Dear Piyush

Velocity Profile through Control Valves -As a liquid travels through a control valve, a ‘vena contracta’ (point of narrowest flow restriction) develops directly downstream of the throttling point. The flow area at this point is smaller than the rest of the flow path. As the flow area constricts, the velocity of the fluid rises. After the fluid passes the vena contracta the velocity drops again. Velocity through a Control Valve, for a velocity profile through a conventional single seated globe-style control valve.

If the speed through the valve is increases enough, the pressure in the fluid drops to a level where the fluid may start to boil, bubble or flash. And when the pressure recovers sufficiently the bubbles w. Cavitation may be noisy but is usually of low intensity and low frequency. This situation is extremely destructive and may wear out the trim and body parts of a valve in short time.

The ideal solution to cavitation is to reduce the pressure from inlet to outlet gradually, thus avoiding a large pres-sure drop at the vena contracta. Cavitation can be avoided entirely by not permitting the pressure to fall below the vapor pressure, thereby eliminating any bubble formation and subsequent collapse. Gradual Pressure Reduction Profile, for an illustrated example of cavitation elimination. Another solution that can be used for lower levels of cavitation involves controlling or dissipating the energy of the imploding bubbles by isolating them away from the metal surfaces. This greatly reduces the amount of energy that the exposed surfaces of a valve need to absorb, allowing the components to resist damage.

Pressure Profile Through Control Valves-Sigma –The Cavitation Index

Various cavitation indices have been used to correlate performance data to improve designs of hydraulic process equipment. A cavitation index, called Sigma (σ), has been developed and applied to quantify cavitation in control valves. Sigma represents the ratio of the potential for resisting cavity formation to the potential for causing cavity formation. This cavitation index is defined as follows

 σ = (P1-Pv)/ (P1-P2)

Where

P1 = Upstream pressure, measured two pipe diameters upstream from the valve

P2 = Downstream pressure, measured six pipe diameters downstream from the valve

PV = Vapor pressure of the liquid at flowing temperature

 A general rule of thumb is that you will have choking if Pin/dP is greater than 2. Of course, choking will also be a function of the valve geometry: ball valves are easier to choke than globe valve. Valve choking occurs when the pressure drop across the valve is very large. For compressible fluids, the gas enters at the inlet of the plug and its volume expands to a pressure below the outlet pressure (vena contracta). The specific volume then recompresses to the valve outlet pressure. Choking happens when the expansion is so large that the outlet pressure has no more effect on the throughput flow (large ∆P). The only variables having an effect at chocked conditions will be the inlet pressure and the valve position: changes at the outlet pressure will have no effect on flow. Disturbance downstream of the valve, the inlet pressure increases, the ∆P decreases, the flow throughput decreases, the disturbance is mechanically caught by the valve fluid dynamics before the controls start working. Choked valve: Disturbance downstream of the valve, the inlet pressure increases, the ∆P decreases, the flow through put does not decrease, the disturbance is not mechanically caught by the valve fluid dynamics and the controls have to do all of the job :large downstream pressure excustions. you might be okay. However, if the disturbances are fast the valve selection is critical to your process stability.

Regards,

Prem Baboo