Questions regarding pipe fitting standards
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For example, for a seamless reducing joint of DN50X25, is the standard GB/T12459 or GB/T14383?
If butt welding is used, it should be GB/T12459; if socket welding or threaded connection is used, it should be GB/T14383. Generally, socket welding is used for diameters below DN50, while threaded connections are used for instrument air; therefore, GB/T14383 should apply in this case. However, some design institutes use butt welding for diameters below DN50, in which case GB/T12459 applies.
In fact, there are no large or small ends in 14383; generally, 12459 butt-welded seamless large and small ends are chosen!
My question now is: since butt welding is generally used for diameters above DN50, and socket welding for those below DN50, doesn’t that mean there are two different standards for pipe fittings? !
You’re right; domestic standards are usually quite detailed. Specifically, the SH standards include SH3410, SH3408, and SH3409. The GB standards are GB 14383, GB12459, and GB13401 (for plate-made pipe fittings). ASME standards include ASME B16.11 and ASME B16.9. I won’t go into detail regarding the names of these standards – explanations can be found on Baidu. As you mentioned, DN50 pipe fittings are generally of the BW type, so GB12459 should be applied in such cases. Instrument air pipelines are a separate matter, of course!