RTDs and TCs Connection Head material Selection


Thread Starter


The connection head of RTDs or Thermocouples are offered in different materials: Aluminum, Cast iron, Cast aluminum and Stainless steel. Any criteria for selection between them? We had in our plant one thermocouple head was stuck close, and we were unable to open it. when using hard tools to open it, part of the cover was broken. is material selection can make also the box easy to be opened by normal tools?

<b>ANY</b> threaded cover is going to eventually get corroded and/or galled if some kind of thread lubricant isn't used when the covers are removed and replaced. For older covers which didn't have the thread lubricant applied, one has to carefully remove the covers, and using brushes to remove as much of the corrosion as possible (this usually occurs primarily on aluminum covers), then apply a proper--but not excessive--film of thread lubricant and re-installing and removing the cover several times, applying a little more thread lubricant as necessary to ensure smooth operation. Then, just lightly seat the cover--don't crank down on it like a bolt that needs 20 N-m of torque. Just seat it lightly and leave it, and the next time--even if it's seven years later the cover should be easier, much easier, to remove.

It's a shame that most construction companies don't or aren't forced to use thread lubricant during construction. It should be a requirement, and the requirement should enforced. And, maintenance crews (mechanics and I&C technicians and electricians) should ALL have a can of thread lubricant in their tool box/bag/pouch--and should use it EVERY time they remove a cover.

Sometimes thread lubricant is called "never-seize" (I think there's even a brand of thread lubricant named something similar). And, remember--some materials require a specific type of thread lubricant (this may apply to Stainless Steel fittings, in particular, or to aluminum fittings--check the thread lubricant container to be sure it's suitable for the application).

But, in general, the use of thread lubricant is ALWAYS recommended to prevent just the type of problem you are describing. It's not really the material (unless there is a corrosive atmosphere in the location), but it's more about the judicious and regular use of a simple thread lubricant. (Some high-temperature thread lubricants have a very fine carbon paste which stains hands and clothes--so, again, judicious use and application will prevent a lot of unnecessary mess!)

Hope this helps!
Thanks Sir for your reply,

but I appreciate if any advice also for material selection criteria for the Cover between :Aluminum, Cast iron, Cast aluminum and Stainless steel. When I use each type of them and what is the difference?

The choice of material--for any application--generally depends on environmental conditions in the area where the device/instrument is to be located. Does the area require explosion-proof or spark-proof fittings? Does it require stainless steel?

I can tell you that GE (not necessarily packagers of GE-design heavy duty gas turbine--but GE-packaged heavy duty gas turbines) generally have only fittings that meet Class 1, Div. 2, Group D guidelines (which is North American standard). They just standardized on that as being the most widely used and applicable standard to keep the number of types and material fittings to a minimum. They have provided other materials optionally--at additional cost--but whatever fitting meets that standard is what was normally supplied with GE-packaged units.

Some vendors--and GE now uses vendors for EVERYTHING--just supply the lowest-cost fittings regardless of what was in the GE Design Specification. There isn't too much in the way of quality control, because GE just drop-ships a lot of equipment to be installed at site--something they didn't use to do. And, they rely on the vendor to certify they meet the Spec--and there are unscrupulous vendors.

A lot of EPCs just buy whatever is least expensive (cheapest)--unless there is some environmental issue or technical regulation or standard. Some equipment suppliers also use a standard offering that meets a wide variety of regulations and requirements.

If you want to know if a specific material is required for a particular application at a particular location in the plant--you need to understand the requirements and regulations and then look at the data sheets of fitting suppliers to find the proper material for the application. A good supply company will be able to help you with that choice--they generally have good knowledge of local requirements and regulations and know what meets those requirements and regulations and can help with choosing the proper material for the application/location. (The supply companies that want to sell the most equipment and keep their Customers coming back for more will do this, anyway!)

If I were specifying fitting materials for a specific application in a particular environment in a particular location I would be investigating the area classification, any hazardous atmospheres, any local technical regulations and standards--and I'd also be talking to the Customer to see if they have any standards they want used in the selection of materials. But, regardless, if I were installing the fittings and commissioning the equipment I would be certain that thread lubricant were used on the fittings--in the same way that bolts have thread-locking solution applied, or high-temperature thread lubricant during assembly. Or that Teflon tape is used in pipe threads, or PTFE thread sealant is used on natural gas pipe threads, etc. Or, thread lubricant is used on stainless steel fittings and bolts to prevent galling during tightening or removal. It's just cheap insurance--and good maintenance practice. And sound plant policy to try to reduce problems in a proactive fashion--instead of having to replace broken fittings when they can't be opened because of corrosion or galling.

Hope this helps!