ASME B16.5 — Pipe Flanges and Flanged Fittings
Covers flanges from NPS 1/2" to 24" in pressure classes 150 through 2500 — materials, dimensions, tolerances, marking, and pressure-temperature ratings.
ASME B16.5 (formally ASME B16.5-2020) is the American Society of Mechanical Engineers standard for pipe flanges and flanged fittings made from cast or forged materials. It is one of the most frequently referenced standards in industrial piping design worldwide and governs flanges in nominal pipe sizes NPS 1/2" through NPS 24" across seven pressure classes: 150, 300, 600, 900, 1500, and 2500. For NPS above 24", the companion standard ASME B16.47 takes over. The standard applies to flanges manufactured from carbon steel, alloy steel, stainless steel, and nickel-base alloys — each material assigned to one of more than 30 material groups that determine the pressure-temperature (P-T) rating tables. The pressure class system is the foundation of B16.5. A Class 150 flange, for example, is rated for 285 psi (19.6 bar) at ambient temperature in Group 1.1 carbon steel (ASTM A105). As service temperature rises, the allowable working pressure decreases according to the P-T rating table for the relevant material group. A Class 300 flange in the same material handles 740 psi (51.0 bar) at ambient, and a Class 600 reaches 1480 psi (102.1 bar). This progression allows designers to select the exact class that meets their design conditions without over-specifying. The seven classes are NOT linearly scaled — Class 900 does not simply equal 6× Class 150. Each class has its own independent set of P-T curves. ASME B16.5 covers all the standard flange types used in piping systems: weld neck (WN), slip-on (SO), blind (BL), socket weld (SW), threaded (THD), and lap-joint (LJ). Each type has a specific dimensional table covering outside diameter, bolt circle diameter, number and diameter of bolt holes, thickness, hub length (for weld neck), and raised face or ring-type joint (RTJ) details. The weld neck flange is the premium choice for critical service — its tapered hub provides excellent stress distribution and is mandatory in most refinery and power-plant piping classes. Slip-on flanges are more economical and easier to align but require a double fillet weld and are limited to lower-pressure, non-critical applications. Blind flanges close off piping ends and must withstand full line pressure, so their thickness is the greatest among all types for a given class. Material groups in ASME B16.5 organize hundreds of ASTM and ASME material specifications into groups with similar high-temperature strength characteristics. Group 1.1 is the most common: it includes ASTM A105 (forged carbon steel for flanges), ASTM A216 WCB (carbon steel castings), and ASTM A350 LF2 (low-temperature carbon steel forgings). Group 1.5 covers the chrome-moly alloys such as ASTM A182 F11 and F22 used in high-temperature refinery service. Group 2.1 through Group 2.3 address austenitic stainless steels like ASTM A182 F304, F316, and F321 for corrosion-resistant and cryogenic applications. The material group directly determines the P-T rating: a Group 1.1 flange has different allowable pressures at 400°F than a Group 1.5 flange of the same class. In Indian procurement, ASME B16.5 is the de facto standard for industrial flanges in refineries (IOCL, BPCL, HPCL, ONGC), power plants (NTPC, BHEL), fertiliser units, and petrochemical complexes. Flanges are procured from forging houses in Rajkot, Ludhiana, and Mumbai, with material conforming to ASTM A105 (carbon steel), ASTM A182 F304/F316 (stainless steel), or ASTM A182 F11/F22 (alloy steel). The procurement specification should include: standard (ASME B16.5), NPS, class, flange type (WN/SO/BL/SW/THD/LJ), face finish (RF, RTJ, or FF), schedule of mating pipe, material grade, and documentation level (MTC 3.1 or 3.2 per EN 10204). Marking per B16.5 Section 6 must include manufacturer, material designation, class, size, bore (for WN), and the ASME "B16.5" stamp. Common procurement errors include: specifying Class 150 where design conditions require Class 300, ordering slip-on flanges for high-vibration or high-temperature service where weld neck is mandatory, mixing ANSI 150 (the older designation) with ASME 150 (they are the same), and failing to specify the face finish — a raised-face flange will not seal properly against a flat-face companion without appropriate gasket modifications. RP Sales stocks and supplies ASME B16.5 flanges in all types and classes, with material test certificates and third-party inspection available for critical projects.
Scope and pressure classes
ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2" (DN 15) through NPS 24" (DN 600) in seven pressure classes: 150, 300, 600, 900, 1500, and 2500. The pressure class defines the flange's maximum allowable working pressure at a reference temperature. As temperature increases, the allowable pressure decreases according to the pressure-temperature rating tables in the standard.
The pressure class number does NOT directly represent psi. A Class 150 flange in Group 1.1 material (ASTM A105) is rated for 285 psi at 100°F, not 150 psi. A Class 300 is rated for 740 psi at the same conditions. The class system provides standardised pressure-temperature envelopes so that designers can select the minimum class that satisfies their design conditions.
For NPS above 24", ASME B16.47 provides two dimensional series (Series A and Series B) covering NPS 26" through NPS 60". These are separate standards — B16.5 dimensional tables do not extend beyond NPS 24".
Flange types covered
Weld Neck (WN): A long, tapered hub is welded to the pipe with a single butt weld. The taper reduces stress concentration and is ideal for critical, high-pressure, and high-temperature service. WN flanges are the standard choice in ASME B31.3 process piping.
Slip-On (SO): The pipe slides into the flange bore, and two fillet welds (inside and outside) secure it. SO flanges are easier to align and less expensive than WN but have approximately one-third lower fatigue life. They are limited to Class 150 and Class 300 in most refinery piping classes.
Blind (BL): A solid disc used to close off a pipeline, nozzle, or vessel opening. BL flanges must withstand full line pressure and are the thickest flange type for any given class and NPS.
Socket Weld (SW): The pipe inserts into a recessed bore and is fillet-welded from the outside. SW flanges are used for small-bore piping (typically NPS 2" and below) in high-pressure service where butt-weld joint preparation is impractical.
Threaded (THD): The pipe is screwed into the flange bore. Used only for low-pressure, non-critical service — vibration can loosen the threaded joint over time. Not permitted in most refinery piping classes.
Lap-Joint (LJ): A loose flange ring sits behind a stub-end. The stub-end contacts the gasket while the flange ring provides bolt-hole alignment. LJ flanges allow easy disassembly and are used where frequent maintenance is expected — particularly with expensive alloy piping where the stub-end is alloy but the flange ring is carbon steel.
Pressure-temperature ratings
The P-T rating is the maximum allowable non-shock working gauge pressure at a specific temperature for a given material group and pressure class. ASME B16.5 Table 2 provides these ratings for each combination of class and material group. The rated pressure at ambient temperature (−20°F to 100°F) is the highest value; as temperature rises, the material's allowable stress decreases and so does the rated pressure.
For example, a Class 150 flange in Group 1.1 (ASTM A105 carbon steel) is rated at 285 psi at 100°F but drops to 260 psi at 400°F, 170 psi at 700°F, and 110 psi at 850°F. A Class 300 flange in the same material is rated at 740 psi at 100°F, declining to 675 psi at 400°F and 440 psi at 700°F. The exact values must always be taken from the current edition of B16.5 rather than from memory.
Interpolation between tabulated temperatures is permitted by linear interpolation. Extrapolation beyond the table is not permitted — if your service temperature exceeds the maximum tabulated temperature for the material group, you must either select a higher class or change to a material group with a higher temperature rating.
Material groups (Groups 1–3)
ASME B16.5 organises materials into groups based on high-temperature strength characteristics. Each group has its own P-T rating table. The main groups relevant to Indian procurement are:
Group 1.1 — Carbon and carbon-manganese steels: ASTM A105 (forged), A216 WCB (cast), A350 LF2 (low-temp forged). This is the default group for general refinery, water, and process piping flanges.
Group 1.5 — Chrome-moly steels: ASTM A182 F11 (1¼Cr-½Mo), F22 (2¼Cr-1Mo). These flanges are used in high-temperature refinery and power-plant service where carbon steel P-T ratings are insufficient.
Group 2.1 — Austenitic stainless steels (low carbon): ASTM A182 F304L, F316L. Widely used in chemical, pharmaceutical, and food-processing piping where corrosion resistance is critical.
Group 2.2 — Austenitic stainless steels (standard carbon): ASTM A182 F304, F316, F321, F347. Higher high-temperature strength than the L-grades but with greater sensitisation risk.
Group 3.x — High-nickel alloys: Covers alloys such as Monel, Inconel, and Hastelloy for aggressive corrosion environments. These are specialty flanges with long lead times and high cost.
| Pressure class | Max WP at 100°F (psi) | Max WP at 100°F (bar) | Max WP at 400°F (psi) | Typical service |
|---|---|---|---|---|
| Class 150 | 285 | 19.6 | 260 | Low-pressure water, air, utility steam |
| Class 300 | 740 | 51.0 | 675 | Medium-pressure process, steam |
| Class 600 | 1480 | 102.1 | 1350 | High-pressure process, refinery |
| Class 900 | 2220 | 153.1 | 2025 | High-pressure hydrocarbon, lethal service |
| Class 1500 | 3705 | 255.5 | 3375 | Very high pressure, HP/HT wells |
| Class 2500 | 6170 | 425.4 | 5625 | Ultra-high pressure, critical service |
| Size range | NPS 1/2" (DN 15) to NPS 24" (DN 600) |
|---|---|
| Pressure classes | 150, 300, 600, 900, 1500, 2500 |
| Standard | ASME B16.5-2020 |
| Flange types | Weld Neck, Slip-On, Blind, Socket Weld, Threaded, Lap-Joint |
| Face finishes | Raised Face (RF), Ring-Type Joint (RTJ), Flat Face (FF) |
| Materials (carbon) | ASTM A105, A216 WCB, A350 LF2 |
| Materials (stainless) | ASTM A182 F304, F304L, F316, F316L, F321, F347 |
| Materials (alloy) | ASTM A182 F11, F22, F91 |
| Marking | Manufacturer, material, class, size, bore, ASME B16.5 stamp |
| Documentation | MTC 3.1 standard; 3.2 with TPI for IBR/export projects |
- ASME B16.5-2020 — Pipe Flanges and Flanged Fittings: NPS 1/2 Through NPS 24 Metric/Inch Standard — American Society of Mechanical Engineers
- ASTM A105/A105M — Standard Specification for Carbon Steel Forgings for Piping Applications — ASTM International
- ASME B16.47 — Large Diameter Steel Flanges: NPS 26 Through NPS 60 (American Society of Mechanical Engineers)
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