Thermowells

[atlas0821]

Do you guys have good information on sizing thermowells (U length)? I am looking for something to help with spec'ing out some themowells. This is a new area for me and would appreciate any help that you could give. Is there any chart that basically gives U lengths for just about all possible combinations of piping and accessories? Not sure if there is something out there like that, but worth asking. Thanks!

 

[controlnovice]

All thermowell, temperature element and temperature transmitter vendor will have charts on standard thermowell lengths with pictures. They will explain the U lengths for flanged, threaded, welded and sanitary wells. Also keep in mind the lag length and extension lengths as well.

Try

http://www.emersonprocess.com/rosemount/document/drawings/91drw.pdf

or

http://www.ashcroft.com/products/qg.cfm?qg=thermowells&sub=14

The U length depends on your process requirements. Typically, for tanks and vessels, we specify the length between 6-8 inches inside the vessel. Be careful on interference with equipment inside the tanks though, such as heating/cooling coils, etc. Thermowells for piping can be a bit trickier.

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[JimCasey]

You want to have the well as long as possible so that conduction effects will be minimized. The Thermocouple junction is in the tip of the T/C. If the well touches something that is not what you intended to measure it will introduce an error.

It is reasonably common for small piping to put the well in one side of a tee and let it protrude upstream a ways. That way you can put a 6" or 8" well in a 1" pipe.... Then the tee drains through the common port.

 

[atlas0821]

Thanks for the information.

 

[JLSeagull]

When selecting thermowells, consider the resonance issues. Generally the shorter the better to avoid wells breaking. Look for "Thermowell Design Considerations per ASME PTC 19.3 Chapter 1, General with Current Well Nomenclature Updates".

 

[Ashereng]

I go with JL's advise. When I choose U length, I go with the shortest length that will do the job.

For inside tanks, usually 6-8 inches off the wall is sufficient to get away from wall effects.

For pipes, usually to mid-line for pipes 4" and under. For 4" and larger, usually 1/4 D is enough. When you get really big, 10" and larger, a flow regime/character study may be needed to determine where exactly the well needs to be to get the reading that is representative of whatever you are trying to measure.

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[Weegie]

A good rule of thumb for thermowells in piping is that the tip of the well should be located in the middle third of the process pipe.

 

[JLSeagull]

The tip should be away from the pipe wall. Mid-pipe is looking for trouble in large pipe. Do not use the third of the pipe rule for 36 NPS pipe. The well would be likely to break.

As most wells these days are flanged, consider a flanged well. The pipe extension (top of pipe to flange face) may be six inches. Frequently the calculation will eliminate a 12-inch insertion length. I try to select thermowell insertion lengths for any temperature element that would match with dial thermometer stem lengths. A 10-inch "U" on a flanged well should match a 12-inch stem. This would be the longest well to consider in most piping cases. This issue is not a problem for vessels.

Consider wells that fit the following pattern except that threaded wells on insulated lines may require longer lag extensions.
Stem lengths in inches:
Stem, "U" NPT "U" Flanged
2.5, 1"
4, 2.5" 2"
6, 4.5 4"
9, 7.5" 7"
12, 10.5" 10"