API 650 - Tank Design & Calculations

API 650 establishes the requirements for the design and construction of welded aboveground storage tanks used in the petroleum and process industries. A proper design involves gathering accurate input data, defining process conditions, and applying mechanical calculations for shell, bottom, and roof components. The code specifies limits on pressure, temperature, and materials, while also addressing seismic, wind, and corrosion considerations. This page outlines the complete design workflow, supported by formulas and an example calculation based on API 650 API 650 provisions.

Table of Contents

  1. Design Basis and Inputs
  2. Tank Geometry and Capacity Selection
  3. Materials and Allowable Stresses
  4. Shell Design and Thickness Calculations
  5. Roof and Bottom Plate Design
  6. Foundation and Settlement Considerations
  7. Wind, Seismic, and Environmental Loads
  8. Welding, Fabrication, and Joint Efficiency
  9. Inspection, Testing, and Documentation
  10. Worked Example of API 650 Tank Design

1. Design Basis and Inputs

The first step in API 650 tank design is establishing the design basis and collecting all required inputs.This document consolidates all necessary inputs that define the operational, environmental, and material requirements, forming the foundational reference for all subsequent calculations and ensuring code compliance.

Key inputs include:

  • Stored Product Data:The fluid's properties are the primary design driver. This includes the fluid type, density (for hydrostatic loading), specific gravity, and operating temperature range. Crucially, the fluid's vapor pressure at the maximum storage temperature determines its suitability for atmospheric storage and informs the selection of the roof type (fixed, floating) and venting requirements. The corrosivity dictates the material of construction and the corrosion allowance.
  • Operating Conditions: This defines the tank's pressure envelope, including the maximum design liquid level, and any small internal pressure (typically up to 2.5 psig) or partial vacuum it must withstand. The design temperature sets the allowable stress values for the selected steel material
  • Tank Capacity: The required storage volume, including working capacity and necessary freeboard, is specified by the client's process requirements. This directly influences the initial sizing of the tank diameter and height, which are optimized based on cost and plot space.
  • Environmental Conditions: Site-specific data is essential for assessing external loads. This includes basic wind speed for wind girder design, seismic parameters for earthquake load analysis, and snow load for roof design. Ambient temperature ranges also influence material selection.
  • Material of Construction: The selection of steel grade (e.g., A36, A573) is based on strength, toughness, and weldability requirements. Any need for internal linings or coatings for corrosion protection or product purity is also defined here.
  • Regulatory or Project Standards: Any additional requirements from local regulations, client specifications, or project specifications that govern aspects like inspection, testing, or documentation are consolidated in this section.

Collecting these inputs accurately is critical as they form the foundation for subsequent mechanical calculations, shell and roof design, foundation considerations, and load assessments.

2. Tank Geometry and Capacity Selection

After defining the design basis, the next step is determining the tank geometry and selecting the appropriate storage capacity. The tank’s diameter, height, and volume must satisfy process requirements while remaining practical for construction and inspection.

Key considerations include:

  • Tank Diameter and Height: The selection is an economic and structural optimization. A larger diameter reduces shell height and hydrostatic stress, often leading to thinner shell courses, but requires a larger foundation and plot area. A taller tank with a smaller footprint may be used where space is constrained, but this increases hydrostatic stress, potentially requiring thicker plates in lower courses. The height-to-diameter (H/D) ratio is a key metric for stability.

  • Tank Volume: The total volume is calculated from the required working capacity plus freeboard.The freeboard is a critical safety margin, which is the extra height above maximum liquid level to prevent overflow during filling, agitation, or wind-induced waves.Typically 3–6% of liquid height.

    Calculate the working volume using the formula:
    V = π × (D/2)2 × Hworking, where D is tank diameter and Hworking is liquid height below freeboard.

  • Course Layout: The shell is designed as a series of horizontal courses. The layout must use standard plate dimensions to minimize scrap. Course heights are set so that the required thickness decreases from bottom to top, following the hydrostatic pressure profile. This often results in a "stair-stepped" thickness diagram.
  • Access and Manways: Tank geometry must accommodate manways, ladders, and platforms for inspection and maintenance without interfering with structural integrity.y.

Proper geometry selection ensures the tank meets operational needs, maintains structural stability, and allows practical fabrication and maintenance.

3. Materials and Allowable Stresses

The selection of materials is governed by the need to safely contain the product while withstanding operational and environmental loads. API 650 specifies approved materials and their allowable stresses to ensure structural integrity.

Key considerations include:

  • Material Selection and Allowable Stress: The choice of steel grade (e.g., A516 Gr. 60, A573 Gr. 70) is directly linked to the design temperature and required strength. The allowable stress for each material, obtained from API 650 tables, is the maximum permissible stress under design conditions and is the fundamental value used in all thickness calculations.
  • Corrosion Allowance: An additional thickness is added to the calculated shell and bottom plate thicknesses to account for uniform corrosion over the tank's design life. This value is based on the stored product's corrosivity and is typically specified by the client or based on historical data.
  • Lining or Coating: For corrosive services where a corrosion allowance alone is impractical, internal linings (e.g., epoxy, phenolic) or coatings are specified. Their selection is critical and depends on full compatibility with the stored product and operating temperature to ensure long-term protection.
  • Weldability and Fabrication: The selected material must be readily weldable and formable into cylindrical courses. The chemical composition and mechanical properties of the plate must be suitable for the rolling, welding, and post-weld heat treatment processes to maintain structural integrity.

Accurately defining materials and allowable stresses ensures the tank can safely contain the product throughout its intended service life while complying with API 650 limits.

4. Shell Design and Thickness Calculations

The shell is designed to resist the primary load of the stored liquid's hydrostatic pressure. API 650 provides specific methods for calculating the required thickness at every elevation, ensuring a safe and code-compliant design.

Key considerations include:

  • Design Method Selection: API 650 allows for the One-Foot Method or Variable Design-Point Method to calculate shell thickness.

    The One-Foot Method calculates the thickness for each shell course based on the hydrostatic head at a point 1 ft (0.3 m) above the course's bottom.

    The Variable-Design Point Method is a more refined approach that determines the exact point of maximum stress in the lower shell courses, often resulting in a more optimized and thinner shell for large-diameter tanks.

  • Hydrostatic Pressure & Thickness Calculation: The internal liquid pressure at a given elevation is calculated using P = ρ × g × h, where ρ is fluid density, g is gravity, and h is liquid height above the point.

    • Shell design ensures the cylindrical wall can withstand the hydrostatic pressure of the stored liquid. The fundamental mechanical principle is based on the thin-walled cylinder formula: t = (P × R) / (S E - 0.6P), where P is the hydrostatic pressure, R is the tank radius, S is the allowable stress, and E is the joint efficiency.

    API 650 adapts this principle into its standard calculation by substituting for pressure (P = ρ g h) and consolidating the constants for water (ρ = 1000 kg/m³). This yields the code's specific equation for the required thickness at a given elevation: t = (4.9 D (H - 0.3) G) / (Sd E) + CA , where D is the tank diameter, H is the liquid height above the course, G is the specific gravity, and CA is the corrosion allowance. The result from either method is rounded up to the nearest commercial plate thickness.
  • Corrosion Allowance: Add the specified corrosion allowance to the calculated thickness. Round up to the nearest available commercial plate thickness.
  • Course Layout: Divide the shell into courses of standard plate widths while maintaining uniform stress distribution. Consider welding sequence and longitudinal seam efficiency.

Proper shell design ensures structural integrity under all operational and environmental conditions while maintaining compliance with API 650 minimum requirements.

5. Roof and Bottom Plate Design

Roof and bottom design ensures the tank is sealed, structurally stable, and capable of handling internal pressures, external loads, and environmental factors. API 650 defines requirements for fixed, cone, and floating roofs as well as bottom plate thickness and reinforcement.

Key considerations include:

  • Roof Types: API 650 covers Cone Roofs and Dome Roofs for fixed roof service, and External Floating Roofs (EFR) and Internal Floating Roofs (IFR) for volatile liquids. The design of fixed roofs must account for the specified internal pressure (typically up to 2.5 psig) and partial vacuum, which directly influences the roof plate thickness and supporting structure.
  • Roof Structural Design:The self-supporting cone or dome roof thickness is calculated per API 650 formulas based on the tank diameter and live loads. For supported cone roofs, the roof plates are designed for specified live loads, while the rafters or supports are sized separately to carry the total load (dead load + live load) over their span.
  • Bottom Plate Configuration: API 650 mandates the use of a Annular Bottom Ring in welded tanks subject to product hydrostatic pressure. The required thickness of this annular ring is calculated based on the product's specific gravity and the yield strength of the material, and it is independent of the corrosion allowance. The remainder of the bottom (sketch plates) has a specified minimum nominal thickness.
  • Venting and Sealing: Fixed roof tanks require Pressure-Vacuum Vents sized per API STD 2000 to handle breathing and emergency relief. Floating roofs require rim seals, deck fittings, and roof drains designed and specified per API 650 appendices to ensure safe operation.

Accurate roof and bottom design ensures the tank maintains integrity under operating conditions while providing safe storage and ease of maintenance.

6. Foundation and Settlement Considerations

A properly designed foundation ensures that the tank is supported uniformly and remains stable under operational and environmental loads. API 650 provides guidelines for bearing pressure, settlement limits, and foundation type selection.

Key considerations include:

  • Foundation Type: The selection between a concrete ringwall and a full reinforced concrete raft is based on soil bearing capacity and load distribution. A ringwall supports the heavily loaded shell region, while a full raft is used for poor soils to distribute the entire tank load.
  • Bearing Pressure and Geotechnical Investigation: The foundation design must be based on a geotechnical report. The maximum bearing pressure under the tank shell, including product hydrotest weight, must not exceed the allowable soil bearing capacity.
  • Settlement Limits: API 650 defines strict limits for uniform and differential settlement. Excessive differential settlement can distort the shell, leading to buckling or failure of bottom-to-shell welds. The foundation must be designed to keep settlements within these code-specified tolerances.
  • Anchor Design:For tanks requiring anchorage per wind or seismic calculations, the foundation must be designed to resist the specified uplift forces. This includes designing the anchor chairs, bolts, and the embedded portion to transfer these tensile loads into the foundation.
  • Subgrade and Bottom Support: The subgrade must be properly compacted and leveled. A sand or asphalt foundation pad is typically required to provide a uniform, corrosion-inhibiting support surface for the tank bottom plates, preventing localized stress points.

Ensuring correct foundation design minimizes the risk of structural deformation, stress concentrations, and operational issues during the tank’s service life.

7. Wind, Seismic, and Environmental Loads

API 650 provides specific methodologies to ensure tank stability and structural integrity under environmental loads. These loads are analyzed independently and in combination with operational loads to determine governing design conditions.

Key considerations include:

  • Wind Loads: Wind pressure is calculated per ASCE/SEI 7, using basic wind speed, exposure category, and tank geometry. This analysis checks for:

      • - Shell Stability: The calculated wind pressure is used to determine the need for intermediate wind girders to prevent shell buckling.

      - Uplift and Anchorage: The roof and upper shell are checked for wind uplift. If the tank is not self-anchoring, a detailed calculation for anchor bolts and chairs is performed.

  • Seismic Loads: API 650 employs a site-specific, multi-mode analysis. THe standard outlines methods to calculate base shear and overturning moments for anchorage design.
  • Snow and Ice Loads: The roof design must account for the specified snow load, combined with a minimum live load, as these contribute to the downward force on the roof structure and supporting elements.
  • Load Combinations The tank is evaluated under critical combinations of loads, such as hydrotest weight plus wind, or operating weight plus seismic, to identify the most severe stresses on the shell, foundation, and anchorage.

Accurate calculation of environmental loads ensures that the tank structure can withstand extreme conditions without compromising safety or operational performance.

8. Welding, Fabrication, and Joint Efficiency

API 650 mandates specific welding, fabrication, and inspection criteria to ensure the structural integrity of the completed tank. Adherence to these procedures is critical for achieving the design strength assumed in the shell and roof calculations.

Key considerations include:

  • Joint Efficiency (E): This factor accounts for the quality of the longitudinal welds in the shell courses. Its value, determined by the extent of radiographic examination (e.g., spot, partial, or full), directly reduces the allowable stress used in the shell thickness formula (t ∝ 1/E). A higher joint efficiency permits the use of thinner plates.
  • Welding Procedure Specification (WPS) All welding must be performed under a qualified WPS, supported by Procedure Qualification Records (PQRs). This ensures the weldment's mechanical properties, such as strength and toughness, are consistent and meet the code requirements for the specified materials.
  • Shell Assembly and Welding Sequence: The fabrication sequence is critical to control distortion and residual stresses. This involves rolling plates into courses, welding vertical (longitudinal) seams first, and then stacking and joining courses with horizontal (circumferential) seams using a specified sequence to maintain roundness.
  • Post-Weld Heat Treatment (PWHT) PWHT may be required for specific steel grades or thicknesses to relieve residual stresses from welding, thereby improving toughness and reducing the risk of brittle fracture or stress corrosion cracking.

Adhering to proper welding and fabrication practices ensures the tank can safely withstand operational loads, environmental effects, and long-term service stresses.

9. Inspection, Testing, and Documentation

API 650 mandates a rigorous program of inspection and testing to verify fabrication quality and structural integrity before the tank is placed into service. Comprehensive documentation provides a permanent record for the owner.

Key considerations include:

  • Hydrostatic Testing: This is the final proof test. The tank is filled with water to the design liquid level and held for a specified time. The primary acceptance criteria is the absence of leaks. The test also serves to settle the foundation and verify the tank's general stability under full load.
  • Non-Destructive Examination (NDE): API 650 requires mandatory NDE at specific points. This typically includes visual inspection of all welds, radiographic testing (RT) of shell longitudinal and circumferential welds, and magnetic particle (MP) or liquid penetrant (PT) testing of critical welds like the shell-to-bottom joint.
  • Material Certificates: Mill test reports (MTRs) must be provided for all pressure-retaining plates and structural components, verifying chemical composition and mechanical properties comply with the specified material grade.
  • As-Built Documentation: The final "Data Book" includes the as-built drawings, material certifications, weld maps, NDE reports, and hydrotest report. This package is essential for future integrity management and any potential fitness-for-service assessments..
  • Documentation of Deviations: Any deviation from design, including repair or modification, should be recorded and evaluated to ensure continued compliance with API 650 standards.

Thorough inspection, testing, and documentation confirm that the tank meets design criteria, ensuring safe operation and long-term reliability.

10. Worked Example of API 650 Tank Design Calculations

Consider designing a cylindrical aboveground storage tank for water with the following specifications: Capacity = 5000 m³, Design temperature = 40°C, Design pressure = atmospheric, Steel grade = ASTM A36, Corrosion allowance = 2 mm, Site wind speed = 133 km/h, Seismic zone = low.

Step 1: Tank Geometry

Select a tank diameter (D) of 25 m. Working height (Hworking) is calculated from required volume:
V = π × (D/2)2 × Hworking
Hworking = 5000 / (3.1416 × 12.52) ≈ 10.2 m
Add freeboard = 0.5 m → Total shell height Htotal ≈ 10.7 m

Step 2: Shell Thickness Calculation

Hydrostatic pressure at maximum height: P = ρ × g × Hworking
ρ = 1000 kg/m³ (water), g = 9.81 m/s², Hworking = 10.2 m
P = 1000 × 9.81 × 10.2 ≈ 100,062 Pa ≈ 0.100 MPa

API 650 shell thickness formula:
t = (P × R) / (S × E - 0.6 × P)
Where:

  • R = tank radius = 12.5 m
  • S = allowable stress for ASTM A36 at 40°C = 138 MPa
  • E = weld joint efficiency = 1.0
  • P = hydrostatic pressure = 0.100 MPa

t = (0.100 × 12.5) / (138 × 1.0 - 0.6 × 0.100) ≈ 1.25 / 137.94 ≈ 0.00906 m ≈ 9.1 mm Add corrosion allowance = 2 mm → t = 11.1 mm Choose commercial plate thickness = 12 mm

Step 3: Bottom Plate Thickness

Minimum bottom thickness:
tbottom = √(3 × ρ × g × H × R² / (4 × S × E)) (per API 650 simplified)
tbottom = √(3 × 1000 × 9.81 × 10.2 × 12.5² / (4 × 138 × 10⁶ × 1))
tbottom ≈ √(4.7 × 10⁶ / 552 × 10⁶) ≈ √0.00852 ≈ 0.0923 m ≈ 9.2 cm Add corrosion allowance = 0.2 cm → 9.4 cm Choose commercial thickness = 95 mm

Step 4: Roof Design (Fixed Cone)

Roof thickness calculation for fixed cone (API 650, Section 5.7):
troof = (p × rroof) / (S × E), where p = design pressure, rroof = roof radius Assuming p = 0.005 MPa (small overpressure), rroof = 12.5 m, S = 138 MPa, E = 1.0
troof = (0.005 × 12.5) / (138 × 1) ≈ 0.0625 / 138 ≈ 0.00045 m ≈ 0.45 mm Minimum API 650 roof thickness = 6 mm, add corrosion allowance = 2 mm → t = 8 mm Choose commercial plate thickness = 8 mm

Step 5: Nozzle Reinforcement

API 650 Section 4.8 requires reinforcement for openings:
Calculate required reinforcement area: Ar = Ao × (P × D) / (S × t), where Ao = nozzle area, D = shell diameter, t = shell thickness.
Provide reinforcement pad or increased shell thickness around the nozzle to satisfy this requirement.

Step 6: Wind and Seismic Load Check

Wind pressure: pwind = 0.613 × V² × C, V = 133 km/h ≈ 36.94 m/s, C = exposure coefficient ≈ 1.0
pwind = 0.613 × 36.94² × 1 ≈ 836 Pa
Check shell and roof thickness against combined hydrostatic + wind stress. Uplift on roof negligible for low wind speed.

Seismic loads for low seismic zone assumed minor. For higher zones, base shear and overturning moment are calculated using:
V = Cseismic × W, W = tank + fluid weight, Cseismic = seismic coefficient.

Step 7: Foundation Design

Ringwall foundation selected. Bearing pressure:
q = (Weight of tank + liquid) / Contact area
Ensure q ≤ allowable soil pressure. Differential settlement < 25 mm as per API 650 recommendations.

Step 8: Welding and Joint Efficiency

Longitudinal and circumferential welds designed with joint efficiency E = 1.0 for fully radiographed welds. Partial inspection joints use E = 0.85. Preheating and NDE applied to critical welds.

Step 9: Hydrostatic Test

Fill tank to 1.1 × Hworking and maintain pressure for 24 hours. Check for leakage, deformation, and verify weld integrity.

This calculation demonstrates step-by-step application of API 650 formulas for shell, roof, bottom, nozzle reinforcement, environmental load checks, and foundation considerations.

Specific Notes and Considerations

The following points summarize the key assumptions, simplifications, and important considerations used in the Section 10 tank calculation:

  • Fluid Properties: Water was assumed as incompressible, with constant density 1000 kg/m³ at 40°C. No temperature gradients were considered.
  • Design Pressure: The tank was treated as atmospheric with negligible internal overpressure, except for hydrostatic head.
  • Shell Thickness Calculation: The API 650 formula t = (P × R)/(S × E - 0.6P) was used assuming longitudinal weld joint efficiency E = 1.0 (full radiography) and uniform shell stress distribution.
  • Bottom Plate: Minimum thickness formula used assumes uniform support over a ringwall foundation and neglects local stress concentrations around nozzles, which were later accounted for with reinforcement calculations.
  • Roof Design: Fixed cone roof thickness used nominal internal pressure of 0.005 MPa for calculation; API 650 minimum thickness of 6 mm was used as baseline before adding corrosion allowance.
  • Nozzle Reinforcement: Simplified API 650 area-based method applied; only standard circular nozzles considered. Irregular openings would require detailed FEA or local reinforcement calculations.
  • Environmental Loads: Wind pressure calculated using 133 km/h site wind speed; seismic load ignored due to low seismic zone assumption. Combination of wind and hydrostatic stress was checked, but dynamic effects were not included.
  • Foundation: Ringwall foundation assumed with uniform soil bearing. Soil settlement effects were neglected except to check allowable pressure; differential settlement assumed within API 650 limits.
  • Welding and Joint Efficiency: All longitudinal and circumferential welds assumed E = 1.0; partial inspection joints would reduce efficiency to 0.85, increasing shell thickness requirements.
  • Hydrostatic Test: Test pressure assumed 1.1 × maximum liquid head; no temperature effects during testing were included.

These assumptions and simplifications are essential to understand the scope and limitations of the calculation example. Any deviation in fluid properties, pressure, environmental loads, or inspection requirements would necessitate recalculation of shell, roof, bottom, and reinforcement thicknesses.

Related Resources