An RTJ flange (Ring Type Joint flange) is a type of pipe flange designed to create a leak-proof metal-to-metal seal in systems that operate under extreme pressure and temperature. Unlike standard flanges that use rubber or fiber gaskets, RTJ flanges use a soft metal ring that sits in a precision-machined groove on the flange face. When the bolts are tightened, this metal ring deforms into the groove surfaces, creating a seal that can handle pressures up to 20,000 psi in certain configurations.
How the Seal Works
The sealing mechanism behind an RTJ flange is surprisingly simple. A metal ring gasket, softer than the flange itself, is placed into a groove machined into the flange face. As the two flanges are bolted together, the compression forces the softer gasket metal to flow into the microfine surface texture of the harder flange groove. This plastic deformation creates an extremely tight metal-to-metal seal with virtually no gaps for fluid or gas to escape.
The gasket must always be softer than the flange groove, typically by 15 to 20 points on the Brinell Hardness scale. This hardness difference is critical: it ensures the gasket deforms rather than the groove. If the gasket is too hard, it can permanently damage the groove, compromising the flange for future use.
Once the flanges are fully tightened, the raised faces of the two flanges may come into direct contact with each other. When this happens, the gasket is fully compressed and won’t bear any additional load from further tightening, which protects the seal from being over-compressed.
Three Gasket Styles: R, RX, and BX
RTJ flanges use one of three gasket designs, each suited to different pressure ranges and operating conditions.
- R Type is the most common and comes in two profiles: oval and octagonal. The oval was the original design, but the octagonal version has largely replaced it because of its superior sealing efficiency. Both seat into a machined groove on the flange face, and the octagonal profile makes better contact with the groove walls.
- RX Type fits the same groove design as the R type, making them interchangeable. The key difference is that RX gaskets use a pressure-energized design, meaning the internal pressure of the system actually pushes the gasket tighter against the groove walls. This makes the seal stronger as pressure increases, rather than weaker. RX gaskets are rated for pressures up to 5,000 psi and perform better in environments with vibration and shock loading.
- BX Type is built for the most extreme applications. These gaskets can only be used with API 6BX type flanges (they are not interchangeable with R or RX grooves) and handle pressures up to 20,000 psi. You’ll find these on wellhead equipment and other high-pressure, high-temperature systems in the oil and gas industry.
Where RTJ Flanges Are Used
RTJ flanges were originally designed for the petroleum and petrochemical industries, and that remains their primary home. They’re the standard choice anywhere a system faces high pressure, high temperature, or both. Common applications include refineries, chemical processing plants, offshore platforms, and subsea infrastructure.
In deepwater oil and gas operations, RTJ flanges are especially critical. Subsea manifolds operating thousands of meters below the surface experience enormous pressure surges and extreme temperature swings. Standard flat-face or raised-face gaskets can’t reliably contain these forces. For wellhead and “Christmas tree” equipment (the assembly of valves that controls flow from an oil or gas well), the API 6A standard governs flange design. API 6A flanges using BX-type gaskets and high-performance alloys are engineered specifically for these conditions.
In terms of pressure classes, RTJ flanges following the ASME B16.5 standard are manufactured in classes ranging from 150 through 2500. Subsea and offshore piping typically uses Class 900 through 2500, while wellhead applications governed by API standards can go much higher, up to API 20K (20,000 psi).
RTJ vs. Raised Face Flanges
The most common alternative to an RTJ flange is a raised face (RF) flange, which uses a flat or semi-metallic gasket compressed between two raised surfaces. RF flanges work well in systems with low to moderate pressure and temperature. They’re simpler to install, the gaskets are cheaper, and they’re perfectly adequate for a wide range of industrial piping.
RTJ flanges take over where RF flanges reach their limits. If a system operates under extreme pressure, extreme temperature, or significant thermal cycling (repeated heating and cooling), the metal-to-metal seal of an RTJ connection provides a level of reliability that a softer gasket simply can’t match. The tradeoff is cost and complexity: RTJ flanges require precisely machined grooves, carefully selected gasket materials, and more exacting installation procedures.
Groove Condition and Inspection
Because the seal depends entirely on the gasket deforming into the groove, the condition of that groove is everything. Groove dimensions on new RTJ flanges must conform to ASME/ANSI B16.5, while the metal ring gaskets follow the ASME B16.20 standard.
Over time, grooves can sustain minor damage from gasket removal, corrosion, or improper handling. If a groove needs reconditioning, there’s a limit to how much metal can be removed before the structural integrity of the flange is compromised. Beyond that limit, the flange either needs weld reclamation and re-machining (an expensive repair) or replacement. This is why protecting groove surfaces during maintenance is a high priority. Even minor scratches or nicks in the groove can create a leak path.
Why You Should Never Reuse an RTJ Gasket
Metal ring gaskets are single-use components. Both API 6A and ASME PCC-1 explicitly recommend against reusing them, and the reasoning is straightforward.
When an RTJ gasket is first compressed, it plastically deforms to fill the groove surfaces. This deformation is permanent. The gasket has a limited amount of built-in interference (slightly oversized relative to the groove) that allows it to seat properly on that first compression. Once that interference is used up, a reused gasket won’t seat correctly the second time around.
There’s a second, less obvious problem. When metal deforms plastically, the surface undergoes work hardening, becoming harder and more brittle. A reused gasket is therefore harder than it was originally, which means it’s closer in hardness to the flange groove. Instead of the gasket deforming into the groove, the hardened gasket can damage the groove itself. Visual inspection of a used gasket isn’t enough to detect work hardening, brittleness, or changes caused by heat and chemical exposure during service. Some fabricators do reuse gaskets during hydrostatic testing (a pressure test done before a system goes live), but this practice carries risk and isn’t endorsed by the major standards.
Common gasket materials include soft iron, low-carbon steel, various chrome-molybdenum alloys, and stainless steel grades like 304, 316, and 347. Each has a specified maximum hardness. Soft iron gaskets, for example, max out at 90 Brinell, while Type 410 stainless can go up to 170 Brinell. The choice of gasket material depends on the flange material, the operating environment, and the fluid or gas being contained.