation that can be overcome by adding a component, the control of flashing requires total modification of the system valve.

Noise

One of the most easily recognizable problems in the valve is noise. Noise other than a disruption to the comfort of work can also cause injury/damage to the hearing system. Noise is caused by turbulence after flowing through the valve. Turbulence causes what causes pressure fluctuations and this will cause vibration. The vibration that is at the threshold frequency of human hearing will cause noise such as images of vibration in the pipe that causes noise.

Noise can be suppressed by adding a component that serves to prevent excessive turbulence in the flow. Equipment that serves to reduce noise is called a noise attenuator

 

News Reporter
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