ASME B16.5 — Pipe Flanges and Flanged Fittings
Valve Standards / Design StandardsWhat Is ASME B16.5
ASME B16.5 is the primary American standard for pipe flanges and flanged fittings. It specifies dimensions, tolerances, materials, pressure-temperature ratings, marking, and testing for flanges from NPS ½ through NPS 24 in pressure classes 150, 300, 600, 900, 1500, and 2500. Virtually every flanged connection in an ASME B31 piping system references B16.5 for dimensional and rating information.
First published in the 1920s and updated regularly, B16.5 is maintained by the ASME B16 committee on standardization of valves, flanges, fittings, and gaskets. It works hand-in-hand with ASME B31 piping codes — B31.1 for power piping and B31.3 for process piping — which govern design, while B16.5 defines the component.
Scope and Coverage
- Size range: NPS ½ (DN 15) through NPS 24 (DN 600)
- Pressure classes: 150, 300, 600, 900, 1500, 2500
- Materials: Carbon steel, alloy steel, stainless steel, nickel alloys — grouped into 34 material groups (1.1 through 3.19)
- Flange types: Welding neck, slip-on, socket welding, threaded, lap joint, blind, and long welding neck
- Temperature range: −29 °C to 649 °C, depending on material group
Flange Types Covered
| Type | B16.5 Designation | Key Feature |
|---|---|---|
| Welding Neck | WN | Butt-welding hub; highest integrity |
| Slip-On | SO | Fillet-welded inside and out; lower cost |
| Socket Welding | SW | Socket for pipe insertion; small-bore |
| Lap Joint | LJ | Used with stub end; free rotation aids alignment |
| Threaded | TH | NPT threads; no welding needed |
| Blind | BL | No opening; for line termination and testing |
| Long Welding Neck | LWN | Extended neck for vessel and equipment nozzles |
Facing Types
| Facing | Description | Typical Use |
|---|---|---|
| Raised Face (RF) | 1.6 mm (Class 150) or 6.4 mm (Class 300+) raised ring | Most common; used with flat or spiral-wound gaskets |
| Flat Face (FF) | Full flat face — no raised ring | Cast iron and low-pressure systems; avoids bending stress on fragile flanges |
| Ring-Type Joint (RTJ) | Groove for metal ring gasket | Class 600+ and high-pressure/high-temperature hydrocarbon |
| Tongue and Groove | Matching tongue and groove pair | Specialty; confined gasket |
| Small Male-Female | Smaller raised face and matching recess | Specialty; confined gasket |
Pressure Classes
The class number indicates the pressure-temperature rating, not the actual pressure in bar or psi. The rating depends on the material group and temperature. For example, a Class 300 carbon-steel flange (Material Group 1.1) is rated for 51.1 bar at 38 °C but only 29.6 bar at 343 °C.
The class system simplifies procurement — engineers specify the class, and B16.5 provides the exact rating for every material and temperature combination. This ensures that all components in a flanged joint (flanges, gaskets, bolts) are matched to the same pressure boundary.
Material Groups and Ratings
B16.5 organizes materials into groups with similar mechanical properties:
- Group 1.x: Carbon and low-alloy steels (A105, A350 LF2, A182 F5/F9/F11/F22/F91)
- Group 2.x: Stainless steels (A182 F304, F316, F347, F321)
- Group 3.x: Nickel alloys (B16.5 — Hastelloy, Inconel, Monel, etc.)
Each group has its own pressure-temperature table. Selecting the correct table is critical — specifying a Group 1.1 rating table for a Group 2.2 stainless flange will overstate the pressure capability, because stainless steels have lower allowable stresses at elevated temperature than carbon steels.
Key Dimensions
- Outside diameter (O.D.): The flange rim diameter, which determines bolt circle and clearance
- Bolt circle diameter (B.C.): The diameter of the circle through bolt hole centers
- Number and size of bolt holes: Specified per NPS and class
- Thickness: Minimum flange thickness at the hub — depends on class and material
- Hub length and bore: For welding-neck flanges, the dimensions of the hub that transitions from pipe wall to flange
All dimensions are tabulated in B16.5 for every combination of NPS and class. Manufacturers must hold these tolerances to ensure interchangeability and proper gasket compression.
Marking Requirements
B16.5 requires each flange to be marked with:
- Manufacturer's name or trademark
- ASME B16.5 designation
- NPS and schedule (or wall thickness designation)
- Pressure class (e.g., Class 300)
- Material grade (e.g., A105, F316)
- Heat number (for traceability)
Marking must be on the outside face or rim of the flange in raised or stamped characters. This information is essential for inspectors and maintenance personnel to verify that installed flanges match the piping specification.
Relationship to Other Standards
| Standard | Relationship |
|---|---|
| ASME B16.9 | Butt-welding fittings — matches B16.5 flanges for end-to-end dimensions |
| ASME B16.10 | Face-to-face dimensions for valves — references B16.5 flanges |
| ASME B16.20 | Gaskets for B16.5 flanges — spiral-wound, RTJ, and sheet |
| ASME B16.21 | Nonmetallic gaskets for B16.5 flanges |
| ASME B16.47 | Large-diameter flanges (NPS 26–60) — extends B16.5 concept |
| ASME B31.1 / B31.3 | Piping codes that specify B16.5 flanges as the default component standard |
| ASME PCC-1 | Bolted flange joint assembly guidelines |
Practical Guidance
- Always verify the material group: A Class 300 flange in F304 (Group 2.1) has a lower pressure rating at elevated temperature than one in A105 (Group 1.1). Check the correct table.
- Mind the temperature derating: Pressure ratings drop with temperature. A Class 150 carbon-steel flange rated at 19.6 bar at 38 °C falls to just 6.9 bar at 343 °C.
- Don't mix classes: Never bolt a Class 150 flange to a Class 300 flange, even if the bolt circle and O.D. happen to match. The gasket contact area, bolt loads, and pressure capability are different.
- RTJ for high-pressure: In Class 600 and above, RTJ facing is the norm for hydrocarbon service. Specify the correct ring number (R or BX series) per B16.20.
- Check for large-diameter: NPS 26 and above falls under ASME B16.47, not B16.5.
Practical tip: When ordering replacement flanges for an existing plant, always specify the exact material grade, class, facing, and schedule — not just "Class 300 flange." A 316 stainless Class 300 flange has a different pressure rating and different corrosion resistance than a carbon-steel Class 300 flange, even though the bolt pattern is identical. The wrong grade can fail under design conditions.