What actually mean by Flow under and Flow over control valve? Is there any typical application of this or i can use any type?
>What actually mean by Flow under and Flow over control
>valve? Is there any typical application of this or i can use
This might help:
Fisher Control Valve Handbook
They might be referring to flow up or flow down through the valve body and trim.
In a globe valve, you can choose to flow the fluid over or under the plug. this is meant by flow over or flow under, taking reference to media over/under the plug. usually it is application dependent; e.g. fail open/close, mild cavitation etc.
If its non-severe service, if it is a fail close, choose flow over. you're likely to get a smaller actuator sizing due to fluid assist. fail open, flow under.
If there is mild cavitation potential and you do not wish to use severe service trims, go for flow under. the recovery factor will likely eliminate the mild cavitation. harden trims for better longevity.
speak to a manufacturer if in doubt. their applications should be able to answer your queries.
Most often Flow Under is chosen so that when the valve is closed it cannot leak to the environment. I have often seen steam fitters replace the packing in a manual valve by closing it and fishing out the old packing with a hook. You couldn't do that with Flow Over.
Others have pointed out the cavitation advantage.