ASME B16.5 is the standard published by ASME (American Society of Mechanical Engineers) covering pipe flanges and flanged fittings for pressure piping systems. It defines dimensions, materials, pressure-temperature ratings, bolt patterns, and facing types for flanges from NPS 1/2 to NPS 24 across seven pressure classes (150, 300, 400, 600, 900, 1500, 2500).

HT PIPE manufactures and supplies ASME B16.5 compliant flanges in carbon steel (A105/WCB), stainless steel (F304/F316L), and alloy steel (F5/F9/F22) across all standard flange types and pressure classes, with full Mill Test Certificates (MTC) per EN 10204 3.1. 

1. Standard Scope & Designation

ASME B16.5 applies to pipe flanges and flanged fittings used in pressure piping systems:

2. Flange Types Covered by ASME B16.5

ASME B16.5 covers six flange types, each suitable for different service conditions:

2.1 Flange Facing Types

3. Flange Materials — ASME B16.5 Material Groups

ASME B16.5 assigns flange materials to groups based on their composition and service capabilities:

3.1 Group 1 — Carbon Steel & Low-Alloy Forgings

3.2 Group 2 — Alloy Steel Forgings

3.3 Group 3 — Stainless Steel Forgings

3.4 Group 4 — Duplex / Super-Duplex & Special Alloys

4. Pressure-Temperature Ratings — Group 1 Carbon Steel (A105 / WCB)

Allowable pressure in bar (MPa) at design temperature. Per ASME B16.5 Tables. Verify against project-specific P-T data.

Note: Class 150 rated to 425°C but with severely reduced pressure. Group 1 (A105/WCB) is limited to 427°C. For higher temps, use Group 2 alloy steel (F9/F22) or Group 3 stainless (F347).

4.1 Pressure-Temperature — Group 3 Stainless Steel (A182 F304/F316)

Note: F304/F316/F321 rated to 540°C. F347/F310 rated to 595°C. For F316L, limit is 425°C. Full tables per ASME B16.5-2020 Tables 2-3.1.

5. Flange Dimensions — Class 150 & Class 300 (Selected NPS)

Key dimensions for Weld Neck (WN) flanges. Full tables per ASME B16.5 Tables. For Class 400/600/900/1500/2500, refer to B16.5 Tables 6 through 12.

5.1 Class 150 — Weld Neck Flange Dimensions (mm)

5.2 Class 300 — Weld Neck Flange Dimensions (mm)

Note: All dimensions per ASME B16.5-2020. Full tables include Class 400/600/900/1500/2500. For dimensions of SO, SW, TH, LJ, and BL flanges, refer to B16.5 Tables 7-12. For Class 600+, studs are typically double-end studs with nuts.

6. Bolt & Gasket Specifications

6.1 Bolt / Stud Specifications by Pressure Class

6.2 Gasket Selection Guide

7. Industries & Applications

8. Flange Marking per ASME B16.5

Per ASME B16.5 Section 7, each flange shall be permanently marked with:

9. Related Standards

10. How to Order — Required Information

11. Frequently Asked Questions

Q: What is ASME B16.5?

ASME B16.5 is the standard for pipe flanges and flanged fittings from NPS 1/2 to NPS 24, covering seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500. It defines flange dimensions, materials, pressure-temperature ratings, bolt patterns, facing types, and marking requirements. The standard is mandatory-referenced by ASME B31.1, B31.3, and B31.5 piping codes.

Q: Weld Neck vs Slip-On flange — which to use?

Weld Neck (WN) flanges are preferred for high-pressure, high-temperature, cyclic loading, and fatigue-critical service because the tapered hub transitions smoothly from the pipe, reducing stress concentration at the weld. Slip-On (SO) flanges are lower cost and easier to fit but require fillet welds on both sides; they are limited to Class 150 and 300 and should not be used for severe service or where vibration is present.

Q: What pressure class flange should I use?

Pressure class is determined by the system's design pressure and temperature using ASME B16.5 pressure-temperature tables. The flange must be rated for the maximum pressure at the design temperature. In practice, use the same pressure class as the piping system rating (e.g., Class 300 piping system uses Class 300 flanges). For high-pressure/high-temp applications, always verify the allowable pressure from the P-T tables.

Q: What is the difference between RF and RTJ flanges?

Raised Face (RF) flanges have a 1.5 mm (1/16 in.) raised circular pad for Class 150–600, using spiral wound, jacketed, or ring gaskets. Ring Type Joint (RTJ) flanges have a groove machined into the face and use metallic oval or octagonal ring gaskets. RTJ flanges are preferred for high-pressure and high-temperature hydrocarbon service (Class 600+) because the metal-to-metal seal is more reliable under extreme pressure/temperature conditions.

Q: What materials are available for ASME B16.5 flanges?

ASME B16.5 Group 1 (carbon steel/low-alloy): A105, A350 LF2, A216 WCB/WCC; Group 2 (alloy steel): A182 F5, F9, F11, F12, F22; Group 3 (stainless): A182 F304, F304L, F316, F316L, F321, F347, F310; Group 4 (duplex/special): A182 F51, F53, F55, F44, and Nickel alloys (B462). Always match flange material to the pipe material and service conditions.

Q: What is the difference between ASME B16.5 and B16.47?

ASME B16.5 covers NPS 1/2 to NPS 24 flanges across all pressure classes. ASME B16.47 covers large diameter steel flanges for NPS 26 through NPS 60 in Series A (MSS SP-44 style, Weld Neck and Blind) and Series B (LAP joint style). B16.47 flanges are generally used for large bore intake/outfall, cooling water, and low-pressure utility piping in the NPS 26–60 range.