A raised face slip-on (RFSO) flange combines two common features. It is a slip-on flange, which slides over the pipe. It also has a raised face (RF), a small raised ring around the bore. This ring is the sealing surface for the gasket. This flange type is standard for many low to medium-pressure applications.
How It Works and Key Parts
An RFSO flange connects by sliding over the pipe’s end. It is fixed with two fillet welds: one inside the hub and one outside on the flange’s face. The raised face, typically 1/16″ high for Classes 150 and 300, or 1/4″ for higher classes, creates a concentrated sealing area. When bolted to another raised face flange, the gasket compresses into this space to form a tight seal.
Common Specifications and Dimensions
RFSO flanges follow standard dimensions for size and pressure rating. Key standards are ASME B16.5 (NPS 1/2″ to 24″) and ASME B16.47 (for larger diameters). The primary specifications are:
| Key Spec | Details for RFSO Flanges |
| Pressure Class | Common: 150, 300. Also available in 600, 900 for specific needs. |
| Raised Face Height | 1/16″ for Class 150 & 300. 1/4″ for Class 600 & above. |
| Facing Finish | Common: 125-250 µin Ra (serrated) for soft gaskets. Smooth finish for metal gaskets. |
| Material Grades | Carbon Steel (A105), Stainless (304/316), Alloy Steels, and other materials. |
| Standards | ASME B16.5 (most common), MSS SP-44, API 605, and custom specs. |
Here is a reference table for common ASME B16.5 RFSO flange dimensions:
| NPS | Class 150 O.D. | Class 150 Thickness | Class 300 O.D. | Class 300 Thickness |
| 2″ | 6.00 in | 0.44 in | 6.50 in | 0.56 in |
| 4″ | 9.00 in | 0.56 in | 10.00 in | 0.69 in |
| 6″ | 11.00 in | 0.62 in | 12.50 in | 0.81 in |
| 8″ | 13.50 in | 0.69 in | 15.00 in | 0.94 in |
| 10″ | 16.00 in | 0.75 in | 17.50 in | 1.06 in |
Applications: Where to Use RFSO Flanges
RFSO flanges are suitable for many systems where conditions are not extreme. Common uses include:
- Water and Wastewater Lines: Treatment plants, pumping stations.
- Building Services: HVAC, fire protection sprinkler systems.
- Industrial Utilities: Plant air, cooling water, low-pressure steam.
- Low-Pressure Process Lines: Non-hazardous chemical transfer.
- General Plant Piping: Where cost and ease of installation are priorities.
Choosing the Right Flange: RFSO vs. Other Types
It’s important to know when an RFSO flange is the correct choice.
| Flange Type | Best Used For | Compared to RFSO |
| Raised Face Slip-On (RFSO) | Low/medium pressure, cost-sensitive projects, easier alignment. | Baseline for comparison. |
| Weld Neck Flange | High pressure, high temperature, severe cycles, critical systems. | Stronger butt weld, better for stress. Higher cost and skill to install. |
| Flat Face Slip-On | Connecting to flat face cast iron equipment or valves. | Prevents bending brittle iron. Not for standard raised face connections. |
| Lap Joint Flange | Systems needing frequent disassembly or with expensive pipe alloys. | Allows pipe stub end rotation. The flange itself is not welded to the pipe. |
Choose an RFSO flange when: Pressure and temperature are within its rating. The system has minimal vibration or thermal cycles. Initial cost and installation speed are important factors.
Installation and Welding Notes
Proper installation is key for a leak-free connection.
- Slide and Align: Slide the flange onto the pipe. Align bolt holes with the mating flange.
- Tack Weld: Lightly tack weld in place to maintain alignment.
- Complete Welds: Perform two fillet welds. Weld the hub interior first, then the hub exterior to the flange face.
- Gasket and Bolt: Insert the correct gasket (e.g., non-asbestos sheet for low pressure). Install and tighten bolts in a crisscross pattern to an even torque.
Key Tip: Avoid excessive welding heat. It can warp the flange face and ruin the sealing surface.
Frequently Asked Questions
Q: What gasket type should I use with an RFSO flange?
A: For Class 150/300 service, soft cut gaskets (non-asbestos sheet, rubber) are common. For higher pressure or temperature, use semi-metallic or spiral-wound gaskets. The gasket ID should match the raised face’s inner diameter.
Q: Can I use a Class 150 RFSO flange for a 300 PSI system?
A: Not necessarily. The “Class” is not a direct PSI rating. It is a pressure-temperature rating. You must check the ASME B16.5 table for your material and operating temperature to find the maximum allowed working pressure.
Q: Are RFSO flanges available in stainless steel?
A: Yes. They are common in materials like ASTM A182 F304/F316 for corrosive service. Texas Flange supplies them in stainless, carbon, and alloy steels.
Q: What’s the main drawback of a slip-on flange?
A: The double fillet weld is inherently weaker in fatigue and stress than a weld neck’s butt weld. It is not ideal for systems with frequent pressure swings or high vibration.
Conclusion
The raised face slip-on flange is a practical, economical choice for standard industrial piping. It provides a reliable seal for a wide range of non-critical services. Success depends on selecting the correct pressure class, material, and ensuring proper installation.
Need to source reliable raised face slip-on flanges? We supply standard and custom RFSO flanges in all pressure classes and materials, from carbon steel to specialty alloys. Get a precise quote based on your specifications.
Submit your RFSO flange requirements for pricing and availability: http://texasflange.com/lp12/