Expander/ Joule Thomson operation

[breizh29]

Hey all,
Here is my question:
A turbine expands a 170bG dry gas to 82 bG. The expander work is recovered by a direct connected compressor boosting the pressure of the separator gas to 125 bG. The expander inlet Pr and the compressor outlet Pr are controlled at 170 bG and 125 bG respectively. The interpressure is 82 bG. The gas can also be expanded over the Joule Thomson valve. In this case the Pr drops to the fixed 125 bG. Less liquid is knocked out than with the expander operation. When the expander shutdown is operated by a trip the JT valve which is already in a fully closed position will open widely as per the controller.

Question : Wouldn't this sudden opening of the JT valve cause an upset in the system and what might be the consequence of this upset condition?

For one plant The outlet Pr of the compressor/JT valve is controlled by a gas turbine driven HP compressor, for the other plant the aforesaid pressure is controlled by a back pressure controller installed next to the exit of the plant.
I would appreciate if you had some experiences or you could give me any tips?
Thanks
nicolas

 

[jsummerfield]

You loose the greater refrigeration from the expander when it trips as you open the JT valve. Thus, less gas is condensed. The controller tuning may be different with the expander than with the JT valve. Some control systems may permit loading different controller tuning constants base upon the expander running status. However, I do not think that it is a real issue.

John