How to use this page
A storage tank is a small number of decisions taken in a fixed order. Most are taken during the design basis and FEED, and most become expensive to reverse well before construction starts. This page sets out that sequence: code selection, roof configuration, foundation, hydrostatic test, commissioning and handover.
It is written for owners, project managers and engineers specifying or evaluating tankage scope. Applicable design envelopes, thresholds and annex provisions are edition- and jurisdiction-dependent and must be confirmed against the governing code and the project specification in every case.
1. Selecting the design code
The choice between API 650 and API 620 is settled by the design pressure and the design metal temperature, not by tank size or by preference.
API 650 governs vertical, cylindrical, aboveground welded tanks in atmospheric to near-atmospheric service. Its annexes extend that envelope, with separate provisions for internal pressure, elevated temperature, stainless steel, seismic design, shop-fabricated tanks, and external and internal floating roofs. The applicable annexes must be confirmed for each project.
API 620 governs large welded low-pressure storage tanks with design pressures up to 15 psig. Its low-temperature annexes extend service downward for refrigerated and cryogenic duty, subject to the material, detailing and inspection provisions attaching to each.
The practical rule:
| Condition | Governing standard |
|---|---|
| Atmospheric to near-atmospheric service | API 650 |
| Low-pressure service above the API 650 envelope, to 15 psig | API 620 |
| Elevated-temperature service | API 650, elevated-temperature annex |
| Refrigerated service | API 620, low-temperature annexes |
| Cryogenic service | API 620 low-temperature annexes, with API 625 governing the tank system |
| Above the API 620 ceiling | Neither. This is pressure vessel design. |
Three points that cause avoidable rework:
Venting is a separate code. API 2000 governs pressure and vacuum relief for atmospheric and low-pressure tanks. A tank designed correctly to API 650 and vented incorrectly is not a compliant installation. Vacuum collapse during rapid product withdrawal remains one of the more common serious tank incidents.
The annexes carry the design, not the standard number. "Designed to API 650" is not a specification. The applicable annexes are the specification.
Selecting the code decides the material specification. Low-temperature service requires impact-tested material at the design metal temperature. Discovering that requirement after plate is ordered is a schedule event, not a design change.
2. Roof configuration
Roof selection follows from the product's vapour pressure, the applicable emissions regime, tank diameter and site climate.
Fixed roof. A cone or dome roof welded to the shell, with a vapour space above the liquid. Simplest, lowest cost, and appropriate for low-volatility products such as diesel, gasoil, heavy fuel oil and water where the emissions regime permits. The vapour space requires venting to API 2000, and blanketing where the product warrants it.
External floating roof (EFR). A deck floating directly on the liquid, eliminating the bulk vapour space. Standard for crude oil and gasoline in large tanks. Design attention concentrates on three items: the rim seal system, roof drainage, and the deck type. Pontoon decks are lighter and cheaper; double decks carry rainwater and snow load better and are the normal choice at large diameter. Rim seals are specified as mechanical shoe, liquid-mounted or vapour-mounted, single or double, and the emissions regime usually dictates which.
Internal floating roof (IFR). A fixed roof for weather protection with a floating deck beneath it for emission control. The normal choice for volatile products where rainfall, sand or dust would make an exposed deck an operating burden. Common across the Gulf and in high-rainfall tropical sites for the same reason from opposite causes.
On emissions control. The capacity and vapour pressure at which controls become mandatory are set by the applicable environmental regulation, not by the tank code, and differ between jurisdictions. The governing regime must be identified in the design basis.
Retrofit. Fitting an internal floating roof into an existing fixed-roof tank is routinely done and is not automatic. It requires review of shell verticality and out-of-roundness, existing appurtenances and internal obstructions, roof support structure, seal clearance, and the venting and inerting arrangement, followed by re-rating under API 653.
3. Foundation
Foundation type is set by the geotechnical investigation and the tank loading: ringwall, crushed stone ringwall, earth pad, or piled with a reinforced concrete slab or ring. On weak or variable soils the options narrow to piling or ground improvement with pre-load.
The decision is taken early because it interacts with everything downstream. Foundation type determines the settlement the tank will experience, and settlement is what the hydrostatic test measures. Where the geotechnical investigation is thin at FEED stage, the risk is not the foundation cost; it is discovering unacceptable differential settlement during hydrotest, when the tank is built.
4. Hydrostatic testing
A new field-erected tank is filled with water before it sees product. Water is used because it is available, controllable, and low hazard relative to the intended product, and because its density loads the shell and foundation at or above the service condition for most hydrocarbons.
What the test proves. Three things simultaneously: integrity of the shell and bottom welds under full hydrostatic head; structural adequacy of the tank; and the behaviour of the foundation under first loading. The third is the reason the test cannot be waived on schedule grounds. It is the only occasion before service on which the foundation is proof-loaded and measured.
How it is run. The tank is filled in stages with hold points defined by the project specification. At each hold point, settlement is measured at equally spaced stations around the shell perimeter, and welds and seams are inspected for leakage. The full level is held for the specified period before controlled draining.
What the settlement survey detects. API 653 provides the evaluation basis and distinguishes modes that have different consequences:
- Uniform settlement. The tank goes down evenly. Least damaging structurally; affects nozzle elevations and connected piping.
- Planar tilt. The tank settles as a rigid plane. Increases hoop stress on the low side and affects floating roof operation.
- Out-of-plane differential settlement. The shell distorts. The mode with the most serious structural consequences.
- Edge settlement and bottom bulging. Localised, and a common driver of bottom plate and weld damage.
Distinguishing these is the purpose of measuring at multiple stations rather than at four points.
Test water. Source, quality and disposal are planned into the commissioning programme, not left to the end. Chloride content governs for stainless steel service. Depending on jurisdiction and site, discharge may require testing and treatment. On a large tank the volume is significant and the disposal route can become a critical path item.
5. Commissioning and handover
Commissioning is a sequence with defined entry and exit criteria at each stage, executed by system or subsystem rather than by tank.
Mechanical completion. Established per system against agreed completion criteria: installation complete to the approved drawings, inspection and NDT records complete, punch list categorised by severity, and the system ready for the next controlled stage. Items affecting safety or the ability to proceed are cleared before moving on; the remainder are carried with agreement.
Pre-commissioning. Preparation without product: line flushing and cleaning, hydrotesting and reinstatement of piping, instrument calibration and loop checking, valve function testing, motor solo runs, cathodic protection energisation, drying and, where required, inerting.
Commissioning. Systems are energised and operated, first on utilities or water, then progressively toward live service. Product introduction and first fill follow, with the fire protection system proven before hydrocarbon is introduced, not after.
Performance testing. Where the contract requires it, the facility is demonstrated against the agreed throughput, transfer rates and operating parameters. The test conditions, the acceptance criteria and the consequences of a failed test are contractual matters that should be settled at award, not negotiated at the point of testing.
Handover. Frequently staged by system or area. The dossier transfers with it: as-built drawings, material and test certificates, NDT and inspection records, settlement survey results, calibration records, spares and operating documentation. The dossier is the record the operator relies on for the life of the asset and, in practice, the item most often incomplete at the point everything else is ready.
The reason the sequence cannot be compressed is that each stage verifies something the next depends on. A skipped step does not remove a fault; it moves the fault downstream to where it is more expensive and more dangerous to find.
6. Where the decisions are actually made
| Decision | Fixed at | Cost to reverse later |
|---|---|---|
| Governing code and annexes | Design basis | Redesign |
| Roof configuration | Design basis / FEED | Redesign, possibly rebuild |
| Material specification | FEED, before plate order | Schedule event |
| Foundation type | FEED, on geotechnical data | Rebuild |
| Venting philosophy | Detailed design | Modification |
| Test and commissioning programme | Pre-construction | Schedule |
Everything in the first four rows is settled before a plate is rolled. That is the argument for putting engineering effort at the front of a tankage project rather than in the middle of it.
7. Track record
Chemie-Tech’s scope at the Dangote Refinery and Petrochemical complex in the Lekki Free Trade Zone, Lagos, covered Tankage Packages I to III, comprising 154 tanks with an aggregate capacity of 36.8 million bbl and a maximum diameter of 92 m; the 50-tank T1 and T2 tank farm package at the Basrah Refinery Upgrading project, maximum diameter 57 m; and terminal EPC and FEED delivery across Africa, the Middle East, Europe, South Asia and Oceania, including completed FEED for a planned 1.2 million m³ development at Sohar with a maximum tank diameter of 90 m.
Common Questions
Can one tank be designed to both API 650 and API 620?
No. One standard governs. Supplementary requirements from the other may be applied by specification, but that is an engineering and purchaser decision requiring formal review, not a dual certification.
Is API 650 always the cheaper option?
Within its envelope, generally yes. Outside it, the question does not arise: applying it beyond its pressure or temperature limits is a code non-compliance, not an economy.
Can hydrostatic testing be waived?
Only within the code's own provisions and with purchaser approval. It is the only pre-service proof load on the foundation, which is why waiver requests on schedule grounds are usually a false economy.
What is most often incomplete at handover?
The dossier. As-builts, certificates and inspection records fall behind physical progress unless they are tracked as deliverables with their own schedule.