ASME B16.5: Pipe Flange Dimensions

ASME B16.5, *Pipe Flanges and Flanged Fittings*, is published by the American Society of Mechanical Engineers and is one of the most widely used piping standards in the world. It is the reason a flange made in one country bolts correctly to a pipe spool, valve or vessel nozzle made in another.

The standard covers flanges and flanged fittings from NPS 1/2 through NPS 24, in both inch and metric units. For those components it specifies:

• Pressure-temperature ratings for each pressure class and material group
• Materials permitted, arranged into material groups
• Dimensions — outside diameter, thickness, hub dimensions, bolt circle, number and size of bolt holes
• Tolerances on those dimensions
• Facings — raised face, flat face, ring joint and others — and their surface finish
• Bolting and gasket requirements for joints
• Marking of flanges and fittings
• Testing requirements for flanged fittings

The standard is revised periodically, and piping codes and project specifications normally call up a specific edition. Always work to the edition named in the governing specification.

What B16.5 does not cover

B16.5 does not cover flanges above NPS 24, which fall under ASME B16.47, and it does not specify the design of the piping system itself — that is the role of piping codes such as ASME B31.3 for process piping and ASME B31.1 for power piping. Valves with flanged ends have their own standard, ASME B16.34, which refers back to B16.5 for end flange dimensions.

The pressure class is the most misunderstood part of B16.5. A Class 150 flange is not rated at 150 psi, and a Class 300 flange is not rated at 300 psi. The class is a designation that points to a table of allowable pressures, and that table has two further dimensions: the material group and the temperature.

As temperature rises, steel loses strength and allowable pressure falls. The same Class 150 flange can be comfortably adequate at ambient temperature and inadequate for the same line at operating temperature.

At −29 to 38°C, group 1.1 is rated at about 153.2 bar for Class 900, 255.3 bar for Class 1500 and 425.5 bar for Class 2500. The ratings of each class are not simply proportional to the class number, but they rise broadly in step with it.

Carbon steel at high temperature

B16.5 cautions that carbon steels exposed for long periods above about 425°C may suffer conversion of carbides to graphite, which reduces strength. Carbon steel flanges are permissible above that temperature in the tables but are not recommended for prolonged service there; chromium-molybdenum or stainless materials are the usual choice for sustained high-temperature service.

The weakest element governs the joint

A flanged joint is only as capable as its weakest component. The rating of an assembled joint also depends on the bolting and gasket, and on the rating of any valve or fitting in the line. Mating a Class 300 flange to a Class 150 flange does not produce a Class 300 joint.

B16.5 arranges permitted materials into numbered groups that share a pressure-temperature table. Forgings, castings and plates of broadly similar composition sit in the same group.

Low-carbon stainless grades in group 2.3 have lower allowable pressures than their standard-carbon counterparts, because low carbon reduces strength slightly. They are specified where welding makes resistance to sensitisation more important. Forged alloy and stainless flange materials are specified under ASTM A182.

Welding neck flanges are bored to match the connecting pipe's wall schedule, and their weld bevel follows ASME B16.25. Socket welding flanges require a gap of about 1.6 mm between pipe end and socket bottom before welding, to prevent weld cracking from thermal expansion. See forged flanges for detail on selecting a type.

• Raised face — the most common facing. The raised portion is 1.6 mm high for Classes 150 and 300 and 6.4 mm high for Class 600 and above. Note that B16.5 flange thickness dimensions for Classes 400 and above do not include the raised face, while for Classes 150 and 300 they do — a frequent source of confusion.
• Flat face — used when mating to cast iron or other flanges that could crack under a raised face.
• Ring joint — a groove carrying a metal ring gasket, for high pressure and temperature.
• Tongue and groove, male and female — confined gasket facings for specific applications.

Raised and flat faces are normally supplied with a serrated concentric or spiral finish in the range of about 3.2 to 6.3 µm Ra (125 to 250 µin). The serrations grip soft gaskets and resist blow-out. Ring joint grooves require a much smoother finish, with side walls commonly specified at no rougher than about 1.6 µm Ra (63 µin).

Bolt circle, number of holes and bolt size are what make a B16.5 flange physically compatible with its mating flange. The table gives values for some common sizes as a reference for what the standard specifies.

Notice that an NPS 4 flange in Class 300 has a larger outside diameter, larger bolt circle and larger bolts than the same size in Class 150. The two classes cannot be bolted together.

B16.5 identifies bolting materials suitable for flanged joints and relates them to the pressure ratings. In practice, for carbon and low-alloy steel flanges in general service, ASTM A193 Grade B7 stud bolts with ASTM A194 Grade 2H heavy hex nuts are the most common combination. Low-temperature service uses grades such as A320 L7, and stainless service uses stainless bolting grades.

Gaskets are specified in companion standards: ASME B16.20 for metallic and spiral-wound gaskets and ring joint gaskets, and ASME B16.21 for non-metallic flat gaskets. The gasket must suit the facing, fluid, temperature and pressure class.

B16.5 requires flanges to be marked so they can be identified in service. Typical markings include:

• Manufacturer's name or trademark
• Material specification and grade, such as A105 or F316L
• Pressure class
• Nominal size
• Wall thickness or schedule for welding neck flanges
• Heat number or identification traceable to the material certificate
• Ring groove number for ring joint flanges

Marking practice commonly follows MSS SP-25. Heat number marking is what connects a flange in a plant back to its EN 10204 material certificate.

B16.5 and EN 1092-1 are not interchangeable

European EN 1092-1 flanges are designated by nominal pressure — PN 6, PN 10, PN 16, PN 25, PN 40 and so on — rather than by class. Bolt circles, hole counts and bolt sizes differ from B16.5, so a PN flange generally will not bolt to a Class flange of the same nominal size. Projects mixing European and American equipment need adapter flanges or a single agreed standard.

Much of the confusion around flange orders comes from terms that sound interchangeable but are not. The table clarifies the pairs we see mixed up most often.

A common example: an order for "100 NB Class 150 weld neck flange" is incomplete without the pipe schedule, because the weld neck bore must match the connecting pipe's wall.

B16.5 specifies tolerances on the dimensions that make flanges fit and seal. Exact values vary by size and feature and should be read from the governing edition, but the features controlled include:

• Outside diameter of the flange
• Bore, particularly on welding neck flanges where it must match the pipe
• Thickness of the flange and height of the raised face
• Hub dimensions — diameter at the weld end and hub length
• Bolt circle diameter and spacing of adjacent bolt holes
• Bolt hole diameter and eccentricity of the bolt circle relative to the bore
• Facing finish, including serration roughness
• Weld bevel geometry on welding neck flanges, per ASME B16.25

Of these, bolt circle and hole spacing are what decide whether a flange physically bolts to its mate, and bore and bevel decide whether it can be welded properly. Surface finish decides whether the gasket seals. Inspection effort should follow that order of consequence.

1. 01Reading the class as a pressure. A Class 150 flange is not rated at 150 psi, and its rating falls with temperature.
2. 02Checking the rating only at ambient temperature. Always read the table at the design temperature for the actual material group.
3. 03Mixing classes in one joint. Different classes have different bolt patterns and do not mate.
4. 04Mixing B16.5 and EN 1092-1 flanges. Class and PN flanges generally do not share bolt patterns.
5. 05Omitting the schedule on weld neck flanges. The bore will not match the pipe.
6. 06Bolting a raised face flange to a flat-face cast iron flange. The concentrated load can crack the cast iron.
7. 07Using carbon steel for sustained high-temperature service. Chromium-molybdenum or stainless materials suit prolonged service at elevated temperature.
8. 08Assuming the flange sets the joint rating. Bolting, gasket and the other components in the line also limit it.

A complete B16.5 flange description follows a standard order: type, facing, size, class, schedule where applicable, material and standard, followed by any finish, testing and certification requirements.

• Weld neck, raised face, NPS 4, Class 300, bore to Schedule 40, ASTM A105N, ASME B16.5, EN 10204 3.1
• Blind, raised face, NPS 6, Class 150, ASTM A182 F316L, ASME B16.5
• Slip-on, raised face, NPS 2, Class 150, ASTM A105, ASME B16.5

Before ordering, confirm the design temperature against the pressure-temperature table, confirm that mating flanges, valves and fittings share the same class and facing, and state the edition of B16.5 required by your project.

Related: ASTM A105, ASTM A182, forged blind flanges and our work for oil and gas. Request a quote.