Regarding DN50 steel pipes, is it mandatory to use socket weld in the following cases?
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This post was last edited by “Little Worker” on November 19, 2024, at 10:20. Dear experts, as the title suggests, in all the projects I’ve worked on in the past, butt welding was used; socket welding wasn’t employed even when dealing with small-diameter pipes. All the pipes used in those projects were metric-sized, and their wall thicknesses increased progressively along with the pipe diameter—the wall thickness remained consistent relative to the pipe diameter. This is quite different from projects that follow American standards. For instance, under American standards, small-diameter pipes have relatively thick walls, and there are cases where larger-diameter pipes actually have thinner walls than smaller ones. This contradicts my theoretical calculations based on wall thickness formulas, which predict that small-diameter pipes should have thinner walls. After asking around, most people mentioned concepts like standardization and normalization. However, I fail to understand the rationale behind this practice. I hope some knowledgeable experts can shed some light on this matter. Moreover, the small-diameter pipes are joined using socket welding. I really don’t understand this. In previous projects, it was stated that socket welding should be used for pipes with a diameter of DN50 or less, because butt welding requires a lot of energy; given that small-diameter pipes have thin walls, they’re prone to being burned through during butt welding. But if you use socket welding, why is the wall thickness still so great? With such a thick wall, butt welding would suffice. Additionally, socket-welding fittings are quite expensive. I simply have no idea why this approach is adopted
Whether butt-welded joints must be used for DN50 steel pipes mainly depends on design requirements, engineering standards, and on-site construction conditions. Generally speaking, both butt welding and socket welding can be used for connecting steel pipes with a DN50 size. However, determining which method is more suitable requires considering various factors, such as the material of the pipe, operating environment, pressure rating, and ease of construction. American and European standards differ in their specifications regarding pipe wall thickness, which leads to differences in design and material selection. It is recommended to determine which welding method to use based on the specific project requirements and relevant specifications. .
The use of socket welding and welding processes is also relevant for pipes with a diameter of less than 50. In cases where cost-effective semi-welding is desired, socket welding is generally more suitable for smaller pipes. Currently, full welding is the commonly used method; it has lower requirements regarding wall thickness. As a result, many now prefer to use butt welding even for small pipe fittings. Socket welding, on the other hand, involves higher material costs. I’ve also noticed the problem you described regarding the American standard; the response I received was also related to standardization. In reality, it’s sufficient for it to meet the requirements of strength calculations—there’s no need to overthink it.
1. For pipes with a diameter of DN50 or less, socket welding isn’t necessarily required. Several previous speakers have already provided the reasons for this; ultimately, a comprehensive judgment must be made based on the medium, temperature, and pressure conditions； 2. For ASTM steel pipes, the outer diameter and wall thickness comply with ASME B36.10 and ASME B36.19. Taking ASME B36.19 as an example, in order to reduce the number of specifications, there are only the following wall thicknesses: (5S, 10S, 40S, 80S, 160S). For CL150, 10S might not be sufficient for the smaller sizes; therefore, 40S was chosen instead. For the larger sizes, however, 10S can be used. This results in the phenomenon where the larger sizes are thinner than the smaller ones. 3. Of course, fundamentally, it’s still necessary to meet the wall thickness calculations; this is the foundation. However, for the sake of standardization, ASME defines fewer “classes”. In most cases, for small-sized valves in CL150, CL300, and CL600, 40S can be used instead. This reduction in specifications facilitates inventory management during manufacturing and allows for the interchangeability of materials across different pressure classes on-site, thereby reducing costs.
I’ve looked at several projects; in all of them, for pipes under DN50, socket weld joints are used. However, the wall thickness of these socket-welded pipes is quite substantial, so there’s no problem using butt welding instead. Moreover, socket-welded fittings are more expensive than those used for butt welding. Previously, I understood that for pipes smaller than DN50, butt welding isn’t used because their wall thickness is thin; using butt welding could easily result in burn-through. However, these pipes have a wall thickness of SCH80 or more. Given this thickness, I’m confused as to why socket welding is still being used
Yes, this is the situation. Many years ago, I remember that my previous company used socket welding; the reason being that for small-diameter pipes, the wall thickness is relatively thin, making butt welding prone to penetration. Later, I changed companies, and we switched to butt welding instead, as the wall thickness met the requirements. However, recently I noticed that other companies still use socket welding for pipes with a diameter of DN50 or less. The problem is that the wall thickness of those pipes isn’t actually thin at all—it’s quite thick, even thicker than the pipes I used to weld using butt welding. I really don’t see the point of using socket welding in this case
Yes, when using the Chinese standard specifications previously, the wall thickness gradually increased; this is also in line with the wall thickness calculation formula. However, under the American standards, small-diameter pipes are joined using socket welding, and their wall thickness is even greater. For pipes with a diameter of DN50 and above, the wall thickness actually decreases. Also, why might the wall thickness of 10s be insufficient for smaller sizes, while it’s adequate for larger sizes? According to the definitions in the pipe schedule, the design pressures for both pipes should be the same; thus, their pipe schedule numbers ought to be identical. How is it possible that the wall thickness of the smaller pipe proves insufficient after just 10 seconds, while that of the larger pipe remains adequate?
You only considered strength, not stiffness.
It’s not that there isn’t enough; there’s just a minimum wall thickness requirement. Small-sized pipes should have at least 40S.