Frangible Roof / Compression Ring Calculator

Small internal pressure design: roof-to-shell compression area, maximum design pressure, and frangible margin (API 650 Appendix F)

◉ API 650 APPENDIX F STYLE CHECK · NOT A REGULATORY SUBMISSION TOOL

⚠ Indicative tool only. Formulas checked directly against the standard's page images. Applies when internal pressure exceeds the weight of the roof plates but not the weight of the shell, roof, and attached framing (§F.1.2) — anchors for pressure are not required in this regime. If pressure exceeds the shell/roof/framing weight, §F.7 (anchored tanks) applies instead, which this tool doesn't cover. This tool also doesn't cover §F.7 or the field pressure test procedure (§F.4.4, purely procedural, no calculation).

Identification

Company

Tank ID

Location

Record Type

Research entries stay private to you for practice/testing. Switch to Official once this is a real design your company should see.

Design Basis

Unit System

D — Nominal Tank Diameter m

θ — Roof-to-Horizontal Angle degrees

F_y — Lowest Min. Yield Strength, Junction MPa

D_LR — Nominal Roof + Structural Weight N

D_LS — Nominal Shell + Framing Weight (not roof) N

M_w — Wind Moment N-m

Participating Width Helpers (Figure F-2, informational)

Helps size the compression ring area A from its actual geometry: A ≈ w_c×t + w_h×t_h (+ the area of any angle/bar member) — see Figure F-2 details a–k for the exact shape.

R_c — Inside Radius of Tank Shell mm

t — Shell/Member Thickness at Joint mm

t_h — Roof Plate Thickness mm

Result

R₂ = R_c/sinθ—

w_c — max participating shell width = 0.6√(R_ct)—

w_h — max participating roof width = 0.3√(R₂t_h), capped—

Maximum Design Pressure From Compression Area (§F.4.1)

A — Actual Compression Area (nominal, less CA) mm²

Result

Maximum design pressure P—

Required Compression Area From Target Pressure (§F.5.1)

P_i — Target Design Internal Pressure kPa

For self-supporting roofs, §F.5.2 additionally requires A be not less than the roof's own cross-sectional area per §5.10.5/5.10.6 (see the Roof Design calculator).

Result

Required compression area A—

Maximum Pressure Limited by Uplift (§F.4.2)

Result

P_max — uplift-limited maximum design pressure—

Your design pressure vs P_max—

Calculated Failure Pressure & Frangible Margin (§F.6, §F.4.3)

P_f is expressed in terms of the design pressure permitted by §F.4.1 (the P computed above from your compression area). The suggested frangible-joint margin is P_max ≤ 0.8P_f.

Result Not evaluated

P_f — calculated minimum failure pressure—

0.8 × P_f—

P_max vs 0.8P_f margin—

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