RF Flange Guide: Raised Face Flange Selection, Gasket Sealing, and RF vs FF vs RTJ Comparison
In industrial piping, the RF flange is often the default sealing-face choice because it offers a practical balance of sealing performance, availability, machining cost, and gasket compatibility. RF means raised face: a narrow circular sealing land is machined above the surrounding flange face so gasket compression is concentrated around the bore instead of across the full flange diameter.
For buyers, engineers, and maintenance teams, understanding the RF flange is useful for much more than naming a flange type. It affects gasket selection, bolt load, face finish, pressure class, leakage risk, inspection requirements, and whether a flange can be paired with FF or RTJ designs. This guide explains how raised face flanges work, where they are normally used, how they compare with FF and RTJ flanges, and what information to include when requesting a quotation from a flange manufacturer.
What Is an RF Flange?
An RF flange, or raised face flange, is a pipe flange with a raised gasket seating surface. The raised area is usually machined with a controlled serrated finish so the gasket can grip the surface and deform enough to create a seal. RF facing is widely used on weld neck flanges, slip on flanges, blind flanges, socket weld flanges, threaded flanges, lap joint backing flanges, and many large diameter flanges.
The raised face does not mean the flange is automatically high pressure by itself. Pressure capability still depends on the standard, class, material, temperature, gasket, bolts, and installation. However, RF facing is common across ASME pressure classes because it provides better gasket seating than a full flat face in many process-service conditions.
Why Raised Face Flanges Are Common in Piping Systems
Raised face flanges are popular because they are versatile. They can be supplied in carbon steel, stainless steel, alloy steel, duplex stainless, and nickel alloy materials. They are available under common standards such as ASME B16.5 for NPS 1/2 through NPS 24 and ASME B16.47 for larger sizes. For many buyers sourcing from a flange factory in China, RF facing is the standard choice unless the drawing or purchase specification calls for FF, RTJ, tongue and groove, or another special face.
The main advantage is gasket compression. Because the gasket seats only on the raised annular surface, the bolt load is concentrated in a smaller area. This helps create a stronger seal with suitable soft, semi-metallic, or metallic-reinforced gaskets. In refineries, chemical plants, water treatment plants, power generation, shipbuilding, and general industrial piping, this makes RF flanges a reliable and economical option.
RF vs FF vs RTJ Flanges: Key Differences
The most common confusion in flange procurement is the difference between RF, FF, and RTJ faces. They are not interchangeable by name only. Each face type has different sealing behavior, gasket requirements, and installation risks.
| Face Type | Sealing Design | Typical Gasket | Common Use | Important Notes |
|---|---|---|---|---|
| RF flange | Raised annular sealing surface around the bore | Spiral wound, PTFE, graphite, compressed fiber, camprofile | General industrial piping, oil and gas, chemical, steam, water, process lines | Most common choice for ASME steel flanges; requires correct gasket OD/ID and face finish |
| FF flange | Entire flange face is flat | Full-face non-metallic gasket | Cast iron equipment, low-pressure water service, some pump and valve connections | Often used to reduce bending stress on brittle equipment flanges |
| RTJ flange | Precision-machined ring groove | Metal ring gasket | High pressure, high temperature, critical oil and gas service | Excellent sealing under severe conditions but higher machining and gasket cost |
In simple terms, RF is the flexible industrial standard, FF is often selected when equipment compatibility or low-pressure service requires a full-face gasket, and RTJ is chosen for severe pressure or temperature where a metal ring gasket is needed.
How an RF Flange Seals
An RF flange joint seals when the bolts create enough gasket stress between two raised faces. The gasket fills small surface irregularities and resists internal pressure. A good seal depends on five linked factors: flange face finish, gasket material, bolt grade, tightening method, and operating temperature.
Many RF flanges are supplied with a serrated concentric or spiral finish. This finish helps grip the gasket and reduce leakage paths. A face that is too smooth can allow some gasket materials to creep, while a face that is too rough can cut or damage softer gaskets. That is why drawings often specify the roughness range, especially for chemical, steam, or hydrocarbon service.
RF Flange Gasket Selection Guide
| Service Condition | Common Gasket Option | Why It Is Used | Buyer Reminder |
|---|---|---|---|
| General water or air service | Compressed fiber, rubber, PTFE | Economical and easy to install | Confirm temperature and chemical compatibility |
| Steam or hot process lines | Graphite-filled spiral wound gasket | Handles heat and compression cycling better than many soft gaskets | Specify inner and outer rings when required |
| Chemical or corrosive fluids | PTFE envelope, modified PTFE, graphite, or suitable semi-metallic gasket | Resists chemical attack while maintaining seal stress | Check media concentration, temperature, and pressure |
| High-pressure RF flange joints | Spiral wound or camprofile gasket | Improves recovery and blowout resistance compared with many soft gaskets | Confirm flange class, bolt material, and torque procedure |
| Large diameter flanges | Segmented or custom gasket depending on size | Supports installation and transport for large OD joints | Control storage, flatness, and installation alignment |
For RF flange procurement, the gasket should never be treated as an afterthought. A high-quality stainless steel flange can still leak if the gasket is underspecified, damaged, installed off-center, or paired with the wrong surface finish.
Where RF Flanges Are Used
RF flanges appear in a wide range of products and systems. A weld neck RF flange is common where strength, alignment, and fatigue resistance matter. A slip on RF flange is often used for moderate-pressure piping where installation economy is important. A blind RF flange closes the end of a line, vessel nozzle, or inspection opening. Socket weld and threaded RF flanges may be selected for smaller bore piping, instrument lines, and maintenance-friendly connections.
Industries using RF flanges include:
- Oil and gas gathering, refining, and petrochemical piping
- Chemical process plants and storage terminals
- Power plants, steam lines, and boiler auxiliary systems
- Water treatment, desalination, and utility piping
- Shipbuilding and offshore modules
- Pressure vessels, heat exchangers, filters, and skid-mounted equipment
Standards and Dimensions for RF Flanges
Most RF flange buyers specify the standard first. Common standards include ASME B16.5, ASME B16.47 Series A or Series B, EN 1092-1, JIS, DIN, and custom drawing requirements. The standard defines dimensions such as outside diameter, bolt circle, bolt hole size, thickness, hub dimensions, raised face diameter, and raised face height.
For ASME flanges, pressure classes such as Class 150, 300, 600, 900, 1500, and 2500 should be specified clearly. Larger sizes may fall under ASME B16.47 rather than B16.5. If a project needs a custom flange manufacturer, the drawing should identify whether the sealing face is RF, FF, RTJ, or another configuration. This avoids costly mismatches during installation.
Common RF Flange Materials
RF flanges can be produced from forged carbon steel, stainless steel, low-temperature steel, alloy steel, duplex stainless steel, and special alloys. A stainless steel RF flange is common for corrosion resistance, sanitation-related piping, and chemical service. Carbon steel RF flanges are widely used in oil, gas, water, steam, and general industrial applications. Duplex and super duplex options may be chosen for chloride-containing environments where strength and pitting resistance are important.
Material should be selected according to pressure, temperature, fluid, corrosion allowance, welding requirements, and applicable codes. Common material specifications include ASTM A105, ASTM A350 LF2, ASTM A182 F304/F316/F321/F347, ASTM A182 F51/F53/F55, and other project-specific grades.
Can RF Flanges Connect to FF or RTJ Flanges?
As a rule, flange face types should match unless an engineer approves a special transition. RF-to-RF is the normal pairing. FF-to-FF uses a full-face gasket and is often connected to cast iron or low-pressure equipment. RF-to-FF can create bending stress on a flat-face equipment flange if bolt load is not controlled. RTJ flanges require matching ring grooves and metal ring gaskets, so RTJ-to-RF is not a normal bolted joint.
If an existing site has mixed flange faces, document the equipment material, pressure class, gasket, bolt grade, and operating condition before ordering replacement flanges. A small face mismatch can become a leakage, cracking, or shutdown problem after installation.
RF Flange Installation Tips
Correct installation is as important as correct manufacturing. Before assembly, inspect the raised face for dents, scratches crossing the sealing path, corrosion, weld spatter, and handling damage. Confirm that the gasket is the correct type and size. Align the flanges without forcing the pipe into position. Use proper bolts and lubricants, and tighten with a cross-pattern sequence in several passes.
| Installation Check | Why It Matters |
|---|---|
| Clean raised faces | Dirt, paint, or corrosion can create a leakage path |
| Correct gasket centering | Off-center gaskets reduce sealing area and may intrude into the bore |
| Controlled bolt tightening | Uneven load can crush one side of the gasket and leave another side under-compressed |
| No forced alignment | Pipe stress can distort the flange joint after startup |
| Recheck after thermal cycling when required | Temperature changes can relax gasket stress in some services |
Common Mistakes When Ordering RF Flanges
The most common purchasing mistake is ordering by size only. A request such as “6 inch RF flange” is incomplete. The manufacturer also needs the standard, class, material, flange type, schedule or bore, facing, quantity, coating, inspection requirement, and any certificates.
Another mistake is assuming all raised faces are the same. RF dimensions and raised face heights are tied to the standard and class. Large diameter RF flanges may also require tighter control of flatness, bolt hole orientation, and packaging to prevent transport damage. For custom forged flanges, the drawing should clearly mark the sealing face finish and tolerance.
RF Flange RFQ Checklist
To get an accurate quotation from a pipe flange manufacturer, include these details:
- Flange type: weld neck, slip on, blind, socket weld, threaded, plate, lap joint, or custom design
- Face type: RF flange, FF flange, RTJ flange, or special facing
- Standard and class: for example ASME B16.5 Class 300 or ASME B16.47 Series A Class 150
- Size and bore: NPS, DN, pipe schedule, or exact inside diameter
- Material grade: carbon steel, stainless steel, duplex, alloy steel, or drawing material
- Gasket and service information if available
- Surface treatment: anti-rust oil, black paint, galvanizing, passivation, or custom coating
- Inspection: MTC, PMI, UT, hardness, dimensional report, or third-party inspection
- Quantity, delivery schedule, packaging, and destination port
Conclusion
The RF flange is a practical and widely used raised face connection for industrial piping. It offers reliable gasket compression, broad gasket compatibility, and availability across many flange types, sizes, pressure classes, and materials. Compared with FF flanges, RF flanges usually provide stronger concentrated sealing for steel piping. Compared with RTJ flanges, they are easier and more economical for many general process applications.
For best results, specify the face type, standard, class, material, flange type, gasket requirement, and inspection needs clearly. As a stainless steel flange supplier and custom flange manufacturer, Songhai Flanges can support RF flanges, blind flanges, weld neck flanges, slip on flanges, large diameter flanges, and custom forged flanges for industrial piping projects.