Backing Strips below -20F

[david339933]

Anybody tell me how they handle backing strips for Cat. B seams for MDMT below -20F? Backing strips are considered a pressure part, but are they considered a base metal and therefore UCS-67(a)(3) is applicable?

Using a Curve D material for backing strip, Shell and Head (Curve B) are exempt per UCS-66.2. MDMT = -40F.

Doesn't make sense to use impact tested consumables just for a backing strip, which would be removed when machining an impact sample.

 

[Morcuse]

How are you determining the backing strip MDMT?

 

[david339933]

Curve D material (SA-516-70N - 1/4" thick): Good to -55F
I asked for an interpretation on this, but they will not answer....this is somehow considered consulting!?!

My issue is that UCS-67(a)(3) states for base metals exempt by Curve C or D and MDMT is colder than -20F and not colder than -55F, the WPS doesn't need impacts if each lot of filler metals is impact tested.

Head and Shell are exempt per UCS-66.2 so no need for impact WPS, but adding the Curve D backing strip, leaves my AI questioning.

I am going with the argument that the backing strip is not a base metal, but backing, based on definitions in Sec. IX. Therefore UCS-67(a)(3) is not applicable as it references base metals only.

 

[Morcuse]

Why would you consider backing strips a base metal anyway? What makes one of these different from a reinforcing pad for MDMT purposes?

 

[david339933]

Because the backing strip is considered a pressure part and must be good for the MDMT. I have never given this much thought until my AI brought up the question. I have always just ensured the backing strip material if the MDMT was below -20F was of a proper material...basically changing from SA-36 (exempt to -20F per UCS-66(h) to SA-516-70N (Curve D material exempt to -55F).

 

[Morcuse]

Reinforcing pads are also pressure parts and must be good for the MDMT, isn't that right?

 

[DekDee]

David,
Would like to try to help but unsure if this is relevant.
We do not include internal or external reinforcement in any calculations be they ASME IX or design code.
Why would a backing strip be any different to internal reinforcement ?
Not part of the base metal so not part of the equation ?
Regards,
Shane

 

[david339933]

Thanks DekDee,
That's the stand I am taking...and to most seems logical. Would have been nice to have a yes or no ASME answer to present to my AI though.

As for repads, there are ways to reduce without changing material groupings.

 

[Morcuse]

On this regard I would like to quote the introduction:

"The Code is not a handbook and cannot replace education, experience, and the use of engineering judgment. The phrase engineering judgment refers to technical judgments made by knowledgeable engineers experienced in the application of the Code. Engineering judgments must be consistent with Code philosophy, and such judgments must never be used to overrule mandatory requirements or specific prohibitions of the Code"

Best of luck on convincing your AI

 

[r6155]

Backing strip is not a good idea for low temperature service.
Avoid backing and use GTAW for the root pass.
You can use Standard Welding Procedure Specifications, see ASME IX APPENDIX E.

Regards

 

[david339933]

Seems the Committee had second thoughts on my question and decided to answer.

Apparently there is a Revision to the 2021 Code stating the coincident ratio for nozzle repads, saddle attachments etc. shall be that of the shell or head to which the component is attached.