Large Storage Tank Inspection

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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.”

©ISTOCKPHOTO.COM In addition to the visual examination of the floor, a vacuum test of the welds seams should be conducted.  In some cases, a complete magnetic flux leakage (MFL) scanning of the floor is necessary to identify bottom side corrosion. Another test to validate or confirm this type of corrosion can be to cut out a small circle of the steel, or a “coupon,” in the floor.

The inspectors have to look upward, too, as the roof rafters, support column(s) and attachments are an area of concern. The roof is an important part of the tank, if for no other reason than keeping water from getting inside. “I cannot tell you how many roof rafters we’ve found partially hanging or completely down in our inspections,” Brooks said.

A settlement survey will check for “planar tilt.” Tilt of a tank puts more stress on one area than others. If there has been settlement, it is common for the tank to be reduced or “de-rated” for a lower fill height than original construction allowed. It is not unusual for tanks to be jacked and foundations built up, he noted. “Foundations are very important for the life of a tank.” Brooks explained that API 650 (new construction) tanks are required to have the floor extend three inches beyond the tank shell or chime.  Having dirt, gravel, and vegetation growing next to a tank will create corrosion damage, especially at the chime. 

“If you are changing product in your tank, you need to let the inspector know the specific gravity of product to be placed in the tank,” Brooks said. This will also determine appropriate fill height. A problem can occur with trying to utilize a tank originally constructed to hold a product different from liquid fertilizer.

Additionally, tanks with bladders must have a bladder compatible with the material being stored. Coatings are another way that tanks are less vulnerable to corrosion but need expert sandblasting before the coating is applied. Both the bladder and coating can encumber the inspection process but can be good investments, he said.

POST INSPECTION PROCEDURES

A full inspection report must have these areas addressed:

  • Tank data page
  • Shell, roof and floor remaining life (RL) calculations
  • Settlement survey data and calculations
  • Maximum fill height
  • A log and mapping of all UT readings taken
  • Pictures taken during inspection
  • NDE (non-destructive examination) certificates by the inspector and NDE level of achievement

Brooks includes much more than numbers and short comments in any inspection report. An in depth executive summary of the inspection is definitely necessary in his opinion. “The summary needs to tell you what was done when inspecting that tank,” he explained.  Inspectors, he said, have an obligation “to try and make it (the summary) as simple and understandable as possible.”

Often there are recommendations for mandatory or discretionary repairs. Mandatory repairs are those items that must be completed per API specifications to place the tank in service.  Discretionary repairs are those that do not need to me made but are encouraged to extend the life of the tank. 

“You need documentation of what repairs are needed and when they were completed.  That repair report should stay with the inspection report and be maintained,” Brooks said.

“As a general rule,” he said, “A tank has to prove to me that it is suitable for continued use.”

NEW CONSTRUCTION

Brooks warned the audience to keep on top of the inspections process as well as new construction projects. He believes companies involved in both are very competitive, and, therefore, cutting corners can happen, especially in new construction.

“You need to know enough about API 650 to ask the right questions,” he said. Brooks said a storage tank engineer is crucial for construction and assessment of a specific tank for a specific use.

A main concern with new construction should be having verified specifications of the steel that is being used, course by course from bottom to top and for the floor. He also suggested a company ask for eight-foot steel for the floor instead of the lower cost six-foot steel because it will decrease the amount of welding. 

As for welding, he said a company should ask for welding procedure information and it should be put with the tank’s records. Whether a tank is new or old, Brooks said, records are extremely important. “Maintain a history and records about every tank.”

DOCUMENTS AND REFERENCE

Inspection and interval for inspection recommendations are outlined in the API 653 document, which Brooks referenced often in his presentation. He also noted API 650, with its specifications for new storage tank construction. Both documents are available through the American Petroleum Institute web site at www.api.org/standards, and The Fertilizer Institute brochure on “Mechanical Integrity Guidelines for Aboveground Storage Tanks of Liquid Fertilizer” that is available under Resources on the www.tfi.org web site.

This article is brought to you in cooperation with the National Agronomic Environmental Health and Safety School (NAEHSS).


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