Backing Strips 3

[DLind2005]

I am in a pinch. My fabricator cannot weld a stainless steel vessel that is 12" nominal with two elliptical heads on each end of the shell, with an overall length of 24". One of the circumferential joints is already welded, and they are concerned that the will not be able to get the full penetration weld correctly and to code. The shell and heads are only 3/16" thick, my question is, can I use a backing strip to help with the welding of the other head? Should it be designed the same thickness and material as the shell? This will not be removed after welding. I am unfamiliar with backing strips and would like to know more and what is required to be to code.

 

[TGS4]

What's your Code of Construction? Where in the world are you located/what are your jurisdictional requirements?

 

[DekDee]

DLind,
Leaving aside the rather critical information regarding the code of construction as noted by TGS4 the person responsible for designing this vessel is the only person who can tell you if a backing strip is acceptable to use,
Cheers,
DD

 

[gr2vessels]

I would be very worried if my fabricator would have problems with the closing weld on an 12" tiny vessel;- even a simple TIG weld would be sufficient for a good closing weld. However, yes, he may use backing strip which will remain in place, as per ASME VIII Table UW-12, type 2 weld (refer also to conditions like UCS-4, UW 35, UW-36, etc.). Obviously, there are other design conditions you haven't mentioned that might restrict the use of backing strip remaining in service, particularly if it would trap liquid causing crevice corrosion on stainless steel or similar. As you see above, there is no straight answer yes or no.
Cheers,
gr2vessels

 

[Duwe6]

. . . and now you know that ASME really stands for Almost, Sometimes, Maybe, Except.

 

[weldstan]

If your fabricator cannot fit, tack and weld out a 12" by 3/16" wall with full penetration from one side using the GTAW process, you've picked the wrong fabricator. While backing is permitted, your design calculations and minimum wall chosen therefrom may not be acceptable based on the reduced joint efficiency. Whether the use of backing strips is acceptable for the corrosive service condition is another concern and the Engineer/Purchaser must be consulted for approval.