References & further reading

• GB/T 9125.1—2020 Steel pipe flange connecting fasteners — Part 1: PN series (partially replaces GB/T 9125—2010); GB/T 9125.2—2020 — Part 2: Class series (partially replaces GB/T 9125—2010). Use informative annex clauses for bolt/stud length stacking guidance in force for your job.
• Historic consolidated edition: GB/T 9125—2010 (superseded in part by the 9125.1/9125.2—2020 split); clause and annex numbering are not identical—always open the PDF you legally hold.
• GB/T 9124 (series) Steel pipe flanges — PN-designated dimensions; obtain C and bolting data from the executed standard edition.
• GB/T 20801 (series) Pressure piping — Industrial metallic piping — overall design scope; use for bolting and materials coordination where applicable.
• ASME B16.5 / B16.47 when Class-rated flanges are executed—keep the flange family consistent end-to-end.
• Listing standards is for traceability and scope boundaries; it is not an endorsement or substitute for licensed review.

Model assumptions

• Contract / governing standard: Projects not governed primarily by Chinese GB flange specs (overseas jobs, ASME/EN/IEC contracts): dimensions, acceptance criteria, and bolting rules follow your project specification, applicable codes, and purchase contract—the GB/T 9125 annex excerpts and in-page helper fills are supplementary only.
• Annex mapping: GB/T 9125.1—2020 and GB/T 9125.2—2020 use informative Annex B for fastener length; GB/T 9125—2010 used Annex A. PN and Class live in different 2020 volumes—formula items and subscripts (e.g. tongue/groove, lap) are not interchangeable without checking the B.x formula in your edition.
• Tables B.1–B.3 (2020): With an annex volume selected, Calculate auto-fills z from B.2 and Δl half-band from B.3 using the computed L; RTJ joints also need DN/pressure for S (B.1 — partial grid). Enter manually when lookup misses. ΔC and Δl remain lumped shortcuts for standard primes such as C′, l′, l₁′ where you do not split fields.
• Engine & disclaimer: Calculator engine v2.0.0. C, F, E, S, f₁, f₂ must match your governing flange standard or purchase order; output is geometric screening only—not code compliance or supplier certification. PN/Class must stay on one standard dimension row; no P–T rating or bolt grade selection.
• Wafer vs non-wafer: Wafer-style means equal flange thickness on both sides (typical wafer valve); non-wafer uses separate C₁/C₂. Mid insert Tₘ is not flange C.
• Fastener weight (optional): From computed Lₛ and joint pattern, approximates n sets of bolt/stud steel, nuts, and washers—excludes flanges, gaskets, and valves. Multiple nuts or washers per side (jam nuts, double washers) scale by per-side count; mass uses m × count and t_w × count. No flange catalog mass.
• GB/T 911x—2010 metallurgical lookup: Platform data/published read model via API; 9124/9125—2020 + HG20592 is a separate roadmap—do not mix DN/PN lines. Lookup covers pipe flange pairs only (non-wafer), no Tₘ; pick sealing face when multiple exist. Apply row refreshes L, empirical mass (kg/100 mm + q), and M×L procurement spec. Gasket T and minimum T₁ are separate; bolts +T₁, studs +2T₁.

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