ASME B16.5 · Standard reference
ASME B16.5-2025 marking requirements (Section 4)
Section 4 of ASME B16.5-2025 defines the identification markings that must be applied to flanges and flanged fittings. It starts from MSS SP-25, the standard marking system for valves, fittings and flanges, and then modifies it with a specific list of items: the manufacturer, the material, the pressure class, the conformance mark, the size and, for ring-joint flanges, the ring-groove number. The 2025 edition's Summary of Changes lists no Section 4 entry, so these are long-standing requirements rather than new 2025 text.
The material marking rule is the one buyers and inspectors most often get wrong. Cast flanges and flanged fittings carry the ASTM specification, the grade identification symbol and the melt number or melt identification. Plate flanges, forged flanges and flanged fittings carry the ASTM specification number and grade symbol only — no melt number is required. A manufacturer may add its own trade designation for the grade, provided the symbols cannot be confused, and a component that meets more than one listed specification or grade may carry more than one symbol.
Some marks are conditional and some are optional. The "B16" or "B16.5" designation is mandatory but the "ASME" prefix is not. Temperature marking is not required, and if it is applied it must be shown together with the corresponding tabulated pressure rating. A Case number goes on only when the component relies on an ASME Case. The requirements are set out below clause by clause, with the exemptions and options in a separate section.
- Standard
- ASME B16.5-2025
- Section
- 4 — Marking
- Base marking standard
- MSS SP-25, as modified by §4.2
- Conformance mark
- "B16" or "B16.5"
- Cast material mark
- Spec + grade + melt number
- Temperature mark
- Optional
Required identification markings
The table lists what the standard requires, clause by clause, in clause order except for the optional temperature marking (4.2.5), which is listed last because it is not mandatory. The standard does not prescribe a layout; 4.2.4 only says the conformance mark is preferably placed next to the class designation.
B16.5 · ASME B16.5-2025
| Requirement | Clause | What must be marked |
|---|---|---|
| Manufacturer identity | 4.2.1 | The manufacturer's name or trademark. |
| Material — cast flanges and flanged fittings | 4.2.2(a) | The ASTM specification, the grade identification symbol (letters and numbers), and the melt number or melt identification. |
| Material — plate and forged flanges, and flanged fittings | 4.2.2(b) | The ASTM specification number and the grade identification symbol. |
| Optional trade designation | 4.2.2(c) | The manufacturer may add its trade designation for the material grade, provided confusion of symbols is avoided. |
| Pressure class | 4.2.3 | The number of the rating class designation: 150, 300, 400, 600, 900, 1500 or 2500. |
| Conformance mark | 4.2.4 | The designation "B16" or "B16.5", preferably next to the class designation. An "ASME" prefix is optional. |
| Size | 4.2.6 | The NPS designation. Reducing flanges and reducing flanged fittings carry the applicable NPS designation for each opening (paras. 3.2 and 3.3). |
| Ring-joint flanges | 4.2.7 | On the edge (periphery): the letter R and the corresponding ring-groove number. |
| Multiple material marking | 4.2.8 | Optional: more than one applicable specification or grade symbol, placed to avoid confusion and following ASME BPVC Section II, Part D, Mandatory Appendix 7. |
| Case number | 4.2.9 | "Case ##" when the flange or flanged fitting relies on and meets the requirements of a Case. |
| Temperature (optional) | 4.2.5 | Not required. If the temperature is marked, it must be shown with its corresponding tabulated pressure rating for the material. |
Paraphrased from the clause text; the clause number is given for each row. "Must" appears here because the clause uses "shall".
Notes, exemptions and what is optional
- General rule (4.1): flanges and flanged fittings are marked as required in MSS SP-25, except as modified by §4.2. MSS SP-25 is the baseline; Section 4 adds and overrides.
- Material marking is an assurance (4.2.2(e)): applying the ASTM specification number or trade designation is the manufacturer's assurance that the material complies with that specification or specifications, including when multiple certifications apply.
- ASME Code substitution (footnote 2 to 4.2.2): an ASME Boiler and Pressure Vessel Code, Section II specification number may be substituted for an ASTM specification number where the ASME requirements are identical to or more stringent than the ASTM specification for the grade, class or type.
- Multiple material marking (4.2.8): permitted at the manufacturer's option, not required. It applies to components meeting more than one specification or grade listed in Table 1.1-1, and the marking must follow BPVC Section II, Part D, Mandatory Appendix 7.
- Temperature (4.2.5): no temperature marking is required. If one is applied, it must be paired with the corresponding tabulated pressure rating for the material — a bare temperature is not a valid mark.
- Case number (4.2.9): required only when the component relies on and meets a Case. Section 1.10 adds that a Case may be used only when agreed by the purchaser and the manufacturer, and that when a Case has been agreed the marking must include the Case number. A Case number is not a routine mark and does not replace the class or conformance marks.
- Reducing flanges (4.2.6): the size marking must identify the NPS of each opening, not only the larger opening, as required by paras. 3.2 and 3.3.
- Ring-joint flanges (4.2.7): the R mark and groove number go on the edge (periphery) of the flange, so they remain visible with the flange bolted up.
- No 2025 change: the 2025 edition's Summary of Changes and List of Changes contain no Section 4 entry, so marking requirements carry over from the previous edition.
What a marked flange looks like in practice
The standard does not prescribe the order or the method — characters may be cast, stamped or laser-marked, and the conformance mark is only required to be preferably next to the class number. The strings below are therefore illustrations of the required content, not a mandated layout.
- Forged weld-neck flange, single material: [manufacturer trademark] A105 150 B16.5 NPS 6.
- Cast flange, single material: [manufacturer trademark] A216 Gr. WCB [melt number] 150 B16.5 NPS 6.
- Ring-joint flange: the same identification marks plus R and the ring-groove number on the edge — for example R followed by the groove number taken from Table 5 (Table 5C) for that size and class.
- Multiple-certification flange: the extra specification or grade symbols are added next to the first, marked so that the combination cannot be misread.
- Case component: "Case ##" is added when the component relies on an ASME Case agreed with the purchaser.
ASME B16.5 marking — questions
Is the "ASME" prefix on the B16.5 mark required?
No. Para. 4.2.4 requires the designation "B16" or "B16.5" to indicate conformance, preferably located next to the class designation, and states that the use of the prefix "ASME" is optional. Either form of the conformance mark is acceptable.
What is the difference between marking a cast flange and a forged flange?
Cast flanges and flanged fittings must carry the ASTM specification, the grade identification symbol and the melt number or melt identification. Plate flanges, forged flanges and flanged fittings must carry the ASTM specification number and the grade identification symbol, with no melt number required (4.2.2(a) and (b)).
Can a flange carry more than one material marking?
Yes, at the manufacturer's option, when the component meets the requirements of more than one specification or grade listed in Table 1.1-1. The marks must be placed so as to avoid confusion and follow ASME Boiler and Pressure Vessel Code, Section II, Part D, Mandatory Appendix 7 (4.2.8).
Must the temperature be marked on a B16.5 flange?
No. Temperature markings are not required. If a manufacturer does mark a temperature, it must be shown together with the corresponding tabulated pressure rating for the material (4.2.5), so a temperature alone is not an acceptable identification mark.