ASME B16.20 Spiral Wound & RTJ Gasket Dimensions and Sealing Stress Guide

Wrong gasket OD on an RF flange, a missing inner ring on a spiral-wound (SWG) order, or an RTJ ring number that does not match the groove class turns a “standard” B16.5 joint into a hydrotest leak or a crushed sealing element. ASME B16.20 gasket dimensions fix the geometry; ASME PCC-1 / VIII-1 Appendix 2 $m$ and $y$ factors bound how hard you must seat that geometry.

Quick Summary (TL;DR)

• Takeaway: Size SWG / RTJ from ASME B16.20 (NPS × class), then check seating with $y$ and operating stress with $m\cdot P$ before locking bolt load.
• Governing relation: Contact area $A_g = \pi/4\,(OD_g^2 - ID_g^2)$; seating floor $\ge \max(y,\sigma_{g,\min})$.
• Code / basis: ASME B16.20 · ASME B16.5 facing · ASME PCC-1 App. O screening · ASME VIII-1 App. 2 $m$, $y$.

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ASME B16.20 gasket dimensions in the field

Piping purchase specs often say “B16.20 spiral wound, graphite fill, 304 SS windings, inner and outer rings” without listing the four diameters the warehouse needs. Fitters then guess from the raised-face OD or reuse a Class 150 outer ring on a Class 300 joint. Outer-ring OD (OR_OD) tracks the raised-face / centering envelope for that NPS × class; sealing-element OD (SE_OD) and gasket ID set the contact band.

Look up the FEK screening row for any listed NPS / class in the Gasket Dimension & Selection calculator (public slug gasket-dimension-selection). Pair geometry with bolt preload using Flange Gasket Stress & Sealing and Bolt Torque & Tensioning.

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Core formulas & parameter definitions

Spiral-wound contact geometry

For an annular sealing element (and for average gasket-stress screening when $OD_g$ / $ID_g$ are the contact diameters):

$$A_g = \frac{\pi}{4}\left(OD_g^{2} - ID_g^{2}\right)$$

FEK spiral-wound lookup reports four B16.20 screening diameters:

Hero string format matches the calculator: ID × SE_OD × OR_OD.

RTJ pitch and section

Ring-type joints are identified by ring number, pitch diameter $P$, and section width × height ($w \times h$). Octagonal rings are the FEK default style for standard R-number grooves; oval rings use the same pitch family but a different section profile — always match ring style to the groove stamped on the flange.

Seating and operating stress (VIII-1 App. 2 / PCC-1 screen)

Appendix 2 gasket factors for a flexible-graphite-filled spiral wound (FEK spiral_wound_filled):

$$m = 3.0,\qquad y = 10{,}000\ \mathrm{psi}\ (69\ \mathrm{MPa})$$

Operating maintenance check (simplified):

$$\sigma_{g,\mathrm{op}} \ge m\cdot P$$

Installation / seating floor used in FEK gasket-stress screening:

$$\sigma_{\mathrm{seat}} = \max\!\left(y,\ \sigma_{g,\min}\right)$$

For filled SWG, FEK uses $\sigma_{g,\min} = 5{,}000$ psi and a PCC-1-style crush ceiling $\sigma_{g,\max} \approx 25{,}000$ psi. Target assembly gasket stress is therefore planned above $y$ and below $\sigma_{g,\max}$, not at an arbitrary “internet” 30 ksi ceiling.

Parameter definitions

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Worked example: NPS 6 Class 300 spiral-wound gasket dimensions

Scenario: ASME B16.5 NPS 6″ Class 300 RF flange pair with an ASME B16.20 spiral-wound gasket (flexible graphite filler; stainless inner / outer rings as specified on the PO).

Inputs

Step 1 — Read the B16.20 screening row (FEK catalog)

From gasketDimension.json / Gasket Dimension & Selection for NPS 6 · Class 300:

Calculator hero (metric): ID × SE_OD × OR_OD ≈ 98.400 × 149.400 × 266.700 mm.

Step 2 — Map seating stress to bolt target

Using FEK Flange Gasket Stress factors for filled SWG:

Plan assembly so average gasket stress sits ≥ 10 ksi and ≪ 25 ksi, then convert that stress into stud preload / torque with Bolt Torque & Tensioning and a star-pattern sequence.

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RTJ gasket dimensions: Octagonal rings by class (NPS 4)

Long-tail trap: crews reuse R-37 on every NPS 4 RTJ flange. In the FEK B16.20 screening table, ring number changes with class:

So RTJ gasket dimensions — Octagonal vs Oval is first a groove / ring-number match, then a section style match. Oval rings are not interchangeable with octagonal grooves of the same nominal size.

Reproduce Class 300 in the calculator: NPS 4, class 300, type RTJ → hero Ring No. R-37 with pitch 123.800 mm.

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Interactive tool CTA

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Frequently Asked Questions (FAQ)

Q1. When is a spiral wound gasket inner ring required under ASME B16.20 practice?

B16.20 / project PMS language typically requires an inner ring when crush control, vacuum, or high-class service needs a solid bore stop. FieldEngineersKit always exposes IR_OD and ID on spiral-wound rows so you can verify the ring is present on the ticket. If the PO says “inner + outer” and the box shows only a centering ring, stop — do not assemble.

Q2. How do minimum gasket seating stress and target bolt load relate under ASME PCC-1?

Minimum seating stress starts at Appendix 2 $y$ (10 ksi for FEK filled SWG). PCC-1-style assembly targets raise average gasket stress above $y$ while staying under the material crush band (FEK $\sigma_{g,\max}$ ≈ 25 ksi for filled SWG). Convert the chosen $\sigma_g$ into stud stress / torque with the bolt-torque tool; do not treat “25–30 ksi on every SWG” as a universal code number — FEK’s filled-SWG ceiling is 25 ksi.

Q3. Why do RTJ gasket dimensions differ for Octagonal vs Oval ring joints?

Pitch diameter can share a family, but section geometry and groove machining differ. FEK lists Octagonal (and RX where tabulated) with explicit $w\times h$. An oval ring in an octagonal groove (or the reverse) will not seat correctly even if the ring number “sounds right.”

Q4. Why don’t some internet ASME B16.20 SWG seating stress / $Y$ factor charts match FEK?

Many blogs mix VIII-1 App. 2 $y$, OEM preferred assembly stress, and PCC-1 Appendix O bands into one column labeled “Y factor.” FieldEngineersKit keeps them separate: $y = 10$ ksi, $\sigma_{g,\min} = 5$ ksi, seating floor $\max(y,\sigma_{g,\min}) = 10$ ksi, crush $\sigma_{g,\max} = 25$ ksi for filled SWG. Always reconcile the OEM sheet for exotic fillers or PTFE-faced windings.

Q5. Why does NPS 4 Class 600 use R-38 instead of R-37?

Because ASME B16.20 / B16.5 groove class — not NPS alone — sets the ring number. In the FEK catalog, NPS 4 Class 300 = R-37, Class 600 = R-38, Class 900 = R-39. Confirm with Gasket Dimension & Selection before issuing the RTJ purchase order.