【Haichuan Chemical Valve Management】Series Posts——Installation Techniques for Welded Valves——The Differences Between Butt Welding and Socket Welding

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Proper management of valves is the foundation for stable operation. Everyone is welcome to engage in discussions and share insights on this series of posts: 【HaiChuan Chemical Valve Management】 series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ---------------------------------------------------------------- Disclaimer: The content contained in this article is intended solely for technical exchange and reference purposes only; it does not constitute any form of professional engineering advice, design basis, or operational guidance. -----------------------------------------------------------------------------------Valves on industrial pipelines can be connected not only by flanges but also by welding. Welded valves are of two types: butt welding (BW) and socket welding (SW). Butt-welded valves: The connection end of butt-welded valves features a groove with dimensions matching the outer diameter of the pipe, and the valve is welded to the pipe at this junction. Butt welding is characterized by high weld strength, and it is suitable for pipes under high pressure, at high temperatures, and with large diameters. The welding end of a butt-welded valve must match the material of the pipeline; typically, the welding end of the valve is made of forgings, with the same material as the valve body. Socket weld valves: The connection end of a socket weld valve is a “socket”; the pipe is inserted into it and then welded. Socket welding is suitable for valves with small diameters (below DN50). The weld strength of socket welding is lower than that of butt welding, but it is easy to align the components during installation, making it suitable for on-site installation. Installation steps for welded valves: Before welding, the welding ends must be cleaned to remove oil and rust. The valve must be in the open position to prevent welding heat distortion from damaging the sealing surface. If the valve has a closing element, it must be protected to prevent welding slag from getting inside. Solder the valve in place and check whether the valve and pipeline are aligned. Weld in segments and control heat input to prevent overheating and deformation of the valve body. Inspect the appearance of the weld after welding, and perform non-destructive testing (radiography or penetrant testing) if necessary. Common problems with welded valves Problem 1: Welding deformation. The valve body deforms due to heat, causing the sealing surface to fail. The solution is to control the heat input, weld in segments, and perform post-weld machining on the sealing surface if necessary. Question 2: Slag damages the sealing surface. Weld spatter falls on the sealing surface, preventing the valve from closing properly. The solution is to insert a protective pad into the valve chamber before welding, in order to close the valve disc and protect the sealing surface. Question 3: Welding cracks in dissimilar steels. Due to the different materials of valves and pipes, welding is prone to cracking. The solution is to choose the right welding material, apply a surfacing transition layer, and control the preheating temperature and cooling rate. Question 4: Excessive weld stress. Welding residual stress causes the valve to deform or crack. The solution is to perform post-weld heat treatment to relieve stress. Material matching for welded valves: Carbon steel valves are welded to carbon steel pipes using direct welding. Alloy steel valves are welded to alloy steel pipes, requiring preheating and post-weld heat treatment. Stainless steel valves are used to weld stainless steel pipes, with heat input being controlled to prevent intergranular corrosion. For carbon steel valves welded to stainless steel pipes, a surfacing layer must be welded on. Acceptance criteria for welded valves: The surface of the weld joint must be smooth, with no cracks, pores, or undercut. The weld dimensions must meet the design requirements. Important valves are to undergo radiographic testing (RT), and acceptance shall be carried out in accordance with ASME B16.34 or GB/T 13927 standards. In one sentence: For large-diameter butt-welded joints and small-diameter socket-welded joints, the sealing surfaces must be protected before welding; transition zones are necessary when welding different types of steel, and flaw detection is essential after welding.