Code of Practice for Design Fabrication and Erection of Vertical Mild Steel Cylindrical Welded Oil Storage Tanks

IS 803: Scope Overview & Key Data for Tank Design

1. Scope (Clause 1)

• Covers design, materials, and construction of steel tanks for water storage.
• Includes open-top and closed-top tanks.
• Provides typical data for nominal sizes, capacities, and shell thicknesses.

2. Key Tables (Clause 3.2 & 6.3.9)

• Tables 1 to 6: Nominal sizes, capacities, shell thickness for tanks.
• Table 7 (Clause 6.3.8.3): Section modulus of wind girders for various member sizes and shell thicknesses (5 mm & 6 mm).

3. Section Modulus Table Excerpt (Wind Girders)

• Detailed wind girder design options include continuous seal welds, intermittent welds, and formed plates.
• Shell thickness influences section modulus and thus strength.

4. Design Notes

• Appendix B provides an alternate shell design method.
• Wind girders are critical for open-top tank stability against wind loads.

flowchart LR
    A[Scope: Steel Tanks] --> B[Nominal Sizes & Capacities]
    B --> C[Shell Thickness Tables]
    C --> D[Wind Girder Section Modulus]
    D --> E[Member Sizes & Welding Details]
    E --> F[Design & Stability]

Summary: IS 803 provides comprehensive data tables and design specifications for steel water tanks, focusing on shell thickness, nominal sizes, and wind girder design to ensure structural stability.

IS 803 Key References:

1. Tables 1 to 6 (Clause 3.2, Pages 11-13):
   Provide nominal sizes, capacities, and shell plate thickness for tanks per IS 803.

2. Table 6 - Minimum Shell Plate Thickness (Clause 2.0):

   • Thickness depends on tank diameter & height.
   • Plates conform to IS 226/IS 2062, E=0.85, sp. gr.=1 (no corrosion allowance).
   • Example: For a 10 m diameter, 6 m height tank, thickness ~6 mm.

3. Table 7 - Section Modulus of Wind Girders (Clause 6.3.8.3):

   • Lists section moduli (cm³) for various ISA angles and shell thicknesses (5 mm, 6 mm).
   • For example: ISA 75×75×10 mm has section modulus ~13.6 cm³ (5 mm shell) and 13.0 cm³ (6 mm shell).
   • Includes details for continuous/intermittent welds and formed plates.

4. Alternate Shell Design:

   • Appendix B covers alternate tank shell design methods (Clause 6.3.9).

Example Table Extract (Shell Plate Thickness)

Wind Girder Section Modulus (cm³) Sample

This data aids in quick design checks for shell thickness and wind girder sizing per IS 803.

IS 803 — Key Definitions & Specifications Summary

1. Definitions (Clause 6.4.1)

• Definitions relate to roof designs of tanks.
• Standard terminology for tank components, wind girders, shell plates, etc., are specified for uniform design understanding.

2. Typical Data for Tanks (Clause 3.2, Tables 1-6)

• Present nominal sizes, capacities, and shell thicknesses.
• Useful for preliminary design and selection of tank dimensions.

3. Wind Girders (Clause 6.3.9 & Table 7)

• Wind girders support tank shells against wind loads.
• Section modulus (Z) values for various angle sections (ISA) are tabulated for shell thicknesses 5 mm and 6 mm.
• Section modulus is critical for bending stress calculations.

Sample Section Modulus Table Extract (cm³)

Important Formula:

Bending Stress in Wind Girder:

[ \sigma = \frac{M}{Z} ]

• (\sigma) = bending stress
• (M) = bending moment
• (Z) = section modulus from Table 7

Summary:

• Use Tables 1-6 for tank sizing.
• Use Table 7 for wind girder section modulus.
• Apply bending stress formula for design checks.

flowchart TD
    A[Tank Design] --> B[Select Tank Size (Tables 1-6)]
    B --> C[Choose Wind Girder Section]
    C --> D[Find Section Modulus (Table 7)]
    D --> E[Calculate Bending Stress \sigma = M/Z]
    E --> F[Check Against Allowable Stress]

This concise guide helps structural engineers apply IS 803 definitions and tables effectively for tank and wind girder

IS 803 Key Materials Data Summary

1. Shell Plate Thickness (Table 6)

• Plates per IS: 226 or IS: 2062, E = 0.85, sp. gr. = 1 (excl. corrosion).
• Thickness depends on Tank Diameter (m) and Tank Height (m).
• Example values for 2 m wide plates:

2. Wind Girders Section Modulus (Table 7)

• For angles ISA sizes and shell thicknesses 5 mm & 6 mm.
• Section modulus (cm³) varies with member size and weld detail.

Notes:

• Use these tables

IS 803: Design Requirements for Wind Girders & Shell Plates

1. Design of Wind Girders (Clause 6.3.8 & Table 7)

• Wind girders are designed using section modulus (Z) values depending on member size and shell thickness.
• Section modulus values (in cm³) are tabulated for various IS angles (ISA) and thicknesses (5 mm, 6 mm).
• Example excerpt from Table 7:

• Weld details (continuous or intermittent) affect the section modulus.

2. Minimum Shell Plate Thickness (Clause 3.2, Table 6)

• Thickness depends on tank diameter and height.
• Plates conform to IS:226 or IS:2062, with modulus of elasticity E = 0.85 and specific gravity = 1.
• Sample thickness values for 2 m height tanks:

3. Alternate Design Method (Clause 6.3.9, Appendix B)

• Allows higher design stress based on product specific gravity.
• Applicable only if purchaser specifies.

Summary Formula for Section Modulus (Z):

[ Z = \frac{I}{y} ] Where:

• (I) = Moment of inertia of the wind girder section
• (y) = Distance from neutral axis

IS 803: Design of Shell Plates and Roofs - Key Points

1. Shell Plates Design (Clause 6.3)

• Shell plates must be rolled to the correct curvature unless purchaser specifies otherwise.
• Minimum nominal plate thickness depends on tank diameter:

2. Wind Girders Design (Clause 6.3.9 & Table 7)

• Wind girders support shell plates against wind loads.
• Section modulus values depend on girder section size and shell thickness.
• Typical sections: ISA angles with sizes like 65×65×6 mm, 75×75×10 mm, etc.
• Section modulus (Z) in cm³ for shell thicknesses 5 mm and 6 mm are tabulated, e.g.:

• Continuous welds and intermittent weld brackets have different section modulus values.

3. Design Formula (General)

• Shell thickness ( t ) and wind girder section modulus ( Z ) are selected to resist bending moments from wind pressure ( p ).
• Bending moment ( M = p \times r^2 ) (where ( r ) = tank radius)
• Required section modulus ( Z = \frac{M}{f_y} ), where ( f_y ) = allowable stress.

Summary Diagram: Shell Plate Thickness Selection

flowchart TD
    A[Tank Diameter (m)] -->|≤ 12| B[Plate Thickness = 5 mm]
    A -->|> 12 and ≤ 20| C[Plate Thickness = 10 mm]
    A -->|

IS 803 - Appurtenances and Mountings Key Points

1. General Requirements (Clause 7.1.1)

• Appurtenances/mountings must conform to IS 803.
• Alternative designs allowed if strength, tightness, and utility are equivalent and purchaser agrees.

2. Attachment Methods (Clause 6.3.7.6)

• Figures 11 & 12 illustrate acceptable attachments.
• Fillet weld sizes for manholes and nozzles are in Tables 9 & 10.
• For other attachments, fillet weld sizes per Clauses 6.3.7.5 to 6.3.7.7.
• For Types B, C, D attachments, fillet weld sizes follow these clauses strictly.

3. Nozzle Pipe Thickness (Note 7)

• For nozzles 650-900 mm diameter on shells > 25 mm thickness, nozzle thickness Tp must be:

4. Roof Manholes (Table 11, Clause 7.4.1)

• Bolt axis always vertical.

flowchart TD
    A[Appurtenances & Mountings] --> B[Conform to IS 803 or equivalent]
    B --> C[Attachment Methods]
    C --> D[Fillet weld sizes per Tables 9 & 10 or Clauses 6.3.7.5-7]
    B --> E[Nozzle Thickness]
    E --> F[Refer to Table for Tp based on shell thickness]
    B --> G[Roof Manholes]
    G --> H[Use Table 11 for

IS 803: Shop Fabrication and Inspection Key Points

1. Workmanship (Clause 8.1.1)

• Fabrication must strictly follow IS 803.
• Workmanship and finish: First class quality.
• Inspection: Manufacturer's inspector must conduct thorough inspection regardless of purchaser's waiver.

2. Shell Nozzle Flanges (Table 10 & Fig. 13)

• Nominal Size (D), Minimum Wall Thickness (Tp), and distances are standardized.
• Includes corrosion allowance for petroleum service.
• Typical dimensions (in mm):

• Notes:
  • Shell opening (D_s = D_o + 65) mm for Type A attachments.
  • Reinforcing plate opening (D_p = D_o + 3) mm.
  • Fillet weld sizes depend on shell thickness (t) and pipe wall thickness (T_p).

3. Welding and Inspection

• Radiographic inspection is mandatory for shell joints.
• Testing and inspection per Clause 12.1 and related welding clauses.
• Ensure all welds meet specified dimensions and quality.

Summary Diagram: Shop Fabrication Inspection Flow

flowchart TD
    A[Start Fabrication] --> B[Follow IS 803 Standards]
    B --> C[Manufacturing Workmanship Check]
    C --> D[Dimensional Verification (Table 10)]
    D --> E[Welding & Radiographic Inspection]
    E --> F[Final Inspection by Manufacturer]
    F --> G[Approval or Rework]
    G --> H[Dispatch]

**Use IS 803 Table 10 and welding clauses as primary references

IS 803 - Site Erection Key Points & Tables

Clause 9: Site Erection

• Clause 9.3 Erection of Plates: Plates must be erected as per manufacturer's instructions and inspected for compliance.
• Clause 9.3.4 Erection Holes and Attachments: Proper holes and attachments should be provided for ease of erection and alignment.

Clause 8.8.6: Gauges & Templates

• Manufacturer provides all gauges/templates for inspection.
• Inspector may verify test results independently; cost borne by manufacturer if material fails, else by purchaser.

Key Table: Section Modulus of Wind Girders (Clause 6.3.8.3)

Practical Notes for Site Erection:

• Use correct gauges/templates for alignment.
• Check weld types: continuous seal welds for better rigidity.
• Use appropriate member sizes from tables for wind girders.
• Ensure attachment holes

IS 803 - Site Welding Key Points

1. Welding Procedure (Clause 10.6 & 10.6.7)

• Welding procedures must ensure weld metal and heat-affected zone (HAZ) have mechanical properties matching the base plate.
• For design metal temperatures < 10℃, welds must pass impact tests:
  • Impact energy ≥ 25.5 N·m (2.5 kgf·m) average at design temperature
• Qualification includes vertical and horizontal welds.

2. Electrodes Specifications

• Use electrodes per IS standards:
  • IS 814 (Part I): For structural steel except sheets.
  • IS 818 (Part II): For welding sheets.

3. Welding Sequence (Clause 10.2)

• Sequence should minimize residual stresses and distortion.
• Typically, symmetrical welding and back-step techniques are recommended.

4. Dimensional Tolerances (Clause 9.4.3)

• Gap between shell and roof periphery ≤ 50 mm or as per manufacturer’s sealing requirements.

Summary Table: Impact Test Requirement

flowchart TD
    A[Start Welding Procedure] --> B[Select Electrode as per IS 814/818]
    B --> C[Prepare Weld Joint]
    C --> D[Follow Welding Sequence to minimize stress]
    D --> E[Perform Weld]
    E --> F[Conduct Impact Test (if T < 10℃)]
    F -->|Pass| G[Accept Weld]
    F -->|Fail| H[Requalify Procedure]

This ensures site welds meet IS 803 structural integrity and safety requirements.

IS 803: Radiographic Inspection of Shell Joints - Key Points

1. Application (Clause 11.1)

• Spot radiographic inspection (X-ray/gamma-ray) applies to shell joints with joint efficiency factor = 0.85.
• Follow procedures per IS 1182-1967 and IS 2595-1963.

2. Preparation of Welds (Clause 11.2)

• Remove weld ripples/irregularities on both sides to avoid masking defects.
• Weld surface must merge smoothly with plate.
• Reinforcement crown thickness limits:

3. Permissible Porosity Limits (Table 17, Clause 11.7(d)(5))

IS 803: Testing of Tanks - Key Points

1. Testing Clauses (Clause 12)

• 12.1 Bottom Testing: Checks for leaks and strength of bottom plates.
• 12.2 Shell Testing: Ensures shell integrity under hydrostatic pressure.
• 12.3 Fixed Roof Testing: Verifies roof tightness and structural soundness.
• 12.4 Repair of Leaks:
  • No welding unless all connecting lines are blanked off (12.4.5).
  • Repairs only on empty, cleaned, and gas-freed tanks.
  • Repairs on oil-contained tanks require purchaser's written approval.

2. Typical Data for Design & Testing (Clause 3.2 & Tables 1-6)

• Table 6: Minimum shell plate thickness (mm) for tanks based on diameter and height (using IS:226/2062 plates, E=0.85, sp gr=1, excluding corrosion allowance).

(Refer to IS 803 Table 6 for detailed values)

3. Purchaser's Information (Appendix A)

• Essential data like tank location, capacity, product, design pressure, corrosion allowance, wind & seismic loads, foundation, and mountings must be provided for testing and design.

Summary Diagram: Testing Flow

flowchart TD
    A[Tank Fabrication Completed] --> B[Bottom Testing]
    B --> C[Shell Testing]
    C --> D[Fixed Roof Testing]
    D --> E{Leak Found?}
    E -- Yes --> F[Repair Leak]
    E -- No --> G[Testing Complete]
    F --> E

Note: Always follow safety protocols during testing and repairs, especially for tanks previously containing oil.

IS 803: Packing and Transportation Key Points

Packing (Clause 8.7)

• Projecting plates and member ends at joints: Must be stiffened.
• Straight plates: Bundled securely.
• Screwed ends & machined surfaces: Packed suitably to avoid damage.
• Small parts (rivets, bolts, railings): Packed separately.
• General tank material: Packed to prevent damage/distortion during transit.

Thermal Venting Capacity (Clause 3.3.2, Table 18)

• Venting capacity is critical for safe transportation/storage.
• Expressed in m³ free air/hr at 10 N/cm² and 15°C.
• Separate values for vacuum (inbreathing) and pressure (outbreathing).
• Different venting for products with flash points ≥40°C and <40°C.
• For intermediate capacities, interpolate values.

Summary Diagram of Packing & Venting Process

flowchart TD
    A[Tank Components] --> B{Packing}
    B --> C[Stiffen projecting plates]
    B --> D[Bundle straight plates]
    B --> E[Pack screwed ends & machined surfaces]
    B --> F[Separate packing for small parts]
    B --> G[Prevent damage during transit]
    A --> H{Thermal Venting}
    H --> I[Calculate venting capacity]
    I --> J[Vacuum (inbreathing)]
    I --> K[Pressure (outbreathing)]
    J --> L[Use Table 18 values]
    K --> M[Use Table 18 values based on flash point]
``

IS 803 - Safety and Access Provisions: Key Formulas, Tables & Specs

1. Tank Accessories (Clause 7.10)

• Accessories like level indicators, foam chambers, gauge hatches, free vents, and earthing bosses must conform to relevant Indian Standards and purchaser agreement.

2. Shell Manhole Cover Plate & Bolting Flange Thickness (Table 8)

• Thickness increases with tank height and manhole size (500, 600, 750, 900 mm).

3. Shell Manhole Dimensional Data (Table 9)

• Neck thickness Tp depends on shell thickness (e.g., 5-20 mm shell → Tp = 8 mm).

4. Emergency Venting (Clause 4.3, Table 19)

• For intermediate values, interpolate linearly.

5. **Wind Girders Section Modulus (Clause 6.3

IS 803: Key Points from Appendices on Floating Roof Design & Venting

Appendix D: Floating Roof Design (Pontoon, Double-Deck, Covered Roofs)

• Applies to pontoon, double-deck, and covered floating roofs inside fixed roof tanks.
• Design focuses on:
  • Minimum structural requirements for roof panels and pontoons.
  • Load considerations: wind, rain, snow, and liquid weight.
  • Stability against uplift and lateral forces.
• Typical design includes:
  • Thickness of steel plates.
  • Spacing and sizing of pontoons.
  • Drainage provisions to avoid water accumulation.

Appendix C: Venting Requirements for Fixed Roof Tanks

• Covers normal and emergency vent sizing for above-ground tanks.
• Vent sizing based on:
  • Tank volume.
  • Expected pressure/vacuum relief.
  • Climatic and operating conditions.
• Vent area (A_v) calculation formula for atmospheric tanks:

[ A_v = \frac{Q}{C_d \times \sqrt{2 \times g \times H \times \rho}} ]

Where:

• ( Q ) = flow rate (m³/s)
• ( C_d ) = discharge coefficient (~0.6-0.8)
• ( g ) = gravity (9.81 m/s²)
• ( H ) = pressure head (m)
• ( \rho ) = fluid density (kg/m³)

Summary Table: Vent Sizing (Indicative)

flowchart TD
    A[Tank with Fixed Roof] --> B[Floating Roof Design]
    B --> C[Pontoon Roof]
    B --> D[Double-Deck Roof]
    B --> E[Covered Floating Roof]
    A --> F[Venting Requirements]
    F --> G[Normal Venting]
    F --> H[Emergency Venting]

For detailed thickness, spacing, and vent sizing tables, refer to Appendices C & D of IS 803.