Why is the outer diameter of DN125 pipes in GB/T 12459 141.3?
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Hello everyone, I would like to ask why the outer diameter of DN125 pipes according to GB/T 12459 is 141.3? This value does not match the outer diameters of 139.7 or 140 Is there anyone in the industry who knows the reason?
Aren’t there the I series and the II series? The II series corresponds to European standard dimensions
The outer diameter of a 5-inch pipe in imperial units is 141.3. The standard series recommended is the imperial pipe series.
This post was last edited by zjq1962 on 2026-1-16 09:33. Did the original poster say that the imperial outer diameter for a pipe flange standard of DN125 is 139.7, but that it’s 141.3 for the pipe fitting? This issue also exists for the DN65 specification: the flange outer diameter is 76.1, but the pipe fitting is 73. It should be standardized in the new version of the flange standards next. In the GB/T17395 standard, these specifications belong to different series respectively.
In the GB 12459-1990 standard, the outer diameter of DN125 pipes is still 139.7; it was changed to 141.3 starting with the 2005 version. I couldn’t find any reason for this change, which puzzles me.
According to GBT 17395-2024, 139.7 or 140 belong to the general series, while 133 belongs to the non-general series； 141.3 belongs to a small number of special, dedicated series.
The outer diameter dimensions of pipes as specified in GB/T 28708, implemented in GB/T 12459 2025.
This post was last edited by zjq1962 on 2026-1-16 at 14:19. Making such changes can ensure consistency with ASME standards; the same applies to the outer diameter of DN65 fittings, which is 73. Another possibility is that for the outer diameter series in the pipe fitting standards, GB/T 28708 for pipes used in pipeline engineering is applied.
Mm-hmm, I’ve learned that a 125 flange can be used with so many different types of pipes