RTJ Flanges: What They Are, When to Use Them, and How to Specify the Right One
If you work in upstream oil and gas, refining, or any application where pressure ratings push past the 300# class, you have probably come across RTJ flanges. They show up on nearly every high-pressure spec sheet, and for good reason. Ring type joint connections deliver a metal-to-metal seal that gasket materials alone cannot match at extreme pressures and temperatures. Specifying the right RTJ flange involves more than just matching a size and class. You need the correct ring groove profile, the right gasket style, and the right material pairing to make the joint work as an optimal seal.
This guide walks through what RTJ flanges are, how they differ from raised face and flat face connections, when they are the right call, and what to watch for when you are putting together a spec.
What Makes an RTJ Flange Different
An RTJ (Ring Type Joint) flange uses a machined metal ring gasket seated in a precision-cut groove on the flange face. Unlike raised face flanges that rely on compressed sheet gaskets or spiral wound gaskets, RTJ flanges achieve sealing through controlled deformation of a soft metal ring against hard groove walls. When the bolts are torqued, the ring gasket yields into the groove surfaces, creating a tight metal-to-metal seal.
The groove itself is the defining feature. Per ASME B16.5, RTJ flanges are manufactured with either R-type or RX-type grooves machined directly into the flange face. The groove geometry, depth, and surface finish are tightly controlled because even small deviations will compromise the seal. This is not like a raised face connection where you have some tolerance in the gasket compression. With RTJ, precision matters.
RTJ flanges are available in all standard ASME B16.5 sizes (1/2″ through 24″) and pressure classes from 150# through 2500#, though they become the standard choice starting at 900# and above. For larger sizes, ASME B16.47 covers RTJ configurations in Series A and Series B flanges. In API 6A wellhead and Christmas tree applications, RTJ connections are essentially the default, rated up to 20,000 PSI.
RTJ Gasket Types: R, RX, and BX
Not all ring gaskets are interchangeable, and getting this wrong is one of the most common mistakes in RTJ specifications. There are three main styles defined by ASME B16.20 and API 6A:
R-type gaskets are the standard oval or octagonal cross-section rings used with ASME B16.5 flanges. Octagonal rings provide slightly better sealing because they make line contact with the groove walls rather than the area contact of oval rings. Both seat in the same R-type groove, but octagonal is the preferred choice for new construction.
RX-type gaskets are pressure-energized versions of the R-type. They fit into standard R-type grooves but have a modified cross-section that allows internal pressure to further energize the seal. The big advantage: RX gaskets are interchangeable with R-type grooves, so you get better performance without changing flanges.
BX-type gaskets are a completely different design used exclusively with API 6BX flanges. They require a dedicated BX groove and are not interchangeable with R or RX grooves. You will find these on wellheads, trees, and other API 6A rated equipment at 5,000 PSI and above.
| Gasket Type | Cross-Section | Groove Type | Standard | Typical Application |
| R (Oval) | Oval | R-type | ASME B16.20 | General high-pressure, legacy installations |
| R (Octagonal) | Octagonal | R-type | ASME B16.20 | Preferred for new construction, better sealing |
| RX | Modified octagonal | R-type (interchangeable) | API 6A | Pressure cycling, thermal cycling, wellhead |
| BX | Square/rectangular | BX-type (dedicated) | API 6A / 6BX | Wellheads, trees, 5,000+ PSI rated equipment |
When to Specify RTJ Over Raised Face
The short answer: when pressure, temperature, or safety classification demands it. Here is the more useful breakdown.
Pressure class 900# and above. ASME B16.5 allows raised face connections at 900# and higher, but most engineering specs default to RTJ at these classes. The metal-to-metal seal handles the higher bolt loads and flange stresses better than gasket-dependent connections. At 1500# and 2500#, RTJ is essentially universal.
Sour service and NACE MR0175/ISO 15156 applications. When hydrogen sulfide is present, the integrity requirements go up significantly. RTJ connections with properly specified gasket materials (typically soft iron, low-carbon steel, or nickel alloys depending on the environment) are preferred because they eliminate the risk of gasket blowout under corrosive attack.
High-temperature service above 750F. Spiral wound and sheet gaskets have temperature limitations that metallic ring gaskets do not. For applications in the 750F to 1,100F range (think FCC units, reformers, high-temperature steam), RTJ connections maintain seal integrity where non-metallic gasket elements would degrade.
API 6A wellhead and tree equipment. If you are specifying anything per API 6A, RTJ (or BX) is not optional. It is the required connection type. This covers wellheads, Christmas trees, chokes, actuators, and related pressure-containing equipment from 2,000 to 20,000 PSI working pressure.
Critical service per ASME B31.3. For Category M fluid service (lethal substances), many owner specifications require RTJ connections regardless of pressure class. The reasoning is simple: the metal-to-metal seal provides an additional layer of safety that non-metallic gaskets cannot guarantee over the life of the joint.
Specifying RTJ Flanges: What to Include
When you are putting together a purchase order or material requisition for RTJ flanges, here is what needs to be on the line item to avoid back-and-forth:
Size and pressure class. Standard format: 4″ 900# RTJ WN A105. Do not leave out the facing designation. If someone writes “4 inch 900 class weld neck” without specifying RF or RTJ, that is a guaranteed back-and-forth.
Flange type. Weld Neck (WN), Blind (BL), and Slip-On (SO) are all available in RTJ configurations per ASME B16.5. Weld Neck is the most common because the integral hub provides better stress distribution at high pressures.
Material grade. Carbon steel (A105, A350 LF2), stainless (SA-182 F304/F304L, F316/F316L), chrome-moly (F11, F22, F91), and nickel alloys (625, 825) are all available. Match the gasket material compatibility to your service conditions.
Ring gasket number. Every size and class combination has a specific ring number per ASME B16.20. A 4″ 900# RTJ flange uses an R-45 ring gasket. This number must match between mating flanges, and the gasket is ordered separately.
Applicable standard. ASME B16.5 for NPS 1/2″ through 24″, ASME B16.47 for NPS 26″ through 60″, or API 6A for wellhead equipment. A B16.5 RTJ groove and an API 6A BX groove are not the same thing.
Common Mistakes with RTJ Flanges
A few things that trip people up, and they are worth knowing before you order:
Reusing ring gaskets is probably the most frequent error. RTJ gaskets are single-use. The controlled deformation that creates the seal means the gasket conforms to the groove surfaces during initial makeup. Reinstalling the same gasket will not give you the same seal. Always use a new ring per ASME PCC-1 guidelines.
Mixing R-type and BX grooves happens more often than anyone wants to admit, especially on projects where both ASME and API equipment are in the same system. An R-type gasket in a BX groove (or vice versa) will not seal. Verify the groove type on the flange marking before installing gaskets.
Incorrect bolt torque on RTJ joints is another common issue. Because the gasket is metallic, bolt torque sequences and values are more critical than with soft gaskets. ASME PCC-1 provides detailed torque procedures for flanged joints, and following a proper star pattern with multiple passes is not optional on RTJ connections.
The Bottom Line
RTJ flanges are the standard for high-pressure, high-temperature, and critical service piping. They cost more than raised face flanges, and the gaskets are more expensive than spiral wounds, but the seal integrity justifies the investment in any application where failure is not acceptable. If you are specifying 900# and above, working with API 6A equipment, or dealing with sour service, RTJ is the way to go.
Need RTJ flanges quoted? Texas Flange carries RTJ configurations across all standard sizes, pressure classes, and material grades. Get in touch with our team and we will have pricing and availability back to you fast.
Texas Flange & Fitting Supply | 281-484-8325 | texasflange.com
