By Ray Delaforce
Software Engineer, Intergraph
Part 1: Joint Efficiency and Welded Joint Category
When correctly applied, joint efficiency determines the required thickness of a vessel component. This article concerns the principles applied by the ASME Section VIII, Division 1 pressure vessel code, because it is the most widely used code worldwide.
The application of joint efficiency is often misunderstood, and results in violations of the code, and even costly rework. This article attempts to clear away some of the misconceptions. We start off by defining the meaning of joint efficiency, denoted by the letter ‘E.’
That definition cannot be found anywhere in the code, but if you think about it, that must be the definition. We know the specified strength of the parent plate, because we can get it directly from the code (ASME Section II-Part D). But, how can we assess the strength of the weld metal? Actually, the code uses another method, but first we need to know a little more before we can apply a joint efficiency.
We need to know the meaning of these two terms first:
- Weld CATEGORY
- Weld TYPE
First, let us deal with the word CATEGORY. This word simply means LOCATION. Perhaps the code should have used the term ‘location’ to make things clearer. Look at this illustration that comes from Figure UW-3:
The longitudinal joints are Category A, and the circumferential joints are Category B. There are other categories, but we shall concern ourselves with these two categories for the sake of this article. What you need to know is that the stress in a Category A weld is twice as large as the stress in a Category B weld. That should tell us that the Category A weld is more critical to the safety of the vessel than the Category B weld. The code pays particular attention to Category A welds. Those stresses are illustrated here:
Stay tuned for next month's newsletter when we discuss welded joint type and quality.
Sign up for the Insider Newsletter today!