External Pressure (API 650 Annex V)

External Pressure / Vacuum (API 650 Annex V)

Design of a flat-bottom tank for uniform external pressure (vacuum) to API 650 Annex V: the roof under the total roof pressure, the roof-to-shell region, the shell by the transformed-shell method for wind plus external pressure and for external pressure alone, the number, spacing and size of intermediate stiffeners, the top and bottom end stiffeners, their attachment welds and the bottom under external pressure and flooding

New here? Read the User Manual — the two shell conditions, the transformed shell, placing stiffeners, worked examples and how to read the results.

◉ API 650 14TH ED. (AUGUST 2025) ANNEX V · V.7 ROOF · V.8 SHELL · V.9 BOTTOM · NOT A REGULATORY SUBMISSION TOOL

⚠ Indicative tool only. Follows API 650, 14th edition (August 2025), Annex V, for design external pressures over 0.25 kPa (1 in. of water) and up to 6.9 kPa (1 lbf/in²), checked against that text and its V.10 example. The units also choose the rule set: SI and US customary each use the printed equations of that system. Annex V does not apply to aluminum tanks. Supported cone roof plates (V.7.1) are designed by agreement and are a record item here.

Identification

Company

Tank ID

Location

Record Type

Units and rule set

Switching converts what is typed in and the rule set (the printed SI or US customary equations). Saved records stay in SI.

Summary of checks

Enter the tank, its shell courses and the design external pressure.

1. Tank and Shell

Nominal diameter, D m

Shell F_y at the maximum operating temperature MPa

Modulus of elasticity, E MPa

Material

Shell courses from the bottom (course 1) up. Annex V numbers them from the top; the tool does that for you.

Deduct corrosion allowance (V.5, Purchaser)

Shell corrosion allowance mm

Roof corrosion allowance mm

Bottom corrosion allowance mm

2. Loads (V.3, 5.2.2)

Specified design external pressure, P_e kPa · excludes wind

External pressure combination factor, F_pe at least 0.4

Design wind speed, V (3-s gust) km/h

or P_WV entered kPa · overrides V

Roof live load, L_r kPa · at least 1.0

Balanced snow load, S kPa

Roof dead load, DL kPa · blank = roof plate

3. Fixed Roof (V.7)

Roof type

Cone angle from horizontal, θ degrees

Dish radius, R m

Roof plate thickness, nominal mm

Roof F_y MPa · blank = shell

Allowable tensile stress, f MPa · blank = 0.6 F_y

4. Shell Buckling (V.8.1)

5. Stiffeners (V.8.2)

Sections are on the outside of the shell. An angle has its horizontal leg against the shell and its vertical leg at the outer end. Or enter the section’s area, its own moment of inertia and its centroid from the shell mid-plane.

Intermediate stiffener positions m from the top of the shell · comma-separated · blank = equal transformed spacing

Stiffener F_y MPa · blank = shell

Allowable compressive stress, f_c MPa · blank = V.3.1

6. Bottom (V.9)

Bottom (annular) plate under the shell, t_b mm

Product required to remain when P_e acts m

Unit weight of the liquid inside, G_in kg/m³

Flood depth outside, H_out m · blank = none

Unit weight of the flood liquid, G_out kg/m³ · blank = 1000

Liquid kept inside during flooding, H_in m

7. Design Record (Annex V)

Tick each item once it is confirmed. Items that don’t apply are hidden.

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