Ring Joint (RTJ) Gasket Dimensions — ASME B16.20 Type R (R11 to R45)

ASME B16.20 Type R ring-joint (RTJ) gasket dimensions by ring number, R11 through R45: average pitch diameter P, ring width A, oval ring height B, octagonal ring height H and the flat width on the octagonal ring C, all in millimetres. RTJ gaskets are identified by R-number, not pipe size — the same NPS has a different R-number at each pressure class — and the oval and octagonal cross-sections share pitch diameter and width but differ in height. Values cross-checked across two independent reference tables — Wermac and piping-world agree on every value for every ring number — with an inch table (Flange Bolt Chart) confirming by conversion.

ASME B16.20 ring joint (RTJ) gasket dimensions — Type R (R11 to R45)

A ring-joint (RTJ) gasket is a solid metal ring that seats in the machined groove of two RTJ flange faces; as the joint is bolted up the ring deforms into the groove and forms a metal-to-metal seal, which is why RTJ joints are used on high-pressure and high-temperature service. ASME B16.20 identifies each ring by a ring number (R-number), not by pipe size: one R-number can serve more than one flange, and a given flange size has a different R-number at each pressure class, so the ring is ordered by its R-number and cross-section (oval or octagonal), not by NPS alone. The Type R ring is made in two cross-sections — oval and octagonal — that share the same pitch diameter and width for a given R-number but differ in height. The values below are ASME B16.20 Type R, cross-checked across two independent reference tables.

P is the average pitch diameter of the ring (the diameter at which the ring contacts the flange groove); A is the ring width; B is the height of the oval ring; H is the height of the octagonal ring; C is the width of the flat on the octagonal ring. ASME B16.20 tolerances are P ±0.18 mm, A ±0.20 mm, C ±0.20 mm and ring height +1.3 / −0.5 mm. The two reference tables (Wermac and piping-world, both in millimetres) agree on every value for every ring number.

Reading the dimensions

• Ring number (R-number) — the ASME B16.20 designation that fixes all of the ring dimensions. The R-number is matched to the flange groove, so a flange is ordered with the same R-number on the flange and the gasket. The same R-number can apply to more than one flange size and class.
• Pitch diameter (P) — the diameter at which the ring seats against the angled walls of the flange groove. It must match the flange groove pitch diameter and is the same for the oval and the octagonal ring of a given R-number.
• Ring width (A) — the radial width of the ring cross-section, the same for the oval and octagonal forms of a given R-number.
• Oval height (B) and octagonal height (H) — the axial height of the ring. The oval ring is the older cross-section and seats on curved contact faces; the octagonal ring is the modern cross-section and seats on flat angled faces. The octagonal ring is generally preferred because its flat seating faces give a more defined contact, but the two are interchangeable in the same groove at the same R-number.
• Octagonal flat (C) — the width of the flat face on the angled sides of the octagonal ring, where it contacts the groove wall.

Ring material is selected for the service and is softer than the flange so the ring, not the groove, takes the deformation — soft iron, low-carbon steel, 304 / 316 stainless and other alloys are common, each marked with its material code. Material does not change the dimensions above. Confirm the ring material, hardness and the oval-or-octagonal cross-section against the service and the flange groove before ordering.

Selecting and matching

A ring-joint gasket matches a flange only when the R-number agrees, so confirm the R-number on the flange and order the same R-number ring. Because the R-number — not the pipe size — sets the dimensions, do not select the ring by NPS alone: the same NPS has a different R-number at each pressure class. Match the ring cross-section (oval or octagonal) to the groove form, use a ring softer than the flange, and bolt the joint up evenly so the ring seats into the groove and forms the metal-to-metal seal. A ring-joint gasket is a single solid ring and is not interchangeable with a flat or spiral wound gasket.

Dimensions are nominal ASME B16.20 Type R values and provided for identification and cross-reference. Confirm against the controlling standard and the specific gasket maker’s current data sheet before manufacture or procurement.

Reference compiled for identification and cross-reference. Dimensions are nominal; confirm against the controlling ASME B16.20 ASME B16.20 revision and the specific component data sheets before manufacture or procurement. All trademarks are the property of their respective owners.