Backing Bar Materials

[muld0020]

I'm revisiting what should be a simple question. We typically use 1/4" thk backing bars on circ. seams (heat exchanger closing seams) since the material is SA-36 and comliant with UG-(5)(a) and UCS-12. The question has come up if we can use thinner material that is easier to work with (3/16" or 1/8"), but that material only comes in HR strip. HR strip has chemicals and cert of compliance, but the backing bar is integral with a pressure boundary and subject to pressure.

Can HR strip material be used? What code section(s) allow this?

 

[trottiey]

What is UG-(5)(a)? I think you're missing a number.
Your proposed material is HR strip of what? SA-36? Isn't SA-36 strip and bar normally hot-rolled?
Are you saying that the backing bar would be removed in one case and left in place in the other case? That could make a difference to your joint efficiencies and your welding procedure qualification.
What objection has been raised? That might help us understand the problem.

 

[SnTMan]

muld0020, see UW-5, esp UW-5(b)(3). I would not consider backing strips which remain in place as pressure retaining as once the weld is made it serves no further purpose. Unless of course you in some way in your design consider the strength of the backing bar as contributing to the vessel integrity. Then UW-5(a) would govern.

Regards,

Mike

 

[trottiey]

SnTMan, see interpretation VIII-1-86-116:
Question (1): Are backing strips left in place after completion of the vessel considered to be material subject to stress due to pressure?
Reply (1): Yes.
Question (2): Are backing strips, whether removed or left in place, considered nonpressure parts, such as skirts, baffles, and supporting lugs?
Reply (2): No.
[question 3 ommitted]

However, UCS-5(b) does allow you to use non-code material for backing strips, with conditions.

 

[SnTMan]

trottiey, well that's interesting, wasn't aware of that one. How can a backing strip that had been removed be considered a pressure part (i.e. not nonpressure) I wonder.

In that case it seems UW-5(a), UCS-5(b), etc. would govern.

Thanks,

Mike

 

[trottiey]

Interesting, I hadn't noticed that.

I think that might be an overzealous interpretation of the interpretation. It doesn't actually say that a removed backing strip is a pressure part. But even if it is a pressure part, ("materials used in the construction of welded pressure vessels" per UW-5,) it clearly is not "material subject to stress due to pressure," which is how UG-5, UCS-5, etc., are worded. So it could still be made of non-code material of proven weldable quality. (Otherwise UCS-5(b) would make no sense.)

I'm not sure what the impact would be of treating it as a pressure part. Maybe the intent is just to require identification of the removed backing strip? Maybe that's what UG-116(l) is getting at? I've puzzled over that one before and never really figured it out.

 

[SnTMan]

I would be more inclined to interpret UG-116(l) as applying to parts attached by bolting, clamps, etc, that can be removed, such as a bolted flat head, channels and shell covers of heat exchangers, etc. such that these parts don't need their own nameplates.

Probably the only "pressure" a removed backing strip is going to be subjected to is when it gets crushed at the scrapyard. At that point nobody cares about it

Have a good weekend, all.

Mike

 

[GenB]

Plate or Bar can be used for buck-up bar if you have documents and in vessels of similar metal. you have to check for the weldability and compatibility of the metals.
If c/s A/SA 36 to any other similar metals, not a problem.
finally: check with your AI for approval.
I see no Code violations for Sect VIII-1 here.
On the other hand there section I will not allow A/SA36.