On crude unit we have for pass fired heater with thermocouples on each pass exit.
We experienced thermowell fracture two time in last three  years. When thermowell is broken  crude oil start leaking. It is very unpleasant situation because of high crude oil temperature and possibility of fire.
Possible cause of fracture is pass vibrations on heater outlet.

I find in literature if there is possibility of thermocouple fracture it is necessary to install valve on thermovell end and in case of fracture it is possible to close valve, cut wires and stop leaking.

What is manufacturer or solution in this case?

A failure analysis of the thermowell should be done to determine if failures are caused by vibration or corrosion attack.

If vibration, a study needs to be done to determine a correction. (eg. thicker material, different length, provide thermowell with guides which support from the nozzle id, etc.) Thermowell supplier should be able to do this given your installation details.

I suppose that it is impracticable to install a plug valve in thermocouples application. If I am wrong let me know. We have already had some thermocouples leaks on heater passes of our CDU heater. Leaks have been caused by erosion due stem wear by coke particles , by broken stems due to vibration e by naphtenic acid corrosion. I agree with Milutin that such failures in thermocouples are very dangerous and can cause a fire. We have changed our thermocouple stems to 317L and all turnarounds thermocouple stems are inspected and replaced if required by inspection.If corrosion, a material change will be needed.

I would recommend strongly against putting a valve on the thermowell.

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