Regarding standard issues
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There are the following standards: HG20615-1997, Steel Pipe Flanges – Types and Parameters (American System). What does the American System refer to? ? At first, I thought this was the American standard, but later I found out it was the standard set by the Ministry of Chemicals. I’ve never been able to figure it out – is it possible to change this American system to a European one? ?
The so-called American system is simply the result of rounding American standards. If you examine it carefully, you will find that it’s nothing more than rounding the British dimensions of B16.5 to the current metric dimensions