Removing Thermowells

[ChEMatt]

We have a situation where our thermocouples are not reading temperature correctly (based on the heat release of the reaction and other related process variables). We cannot directly determine the temperature based on other information so we do need these temperature measurements.

One suggestion is to use thermal paste to fill the thermowell. Another suggestion is to remove the thermowell and mount the thermocouple directly to the process. We would tag the thermcouple as such and ensure all documentation is up to date (P&IDs, of course).

Some folks consider a direct-mounted thermocouple to be a very dangerous safety hazard. Some folks believe it can be done safely.

What is your opinion on the subject? What steps would you take to ensure that it is done safely, if it can be done safely?

Thanks.


Onwards,

Matt

 

[JLSeagull]

You need two seals between the process fluid and the wiring conductors. I think that this is an OSHA requirement and I don't have time to look for it now.

Regardless, the old low density polyethylene process is among the highest pressure processes. As I recall there are direct insertion temperature elements for LDPE. These may be provided by Autoclave Engineering.

 

[skearse]

I'm not aware of any OSHA requirements for two seals, but that could be the case...

I think that they may both be viable alternatives, and the question for my decision would be based in part also on the need to recal/certify/repair/replace the TE/TT without taking the process completely offline (again, depending on process specs, plant safety reqs, etc.). Are these TCs or RTDs? With a TW, you may be allowed to swap a TC/RTD while the kettle is online (or at least with minimal downtime that may not require a line break); without one, you have to take a shutdown and deinventory/decon before you can swap out a $400 RTD...how much is the downtime worth? Just something else to consider.

 

[ChEMatt]

We're meeting next week to discuss the safety issues. I will research the thermal paste option, of course. I wanted to find out whether or not the direct measurement sans TW is "taboo". Seems like it is not.

Process runs at about 350 to 400 psig. Not too too high of a pressure, but nothing you want to trifle with either.

With regards to "what do we do if it fails", it's a pilot plant so shutdowns come fairly often. If it dies mid-run we will replace it soon enough, or we could idle the process if it is truly critical.

After reviewing the forums I think perhaps this should have gone in the "Plant Design and Operations". I'm going to cross post it there as well.

Onwards,

Matt