What you should know about your petroleum storage tanks
Are your storage tanks fit for service or are they an impending disaster? The purpose of this article is to advise you on the inspection of your storage tanks and how to minimize risks that they may pose to you, the environment and other stakeholders.

Introduction
Aboveground Storage Tanks (AST) are used for storage of crude oil and its derivatives such as gasoline, kerosene, diesel and fuel oil. The material of construction is carbon steel, with tank sizes ranging from 3m to 60 m in diameter. They are constructed to BS 2654, API 650 or its predecessor API 12C. There are approximately 300 storage tanks in Kenya. The main components of storage tanks are the shell, bottom, and the roof. Other components include nozzles, roof trusses, electrical shunts, stair ladders, roof seals, among others.

Owing to the impurities present in hydrocarbon such as salts and sulphur, tanks are prone to damage mechanisms such as corrosion, which if not adequately mitigated, leads to failure of the tank.Corrosion in tanks occurs at rates and patterns unique to every component and product stored. Different products have different corrosivity and corrosion patterns, thus they dictate different life spans for the tanks. The difference in product corrosivity results from the difference in the percentage composition and nature of impurities (corrosive media) embedded in each product.

As a result, some products may be more aggressive on bottom plates while others may attack the roof or the shell more. For instance, kerosene tanks corrode heavily on the bottom plates but negligibly on the roof plates whereas diesel tanks corrode heavily of the roof plates internally. While the shell plates for kerosene and diesel tanks may suffer mild corrosion, gasoline tanks on the other hand may suffer heavy uneven corrosion on the shell plates.

Consequence of Failure

Unmitigated corrosion and lack of sufficient inspection of storage tanks eventually results in failure by product leakage. Loss of containment is an undesired eventuality with unpleasant consequences such as;

?    Environmental pollution from spillage of hazardous product
?    Product loss
?    Environment decontamination/ clean-up cost
?    Negative publicity
?    Possible hazards e.g. fires, fatalities
?    Legal implications
?    Loss of asset, costly repairs/ replacement
?    Loss of operation time due to unplanned outage

Case studies from around the world show that the major contributor to failure of storage tanks is failure to carry out sufficient, timely inspection. For instance in 1919, USA, 21 people died and 12 million litres of product lost after a storage tank exploded. This was attributed to inadequate inspection and hefty fines were incurred in legal suits by the owner. Several other failures have occurred in the recent past, including in Kenya resulting in various consequences listed above.

Tanks Inspection

A common malpractice among tank owners has been carrying out in-service external inspection that is assumed sufficient for evaluating the fitness of the tank for continued service. Similarly, during out-of-service inspection, inspection is carried out without sweep blasting of bottom plates, in which case, visual inspection results are compromised.Inspection, Repair, Reconstruction, and Alteration of AST is governed by API 653, which gives guidance on inspection methodology, frequency, evaluation for suitability for service (acceptance and rejection criteria for components), repair methods, welding guidelines, and QA/QC measures during and after repairs.
Inspection and tank evaluation must be done by an API 653 Certified Tanks Inspector.

Evaluation of Suitability for Service

When tank inspection results show that a change has occurred from the original physical condition of the tank, an evaluation must be done to determine its fitness for continued use or change of service. In addition, when making decisions regarding repairs, alterations, dismantling, relocating, or reconstructing an existing tank, this evaluation must be done.

Roof Plates

Plates corroded to an average thickness of less than 2.3 mm in any 100 m2 area or plates with through hole(s) must be repaired or replaced.

Tank Shell

Flaws, deterioration, and corrosion greater than the original corrosion allowance that might adversely affect the performance or structural integrity of the shell must be evaluated and fitness for service determined. The tank shell must be evaluated for strength, stability, remaining life and the maximum filling height allowable after corrosion.

Bottom Plates

The integrity of tank bottoms must be determined to prevent perforation and subsequent leakage of hydrocarbon. The bottom may fail from;
?    Internal corrosion (product side) – pitting or uneven corrosion
?    External (Soil side) corrosion
?    Uneven settlement resulting in high localized stresses in bottom plates/ welds

Product side corrosion may be minimized or delayed by use of internal lining protection whereas soil side corrosion may be mitigated by installation of cathodic protection system on bottom plates. Consequences associated with bottom leakage may be minimized by installation of leak detection systems and leak prevention barriers which prevent escape of released material and/ or contain/ channel it for leak detection.

API 653 stipulates that the minimum bottom plate thickness at the time of next inspection must be 2.54 mm for sketch and rectangular plates and not less than 5mm for annular plates. This implies that the thickness at current inspection must be such that after incorporating corrosion rate over the interval to next inspection, the minimum thickness at next inspection must be as stated above. Therefore, evaluation of tank bottom integrity must be done such that all sections that do not meet API 653 requirements are repaired or replaced.

Inspection Frequency

When determining the inspection intervals for AST, several factors must be considered e.g.;
?    Nature of product stored
?    Corrosion allowance and corrosion rate
?    Corrosion prevention systems and leak detection systems in place
?    Condition at previous inspection and repairs done
?    Location of tanks and level of risk associated with failure

Service history of a tank or that of others in similar service can help in determining the inspection intervals.
External Inspection
Routine in-service external inspection must be done monthly by the owner/ operator personnel. This aims to observe for evidence of leak, shell distortion, signs of settlement, corrosion, condition of the foundation, paint coating insulation and grounding systems.
External Ultrasonic Thickness Inspection must be done at intervals not exceeding five years when the corrosion rates are unknown.

Internal Inspection

Internal inspection is required to ensure that the bottom is not severely corroded and leaking. It helps gather data necessary for minimum bottom and shell thickness assessment, as well as identify and evaluate bottom settlement.
The interval from initial service to the initial internal inspection must not exceed 10 years unless special conditions outlined in API 653 are met. The interval between subsequent inspections is determined in accordance with the established corrosion rates. Unless a Risk Based Inspection assessment is performed as described in API 653, the interval must not exceed 20 years.

Reporting
A written report must be prepared by the Tank Inspector after every external or internal inspection. This report must include;
?    Date of inspection
?    Type of inspection (internal or external)
?    Scope of inspection including mention of areas that were not inspected and reasons.
?    Description of the tank (number, size, capacity, year of construction, material of construction, service history, roof and bottom design)
?    List of components inspected and their condition
?    Inspection methods and tests used e.g. visual, MFL, UT and the results
?    Corrosion rate of bottom and shell
?    Settlement survey measurements and analysis
?    Recommendations for repair, replacement, monitoring, calculated inspection interval
?    Name, company, API 653 certification number and signature of the Authorized Inspector responsible for the inspection
?    Drawings, photos, NDE reports and other pertinent information appended

Tank Repair and Alteration

All repair work must be authorized by the Authorized Inspector (AI) or an engineer experienced in storage tank design before commencement of work by a repair organisation. All proposed design, work execution, materials, welding procedures, examination, and testing methods must also be approved. The AI must designate inspection hold points required during the repair or alteration sequence and minimum documentation that must be submitted upon job completion. During repair and alteration, various minimum requirements of API 653 must be met.

Welding and Testing

Welding procedure specifications, welders and welding operators must be qualified in accordance with ASME IX. Preheat and PWHT requirements must be observed where required.

Examination and testing of repaired components shall be performed in accordance with sec 12 of API 653. Among techniques that are commonly used for testing are Visual Inspection, Magnetic Particle Inspection, Dye Penetrant Testing, Diesel Test, Vacuum Box Test, Radiography, hydrostatic test.

Conclusion

Storage tanks are a critical asset to the energy sector. Proper maintenance and care is vital to ensure continued availability in service and prevent costly failures. Timely inspection by an authorized inspector is important to evaluate the corrosion rates, remaining life, strength, stability, fitness for service and maximum filling height.

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