Understanding Flange Face Finish
The surface finish of a flange face (also called flange serration or flange texture) is a critical factor in achieving leak-tight bolted joint integrity. The microscopic peaks and valleys on the flange face create a sealing path that works with the gasket to prevent fluid or gas leakage. Incorrect flange face finish is one of the leading causes of flange joint leaks in steel pipe systems.
ASME B16.5, ASME B16.47, and other flange standards specify surface finish requirements that must be met for proper gasket seating and sealing. Understanding these requirements is essential for flange manufacturers, inspectors, and maintenance engineers.
Flange Face Types and Their Finish Requirements
Raised Face (RF) Flanges
Raised face flanges are the most common type in process piping. The raised face provides a defined gasket seating area with specific finish requirements.
- Raised face height: 1/16″ (1.6 mm) for Class 150-800; 1/4″ (6.4 mm) for Class 900-2500
- Surface finish: 125-500 AARH (American Association of Refinery Hotels) roughness average
- Standard finish: 250-350 AARH (most common specification)
- Conversion: 250 AARH ≈ Ra 6.3 µm; 500 AARH ≈ Ra 12.5 µm
Flat Face (FF) Flanges
Flat face flanges have the entire face as the gasket seating surface. They are used with flat-face mating equipment (cast iron, ductile iron) or with full-face gaskets.
- Surface finish: Same as raised face: 125-500 AARH
- Standard finish: 250-350 AARH
- Special consideration: Flat face flanges must be perfectly flat; any warpage causes leaks
Ring Type Joint (RTJ) Flanges
RTJ flanges use a machined groove that accepts a metal ring gasket. The groove surfaces have different finish requirements than RF faces because the sealing mechanism is metal-to-metal contact.
- Groove bottom finish: Maximum 63 AARH (Ra 1.6 µm)
- Groove side walls: Maximum 125 AARH (Ra 3.2 µm)
- Sealing principle: Initial line contact between ring gasket and groove, progressing to full contact under bolt load
Serration Types
Spiral Serration (Phonographic Finish)
Spiral serration creates a continuous spiral groove pattern across the flange face, similar to a phonograph record. This is the preferred and most common finish type.
- Pattern: Continuous spiral from outside to inside diameter
- Groove depth: 0.002″ to 0.004″ (0.05-0.10 mm)
- Groove spacing: 0.032″ to 0.040″ (0.8-1.0 mm) between grooves
- Advantage: Provides uniform compression of the gasket, preventing radial leakage paths
- Gasket compatibility: Works with all gasket types (soft, semi-metallic, metal)
Concentric Serration (Stockade Pattern)
Concentric serration creates a series of concentric circular grooves on the flange face. This pattern is less common but specified for certain applications.
- Pattern: Concentric circular grooves from outside to inside diameter
- Groove dimensions: Same as spiral (0.002-0.004″ depth, 0.032-0.040″ spacing)
- Advantage: Creates multiple independent sealing barriers (barriers in series)
- Gasket compatibility: Best with soft gasket materials that can flow into grooves
- Limitation: Not compatible with spiral wound gaskets (grooves interrupt the winding)
Surface Roughness Measurement
AARH System
The AARH (Arithmetic Average Roughness Height) system is used in North America to specify flange face finish. It is equivalent to the Ra (Arithmetic Average) parameter in the ISO system, measured in micro-inches (µin).
Ra System (ISO)
The international system uses Ra (arithmetic average roughness) measured in micrometers (µm):
| AARH (µin) | Ra (µm) | Application |
|---|---|---|
| 125 | 3.2 | RTJ groove sides, smooth RF |
| 250 | 6.3 | Standard RF flange finish |
| 350 | 8.8 | Standard RF flange finish |
| 500 | 12.5 | Maximum allowed for RF |
| 63 | 1.6 | RTJ groove bottom |
Measurement Methods
- Profilometer (contact): Diamond stylus traces across the surface, measuring peak-to-valley height. Most accurate and standard method.
- Comparator blocks: Visual and tactile comparison against reference specimens with known roughness. Quick field method but subjective.
- Optical profilometer: Non-contact measurement using light interference patterns. Useful for delicate surfaces.
Gasket Compatibility by Flange Finish
Soft Gaskets (Non-Asbestos Fiber, PTFE, Elastomer)
- Compatible with both spiral and concentric serrations
- Best performance with 250-350 AARH (Ra 6.3-8.8 µm) finish
- Gasket material flows into serrations creating multiple sealing barriers
Semi-Metallic Gaskets (Spiral Wound)
- Compatible with spiral serration only (concentric serration can damage winding)
- Best performance with 250-350 AARH finish
- ASME B16.20 requires spiral serration for spiral wound gaskets
Metal Jacketed Gaskets
- Compatible with spiral serration
- Require 125-250 AARH finish for proper sealing
- Metal filler material must conform to serration profile
Ring Joint Gaskets (RTJ)
- Require machined grooves with specific finish (max 63 AARH for groove bottom)
- Sealing is metal-to-metal, not dependent on serration pattern
- Groove dimensions per ASME B16.5 Table 16
Quality Control and Inspection
In-Process Inspection
- Visual examination for tool marks, scratches, and gouges
- Roughness measurement using profilometer (minimum 3 readings per face)
- Serration type verification (spiral vs concentric)
- Flatness check for flat face flanges (dial indicator method)
Common Defects
- Excessive roughness (>500 AARH): Caused by worn tooling or incorrect feed rate
- Insufficient roughness (<125 AARH): Over-finished surface cannot grip gasket
- Circumferential scratches: Radial leak paths that bypass gasket sealing
- Tool marks in RTJ grooves: Prevent proper ring gasket seating
Field Repair of Flange Faces
When flange faces are damaged in service, field machining can restore the surface finish:
- Portable flange facers can machine raised face flanges in-place
- Remove minimum material to restore smooth, continuous serration pattern
- Verify finish with profilometer after machining
- Check raised face height remains within specification
- RTJ groove repair requires specialized grooving tools and is more complex
CoreMetal Steel Flange Products
CoreMetal Steel manufactures and supplies carbon steel, alloy steel, and stainless steel flanges to ASME B16.5, B16.47, EN 1092-1, and JIS B2220 standards. All flanges are machined to specified face finish requirements with full dimensional inspection and material certification. Our product range includes weld neck, slip-on, blind, socket weld, threaded, and lap joint flanges in all pressure classes.
Conclusion
Flange face finish is a critical parameter that directly affects the leak-tightness of bolted pipe connections. By understanding the relationship between surface finish, serration type, and gasket selection, engineers can design and maintain reliable flange joints that prevent leaks, reduce maintenance costs, and ensure safe operation.
Contact CoreMetal Steel for flange specifications, face finish requirements, and technical support for your piping project.