Class 300 vs PN40: Pressure Equivalence

Quick Answer

ASME Class 300 and EN PN40 are the closest equivalent pressure ratings between the US and European flange standards. At ambient temperature in carbon steel, Class 300 is rated for 51 barg (740 psig) and PN40 for 40 barg (580 psig). Class 300 has a higher pressure capacity, so a PN40 flange does not fully replace a Class 300 flange at all conditions.

Detailed Comparison

ASME B16.5 Class 300 and EN 1092-1 PN40 are often paired in cross-reference tables, but the equivalence is approximate. The ASME system uses a class-based designation with full pressure-temperature curves per material group. The EN system uses a fixed PN number representing the maximum allowable pressure in bar at ambient temperature.

Feature	ASME Class 300	EN PN40
Standard	ASME B16.5	EN 1092-1
Max pressure at 20°C (CS)	51 barg (740 psig)	40 barg (580 psig)
Max pressure at 200°C (CS)	41.1 barg (596 psig)	36.3 barg (527 psig)
Max pressure at 300°C (CS)	35.2 barg (510 psig)	31.2 barg (453 psig)
Flange OD (6”/DN150)	381 mm	375 mm
Bolt circle (6”/DN150)	317.5 mm	320.0 mm
Number of bolts (6”/DN150)	12	12
Bolt size (6”/DN150)	7/8” (M22)	M27
Raised face height	1.6 mm (1/16”)	2 mm
Flange thickness (6”)	52.3 mm	57 mm

Pressure Equivalence at Temperature

The ASME system provides higher pressure ratings than the EN system at the same nominal equivalence level. At ambient temperature, Class 300 exceeds PN40 by approximately 27%. This gap narrows at elevated temperatures but never closes entirely.

This means that:

• A Class 300 system can safely accept a PN40 component only if the design pressure stays within the PN40 limit.
• A PN40 system cannot accept a Class 300 flange at face value because the bolt patterns and gasket dimensions differ.

Dimensional Compatibility

Unlike Class 150/PN16, where some sizes have close bolt hole patterns, Class 300 and PN40 flanges are dimensionally different in most sizes. The bolt circle diameters, bolt sizes, and flange ODs do not align, making field mating impractical without a transition spool.

For dimensional details on ASME flanges, see the ASME flange rating reference. For EN flanges, refer to EN 1092-1 tables.

Gasket Differences

ASME Class 300 RF flanges typically use spiral wound gaskets per ASME B16.20 with a carbon steel outer ring and flexible graphite filler. EN PN40 flanges use spiral wound gaskets per EN 1514-2, which have different OD and ID dimensions. Gaskets are not interchangeable between standards.

Common Mistake

Assuming Class 300 and PN40 flanges can bolt together directly. The dimensional differences in bolt circle, bolt size, and flange thickness make them physically incompatible. Always use a transition spool with the correct flange on each end.

Read the full guide to pipe flanges