The connection between two flanges is only as good as the surface that seals them. The two most common sealing surfaces are the Raised Face (RF) and the Flat Face (FF). Choosing the correct one is essential for a leak-proof and safe piping system.
This guide explains the differences to help you make the right choice.
Quick Comparison: RF vs. FF Flanges
This table summarizes the key differences between the two flange face types.
| Feature | Raised Face (RF) Flange | Flat Face (FF) Flange |
| Physical Design | Has a raised ring around the bore. | The sealing surface is completely flat and level. |
| Primary Purpose | Concentrates bolt pressure on a smaller gasket area for a tighter seal. | Distributes bolt pressure evenly to protect brittle materials from cracking. |
| Pressure & Temperature | Suitable for medium to very high pressure and temperature systems. | Used for low-pressure, low-temperature applications (commonly under 300 psi). |
| Common Applications | Oil & gas, petrochemical plants, power generation, high-pressure process lines. | Water treatment, HVAC, fire protection, plumbing, systems with cast iron or plastic components. |
| Gasket Compatibility | Works with ring gaskets (inside the bolts), including spiral-wound and other semi-metallic types. | Requires a full-face gasket that covers the entire surface and has holes for the bolts. |
| Material Suitability | Carbon steel, stainless steel, alloy steels. | Cast iron, ductile iron, plastics (PVC, HDPE), and other brittle materials. |
A Closer Look at Raised Face (RF) Flanges
The RF flange is the most common type in industrial plants. The raised surface is typically 1.6 mm (1/16 inch) high for Class 150 and 300 flanges, and 6.4 mm (1/4 inch) for higher pressure classes.
By focusing the clamping force on a smaller area, an RF flange creates a much higher sealing stress on the gasket. This makes the joint stronger and more reliable for containing high pressures. RF flanges are compatible with a wide range of gasket types, from simple non-asbestos sheets to robust spiral-wound gaskets for demanding services.
Common uses: Refineries, chemical processing, high-pressure steam lines, and any application where preventing leaks is critical.
A Closer Look at Flat Face (FF) Flanges
FF flanges have a sealing surface that is completely flush with the bolt circle. The main advantage is the large, even contact area. When bolts are tightened, the force is spread out across the entire flange face. This prevents high stress points that could bend or crack brittle materials like cast iron valves or plastic pipes.
Because the sealing pressure is lower, FF flanges are paired almost exclusively with soft, full-face gaskets made of rubber, fiber, or PTFE. These gaskets must match the flange’s outside diameter and have holes for every bolt.
Common uses: Municipal water systems, building HVAC, fire sprinkler lines, pump suctions, and connections to cast iron equipment.
Critical Rule: Never Mix RF and FF Flanges
You should never directly bolt a Raised Face flange to a Flat Face flange.
If you try, the raised face will create a gap. This puts all the bending stress on the center of the flat face flange when the bolts are tightened. For a brittle material like cast iron, this will likely cause it to crack.
If you must connect a carbon steel line (with RF flanges) to cast iron equipment (with FF flanges), the correct method is to use a full-face gasket and machine off the raised face from the steel flange. This creates two flat, matching surfaces.
Frequently Asked Questions
Q: Can I use a ring gasket on a flat face flange?
A: It is not recommended. A ring gasket only contacts part of the flat surface, leaving an unsealed area between the gasket’s outer edge and the bolt holes. This can lead to leaks.
Q: How do I identify the pressure class of a flange?
A: The class (150, 300, etc.) is often marked on the flange’s outer rim. You can also identify it by measuring the flange’s outside diameter, thickness, and bolt hole pattern, then comparing these to standard dimension tables.
Q: Are there other types of flange faces besides RF and FF?
A: Yes. Other common types include Ring-Type Joint (RTJ) for extremely high pressure, and Tongue-and-Groove (T&G) for precise alignment. Like RF and FF, these are not interchangeable with each other.
Q: What’s the most important factor when choosing between RF and FF?
A: Start with the material of the equipment you are connecting to. If it’s cast iron, plastic, or another brittle material, you likely need an FF flange. For all-carbon steel or alloy systems, the decision is based on your system’s pressure and temperature rating.
Conclusion
Choosing between an RF and an FF flange is a fundamental decision in piping design. RF flanges provide a high-integrity seal for demanding industrial services. FF flanges offer protection and reliable sealing for low-pressure systems, especially those with brittle components. Remember the cardinal rule: these two face types are not designed to be connected to each other.
Need help selecting the correct flange type for your specific pressure, temperature, and material requirements? Our technical team can guide you to the right choice to ensure a safe, leak-free connection. Submit your project details for expert support and a quote: http://texasflange.com/lp12/