Pipe Fitting Marking and Identification: What Every Mark Means and Why It Matters
Every compliant pipe fitting carries a stamped or stencilled code that tells a trained reader its material grade, size, schedule, manufacturer, and applicable standard — without opening the certificate. Knowing how to decode these marks is essential for goods receipt inspection, piping audit, and in-service investigation. An unmarked or illegible fitting is a non-compliant fitting under both ASME B16.9 and EN 10253.
Mandatory Marks Under ASME B16.9
ASME B16.9 Section 9 specifies the following required marks on all fittings:
- Manufacturer's name or trademark — identifies the producing mill
- Material designation — e.g. WPB, WP316L, WP11 Cl.1 — the ASTM grade designation
- NPS size — e.g. 6 for NPS 6; for reducers both sizes (6×4)
- Schedule or wall thickness — e.g. SCH 40, SCH 80, or the actual wall in inches/mm
- Heat or heat code number — links the fitting to its melt heat and the EN 10204 3.1 certificate
Additionally, for alloy steel (P11, P22, P91, P92), the Class designation must be marked (e.g. WP11 Cl.1 = seamless; Cl.3 = welded and solution-treated). P91 is marked WP9V (for the vanadium-bearing grade). Missing the Cl. designation on alloy steel is a non-compliance.
Colour Coding by Grade
Many manufacturers apply paint stripe colour coding in addition to stamping. These are not standardised by ASME — they are manufacturer-specific — but the following are widely used in industry:
| Grade | Common Colour Code | Note |
|---|---|---|
| WPB (Carbon Steel) | Black or no colour | Primer coated as standard |
| WP11 (1¼Cr-½Mo) | Green stripe | Varies by manufacturer |
| WP22 (2¼Cr-1Mo) | Yellow stripe | Varies by manufacturer |
| WP91 (9Cr-1Mo-V) | Purple or violet stripe | Widely adopted convention |
| WP316L | White or stainless natural | Often no paint on SS |
| Duplex 2205 | White + blue stripe (common) | Not standardised |
Critical note: Colour codes are never the basis for material acceptance — they are only an aid to sorting and handling. Always read the stamped designation and verify against the EN 10204 3.1 certificate. Colour can fade, be over-painted, or be applied incorrectly. PMI (XRF) is the definitive check if there is any doubt about grade identity.
EN 10253 Marking Requirements
EN 10253-2 Section 10 requires: manufacturer name or trademark, material grade (EN designation, e.g. X2CrNiMo17-12-2 or the EN number 1.4404), the standard number (EN 10253-2), fitting type and size, and cast or heat number. For Type B fittings, the wall schedule (or nominal wall) must also be marked — confirming the bore-matching requirement has been fulfilled for the stated schedule.
Goods Receipt Inspection Checklist
- Confirm manufacturer mark is legible and matches the certificate
- Confirm material designation on fitting matches material designation on EN 10204 3.1 certificate
- Confirm heat number on fitting matches heat number on certificate — cross-check at least 10% of fittings per lot
- Confirm size and schedule marking matches PO
- For alloy steel: confirm Class or grade suffix (Cl.1 vs Cl.3; WP9V vs WP9)
- Perform PMI spot-check on alloy and stainless fittings — minimum 5% or per project ITP
- Reject any fitting with illegible heat number — it cannot be traced to a certificate