Steel Storage Tank Design — API 650 Atmospheric Tanks, API 620

Q: What is the 1-foot method for tank shell design?

The 1-foot method per API 650 calculates required shell thickness at a design point 1 foot above the bottom of each shell course. Formula: td = 2.6D(H-1)G/Sd + CA for design, tt = 2.6D(H-1)/St for hydrostatic test. The 1-foot rule reflects stress relief from the adjacent lower course.

Steel storage tanks store liquids at atmospheric or low pressures. This guide covers design of field-erected welded steel tanks per API 650 and API 620 standards.

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Tank Shell Design Methods

API 650 provides two methods for calculating shell thickness:

1-Foot Method: The traditional approach, applicable to all tank diameters. The design shell thickness td is calculated at a point 1 ft (0.3 m) above the bottom of each shell course:

td = 2.6 × D × (H - 1) × G / Sd + CA
tt = 2.6 × D × (H - 1) / St (hydrostatic test condition)
Where D = tank diameter (ft), H = liquid height (ft) from course bottom, G = specific gravity of stored liquid, Sd = allowable stress for design condition, St = allowable stress for hydrostatic test, CA = corrosion allowance

Variable-Point Method: More economical for tanks over 200 ft (61 m) diameter. The design point for each course is at a height x above the bottom of the course where x = min(1 ft, 0.045 × D × (H - 1)^0.5). This reduces the required thickness of upper courses by optimizing the design point location.

Minimum shell thickness requirements per API 650 Table 5.2:

Tanks ≤ 50 ft diameter: 3/16 in (4.8 mm) min
50 ft < D ≤ 120 ft: 1/4 in (6.4 mm) min
120 ft < D ≤ 200 ft: 5/16 in (7.9 mm) min
D > 200 ft: 3/8 in (9.5 mm) min plus additional requirements

Bottom Plate Design

The tank bottom consists of annular plates (under the shell) and remaining bottom plates (within the ring):

Annular bottom plates per API 650 5.4:

Minimum thickness: 1/4 inch (6 mm)
Radial width: minimum 24 inches (610 mm) beyond the tank shell
For tanks > 80 ft diameter or with corrosion allowance > 1/8 in, the annular plate must be thicker
Annular plates are butt-welded to each other and fillet-welded to the shell

Bottom plates: Rest on prepared foundation. Minimum 1/4 inch thick for tanks with non-corrosive service. Welded lap joints with 1-inch overlap minimum. The bottom plates are sloped 1:120 toward a sump for drainage.

Tank Appurtenances and Fittings

Nozzles: Per API 650 5.7, nozzles for tank inlet, outlet, and connections must be:

Minimum schedule 40 for threaded connections
Reinforced with pad plates when nozzle size exceeds the reinforcement limit
Located at least 6 inches above the bottom plate for bottom nozzles
Sized for maximum filling and emptying rates

Manways and roof openings: Per API 650 5.8, tanks over 10 ft diameter require at least one shell manway. Roof manways provide access for inspection. Reinforced roof openings for vents and gauging equipment.

Vents: Emergency venting per API 2000 for fire exposure and normal venting for filling/emptying rates. Vent sizing based on flow rate and pressure rating.

Wind girders: Required at the top of the tank shell per API 650 5.9 to prevent buckling under wind load. The intermediate wind girder (if required) is located at a distance from the top based on shell thickness.

Tank Erection and Testing

Field erection sequence:

Foundation preparation and anchor bolt installation
Bottom plate layout, welding, and leak testing
Shell plate rolling, positioning, and vertical welding
Roof structure assembly (self-supporting or column-supported)
Internal floating roof installation (if specified)
Hydrostatic test — fill to design liquid level with water, hold for 24 hours, inspect for leaks
Final connection of nozzles, manways, and appurtenances

Inspection requirements:

Visual inspection of all welds per API 650 8.2
Radiographic or ultrasonic testing of shell butt welds per API 650 8.3 (percentage depends on shell plate thickness)
Leak testing of bottom plates (vacuum box testing per API 650 8.5)
Hydrostatic test verifies shell integrity and foundation settlement

Corrosion Allowance Considerations

Per API 650 5.3.2, minimum corrosion allowance is typically 1/16 inch (1.6 mm) for standard service. For aggressive environments (sour crude, acidic products), increase to 1/8 inch (3.2 mm) or more. The corrosion allowance is added to the calculated shell thickness and applies to both inner and outer surfaces if both surfaces are exposed to corrosive environments. Internal coatings or cathodic protection can reduce the required corrosion allowance.

API 620 vs API 650

While this guide focuses on API 650 (atmospheric storage tanks), API 620 covers low-pressure storage tanks operating at pressures up to 15 psi (103 kPa). Key differences:

API 620 design: Uses the double-curvature membrane theory (shells of revolution) rather than the beam-theory approach of API 650. The shell thickness is governed by the maximum tensile stress in the meridional and latitudinal directions
Internal pressure: API 620 tanks can operate at low positive pressures (typically 0.5-15 psi), requiring additional design checks for the roof-to-shell junction and bottom-to-shell junction under internal pressure
Refrigerated tanks: API 620 covers refrigerated liquid storage (LNG, LPG) requiring specialized materials with low-temperature toughness per API 620 Appendix R or Q
Applications: API 620 tanks are used for pressurized liquid storage, refrigerated storage, and specialized chemical service. API 650 covers the vast majority of field-erected atmospheric storage tanks

The Steel Calculator's tank design tools currently implement API 650 methods. For API 620 design, refer to the full standard or engage a specialist tank design engineer.

Material Selection for Storage Tanks

Tank shell plates must meet ASTM material specifications with adequate toughness for the design metal temperature:

Common materials per API 650 Table 4.1:

ASTM A36 (Fy = 36 ksi): General purpose, limited to tanks with shell plates ≤ 1 inch thick. Not recommended for low-temperature service
ASTM A573 Grade 70 (Fy = 42 ksi): Improved notch toughness, commonly used for tanks in moderate climates
ASTM A516 Grade 70 (Fy = 38 ksi): Pressure vessel quality steel, excellent toughness. Required for tanks storing refrigerated products and for tanks in cold climates with minimum design temperatures below 40°F (4°C)
ASTM A537 Class 1 (Fy = 50 ksi): Heat-treated, high-strength, for large-diameter tanks requiring thinner shell plates

Impact testing requirements per API 650 4.2.3: Full Charpy V-notch (CVN) impact testing is required for group IV and V materials (A516, A537) and for any material used at design metal temperature below 40°F (4°C). The minimum CVN energy is 20 ft·lb (27 J) for longitudinal specimens.

Frequently Asked Questions

What is the API 650 tank design method? Per API 650 13th Edition: (1) The 1-foot method calculates shell thickness at design points 1 ft above each shell course: td = 2.6D(H-1)G/Sd + CA for design condition, tt = 2.6D(H-1)/St for hydrostatic test, (2) Minimum shell thickness per API 650 Table 5.2 — 3/16 inch (4.8 mm) for tanks ≤ 50 ft diameter, (3) Annular bottom plates — minimum 1/4 inch (6 mm) thick, extend minimum 24 inches (610 mm) beyond tank shell, (4) Wind girders required at the top of the shell per API 650 5.9, (5) Seismic design per API 650 Appendix E with sloshing wave height calculation.

How are tank foundations designed? Per API 650 Section 7 and Appendix B: (1) Ringwall foundation — reinforced concrete ring under the tank shell with compacted granular fill inside, (2) Tank bottom on compacted sand or crushed stone with slope of 1:120 for drainage, (3) Annular plate ring under the shell — minimum 1/4 inch (6 mm) thick, bears on the ringwall, (4) Soil bearing pressure — typically limited to 2000-4000 psf (96-192 kN/m²) for controlled fill, (5) Settlement limits — total settlement 12-24 inches (300-600 mm), differential settlement 1/2 inch (12 mm) in 30 ft (9 m). Cathodic protection system embedded in the ringwall.

What wind and seismic loads apply to storage tanks? Per ASCE 7-22 and API 650 Appendices E and F: (1) Wind — overturning moment on empty tank governs, computed using GCp per ASCE 7-22 Figure 27.3-1 and projected area, (2) Seismic design basis earthquake (DBE) — two mass model: impulsive (rigid shell + bottom portion of contents) and convective (sloshing upper portion of contents), (3) Sloshing wave height — δmax = 0.5D × AI × S × Ts² per API 650 E.6.1, freeboard must accommodate slosh height, (4) Overturning — checked at the shell-to-bottom junction per API 650 E.5.2, (5) Anchor bolts — required if seismic overturning exceeds tank stability.

What is the 1-foot method for tank shell design? The 1-foot method per API 650 calculates the required shell thickness at a design point located 1 foot above the bottom of each shell course. The formula is td = 2.6D(H-1)G/Sd + CA for design and tt = 2.6D(H-1)/St for hydrostatic test. The 1-foot rule reflects the fact that the stress at the bottom of a course is partially resisted by the adjacent lower course. It is the default method for tanks of all diameters per API 650.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice. All results must be independently verified by a licensed Professional Engineer.