Can the branch pipe fitting be welded directly to the flange?
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Dear experts, during the on-site construction of a chemical plant, it was found that there were two branch pipes coming off the same pipeline. In one case, the branch pipe connection was made by directly welding the branch pipe fitting to the flange, while in the other case, the branch pipe fitting and a short pipe were used as an intermediate element before welding to the flange. The specifications for the branch pipe fitting are carbon steel A105, 50X25, SCH80; the welding method used is butt welding, in accordance with MSS SP-97. The specifications for the flange are DN25, CL300, A105, WN-RF, SCH80, HG/T 20615. I would like to know whether it is possible to weld the flange directly to the branch pipe fitting, and whether there are any negative consequences associated with such welding. I have checked the relevant standards for flanges and branch pipe fittings, but could not find any rules prohibiting such welding. However, I have a feeling that it might not be a good idea, though I am not sure why. I would appreciate it if you could give me some advice on whether it is appropriate to weld the flange directly to the branch pipe fitting. Thank you. For details, see the screenshot below:
I took a closer look; there is a problem with the first drawing. The main pipe fitting of DN50*25, along with a DN25 flange, should not have a distance of less than 119 mm between them – it seems that a short pipe segment was missing. The single-line diagram (pipe section diagram) in the design is incorrect; you should check whether the length of the pipe fitting plus the length of the flange is less than 119 mm. If the several drawings you listed are installed at the same elevation, the piping layout will not look good either.
I took a closer look; there is a problem with the first drawing. The main pipe fitting of DN50*25, along with a DN25 flange, should not have a distance of less than 119 mm between them – the design team failed to add the necessary short pipes. The pipeline layout diagrams drawn by the design team are incorrect; you should check whether the length of the pipe fitting plus the length of the flange is less than 119 mm. If all the drawings you have are at the same elevation, the piping layout will not look good either.
This is not the usual way of welding; there must be a straight pipe section in the middle. Even if it is designed this way, it won’t be welded like that in practice.
If the branch pipe fitting is socket-welded, it cannot be welded either.
Okay, thank you all. I’ll get in touch with the designer again
We have welded it this way on-site countless times; the design drawings show it exactly like this
There’s definitely a problem with the first diagram; the pipes were omitted. The combined dimensions of the DN25 pipe socket and flange should not be 119. Generally, it still can’t be welded directly!
It usually comes with a short tube; it cannot be welded directly. The diagram is incorrect
Generally, straight pipe sections need to be added. If it is butt welding with aligned diameters, rather than socket welding (SW) or surfacing welding (SO), direct welding is possible; however, almost no one does this, as it is likely that the machinery will not be able to reach that area during subsequent non-destructive testing or heat treatment (I can’t remember exactly what it is). Therefore, it is recommended not to carry out direct welding.