Changing the service or operating conditions of an existing aboveground storage tank (AST) is very common. The American Petroleum Institute (API) and other worldwide industry standards recognise the need to change the service of an AST and provide guidelines for evaluating the fitness-forservice of the tank. When industry standards are not observed, changing the service or operating conditions of an AST can sometimes lead to adverse or even disastrous results. ASTs are re-rated for a variety of reasons, the three most common ones are: a change in the liquid to be stored; a change in the operating conditions; or a significant change in the physical condition of the tank. The magnitude and extent of changes to the AST may range from simple to complex and any combination of the above three reasons may be cause for re-rating the AST.
The API has developed industry standards for performing fitness-for-service evaluations that are used throughout the world. The most widely recognised industry standard for the evaluation of field-erected ASTs is API Standard 653, ‘Tank Inspection, Repair, Alteration and Reconstruction’. This also references API Standard 579, ‘Fitness-for-Service’ for performing more detailed fitness-for-service evaluations when circumstances warrant a more comprehensive assessment. Other standards exist and are widely used, such as those of the Steel Tank Institute (STI) for shop-fabricated tanks and other non- API type tanks. API 653 explicitly employs the principles of API Standard 650, ‘Welded Tanks for Oil Storage’. However, API 653 states that storage tank owners/operators may apply API 653 to any steel tank constructed in accordance with a tank specification…..
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