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Up Topic Welding Industry / Technical Discussions / Welded Pressure Vessel Joint Design
- - By TimGary (****) Date 08-07-2000 19:27
I am thinking about qualifying a procedure for ASME category A and B welds on carbon steel pressure vessels utilizing backing strips.
My thought direction is to reduce man-hours by eliminating back-gouging/grinding. As I would be working with 5/16" to 7/16" base metal thicknesses, with a backing strip, I could make the weld from the exterior with sub-arc in 1 or 2 passes.
ASME Sect VIII UW-12 (2), UW-35 (c), and UW 37 (d) are all I can find that regulate this joint design, so I'm hoping that someone can answer a couple of questions for me.
Would the edges of the backing strip have to be seal welded on the inside of the vessel?
UW-35 (c) says that "...when a backing strip is left in place, the requirement for reinforcement applies only to the opposite side of the backing strip". Can anyone explain that? What do they mean by "requirement for reinforcement"?
With a 3/16" to 1/4" root opening, would a bevel be necessary on both pieces?
Can anyone suggest welding parameters that would help make this weld in one pass, or would that much heat at once cause excessive warpage or base metal damage. (A514 Gr-70 base metal)
Does anybody out there use this joint design? Does it work well?
Any comments or suggestions would be greatly appreciated.
Thanks in advance for your time,
Parent - By G.S.Crisi (****) Date 08-07-2000 21:35
Dear Tim,
I'll give you some opinions based on many years of experience. I won't speak of the ASME paragraphs for now because I don't have the Code at hand, so I'll do it later, after having read the pertinent paragraphs.
1st. If I've understood well, you're thinking of putting the backing strip to avoid backgouging the weld. This is NOT mandatory, i.e., it's entirely up to you to decide whether or not to put the strip. If the welds are sound, and using submerged arc the chances are in favor of being so, then, if I were you, I wouldn't put the backing strip. You say nothing on whether or not the welds will be radiographed. In case they are, a backing strip should not definetely be used.
2nd. The seal weld on both sides of the backing strip depends on the fluid the vessel's going to contain. A strong, hot caustic solution, as caustic soda or potash, for example, would strongly suggest to make the seal welds in order to prevent a well known problem called "caustic embrittlement", which happens when such a solution gets into the very narrow gap left between the strip and the vessel wall. Many years ago, when boilers drums were riveted instead of welded, many boilers exploded because boiler water (a strong caustic solution with a pH above 11) got into the tiny narrow gap between the overlap of the riveted section, thus producing the caustic embrittlement.
Now, if the fluid is either neutral or lightly caustic (pH of, say, 8 or less), then you don't need to seal weld the sides.
3rd. When the base metal thickness (a plate in this case) is above 1/4 inches, as in your case, a bevel is needed. It can be neglected when the plate thickness is 1/4 inches or less.
4th. With a thickness of 5/16 to 7/16 in, I don't see any problem in making the weld in just one pass using SAW. You'll have, of course, to regulate correctly the travel speed of the piece and the feed speed of the wire.
I ough you the answer on the meaning of the Code paragraphs. I'll take a look at the Code and get back with the answer.
Giovanni S. Crisi
Sao Paulo - Brazil
Parent - - By TimGary (****) Date 08-08-2000 18:39
Thankyou Mr. Crisi for your reply,
In further explination, I'm trying to figure out the quickest method with the least probability of repairs due to operator error, for making these welds. A high repair rate due to operator error, associated with backgouging, has gotten old very fast. Fit-up irregularities such as uneven beveling and mismatch make applying an even root pass without the need for backgouging not an option. Though employee training and grooming will pay off in the future, I'm hoping to simplify the process for now. My thought is that utilizing a backing strip will help with the fit-up and leave the full penetration weld solely to the sub-arc, thus reducing repairs and man-hours.
These welds would be subject to ASME spot-xray requirements. Why is xray a problem with the backing strip?
Thanks for the advice about the caustic related embrittlement. These vessels are for water softening and will be periodocally flushed with acid.
Thanks again for your time,
Any suggestions are greatly appreciated.
Parent - By G.S.Crisi (****) Date 08-09-2000 00:35
Dear Tim,
I'm answering you in a hurry because my lessons will start in a few minutes.
Let me clear up something. If the vessels will be used for water softening, then they won't be flushed with acid but with brine (solution of table salt in water). In this case the vessel doesn't need to be rubber lined and you can use a backing strip provided the welds won't be radiographed. Where is it written that they can't be X-rayed when using a backing strip I don't remember in this moment. However, I've always seen that procedure. I'll make some further research on the matter and come back to you.
Now, if the vessel's going to be flushed with weak acid, this means that it's not a softener but the first column of a demineralizer, i.e., a cation exchanger. In this case, it'll be rubber lined to prevent corrosion and a backing strip can not be used because it'll leave air pockets under the lining. Ask any rubber applicator.
OK, it's enough for now, I've got to rush to my lesson. I'll be in touch with you again tomorrow.
Good night
Parent - By G.S.Crisi (****) Date 08-11-2000 20:40
Dear Tim,
I've just looked at the Portuguese translation of an old edition of ASME VIII dating back to 1983 and I believe I can be of help to you. The paragraph you're talking about (UW-35.c) is the UW-35.b in that old edition. It refers to the "reinforcement" of the weld. What the "reinforcement" is, it's cleared up in the next paragraph (UW-35.c of the 1983 edition). It says that in order to make sure that the welding bevel has been totally filled up so that the surface of the weld seam is not below the surface of the adjacent base metal, some weld metal should be added as reinforcement ON BOTH SIDES OF THE JOINT (I'm translating directly from Portuguese to English so the words may not be the same used by the English edition of the Code). It keeps on by showing a table stating the reinforcement thickness as a function of the base metal thickness.
So, what paragraph UW-35.c (UW-35.b in the old edition) points out is that when you use a backing strip covering the inner side of the weld you don't need to apply a reinforcement on that side, which, to tell the truth, is more than obvious, because the strip has been put there BEFORE the weld is done.
By reading paragraph (2) of Table UW-12, as you mention, one would deduct that it's not forbidden to use a backing strip when the welds are going to be X-rayed. However, this is not what I've seen in my professional life. I'll make some research on it and come back with an answer.

Up Topic Welding Industry / Technical Discussions / Welded Pressure Vessel Joint Design

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