Hi All,

I am trying to understand what is wrong with a control valve that does not seem to seal properly since a recent rebuild (see attached image). The valve is a globe valve that is being used to control the level in a Knock Out Pot. The control scheme is On/Off.
Previously this valve would open and close every hour or so, but now the close command is given, but the level continues to fall for hours sometimes hitting the LL trip point. Eventually, the valve seals up and the level rises again, but not in the linear way it would previously. When the LL trip is hit the level rises just as quickly as it used to in a linear way.

During a previous rebuild the decision was made by the team on site to modify the plug to shorten the long nose by cutting it. The logic was that there was dirt getting clogged in the outlet due to the long nose, resulting in erosion of the seat and poor sealing performance (after two years operation). Performance after this modification was normal.

After the most recent rebuild no modifications were made to the plug, but the performance is now much worse.

Do you think I should plan in for the valve to be pulled out again and inspected?
Is there an issue with the air pressure regulator?

 

Check your controller settings

 

Hi Dave,
Could you please elaborate a bit more on which settings to check?

 

Make a sketch of your control circuit, with make and model of the controller together with intermediate components including transmitter and any pneumatic transducers to drive the valve.

 

Hi Dave,
Thanks for your help, here is the sketch.

 

Bump

 

as long as you have proper air supply pressure and the valve piston actuator has been tested, the limitation must be in the valve stem friction of the valve.

 

Hi All,

I am trying to understand what is wrong with a control valve that does not seem to seal properly since a recent rebuild (see attached image). The valve is a globe valve that is being used to control the level in a Knock Out Pot. The control scheme is On/Off.
Previously this valve would open and close every hour or so, but now the close command is given, but the level continues to fall for hours sometimes hitting the LL trip point. Eventually, the valve seals up and the level rises again, but not in the linear way it would previously. When the LL trip is hit the level rises just as quickly as it used to in a linear way.

During a previous rebuild the decision was made by the team on site to modify the plug to shorten the long nose by cutting it. The logic was that there was dirt getting clogged in the outlet due to the long nose, resulting in erosion of the seat and poor sealing performance (after two years operation). Performance after this modification was normal.

After the most recent rebuild no modifications were made to the plug, but the performance is now much worse.

Do you think I should plan in for the valve to be pulled out again and inspected?
Is there an issue with the air pressure regulator? View attachment 3104

Thanks for posting this,
First I have comment about the design of select Knock Out Pot drain valve type globe and not butterfly type!? In such process normally content of heavy material, dirty.
Back to your concern, cutting the plug end to shorten to avoid the problem is wrong idea consequences valve passing and maybe it got stuck in closed position, check also the SOV operation for the coil and the body insure air is passing when SOV energized. If everything fine. You have to remove the actuator from the body and inspect the seat and body.
To have no more issue with this valve in future, good to change the valve with butterfly valve.