Why don’t the steel pipes with American standards DN150 and DN125 have a Sch30 wall thickness?
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It was found that whether it is national standards or chemical industry standards, for steel pipes under American standards, except for the DN150 (NSP6) and DN125 (NSP5) sizes, pipes of other sizes all have a wall thickness of grade Sch30; yet these two sizes do not have such a thickness, creating a discrepancy. It is unclear why this arrangement was made.
This is a quite professional question; it’s clear that you’ve studied steel pipe standards in great detail! I was also confused by this when working on pipeline design. My understanding is as follows: The Sch30 wall thickness actually represents a non-standard, simplified series defined in ASME B36.10M; it is primarily used for certain commonly used large-sized components in order to reduce weight or improve cost-efficiency. However, the DN150 (6") and DN125 (5") sizes are special from an engineering perspective; the critical points regarding pressure resistance and pipeline support design differ for sizes over 6 inches and those under 5 inches. It is possible that Sch30 does not provide sufficient strength redundancy for these two diameter ranges, or it may not be economical from a supply and demand standpoint, which is why these sizes were omitted in the standard formulation. Additionally, you can check the table headings in ASME B36.10M; sometimes the notes indicate that certain wall thicknesses are applicable only to specific size ranges. If the actual project requires this wall thickness, it is also possible to consider using Sch20 or Sch40 wall thicknesses as a substitute, but the pressure-bearing capacity and weight will need to be recalculated.