Can you have joint efficiency of 1.0 on a spot radiography joint per uw-11(a)5(b)? 2

[asmeqc1]

Hey everyone, I have a question about joint efficiency. Take the case of a vessel with an elliptical head attached with a type 2 joint that meets spot radiography requirements per UW-11(a)5(b). The vessel also has a single longitudinal joint and all Cat A welds are type 1 with full radiography.

Now I am being told that the lowest efficiency on this vessel is 1. However, my understanding that spot radiography (6” spot) qualifies at 0.7. Where am I going wrong? Is it really possible to have a joint efficiency of 1 and still have spot radiography?

Cheers

 

[asmeqc1]

Nevermind, just answered my own question.

Turns out yes, a type-1 joint using spot radiography can indeed be type 1, which might throw you off if you were looking at the table in uw-12 which says its 0.85. This is why. UW-12(d) states:

"For calculations involving circumferential stress in seamless vessel sections or for thickness of seamless heads, E=1.0 whethe spot radiography requirements of UW-11(a)(5)(b) are met."

I thought I was going crazy for a sec!

 

[TomBarsh]

I don't have my Code book in front of me and it's way past my bedtime (which affects my thought processes). But this is likely the single most-asked question in my experience. At our office I've had this same type of question several times a week (for many years).

The joint on which the "special" spot radiograph is performed will not itself obtain E = 1.0. But if this is performed on the circ seam attaching a seamless head to the shell then the head thickness calculation can be based on 1.0 (otherwise, 0.85 as you describe). Further, for a seamless shell or a welded shell with full RT on the long seam the two end circ seams can be the "special" spot radiograph and the shell will obtain E=1.0 for purposes of required thickness based on circumferential stress. But if there is no RT or the rules of UW-11(a)(5)(b) are not met then that shell will only obtain E = 0.85.

The circ seam that the "special" spot radiograph is performed on will itself have E either something like 0.7 or 0.85, depending on whether the joint is otherwise "spot" or "none" (and those values E will decrease for a Type 2 joint).

A situation that could arise is a vessel with seamless heads, seamless shell, no RT except on the two head-shell seams that meets UW-11(a)(5)(b). E = 1.0 for the heads and for the shell for circ stress. But mount this vessel on a skirt attached to the bottom head, now you need to consider the longitudinal stresses in the shell. For this calculation (required thickness based on longitudinal stress) E = 0.7 (for Type 1 "None") or E = 0.85 (for Type 1 Spot). Remember that the "special" spot radiographs cannot also be used to meet the normal Spot RT requirements.

Having spot with the "special" spot radiographs is often called "spot-plus". There was a Code Interpretation from about 12 years ago that permitted having no radiography but with the special spot radiographs for UW-11(a)(5)(b). I refer to this as "none-plus". I have only heard of a 3-4 people at most asking me about this condition.

The firm lesson is that vessel designers need to read the paragraphs of UW-11 and UW-12, and not just refer to Table UW-12