The flange face is the sealing surface where the gasket sits. It's one of the most overlooked details in piping design, yet specifying the wrong face type can compromise joint integrity. Three face types dominate industrial piping: raised face (RF), flat face (FF), and ring joint (RTJ). This guide explains when to use each and why they matter.
Raised Face Flanges (RF)
Raised face is the default face type in ASME B16.5 and the most common in general industrial piping. The raised face is a machined protrusion (typically 1/16" height and 1/4" width on the face) that concentrates gasket pressure to the raised area, helping the gasket create a tight seal. The entire bolt circle is included in flange thickness, but only the raised area seals.
RF flanges pair with soft gaskets: spiral-wound gaskets (with or without filler), corrugated metal gaskets, and graphite-filled gaskets. These gaskets conform to the raised face and are seated reliably under moderate bolt load. RF is the right choice for most piping: water systems, compressed air, steam, light hydrocarbons, and general industrial service below 1500 psi and 600°F.
The advantage of raised face is cost and availability. Nearly every stock flange has an RF face. Gaskets are inexpensive and widely available. Joining RF flanges is straightforward—torque the bolts to the standard specification, and the gasket seats. The disadvantage is that raised face does not seal as well as ring joint at very high pressure; if your system runs above 2500 psi, ring joint is preferred.
A critical detail: raised face dimension varies by size and class. A 1" Class 150 RF has different face geometry than a 1" Class 600 RF. Always specify the flange size and class, and match gasket dimensions to the rated face.
Flat Face Flanges (FF)
Flat face flanges have no raised area. The entire face is flat, at the same level as the outer diameter. The face is still precision-machined for flatness, but there is no protrusion. Flat face flanges are common in large-diameter piping (24" and above) and in systems where two dissimilar materials (e.g., steel and cast iron) are bolted together.
The key advantage of flat face is that less bolt load is required to create a seal. In large-diameter applications, raising the bolt load to seat a raised face gasket would require enormous bolt tensioning and risk bolt failure. Flat face reduces the required sealing pressure, making large connections more practical. FF flanges also pair better with rigid gasket materials like corrugated metal or solid metal-jacketed gaskets.
The downside is that flat face gaskets must be thicker and more rigid than the soft gaskets used with raised face. This increases gasket cost and can complicate installation if the gasket does not sit evenly on the face. Flat face is also less common; not all flanges are offered in FF, so lead time and custom work are more likely.
When bolting a flat face flange to a raised face flange (mixed face connection), special care is required. The gasket must bridge the raised face and rest partially on the flat face and partially on the raised area. Using an oversized gasket that covers both faces is safer than attempting to seat a gasket that straddles the step.
Ring Joint Flanges (RTJ)
Ring joint flanges have a grooved sealing surface rather than a flat or raised face. The groove is an octagonal or oval depression in the face that accepts a metal ring gasket. RTJ is specified for very high-pressure service: 2500 psi and above. The metal ring gasket sits in the groove and is locked in place by the flange faces, creating an extremely tight seal under high pressure.
RTJ flanges are mandatory in oil and gas wellhead applications (API 6A/16A standards) and common in high-pressure piping in refineries, chemical plants, and compressor discharge lines. The metal ring gasket is resilient enough to withstand extreme pressure without weeping or blowing out. RTJ can handle temperature swings better than soft gaskets because the metal ring does not degrade as easily as elastomer-filled gaskets.
The disadvantages of RTJ are cost, availability, and complexity. A ring joint flange costs 40–60% more than the equivalent raised face flange. Metal ring gaskets are specialized components; you cannot substitute a spiral-wound or flat gasket on an RTJ flange. Installation requires careful alignment and proper bolt tensioning using a torque wrench or hydraulic bolt tensioner. Over-torquing can damage the groove; under-torquing leaves a leaking joint.
RTJ is not interchangeable with RF or FF without special adapters. If your specification calls for RTJ, all flanges in that line must be RTJ. Mixing face types is a common source of field failures.
Gasket Compatibility and Sealing Performance
The face type and gasket type are paired. A spiral-wound gasket on an RTJ flange will not seal properly—the gasket is too soft and will extrude into the groove. A metal ring gasket on a raised face flange is wasteful and ineffective. Always match gasket type to face type per ASME B16.5, API 6A, or the flange standard applicable to your application.
Sealing performance improves with higher pressure and better face preparation. At low pressure (under 300 psi), a soft gasket on any face type seals adequately. At high pressure (above 1500 psi), surface finish and flatness become critical. Raised face surfaces must be finished to 62–125 micro-inch Ra (root mean square roughness) to prevent gasket extrusion. Ring joint grooves must be within tight dimensional tolerances (±0.005" in many cases) for the metal ring to function properly.
Temperature cycling can degrade soft gaskets and cause gasket creep (gradual displacement under sustained load), leading to weeping leaks. Metal ring gaskets resist creep better and are preferred where thermal cycling is a factor.
Pressure Class Limitations
ASME B16.5 limits flange face types by pressure class. Flanges Class 150 through Class 600 are typically offered in raised face. Flat face is less common at high classes (Class 600 and above) because the flat geometry cannot concentrate sealing pressure effectively. Ring joint is specified for Class 900 and above, where metal-to-metal sealing is essential.
A Class 150 RF flange is suitable for low-pressure, low-temperature service. A Class 2500 RTJ flange is required for wellhead or compressor discharge service. Attempting to use a Class 150 RF flange in Class 600 service is a failure waiting to happen. Always cross-check pressure class against operating pressure and select the appropriate face type for that class.
Common Mistakes in Face Specification
1. Mixing face types without awareness. A piping system with both RF and RTJ flanges requires special adapters or spacer flanges to connect them. Bolting them directly together will leak. Always specify a uniform face type throughout a pressure boundary.
2. Under-specifying at high pressure. Using Class 600 RF flanges in a system that runs 2500 psi leads to gasket extrusion and failure. High-pressure lines must have Class 2500 RTJ flanges and metal ring gaskets. There is no workaround.
3. Over-specifying at low pressure. Specifying RTJ flanges for 50 psi service is excessive and wasteful. Use Class 150 RF for low-pressure piping. Save RTJ for where it is needed.
4. Incorrect gasket selection. Installing a soft gasket on an RTJ flange or a metal ring on an RF flange compromises sealing. Always match gasket type to the face type and flange class.
5. Poor surface preparation. Even the right flange and gasket combination will leak if the sealing surface is dirty, corroded, or rough. Clean the face with a wire brush, dry it, and inspect for pitting or scoring before assembly.
Selection Quick Reference
Use Raised Face (RF) for:
- Pressure below 1500 psi
- General industrial piping (water, air, steam)
- Temperature below 600°F
- Cost and availability are priorities
Use Flat Face (FF) for:
- Large-diameter flanges (24" and above)
- Mixed metal connections (steel to cast iron)
- Where soft gasket sealing is inadequate
Use Ring Joint (RTJ) for:
- Pressure above 2500 psi
- Oil and gas service (API 6A/16A)
- Extreme temperature cycling
- Hydrogen service or sour gas applications
Final Thoughts
The flange face type is not an afterthought—it is a core design decision that affects gasket selection, bolt load, system reliability, and cost. Match the face type to your pressure class and service conditions. Use raised face for standard piping, flat face for large connections, and ring joint for high-pressure service. Specify compatible gaskets and ensure proper installation. A well-designed flange connection, properly torqued and inspected, will hold pressure and temperature for decades.