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tube with ribs welded to flange

tube with ribs welded to flange

(OP)
Hi, I've a doubt. We've an application welding a round tube to a round flange with ribs (see image). We use to design this with clips in the ribs so the weld of the ribs do not go over the flange weld. But recently I've found a document from Corus with the detail in image. Welding on weld and recommending it to avoid corrosion problems. Now I'm not sure if there's any limitation in the structural or ASME pressure code to do or not to do one or the other (I haven't found anything but maybe I'm missing something).
Thanks very much,
Manu

RE: tube with ribs welded to flange

I would tend to think the 'preferred detail' where the rib is ground (I'm assuming by hand by the welder/operator?) to clear the weld is not ideal- you're creating a crevice and thus bringing crevice corrosion into the equation unnecessarily. If the rib is hand ground, you're going to get inconsistencies. Some ribs may experience crevice corrosion, and some may not.

I would either use the preferred detail and weld the full rib perimeter, or use the second detail.

RE: tube with ribs welded to flange

ManoloGalarraga:
The other side of this argument is the reason the stiffener with the clipped corner have existed for so long. The old detail prevents anyone from welding into that corner, onto that existing fillet weld, where a really nasty tri-axial stress condition is set up, and where stress raisers will likely exist. From the fatigue and high stress concentration standpoint, your new found detail is far from ideal. This has been a long standing quandary, do you want this nasty welding detail and high stress concentrations or do you try to do something to try to minimize the potential corrosion problems. I’m most concerned with high stress concentrations which I might be causing by my design and their detrimental effects on the structure. In some locations, your ribs are called base plate stiffeners, and they have some pretty high stresses in the region you are talking about. From the cost standpoint, I would just use a 45̊ corner sheared clip to adequately clear the existing weld, not the quarter round which is much more difficult to make. I would also shear/clip the two sharp ends of the stiffeners, leaving about a .5" flat end, that’s much cleaner and neater. Generally, you should not weld all around that stiffener to the base plate either; rather, stop the fillet weld .5" short of the end of the butted joint surfaces, and do a good job of filling start/stop craters at these ends. When you try to weld around the sharp corners and narrow edge of that plate, you invariably cause/leave small sloughed notches on the sharp corners, and these are another serious stress raiser at those locations. Some people/inspectors object to this, for fear of hiding starting cracks. I think it would be better to spec. (clear notes on drawing about methods and intentions, etc.) to lightly caulk locations where water can enter a joint/faying surface, to try to keep water out. A light bead of a good grade of caulking, finger pressed down into the open joint, and not obscuring surrounding weld should do the trick. Do not caulk the lower ends of those joints, you do want any water to drain out. Don’t forget, those stiffeners and welds do actually have to be designed for the loads they will see.

RE: tube with ribs welded to flange

From time-to-time I've read some discussions about "welding over welds" at intersections and especially repairs that have gotten folks kind of agitated about things like HAZ etc.

Maybe those discussions were about fancy pressure vessels and stainless steels, not good ol' structural steel.

FWIW AWS D1.1-98 section 5.28 prohibits caulking of welds

RE: tube with ribs welded to flange

My 0.02: If the details are being produced on burn table, either a clip or a cope is the same amount or work, none :)

I'd prefer the clipped / coped detail to permit inspection of the tube-flange fillet, assuming the gusset can be 100% seal welded.

If you've ever seen rust runs from a welded joint you'd know how ugly they are, particularly on new equipment :)

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: tube with ribs welded to flange

(OP)
Hi, thanks for the answers. I was always taught to avoid weld over weld but that Corus document put the doubt in me and I wanted to see some other opinions.
(Corrosion and protection of steel bridges http://resource.npl.co.uk/docs/science_technology/... )

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