Large Storage Tank Inspection
Chris Brooks, president of Heartland Tank Services, Inc. stressed the type of inspection routinely needed for large storage tanks that have been in service for a few years during the 2011 National Agronomic, Environmental, Health and Safety School.
Brooks said, “Due to the corrosive nature of most liquid fertilizer solutions and weight (specific gravity) of those solutions, liquid fertilizer storage tanks require routine and qualified mechanical integrity practices to maintain a safe storage environment.” A major concern in storing liquid fertilizer he explained is having a tank that will withstand the specific gravity of liquid fertilizer usually 1.3 to 1.5. For instance, oil has a much lower specific gravity, and thus requires less thickness of steel for proper containment.
Brooks conveyed detailed information on API 653 (American Petroleum Institute guidelines) inspections for above ground storage tanks. A very important part of receiving the API 653 Certified Inspector designation that he holds is obtaining an NLPA Level II Ultrasonic Thickness (UT) technician certification.
UT measurements need to be taken of the shell, floor and roof of large storage tanks, and the location of those results must be logged and recorded. Subsequent UT tests should be conducted within the same area as previous tests to compare any changes over time.
An API 653 inspection of a tank usually requires more than an empty and rinsed tank. “If you cannot see the steel, how are you going to inspect it? The tank has to be really clean,” Brooks said. Tanks without coatings or bladders must be power-washed from the top of the internal shell down to and including the floor. An inspection could require a brush sand blast of the interior, which means the cleaning can sometimes cost more than the contracted inspection. It is not unusual to find corrosion under hardened scale deposits that did not dislodge with a rinse.
All areas inside the tank need to be accessible so that the inspector can reach high and low for the inspection. Only looking at the bottom two courses of steel is not acceptable.
AREAS OF CONCERN
“We are looking for shell and floor plate pitting and weld deterioration,” Brooks said in explaining one of the major tasks of a certified inspector. The inspector looks for proper weld spacing, proper reinforcement pads on nozzles and man-ways, and more. Inspection of the welding has to be completed internally as well as externally on the tank. Inspectors are looking for any shell distortions at weld joints such as peaking and banding. “Joint efficiency” is the terminology for weld quality, Brooks explained, “A lower joint efficiency given to the tank could result in a reduced maximum allowable fill height.”
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