What are PN10 , PN16 and PN25 Standards?

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PN10, PN16, PN25 and PN40 specify both pressure class in bars of pressure, and metric flange dimensions (such as bolt circle diameter, hole size, bolt size etc) according to international flange standards such as ISO 7005-1 or DIN 2501 that use the metric system of measurements. Flanges produced to ANSI, AWWA, ASA, or old British standards that are dimensioned in inches will not mate up with metric flanges in general. It is good practice when specifying a PN flange standard to indicate the actual working pressure since it is a common occurrence that a specific PN, say PN16, flange drilling is required on a fitting or valve to match adjacent flanges, but the pressure requirement is less than PN16.