By Vince Carucci
API 653, “Tank Inspection, Tank Repair, Tank Alteration and Tank Reconstruction,” was first published by the American Petroleum Institute (API) in January, 1991. Its scope encompasses the maintenance, inspection, repair, alteration, relocation and reconstruction of existing, above ground, atmospheric storage tanks constructed per API 650, or its predecessor API 12C. If API 653 is intended to cover existing storage tanks, why should it be considered in the design of new storage tanks? For one simple reason: Ignoring API 653 when developing the design requirements for a new storage tank could result in operating or maintenance penalties later. Here's how.
We'll start with the premise that an owner intends to comply with API 653. API 653 specifies minimum acceptable bottom plate thicknesses before repairs are required, and requires that an internal inspection be made before the bottom can corrode below these levels. The acceptable bottom plate thickness (i.e., 0.1 or 0.05 in.) depends on whether the bottom is bare steel, has an internal lining, or a secondary containment and leak detection system is installed. The minimum bottom plate thickness and inspection frequency requirements of API 653 can have a direct impact on several design decisions that must be made with respect to the bottom of a new tank. These, in turn, directly affect maintenance/inspection planning and operational flexibility for the tank.
API 650 requires a minimum bottom plate thickness of 0.25 in. It does not mandate an internal lining, cathodic protection system, nor secondary containment and leak detection system. An owner wants to maximize the required interval between internal inspections, typically targeting for 10 years. The owner will typically be able to estimate anticipated corrosion rates for both the liquid and soil sides of the tank based on past experience. The question that must be answered is whether a standard API 650 tank design is acceptable, considering API 653 requirements and the owner's maintenance/ inspection planning and operational requirements.
Depending on the situation (e.g., corrosion rates), it may be necessary to exceed the basic API 650 requirements to achieve the internal inspection frequency goal. The choice of the approach to take will be based primarily on economics. Options which may be considered include:
Note also that local regulatory requirements must also be met as part of a new tank design. This will also affect meeting API 653 requirements. For example, storage tanks storing hazardous waste must have a secondary containment and leak detection system installed to comply with Federal regulations. With this system, API 653 would permit the bottom to thin to 0.05 in. before repairs are required.
Based on these considerations, there should be coordination among the maintenance, inspection, operations, project and engineering staff to ensure that the storage tank built today does not result in unnecessary costs later.