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Pressure Class & Flange Ratings Explained

What ASME flange pressure classes mean, why the allowable pressure depends on material group and temperature, and where to find the governing pressure-temperature table. It deliberately gives no pressure values.

Ref PV-0003ASME B16.5ASME B16.47ASME B16.34

Safety-critical sheet, not yet signed off by an expert reviewer. Use it to understand the topic, then take every value and decision from the governing standard, the manufacturer’s instructions and a qualified person.

What it is

A pressure class (150, 300, 400, 600, 900, 1500, 2500) is a rating designation for flanges, flanged fittings and valves under ASME B16.5 (NPS 1/2 through 24), ASME B16.47 (NPS 26 through 60) and ASME B16.34 (valves). The class number is not the working pressure in psi. The allowable working pressure for a class depends on:

the material group of the flange (for example carbon steel, low-alloy or stainless), and
the design temperature. As temperature goes up, allowable pressure goes down.

Those values are in the pressure-temperature (P-T) rating tables of the governing standard. That table, together with the piping code (ASME B31.1, B31.3, etc.), is the only correct source.

How to identify

Markings: forged flanges are stamped with the manufacturer, size, class, material specification and grade (e.g. ASTM A105, A182 F316L), facing, bore or schedule, and heat number (MSS SP-25 practice).
Geometry: a higher class has a thicker flange, a larger bolt circle and more or larger bolts for the same NPS. Bolt count and bolt circle can confirm the class when the stamp is unreadable. Look them up in the dimensional tables of the standard.
Facing: raised face (RF), flat face (FF) or ring-type joint (RTJ) groove. RTJ is common in high classes.

What determines a flange's rating

Factor	Why it matters	Where to look
Pressure class	Sets the dimensions and the rating column	ASME B16.5 / B16.47
Material group	Different materials have different allowable stresses	Material table in the standard
Design temperature	Allowable pressure drops as temperature rises	P-T rating table
Gasket and bolting	The joint is only as good as its gasket and bolts	ASME B16.20 / B16.21; ASTM A193 / A194
Piping code	May add limits or require a lower rating	ASME B31.1, B31.3, B31.9
Mating components	Valves and equipment nozzles have their own ratings	ASME B16.34; equipment nameplate

Key points

Bolt holes may line up between classes in some sizes, but the classes are not interchangeable. A Class 150 flange bolted to a Class 300 nozzle derates the joint to the lower class, or may not fit at all.
Cast iron (ASME B16.1), ductile iron (B16.42) and AWWA C207 flanges have their own ratings and dimensions, separate from B16.5.
A flat-face cast iron flange should be mated to a flat-face flange with a full-face gasket. Bolting a raised-face steel flange to it can crack the cast iron.
When in doubt, give the customer the class, material grade and standard, and point them to the engineer of record for the rating.

Common mistakes

Reading "Class 150" as "150 psi".
Quoting a flange rating without asking for the design temperature and material.
Substituting a stainless flange for carbon steel, or the reverse, without checking the material-group rating and the line class.
Mixing B16.47 Series A and Series B flanges, which have different dimensions.

Governing standards: ASME B16.5; ASME B16.47; ASME B16.34; ASME B16.1 and B16.42 for iron flanges; ASME B31.1 / B31.3 / B31.9; MSS SP-25 for markings.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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