1. Introduction to Flange Face Types
The flange face — the sealing surface where two flanges meet — is arguably the most critical design feature of any flange connection. While engineers often focus on pressure ratings, materials, and dimensions, the face type ultimately determines whether a joint will seal reliably under process conditions.
ASME B16.5 defines five principal flange face types, each engineered for specific pressure classes, temperature ranges, and gasket compatibility. This guide covers all five in detail, with practical guidance for engineers, procurement professionals, and quality inspectors.
2. Raised Face (RF) — The Industry Standard
The Raised Face (RF) flange is the most widely used face type in industrial piping systems, accounting for an estimated 70–80% of all flange connections in oil & gas, petrochemical, and power generation applications.
2.1 Design and Dimensions
The RF flange features a raised circular seating surface around the bore. This raised area concentrates bolt load onto a smaller gasket area, increasing gasket contact stress and improving seal integrity. Per ASME B16.5:
| Pressure Class | Raised Face Height | Common Size |
|---|---|---|
| Class 150 | 1.6 mm (1/16 in) | NPS 1/2" – 24" |
| Class 300 | 1.6 mm (1/16 in) | NPS 1/2" – 24" |
| Class 400 | 6.4 mm (1/4 in) | NPS 1/2" – 24" |
| Class 600 | 6.4 mm (1/4 in) | NPS 1/2" – 24" |
| Class 900 | 6.4 mm (1/4 in) | NPS 1/2" – 24" |
| Class 1500 | 6.4 mm (1/4 in) | NPS 1/2" – 24" |
| Class 2500 | 6.4 mm (1/4 in) | NPS 1/2" – 12" |
2.2 Surface Finish Requirements
The raised face of RF flanges must be machined with a serrated finish. Per ASME B16.5, the standard facing specification requires:
AARH 125–250 μin
Arithmetic Average Roughness Height — the industry-standard range for RF flange serration. Also expressed as RMS 125–250 μin.
3.2–6.3 μm Ra
Metric equivalent of AARH 125–250. Most international specifications use Ra (Roughness Average) in micrometers.
30–55 Grooves/inch
The serration pattern — typically concentric or spiral (phonographic) — should have approximately 30 to 55 grooves per linear inch.
Spiral or Concentric
Both patterns are acceptable. Spiral (phonographic) is produced by lathe turning and is the most common in commercial production.
2.3 Gasket Compatibility
RF flanges work with virtually all non-metallic and semi-metallic gasket types: compressed fibre, PTFE, spiral-wound, kammprofile, and ring-type gaskets with centering rings. The serrated finish bites into soft gasket materials, preventing blowout and providing mechanical grip.
2.4 Common Applications
RF flanges are the default choice for:
- General process piping (water, steam, air, hydrocarbons)
- Utility systems and plant services
- Pressure classes from Class 150 through Class 600
- Temperatures from cryogenic to moderate-high (up to ~400°C / 750°F)
- Where standard gasket materials provide adequate chemical compatibility
3. Flat Face (FF) — For Brittle Materials
The Flat Face (FF) flange has a completely flat gasket seating surface across the entire face. Unlike RF flanges, there is no raised portion — the entire flange face makes contact with the full gasket area.
3.1 Why Flat Face Exists
FF flanges exist primarily for one engineering reason: preventing flange damage when mating to brittle materials. Materials like cast iron, bronze, and some FRP (fibre-reinforced plastic) components have low ductility and cannot withstand the concentrated bending stress that an RF flange would impose.
3.2 Dimensional Standards
FF flanges maintain the same overall dimensions (bolt circle, number of bolts, flange outer diameter) as their RF counterparts in the same class and size. The only difference is the absence of the raised face. Flat face flanges are most common in Class 125 and Class 250 (cast iron standards), but are also available in steel Class 150 and 300 for equipment interface requirements.
3.3 Gasket Requirements
FF flanges require full-face gaskets — gaskets that cover the entire flange face, including bolt holes. This distributes bolt load across the full flange area and eliminates bending stress. Typical materials include:
- Compressed non-asbestos fibre sheet (1.5 mm / 1/16 in thick)
- PTFE sheet (for chemical services)
- Reinforced graphite sheet (for elevated temperature)
3.4 Typical Applications
FF flanges are found in:
- Cast iron pump casings and valve bodies
- Bronze valve connections in marine and seawater systems
- FRP tank nozzles and piping
- Water treatment plant piping (ductile iron lines)
- Equipment interfaces that specify flat face by manufacturer requirement
4. Ring Type Joint (RTJ) — High-Pressure Metal Seal
The Ring Type Joint (RTJ) flange face type is engineered for the most demanding pressure and temperature conditions. It uses a precision-machined groove in the flange face that accepts a metallic ring gasket, creating a metal-to-metal seal.
4.1 How RTJ Seals Work
When bolts are tightened, the metallic ring gasket is compressed into the tapered groove. The softer ring material (typically softer than the flange material) deforms plastically, filling micro-irregularities in the groove surface and creating a leak-tight metal-to-metal seal. The seal actually improves as internal pressure increases, because pressure acting on the ring gasket inside the groove creates an energising effect.
4.2 RTJ Groove Geometry
The groove is machined to precise tolerances per ASME B16.5. The groove depth, width, and taper angle are standardised for each ring type (R, RX, BX designations):
| Ring Type | Standard | Typical Class Range | Groove Angle |
|---|---|---|---|
| Type R (Octagonal) | ASME B16.20 | Class 150–2500 | 23° |
| Type R (Oval) | ASME B16.20 | Class 150–2500 | 23° |
| Type RX | ASME B16.20 | Class 720–5000 | 23° (pressure-energised) |
| Type BX | API 6BX | Class 5000–20000 | 23° (face-to-face) |
4.3 Surface Finish Requirements
RTJ grooves demand a much smoother finish than RF faces:
RMS 63 μin Maximum
RTJ groove finish must not exceed 63 μin RMS (1.6 μm Ra). This fine finish ensures proper metal-to-metal sealing contact.
No Cross-Grain Marks
Groove surface finish must be free of radial tool marks, scratches, or imperfections that could create leak paths. Any defect makes the flange unusable.
4.4 When to Specify RTJ
RTJ is specified when any of the following apply:
- Pressures at Class 900 and above
- Temperatures above 427°C (800°F)
- Services containing hydrogen (H2), hydrogen sulphide (H2S), or other highly penetrating gases
- Thermal cycling service where gasket relaxation is a concern
- High-vacuum or high-integrity leak-tightness requirements
- Refinery hydrocracker, hydrotreater, and reformer circuits
5. Tongue & Groove (T&G) and Male & Female (M&F)
Tongue & Groove (T&G) and Male & Female (M&F) are specialised face types that provide self-centering alignment and positive gasket retention. Though less common than RF and RTJ in general service piping, these face types have important niche applications.
5.1 Tongue & Groove (T&G) Design
In a T&G pair, one flange has a raised tongue that fits into a matching groove machined into the mating flange. The gasket sits in the grooved flange and is fully contained — it cannot blow out or extrude. Key characteristics:
- Self-centering: The tongue-and-groove geometry automatically aligns the flanges during assembly
- Positive gasket retention: The gasket is mechanically retained within the groove, preventing extrusion even under severe pressure surges
- Controlled gasket compression: The metal-to-metal contact between tongue and groove faces limits gasket compression, preventing over-compression and damage
5.2 Male & Female (M&F) Design
M&F flanges are similar to T&G but with a simpler geometry. The male flange has a raised face that fits inside a recessed female flange face. Unlike T&G, the male face does not bottom out in the female recess, so gasket compression is controlled by bolt torque rather than metal-to-metal contact.
5.3 Application Comparison
| Feature | Tongue & Groove | Male & Female |
|---|---|---|
| Gasket Containment | Full — gasket in groove | Partial — recessed face |
| Self-Centering | Excellent — tongue guides alignment | Good — male fits into female |
| Gasket Compression Control | Metal-to-metal positive stop | Bolt torque dependent |
| Cost | Higher — precision machining | Moderate |
| Typical Application | High-integrity chemical seals, heat exchangers | Shell-and-tube exchangers, vessel manways |
5.4 Limitations
Both T&G and M&F face types have the same practical limitation: the flanges are not interchangeable. A male or tongue flange can only mate with its corresponding female or groove flange — unlike RF and RTJ where any two flanges of the same face type, class, and size are compatible. This increases spares complexity and cost.
6. Face Type Selection Guide
Selecting the correct flange face type depends on a combination of process conditions, material constraints, and economic considerations. Use the following decision framework:
| Condition | Recommended Face Type | Alternative |
|---|---|---|
| General service, Class 150–600, T < 400°C | Raised Face (RF) | Flat Face (if mating cast iron) |
| High pressure, Class 900+ | Ring Type Joint (RTJ) | RF with spiral-wound gasket (Class 900 only, with careful engineering review) |
| High temperature (>427°C) | Ring Type Joint (RTJ) | RF with kammprofile gasket (limited to ~500°C) |
| Hydrogen / H2S service | Ring Type Joint (RTJ) | T&G with PTFE-envelope gasket |
| Cast iron / bronze equipment | Flat Face (FF) | RF only with full-face gasket + engineering approval |
| Toxic / lethal service | RTJ or T&G | RF with spiral-wound gasket per ASME B31.3 |
| High vacuum | RTJ | T&G with metallic gasket |
| Thermal cycling | RTJ | T&G with spring-energised gasket |
7. Industry Standards Governing Flange Faces
Several international standards define the requirements for flange face dimensions, tolerances, and surface finish. Engineers and inspectors should be familiar with the following:
| Standard | Scope | Face Types Covered |
|---|---|---|
| ASME B16.5 | Pipe flanges NPS 1/2"–24" | RF, FF, RTJ, T&G, M&F |
| ASME B16.47 | Large-diameter flanges NPS 26"–60" | RF, RTJ |
| ASME B16.20 | Metallic gaskets for pipe flanges | RTJ ring dimensions, RF gasket dimensions |
| EN 1092-1 | European flange standard | Type A (FF), B1 (RF), and others |
| API 6A / 6BX | Wellhead and Christmas tree flanges | RTJ (Type BX), API flanges |
| MSS SP-44 | Steel pipeline flanges | RF, RTJ |
8. Quality Inspection of Flange Faces
At JIAJI FORGING, every flange face is inspected before shipment. The following checks are critical to ensure a reliable seal:
8.1 Surface Roughness Verification
RF face finish is verified using a profilometer or surface roughness comparator gauge. Measurements are taken at multiple points on the serrated face. For RTJ grooves, the smoother RMS 63 finish requires profilometer verification with a smaller stylus radius.
8.2 Visual Inspection
Inspectors check for: radial scratches or score marks (unacceptable on RTJ grooves), corrosion pitting on the face, incomplete serration coverage, mechanical damage from handling, and improper gasket seating surface.
8.3 Dimensional Inspection
Raised face height, groove depth (RTJ), groove angle, and concentricity are verified using calibrated gauges. RTJ groove dimensions are checked against the ring number chart in ASME B16.20.