FieldEngineersKitEngineering Reference & ASME Code Basis
1. Core Formula & Variable Definitions
ASME B16.20 (Metallic Gaskets for Pipe Flanges) · ASME B16.5 (Flange Facings & Bolt Circle Matching) · ASME Section VIII, Div 1, Appendix 2 (Gasket Factors m & y) · ASME PCC-1 (Flange Joint Assembly Guidelines)
Wm1 = π4G²P + 2bπGmP · Wm2 = πbGy
ASME B16.20 Metallic Gasket Dimensions & ASME VIII-1 App. 2 Seating Rules
ASME B16.20 governs metallic and semi-metallic gaskets for ASME B16.5 and B16.47 flanged joints, including spiral-wound gaskets (SWG) with centering/inner rings and Ring-Type Joint (RTJ) metallic rings. Gasket seating design follows ASME Section VIII Division 1 Appendix 2 / ASME PCC-1. The gasket maintenance factor m ensures sealing under internal operating pressure P, while yield factor y defines the initial minimum compressive seating stress.
- OD_g / ID_g (Sealing Element Dimensions) — Outside and inside diameter of the active spiral-wound metal/filler sealing package (mm or in).
- OD_outer (Centering (Outer) Ring OD) — Outer guide ring diameter sized to fit inside flange bolts for concentric alignment (mm or in).
- ID_inner (Internal (Inner) Ring ID) — Solid metal inner ring bore preventing radial inward buckling into the pipe flow stream (mm or in).
- G (Gasket Pitch Reaction Diameter) — Mean diameter of gasket contact face per ASME VIII-1 App. 2 (mm or in).
- b (Effective Gasket Width) — Effective seating width calculated from basic width b0 (mm or in).
- m (Gasket Maintenance Factor) — Multiplier factor representing residual compressive stress required to hold process pressure P.
- y (Gasket Seating Stress) — Minimum initial compressive stress required to yield filler and conform to flange face serrations (MPa or psi).
2. Allowances, Tolerances & Standards
ASME B16.20 sets strict manufacturing tolerances on active winding thickness, centering ring diameters, and groove machining to ensure gasket containment and prevent catastrophic blowout.
Standard spiral-wound sealing elements are manufactured at 4.5 mm (0.177 in) uncompressed thickness. Under target bolt load, windings compress to the thickness of the 3.2 mm (0.125 in) solid steel outer guide ring.
ASME B16.20 mandates solid internal rings (Style CGI) on all Class 900+ flanges, all NPS 24+ Class 150–600 flanges, and all PTFE-filled spiral-wound gaskets to eliminate inward radial buckling into the pipe bore.
Centering ring outer diameter is machined to fit snugly inside the flange bolt circle (PCD minus bolt hole diameter) with a ±0.8 mm tolerance, ensuring automatic centering during vertical/horizontal makeup.
Per ASME B16.20, metallic RTJ rings must have lower hardness than the mating flange groove (Soft Iron max 90 HRB / 56 HBW, 316 SS max 160 HBW) so the ring deforms plastically into the groove without gouging the flange facing.
Quick Reference Lookup Table
| NPS | Inner ring OD (mm) | Sealing OD (mm) | Outer ring OD (mm) | ID (mm) |
|---|---|---|---|---|
| 2" | 60.3 | 92.1 | 104.8 | 52.4 |
| 4" | 87.0 | 117.5 | 190.5 | 78.0 |
| 6" | 130.2 | 167.0 | 241.3 | 116.8 |
| 8" | 161.9 | 215.9 | 298.5 | 146.1 |
| 10" | 215.9 | 266.7 | 362.0 | 193.7 |
| 12" | 254.0 | 320.7 | 431.8 | 242.9 |
NPS 4 Class 150 matches this app (outer ring 190.5 mm = Class 150 bolt circle). Other rows are B16.20 screening; confirm winding ID with the gasket OEM.
3. Material & Code Limitations
Gasket selection depends on process fluid corrosiveness, operating temperature limits, and cyclic thermal shock. Gasket windings and fillers have distinct upper temperature thresholds.
| Material Group | Temperature Range | Allowable Stress / Limit | Engineering Notes |
|---|---|---|---|
| 316L SS Winding + Flexible Graphite Filler | -200 °C to 450 °C in air (650 °C in steam/inert) | Seating Stress y = 69.0 MPa (10,000 psi), m = 3.0 | Workhorse gasket for hydrocarbons, steam, and general process utilities. Graphite oxidizes in air above 450 °C. |
| 316L SS Winding + Expanded PTFE Filler | -200 °C to 260 °C (-320 °F to 500 °F) | Seating Stress y = 17.2 MPa (2,500 psi), m = 2.0 | Universal chemical resistance for strong acids, caustics, and high-purity media. Subject to creep/relaxation above 200 °C; inner ring strictly required. |
| Inconel 625 / Monel 400 Winding | -200 °C to 650 °C (Inconel 625) | Seating Stress y = 69.0 MPa, m = 3.0 | Severe sour gas (NACE MR0175), hydrofluoric acid (Monel), and seawater cooling piping. |
| RTJ Soft Iron / 316L Octagonal Rings | -196 °C to 538 °C (per metal grade) | Seating Stress y = 124.1 MPa (18,000 psi), m = 5.5 ~ 6.0 | High-pressure Class 600–2500 services, offshore wellheads, and lethal gas pipelines. |
Code Applicability & Safety Boundaries
- Strict Reuse Prohibition: Spiral-wound and metallic RTJ gaskets undergo irreversible plastic deformation during initial bolt makeup. Reusing a compressed gasket is a primary cause of joint leaks and is strictly prohibited by ASME PCC-1.
- Style CG vs Style CGI: On high-pressure lines (Class ≥ 300) and vacuum systems, Style CG (outer ring only) must not be substituted for Style CGI (inner + outer ring) due to inward radial collapse risks.
- Flange Facing Finish Matching: Spiral-wound gaskets require a serrated concentric or phonographic spiral finish (Ra 3.2 ~ 6.3 µm / 125 ~ 250 µin). Smooth finishes (< 1.6 µm) allow graphite filler extrusion under compression.
4. Step-by-Step Worked Example
Field VerificationSelect the ASME B16.20 spiral-wound gasket dimensions and calculate the minimum required bolt seating load (Wm2) and operating bolt load (Wm1) for an NPS 4 (DN 100) Class 300 flange operating at 3.0 MPa (30.0 bar) design pressure using 316L/Graphite filler (m = 3.0, y = 69.0 MPa).
Extract ASME B16.20 Standard Gasket Dimensions
Outer ring OD fits inside the 190.5 mm PCD bolt envelope; inner ring matches Sch 40 pipe bore.
Calculate Basic and Effective Gasket Width (b)
Effective width accounts for contact pressure distribution across the compressed winding spiral.
Calculate Gasket Pitch Reaction Diameter (G)
Mean reaction line where the total compressive force acts against hydrostatic fluid end-thrust.
Calculate Minimum Initial Bolt Seating Load (Wm2)
Total minimum tensile load across all 8 bolts to seat the graphite filler into the flange face serrations.
Calculate Minimum Operating Bolt Load (Wm1) & Per-Bolt Target
Seating load governs over operating pressure for low/medium pressure joints.