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Buyers who source flanges for both European and American-designed plants meet two systems that look similar but are not the same: EN 1092-1, where flanges are rated by PN and sized by DN, and ASME B16.5, where flanges are rated by Class and sized by NPS. Ordering the wrong one is one of the most common reasons a flange arrives on site and does not bolt up.

Scope of each standard

EN 1092-1 covers circular steel flanges in PN ratings such as PN 6, PN 10, PN 16, PN 25, PN 40, PN 63 and PN 100, from small DN sizes up to very large diameters. Bolt holes, bolt circles and dimensions are metric, and bolting is metric (M12, M16, M20 and so on).

ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 to NPS 24 in Classes 150, 300, 400, 600, 900, 1500 and 2500. Larger sizes are covered by ASME B16.47. Dimensions are based on inches and bolting is usually unified inch thread (UNC/8UN).

How the flange types are named

EN 1092-1 identifies flange types by number, while ASME B16.5 uses descriptive names. The common equivalents are:

Facings are also coded differently: EN 1092-1 uses letters (Type A flat face, Type B raised face, Types C/D tongue and groove, Types E/F spigot and recess, Types G/H O-ring), while ASME B16.5 uses RF, FF, RTJ, tongue and groove, and male and female.

PN and Class are not converted one-to-one

A PN number is not the allowable pressure at every temperature, and neither is a Class number. Both systems give pressure–temperature ratings that depend on the material group and the design temperature. For example, the allowable pressure of a carbon steel flange falls as temperature rises, and a stainless steel flange of the same rating can have a different curve. Always check the rating table for the exact material group in the edition of the standard named on the purchase order.

Can EN and ASME flanges be bolted together?

As a rule, no. Bolt circle diameters, number of bolt holes, hole sizes and flange thickness differ between PN and Class flanges. In a few sizes the dimensions come close, but close is not enough for a pressure joint. Never assume interchangeability; compare the drawings or, better, order matching flanges for both sides.

Where a European project needs class-designated flanges, EN 1759-1 exists for that purpose. It is a different standard from EN 1092-1 and should be named explicitly.

Pipe outside diameters

Weld neck flanges must match the pipe outside diameter and wall thickness at the weld end. Many DN sizes share the same outside diameter as the corresponding NPS size, but not all pipe series do. State the pipe OD and wall (or schedule) in the order so the bore of the weld neck is machined correctly.

Buyer checklist

• Name the standard and edition: EN 1092-1 or ASME B16.5 (or B16.47 for large sizes).
• Give DN and PN, or NPS and Class – never a mix of both.
• State flange type and facing (for example Type 11 / B1, or WN RF).
• For weld neck flanges, state pipe OD and wall thickness or schedule.
• Specify the material standard and grade, and the inspection certificate (for example EN 10204 3.1).
• Confirm the bolting system (metric or inch) matches the mating flange.