Removing Thermowells
Removing Thermowells
(OP)
This is a cross post from: http://www .eng-tips. com/viewth read.cfm?q id=254711& amp;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.
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





RE: Removing Thermowells
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.
RE: Removing Thermowells
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?
RE: Removing Thermowells
RE: Removing Thermowells
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.
RE: Removing Thermowells
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.
RE: Removing Thermowells
Regards
Stonecold
RE: Removing Thermowells