API 650 is the American Petroleum Institute standard that governs the design, fabrication, erection and inspection of welded steel atmospheric storage tanks used across the oil & gas, petrochemical, water and power generation industries. When engineers refer to "API 650 tank engineering," they mean the full set of design activities required to produce a code-compliant tank — from defining design inputs through to issuing stamped drawings for construction.
In Saudi Arabia and the wider GCC region, API 650 is the predominant standard for above-ground storage tanks (ASTs) handling crude oil, refined products, water, chemicals and other fluids at or near atmospheric pressure. Understanding what the engineering scope actually covers helps clients issue accurate scopes of work, and helps procurement managers evaluate proposals correctly.
Scope of API 650 — What Does It Cover?
The standard applies to tanks built of carbon steel, stainless steel and some aluminium alloys with a design pressure not exceeding 2.5 kPa (1 oz/in²) above atmospheric at the liquid surface. It covers:
- Shell plates, annular bottom plates, and floor plates
- Fixed roofs (cone, dome) and open-top configurations
- Floating roofs (internal and external) under Appendix C
- Nozzles, manholes and structural attachments
- Wind girders and stiffening rings
- Seismic design per Appendix E
- Foundation loads and tank settlement criteria
It does not cover pressure vessels (governed by ASME VIII), pipelines or underground tanks — a distinction that matters when scoping work in a new facility.
Key Design Inputs the Engineer Needs
Before any calculation begins, the engineer must gather a defined set of process and site data. Missing or incorrect inputs are the most common cause of rework in tank projects:
- Fluid properties: stored product, specific gravity, design liquid level
- Geometry: nominal diameter, nominal height, number and size of nozzles
- Material grades: plate grades (A36, A283C, A516 Gr.70, duplex stainless, etc.)
- Site conditions: wind speed, seismic zone, snow load (if applicable), ambient temperature range
- Operating conditions: design temperature, corrosion allowance, vacuum/pressure relief settings
- Client/project specifications: any deviations from the standard, coating requirements, inspection class
Principal Design Checks Performed
Shell Thickness
Each shell course is checked for hydrostatic pressure from the stored liquid. The engineer calculates a design thickness and a hydrostatic test thickness for each course, then selects the thicker of the two. Minimum thickness requirements (3 mm to 6 mm depending on diameter) also apply.
Roof and Bottom Plates
Cone-roof rafter design follows API 650 Section 5.10, while self-supporting dome or cone roofs use stress analysis per Appendix F. Bottom plates are generally a minimum 6 mm for the annular ring and 5 mm for interior plates, with lap-weld or butt-weld configurations depending on diameter.
Wind and Seismic Loads
Wind stability is checked by comparing the wind overturning moment against the resisting moment from the tank weight and liquid contents. Appendix E provides a pseudostatic or dynamic seismic analysis approach that addresses impulsive and convective (sloshing) liquid mass — critical in Saudi Arabia where seismic hazard varies from low in the Eastern Province to moderate in the Hejaz region.
Nozzle Reinforcement
Nozzles introduce local stress concentrations in the shell. API 650 Section 5.7 defines reinforcement area requirements. Larger nozzles, or nozzles in thin shell courses, may require reinforcing pads or insert plates with material and weld requirements defined by the code.
Fabrication Support and Typical Deliverables
API 650 engineering does not end with calculation sheets. The engineer is expected to produce a coordinated set of deliverables that the fabricator and site team can use directly:
- General arrangement (GA) drawing showing tank geometry, nozzle schedule and orientation
- Shell plate layout drawing (strake development, plate dimensions, weld seam positions)
- Roof framing drawing (rafter layout, central column if required, wind girder)
- Nozzle and manhole detail drawings
- Stairway, ladder and platform drawings (often to client standard)
- Design calculation report stamped by a licensed engineer
- Material take-off (MTO) for procurement
- Inspection and test plan (ITP) referenced to API 650 Chapter 6
SLETEC's team prepares full API 650 design packages for fixed-roof, open-top and floating-roof tanks. Our API 650 tank engineering service covers everything from initial design inputs to issued-for-construction (IFC) drawings, supporting clients in Saudi Arabia, Bahrain, UAE and India.
For tanks already in service that require assessment, repair or modification, API 653 (Tank Inspection, Repair, Alteration and Reconstruction) applies — a topic covered in our article on tank repair engineering and method statement preparation.