Thermowell Selection


Thread Starter


How to decide the different type of connection provided to thermowell for high pressure line?
Should thermowell be welded or flanged?
How to decide the rating of flanged?
It's normally spelled out in the Pipe Specification from the mechanical and process engineers.

Having said that I prefer weld in rather than high pressure flanges. I sometimes use a weld on temperature element to measure the pipe material rather than breech the high pressure line. Once it's well insulated there is little difference.
Currently that is decided by the piping codes in your area and your firms design standards.

The specific options can only be resolved by finite element calculations. Such calculations require details of your process that are not publically available. A full assessment generally requires testing various design options computationally.

Most companies wait until a failure occurs before doing that, regrettably.
Weld-on measurements are not uncommon with furnace tubes, but with heavy walled high pressure piping. The response time might be longer than most processes can tolerate.
>Weld-on measurements are not uncommon with furnace tubes,
>but with heavy walled high pressure piping. The response
>time might be longer than most processes can tolerate.

I won't argue with you, I simply put it forth as another option. Quite ok for a local indicator, perhaps not for temperature control or critical alarm. A 2500# flanged thermowell also has issues.
Indeed, like the fellow said, if it has wires, it's someones headache!

But you are right on, there are some measurements that simply defy decent temperature measurement, and skin temperature is only affordable option.

it is definitely used.
Some things to consider when you make a choice:

1. O-rings between flanges can leak, well-welded thermowells will not.

2. In case you are in the food industry: properly welded thermowells are hygienic while flanged thermowells can have dead zones where bacteria can grow.

3. Flanged thermowells can be easily taken out of the pipe if necessary while welded ones are much more difficult to remove.

For the functioning of the temperature measurement, I don't see much difference.
Concerning your questions about the rating of a flanged thermowell, I should consider choosing a rating equal to the design pressure of the pipe or vessel.
Welding thermowells in the pipe is not a good idea when two different metals are involved. The welds will fail.

Van Stone flanges and thermowell in high temperature processes are common. Furnaces require other methods if extreme temperatures are involved.