A thermocouple well assembly with a sealing coupling and a method for eliminating leaks in hydroconversion reactors while continuing to hydroprocess

A thermowell assembly and method which allows for arresting leakage in the case a thermowell begins to leak in a hydroprocessing process without having to shut down a flow of a hydrocarbon feed stream through a hydroconversion reaction zone in the hydroprocessing process. The thermowell assembly comprises a first hollow sleeve section which supports a thermowell member, a ferrule sealing member which engages the first hollow sleeve section, a second generally cup-shaped second sleeve section wherethrough a thermocouple member slidably passes, and an outer sleeve member for maintaining the union of the first sleeve section, the ferrule sealing member, and the second sleeve section. The method comprises severing and/or removing the thermocouple member from the commenced-leaking thermowell member and placing a high pressure cap over an aperture left vacant by the thermocouple member to seal-off the leaking thermowell member from the atmosphere.