The issue of imperial and metric systems
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Today I heard that British standard 4/8 pipe sizes correspond to metric DN15 and DN20 pipes. However, DN denotes the diameter of the pipe, and it is not a way of indicating either the inner or outer diameter; it’s not as clear as using Φ to represent the outer diameter. So, do British standard 4/8 pipe sizes refer to the inner diameter, outer diameter, or the diameter overall? I really don’t know much about this – please don’t laugh at me
DN refers to the valve diameter, that is, the inner diameter. 1/2\" and 3/4\" in imperial units correspond to what you call 4mm and 6mm; these correspond to DN15 and DN20 in metric units, with DN also referring to the inner diameter
Thank you, sir upstairs, but the measurement we obtained using a caliper for the inner diameter of the 4-inch pipe is not 12.7 mm; it’s slightly larger. Why is that? The wall thickness of the 4-inch British standard pipe we have is 1 mm, and its inner diameter is approximately 13 mm
What was said upstairs seems incorrect! ! The nominal diameter is neither the inner diameter nor the outer diameter of a pipe; it is merely a common term used to describe a pipe’s size. However, pipes with the same nominal diameter generally have the same outer diameter ； There is no inherent connection between the dimensions of imperial pipes and metric pipes; it’s just a rough comparison! ! !
What was said on the 4th floor is correct; this refers to the nominal diameter. Pipes with different pressure ratings may have the same nominal diameter, but their inner and outer diameters will vary. You need to determine the standards first, and then you can find the relevant pipe standards to find out their inner and outer diameters.
The original poster can take a look at HG20553-1993; actually, the LA series corresponds to what we call the imperial pipe series, with detailed specifications for the outer diameter, while the LB series represents the metric pipe series. For example, for a DN15 pipe, its inner diameter is not fixed; it depends on the wall thickness calculated based on the pipe’s corresponding temperature and pressure rating, while the outer diameter remains constant.
The nominal diameter is not the outer diameter nor the inner diameter, but rather a nominal size that approximates the inner diameter of ordinary steel pipes. For each nominal diameter, there is a corresponding outer diameter, while the inner diameter value varies depending on the thickness. The nominal diameter can be expressed in metric mm or imperial inches.