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.
Thanks.





RE: Relief line thermal bowing
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RE: Relief line thermal bowing
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
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RE: Relief line thermal bowing
RE: Relief line thermal bowing
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?
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RE: Relief line thermal bowing
RE: Relief line thermal bowing
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
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