Code of Practice for Design Fabrication and Erection of Vertical Mild Steel Cylindrical Welded Oil Storage Tanks
1976 Edition

The minimum thickness for shell plates varies with tank diameter as follows: less than 15 meters requires 5 mm; between 15 and 36 meters requires 6 mm; from 36 to 60 meters requires 8 mm; and above 60 meters requires 10 mm. Additionally, the shell plate thickness is calculated using the formula t = (4.9 (H - 0.3) D G) / (S E), where t is thickness in mm, H is vertical height in meters, D is tank diameter in meters, G is specific gravity of the stored liquid (≥1.0), S is allowable stress, and E is joint efficiency. The final thickness used should be the greater of the calculated value or the minimum from the table. Maximum shell plate thickness allowed is 40 mm, with insert plates permitted up to 75 mm depending on steel grade. Roof and bottom plates thickness are determined based on design requirements and product stored, without a fixed minimum specified.

Welding procedures and welder qualifications must comply with Indian Standards IS 823-1964 and IS 817-1966. Radiographic inspections, specifically spot X-ray or gamma-ray testing, apply to shell joints with a joint efficiency of 0.85, following IS 1182-1967 and IS 2595-1963. Acceptance criteria prohibit cracks, incomplete fusion, or penetration defects. Inclusions must not exceed two-thirds of the thinner plate thickness and should be no longer than 20 mm. Porosity is limited to specific maximum areas and pore sizes, with detailed limits for isolated, clustered, and aligned pores. If defects are detected, additional radiographs are taken adjacent to the defect area until the weld meets acceptance or full replacement is required.

Design loads include dead weight plus a uniform live load of 1,225 N/m² (125 kgf/m²) for roofs and supports. Legs and attachments supporting floating roofs must sustain a live load of 1,250 N/m² for single or double-deck roofs and 625 N/m² for pan-type floating roof decks. Roof loads are preferably transmitted through bulkheads or diaphragms to legs, with steel pads welded continuously to the tank bottom to distribute loads and prevent damage. Wind loads should be determined per IS 875-1964, and internal vacuum loads of at least 500 N/m² must be accounted for as specified by the purchaser.

Vent capacity must accommodate both oil movement and thermal effects. Oil movement venting capacity is calculated as 15 m³/hr per m³/hr of maximum emptying rate. Thermal venting capacities, varying by tank volume and liquid flash point, are listed in tabular form—covering vacuum (inbreathing) and pressure (outbreathing) scenarios. For example, a 60 m³ tank requires 15 m³/hr vacuum venting, 8.5 m³/hr pressure venting for liquids with flash points ≥40°C, and 15 m³/hr for flash points below 40°C. For fire exposure venting on tanks with design pressure ≤10 N/cm² and wetted area ≤280 m², specific tabulated values apply; for larger wetted areas, venting is calculated by the formula Venting (CMH) = 220 × A^0.82, with A being the wetted surface area in m² up to 10 m height.

Floating roofs covered include pontoon, double-deck, and covered types situated inside fixed roof tanks. Legs and attachments must support the roof plus a uniform live load of 1,250 N/m² for single and double-deck roofs, while pan-type roofs must bear 625 N/m² on the deck. Load transmission should be via bulkheads or diaphragms to legs, with steel pads welded continuously to the tank bottom to distribute loads and prevent damage. Overflow provisions must enable liquid to return safely without manual intervention or harm. For covered floating roofs, the outer rim and appurtenance necks require a minimum height of 200 mm to ensure structural integrity and safety.