In this this article, guest author, Ramesh Tiwari discusses the issues in calculating the weld efficiency of a seamless section according to the ASME Pressure Vessel Code. Specifically he focuses on how to do it using radiographic and ultrasonic examination. For the non technical, the efficiency of welded joint (weld efficiency) is an artificial number that defines it strength based on the amount of non-destructive testing that is undertaken on it. With no testing the weld efficiency could be as low as 60% but with increasing levels of ultrasonic and radiographic testing this can increase to 100%.
Paragraph UW-11(a)(5)(b) and Weld Efficiency
The following sample vessel illustrates how to calculate the efficiencies of a seamless section according to UW-12 when UW-11(a)(5)(b) is utilized.
The vessel is made from 3 components — A rolled and welded shell and two seamless elliptical heads. The long seam in the shell and the head to shell welds are spot radiographed according to rules for radiography of paragraph UW-52. All welds are Type 1 butt welds. What is the weld efficency (E) for the rolled shell, and for the seamless heads? Efficiencies of seamless sections are found in UW-12(d) – the two heads are seamless. UW-12(d): Seamless vessel sections or heads shall be considered equivalent to welded parts of the same geometry in which all category A welds are Type No 1. For calculations involving circumferential stress in seamless vessel sections or for thickness of seamless heads, E = 1.0 when the spot radiography requirements or UW-11(a)(5)(b) are met. E = 0.85 when the spot radiography requirements of UW-11(a)(5)(b) are not met, or when the category A or B welds connecting the seamless vessel sections of heads are Type no 3, 4, 5 or 6 of Table UW-12. The sample vessels welds are Type 1 so the bit about Type 3, 4,5 or 6 welds can be ignored, but UW-11(a)(5)(b) can be utilized. UW-11(a)(5)(b) can not be read on its own. UW-11, UW-11(a) and UW-11(a)(5) are also needed as reference in order to understand the context of UW-11(a)(5)(b). To quote UW-11(a) — Full Radiography
Radiographic Examination of Welded Joints
The following welded joints shall be examined radiographically for their full length. Starting with UW-11(a)(5): All Category A and D buttwelds in vessel sections and heads where the design of the joint or part is based on a joint efficiency permitted by UW-12(a), in which case: and finally — UW-11(a)(5)(b): Category B or C butt welds [but not including those in nozzles or communicating chambers except as required in (2) above] which intersect the Category A butt welds in vessel sections or heads or connect seamless vessel sections or heads shall, as a minimum meet the required for spot radiography in accordance with UW-52. Spot radiographs required by this paragraph shall not be used to satisfy the spot radiography rules as applies to any other weld increment.
Simplification of UW-11(a)(5)
Definitely a mouthful, start simplifying UW-11(a)(5). A Cat A weld is a long seam. Combine it with UW-11(a)… All long seam welds in heads or shells shall be type 1 or type 2 and shall be radiographed for their full length. Simplify paragraph UW-11(a)(5)(b) Spot radiographs required by this paragraph shall not be used to satisfy the spot radiography rules as applies to any other weld increment. This means that the radiography used to satisfy paragraph UW-11(a)(5)(b) cannot be used to satisfy other weld increments that have spot radiography rules applied to them. For example. If the vessel was to be stamped RT-3 then all welds are required to be spot radiographed. You cannot use the radiographic shots for welds satisfying RT-3 stamping. Paragraph UW-11(a)(5)(b) radiography MUST be separate shots…
3 Simple Requirements for Weld Efficiency
…so to continue on… Ignoring the possibility of nozzles or communicating chambers intersecting welds, we get the following: Category B or C butt welds which intersect the Category A butt welds in vessel sections or heads or connect seamless vessel sections or heads shall, as a minimum meet the required for spot radiography in accordance with UW-52. Combining the simplified statements (the possibility of intersecting chambers and nozzles has been ignored) — 3 simple requirements: To meet UW-11(a)(5)(b) for the long seam in question: (1) Fully radiograph the long seam or choose seamless materials (2) Long seam must be Type 1 or Type 2 (3) Spot or fully radiograph the joining circ weld For the vessel shell, the long seam is not fully radiographed, and is not a seamless section, so UW-11(a)(5)(b) is not met for the shell long seam. For each vessel head, the long seam in the head is “assumed” to be Type 1 and is seamless, so UW-11(a)(5)(b) is met for the imaginary head long seam. Getting back to the sample vessel and UW-12. Imaginary (assumed) Type 1 seams have been added to the heads:
UW-12(d) is readable once UW-11(a)(5)(b) is known. To paraphrase for the sample vessel: The imaginary welds in the head have an efficiency of 1.0 if UW-11(a)(5)(b) is met and 0.85 if it is not. For the sample vessel, the head efficiency is 1.00. The shell long seam efficiency is read directly from table UW-12 – it is 0.85.