Understanding Raised Face Slip-On Flange Design
Raised face slip-on (RFSO) flanges represent a fundamental design in industrial piping systems. These flanges feature a low hub that allows the component to slide directly over the pipe, with two fillet welds securing it in place. The raised face—a machined ridge on the flange surface—creates a concentrated contact area for gasket sealing, significantly improving joint integrity and reducing leakage potential.
ASME B16.5 Specifications
RFSO flanges comply with ASME B16.5 standards for pipe flanges and flanged fittings. The following table presents key dimensional specifications for common nominal pipe sizes:
| NPS | OD (in) | Thickness (in) | Bolt Circle (in) | Bolt Count |
|---|---|---|---|---|
| 1/2 | 3.88 | 0.5 | 2.75 | 4 |
| 2 | 6.0 | 0.69 | 4.75 | 8 |
| 4 | 9.0 | 0.81 | 7.5 | 8 |
| 6 | 11.0 | 0.92 | 9.5 | 12 |
| 8 | 13.5 | 1.06 | 11.75 | 12 |
| 12 | 18.0 | 1.31 | 16.0 | 16 |
Primary Applications
RFSO flanges excel in low-pressure applications including plant water systems, cooling loops, HVAC distribution networks, and fire protection piping. Their ease of installation and cost-effectiveness make them the standard choice in these service conditions where cyclic stresses and extreme pressures are not limiting factors.
Advantages and Limitations
The primary benefits of RFSO flanges include straightforward field installation, minimal machining cost, and reliable sealing performance in moderate-pressure environments. However, their low hub design makes them unsuitable for high-pressure applications, cyclic loading scenarios, or hazardous fluid service where safety margins require more robust flange geometry.
Material Options
Manufacturers provide RFSO flanges in multiple materials to match service requirements. Carbon steel ASTM A105 serves general applications. Stainless steel 304 and 316 grades address corrosive environments. Alloy steels including A350 LF2 extend service into cryogenic applications. Material selection ensures compatibility with the process fluid and operating temperature range.
Frequently Asked Questions
Q: Can RFSO flanges be reused after disassembly?
A: Yes, provided the flange shows no cracks, excessive corrosion, or damage to the raised face. Inspect the welding surfaces and replace gaskets before reinstallation.
Q: What gasket material pairs with RFSO flanges?
A: Compressed fiber and aramid-based gaskets perform well with the raised face geometry. Full-face gaskets work but waste material; ring gaskets are more economical.
Q: How many fillet welds are required per RFSO flange?
A: Two fillet welds, one on each side of the flange hub, satisfy code requirements. Both welds must pass visual and radiographic inspection per ASME B31.1.
Q: Are RFSO flanges suitable for 300 PSI steam service?
A: No. RFSO design limits pressure ratings to approximately 150 PSI maximum. For higher-pressure steam, specify weld neck or threaded flanges with thicker cross-sections.
Q: What bolt specification is standard for RFSO flanges?
A: ASTM A307 carbon steel bolts suit general service. Stainless steel bolts match stainless flanges to prevent galvanic corrosion in corrosive or marine environments.