PVF Reference Library · Wall Charts

Flange Class Identification Chart

How to identify an ASME B16.5 flange's pressure class from its stamped markings and its geometry, without giving pressure-temperature values.

Ref PV-0054ASME B16.5

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

ASME B16.5 steel pipe flanges come in seven pressure classes: 150, 300, 400, 600, 900, 1500 and 2500. The class is a rating designation, not a pressure in psi. A higher class flange for the same pipe size is thicker, usually has a larger outside diameter and bolt circle, and often has more or larger bolts. Each class has its own bolt pattern, so flanges of different classes do not bolt together. The allowable pressure for each class depends on material and temperature and is listed in the B16.5 rating tables.

How to identify

Read the stamp. B16.5 flanges are marked with the maker, material, class, size and other data. The class number is usually on the rim or the hub.
Check the facing. Raised face (RF), flat face (FF) or ring type joint (RTJ).
Measure the raised face height if there is one.
Measure the flange OD, thickness and bolt circle, and count the bolt holes. Compare to the B16.5 dimension tables for the size.
Check for a ring groove number (R-number) on RTJ flanges.

Identification clues

Clue	What to look for	What it tells you
Class stamp	150, 300, 400, 600, 900, 1500 or 2500	The class directly
Raised face height	1/16 in on Class 150 and 300; 1/4 in on Class 400 and higher	Separates low classes from higher classes
Flange thickness	Thicker for higher class at the same size	Relative class
Bolt circle and OD	Larger for higher class at the same size	Match to B16.5 tables
Bolt hole count and size	Often more or larger holes at higher class	Match to B16.5 tables
RTJ groove	Groove for a metal ring gasket	Common on higher classes; check R-number
Material stamp	For example A105, A182 F316L	Needed with class to find the rating

Key points

Raised face height is not included in the flange thickness in the B16.5 tables for some classes. Know which way your table reads before you compare.
Class 400 is uncommon. A flange that looks like Class 600 may be Class 400. Read the stamp.
Cast iron flanges (ASME B16.1, Classes 125 and 250) are flat face and share bolt patterns with some steel classes, but they are not interchangeable in rating.
Large flanges above NPS 24 follow ASME B16.47, Series A or B, which have different patterns.
Never pick a class from looks alone when the stamp is missing. Confirm by measurement against the tables.

Common mistakes

Reading the class as the working pressure in psi.
Bolting a raised face steel flange to a flat face cast iron flange without the right gasket and facing.
Mixing up Class 300 and Class 400 when only the bolt count is checked.
Ordering by size only, without class or facing.

Governing standards: ASME B16.5 (pipe flanges and flanged fittings, NPS 1/2 through 24); ASME B16.47 (large diameter flanges); MSS SP-25 (marking).

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.

More in Wall Charts

← All PVF sheets