3 replies to this topic

[PENGG]

What is the effect of max shutoff pressure on control valve ? How it is important for control valve ?

[Steve Hall]

If the direction of flow is such that the valve could open with excessive pressure then this shutoff pressure is of practical value. Consider a globe valve with conical plug. If the direction of flow is toward the opening plug (usually up), then the shutoff pressure is related to the actuator and pressure of the instrument air. The supply is fighting the instrument air, and like an arm wrestling match the stronger wins. On the other hand, if the flow is down through the valve, in the direction of the plug closing, then shutoff pressure should be the same as the pressure rating of the valve body. However, for very tight shutoff specifications, the pressure may still be important since leakage may increase with increased pressure.

Butterfly and ball valves should be unaffected by shutoff pressure, but diaphragm valves, where the actuator controls the tightness of the valve seat, do have a maximum shutoff pressure.

[fallah]

PENGG,

The max shut off pressure of a control valve is the max differential pressure (across the valve) that relevant actuator could still withstand the closed position. Then it pointed out beyond that max differential pressure value, the valve might not be still in closed position.Valve manufacturers usually would specify the the max delta P for valves actuators.

[kkala]

In the process control valve data sheet (basic engineering) we generally fill the space after "max delta P" with max shut off pressure (gauge) of the relevant (centrifugal) pump; this on the ground that the valve may be closed while downstream line is empty, or its pressure is close to zero. This value affects the choice of actuator.
Note: Supplied instrument air can have lower pressure, seeing that force=pressure×surface.
Above concerns the common case seen (globe valve, unbalanced plug, Fig 8-65 in Perry 7th ed). Post No 2 by Steve Hall gives a wider picture.

Additional note: Following can be useful on flow direction of control valves (they can operate with reverse flow too).
http://cr4.globalspec.com/thread/32095 '> http://cr4.globalspec.com/thread/32095 .

Edited by kkala, 29 December 2012 - 04:59 PM.