Spherical storage tanks are among the most iconic and technically advanced structures in the oil, gas, and petrochemical world. This article explains the end-to-end fabrication and construction process for giant sphere tanks, showing the methods, checks, and codes that make them reliable for large-scale storage.
Why a Sphere?
Because a sphere shares pressure uniformly, it avoids stress concentrators found in cylinders and flats. The geometry reduces localized peaks and cuts down on stiffeners.
Spherical tanks are used worldwide for pressurized gases such as LPG, LNG, ammonia, and industrial gases. They’re compact for the volume they offer, and their footprint is easy to protect with safety setbacks and firefighting access.
The Rulebook: Codes & Criteria
Before steel is cut, the design team locks down key inputs: P, T, corrosion allowance, materials, environmental loads, penetrations, support legs, and maintenance access.
Depending on service and temperature, engineers reference ASME VIII and API 620, with project specs layering in client and authority requirements.
Relief valves, emergency depressurization, and water spray rings are sized for fire case and blocked-in conditions.
Making the “Petals” of the Sphere
Spheres aren’t stamped; they’re assembled from many curved “petals” (gores and crown segments).
Plate cutting: oxy-fuel nests each piece to minimize scrap. Heat input is controlled to prevent HAZ issues.
Cold rolling & pressing: Plates are rolled/pressed in multi-pass sequences to hit the target radius with tight tolerances; trial fits and templates verify curvature.
Edge prep: Machining prepares welding edges (V, double-V, or U) per WPS/PQR; fitter’s marks align circumferential and meridional seams.
Tip for quality: Curvature tolerance matters; small deviations multiply across the shell—tight QC here saves time at elevation.
Lifting, Scaffolding & Fit-Up
Most spheres stand above grade on steel legs for access, fire protection, and thermal relief.
Scaffolding & access: Full-wrap scaffolds provide safe access for fitters pe tank and welders; edge protection and lifelines are mandatory.
Cranes & strand jacks: Mobile cranes lift segments to the upper crown first, then work downward.
Fit-up control: Strongbacks, dogs, and come-alongs pull seams true; Hi-Lo gauges check misalignment. Pre-heat is applied when required by the WPS.
Each closing seam is a small project: alignment, root pass, fill passes, cap, then immediate NDT per hold-points. Dimensional surveys with total stations confirm spherical accuracy after every few segments.
Welding That Holds Pressure
Welding drives the vessel’s integrity, so the paperwork comes first: WPS (how to weld), PQR (prove it works), and WPQ (welder is qualified).
Processes: TIG for roots, stick or flux-core for fill, submerged arc for long seams, chosen per position and thickness.
Controls: Preheat, interpass temperature, heat input, and PWHT (when specified) control microstructure and reduce residual stresses.
Consumables & traceability: Low-hydrogen electrodes baked and logged; heat numbers carried from mill certs to final databook.
Good welding is invisible after the coat goes on—but NDT sees everything.
Quality Control You Can Trust
Hold points and witness points are agreed with the client and third-party inspector. Typical NDT includes:
Visual (VT): root, fill, cap, undercut, profile.
Magnetic particle (MT) or Dye penetrant (PT): surface cracks, toes, stop-starts.
Ultrasonic testing (UT) or Radiography (RT): volumetric defects.
Hardness tests where PWHT isn’t used; Ferrite checks for certain alloys; Positive Material Identification (PMI) on nozzles.
Hydrostatic or pneumatic tests per code: careful pressurization, calibrated gauges, barricades, and exclusion zones.
Repair rates are tracked; any trend triggers a root-cause review—procedure, welder, consumable, environment.
Protecting the Investment
Blasting & surface prep: Near-white metal profile verified with replica tape or roughness gauge.
Primer & topcoats: Epoxy primer + polyurethane topcoat for UV and chemical resistance; stripe coats along edges and around nozzles.
Fireproofing (PFP): PFP thickness per fire curve and exposure; legs are common PFP targets.
Cathodic protection for supports and anchors when needed; drainage and earthing detailed in civils.
Insulation (if service demands): hot service mineral wool or cellular glass with stainless jacketing.
A good coating spec saves millions over a lifespan; corrosion is a marathon, not a sprint.
Making the Sphere Operable
Nozzles & manways: Oriented for process, inspection, and maintenance.
Ladders, platforms, and handrails: Designed to OSHA-style safety with toe boards and mid-rails; hot-dip galvanized or coated.
Piping tie-ins: Spring supports, expansion loops, and flexible connectors.
Instrumentation: Level gauges (displacer, radar), temperature elements, pressure transmitters, ESD valves, and gas detection around the sphere.
Fire protection: Ring main hydrants, deluge spray, monitors, and remote isolation.
From Construction to Operations
Build complete; now prove it works.
Leak tightness & strength: Final pressure test per code with calibrated instruments and documented hold times.
Functional checks: proving interlocks and shutdowns.
Drying & inerting: For certain services, nitrogen purging and moisture specs verified.
Databook handover: Mill certs, WPS/PQR/WPQ, NDT reports, test packs, coating DFT charts, and as-built drawings all compiled.
Operator training: Safe startup, normal ops, emergency procedures, and maintenance intervals.
Paperwork equals safety history—keep it clean and complete.
Where Projects Win or Lose Time
Geometry & fit-up: Small curvature errors turn into hard-to-close gaps; proactive survey and template controls avoid schedule hits.
Welding productivity: Position, wind, and heat management determine repair factors and throughput; sheltering and preheat rigs pay back fast.
Weather & logistics: Lifts depend on wind windows; tower crane reach and crawler capacity dictate segment sizes.
Interface risk: Foundations, legs, and anchor chairs must be finished and surveyed before shell work; late civils = idle welders.
Safety planning: Work at height, hot work, and heavy lifts require JSA/PTW discipline; near-miss reporting keeps the curve flat.
Who Uses Spheres?
Common use cases include LPG terminals, LNG satellite storage, ammonia plants, mixed-gas hubs, and industrial gas yards. When footprint is tight and pressure is high, spheres are often the right answer.
Why Choose a Sphere?
High pressure capacity thanks to uniform stress distribution.
Material efficiency relative to cylindrical shells at similar pressure.
Compact footprint for big volumes, easier firefighting access.
Long service life with proper coatings, maintenance, and inspection.
Predictable behavior in thermal and fire scenarios under proper design.
People First
The geometry is forgiving; construction isn’t. Discipline saves lives: confined-space permits, hot-work controls, daily toolbox talks. Good housekeeping is good safety.
Fast Facts
Why spheres over bullets (horizontal bullets)? Spheres handle higher pressures more efficiently; bullets can be more modular and simpler to site—choose per service and logistics.
Can spheres go cryogenic? With the right materials and insulation systems, spheres can handle low temperatures—project specs govern feasibility.
What’s the typical NDT scope? VT for all passes, MT/PT for surface, and UT/RT for volumetric per code and owner spec.
How long does a build take? Depends on size, weather, and logistics; the critical path is usually shell fit-up, welding, and NDT.
Don’t Miss the Footage
Watching plates become petals, petals become a pressure sphere—it’s the kind of engineering that sticks. It’s a front-row seat to modern fabrication, quality control, and commissioning.
Want more sphere builds, WPS templates, ITP checklists, and NDT cheat sheets? Add this walkthrough to your toolbox. Download now—and bring your next spherical tank project in safer, faster, and right-first-time.
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