How to Order ASME Flanges
ASME flanges are forged or cast piping components used to create bolted, demountable connections in piping systems. They are manufactured per ASME B16.5 (NPS 1/2” through 24”, Class 150 through 2500) and ASME B16.47 (NPS 26” through 60”, Class 75 through 900).
The flange standard defines dimensions, tolerances, pressure-temperature ratings, and facing details. The material standard (ASTM) defines the mechanical and chemical properties.
Core Attributes
Every ASME flange line item must include all of the following attributes.
| # | Attribute | Values / Options | Notes |
|---|---|---|---|
| 1 | Flange type | Weld Neck (WN), Slip-On (SO), Blind (BL), Socket Weld (SW), Threaded (THD), Lap Joint (LJ), Orifice (per ASME B16.36) | WN is preferred for critical services due to the integral hub and smooth stress transition. SO requires two fillet welds |
| 2 | Dimensional standard | ASME B16.5 (NPS 1/2”–24”) or ASME B16.47 Series A (MSS SP-44) / Series B (API 605) for NPS 26”–60” | Series A and Series B have different bolt circles and bolt quantities for the same NPS and class; they are not interchangeable |
| 3 | Nominal size | NPS 1/2” through 24” (B16.5) or NPS 26” through 60” (B16.47) | Always state NPS; do not use “6 inch” or “DN 150” when ordering to ASME standards |
| 4 | Pressure class | 150, 300, 600, 900, 1500, 2500 | The class defines the pressure-temperature rating per ASME B16.5 Table 2. Higher classes have heavier flanges with more and larger bolts |
| 5 | Face type | Raised Face (RF), Ring Type Joint (RTJ), Flat Face (FF), Tongue & Groove (T&G) | RF is the most common for steel flanges. FF is mandatory when mating with cast iron flanges. RTJ is used for high-pressure and high-integrity services |
| 6 | Face finish | Per ASME B16.5 Table E1: RF standard is 125–250 AARH (3.2–6.3 μm Ra) concentric or spiral serrated; RTJ groove finish per ASME B16.5 para. 6.4 | Specifying the finish is critical for gasket seating; a smooth finish on an RF flange may not grip a spiral wound gasket properly |
| 7 | Bore | Standard bore (per B16.5 default) or matching pipe schedule (e.g., Sch 40, Sch 80, Sch 160, XXS) | Applies primarily to WN flanges. The bore must match the connecting pipe ID to avoid turbulence, erosion, and weld fit-up issues |
| 8 | Material specification | See material table below | Always specify the full ASTM designation and grade |
Material Specifications
| Service | ASTM Standard | Common Grades | B16.5 Material Group |
|---|---|---|---|
| Carbon steel | ASTM A105 | A105 (single grade) | Group 1.1 |
| Low-temperature CS | ASTM A350 | LF2 (to -46°C), LF3 (to -101°C, 3.5% Ni) | Group 1.9 / 1.10 |
| Alloy steel (Cr-Mo) | ASTM A182 | F11 (1-1/4Cr-1/2Mo), F22 (2-1/4Cr-1Mo) | Group 1.2 / 1.3 |
| High-alloy steel | ASTM A182 | F5 (5Cr-1/2Mo), F9 (9Cr-1Mo), F91 (9Cr-1Mo-V) | Group 1.4 / 1.14 |
| Austenitic stainless steel | ASTM A182 | F304, F304L, F316, F316L, F321, F347 | Group 2.1 / 2.2 |
| Duplex stainless steel | ASTM A182 | F51 (2205), F53 (2507), F55 (S32760) | Group 2.5 / 2.6 |
Optional and Special Attributes
| Attribute | When Required | Details |
|---|---|---|
| NACE MR0175 / ISO 15156 | Sour service (H₂S) | Limits hardness for CS flanges (22 HRC / 248 HBW max for A105). Requires solution-annealed condition for SS and duplex |
| Hardness testing | NACE service, or per project spec | Report on MTR. Specify test method and acceptance criteria. NACE requires each piece to be tested |
| Special bore | When pipe schedule differs from the B16.5 default bore | Specify the matching schedule: “Sch 80 bore,” “Sch 160 bore,” “XXS bore.” The bore must match the connecting pipe ID exactly |
| Hub end bevel | WN flanges mating with pipe | Standard bevel per ASME B16.25 (37.5° bevel, 1/16” root face) is typically default. Specify if a special bevel or J-prep is required |
| Flange protectors | Storage and shipping | Plastic or wooden covers to protect the flange face (especially RTJ grooves) during transport and storage |
| Certificates | Almost always required | EN 10204 Type 3.1 (MTR) minimum. Type 3.2 for critical services. Specify PMI requirements |
Example Line Items
Correct
30 pcs - Weld Neck Flange, ASME B16.5, NPS 6, Class 300, Raised Face, Sch 40 bore, ASTM A105, MTR EN 10204 3.1
This line item is complete: flange type, dimensional standard, size, class, face type, bore, material, and certificate are all specified.
Incorrect
30 - CS flange 6 inch 300#
This line item is missing: the flange type (WN, SO, BL?), the dimensional standard (B16.5 or B16.47?), the face type (RF or RTJ?), the bore specification, the exact material grade, and the certificate requirement. The supplier cannot quote this without extensive clarification.
Common Pitfalls
| Pitfall | Why It Matters |
|---|---|
| Bore mismatch on WN flanges | A WN flange bored for Sch 40 pipe has a different ID than one bored for Sch 80. If the bore does not match the pipe, the internal step at the weld joint causes turbulence, erosion, and concentration of stresses. It also creates weld fit-up problems |
| Face type omitted | RF, RTJ, and FF flanges have different dimensions (RF flanges are thicker by the raised face height), require different gaskets, and are not interchangeable. Omitting the face type leads to the wrong flange being supplied |
| Confusing ASME B16.47 Series A and Series B | For NPS 26” and above, Series A (based on MSS SP-44) and Series B (based on API 605) have different bolt circle diameters, bolt sizes, and bolt quantities. A Series A flange will not bolt up to a Series B flange of the same size and class |
| Specifying ASME Class as “PN” | ASME classes (150, 300, 600…) and EN PN ratings (PN 10, PN 16, PN 25…) are different systems with different pressure-temperature curves. “PN 300” does not exist; “Class 150” is not equivalent to “PN 20.” Always use the correct system for the applicable standard |
| Omitting the face finish | A standard RF finish of 125–250 AARH is suitable for spiral wound gaskets. If a rubber sheet gasket or PTFE gasket is used, a smoother finish (32–63 AARH) may be required. The finish must be stated if it differs from the B16.5 default |
| FF flange bolted to RF flange | A flat face flange (typically cast iron, ASTM A126) must never be bolted to a raised face flange with standard bolting. The raised face creates a bending moment that can crack the cast iron flange. When mating steel RF flanges with cast iron FF flanges, the raised face must be machined off |
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