Control valves leakage class VI

Dears;
we have recently change the plug and seat of a globe valve (gas CH4; leakage class VI).
the DP pressure in the data sheet is 39 bars.
once put in service the pressure transmitter downstream the valve is showing 2.5 bars, even if the valve is in close position, pressure upstream is 41.5 bars.
is it normal ? for your info, the manual valve downstream the valve is in close position.
what is the downstream pressure tolerated in this case?
 
Have you tested the valve on full pressure in workshop after replacement of Plug & Seat? I suggest following;
1. Sometimes indicator showing 100% close but valve is still minor open
2. If Hand Jack available try to close valve on hand jack and check pressure at downstream transmitter
3. You may increase actuator pressure if you find improvement by above action
4. Is there any Bypass valve in the circuit? Check its condition too
 
Have you tested the valve on full pressure in workshop after replacement of Plug & Seat? I suggest following;
1. Sometimes indicator showing 100% close but valve is still minor open
2. If Hand Jack available try to close valve on hand jack and check pressure at downstream transmitter
3. You may increase actuator pressure if you find improvement by above action
4. Is there any Bypass valve in the circuit? Check its condition too
Dear Samran;
thank you for the reply.
1. the valve has been tested without positioner. the force applied on the the plug is from actuator springs. during this test (operating pressure 41.5 bars, air) we saw air bubbles. I have checked the ANSI standard FCI 70-2: Control Valve Seat Leakage it was written "50 psig or max rated differential pressure across valve plug whichever is lower. " means pressure to be applied on the valve during the test; means maximum 3.5 bars. if I am right !! we put this value on the valve, no leakage. but with 41.5 bars , leakage.
2. there no hand jack.
3. I will check the pressure.
4. P&ID checked, no bypass.
 
Dear Samran;
thank you for the reply.
1. the valve has been tested without positioner. the force applied on the the plug is from actuator springs. during this test (operating pressure 41.5 bars, air) we saw air bubbles. I have checked the ANSI standard FCI 70-2: Control Valve Seat Leakage it was written "50 psig or max rated differential pressure across valve plug whichever is lower. " means pressure to be applied on the valve during the test; means maximum 3.5 bars. if I am right !! we put this value on the valve, no leakage. but with 41.5 bars , leakage.
2. there no hand jack.
3. I will check the pressure.
4. P&ID checked, no bypass.
Increase the actuator pressure manually through positioner by varying output pressure. If no positioner then thru regulator and check leakage behavior
 
Dear Samran;
thank you for the reply.
1. the valve has been tested without positioner. the force applied on the the plug is from actuator springs. during this test (operating pressure 41.5 bars, air) we saw air bubbles. I have checked the ANSI standard FCI 70-2: Control Valve Seat Leakage it was written "50 psig or max rated differential pressure across valve plug whichever is lower. " means pressure to be applied on the valve during the test; means maximum 3.5 bars. if I am right !! we put this value on the valve, no leakage. but with 41.5 bars , leakage.
2. there no hand jack.
3. I will check the pressure.
4. P&ID checked, no bypass.
If the downstream gate valve is closed properly without any leakage, then over a period of time the cavity between CV and Gate Valve should be pressurised to the same value as the upstream of Control Valve. Have you checked for enough period of time to let this happen? I donot think the cavity will still remain pressurised to 3.5 bar as you have indicated.
If it remains pressurised to 3.5 bar for a long period of time then continue with your operation. There is no issue with the CV. The pressure shown by the gauge is some residual pressure which might have been stuck there while closing the valve.
 
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