Why do the grades of pipes and fittings for socket welding be relatively high?

Socket welding is mainly used for small-diameter pipes, as it offers better sealing and greater strength. Choosing pipes with higher wall thicknesses (such as Sch80) and fittings of a higher grade (such as Class 3000) can provide a higher pressure rating and better pressure resistance. Even in low-pressure environments, this design ensures safety and durability. Compared to butt welding, socket welding is easier to install, and it requires relatively lower skills in welding operations. Regarding the material selection for valves, forged steel is commonly used for small-diameter valves, as forging enables a more uniform and dense material structure, thereby enhancing strength and pressure resistance. Large-diameter valves, due to their larger size, have lower casting costs and are easier to manufacture. If a butt-welded connection is chosen, pipes with DN40 size can use thinner wall thicknesses such as SCH40, and fittings and valves of lower standard grades, such as 150lb, can be used; this reduces costs but may compromise strength and pressure resistance to some extent. In summary, the choice of welding method and material standards should be determined based on the actual project requirements and safety standards. .

If the pipe is too small, its strength is poor. It can be understood in this way: when small pipes are welded using butt welding, the welds are relatively short, and due to issues related to the weld coefficient, under the same torque, the welds on small pipes are more likely to crack. That’s why socket welding is used to ensure strength, effectively providing reinforcement at the root of the pipe. I would like to ask which standard specifies the use of socket welding for pipes below DN40; I remember it being for pipes below DN50.

With the wall thickness increased by so much, isn’t it sufficient to just perform direct welding?

This post was last edited by Wang Er_O9u3H on 2024-12-2 at 23:54. After reviewing various domestic and international standards, the main reasons were found to be as follows: 1. Not all valve pressure ratings specified in standards correspond one-to-one with those of the flanges; many small-sized socket-welded valves are only available in CL800/CL1500 ratings. 2. By comparing some standards for valves that have both CL800 and other pressure ratings, it was found that there are no significant differences in parameters such as size, wall thickness, and flow channel diameter between those rated CL800 and those rated CL1500. Only minor differences arise when using extended valve covers and bellows gaskets. However, CL800 allows for higher operating conditions, and in socket-welded and threaded connection systems, there is no need to consider compatibility with corresponding flanges, which in turn expands the range of applications. Thus, there is no need to use valves with lower pressure ratings. 3. As for why the weight class and wall thickness of socket weld pipe fittings and valves are significantly higher than those of pipes and flanges, an important reason is that there is no need for more detailed gradations at small sizes; too many gradations would result in an excessive number of product lines during manufacturing, thereby increasing production costs. This is the result of a comprehensive consideration of various factors.