What is an RTJ flange

An RTJ (Ring Type Joint) flange is a flange sealing design for high-pressure, high-temperature and flammable service. Instead of a flat sealing face, the flange carries a precision-machined ring groove that holds a solid metal ring gasket; the seal is made by line contact between machined metal surfaces and tightens as pressure rises. Soft flange gaskets cannot carry that duty, which is why the ring joint is the default for gas, hydrogen and high-temperature steam lines.

How an RTJ Flange Seals

A raised-face (RF) flange seals by compressing a flat gasket between two bolted faces. An RTJ flange replaces the flat face with a machined ring groove — oval or octagonal in section — and the groove walls bite slightly into the metal ring gasket as the bolts are tightened. The result is a line-contact seal built on small plastic deformation of the ring: the higher the internal pressure, the harder the ring is pressed against the groove. That is the practical difference from the raised-face arrangement used on most pipe flanges.

• Metal ring gasket — a solid oval or octagonal ring, not a flat sheet or a spiral wound gasket.
• Ring groove — a machined circular trapezoidal groove on the flange sealing face.
• Pressure range — commonly used from Class 600 (PN110) to Class 2500 (PN420) and above.
• Assembly tolerances — parallelism deviation ≤ 1.5 mm and bolt hole misalignment ≤ 3 mm.

Ring Type Joint flange face vs raised face flange

Advantages and Limits

The design pays off at heavy duty, and it demands precision in the machine shop and on site.

• High-pressure resistance: the metal-to-metal seal holds from Class 600 (PN110) upward.
• High-temperature resistance: metal ring gaskets tolerate far higher temperatures than non-metallic gaskets.
• Low leakage rate: suited to natural gas, hydrogen, toxic media and high-temperature steam.

The price of those advantages:

• Higher cost: the flange face needs tight machining, and the ring cannot be reused.
• Strict installation: high bolt preload is required, and a damaged groove is difficult to repair.

How to Identify an RTJ Flange

Two details identify the joint in the field.

• The sealing face is not flat — it carries a circular trapezoidal ring groove.
• The gasket is a metal ring, oval or octagonal in section, not a flat sheet.

How to Install an RTJ Flange

Careful assembly protects both the groove and the ring.

• Inspect and clean: check the ring groove for scratches, dents and corrosion, and remove all foreign matter.
• Use a new ring gasket: the ring takes a permanent set after tightening and must never be reused. The ring material should be softer than the flange material.
• Seat the ring: lower it into the groove centred and level, without catching on the groove edge.
• Align the flanges: bring the two flanges together without knocking the ring out of place, keeping parallelism within 1.5 mm and bolt hole misalignment within 3 mm. Never drag a badly misaligned joint into line with the bolts.
• Lubricate the bolts: coat threads, nut bearing faces and washers with high-temperature anti-seize such as molybdenum disulfide or graphite paste. Dry tightening wastes more than 70% of the torque on friction and leaves too little clamping force.

Ring gasket seated in the RTJ groove during assembly

Tighten with a calibrated torque wrench in stages and in a cross pattern:

Table: staged torque sequence for an RTJ bolted joint.

Torquing sequence

Frequently Asked Questions

What is an RTJ (Ring Type Joint) flange?

It is a flange whose sealing face carries a machined ring groove for an oval or octagonal metal ring gasket. The joint seals by line contact and is used for high-pressure, high-temperature or flammable media.

When should an RTJ flange be preferred over a raised-face flange?

From Class 600 (PN110) upward, at high temperature, or wherever leakage must stay very low — natural gas, hydrogen, toxic media and high-temperature steam. Raised-face joints with soft gaskets remain the economical choice for utility lines.

Can a metal ring gasket be reused?

No. The ring is plastically deformed by the groove during tightening, so a new gasket must be fitted at every reassembly. The ring material should be softer than the flange material.

What tightening sequence is used on an RTJ flange?

Use a calibrated torque wrench and tighten in stages: 30%, 60% and 100% of the final torque in a cross pattern, then a final clockwise check at 100%. Lubricate the threads first — dry tightening wastes over 70% of the torque on friction.