Removing Thermowells

[ChEMatt]

This is a cross post from:

http://www.eng-tips.com/viewthread.cfm?qid=254711&page=1

It seemed that after I posted it perhaps the operation forum would be more appropriate.

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

 

[unclesyd]

We use a lot of "live thermowells" in our polymerization process product lines and have done so for over 50 years with no problem. The cardinal rule is that these couples not touched if the unit is on line. the same holds true for the heating media in jackets where we use both local and indicating temperature measurements. The local indication if provided by Palmer Dial Thermometers. Again nothing is touched while the process is on line.
All our thermocouples of the dual calibrated type.

Any new thermowell installation, if live, has to go through a very tough justification process.

Each installation is a separate entity so you generally can't have blanket approval as every one has to be considered separately due to changing physical parameters.

 

[TD2K]

What's the process fluid and what's its pressure and temperature?

How much of an offset are you seeing from your thermocouple readings in the thermowells versus what you think they should be and what sort of temperature differences are the two process thermocouples trying to measure?

 

[JLSeagull]

See duplicate posts.

 

[JimCasey]

Shall we assume you have looked at your thermocouple spec?

Grounded thermocouples are more sensitive than nongrounded. Heat transfers to the junction more quickly in a grounded t/c since the junction is placed against the tip of the t/c tube. However grounded t/cs are sensitive to electrochemical proceses and poor ground loops.

Spring-loaded thermocouples are available to keep the T/C junction firmly in contact with the bottom of the bore, again to promote rapid thermal equalization between the well and the junction.

I you still feel the need to expose the t/c directly to the process, the major tube-fitting vendors all make a t/c fitting. It is bored all the way through at the t/c's diameter instead of having a shoulder inside as normal to limit tube insertion. For safety it is possible to mount the t/c with a ball valve so it can be withdrawn a bit, the valve closed, then completely withdrawn. If there is a lot of force on the t/c, there are hot-tap assemblies that have a couple of jack bolts to allow you to remove the t/c slowly and safely.

lastly, the smaller the t/c is, the faster it responds. so a 1/16" t/c will pick up minor fluctuations where your typical 1/4" may not show them at all. Select diameters with consideration of your flow velocity or you may just bend them over against the pipe wall-again getting an artificially low reading and making them difficult to withdraw.

 

[moltenmetal]

The key purpose of a thermowell is to permit replacement of a defective thermocouple element without requiring depressurization. It also provides some mechanical robustness to the installation, protecting the device from mechanical damage during maintenance, filling, agitation etc.

Direct wetted thermocouples, where the sheath of the element itself forms part of the pressure boundary of the vessel, CAN and ARE used safely in a wide variety of process installations.

Thermocouple elements don't fail that frequently unless you're at high operating temperatures, so the real issue is mechanical integrity and damage resistance. If this is a manageable problem in your situation, you should not be afraid to use a direct wetted element. Multiple points in each element sheath might be prudent.

 

[StoneCold]

If you are going the direct measurement route, I can tell you that we have had much better luck with that in the gas phase than in the liquid phase. Liquid phase applications tend to either corrode away or get busted by vibration, sometimes very quickly. But that is just my two cents.


Regards
Stonecold

 

[skibum2b]

Re: JimCasey - you can also go with a "reduced tip thermcouple. Only the tip is reduced, thus providing the quicker response time, while still providing enough strenth to avoid bending the whole thing against the wall. The only drawback is the slightly higher cost - which will depend on the diameters you need/want.