Pipe Fitting Colour Coding and Paint Marking: Industry Practice and Its Limitations
Colour coding of pipe fittings is an informal but widely used site practice for rapid grade identification during construction. Colour bands painted on fitting ends identify the material grade at a glance, reducing the risk of wrong-grade installation during multi-grade projects. However, colour codes are not standardised by ASME B16.9 or EN 10253 — they are manufacturer conventions and project-specific systems. A colour code that means one grade on one project may mean something entirely different on another. Understanding both the utility and the critical limitations of colour coding prevents over-reliance on an unreliable identification method.
Common Industry Colour Conventions (Not Standardised)
| Grade | Common Colour Convention | Notes |
|---|---|---|
| WPB (Carbon Steel) | Black or unpainted | No colour paint = carbon steel in most conventions |
| WP304L | Green | Some manufacturers use light blue — never assume |
| WP316L | Blue or white | Widely used but not universal |
| WP321 | Sky blue or turquoise | Varies significantly by manufacturer |
| WP11 (P11) | Yellow + green stripe | Alloy steel colours are more consistently applied in UK/Indian practice |
| WP22 (P22) | Yellow + blue stripe | — |
| WP91 (P91) | Purple or violet | Most consistently applied across manufacturers — purple for P91 is near-universal |
| WP5 (P5) | Red | — |
| Duplex 2205 | Orange or red-brown | Not consistent across suppliers |
| Super Duplex 2507 | Orange + white stripe or brown | — |
Critical Limitations of Colour Coding
- Not standardised: There is no ASME, EN, or ISO standard for pipe fitting colour codes. The conventions above are common practice, not code requirements.
- Paint fades, chips, and is overpainted: Colour bands applied in the factory are frequently overpainted during site blasting and coating operations. By the time the fitting is ready to install, the colour code may be gone.
- Same colour, different grade: "Blue" means 316L on one project's material management system and 304L on another. Without the project-specific colour code register, the colour is meaningless.
- Colour codes are not basis for acceptance: No inspection standard permits colour code alone as grade verification. PMI (XRF) or heat number verification against the 3.1 certificate is always required for grade confirmation.
Correct Use of Colour Coding
Colour codes are a rapid visual screening tool — they flag that something may need investigation, or provide fast sorting confidence during installation when combined with other verification. They should never be the sole basis for grade acceptance. The correct workflow: (1) confirm grade by heat number against 3.1 certificate at goods receipt; (2) apply project-specific colour band in addition to (not instead of) the stamped heat number; (3) use colour band for rapid visual sorting during construction; (4) verify final installed grade by PMI spot-check before system handover. PMI is the only field-portable grade verification method that provides chemical analysis without destructive testing.