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In the pipe fitting and flange industry, raised face (RF) and flat face (FF) flanges are two common types of flange facings. The key difference lies in the gasket seating surface: a raised face extends above the bolting circle, while a flat face is level with it. This post details the distinctions and applications of each type.
The raised face (RF) flange is the most prevalent type of flange facing. Its defining characteristic is the raised surface, or "raised face," located above the bolting circle where the gasket sits. This raised portion serves a crucial purpose: it concentrates the pressure from the bolts onto the smaller gasket area. This concentrated pressure creates a tighter, more reliable seal, making RF flanges suitable for a wide range of applications, including high-pressure and high-temperature environments. They are a staple in oil and gas pipelines.
The dimensions of the raised face, including its diameter and height, are standardized by ASME B16.5 (for smaller flanges) and ASME B16.47 (for larger flanges) and are determined by the pressure class and nominal pipe size. Higher pressure ratings necessitate larger flange diameters, more bolts, increased flange thickness, and a more pronounced raised face. The standard surface finish for ASME B16.5 and ASME B16.47 raised face flanges is 125-250 RMS (Roughness Mean Square).
Flat face (FF) flanges, as the name suggests, have a flat surface for the gasket. The gasket surface is in the same plane as the bolting circle face. This simpler design has a different purpose than the raised face.
The primary intention of a flat face flange is to minimize or prevent bending stress on the flanges as the bolts are tightened. This makes them particularly suitable for applications where the mating flange or fitting is cast iron or another relatively brittle material. Flat face flanges are typically found in low-pressure and low-temperature environments, such as in water treatment facilities. Like raised face flanges, the standard surface finish for ASME B16.47 flat face flanges is also 125-250 RMS.
| Feature | Raised Face (RF) Flange | Flat Face (FF) Flange |
|---|---|---|
| Gasket Surface | Raised above bolting circle | Level with bolting circle |
| Pressure Rating | High to Low | Primarily Low |
| Temperature Rating | High to Low | Primarily Low |
| Applications | Versatile, Oil & Gas, etc. | Cast Iron Fittings, Water Treatment |
| Sealing | Enhanced, Concentrated Pressure | Less Concentrated Pressure |
| Bending Stress | More Susceptible | Less Susceptible |
Selecting the correct flange facing depends on the specific application requirements. Consider factors like pressure, temperature, mating flange material, and the potential for bending stress. For most applications, the raised face flange is the preferred choice due to its superior sealing capabilities. Flat face flanges are best suited for specific low-pressure, low-temperature scenarios, especially when mating with cast components. Consulting relevant industry standards and engineering specifications is crucial for proper flange selection.
Need assistance choosing the right Flange for your system? Shribhairav Metal Corporation provides expert support and customized quotes.
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