To control the downstream temperature of a JT valve by opening or closing the valve itself.
An understanding of the application could be helpful.  JT valves exist in refrigeration for HVAC, natural gas processing and other applications.  Also, it would help to clarify whether the JT valve is manual like a fixed choke, or variable such as a control valve operated by an electro/pneumatic signal.
I worked on a fuel gas system where the initial startup conditions could exceed the carbon steel limits.  This was due to changes in the installed case from the design case.  One could control the downstream temperature as an override to maintain the outlet temperature above the minimum temperature limits if the downstream system is high carbon steel.
Install a temperature transmitter in the outlet stream as the input to a PID controller.  Assume that the JT control valve is controlled by another variable, perhaps pressure or flow.  For a fail-closed control valve, a signal selector could select the lower output signal of the primary process controller or the outlet temperature controller.

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