A Question About Pressure Vessels

Not sure, but I think it is 50mm (2") between the closest parts of the two welds.

In my humble opinion the distance should be related to the TAZ width which is at its turn related to the thickness and welding profile. It should be > 2 TAZ if not a weak zone is generated.

TAZ = thermally afected zone.

That's probably a better criterion than mine, though I think I have read mine in some code or spec. TAZ = HAZ (heat affected zone, as used in some other code languages).

It would be alarming to discover that pressure vessels are being designed and constructed without a copy of the relevant codes being to hand at the time.

Get a copy of the applicable specification and read it!

Keep in mind that hoop stresses are higher in a cylinder so your longitudinal welds had better be right or they will fail first!

Drew K

Having had experience with submarines - American and British - at both design and construction levels - that also involved other pressure vessels, the Rule of Thumb was and still is 3" or 80 mm. However, preheat treatment was executed to eliminate the effects of associated residual stresses.

I don't know - but if I were working on the project - even just a theoretical one - I would get the spec and study every word of it!

Then I would know!

I guess a second option would be to anonymously ask the advice of strangers on the internet. That typically works out quite well!

Show me an anonymous poster who displays intelligence and I'll show you a miracle.

I wholeheartedly agree with your post: There is no substitute for valid specifications. However I stand by my original statement of which I am certain, and gleaned from said specifications and my extensive, tried and tested experience and practise.

From your extensive, tried and tested experience and practice, please explain the difference between weld preheating and post-weld heat treatment, in light of your first post.

Anonymous, The ASME Code, Dection VIII, Div. 1, Part UW addresses your issue. FIG. UW-3 illustrates pressure vessel welded joint locations typical of categories A,B,C, and D. Paragraph UW-9(d) states that unless the longitudinal joints are radiographed for a distance of 4" (100 mm) each side of each circumferential welded intersection, courses are to have the centers of the welded longitudinal joints staggered, or separated, by a distance of at least five times the thickness of the thicker plate.

Oops! Sorry, guys.. I should have reviewed my post more closely before submitting! I don't find my "Dection" VIII for verification. I did find my "Section" VIII, however. Slip of a hunt and peck finger, I guess.

Thank you for this comment it reinforces what I wrote (although nobody appreciated it a s a good answer) since it is logical to consider the HAZ which is proportional to the thickness of the welded wall. The distance is such that the temperature in the zone of the new weld will not affect the first one. I assumed (based on heat conduction estimations for low carbon steel) that the HAZ could be around 2 x thickness. The value of 5 t supports my estimation. I am glad to notice once more that practice and theory go hand in hand although several participants do not appreciate a theoretical approach.