×
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

Relief line thermal bowing

Relief line thermal bowing

Relief line thermal bowing

(OP)
We have safety relief valves which can relieve large amounts (1000 m3/hr) of cryogenic liquids (-150 degC) to a 150# flare header (48").  The flare header is sized for other gas relief cases.   Is it normal practice to design the flare header supports to take into account potential thermal bowing due to the cryogenic liquid releases even though these will be short releases from the safety relief valves(less than 30 minutes)?

Thanks.

RE: Relief line thermal bowing

If you mean in regards to the headers axial movement placing frictional loads on the supports in the direction of pipe movement, ie. lateral load on the supporting beams, yes.  

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

RE: Relief line thermal bowing

(OP)
I am referring to the requirement in B31.3 which states "Piping shall be designed to accomodate or eliminate an unequal temperature
distribution at any cross section that may result in thermal bowing. This often occurs due
to thermal stratification, partial filling, or during startup or shutdown"

Is it normal practice to follow this B31.3 requirement for flare headers that will only see cold liquids for short durations?   My reading of B31.3 is that it would also apply to short term reliefs but would like to know what others typically do for this case on flare headers.  

RE: Relief line thermal bowing

Yes I know of the B31.3 requirements.  Thermal bowing can be caused simply by sun or rain hitting the top of the pipe too.  What I didn't understand was when you referred to, "design the flare header supports" as I thought you only needed information on how to do that, but now you mention piping.  Anyway, no matter, both the supports and the pipe must be designed for thermal bowing.  There is no overstress allowance for short time durations.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

RE: Relief line thermal bowing

Our ethylene relief headers had a expansion loop in it.  Make sure you do a complete stress analysis and remove all those anchors if you want a design that works.

RE: Relief line thermal bowing

dcasto,

Maybe we leave one, or better yet, two anchors.  Two guarantees that the expansion loops tend to work like they should.  What do you say?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

RE: Relief line thermal bowing

biginch, yes, around the loop.  Upstream of the loop, we had about 6 psv's tied in, thats where the problem of anchors were.  Now, slides that allow unidirectional are needed too.

RE: Relief line thermal bowing

There is no overstress allowance for short time durations.  

The above statement is not quite correct.

From B31.3:

302.2.4 Allowances for Pressure and Temperature Variations.

(1) Subject to the owner's approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than:
(a) 33% for no more than 10 hr at any onetime and no more than 100 hr/yr; or...


(2) When the variation is self-limiting (e.g., due to a pressure relieving event), and lasts no more than 50 hr at any one time and not more than 500 hr/year, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than 20%.
(h) Temperature variations below the minimum temperature shown in Appendix A are not permitted unless the requirements of para. 323.2.2 are met for the lowest temperature during the variation.


So, provided all of the requirements of 302.2.4 are met, you basically get a 33% overstress allowance for short time durations with owner's approval.  And the way I read the code, you get 20% overstress for self limiting pressure relieving events provided all of the listed requirements are met without the owner's approval.  The devil is in the details, and there are many, to ensure that you can take advantage of pressure and temperature variations for short time durations.
 

RE: Relief line thermal bowing

Zapster,  Thanks for reminding me about this.  I have applied 10% temporary overstress allowances to pressure stresses, but I have not been doing that for thermal stresses.  As you point out, there are quite a few restricting clauses, but they're not impossible to meet, so I must agree with your conclusion.

 

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

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