Flange Surface Finish Requirements
Flange surface finish requirements are defined by ASME B16.5, ASME B16.47, and MSS SP-6 to ensure reliable gasket sealing in flanged piping joints. The standard specifies both the roughness value (measured in AARH) and the groove pattern (concentric serrated or phonographic spiral) for each flange face type. Meeting these requirements is required for leak-free operation.
The surface finish of a flange face directly affects gasket performance. Non-conforming surfaces are one of the leading causes of flange leakage in process piping.
Surface Finish Requirements by Standard
| Standard | Flange Face | AARH Range | Ra (um) | Groove Pattern |
|---|---|---|---|---|
| ASME B16.5 | Raised face (RF) | 125-250 | 3.2-6.3 | Concentric serrated |
| ASME B16.5 | Flat face (FF) | 125-250 | 3.2-6.3 | Concentric serrated |
| ASME B16.5 | RTJ groove | 63 max | 1.6 max | Smooth machined |
| ASME B16.5 | Tongue & groove | 63 max | 1.6 max | Smooth machined |
| ASME B16.47 | Series A (RF) | 125-250 | 3.2-6.3 | Concentric serrated |
| ASME B16.47 | Series B (RF) | 125-250 | 3.2-6.3 | Concentric serrated |
| MSS SP-6 | All standard faces | 125-500 | 3.2-12.5 | Per applicable std. |
| API 6A | RTJ (wellhead) | 63 max | 1.6 max | Smooth machined |
Serration Requirements
ASME B16.5 specifies that the serrated finish on RF and FF flanges must consist of concentric circular grooves (not spiral). The groove characteristics are:
| Parameter | ASME B16.5 Requirement |
|---|---|
| Groove shape | Concentric circles |
| Approximate pitch | 45 grooves per inch (1.8 grooves/mm) |
| Groove depth | 1/64 in. (0.4 mm) nominal |
| Tool radius | 1/64 in. (0.4 mm) |
| Surface roughness | 125-250 AARH across grooves |
A phonographic (spiral) groove is also acceptable in practice and is easier to machine on a lathe. Both patterns create the labyrinth effect necessary for gasket sealing.
RTJ Face Requirements
Ring type joint (RTJ) flange faces require a much finer finish than RF or FF flanges. The groove walls and bottom must be machined to 63 AARH (1.6 um Ra) or finer. Any radial scratch or tool mark in the groove can create a direct leak path past the metallic ring joint gasket.
RTJ grooves must also comply with the dimensional tolerances in ASME B16.5 Table E-1 for groove width, depth, and groove-to-bore concentricity.
Common Defects That Fail Inspection
| Defect | Effect | Action |
|---|---|---|
| Radial scratch | Direct leak path across serrations | Re-machine or reject |
| Corrosion pitting | Gasket cannot seal over pit | Re-machine if within minimum thickness |
| Embedded foreign material | Prevents gasket seating | Clean or re-machine |
| Over-machined (below 125 AARH) | Gasket slippage, insufficient grip | Re-machine with correct tool |
| Under-machined (above 250 AARH) | Gasket cannot fill deep valleys | Re-machine to specification |
| Warpage | Uneven bolt load distribution | Re-face or replace flange |
Field Re-Machining
When a flange face is damaged in the field, portable flange facing machines can re-machine the surface to the required 125-250 AARH serrated finish. After re-machining, verify that the remaining raised face height still meets the minimum per ASME B16.5 (1/16 in. for Class 150-600, 1/4 in. for Class 900-2500).
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