Issues regarding the selection of HG/T20553 Ⅰa versus its supply as Ⅰb
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For industrial pipeline design, the pipe size standards HG/T20553 are applied, with the Ia series being the preferred choice. Pipe manufacturing follows GB/T17395; the dimensions specified in GB/T17395 are divided into Series I, II, and III. The dimensions of Series I are identical to those of HG/T20553 Ib. In other words, choosing tubes from series Ia is essentially equivalent to using Ib tubes as a substitute. So the question is, are pipes of size 20553Ⅰa produced domestically? If it isn’t produced, why is the Ia series still given priority for 20553?
The outer diameter of the Ia series in HG 20553 2011 is the same as that of the I series in GB 17395 2008. These two are not contradictory; you can check by comparing the two standards. Basically, they all use imperial standards for outer diameter; the current trend is to employ imperial pipe diameters.
With low demand, manufacturers won’t produce it voluntarily. Since it’s not available in the market, customization is necessary, and the cost of customization is high; therefore, it’s generally not chosen. That’s the reason
You can look for papers on this topic. In particular, the introductions given by several experts from the design institutes were excellent; they provided detailed explanations on the origins and development of metric and imperial pipes, as well as the current situation in China. The metric pipe series is being phased out (it is no longer used internationally), which is why domestic standards related to this area prefer the use of imperial pipes. The flange series that are closely related to pipes have also gradually become standardized; originally, flanges were of various designs, but now they have increasingly adopted the European and American systems. The HG standard was established in 1997, followed by the 2009 version. Then GB also changed in 2019, dividing it into two series.