Flange Face Finish for Different Gaskets
The flange face finish for different gaskets refers to the specific surface roughness (AARH) that must be present on the flange sealing face to ensure proper gasket seating and a leak-free connection. Each gasket type (spiral wound, RTJ, CNAF, PTFE, or graphite) requires a different finish range because their sealing mechanisms differ fundamentally. Specifying the wrong flange face finish is one of the most common causes of flange leakage in process piping.
The gasket material, construction type, and sealing mechanism each dictate a specific roughness range. Soft gaskets tolerate rougher finishes, while metallic gaskets demand precision-smooth surfaces.
Flange Face Finish by Gasket Type
| Gasket Type | Recommended AARH | Ra (um) | Flange Face | Sealing Mechanism |
|---|---|---|---|---|
| Spiral wound (graphite filler) | 125-250 | 3.2-6.3 | RF | Filler conforms to serrations |
| Spiral wound (PTFE filler) | 125-250 | 3.2-6.3 | RF | Filler conforms to serrations |
| CNAF (compressed fiber) | 125-250 | 3.2-6.3 | RF/FF | Soft material fills grooves |
| Flexible graphite (Grafoil) | 63-125 | 1.6-3.2 | RF | Thin sheet requires smoother face |
| PTFE envelope | 63-125 | 1.6-3.2 | RF/FF | PTFE cold flows; needs smoother face |
| Solid PTFE | 63-125 | 1.6-3.2 | RF/FF | Cold flow risk on rough surfaces |
| Elastomeric (rubber) | 250-500 | 6.3-12.5 | FF | Soft rubber fills deep grooves |
| RTJ (oval/octagonal ring) | 63 max | 1.6 max | RTJ groove | Metal-to-metal contact in groove |
| Lens ring | 32-63 | 0.8-1.6 | Lens groove | Precision line contact seal |
| Kammprofile (grooved metal) | 63-125 | 1.6-3.2 | RF | Soft facing seals against flange |
| Double-jacketed | 125-250 | 3.2-6.3 | RF | Metallic jacket with filler |
Why Different Gaskets Need Different Finishes
The sealing mechanism determines the required finish:
Soft gaskets (CNAF, rubber, PTFE) deform plastically under bolt load and fill the surface irregularities. They tolerate (and sometimes benefit from) rougher finishes because the grooves help anchor the gasket and resist blowout. However, very thin soft gaskets like flexible graphite sheets cannot fill deep grooves, so they require smoother faces.
Semi-metallic gaskets (spiral wound, kammprofile) combine a metallic structure with soft filler. The filler material (graphite or PTFE) conforms to the serrations while the metal component provides resilience. The standard 125-250 AARH finish works well for this sealing mechanism.
Metallic gaskets (RTJ, lens ring) seal through metal-to-metal deformation. The softer gasket metal (typically a lower-hardness alloy than the flange) plastically deforms against the precision-machined groove. Any scratch or roughness on the groove surface creates a direct leak path that the hard gasket cannot fill.
Common Mismatches to Avoid
| Mismatch | Problem | Solution |
|---|---|---|
| RTJ gasket on 250 AARH groove | Metal cannot fill rough grooves | Re-machine groove to 63 AARH max |
| Flexible graphite on 250 AARH face | Thin sheet tears on rough peaks | Use smoother face or thicker gasket |
| Spiral wound on polished face (<63 AARH) | Filler cannot grip; gasket may blow out | Re-machine to 125-250 AARH serrated |
| Rubber gasket on RF flange | Rubber extruded by pressure | Use FF flange or confined gasket |
For a complete reference on flange face types and their finish specifications, see the detailed guide covering all ASME flange face configurations.
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