Hydro Testing & API-653

Understanding API 650 and API 653 Hydro Testing

Second of two parts

The API 653 standard covers steel storage tanks built to API 650 and its predecessor API 12C. API 653 is the referenced standard for tanks after the been placed in service providing the minimum requirements for maintaining the integrity specifically addressing the inspection, repair, alteration, relocation, and reconstruction of tank assets. API-653 is intended to address the scope concerning, tank foundation, bottom (floor), shell, structural supports, roof, attached appurtenances, and nozzles (only the face of the first flange) or first welding / threaded connection.

The API 653 Standard was first published by the American Petroleum Institute (API) in January 1991. With a specific scope around the inspection, maintenance, repair, alteration, relocation and reconstruction of existing above ground storage tanks constructed per API 650, or its predecessor API 12C. Within the API-653 Standard Section 10 references Examination and Testing requirements for all repairs or alterations. This section, is sometimes referred to as the Hydro Testing Section and has been revised from the original standard over the years to include fit for service consideration around the testing requirements. Section 10 defines "Major Repairs" as "operations that require cutting, addition, removal and/or replacement of the annular plate ring, the shell-to-bottom weld, or a sizeable portion of the shell." These operations include such situations as cutting and installing large shell nozzles or penetrations, large shell-plate replacement, extensive repairs to shell butt welds, extensive repairs to the shell-to-bottom weld, annual levelling activities, installing a new bottom, and lifting or jacking the shell or tank. The Standard states that a contractor must conduct a full hydrostatic test after major repairs unless the tank owner waives the procedure. Hydro testing after major repairs is a timely and cost-effective way to get a tank asset back into service as well hydro testing is a useful for testing for leaks, re-establishing / checking for settlement of foundations that have been upgraded or reworked, and confirming the structural integrity of tank repairs.

Some end user specifications do consider API 653 when designing and constructing new atmospheric above ground tanks for many reasons but quite simply “these are long term assets with significant operational value and overlooking API-653 when developing the design requirements for a new API 650 tank could result in operating or maintenance issues later in an assets lifecycle”.  For example, API 650 requires a minimum bottom plate thickness of 0.25 in. API does not mandate any internal protection like internal coating or cathodic protection system or leak detection systems. (not withstanding some state and provincial requirements). Taking API-653 into consideration an owners designers / engineers who wants to maximize the required interval between internal inspections may estimate the anticipated corrosion rates for both the cargo and tank base soil sides of the floor, this can be determined based on theory or practical experience. API 653 provides a 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. Therefore, if the determination is such that the minimum floor thickness in API 650 will not achieve the desired inspection / maintenance goal then, considering API 653 requirements may provide insight into added value in additional CA or linings. For example, how much more will a 0.375 in. thick bottom cost as part of the original construction, and how much will that increase the required internal inspection interval.