ASME B16.5 is the most widely used flange standard in the world, governing pipe flanges and flanged fittings from NPS ½ through NPS 24 in oil, gas, petrochemical, and process piping systems. It defines everything a procurement team or piping engineer needs to specify a flange correctly: the seven pressure classes (150 through 2500), pressure-temperature ratings by material group, face types, bore dimensions, bolting requirements, and marking. Getting any of these parameters wrong can mean a costly mismatch at the flanged joint or, worse, a leak in service.
ZC Steel Pipe manufactures and exports ASME B16.5 flanges in carbon steel, alloy steel, and stainless steel, covering all seven pressure classes and all standard flange types — weld neck, slip-on, blind, socket weld, threaded, and lap joint. We supply flanges for oil and gas pipelines, petrochemical plants, and process piping projects across Africa, the Middle East, and South America.
CONTENTS
What Is ASME B16.5?
The Seven Pressure Classes
Pressure-Temperature Ratings by Material Group
Flange Types Covered by B16.5
Flange Face Types — RF, FF, RTJ, and Others
Dimensions and Bore Sizes
Materials for ASME B16.5 Flanges
Bolting and Gasket Requirements
ASME B16.5 vs ASME B16.47 — Which Standard Applies?
FAQ
1. What Is ASME B16.5?
STANDARD DEFINITION — ASME B16.5ASME B16.5 (Pipe Flanges and Flanged Fittings: NPS ½ Through NPS 24 Metric/Inch Standard) is published by the American Society of Mechanical Engineers. It establishes pressure-temperature ratings, materials, dimensions, tolerances, marking, and testing for flanges in NPS ½ through NPS 24. The standard applies to flanges made from cast or forged materials. It is referenced by ASME B31.3 (Process Piping), ASME B31.4 (Liquid Pipeline), ASME B31.8 (Gas Transmission), and virtually all major piping codes globally.
ASME B16.5 organises its pressure ratings around two variables: the pressure class and the material group. A flange's actual allowable working pressure at a given temperature is determined by looking up the intersection of these two variables in the standard's pressure-temperature rating tables. This means that two flanges stamped "Class 300" but made from different materials can have significantly different pressure ratings — the class number alone is not sufficient to determine the actual rating.
The standard covers NPS ½ through NPS 24. For flanges above NPS 24, see ASME B16.47 (Series A or B), which extends coverage to NPS 60.
2. The Seven Pressure Classes
ASME B16.5 defines seven pressure classes. The class number originated from the approximate pressure rating (in psi) of a Class X carbon steel flange at 850°F (454°C) — a now largely historical convention. What matters in practice is the pressure-temperature table for the specific material group, not the class number alone.
| Class | Approx. MAWP at 38°C — A105 Carbon Steel (bar / psi) | Typical Application |
| 150 | 19.6 bar / 285 psi | Low pressure utility lines, water, steam ≤ 200°C |
| 300 | 51.1 bar / 740 psi | Gas distribution, moderate pressure process piping |
| 400 | 68.1 bar / 990 psi | Less common; bridging class between 300 and 600 |
| 600 | 102.1 bar / 1480 psi | High-pressure gas and oil transmission, refineries |
| 900 | 153.4 bar / 2220 psi | High-pressure steam, gas injection, HPHT applications |
| 1500 | 255.6 bar / 3705 psi | Very high pressure — wellheads, gas injection manifolds |
| 2500 | 425.5 bar / 6170 psi | Extreme pressure — subsea, deep well HPHT |
Procurement Note — Class 400 Is Rarely SpecifiedClass 400 was introduced to bridge the gap between Class 300 and Class 600 but is rarely specified in modern projects. Many mills carry limited or no stock in Class 400 — it typically requires a minimum order quantity and longer lead time. If your design pressure falls between Class 300 and Class 600, most engineers simply step up to Class 600. Specify Class 400 only if your project code or client specification explicitly requires it and be prepared for longer procurement lead times.
3. Pressure-Temperature Ratings by Material Group
ASME B16.5 organises materials into groups — currently 31 groups for cast and forged materials. Each group has its own pressure-temperature rating table. The most commonly used material groups in oil and gas piping are:
| Material Group | Typical ASTM Material | Description | Class 150 MAWP at 38°C | Class 300 MAWP at 38°C |
| 1.1 | A105, A181 Gr.70 | Carbon steel — standard | 19.6 bar (285 psi) | 51.1 bar (740 psi) |
| 1.2 | A216 WCB, A515 Gr.70 | Carbon steel — cast | 19.6 bar (285 psi) | 51.1 bar (740 psi) |
| 1.13 | A350 LF2 | Carbon steel — low temperature | 19.6 bar (285 psi) | 51.1 bar (740 psi) |
| 2.1 | A182 F11, F22 | 1¼Cr–½Mo / 2¼Cr–1Mo alloy steel | 19.6 bar (285 psi) | 51.1 bar (740 psi) |
| 2.3 | A182 F5, F9 | 5Cr–½Mo / 9Cr–1Mo alloy steel | 19.6 bar (285 psi) | 51.1 bar (740 psi) |
| 3.1 | A182 F304, F316 | Austenitic stainless steel (300 series) | 15.1 bar (220 psi) | 39.8 bar (578 psi) |
| 3.2 | A182 F304L, F316L | Low-carbon austenitic SS | 13.8 bar (200 psi) | 36.2 bar (525 psi) |
Critical Engineering Point — Stainless Flanges Have Lower Ratings Than Carbon SteelA common specification error: austenitic stainless steel flanges (Groups 3.1, 3.2) have lower pressure-temperature ratings than carbon steel flanges of the same class. A Class 150 A182 F316L flange is rated at only 13.8 bar at 38°C — versus 19.6 bar for the same class in A105 carbon steel. This is because austenitic stainless has lower yield strength at elevated temperatures than carbon steel. Always verify the PT rating against the material group table, not just the class number, when mixing materials in a piping system.
4. Flange Types Covered by B16.5
Weld Neck (WN)
Hub: Long tapered bore
Weld: Butt weld to pipe
Strength: Highest — full penetration weld
Best for: High pressure, cyclic, critical service
Slip-On (SO)
Hub: Short — pipe slides through
Weld: Two fillet welds — inside and outside
Strength: Approx. 2/3 of weld neck
Best for: Lower pressure, general service, easy alignment
Blind (BL)
Hub: Solid plate — no bore
Weld: Bolted — no welding
Use: Terminate or isolate pipeline ends
Best for: All pressure classes, test blanking
Socket Weld (SW)
Hub: Recessed socket bore
Weld: Single fillet weld — outside only
Size limit: NPS ½ – NPS 3 typical
Best for: Small bore high-pressure instrument lines
Threaded (TH)
Hub: Internal NPT thread
Weld: No welding required
Use: Low-pressure utility, temporary connections
Best for: Where welding is not possible
Lap Joint (LJ)
Hub: Flat bore — used with stub end
Weld: Stub end welded to pipe; flange loose
Use: Frequent disassembly; clad/lined pipe
Best for: Where flange rotation aids bolt alignment
Engineering Insight — Weld Neck Is Not Always MandatoryWeld neck flanges are the strongest type and are often specified by default in oil and gas projects. However, slip-on flanges are fully acceptable under ASME B16.5 for Class 150 and 300 service in non-cyclic, non-shock loading conditions and can reduce cost and installation time for large-bore, lower-pressure systems. The restriction on slip-on flanges typically comes from company specifications or project standards — not from B16.5 itself. Review your applicable piping specification before automatically upgrading all flanges to weld neck.
5. Flange Face Types — RF, FF, RTJ, and Others
| Face Type | Abbreviation | Description | Typical Gasket | Application |
| Raised Face | RF | Raised circular contact area — 1.6 mm for Class 150/300; 6.4 mm for Class 400–2500 | Spiral wound, sheet gasket | Most common — used in the majority of oil and gas piping |
| Flat Face | FF | Full-face seating — entire flange face contacts gasket | Full-face soft gasket | Connecting to cast iron or non-metallic flanges — prevents bending stress |
| Ring Type Joint | RTJ | Machined groove accepts metallic ring gasket (oval or octagonal) | Metallic ring (R, RX, BX) | High pressure, high temperature, critical service; wellhead and XT connections |
| Male/Female | M/F | One flange has raised face, other has matching recess | Sheet or spiral wound | Pumps, compressors — self-centring gasket |
| Tongue & Groove | T&G | Narrow raised tongue on one flange, matching groove on other | Narrow ring gasket | Heat exchangers, vessels — confined gasket |
Critical Engineering Point — Never Mate RF and FF FlangesRaised face and flat face flanges must never be bolted together directly. A raised face flange bolted to a flat face flange concentrates the gasket load on the raised portion only, causing the flat-face flange (often cast iron or lower-strength material) to bend and crack under bolt load. If your project requires connection of RF and FF flanges, the raised face must be machined flat, or a full-face gasket and controlled bolt torque must be used. This is a common field error with real failure consequences.
6. Dimensions and Bore Sizes
ASME B16.5 sets outside diameter, bolt circle diameter, number and diameter of bolt holes, and minimum flange thickness for each NPS and class combination. Bore dimensions are set by the pipe schedule being used — B16.5 provides bore tables for standard, XS (extra strong), and XXS schedules, as well as a range of intermediate schedules.
| NPS | Class 150 OD (mm) | Class 300 OD (mm) | Class 600 OD (mm) | No. of Bolts (Cl. 150) | No. of Bolts (Cl. 300/600) |
| ½" | 89 | 95 | 95 | 4 | 4 |
| 1" | 108 | 124 | 124 | 4 | 4 |
| 2" | 152 | 165 | 190 | 4 | 8 |
| 4" | 229 | 254 | 273 | 8 | 8 |
| 6" | 279 | 318 | 356 | 8 | 12 |
| 8" | 343 | 381 | 419 | 8 | 12 |
| 10" | 406 | 444 | 508 | 12 | 16 |
| 12" | 483 | 521 | 559 | 12 | 16 |
| 16" | 597 | 648 | 711 | 16 | 20 |
| 20" | 711 | 775 | 864 | 20 | 24 |
| 24" | 813 | 914 | 1016 | 20 | 24 |
Note: Dimensions above are for RF weld neck flanges. Slip-on, blind, and other types have the same OD and bolt circle but differ in thickness and bore. Always reference the applicable B16.5 table for the specific flange type and class required.
7. Materials for ASME B16.5 Flanges
7.1 Carbon Steel
The dominant material for B16.5 flanges in oil and gas service. ASTM A105 (forged) and A216 WCB (cast) are the standard grades for ambient to moderate temperature service. For low-temperature service below −29°C, ASTM A350 LF2 (impact tested) is specified. For sour service (H₂S), additional HIC and SSCC requirements apply.
7.2 Alloy Steel
For elevated temperature or high-pressure service, alloy steel flanges extend the pressure-temperature capability beyond carbon steel. Common grades include A182 F11 (1¼Cr–½Mo), A182 F22 (2¼Cr–1Mo), and A182 F5 / F9 (5Cr and 9Cr–1Mo) for refinery and high-temperature process service.
7.3 Stainless Steel
Austenitic stainless steel flanges to A182 F304, F316, F304L, and F316L are used for corrosive service, cryogenic applications, and food/pharmaceutical piping. Note that stainless flanges have lower pressure ratings than carbon steel of the same class — see Section 3 above. Duplex and super-duplex stainless grades are used for highly corrosive offshore and sour service environments.
| Material | ASTM Grade | B16.5 Group | Temp Range | Typical Application |
| Carbon steel | A105 | 1.1 | −29°C to 425°C | Standard oil and gas, process piping |
| Low-temp CS | A350 LF2 | 1.13 | −46°C to 343°C | LNG, cryogenic, cold climate |
| 1¼Cr–½Mo | A182 F11 | 2.1 | −29°C to 593°C | Refinery, elevated temp process |
| 2¼Cr–1Mo | A182 F22 | 2.1 | −29°C to 593°C | High-temp hydrocracking, reformers |
| 316 SS | A182 F316 | 3.1 | −196°C to 538°C | Corrosive service, offshore |
| 316L SS | A182 F316L | 3.2 | −196°C to 454°C | Sour service, chloride environments |
8. Bolting and Gasket Requirements
ASME B16.5 specifies the bolt size, length, and quantity for each NPS and class. Standard bolt materials are ASTM A193 B7 studs with A194 2H heavy hex nuts — the default for most carbon and alloy steel flanged joints. For low-temperature service, A320 L7 / A194 4 bolting is used. For stainless flanges, A193 B8M / A194 8M (316 stainless) bolts are common.
Procurement Note — Gasket Type Must Match Face Type and ServiceSpecifying the flange class and material is only half the job — the gasket selection is equally critical and must be specified on the purchase order. Spiral wound gaskets (SWG) with a carbon steel or stainless inner ring are standard for RF flanges in oil and gas service. Sheet gaskets (compressed non-asbestos fibre, CNAF) are used for lower-pressure utility service and full-face flanges. RTJ metallic ring gaskets are specified by ring number (e.g., R-24 for NPS 4 Class 300) and material. A gasket specified incorrectly for the service temperature or pressure class is a common root cause of flange leaks.
9. ASME B16.5 vs ASME B16.47 — Which Standard Applies?
| Standard | NPS Coverage | Series / Equivalent | Typical Application |
| ASME B16.5 | NPS ½ – NPS 24 | — | Most process and pipeline flanges |
| ASME B16.47 Series A | NPS 26 – NPS 60 | Equivalent to MSS SP-44 | Large-diameter pipeline flanges (API-type) |
| ASME B16.47 Series B | NPS 26 – NPS 60 | Equivalent to API 605 | Large-diameter process piping flanges |
Series A (MSS SP-44 equivalent) flanges are heavier and more expensive than Series B for the same NPS and class, but they match the bolt circle of B16.5 flanges at the NPS 24 boundary — meaning a Series A NPS 26 flange will bolt directly to a B16.5 NPS 24 flange of the same class. Series B flanges do not share this compatibility. This matters for projects where large-diameter pipe transitions to smaller-diameter header sections.
10. Frequently Asked Questions
What is ASME B16.5?
ASME B16.5 is the American standard for pipe flanges and flanged fittings from NPS ½ through NPS 24. It defines seven pressure classes (150 through 2500), pressure-temperature ratings by material group, flange face types, bore dimensions, bolting requirements, and marking. It is the most widely referenced flange standard in oil, gas, petrochemical, and process piping globally, and is cited by all major piping design codes including ASME B31.3 and B31.8.
What are the ASME B16.5 pressure classes?
ASME B16.5 defines seven pressure classes: 150, 300, 400, 600, 900, 1500, and 2500. The class number does not directly state the pressure rating — the actual allowable working pressure depends on both the class and the material group. For standard A105 carbon steel at 38°C, the ratings range from approximately 19.6 bar (Class 150) to 425.5 bar (Class 2500). Ratings decrease as operating temperature increases.
What is the difference between RF, FF, and RTJ flange faces?
RF (Raised Face) is the most common type, with a small circular raised contact area that concentrates bolt load on the gasket. FF (Flat Face) has no raised area and is used when connecting to cast iron or non-metallic flanges. RTJ (Ring Type Joint) has a machined groove accepting a metallic ring gasket, used for high-pressure and critical service. RF and FF flanges must never be bolted together without modification, as doing so will damage the lower-strength flat-face flange.
What is the difference between ASME B16.5 and ASME B16.47?
ASME B16.5 covers NPS ½ through NPS 24. ASME B16.47 covers large-diameter flanges from NPS 26 through NPS 60, in two series: Series A (equivalent to MSS SP-44, heavier, pipeline use) and Series B (equivalent to API 605, lighter, process piping use). Dimensions and bolt patterns are not interchangeable between B16.5 and B16.47, except at the NPS 24/26 transition for Series A flanges, which are designed to be bolt-compatible with B16.5 at that boundary.
What is ASTM A105 and when is it used?
ASTM A105 is the most common forged carbon steel material for ASME B16.5 flanges. It provides minimum tensile strength of 485 MPa and minimum yield of 250 MPa, and is suitable for service from −29°C to approximately 425°C. A105 falls in Material Group 1.1 in B16.5. It is the default flange material for most oil and gas and process piping applications at moderate temperature. For lower temperatures, A350 LF2 (impact tested) is used. For elevated temperature service, alloy steel grades such as A182 F11 or F22 are specified.
What flange types are covered by ASME B16.5?
ASME B16.5 covers Weld Neck (WN), Slip-On (SO), Socket Weld (SW), Threaded (TH), Blind (BL), Lap Joint (LJ), and Reducing Flanges. Weld Neck is the strongest and most widely specified for pressure piping. Blind flanges are used to terminate or blank off a pipeline end. Socket Weld and Threaded are used for small-bore connections. Lap Joint flanges are used with stub ends for applications requiring frequent disassembly or where flange rotation aids bolt hole alignment.
Source ASME B16.5 Flanges from ZC Steel Pipe
ZC Steel Pipe manufactures and exports ASME B16.5 flanges in all seven pressure classes (150 through 2500) and all standard types — weld neck, slip-on, blind, socket weld, threaded, and lap joint. Materials include ASTM A105, A350 LF2, A182 F11/F22/F316L, and duplex/super-duplex stainless. Full MTC documentation, third-party inspection, and custom bore dimensions available.
We supply flanges for oil and gas pipelines, refineries, petrochemical plants, and process piping projects across Africa, the Middle East, and South America.
mandy.w@zcsteelpipe.com
WhatsApp: +86-139-1579-1813
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