Geometric length vs. pressure design
**Length basis:** Choose the **Length calculation basis** control at the top of the calculator first (GB/T 9125, generic axial stack, EN, or ASME context)—the tips row and annex helpers follow that selection so descriptions stay on one standard at a time. This page sums axial stack-up items—flange thicknesses (and deviations), gasket thickness, washer stacks, nut height, thread-end allowance, and length tolerance—using branch patterns aligned with the informative annex of the GB/T 9125 fastener standards for common pipe-flange joints when you work in the GB mode: **GB/T 9125.1—2020** (PN series) and **GB/T 9125.2—2020** (Class series), which partially replace **GB/T 9125—2010**. **9125.1 / 9125.2—2020** publish length guidance in **informative Annex B** (formulas B.1…, tables **B.1** ring-joint gap **S**, **B.2** chamfer **z**, **B.3** length tolerance). The older **GB/T 9125—2010** placed the same role under **Annex A**—clause and table IDs do not line up 1:1. You must enter C, raised-face height F, RTJ dimensions E and S, tongue-and-groove heights, lap-joint thicknesses, and related items from the flange product standard or vendor data that governs your purchase. For RTJ metal-ring joints you may set gasket thickness to zero when your drawing shows no soft packing. The commercial length suggestion rounds up to a 5 mm coarse purchasing series. Optional mass rows approximate stud/bolt steel, nuts, washers, and manually entered flange masses; automated catalog masses by standard row are still on the roadmap.
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):** annex helpers can fill excerpts for ring-joint **S** (B.1—partial grid), chamfer **z** (B.2), and **Δl** half-band from Table B.3 versus calculated **L**—always verify missing combinations against your PDF. **ΔC** and **Δl** remain lumped shortcuts for standard primes such as **C′**, **l′**, **l₁′**, **f′₁**, **t′₂** where you do not split fields.
- A pressure class (PN in EN/GB-style tables or Class in ASME-style tables) selects one full row of dimensions: flange thickness C, bolt-circle diameter K, hole count n, and hole diameter must all belong to that same row for the chosen nominal size. Do not combine thickness from one class with bolting from another, and do not mix GB dimension tables with ASME tables on the same joint.
- The joint-pattern dropdown must match the actual facing pair (e.g. raised face with studs, ring-joint with studs, lap joint). Using a formula branch that does not match the installed flanges invalidates the length.
- This tool does not compute pressure–temperature ratings, required gasket seating stress, tightening torque sequences, or stud/bolt property class and material—those come from piping design codes (for example GB/T 20801), company procedures, and qualified engineering review, especially for high temperature or high pressure.
- Weight screening uses simplified geometry; until standard-pack datasets land, flange kilograms remain user-supplied.
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.
Shareable URL
This tool does not encode inputs in the URL yet; bookmark the localized page path if you need to return. All calculations execute locally in your browser.
Frequently asked questions
- Does the calculator pick PN or Class for me?
- No. You must align thickness C and bolting with one rating row from your governing standard. The tool only stacks the numbers you enter.
- Is this enough for high temperature or high pressure service?
- Length screening alone is never sufficient. You still need code-compliant material selection, bolting grades, gasket rules, and torque or tensioning procedures from your project specifications.
- Why might my result differ from the workshop drawing?
- Different tolerance conventions, extra washers, revised gasket thickness, or revised C after rerating will change L. Always reconcile with the issued drawing and BOM.
- Are calculations sent to a server?
- No. The math runs entirely in your browser session.
Extended copy on this page (headings, assumptions, references, FAQs) may be drafted or localized with AI assistance; engineering judgment and governing codes still apply. Numerical models run locally in your browser as implemented. For contract-critical work, cite primary standards and qualified review.