“`html

What Is ASME B16.5?

ASME B16.5 is an American Society of Mechanical Engineers standard that specifies dimensions, pressure-temperature ratings, materials, tolerances, marking, and testing requirements for pipe flanges and flanged fittings from NPS 1/2 through NPS 24 at Class 150 through Class 2500. It defines the bolt circle diameter, bolt hole pattern, flange outside diameter, flange thickness, raised face dimensions, and ring-type joint groove geometry that ensure any compliant flange from any manufacturer will mate correctly with any other compliant flange of the same nominal size and pressure class — forming the flanged connection standard that underlies the entire ASME-rated industrial piping system and a foundational reference within the valve standards overview hub.

Key Takeaways

Defines dimensions and pressure classes for pipe flanges — ASME B16.5 Tables 1 through 15 provide complete dimensional data for each NPS and Class combination: bolt circle diameter, number of bolts, bolt diameter, flange outside diameter, flange thickness, raised face diameter, and ring-type joint groove dimensions, all standardized to ensure bolt hole alignment and gasket contact compatibility between any two compliant flanges.
Standardizes bolt patterns, flange facings, and tolerances — the bolt hole count and diameter, bolt circle diameter, and flange outside diameter at each NPS and Class combination are fixed values with defined manufacturing tolerances, ensuring that any NPS 4 Class 600 flange from any compliant manufacturer will bolt up correctly to any other NPS 4 Class 600 flange without bolt hole misalignment or gasket seating interference.
Covers pressure-temperature ratings for materials — ASME B16.5 Table A1.1 provides pressure-temperature rating tables for flanges in the same material groups as ASME B16.34, giving the allowable working pressure at temperatures from −20°F to the material’s maximum rated temperature for Class 150 through Class 2500 at each material group.
Ensures compatibility with flanged valves and piping systems — all API and ASME flanged valves reference ASME B16.5 for their end flange dimensions, making B16.5 the universal flanged interface standard that connects valves, fittings, and pipe in every ASME-rated industrial piping system.

How It Works

Pressure-Temperature Rating System

ASME B16.5’s pressure-temperature rating tables use the same material group classification and allowable stress basis as ASME B16.34 — the Class designation has identical meaning in both standards, and a Class 300 flange in Group 1.1 carbon steel has the same rated working pressure at any given temperature as a Class 300 valve body in the same material group. This intentional alignment means that a correctly selected valve-to-pipe flanged connection rated at the same Class and material group will have matching pressure-temperature ratings at both the valve body (per ASME B16.34) and the flange connection (per ASME B16.5), ensuring that neither the valve body nor the flange connection is the weak link in the piping system at any operating condition. The complete pressure class designation framework and how to use pressure-temperature tables for valve and flange selection is explained in the ASME pressure class explained reference. The ASME B16.34 valve body pressure rating framework that is complementary to and consistent with ASME B16.5 flange ratings is addressed in the what is ASME B16.34 reference.

Flange Dimensions and Bolt Patterns

The bolt pattern dimensions — bolt circle diameter, bolt hole count, and bolt hole diameter — are the most critical dimensional parameters in ASME B16.5 because they determine whether two flanges will physically mate with each other when bolted together. ASME B16.5 specifies that bolt holes shall be located on a common bolt circle diameter (BCD) at equal angular spacing, with the BCD, hole count, and hole diameter uniquely defined for each NPS and Class combination. For NPS 4 Class 150, the bolt circle diameter is 190.5 mm (7.5 inches) with eight bolt holes of 22.4 mm (7/8 inch) diameter; for NPS 4 Class 600, the bolt circle diameter increases to 234.9 mm (9.25 inches) with the same eight bolt holes but larger 25.4 mm (1 inch) diameter — the larger BCD and bolt diameter at the higher pressure class reflects the increased bolt load required to maintain gasket seating stress at higher pressure. ASME B16.5 specifies that bolt holes shall straddle the flange’s natural horizontal and vertical centerlines (not centered on them) — this “straddle pattern” is universal across all ASME B16.5 flanges, ensuring that bolts can always be inserted and turned regardless of valve position orientation. The flange outside diameter and thickness at each NPS and Class combination are dimensioned to accommodate the bolt circle and bolt head bearing area while providing sufficient flange stiffness to maintain gasket seating uniformity around the full flange face under bolting load.

Facing Types and Surface Finish

ASME B16.5 defines three primary flange facing types, each requiring a specific gasket geometry and surface finish. Raised face (RF) is the most common facing type in process plant service — a circular raised platform at the center of the flange face, 6.4 mm (1/4 inch) high for Class 150 and 300 and 6.4 mm for Class 600 through Class 2500, with the gasket seated only on the raised face area. The raised face concentrates the bolt load on the smaller gasket area rather than spreading it over the full flange face, increasing the gasket contact stress and improving sealing performance compared to flat face configuration. The raised face surface finish for spiral wound gaskets is specified as 3.2 to 6.3 µm Ra (125–250 µin AA) serrated phonographic finish — the fine concentric serrations provide mechanical interlock with the spiral wound gasket facing, preventing gasket blowout under pressure pulsation. Flat face (FF) is used when one of the mating flanges is cast iron or other brittle material that could crack under the bending loads generated by a raised face bolted joint — flat face flanges use full-face gaskets that extend to the bolt holes, distributing the bolt load over the full flange face area and reducing the bending stress on the brittle flange. Ring type joint (RTJ) facing uses a machined groove in the flange face to seat an oval or octagonal metal ring gasket — RTJ connections provide superior sealing performance at high pressure and high temperature because the ring gasket deforms plastically into the groove under bolt load, creating a metal-to-metal seal that is not dependent on gasket elasticity. RTJ facing is standard at Class 900 and above in oil and gas service where the combination of high pressure, high temperature, and flammable fluid makes metal-to-metal sealing performance mandatory.

Main Components

Dimensional Tables and Material Requirements

ASME B16.5’s dimensional tables are the primary engineering reference for flanged connection design — they provide every dimension needed to specify, fabricate, and inspect a compliant flanged joint. The tables are organized by nominal pipe size (NPS 1/2 through NPS 24) and pressure class (150 through 2500), with separate dimensional entries for each combination. For any given NPS and Class, the table provides: raised face diameter and height; ring groove dimensions (number, width, depth) for RTJ facing; flange outside diameter; bolt circle diameter; number of bolt holes; bolt hole diameter; flange thickness; hub diameter at large end; and approximate flange weight. These dimensions are absolute values (not formulas) — they can be read directly from the table and applied to flange fabrication or inspection without calculation. Material specifications for ASME B16.5 flanges reference ASTM material standards — ASTM A105 for carbon steel forgings, A182 for alloy and stainless steel forgings, A216 for carbon steel castings — with chemical composition and mechanical property requirements inherited from the referenced ASTM specification rather than defined within B16.5 itself. Material certification requirements mandate that flange material be traceable to its mill test certificate — the EN 10204 material certificate framework supporting this traceability requirement is addressed in the what is EN 10204 3.1 reference.

Integration With Valve Testing Standards

ASME B16.5 is primarily a dimensional and pressure rating standard — it defines the flange geometry and allowable working pressure but does not in itself mandate pressure testing of individual flanges before shipment. However, the flanged connections of industrial valves are pressure tested as part of the valve’s shell hydrostatic test, which subjects the complete valve including its end flanges to 1.5 times rated pressure and verifies that no leakage occurs through the flange body, hub, or blind flange used to close the test assembly. This integrated valve and flange pressure testing per what is API 598 provides the practical pressure integrity verification for ASME B16.5 flanged valve ends. The broader hydrostatic testing methodology is addressed in the hydrostatic testing standard reference, and detailed production testing procedures are addressed in the valve pressure testing procedure reference.

API Product Standard Integration

All major API valve product standards specify ASME B16.5 compliance for flanged end connections — this ensures that the valve’s end flanges will mate correctly with the ASME B16.5 compliant pipe flanges specified for the connecting piping, without requiring individual dimensional verification of each valve-to-pipe flange interface. API 6D specifies that pipeline valve flanged ends shall comply with ASME B16.5 for NPS 1/2 through NPS 24 and ASME B16.47 Series A or B for NPS 26 and above — complete API 6D flange requirements are addressed in the what is API 6D reference. API 600 specifies that bolted-bonnet steel gate valve flanged ends shall comply with ASME B16.5 — complete API 600 flange requirements are addressed in the what is API 600 reference. API 602 specifies that compact forged steel valve flanged ends shall comply with ASME B16.5 for the applicable NPS and Class combinations — complete API 602 flange requirements are addressed in the what is API 602 reference.

Advantages

Interchangeability and Regulatory Compliance

ASME B16.5’s standardized bolt patterns and flange dimensions provide the flanged interface interchangeability that makes ASME-rated piping systems globally practical — any NPS 4 Class 300 raised face flange from any ASME B16.5 compliant manufacturer will bolt directly to any other NPS 4 Class 300 raised face flange, with matching bolt hole alignment, identical raised face diameters for correct spiral wound gasket sizing, and consistent flange stiffness for uniform gasket compression. This interchangeability eliminates the project-specific flange dimensional verification that would be required without standardization, saving engineering time, reducing procurement risk, and enabling flange components from multiple manufacturers to be used within a single piping system without compatibility issues. For European projects where ASME B16.5 flanged components must carry PED CE marking, the ASME B16.5 pressure rating tables provide the technical basis for demonstrating compliance with PED essential safety requirements — the integration of ASME B16.5 with EU conformity assessment is addressed in the what is PED 2014/68/EU reference. Flanged valves in flammable service combining ASME B16.5 flange compliance with fire-safe certification per what is API 607 and fire-safe certification represent the standard specification for critical isolation valves in refinery and petrochemical service. Fugitive emission certification per fugitive emission testing and what is ISO 15848 supplements ASME B16.5 flange dimensional compliance with stem seal emission performance qualification for environmentally regulated installations.

Typical Applications

Oil, Gas, Petrochemical, Power, and Chemical Systems

ASME B16.5 applies to every flanged connection in ASME-rated industrial piping — its scope encompasses any service where flanged pipe, fittings, and valves are connected using bolted flanged joints. In oil and gas pipeline systems, ASME B16.5 flanges at Class 600 and 900 in ASTM A105 carbon steel provide the standardized flanged interfaces at valve positions, instrumentation connections, and pipe-to-fitting connections throughout the above-ground sections of transmission and distribution pipelines. In refinery process units, ASME B16.5 Class 150 through Class 1500 flanges in carbon steel, alloy steel, and stainless steel provide the universal flanged connection standard for the thousands of flanged joints in a typical refinery process unit — the standardized bolt patterns allow maintenance crews to unbolt and replace any flanged component in the system using standard tools and standard bolt dimensions without project-specific tooling. In power generation steam systems, ASME B16.5 Class 900 and 1500 flanges in ASTM A182 F22 alloy steel provide the flanged connection standard for high-temperature, high-pressure steam and feedwater piping, where the ring-type joint facing and oval ring gaskets standard for Class 900 and above provide the metal-to-metal sealing integrity required at these extreme operating conditions. In chemical processing plants handling corrosive fluids, ASME B16.5 flanges in ASTM A182 F316L or F51 duplex stainless steel provide corrosion-resistant flanged connections with standardized dimensions compatible with the full range of chemical service piping components. In maintenance and retrofit operations, ASME B16.5 dimensional standardization enables direct replacement of worn or damaged flanged components without pipe modification — a maintenance benefit that reduces planned turnaround duration and minimizes unplanned shutdown time for flanged connection leaks.

Frequently Asked Questions

What does ASME B16.5 cover?
ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 through NPS 24 at Class 150 through Class 2500 — defining dimensions (bolt circle diameter, bolt hole count and size, flange outside diameter, flange thickness, raised face and RTJ groove geometry), pressure-temperature ratings by material group, material specifications, surface finish requirements for gasket seating faces, marking requirements, and testing requirements. It applies to slip-on, weld neck, socket weld, threaded, lap joint, and blind flange types in all standard body materials from carbon steel through nickel alloys.

Does ASME B16.5 apply to valves?
ASME B16.5 applies to the flanged ends of valves — the end flanges that bolt the valve into the piping system. All ASME and API flanged valve standards specify that the valve’s end flanges shall comply with ASME B16.5 dimensions and pressure-temperature ratings for the applicable nominal size and pressure class, ensuring that the valve’s end flanges will mate correctly with the ASME B16.5 compliant pipe flanges in the connecting piping. ASME B16.5 does not govern the valve body design, closure mechanism, or seat sealing — these are governed by ASME B16.34 and the applicable API product standard.

How is pressure class determined in ASME B16.5?
Pressure class under ASME B16.5 is determined the same way as under ASME B16.34 — by identifying the lowest Class designation whose rated pressure at the maximum operating temperature for the selected flange material group exceeds the maximum operating pressure with adequate margin. The ASME B16.5 pressure-temperature tables use the same material group classification and allowable stress basis as ASME B16.34, ensuring consistent pressure class selection for both the valve body and its connecting flanges.

Is ASME B16.5 mandatory?
ASME B16.5 has no inherent legal mandatory status as a voluntary consensus standard. It becomes effectively mandatory when incorporated by reference into project piping specifications, referenced by API product standards (API 6D, API 600, API 602) that are specified in purchase orders, or required by piping system design codes (ASME B31.3 Process Piping specifies that flanges shall comply with ASME B16.5). In practice, ASME B16.5 compliance is a contractual requirement for all flanged industrial piping components in process and pipeline service.

Conclusion

ASME B16.5 is the universal flanged interface standard for ASME-rated industrial piping systems — its standardized bolt patterns, flange dimensions, pressure-temperature ratings, and facing type requirements ensure that any compliant flange from any manufacturer will mate correctly with any other compliant flange of the same nominal size and pressure class, providing the dimensional interchangeability and pressure integrity assurance that industrial piping systems require. When combined with ASME B16.34 valve body pressure rating compliance, ASME B16.10 valve face-to-face dimensional compliance, and API 598 pressure testing certification, ASME B16.5 forms part of the integrated standard stack that defines a fully compliant, dimensionally interchangeable, pressure-rated industrial flanged valve and piping assembly. Engineers requiring a comprehensive framework that integrates ASME B16.5 within the full landscape of valve design, testing, dimensional, and certification standards should consult the valve standards overview hub as the governing reference for all flanged connection standards navigation.