The critical engineering difference between raised face vs flat face flanges is the facing geometry (contact area), which controls gasket selection, required gasket seating stress, and bolt-load/torque limits. A frequent field failure is mating a steel Raised Face (RF) flange to a cast iron Flat Face (FF) flange without correcting the mismatch; the resulting outer-edge gap introduces bending stress at the cast iron flange and can trigger cracking during tightening.
- Raised Face (RF): Common on steel process piping; concentrates bolt load to achieve higher gasket seating stress for medium-to-high pressure service.
- Flat Face (FF): Typically specified when the mating equipment/material is brittle or low-ductility (cast iron, FRP, some plastics/linings) to minimize flange rotation and bending moments.
Sunhy’s experience in stainless steel flanges focuses on machining RF and FF facings to the dimensional, finish, and flatness requirements commonly referenced by ASME B16.5 and EN 1092-1. When specifying, treat “RF vs FF” as an interface decision: it must match the gasket style, bolt load strategy, and—most importantly—the rating and material behavior of the mating equipment.
Definitions and Features
Raised face flange overview
A raised face flange features a sealing surface machined to a defined height above the bolting circle, designed to concentrate load on a smaller gasket area.
In steel process piping, RF is the most common facing because the reduced contact area increases gasket seating stress for a given bolt preload. That higher seating stress is often necessary for semi-metallic gasket styles (for example, spiral wound gaskets) to conform into the facing serrations and resist blowout under pressure cycling. For gasket selection references, many projects align gasket type and dimensions to ASME B16.20 and assembly practice to ASME PCC-1.
Per ASME B16.5, the raised face height is determined by the pressure class:
| Pressure Class | Raised Face Height | Surface Finish (Typical) |
|---|---|---|
| Class 150 & 300 | 1.6 mm (1/16 inch) | 125 to 250 µin Ra (Phonographic/serrated) |
| Class 400 – 2500 | 6.4 mm (1/4 inch) | 125 to 250 µin Ra (Phonographic/serrated) |
| Function | Concentrates Sealing Pressure | Promotes gasket grip and seating |
RF flanges are typically forged from Carbon Steel (A105) or Stainless Steel (A182 F304/F316). The serrated/“phonographic” finish is not cosmetic—it affects friction, gasket seating, and leak performance. In practical terms: a face that is too smooth can allow gasket slip and relaxation; a face that is too rough can damage softer gasket materials and create leak paths. When a client requests a specific Ra range, confirm the gasket manufacturer’s recommendation and align tightening procedures with ASME PCC-1.
Field Case #1 (Leak after first heat cycle): A Class 300 RF steam line sealed tight during commissioning but began “weeping” after the first thermal cycle. Root cause was bolt preload loss from gasket relaxation and uneven tightening sequence. Correction was re-torque after the first heat cycle (per site procedure aligned to PCC-1 guidance), plus verification that bolt lubrication was consistent so torque correlated to preload.
Flat face flange overview
A flat face flange has a gasket surface in the same geometric plane as the bolting circle, providing full face-to-face contact.
Engineers specify FF facings when mating to brittle or low-ductility materials like Cast Iron (ASTM A126), bronze, FRP, or lined equipment. By extending the gasket support to the OD, FF reduces flange rotation and the local bending that brittle materials cannot tolerate. In waterworks and fire service, FF interfaces are commonly referenced by ASME B16.1 and in some systems by AWWA C207 (verify the governing spec for your project).
Here are the defining engineering characteristics:
| Characteristic | Engineering Reason |
|---|---|
| Surface Design | Flush (No gap) to reduce flange rotation and bending load on brittle mates. |
| Mandatory Gasket | Full Face Gasket (with bolt holes) to support the entire face and reduce edge loading. |
Field Case #2 (Cast iron crack during tightening): A cast iron pump casing (FF) was bolted to a steel RF flange with a spiral wound gasket. Crack initiated near a bolt hole after tightening. Immediate fix was replacing the damaged component. Preventive fix was machining the steel facing flat (or using an approved spacer strategy), switching to a full-face gasket compatible with the service, and applying a torque limit aligned to the equipment OEM.
Key design features
The structural difference dictates the mechanics of the joint: Concentration vs. Distribution.
An RF flange concentrates bolt load to generate higher seating stress (useful for many semi-metallic gasket styles). An FF flange distributes load to reduce rotation and protect brittle or thin-wall mating equipment. The correct choice is not “preference”—it is dictated by the mating component’s rating, material behavior, and gasket requirements.
| Feature | Raised Face (RF) | Flat Face (FF) |
|---|---|---|
| Geometry | Step change at gasket surface | Single continuous plane |
| Bolt Load Efficiency | High (Force ÷ Small Area = Higher Seating Stress) | Lower (Force ÷ Large Area = Lower Seating Stress) |
| Typical Mating Material | Forged Steel, Stainless Steel | Cast Iron, Bronze, PVC/CPVC, FRP |
| Common Gaskets | Spiral Wound, Ring Joint, Kammprofile | Full Face elastomer/fiber/PTFE styles (service dependent) |
Field Rule: If you can slide a feeler gauge between the outer edges of two tightened flanges, they are often a Raised Face joint. If the outer edges touch tight with full support to the OD, it is typically a Flat Face joint. (Always confirm by spec and facing measurement—do not rely on appearance alone.)
Evidence point (ratings and traceability): Always confirm the stamped markings and traceability documents for the flange and bolts. Many projects require MTR/CMTR and—where alloy mix-up is a risk—PMI (Positive Material Identification). This is common practice under quality systems aligned to ISO 9001 and pressure equipment compliance such as PED in Europe.
Flat face flange vs raised face flange: Main Differences
Engineering Insight: RF flanges concentrate gasket seating stress (localized), while FF flanges distribute bolt load and reduce flange rotation (global support).
Sealing surface and construction
The main difference is Gasket Seating Stress vs. Flange Rotation/Bending Risk.
In an RF joint, the smaller facing area increases gasket seating stress for a given bolt preload, which is why RF is common for process services and pressure cycling. In an FF joint, the full-face support reduces flange rotation and edge bending, which is why FF is used when the mating component is brittle or has limited allowable flange load.
| Flange Type | Sealing Mechanism Description |
|---|---|
| Raised Face Flange | Concentrates bolt load on the ring area (IBC – Inside Bolt Circle). Higher gasket seating stress. |
| Flat Face Flange | Distributes bolt load across the full face. Lower seating stress; improved support for brittle mates. |
Gasket selection is non-negotiable here. You must use full-face gaskets on Flat Face joints when the mating component is cast iron/FRP/low-ductility or when the specification requires full support to the OD. Using a ring gasket (IBC) on a flat face cast iron flange can bow the flange into the unsupported area and initiate cracking. Conversely, many RF joints use ring gaskets for efficiency and easier handling during maintenance.
| Flange Type | Gasket Type | Installation Note |
|---|---|---|
| Raised Face | Ring-Type (IBC) | Common. Fits inside bolts. Often easier to replace with partial bolt removal (site practice varies). |
| Raised Face | Full-Face | Used when the mating face or spec requires full support; verify torque/load capability and gasket recommendations. |
| Flat Face | Full-Face | Mandatory for cast iron/FRP/low-ductility mates and many utility standards. |
Warning (qualified): Avoid using semi-metallic spiral wound gaskets directly against cast iron flat faces unless the OEM/spec explicitly permits it and the gasket design is appropriate for the facing and allowable flange load. A hard gasket edge or excessive seating stress can score the face or initiate cracking.
Field Case #3 (PTFE-lined/FRP service leak): A utility chemical line with FRP/lining used a ring gasket on an FF interface. Leak started at the bolt-hole quadrant after 24–48 hours due to flange rotation and gasket creep. Correction was switching to an appropriate full-face gasket (service compatible) plus controlled tightening sequence and a verified torque limit based on the equipment allowable flange load.
Pressure and temperature suitability
Raised Face flanges are commonly used across the full range of ASME B16.5 pressure classes, but the actual pressure-temperature limit is material-group dependent and must be taken from the rating tables.
Flat Face interfaces are commonly associated with cast iron and utility standards (e.g., Class 125/250 in ASME B16.1), and are frequently used in water, HVAC, and fire service. Treat the table below as engineering guidance—always verify with the governing standard, material grade, gasket, and the equipment OEM allowable flange load.
| Flange Type | Max Pressure Rating | Temp Limit (Typical) | Typical Service |
|---|---|---|---|
| Flat Face (Cast Iron) | Class 125 / 250 (per ASME B16.1, where applicable) | Often limited by gasket/elastomer and OEM allowable flange load (service dependent) | Water, Fire Mains, HVAC |
| Raised Face (Steel) | Class 150 – 2500 (per ASME B16.5, material dependent) | Use ASME B16.5 pressure-temperature ratings for the specific material group | Steam, Hydrocarbons, Chemical |
Always verify the flange rating against the valve/equipment rating and allowable flange load. Some bolt patterns may appear compatible across standards for certain sizes, but rating and facing requirements can differ—verify BCD, hole count, and the governing code before assembly.
Manufacturing process
Sunhy employs precision CNC machining to ensure the surface finish (Ra) aligns with gasket requirements and the flatness/face geometry meets the drawing/specification.
For Raised Face flanges, the facing finish is commonly specified as a serrated/phonographic finish (often in the range of 125–250 µin Ra, project/gasket dependent). For Flat Face flanges, flatness and full-face support are critical—especially at bolt-hole regions where localized leakage can initiate if the face is distorted.
| Certification | Why it matters for Flanges |
|---|---|
| ISO 9001 | Traceability of heat numbers and controlled inspection records. |
| PED 2014/68/EU | Compliance framework for pressure equipment in Europe (where applicable). |
| PMI Testing | Positive Material Identification to reduce alloy mix-up risk on critical projects. |
Sunhy performs dimensional inspection to confirm facing height and geometry. Even small deviations in raised face height or facing width can change gasket contact width, bolt engagement behavior, and the achieved seating stress. For critical services, align the machining acceptance criteria to the project spec and verify with calibrated measurement methods.
Quality Check: Material Test Reports (MTRs) provide chemical composition and mechanical properties. For alloy-sensitive service (chlorides, sour service, high temperature), PMI is commonly used as an additional verification layer.
Comparison of the advantages and disadvantages of Raised Face vs Flat Face Flange
I. Raised Face Flanges (RF): Pros and Cons
The Raised Face (RF) is the most common facing on steel process piping, distinguished by the step on the sealing face.
Advantages
- Superior Sealing Performance: Higher gasket seating stress for a given bolt preload helps control leakage under pressure and thermal cycling (gasket dependent).
- Wide Gasket Compatibility: Commonly paired with spiral wound, camprofile, and other semi-metallic gaskets used in critical service (verify to ASME B16.20 and gasket manufacturer data).
- Standardization: Widely used across ASME B16.5 pressure classes and common in process industries.
Disadvantages
- Risk to Brittle Equipment: Cannot be bolted directly to cast iron/FRP/low-ductility mates without an approved mitigation (machining flat, spacer strategy, gasket selection, torque limit).
- Alignment Sensitivity: Angular misalignment can concentrate load on a sector of the gasket and drive localized leakage; do not “pull-in” misalignment with bolts.
II. Flat Face Flanges (FF): Pros and Cons
The Flat Face (FF) is a facing choice used to protect equipment made from low-ductility or brittle materials and to maintain full-face support.
Advantages
- Protection for Brittle Materials: Commonly used for cast iron, glass-lined, FRP, and some plastic/lined equipment to minimize flange rotation and bending moments.
- Rigidity: Full-face contact can help stabilize joints where vibration is present (for example, pump suction/discharge), provided allowable flange load is respected.
- Easy Visual Check: Face-to-face contact at the OD can provide a quick visual check for uniform tightening (still verify torque and sequence).
Disadvantages
- Limited Pressure Capability (application-driven): Many FF applications are tied to cast iron/utility standards and elastomeric gaskets, which can limit service envelope compared to steel process RF joints.
- Strict Gasket Requirement: Full-face gaskets are often mandatory; high-temperature full-face options may be more limited and must be selected by service compatibility.
- Surface Area Penalty: Because seating stress is distributed over a larger area, achieving the same local gasket stress can require higher total bolt load—often not permitted by brittle equipment.
| Feature | Raised Face (RF) | Flat Face (FF) |
| Sealing Ability | High (higher seating stress, gasket dependent) | Moderate (distributed load, equipment-protective) |
| Primary Use | Steel Piping (process lines) | Cast Iron/FRP/Utility Equipment |
| Risk Factor | Can crack brittle mates if misapplied | Can leak under high cycling if gasket/bolt load strategy is inadequate |
| Gasket Types | Versatile (semi-metallic/metallic/non-metallic per service) | Often restricted to full-face styles (service dependent) |
| Material Compatibility | Ductile materials (steel, alloy) | Brittle/low-ductility materials (iron, FRP, plastics) |
Applications and Use Cases
Raised face flange in the industry
RF flanges are common in industries requiring leak-resistant performance under pressure and thermal cycling.
Engineers specify RF where containment, repeatable assembly, and gasket compatibility are priorities. Typical applications include:
- Oil & Gas: Process and transport lines where leakage consequences are high.
- Chemical Processing: Corrosive services using appropriate alloys and gasket systems (often referencing ASME B16.20 gasket styles).
- Steam Systems: Plant steam lines where thermal expansion and cycling are expected; tightening procedures should follow a controlled sequence and verified torque.
Sunhy supplies RF flanges in Stainless Steel (304/316) and Duplex Steel (2205) where specified, with face finish controlled to match the gasket requirement and project specification.
Flat face flange in the industry
FF flanges are found where piping connects to utility equipment or rotating machinery with limited allowable flange load.
They are common in balance-of-plant systems and utility services. Common applications include:
- Water Treatment: Connections to cast iron gate valves and check valves.
- Fire Protection: Hydrants and sprinkler systems often use ASME B16.1 Class 125 FF interfaces.
- Marine/HVAC: Cooling water loops utilizing bronze or copper-nickel systems, where flange load control and corrosion considerations matter.
In these systems, the piping material (often PVC, FRP, or thin-wall stainless) benefits from distributed load, provided gasket selection and tightening limits are controlled by the specification and equipment OEM.
Compatibility and installation
The Golden Rule: Do not bolt a steel Raised Face flange directly to a Flat Face cast iron flange unless the mismatch is properly mitigated.
Without mitigation, the RF step can leave an OD gap. Tightening then pulls the cast iron flange into that gap, introducing bending stress and a crack-initiation risk. This is a frequent and costly field error.
| Scenario | Correct Solution |
|---|---|
| Connecting Steel Pipe to Cast Iron Valve | 1. Remove Raised Face: Machine the steel flange face flat (where permitted). 2. Use Spacer (if specified): Install an approved spacer strategy to eliminate the gap. |
| Gasket Selection | Use a Full Face Gasket to support the cast iron flange and reduce rotation. |
| Torque Control | Use a torque wrench and respect equipment allowable flange load. Do not apply steel flange torque values to cast iron equipment. |
If you must connect different types, machining the RF to a flat face (subject to design/spec approval) is a common method to create a true Flat-Face to Flat-Face interface. Tightening procedures should be controlled (sequence + staged torque). Many sites align assembly discipline to ASME PCC-1 for critical joints.
Tip: When ordering flanges for pump suction/discharge, confirm the pump casing material (cast iron vs steel) and the OEM facing requirement. If the casing is cast iron, Flat Face is commonly required.
Choosing the Right Flange
Key selection factors
Flange selection is dictated by the weakest link in your system—often the valve or equipment connection (rating, material, allowable flange load).
Use this engineering checklist:
| Factor | Decision Rule |
|---|---|
| Mating Equipment | Cast iron/FRP/plastic/lined equipment often requires Flat Face and full-face gasket support. |
| System Pressure | Higher pressure cycling typically favors Raised Face on steel systems (verify to governing rating tables). |
| Gasket Type | Spiral wound/camprofile/metallic gaskets often pair with Raised Face facings (confirm to ASME B16.20 and gasket data). |
| Fluid Service | Hazardous service generally requires a conservative sealing strategy (face finish, gasket type, bolt preload control). |
Match the flange not just to the pipe, but to the component it connects to. Sunhy’s technical team can cross-reference pump and valve cut-sheets to help confirm facing and gasket selection before fabrication and site assembly.
Tip: Verify flange dimensions and standards in the field. Some bolt patterns may look similar between systems, but always verify the governing standard, facing, and rating before installation.
Industry standards and codes
Your flanges must meet the governing codes to pass inspection and to ensure interchangeability.
Sunhy commonly supports projects aligned to:
| Standard/Code | Description |
|---|---|
| ANSI/ASME B16.5 | Steel pipe flanges (defines RF heights/dimensions and pressure classes by material group). |
| ASME B16.1 | Gray iron pipe flanges (commonly FF in Classes 125/250). |
| AWWA C207 | Waterworks flanges (utility service; verify project requirement). |
| DIN EN 1092-1 | European standard (Type 11 commonly RF; Type 01 commonly FF—confirm by drawing/spec). |
Ensure your purchase order specifies the standard and facing explicitly (for example: “ASME B16.5, Class 150, RF, A182 F316L, NPS 4, bore per schedule, serrated finish per spec, MTR/PMI as required”).
Common mistakes to avoid
Avoid these field failures by following proper specification and assembly discipline:
Many leaks and cracks are not “material problems”—they are interface and assembly problems.
- The “Gap” Error: Connecting RF to FF (cast iron/FRP) without an approved mitigation (machining/spacer + gasket + torque limit).
- The “Wrong Gasket” Error: Using a ring gasket where full-face support is required (can increase flange rotation and edge loading).
- Over-Torquing: Applying steel torque practices to cast iron equipment; respect OEM allowable flange load and tightening guidance (many sites align tightening discipline to PCC-1).
- Reusing Gaskets: Once compressed, most gaskets lose recovery; reuse is a frequent cause of maintenance leaks.
Improper flange selection leads to downtime and safety risk. Always tighten in a controlled cross/star pattern and in stages (for example 30% / 60% / 100%), with consistent lubrication practices so torque correlates to preload.
Note: Contact Sunhy’s experts to review your flange facing and gasket selection to reduce mismatch risk before fabrication and site work.
FAQ
What is the main difference between raised face and flat face flanges?
The sealing surface geometry and how bolt load is applied to the gasket. Raised Face (RF) flanges use a stepped sealing land to concentrate bolt load and achieve higher gasket seating stress. Flat Face (FF) flanges are flush to the OD to distribute load and reduce flange rotation—especially important when mating to cast iron/FRP/low-ductility equipment.
Can you connect a raised face flange to a flat face flange?
Not directly, unless the mismatch is properly mitigated and approved by spec/OEM. Direct RF-to-FF (especially steel RF to cast iron FF) can create an OD gap and introduce bending stress that can crack cast iron during tightening. Typical mitigations include machining the RF flat (where permitted) or using an approved spacer strategy, plus using a full-face gasket and controlled torque.
Which industries use raised face flanges most often?
Process industries. Oil & Gas, Chemical, and Power Generation commonly use RF facings on steel piping because RF supports a wider range of gasket types (including many semi-metallic styles) and is commonly specified across ASME B16.5 pressure classes. Actual selection still depends on mating equipment material and allowable flange load.
What gasket should you use with each flange type?
Match the gasket to the facing and the mating component’s allowable load.
RF: Often uses ring gaskets and semi-metallic gaskets (e.g., spiral wound) when permitted by service/spec.
FF: Commonly requires a full-face gasket (with bolt holes), especially for cast iron/FRP/plastics/linings, to support the full face and reduce flange rotation.
How do you choose the right flange for your system?
Start with the mating component (equipment/valve/pump) and the governing standard. If connecting to steel valves/piping with process gaskets, RF is commonly used. If connecting to cast iron/FRP/plastic/lined equipment, FF is commonly required. Always verify the pressure-temperature rating tables for the specific material group and confirm gasket compatibility.
| Factor | Raised Face Flange | Flat Face Flange |
|---|---|---|
| Pressure | Common across ASME B16.5 classes (material dependent) | Often tied to cast iron/utility standards (service dependent) |
| Material | Steel / Stainless | Cast Iron / FRP / Plastic / Lined equipment |
Tip: Consult Sunhy’s engineering team for torque strategy, gasket compatibility, and facing verification—especially for dissimilar joints and brittle equipment interfaces.
For a full overview of flange types and standards, see our complete guide
