A flange rating designation is not a complete statement of allowable operating pressure. Give the governing standard, material and design temperature together with the specified rating when requesting a quotation.
Keep the designations separate
Record PN or Class exactly as stated in the project specification. Do not replace one with the other through a simple arithmetic conversion. Matching nominal size alone does not confirm that two flange connections are interchangeable.
Prepare the rating review
Provide flange type, material specification and grade, standard edition, design pressure and temperature, facing and mating connection details. ASME B16.5 includes pressure-temperature ratings. The applicable limits need to be checked in the specified standard for the actual material and conditions; this guide does not supply a substitute rating table.
Confirm the complete joint
Ask the project engineer to check the mating flange, gasket, bolting and connected equipment as a complete assembly. Where an existing connection must be retained, send its verified drawing and marking details. State any proposed change before ordering rather than treating a different class as an automatic upgrade.
Common question
Can I order by a pressure value only? Include the standard and rating designation as well as the design conditions. A quotation should clearly state the supplied specification and any unresolved technical points.