ASME B16.5 & B16.47 Flange Standards
Comprehensive technical guide to ASME steel pipe flange standards, specifications, materials, and applications
ASME B16.5 Overview
ASME B16.5 (Specification for Pipe Flanges and Flanged Fittings: NPS 1/2 through 24) is the predominant flange standard in North America for industrial piping systems. Developed by the American Society of Mechanical Engineers, this standard specifies dimensions, pressure-temperature ratings, materials, and performance requirements for carbon, stainless, and alloy steel flanges.
ASME B16.5 covers nominal pipe sizes (NPS) from 1/2 inch through 24 inches, with seven standard pressure classes: 150, 300, 400, 600, 900, 1500, and 2500 PSI. The standard also specifies flange face types, bolt patterns, bolt diameters, and gasket seating surfaces to ensure interchangeability and compatibility across manufacturers.
Key Facts
- Sizes: NPS 1/2 through 24 (nominal pipe sizes)
- Pressure Classes: 150, 300, 400, 600, 900, 1500, 2500 PSI
- Materials: Carbon, stainless, and alloy steels
- Temperature Range: -20°F to roughly 850°F for carbon steel (Group 1.1). Cr-Mo grades (1.5 / 1.9) extend higher; consult the relevant ASME B16.5 P-T table for the exact group.
- Face Types: Flat, Raised, Ring Joint, Tongue & Groove
Note on Nomenclature: You may encounter both "ASME B16.5" and "ANSI B16.5" designations. Modern standards use the ASME designation, but the specifications are identical. Both refer to the same flange standard.
ASME B16.5 vs. ASME B16.47
ASME B16.5
- •Sizes: NPS 1/2 through NPS 24
- •Pressure Classes: 150 through 2500 PSI
- •Wide material selection available
- •Most common standard globally
- •Used in: industrial piping, power, refining, chemicals
ASME B16.47
- •Sizes: NPS 26 through NPS 60
- •Series A (150# and up) & Series B (75# and up)
- •Limited material options (primarily carbon steel)
- •Covers large-diameter applications
- •Used in: pipelines, waterworks, large systems
When to Use ASME B16.47
ASME B16.47 is required when your piping system uses nominal pipe sizes larger than NPS 24. These flanges feature different mounting patterns and pressure ratings optimized for large-diameter, lower-pressure applications common in water transmission and pipeline distribution systems. If your project involves flanges larger than 24 inches, B16.47 is your standard.
Size Range & Applications
Small Bore
NPS 1/2 to 2
Instrumentation, small systems
Medium
NPS 3 to 8
General industrial piping
Large Bore
NPS 10 to 24
Major process systems, waterworks
Flange Availability: While ASME B16.5 nominally covers NPS 1/2 through 24, standard manufacturing typically begins at NPS 1/2 with most industrial applications concentrated in NPS 1 through 12. Very large sizes (20-24 inches) may require longer lead times or special orders.
Pressure Classes & Design Ratings
ASME B16.5 defines seven pressure classes, each with specific design pressures, temperature deration curves, and intended applications. The pressure class selected should match or exceed the maximum operating pressure and temperature of your piping system. Hydrostatic test pressure is 1.5 times the published pressure values in the spec.
Flange Face Types
The flange face type defines the sealing surface where the gasket sits. Different face types optimize for different gasket materials, sealing requirements, and pressure classes. ASME B16.5 specifies four standard face types.
Face Type | Designation | Description |
|---|---|---|
| Flat Face (FF) | FF | Completely flat sealing surface, used with full-face gaskets. The critical rule: when a steel flange mates to a cast-iron Class 125 or 250 flange, the steel flange must be flat face. A raised face steel flange against a flat cast-iron flange will crack the cast iron when bolted up. Low and moderate pressure service. |
| Raised Face (RF) | RF | Has a raised ring with flat sealing surface. Reduces gasket area and improves sealing performance. |
| Ring Joint (RJ) | RJ | Contains a grooved seat for metallic ring gasket. Used in high-pressure, high-temperature applications. |
| Tongue & Groove (T&G) | T&G | One flange has tongue, other has groove. Provides alignment and centering of gasket. |
Raised Face (RF) - Most Common
Raised face is the standard in ASME B16.5 and is used in the majority of industrial applications. The raised ring is 1/16 inch high on Class 150 and 300 flanges, and 1/4 inch high on Class 400 and above. The raised ring concentrates the gasket load in a smaller area.
Advantages:
- Superior gasket sealing performance
- Lower gasket stress
- Excellent for high pressure/temperature
- Standard across most industries
Ring Joint (RJ) - High Performance
Ring joint faces use metallic ring gaskets in a grooved seat. Extremely reliable for critical, high-pressure, high-temperature service.
Advantages:
- Maximum sealing reliability
- Superior temperature resistance
- Metallic gasket reusability
- Oil & gas industry standard
Gasket Compatibility: Always match your gasket material to the flange face type. Soft gaskets pair with raised and flat faces; metallic ring gaskets pair with ring joint faces. Mixed gasket-face combinations can fail.
Material Groups & Selection
ASME B16.5 defines material groups based on chemical composition and mechanical properties. Each group accommodates different temperature ranges and operating conditions. Material selection affects both cost and performance. The groups listed below are sourced from ASME B16.5 Mandatory Appendix II via wermac.org. For pressure-temperature lookups, use the group number that matches the actual forging spec on the MTR, not the family name.
Material Group | Composition | Temperature Range | Applications |
|---|---|---|---|
| 1.1 | C-Mn-Si carbon steel - A105 forgings, A350 LF2 / LF6 Cl.1, A216 WCB castings, A516 Gr.70 plate | -20°F up to ~800°F (deration above 500°F) | General industrial piping, most common forged carbon flange group |
| 1.5 | 1.25Cr-0.5Mo low alloy - A182 F11, A217 WC6, A387 Gr.11 | Elevated temperature service, up to ~1100°F | Power, refining, mild high-temperature service |
| 1.9 | 2.25Cr-1Mo low alloy - A182 F22, A217 WC9, A387 Gr.22 | High-temperature creep service, up to ~1100°F | Power generation, hydrocracker, hot hydrogen service |
| 2.1 | Austenitic stainless - A182 F304, A240 304 (carbon ~0.08 max) | -425°F to ~1500°F (per group P-T table) | General corrosion service. Not permitted for API 6A flange bodies. |
| 2.3 | Austenitic stainless low-carbon - A182 F316L, A182 F317L, A240 316L | -425°F to ~850°F | Chloride and acidic service, sanitary, food / pharma |
Group 1.1 (Carbon Steel)
ASTM A105 is the most economical choice for moderate temperature applications. Excellent strength and availability.
Best for: General industrial use, utility systems, most process piping
Group 1.9 (2.25Cr-1Mo)
Grade F22 (2.25Cr-1Mo) supports high-temperature creep service. Group 1.5 (F11 / 1.25Cr) is the lighter-alloy cousin. Cost-effective for elevated-temp service.
Best for: Power generation, petrochemical plants, high-temp steam
Group 2.3 (316L stainless)
316L (Group 2.3) is the workhorse for chloride and acidic service. 304L sits in Group 2.1. Note that austenitic stainless is not permitted for API 6A flange bodies - F6a (410ss) is the API stainless variant.
Best for: Food processing, pharmaceutical, coastal, sanitary
Flange Types Covered by ASME B16.5
ASME B16.5 specifies dimensions and design for multiple flange types, each serving different installation and functional requirements:
Weld Neck (WN)
Features a tapered hub that transitions to the pipe. Excellent for vibration and thermal stress resistance.
Use Case: High-temperature, high-pressure, critical service
Advantage: Superior fatigue resistance
Slip-On (SO)
Flange slips over pipe and is welded on both inside and outside bores. Most economical option.
Use Case: General industrial applications, low-cost
Advantage: Lower cost, easy installation
Socket Weld (SW)
Pipe inserts into a socket in the flange and is welded at the end. Compact design.
Use Case: Small bore piping, instrumentation
Advantage: Compact, good for small sizes
Threaded (Th)
Pipe threads into the flange bore. Used when welding is not possible or practical.
Use Case: No-weld installations, retrofit
Advantage: Field removable, quick assembly
Lap Joint (LJ)
Loose flange slides over a stub end that is welded to the pipe. Allows flange rotation.
Use Case: Stainless steel piping, frequent flange changes
Advantage: Cost-effective for stainless applications
Blind (Bl)
Solid face with no bore opening. Used to cap piping ends or as isolation flanges.
Use Case: End caps, blank-off plates
Advantage: Complete system closure
Selection Guidance: Weld neck flanges are preferred for critical, high-pressure service due to superior stress distribution. Slip-on flanges are economical for general piping. Consult your design code (ASME B31.3 for process piping, B31.1 for power plant) for specific requirements.
ASME B16.5 vs. ANSI B16.5: Clarifying the Standards
ASME B16.5
- •Published by American Society of Mechanical Engineers
- •Current designation for the standard
- •Regularly updated and maintained by ASME
- •Use this designation in new specifications
ANSI B16.5
- •Adopted by American National Standards Institute
- •Identical to ASME B16.5 (same dimensions and specs)
- •Legacy designation (pre-2000s standards)
- •Still used in older drawings and legacy systems
The Bottom Line
ASME B16.5 and ANSI B16.5 specify identical flange dimensions and design criteria. The difference is purely in designation: ANSI adopted ASME's standard for broader national recognition, but modern standards are published by ASME. For new projects, specify ASME B16.5. In the field, you'll encounter both designations on older equipment and drawings.
Practical Impact: None. A flange manufactured to ASME B16.5 is interchangeable with one manufactured to ANSI B16.5.
Frequently Asked Questions
Common questions about ASME B16.5 flanges and specifications
Ready to Specify ASME B16.5 Flanges?
Our technical team can help you select the right flange type, material, and pressure class for your application.