What should be the joint efficiency for below type circumferential joint with thickness not exceeding 16mm?
Abhishek Singh
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In today’s article we will look at Type of joints and efficiency in pressure vessels.
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Let's begin
What is Subsection B?
Subsection B is a section of the ASME SEC VIII Div 1 which talks about requirement pertaining to methods of fabrication of pressure vessels. It consists of
Part UW: Requirements for pressure vessels fabricated by Welding
Part UF: Requirements for pressure vessels fabricated by Forging
Part UB: Requirements for pressure vessels fabricated by Brazing
?Part UW which is used widely. Part UW shall be used in conjunction with Subsection A and Subsection C. To read and understand Part UW, we need to be very thorough in UW-3 (Welded joint category) and UW-12 (Joint efficiencies). This article is focused on the understanding of UW-12.
UW-12: Type No. of Joints
In this, we will discuss the different types of welds that determine joint efficiency, and the factors that impact it. We will also explore how we can achieve full penetration in welds.
Type Number of Joints:
The type number of joints in welding determines the joint efficiency along with radiography requirement. We need to be careful while designing and fabricating, ensuring that the type number of joints is correctly mentioned as per code so that we can get the joint efficiency as one. However, it is a common misconception that full radiography means joint efficiency as one.
Type No. (1):
If we want to achieve joint efficiency as one by full radiography, we need to select type number one.
This type requires welding ensuring full penetration, and complete fusion of the root. The configuration for this type of joint is a butt weld joint with both sides of the welding. We weld it from one side, chip back, and grind from the other side to remove the unpenetrated or lack of fusion. After this, we do PT to ensure that all the lack of penetration area has been removed and then we build again from the other side, ensuring that the root is completely fused. This joint is most suitable if both sides is accessible for welding.
If the vessel being welded is small or has a closing joint without a manway opening, it may be difficult to access the inside to do the necessary grinding. In these cases, a temporary backing strip can be used to provide a base for fusing the root. This backing strip is only temporary and must be removed after the welding is complete. Using a backing strip can help ensure the complete fusion of the root and achieve full penetration.
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Now, let's talk about the third type of welding, which is a little trickier because there is no temporary backing strip and the welding is also from one side. In this type of welding, we need to ensure that the blue area shown in the figure, which is the root, is completely fused. This depends on the expertise that a fabricator should have, and it has to be demonstrated by using mock-up welding, which afterward needs to be tested. One of the way to achieve full penetration in this case is by using Tungsten Inert Gas (TIG) welding.
Type No. (2):
This type of joint is used when we have limited access to the other side of the material, like closing joint, we cannot access the joint once we weld it. We can use a backing strip that will remain in place. The backing strip is crucial to ensure that the root of the joint is fused properly. However, the downside of using a backing strip is that we cannot remove it once we've welded it. This means that we have to leave it in place. As a result, this type of weld joint is called Type No 2. Unfortunately, the use of a backing strip means that we will not be able to achieve a joint efficiency of one. In fact, even if we perform full radiography, we will only get a joint efficiency of 0.9.
Limitation: Please note that these types of joints are limited only to, circumferential butt joints not exceeding 16 mm and with one plate offset as shown in below image.
Please note that this type of welds qualifies as Type 2 butt weld joints and with full radiography we can achieve efficiency of 0.9.
Type No. (3):
In this type of joint, welding is done from one side only, as there is no access available to the other side of the joint, not even to place a backing strip. Even if we do 100% radiography for this type of joint, the joint efficiency will be only 0.60 as good as no radiography.
Limitation: Please note that these types of joints are possible only for Circumferential butt joints only, not over 16 mm thick and not over 600 mm outside diameter.
Other types of Joints:
Now, there can be a variety of different welds types like lap joints or corner joints. They do not offer any advantage in terms of radiography so for these welds even if we do radiography no advantage can be taken in terms of joint efficiency.
Thank You…
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