Equipment Design

API 650 Tank Wall Thickness

Design atmospheric storage tank shell courses per API 650 using the 1-Foot Method and Variable-Design-Point Method. Covers material selection, hydrostatic testing, wind girder analysis, and construction practices for welded steel tanks.

Launch Calculator 1-Foot Method

Shell design

Hydrostatic Load

Shell thickness governed by liquid head pressure at each course elevation.

Design methods

1-Foot / VDP

Two API 650 methods: conservative 1-Foot and optimized Variable-Design-Point.

Standard

API 650, 12th Ed.

Welded Tanks for Oil Storage -- the definitive standard for atmospheric tank design.

Contents

Use this guide when you need to:

  • Design shell courses for new atmospheric storage tanks
  • Select between 1-Foot and Variable-Design-Point methods
  • Choose appropriate shell plate materials
  • Evaluate wind girder and seismic requirements
  • Understand hydrostatic test requirements

1. API 650 Overview

API Standard 650, "Welded Tanks for Oil Storage," is the definitive industry standard for the design, fabrication, erection, and inspection of vertical, cylindrical, aboveground, closed- and open-top, welded carbon steel storage tanks. It applies to tanks operating at atmospheric pressure (internal gas pressure not exceeding the weight of the roof plates).

Scope and Applicability

Tank types

Atmospheric Storage

Fixed-roof and floating-roof tanks for petroleum, chemicals, and water. Internal pressure limited to approximately 1 oz/in2.

Size range

15 ft to 400 ft Diameter

From small field tanks (100-500 bbl) to large terminal tanks (500,000+ bbl). No upper size limit in API 650.

Design temperature

Ambient to 500°F

Standard carbon steel materials. Appendix M covers elevated temperature service; Appendix R covers refrigerated tanks.

Foundation

Ringwall or Slab

Appendix B covers foundation design. Ringwall foundations are most common for large tanks.

Shell Design Philosophy

The tank shell resists hydrostatic pressure from the stored liquid. Like a thin-walled pressure vessel, the circumferential (hoop) stress in the shell is:

Hydrostatic Hoop Stress: σ = P × r / t Where: P = Hydrostatic pressure = γ × h = (62.4 × G × h) / 144 psi r = Tank radius (in) t = Shell thickness (in) G = Specific gravity of stored liquid h = Liquid head above point of interest (ft) Rearranging for thickness: t = P × r / (σallow × E) t = 4.6 × D × H × G / (Sd × E) The constant 4.6 comes from unit conversions: 4.6 = 62.4 / (144 × 2) × 12 = combining lb/ft3, in2/ft2, diameter-to-radius, and ft-to-in conversions.

The bottom shell course experiences the highest liquid head and is always the thickest. Each successive upper course sees less liquid head and is progressively thinner. This stepped-thickness design is characteristic of large welded tanks.

Key concept: Shell thickness is determined by two conditions: (1) the design condition with the specified liquid at the design liquid level, and (2) the hydrostatic test condition with water filled to the top of the shell. The required thickness is the maximum of these two conditions, subject to minimum thickness requirements.

2. 1-Foot Method (Section 5.6.3.1)

The 1-Foot Method is the most commonly used approach for calculating shell course thicknesses. It calculates the required thickness at a point one foot above the bottom of each shell course, which provides a reasonable approximation of the average stress in the lower portion of each course.

Design Shell Thickness

API 650 Equation 5.6.3.1-1 -- Design Condition: td = 4.6 × D × (H - 1) × G / (Sd × E) + CA Where: td = Design shell thickness (in) D = Nominal tank diameter (ft) H = Design liquid level measured from bottom of the course under consideration (ft) G = Design specific gravity of stored liquid Sd = Allowable design stress (psi) per Table 5-2a E = Weld joint efficiency CA = Corrosion allowance (in) The "(H - 1)" term means the thickness is calculated at a point 1 foot above the bottom of the course, where H is measured from the bottom of the specific course being designed to the maximum design liquid surface.

Hydrostatic Test Thickness

API 650 Equation 5.6.3.1-2 -- Hydrostatic Test: tt = 4.6 × D × (H - 1) / (St × E) Where: tt = Hydrostatic test shell thickness (in) H = Height from bottom of course to top of shell (ft) (tank filled with water to overflow) St = Allowable test stress (psi) per Table 5-2a Key differences from design condition: 1. G = 1.0 (water) -- no specific gravity factor 2. H = distance to TOP of shell (tank filled completely) 3. St is higher than Sd (short-duration test load) 4. No corrosion allowance (tank is new at time of test)

Required Thickness

The required thickness for each course is the maximum of three values:

  1. Design thickness (td): From the design liquid level equation with corrosion allowance
  2. Hydrostatic test thickness (tt): From the water-fill test equation without corrosion allowance
  3. Minimum thickness: Per API 650 Table 5.6.1.1 based on tank diameter

Minimum Shell Thickness (Table 5.6.1.1)

Nominal Tank Diameter Minimum Thickness
≤ 50 ft (15 m) 3/16" (4.8 mm)
50-120 ft (15-36 m) 1/4" (6.4 mm)
> 120 ft (36 m) 5/16" (7.9 mm)

Worked Example: 1-Foot Method

Given: Tank diameter: D = 100 ft Tank height: 48 ft (6 courses x 8 ft) Stored liquid: Water (G = 1.0) Material: A516-70 (Sd = 25,400 psi, St = 29,200 psi) Joint efficiency: E = 0.85 Corrosion allowance: CA = 1/16" = 0.0625" Course 1 (bottom): H = 48 ft (liquid level above bottom of course 1) td = 4.6 x 100 x (48 - 1) x 1.0 / (25,400 x 0.85) + 0.0625 td = 21,620 / 21,590 + 0.0625 = 1.001 + 0.0625 = 1.064" tt = 4.6 x 100 x (48 - 1) / (29,200 x 0.85) = 21,620 / 24,820 = 0.871" Required = max(1.064, 0.871, 0.25) = 1.064" Selected: 1-1/8" (1.125") Course 3: H = 48 - 16 = 32 ft (liquid level above bottom of course 3) td = 4.6 x 100 x (32 - 1) x 1.0 / (25,400 x 0.85) + 0.0625 td = 14,260 / 21,590 + 0.0625 = 0.660 + 0.0625 = 0.723" tt = 4.6 x 100 x (32 - 1) / (29,200 x 0.85) = 14,260 / 24,820 = 0.574" Required = max(0.723, 0.574, 0.25) = 0.723" Selected: 3/4" (0.750") Course 6 (top): H = 48 - 40 = 8 ft td = 4.6 x 100 x (8 - 1) x 1.0 / (25,400 x 0.85) + 0.0625 td = 3,220 / 21,590 + 0.0625 = 0.149 + 0.0625 = 0.212" tt = 4.6 x 100 x (8 - 1) / (29,200 x 0.85) = 3,220 / 24,820 = 0.130" Required = max(0.212, 0.130, 0.25) = 0.25" (minimum controls) Selected: 1/4" (0.250")
Design tip: For most midstream tanks storing water, crude oil, or produced water, the design condition with corrosion allowance governs. The hydrostatic test condition governs when (a) the test stress St is not much higher than Sd, or (b) the design liquid level is significantly below the top of the shell.

3. Variable-Design-Point Method (Section 5.6.3.3)

The Variable-Design-Point (VDP) Method provides a more refined and often more economical design by calculating the shell thickness at a variable location that depends on the actual plate geometry and the thickness relationship between adjacent courses.

Concept

The 1-Foot Method calculates stress at a fixed point (1 foot above the bottom weld), which is conservative because it does not account for the restraining effect of the thicker lower course on the thinner upper course. The VDP Method recognizes that:

  • A thick lower course restrains deformation at the bottom of the thinner upper course
  • The maximum stress in the upper course actually occurs at some distance above the junction
  • This "variable design point" moves upward as the thickness ratio between courses increases

VDP Calculation

Variable Design Point Location: x = 0.61 × √(r × tlower) + 0.32 × C × h Where: x = Distance above bottom of course to design point (in) r = Tank radius (in) tlower = Thickness of adjacent lower course (in) C = Factor depending on thickness ratio (0 to 1) h = Course height (in) The factor C is determined from the ratio of upper to lower course thickness. When tupper/tlower = 1, C = 1 and the design point is at the same location as the 1-Foot Method. As the ratio decreases (thinner upper course), C decreases and the design point moves lower. Shell thickness at VDP: td = 4.6 × D × (H - x/12) × G / (Sd × E) + CA Where (H - x/12) replaces (H - 1) from the 1-Foot Method.

When to Use VDP

Factor 1-Foot Method VDP Method
Complexity Simple hand calculation Iterative, computer-aided
Result Conservative (thicker) Optimized (thinner uppers)
Bottom course Same for both methods Same as 1-Foot
Best for Tanks ≤ 200 ft diameter Large tanks > 150 ft
Material savings Baseline 5-15% shell weight reduction
Industry usage Most common Terminal/refinery tanks
Important: The VDP method always yields the same bottom course thickness as the 1-Foot Method. The savings come from upper courses where the restraining effect of the thicker lower course allows thinner plates. For small tanks, the savings may be marginal compared to the engineering effort.

4. Material Selection

API 650 Table 5-2a lists approved materials and their allowable stresses. Material selection depends on service temperature, corrosion environment, thickness requirements, weldability, and cost.

Common Shell Materials

Material Sd (psi) St (psi) Min Yield (psi) Notes
ASTM A36 23,200 26,700 36,000 Most economical; good for small tanks
ASTM A283 Gr. C 21,300 24,500 30,000 Lower strength; used for non-critical service
ASTM A516 Gr. 60 22,700 26,100 32,000 Improved notch toughness; cold service
ASTM A516 Gr. 70 25,400 29,200 38,000 Most popular for new tanks; best strength/cost
ASTM A573 Gr. 70 25,400 29,200 38,000 Similar to A516-70; structural quality

Material Selection Guidelines

  • A516 Gr. 70: Default choice for most new construction. Good weldability, adequate notch toughness, and highest allowable stress among common grades.
  • A36: Cost-effective for small field tanks where thickness is governed by minimums rather than stress calculations.
  • A516 Gr. 60: Preferred for low-temperature service (down to -20 deg F) due to superior notch toughness when normalized.
  • Impact testing: Required when design metal temperature falls below certain limits per API 650 Section 5.2.3. Check Charpy V-notch requirements for the selected material and plate thickness.

Weld Joint Efficiency

Joint Efficiency Values: E = 1.0 Double-welded butt joints with full radiographic examination (RT) per API 650 Section 8.1 E = 0.85 Single-welded butt joints with backing strip, or double-welded butt joints without full RT (most common for field-erected tanks) E = 0.70 Spot radiography only Note: Using E = 1.0 requires 100% radiographic examination of all vertical and horizontal shell welds. This adds inspection cost but can reduce plate thickness by 15%, which saves material on large tanks. Break-even analysis: RT cost savings typically justify E = 1.0 when bottom course thickness exceeds 1/2" (approximately D > 80 ft with water storage).
Cost optimization: For large tanks, selecting A516-70 with E = 1.0 (full RT) often provides the most economical design despite the higher inspection cost. The material savings from thinner plates, reduced welding time, and lower transportation weight typically outweigh the radiographic examination cost.

5. Wind & Seismic Design

Empty or partially filled tanks must resist wind and seismic overturning forces. API 650 provides methods for evaluating stability and designing stiffening members.

Wind Design (Section 5.9.6)

Wind loads can cause buckling of the thin upper shell courses and overturning of empty tanks. Two types of wind stiffening are addressed:

Top wind girder

Top Stiffener Ring

Required on all open-top tanks and most fixed-roof tanks to prevent shell ovaling from wind pressure. Size calculated from shell diameter and design wind speed.

Intermediate wind girder

Shell Stiffener

Required when the maximum unstiffened shell height exceeds limits based on transformed shell analysis. Prevents wind buckling of tall, thin shells.

Transformed Shell Method (Simplified): The maximum height of unstiffened shell (H1) is determined by transforming each course thickness to an equivalent height of the thinnest course: Wtr = Wactual × (tuniform / tactual)5/3 If the total transformed height exceeds the critical buckling height, an intermediate wind girder is needed. Rule of thumb: Wind girder analysis is typically required when H/D > 1 or when top courses are at minimum thickness on tanks exceeding 60 ft diameter.

Seismic Design (Appendix E)

API 650 Appendix E provides a comprehensive seismic design procedure that addresses:

  • Impulsive component: The lower portion of liquid moves with the tank shell (short-period response)
  • Convective component: The upper portion of liquid sloshes independently (long-period response)
  • Overturning moment: Combined effect determines anchorage requirements
  • Freeboard: Required clearance above liquid level to prevent sloshing overflow
  • Anchorage: Anchor bolt or self-anchored design depending on overturning stability
Seismic zones: Tanks in high seismic zones (Seismic Use Group II or III) require more detailed analysis per Appendix E. The spectral acceleration parameters SS and S1 from ASCE 7 determine the design seismic loads. Always check local building code requirements, which may exceed API 650 minimums.

6. Construction Practices

API 650 covers construction requirements including plate preparation, welding, erection methods, tolerances, and testing. Key considerations that affect the design engineer:

Erection Methods

Conventional

Bottom-Up Erection

Courses erected from bottom to top using cranes. Traditional method for all tank sizes. Bottom course set first on foundation.

Jacking

Top-Down (Jack-Up)

Top course and roof assembled first at ground level. Each subsequent course inserted underneath and jacked up. Preferred for large tanks -- minimizes work at height.

Shell Tolerances

Parameter Tolerance
Shell roundness (radius variation) ±1/2" on radius for D ≤ 30 ft; ±3/4" for 30-60 ft; ±1" for > 60 ft
Shell plumbness 1/200 of total height (H/200)
Local shell deviation (peaking/banding) Horizontal: ±1/2" per 10 ft; Vertical: ±1/4" per foot
Bottom plate levelness ±1/2" across bottom, uniform slope allowed

Hydrostatic Testing

Every new tank must be hydrostatically tested per API 650 Section 8.5. The test consists of filling the tank with water to the top of the shell and holding for a specified period while inspecting for leaks. Key points:

  • Test medium: Water (SG = 1.0) unless otherwise specified
  • Fill level: To the top of the shell (maximum liquid head on each course)
  • Duration: Minimum 24 hours at maximum fill level
  • Inspection: Visual examination of all shell welds, bottom welds, and shell-to-bottom junction for leaks
  • Settlement: Foundation settlement is measured during and after test to verify uniform support
  • Temperature: Water and ambient temperature must be above 40 deg F to prevent brittle fracture concerns

Bottom Plates

Bottom Plate Requirements (Section 5.4): Minimum thickness: 1/4" (6 mm) excluding corrosion allowance Annular plates (under shell): Thickness depends on shell thickness and hydrostatic test stress. Table 5-1a provides minimum annular plate thickness. Rule of thumb: - Shell thickness ≤ 3/4": Annular plate = same as bottom - Shell thickness 3/4" to 1-1/2": Annular plate = 1/4" thicker - Shell thickness > 1-1/2": Annular plate per Table 5-1a Annular plate width: Minimum 24" radial from shell inside (allows proper support and weld inspection).
Corrosion monitoring: The bottom-side corrosion of the tank bottom is often the life-limiting factor, not the shell. Install cathodic protection, consider tank linings for corrosive service, and plan for periodic bottom inspections per API 653 (Tank Inspection, Repair, Alteration, and Reconstruction).

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