The flange face surface finish has an influence on a safety and tight flange connection. For standard requirements, the design of the flange surface determines the required roughness.
This is taken into account in the standards for flanges (see the table below). However, the standards allow special agreements to be made between the purchaser and the manufacturer of the flanges for the surface finish.
General information about flange face surface finish
- Smooth surfaces are not recommended and are only suitable for low pressures.
- Machine turned serrated spiral, or even better serrated concentric surface is recommended (except for ring joint surface finish).
- The recommended roughnesses should be visual verified using surface comparators and not by instruments having stylus tracers and electronic amplification.
- Rougher surfaces can increase gasket blowout.
- The closer the spacing of concentrically serrated surfaces, the greater the contact area and therefore the higher the bolt load required.
- The roughness of the flange faces can have a dramatic effect on gasket creep relaxation.
- Recommendations from gasket manufacturers and/or flange connection designer can be helpful.
Table: The flange face surface finish contains in different specification
| Flange facing types | EN 1092-1 (PN) Sect 5.7.2 EN 1759-1 (Class) | ASME B16.5 Sect. 6.4.5 | ASME B16.47 Sect. 6.1.4 |
|---|---|---|---|
| Flat face A max. PN 40 | 3,2 ≤ Ra ≤ 12,5 [µm] 12,5 ≤ Rz ≤ 50 [µm] serrated groves | – | – |
Race face B1 (standard) | 3,2 ≤ Ra ≤ 12,5 [µm] 12,5 ≤ Rz ≤ 50 [µm] serrated groves | – | – |
| Race face B2 (if agreed) | 0,8 ≤ Ra ≤ 3,2 [µm] 3,2 ≤ Rz ≤ 12,5 [µm] serrated groves | – | – |
| Flat face (FF) Race face (RF) | – | 3,2 ≤ Ra ≤ 6,3 [µm] serrated groves 2) | 3,2 ≤ Ra ≤ 6,3 [µm] 125 ≤ Ra ≤ 250 [µin.] serrated groves 2) |
| Face Type C Tongue Type D Groove | 0,8 ≤ Ra ≤ 3,2 [µm] 3,2 ≤ Rz ≤ 12,5 [µm] serrated groves | Ra ≤ 3,2 [µm] Ra ≤ 125 [µin.] | – |
| Face Type E Small Male | 3,2 ≤ Ra ≤ 12,5 [µm] 12,5 ≤ Rz ≤ 50 [µm] serrated groves1) | Ra ≤ 3,2 [µm] Ra ≤ 125 [µin.] | – |
| Face Type F Female | 3,2 ≤ Ra ≤ 12,5 [µm] 12,5 ≤ Rz ≤ 50 [µm] serrated groves1) | Ra ≤ 3,2 [µm] Ra ≤ 125 [µin.] | – |
| Ring Joint (Type R and RX) | – | Ra ≤ 1,6 [µm] Ra ≤ 63 [µin.] | Ra ≤ 1,6 [µm] Ra ≤ 63 [µin.] |
| Ring Joint (Type BX) | – | Ra ≤ 0,8 [µm] Ra ≤ 32 [µin.] | Ra ≤ 0,8 [µm] Ra ≤ 32 [µin.] |
1) The cutting tool employed should have an approximate 1,0 mm (0,4 in.) or larger radius;
2) The cutting tool employed should have an approximate 1.5 mm (0.06 in.) or larger radius, and there should be from 1,8 grooves/mm through 2,2 grooves/mm (45 grooves/in. through 55 grooves/in.);
Effect of roughness
When working with a smoother finish, it is important to consider using a thinner gasket to lessen the effects of creep and cold flow. It should be noted, however, that both a thinner gasket and the smooth finish, in and of themselves, require a higher compressive force (i.e. bolt torque) to achieve the seal.
Therefore, due to the flange design, one may have to resort to a thicker gasket, which requires a lower compressive force to seal the gasket.
Recommendations flange face surface finish for different gaskets
It is not possible to make a generally valid statement about the required surface finishes for certain types of gasket. Each of these gasket types require specific ranges of surface finish for optimum gasket performance with lowest leakage. It is always best to consult with the gasket manufacturer to determine the best combination of flange surface finish, gasket design and gasket material(s). A very general rule is listed in the table below for steel flanges:
| Gasket class | Gasket Type | Standard | Typical Roughness 3) |
|---|---|---|---|
| Nonmetallic soft gaskets | Flat gaskets e. g. Rubber, PTFE | EN 1514-1 (PN) EN 12560-1 (Class) ASME B16.21 | 3,2 ≤ Ra ≤ 6,3 µm Gasket thickn. ≤ 1,5 mm or 3,2 ≤ Ra ≤ 12,5 µm Gasket thickn. > 1,5 mm |
| Nonmetallic gaskets | non-metallic reinforced materials e.g. Carbon Fiber/NBR, Aramide Fiber/NBR, PTFE | EN 1514-3 (PN) EN 12560-3 (Class) ASME B16.21 | 3,2 ≤ Ra ≤ 6,3 µm Gasket thickn. ≤ 1,5 mm or 3,2 ≤ Ra ≤ 12,5 µm Gasket thickn. > 1,5 mm |
| Semi-Metallic gaskets | Flat metal jacketed gasket | EN 1514-7 (PN) EN 12560-7 (Class) ASME B16.20 | Ra ≤ 2,5 µm |
| Semi-Metallic gaskets | Spiral wound gaskets e.g. PTFE, Graphite, | EN 1514-2 (PN) EN 12560-2 (Class) ASME B16.20 | Ra ≤ 2,0 µm (Vacuum) Ra ≤ 3,2 µm (Gas) Ra ≤ 6,3 µm (General) |
| Semi-Metallic gaskets | Covered serrated metal gaskets with diff. layer materials (Kammprofile) | EN 1514-6 (PN) EN 12560-6 (Class) | 3,2 ≤ Ra ≤ 6,3 µm |
| Metallic gaskets | Corrugated metal Grooved metal | EN 1514-4 (PN) EN 12560-4 (Class) | 3,2 ≤ Ra ≤ 6,3 µm |
| Metallic gaskets | Flat solid gasket | DIN 7603 | Ra ≤ 1,6 µm |
| Metallic gaskets | Ring Joint gasket Type R (Oval), Type R (Octogonal), BX, RX | EN 1514-5 (PN) EN 12560-5 (Class) ASME B16.20 | Ra ≤ 1,6 µm |
3) Ra (AARH) is the measure arithmetic average roughness height of a surface.
