General engineering orientation to help you specify and enquire with confidence. Guidance only — selection, configuration and scope are confirmed against your project specification at quotation.
Class and PN are designation systems
ASME flanges carry a pressure class (Class 150, 300, 400, 600, 900, 1500 or 2500); EN 1092-1 flanges carry a PN designation. Both are labels for a pressure-temperature rating group, not a single working pressure. The allowable pressure of any flange falls as service temperature rises and depends on the material group under the governing standard.
ASME pressure classes
Class numbers order the rating groups from lightest to heaviest. A higher class means a thicker, heavier flange with a higher allowable pressure at a given temperature. The exact rating is read from the pressure-temperature tables of the governing standard for the applicable material group — it is not the class number expressed in bar or psi.
EN PN designations
PN numbers follow the same principle for metric flanges: PN 10, PN 16, PN 25 and PN 40 are the common designations for general industrial duty, with higher PN ratings available for higher-pressure metric systems. The applicable EN 1092-1 flange type and material determine the actual rating at temperature.
Why Class and PN are not equivalents
Approximate correspondences between Class and PN designations are sometimes quoted for orientation, but the two systems use different dimensional bases, bolting and rating tables. Treating them as interchangeable can produce a joint that does not seal or bolt correctly. Where a system mixes ASME and EN equipment, the interface must be engineered deliberately and confirmed against both standards.
What to confirm at enquiry
- Governing standard and edition
- Pressure class or PN designation
- Service temperature and fluid
- Material grade (rating tables vary by material group)
- Counter-flange details where matching existing pipework
These are buyer-input prompts only and are confirmed against your project specification. Send them with your enquiry for engineering review.