What is the API 650 1-Foot Method for tank shell design?

The 1-Foot Method (API 650 Section 5.6.3.1) calculates shell thickness at a point one foot above the bottom of each shell course. This simplified approach uses the formula td = 4.6 D(H-1)G / (Sd E) + CA, where D is tank diameter, H is design liquid level above the calculation point, G is specific gravity, Sd is allowable stress, E is joint efficiency, and CA is corrosion allowance. It is the most commonly used method for tanks up to 200 feet in diameter.

What is the difference between the 1-Foot Method and Variable-Design-Point Method?

The Variable-Design-Point Method (API 650 Section 5.6.3.3) calculates shell thickness at a variable point that depends on plate geometry rather than a fixed 1-foot location. It typically yields thinner upper courses compared to the 1-Foot Method, resulting in material savings on large tanks. The 1-Foot Method is simpler and more conservative; the VDP method requires iterative calculation but is more economical for large-diameter tanks.

What are the minimum shell thicknesses per API 650?

API 650 Table 5.6.1.1 specifies minimum shell plate thicknesses based on tank diameter: 3/16 inch (4.8 mm) for tanks up to 50 ft diameter, 1/4 inch (6.4 mm) for 50-120 ft diameter, and 5/16 inch (7.9 mm) for tanks over 120 ft diameter. These minimums apply regardless of the calculated design or hydrostatic test thickness.

What materials are commonly used for API 650 tank shells?

Common API 650 shell materials include ASTM A36 (Sd = 23,200 psi), A283 Grade C (Sd = 21,300 psi), A516 Grade 60 (Sd = 22,700 psi), A516 Grade 70 (Sd = 25,400 psi), and A573 Grade 70 (Sd = 25,400 psi). A516-70 is the most widely used for new construction due to its good strength, weldability, and notch toughness.

API 650 Tank Wall Thickness Calculator

Tank Geometry

Tank Nominal Diameter

Tank Shell Height

Number of Shell Courses

Shell Course Height

Standard: 8 ft (96" plate width). Range: 4-10 ft.

Design Parameters

Design Specific Gravity of Liquid

Water = 1.0, Crude oil = 0.82-0.95, Brine = 1.05-1.20

Design Liquid Level

Default = tank height (worst case). May be less for overflow protection.

Corrosion Allowance

Typical: 1/16" (0.0625). Sour service: 1/8" (0.125).

Material & Welding

Shell Plate Material

Allowable Design Stress (Sd)

psi

Auto-filled from material selection. May override manually.

Joint Efficiency (E)

0.85 = single-welded butt (typical), 1.0 = double-welded butt with RT

Design Method & Test

Shell Thickness Design Method

Hydrostatic Test Specific Gravity

Typically 1.0 (water). Test fill to top of shell.

Understanding API 650 Shell Design

API 650 Scope
Covers design, fabrication, erection, and inspection of welded steel tanks for atmospheric storage of petroleum, chemicals, and water. Applies to tanks with internal pressures not exceeding the weight of the roof plates (approximately 1 oz/in2).

Shell Design Basis
Shell thickness is governed by hydrostatic pressure from the stored liquid. The bottom course sees the highest liquid head and is always the thickest. Upper courses progressively thin as the liquid head decreases.

When to Use Each Method:
The 1-Foot Method is simpler and more conservative -- used for most tanks up to 200 ft diameter. The Variable-Design-Point Method optimizes upper course thicknesses by calculating at a variable location instead of a fixed 1-foot point, saving material on large tanks.

Learn the Theory

Understand API 650 design basis, 1-Foot vs VDP methods, material selection, wind and seismic design, and construction practices

Read Engineering Guide →

Results

Bottom Course Thickness

inches (selected)

Top Course Thickness -

Total Shell Weight -

Shell Surface Area -

Tank Volume -

Design Method -

Governing Condition -

Bottom Plate Thickness -

Wind Girder Required -

Formula

td = 4.6 D(H-1)G / (Sd E) + CA

td = Design shell thickness (in)

D = Tank nominal diameter (ft)

H = Design liquid level above bottom of course (ft)

G = Design specific gravity

Sd = Allowable design stress (psi)

E = Joint efficiency

CA = Corrosion allowance (in)

Standards & References

API 650, 12th Edition
Welded Tanks for Oil Storage
API 650 Section 5.6.3.1
1-Foot Method for shell design
API 650 Section 5.6.3.3
Variable-Design-Point Method
API 650 Table 5.6.1.1
Minimum shell plate thickness
ASTM A516/A516M
Pressure vessel plates, carbon steel

Engineering Notes

Min thickness: 3/16" for D ≤ 50 ft, 1/4" for 50 < D ≤ 120 ft, 5/16" for D > 120 ft
Practical limit: Bottom course > 1.75" is impractical per API 650
Joint efficiency: E = 0.85 (single-welded butt), E = 1.0 (double-welded with full RT)
Wind girder: Required when H/D > 1 or per intermediate shell stiffening analysis
Course height: Standard 8 ft (96" mill plate). 10 ft plates available for large tanks.
Bottom plates: Typically 1/4" minimum + CA per API 650 Section 5.4

Quick Reference -- Material Properties

A36: Sd = 23,200 psi, St = 26,700 psi
A283-C: Sd = 21,300 psi, St = 24,500 psi
A516-60: Sd = 22,700 psi, St = 26,100 psi
A516-70: Sd = 25,400 psi, St = 29,200 psi
A573-70: Sd = 25,400 psi, St = 29,200 psi

API 650 Tank Wall Thickness Calculator

Tank Geometry

Design Parameters

Material & Welding

Design Method & Test

Understanding API 650 Shell Design

Learn the Theory

Results

Formula

Standards & References

Engineering Notes

Quick Reference -- Material Properties

Related Calculators

Tank Volume

Vessel Sizing

Hoop Stress

Corrosion Rate