Raised Face vs. Flat Face Flanges: Differences and Applications
When selecting a flange, one critical feature is the flange face type-especially Raised Face (RF) and Flat Face (FF). These face types affect sealing surface, gasket selection, and application suitability.
🔷 1. Raised Face (RF) Flanges
Description:
The gasket seating area is raised slightly (typically 1/16" or 1/8") above the bolt circle face.
Most common in ASME B16.5 flanges.
Purpose:
Concentrates more pressure on a smaller gasket area, improving the seal.
Materials:
Compatible with metallic or semi-metallic gaskets (spiral wound, RTJ, etc.)
Applications:
Process plants, oil & gas, chemical, petrochemical industries.
Suitable for high-pressure and high-temperature environments.
🔸 2. Flat Face (FF) Flanges
Description:
The entire face is flat and smooth, with no raised section.
Often used in cast iron systems or non-metallic piping.
Purpose:
Designed to mate with another flat surface, reducing bending stresses.
Materials:
Requires full-face gaskets made of soft material (rubber, PTFE).
Applications:
Low-pressure applications (e.g., waterworks, fire protection)
Common in AWWA and DIN/EN flange standards.
🔍 Key Differences at a Glance
| Feature | Raised Face (RF) | Flat Face (FF) |
|---|---|---|
| Gasket area | Raised above flange surface | Same plane as bolt face |
| Gasket type | Spiral wound, RTJ, metallic | Full-face rubber or soft gasket |
| Pressure rating | Medium to high | Low to medium |
| Applications | Industrial, high-temp/pressure | Waterworks, low-stress systems |
| Risk of deformation | Lower (if properly torqued) | Higher if mismatched with RF |
⚠️ Mating Note:
Flat Face flanges should always be bolted to other Flat Face flanges, not Raised Face ones. Mismatching may lead to gasket failure or flange damage.
