×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Thermowells

Thermowells

Thermowells

(OP)
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!

RE: Thermowells

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.

______________________________________________________________________________
This is normally the space where people post something insightful.

RE: Thermowells

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.  

RE: Thermowells

(OP)
Thanks for the information.

RE: Thermowells

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".

RE: Thermowells

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.

"Do not worry about your problems with mathematics, I assure you mine are far greater."   
Albert Einstein
Have you read FAQ731-376: Eng-Tips.com Forum Policies to make the best use of Eng-Tips Forums?

RE: Thermowells

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.

RE: Thermowells

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"

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources