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API 650 shell seams using a backing bar?

API 650 shell seams using a backing bar?

(OP)
Quick question: Are you allowed to use a steel backing bar to weld horizontal seams in an API 650 tank shell?
This would be without taking the backing bar off, just leaving it in. I can't find anywhere in API 650 where it says you can't do this, but it just doesn't seem right to me.

Here's some background how this came up:

I'm somewhat involved with a tank the company I work for is building. We normally weld the horizontal shell seams with a SAW girth welder made to weld in the horizontal position inside and outside. The issue with this tank is that there is not enough room to run the machine on the outside of the tank shell due to other tanks and equipment being too close to the tank we're building.

I'm not engineering this tank in particular, but I was asked a while ago about welding only from the inside using a ceramic backing on the outside. I said yes you "can" as long as you determine a proper joint prep by trying it out to make sure you get a good quality weld.

So now come to find out, the shell plates are already burned and beveled and we're only testing this out now. Same old story, "we don't have time to figure things out ahead of time and actually do things properly." What they are getting (with the prep someone in their infinite wisdom has somehow figured out) is a very poor weld on the outside when the ceramic backing is removed. Incomplete penetration and full of porosity and pin holes.

When I look at it, basically the prep is wrong, the bevel is not wide enough, the land is much too thick and they should have allowed for a gap. The plates have already been burned and prepped and, as usual, they only get me involved after they've made a mess of things on their own.

Now their solution is we'll use a steel backing bar on the outside and just leave it in. I've told them that they'll still get the same crappy weld, you just won't see it. However, the RT technician will see it when he X-Rays it. The problem isn't the ceramic backing, it's the prep!

RE: API 650 shell seams using a backing bar?

See 5.1.3.8. Also note that per 5.1.3.7(a), the backing strip would have to be seal-welded on the outside.
You might run into problems with weld distortion. Otherwise, I agree with your assessment.

RE: API 650 shell seams using a backing bar?

(OP)
Yes I found these clauses. They would be covered by insulation, and we are asking the customer if they are OK with leaving a backing bar on the outside of the horizontal seams. We are welding some coupons to what the welds look like, I had a look at a cross section of the weld and it actually looks quite good.

I still don't personally agree with the whole thing.

RE: API 650 shell seams using a backing bar?

Hand weld it where the 3 O'Clock won't fit. Best to do it right now than have problems in the future.

RE: API 650 shell seams using a backing bar?

Prepare welding procedure specifications and perform tests documented by procedure qualification records as required by Section IX of the ASME Code.
Regards
r6155

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