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What a pressure class really means

ASME B16.5, "Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard", defines flanges by pressure class, not by a fixed pressure in bar or psi. The seven classes are 150, 300, 400, 600, 900, 1500 and 2500. The class number is a dimensionless label. The pressure a flange may actually carry depends on two further factors: the material group and the operating temperature.

A Class 300 flange therefore does not mean 300 psi. It means a flange whose allowable pressure at a given temperature is read from the standard's rating tables.

Why ratings fall as temperature rises

Steel loses strength at elevated temperature, so the permitted working pressure decreases as temperature increases. Each material group in ASME B16.5 has its own pressure-temperature table. The tables list the maximum allowable gauge pressure at stepped temperatures, and they apply to the flange, the bolting and the gasket as a joint.

Two points are often missed:

• The highest value in a table applies at a low reference temperature, not at the actual design temperature.
• Values between listed temperatures may be interpolated only as permitted by the standard. Check the wording in the relevant clause.

Material groups

ASME B16.5 sorts the materials it covers into groups (Table 1A of the standard lists the materials and assigns each a group number). Examples of the forged materials commonly assigned to groups are:

Do not assume that two materials with similar names share a rating. A carbon steel flange and a stainless flange of the same class have different allowable pressures at the same temperature. Some stainless grades also have separate ratings depending on the carbon content or the heat-treatment condition. Always find the group number first, then the table.

Choosing a class

The sequence is simple:

• Establish the design pressure and the design temperature, including upset conditions.
• Identify the flange material and its ASME B16.5 group.
• Read the table for that group and find the lowest class whose allowable pressure at the design temperature meets or exceeds the design pressure.

Class 150 is the lowest class. At higher temperatures its capacity falls markedly, so it is not interchangeable with a class chosen by pressure alone. Classes 900, 1500 and 2500 are used for severe duty, and for these classes the standard has specific rules on dimensions and on the use of larger sizes. Note that Class 2500 is covered only up to a limited size range in the standard; confirm the size limits in the edition you work to.

Other things that affect the rating

• Facing: raised face, flat face and ring-type joint faces have different dimensional rules, including facing height in Classes 150 and 300 versus the higher classes.
• Flange type: weld neck, slip-on, socket weld, threaded, lap joint and blind flanges are all rated by class, but each type has its own size and use limits.
• Bolting and gaskets: the rating assumes suitable bolting and gasket selection. Gasket dimensions are covered by ASME B16.20 (metallic) and ASME B16.21 (non-metallic).
• Pipe wall: a weld neck flange should be bored to match the pipe schedule or wall thickness in ASME B36.10M or ASME B36.19M.
• Larger sizes: flanges above NPS 24 are covered by ASME B16.47 (Series A and B), not B16.5.

Comparison with other systems

EN 1092-1 uses PN designations and its own material groups, with ratings given in EN 1092-1 tables. DIN flanges (the national predecessors of the EN series) and GOST 33259 flanges follow a PN logic too. AS 4087 applies in Australia. PN and class are not directly equivalent: class 150 and PN 20 or PN 16 flanges may share some bolt-hole patterns at certain sizes, but they should not be treated as interchangeable without checking dimensions and ratings in each standard.

Buyer's checklist

• State the standard and edition, for example ASME B16.5 with the year.
• Give the class, size (NPS), type and facing.
• Specify the material to the exact ASTM designation, including grade and any supplementary requirements such as impact testing.
• Provide design pressure and temperature, and check them against the correct group table.
• Give the pipe schedule for weld neck bores.
• Define the gasket, bolting and face finish requirements.
• Request the material certificate type, such as EN 10204 3.1, and any marking requirements.
• Confirm size limits for the class, and use ASME B16.47 for NPS above 24.