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🏭 API 650 Tank Thickness

Calculate the minimum required shell thickness for welded steel storage tanks per API 650.

Inputs

Results

Design Thickness (t_d)
0.000 in
Hydrotest Thickness (t_t)
0.000 in
Required Minimum Thickness (Bottom Course)
0.000 in

(Max of td, tt, and nominal minimum based on diameter)

Tank Shell Profile

HD1-ft point

Calculator Description

The API 650 tank-thickness calculator finds the minimum required shell-plate thickness of an aboveground welded steel storage tank using the API 650 "1-Foot" method. Because hydrostatic pressure grows toward the bottom, the shell is built as a stepped stack of plates of increasing thickness with depth.

What this calculator finds

For the design inputs (tank diameter, liquid height, specific gravity, allowable stress, corrosion allowance) it computes the required operating thickness t_d and the hydrostatic-test thickness t_t.

Why it matters

  • Ensuring minimum shell thickness to prevent collapse/leakage while optimizing plate cost
  • Checking safety margin under hydrostatic test pressure (test thickness)
  • Designing for service life by including a corrosion allowance (CA)

Formula

API 650 1-Foot Method

It is an empirical relation that evaluates hydrostatic pressure at the point 1 foot (about 0.3 m) above the bottom of the course. Separate constants are given for US customary and SI units.

Design Thickness (t_d)

Imperial:β€…β€Šβ€…β€Štd=2.6 D (Hβˆ’1) GSd+CA\text{Imperial:}\;\; t_d = \dfrac{2.6\,D\,(H-1)\,G}{S_d} + CA
Metric:β€…β€Šβ€…β€Štd=4.9 D (Hβˆ’0.3) GSd+CA\text{Metric:}\;\; t_d = \dfrac{4.9\,D\,(H-0.3)\,G}{S_d} + CA

Hydrostatic Test Thickness (t_t)

Imperial:β€…β€Šβ€…β€Štt=2.6 D (Hβˆ’1)St\text{Imperial:}\;\; t_t = \dfrac{2.6\,D\,(H-1)}{S_t}
Metric:β€…β€Šβ€…β€Štt=4.9 D (Hβˆ’0.3)St\text{Metric:}\;\; t_t = \dfrac{4.9\,D\,(H-0.3)}{S_t}
  • D, H β€” Tank Diameter and Liquid Height [ft (Imperial) or m (Metric)]
  • G β€” Specific Gravity of liquid (water = 1.0)
  • S_d, S_t β€” Allowable Design and Test Stress [psi (Imperial) or MPa (Metric)]
  • CA β€” Corrosion Allowance (CA) [in (Imperial) or mm (Metric)]

How the formula works

  • Larger diameter D and liquid height H raise hydrostatic pressure, increasing required thickness proportionally.
  • A larger specific gravity G (heavier-than-water liquids) increases the thickness.
  • Higher allowable stress S_d, S_t permits thinner plates. The corrosion allowance CA is always added to the thickness.
  • Test thickness t_t ignores specific gravity and is based on the test water head (usually to the top of the roof course).

Worked example

For a tank with D = 40 m, H = 12 m, G = 1.0, design stress S_d = 165 MPa and CA = 1.5 mm, the bottom course (SI constant 4.9) gives t_d = 4.9 Γ— 40 Γ— (12 βˆ’ 0.3) Γ— 1.0 / 165 + 1.5 β‰ˆ 15.5 mm required.

Useful Tips

Practical tips

  • Compute thickness course by course from the bottom up, then round up to commercial plate thicknesses (e.g. 6, 8, 10 mm).
  • Corrosive services often use CA β‰₯ 3 mm, which adds directly to every course thickness.
  • Note the separate minimum-thickness and roof/floor rules in the code (typically β‰₯ 6 mm shell minimum).

Limitations & cautions

  • The 1-Foot method is an approximation for axisymmetric hydrostatic load on unanchored circular tanks; wind, seismic and vacuum designs are handled separately.
  • At low temperatures (toughness requirements) or special steels, allowable-stress selection needs care.
  • Never mix unit systems with constants β€” use the 2.6 form for US units and 4.9 for SI.