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On a crude unit we have a fired heater with thermocouples on each pass exit.
We have experienced thermowell fractures twice in the past three years. When the thermowell breaks, crude oil starts leaking. It is very unpleasant situation because of the high crude oil temperature and possibility of fire.
The possible cause of the fracture is pass vibrations on heater outlet.
I find in the literature that if there is the possibility of a thermocouple fracture it is necessary to install a valve on the thermowell end and in case of fracture it is possible to close valve, cut wires and stop leaking.
Does anybody know of a manufacturer or solution for this problem?
 
Answers
22/11/2007 A: Jim Hughes, Foster Wheeler Energy Limited, jameshughes1972@hotmail.com
It is common to pass the thermocouple through a stainless steel ball valve outside the thermowell where thermowell rupture is possible (e.g. long thermocouples on FCCU regenerators). Turn the ball valve 90 degrees and this will cut the thermocouple and provide a seal. State your requirements to any recognised temperature instrumentation supplier and there should be no problem getting supplier to manufacture. Remember to order additional quantities to have sacrificial assemblies available for testing.
Thermowells in lines tend to fail due to vibrations caused by velocity of fluid. There is a check that can be performed to check that frequency of induced vibrations is not close to natural frequency of thermowell. Thermowell length and mass can be adjust to vary the natural frequency, alternatively a velocity collar can be attached to the thermowell. Again, check this with any recognised temperature instrument supplier.
11/10/2007 A: Shailesh Shah, Shailesh B Shah & Co , shaileshbshah@gmail.com
Where is the location? In India? I am available to help
I would need to study the source of pass vibrations on heater outlet.......