I'm carrying out a research for my Company on Joule-Thomson valves as severe service, and what are the most common problems experienced in this application. helpful information are : what kind of plant is the valve installed, trim style, design style and of course nature of the problem you are experincing (noise, vibration, plug sticking, clogging, thermal binding...), and how you fixed it (if you did!). if you have any recommendations regarding valve manufacturers, or the experience you have with the currently installed one it would be great! 

The only problem is getting the split control signal between the JT valve and expander.  Most people stay with a Fisher ED body valve with the proper sized trim.  Because the plants are outside and the JT valve isn't used much, noise dosen't enter the picture.  Besides, the expander is much more noisier.

Check into extended bonnets for cold services.  Many Fisher representatives have engineers familiar with the JT valves.

Some people use variable chokes in a similar non-expander application.

Use individual controller outputs to avoid split ranged positioners, etc.  If you have independent PID controller blocks you can also tune differently for the expander and JT control.

You ask what are the most common problems experienced in a Joule-Thomson valve application and after many, many years of operating with them I can recall the following:

1) The basic problems that appear during their use are due to a contaminated fluid;
2) External water ice formation;
3) Valve packing leaks; and,
4) Operator mechanical abuse.

You avoid #1 by having a clean, filtered, and dry fluid;  this, of course, means that you must have dehydrated the gas well below the recommended dew point.  Otherwise, water ice will plug the valve.  This also applies to other components, such as CO2, which could form dry ice upon expansion and subsequently plug up too.  The cleaner and purer the fluid, the easier and safer the operation.

#2 is a no-brainer; you should, as jsummerfield recommends, be using nothing but extended stems on the J-T valves.  Otherwise, the hard water ice formation on the valve is a target for mechanical abuse by the use of torches, wrenches, or even hammers in the attempt to remove the ice cover to make the valve(Gate Valves) operable.

#3 is resolved by a proven extended stem design and religious stem packing maintenance.

#4 is applied by furnishing a reliable, safe design to your operators and training them on the importance of dealing with such a delicate instrument with care and intelligence.

The rest should be a safe and reliable operation.  I have never experienced excessive or loud expansion noise.  But then, I always made sure I had a liberal "cold box" design around the valve and its piping.  Sheetmetal and insulation are cheap and help combat moisture and noise.

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