Preventing Aboveground Storage Tank Replacements with API-653 Tank Inspections

API Certified welder adding a steel patch over a corroded area

This article details how aboveground storage tank failures in Massachusetts and New Hampshire tanks can be identified with an API-653 inspection and repaired, avoiding tank replacement.  Tank failures in the primary areas of a tank will be addressed:

    A tank inspection is the first step to determine whether a tank can be repaired, or should be replaced. Evaluating the tank shell for distortions, flaws, deterioration and corrosion, will provide valuable data to assess the suitability of repairing the tank and keeping it in service. Corrosion may be found in the form of pitting or uniform loss of metal over a large surface area. The tank inspector performs an analysis of shell plate thickness and material. A thorough aboveground storage tank inspection that includes averaged measurements of the tank shell is needed to satisfy API 653 requirements for continued operation of the tank.

    Meeting the API-653 Standard for Aboveground Storage Tank Repairs

    Implementing an inspection program based on a particular industry standard is ultimately up to the owner/operator. When an owner/operator indicates in the Spill Prevention, Control and Countermeasure Plan (SPCC) that he intends to use a standard to comply with a particular EPA rule requirement (e.g. integrity testing), then it is mandatory to implement the relevant portions of the standard.

    A close-up of the plate shell and seam repairs.

    The American Petroleum Institute (API) Standard 653, “Tank Inspection, Repair, Alteration, and Reconstruction” and the Steel Tank Institute (STI) “SP001 Standard for the Inspection of Aboveground Storage Tanks” (STI SP001) are two commonly used inspection standards for aboveground bulk storage. The scope of STI SP001 Standard for the Inspection of Aboveground Storage Tanks by the Steel Tank Institute (STI) includes the inspection and testing of: aboveground shop-fabricated tanks, small field-erected tanks, portable containers, and associated secondary containment. API-653 applies to carbon and low-alloy steel tanks that were built in compliance with the requirements of API-650 and are over 50 feet tall or having diameter greater than 30 feet. The APi-653 standard also covers inspection, alteration, relocation and reconstruction. 

    Types of Aboveground Storage Tank Failures Identified During Tank Inspections and Where They Occur

    The primary structural components evaluated during tank inspections are those structural components that directly affect the tank's capability to store liquid. These components are as follows:

    • Bottom
    • Shell
    • Foundation
    • Roof

    Drawing of a Typical Aboveground Storage Tank

    Illustration of an aboveground storage tank diagram

    Tank Bottoms

    Evaluation of the tank bottom is performed using the methods in the API standard and if done properly will ensure detection of leaks that may cause environmental damage. Shell-to-bottom welds are critical and any cracks in this area must be removed and the welds repaired. 

    Primary Sources of Tank Bottom Failure

    Here are a few reasons why a tank bottom may fail:

    • Corrosion of welded joints
    • Poor drainage around the tank base
    • Irregular settlement that causes the base to become unbalanced
    • Deficient compaction underneath foundation pads 
    • Inconsistent fill material under the tank bottom
    • Shell settlement due to roof loads
    • Cracks in weld joints
    • Excessive corrosion and pitting (exceeds predicted service rates)

    Fluid level overflows the tank containment area

      Determining Tank Bottom Plate Thickness

      The inspector will use tools like the ultrasonic thickness measurement or magnetic flux leakage tool to examine a tank bottom.  Ultrasonic thickness measurement tools are more precise and are typically used to confirm measurements taken by the magnetic flux leakage tool. Data taken from tank bottom scan is used to project the remaining thickness by way of statistical analysis.

      Below is an example of bottom plate thickness readings taken from a one million gallon steel aboveground tank that was built in Massachusetts to API Standard 650. Bottom plate ultrasonic thickness data was collected utilizing random readings. At least three (3) readings per bottom plate were taken. Nominal bottom plate thickness of this bottom is 0.375 inch.

      Bottom Plate Thickness Readings (in inches)
      Plate Reading 1 Reading 2 Reading 3 Reading 4 Reading 5
      1
      0.326
      0.343
      0.336
      0.331
      0.345
      2
      0.302
      0.326
      0.341
      0.337
      0.344
      3
      0.336
      0.331
      0.326
      0.352
      0.335
      4
      0.326
      0.321
      0.328
      0.331
      0.323
      5
      0.340
      0.342
      0.328
      0.331
      0.337
      6
      0.331
      0.339
      0.329
      0.331
      0.336
      7
      0.343
      0.345
      0.327
      0.346
      0.320
      8
      0.326
      0.331
      0.328
      0.329
      0.329
      9
      0.302
      0.335
      0.332
      0.327
      0.356
      10
      0.317
      0.319
      0.334
      0.318
      0.343
      11
      0.334
      0.338
      0.330
      0.328
      0.334
      12
      0.322
      0.338
      0.340
      0.342
      0.337
      13
      0.313
      0.317
      0.334
      0.318
      0.320
      14
      0.328
      0.314
      0.325
      0.318
      0.334
      15
      0.331
      0.329
      0.430
      0.334
      0.330
      16
      0.338
      0.343
      0.342
      0.338
      0.308
      17
      0.337
      0.335
      0.328
      0.327
      0.334
      18
      0.337
      0.328
      0.341
      0.333
      0.358
      19
      0.334
      0.338
      0.326
      0.322
      0.330
      20
      0.319
      0.328
      0.332
      0.319
      0.322
      21
      0.326
      0.328
      0.325
      0.330
      0.306
      22
      0.328
      0.329
      0.340
      0.325
      0.325
      23
      0.325
      0.328
      0.319
      0.327
      0.307
      24
      0.322
      0.319
      0.315
      0.323
      0.325
      25
      0.327
      0.337
      0.332
      0.349
      0.335
      26
      0.351
      0.332
      0.324
      0.320
      0.356
      27
      0.341
      0.320
      0.327
      0.334
      0.338
      27
      0.341
      0.320
      0.327
      0.334
      0.338

      Note:  Readings in bold red are within thickness tolerances of API-653 and were found to be the lowest recorded after MFL test discoveries.

      Tank Diagram of TransCanada's Aboveground Storage Tank

      Tank Shells

      Tank shell distortions include buckling, flat spots, out-of-roundness and peaking at welded joints. These can be caused by high wind, foundation settlement, or poor shell fabrication or repair methods. Compromised lower tank shell near the tank seam

      Areas of failure in Tank Shells include:

      • Tank thickness
      • Distortions
      • Flaws
      • Welds
      • Shell penetrations (ie: nozzles, manways, cleanout openings)

      Assessment of a tank shell is done through measuring shell thickness and taking into account the static fluid head pressure, settlement, roof, wind, and seismic loads. Uniform loss of metal usually occurs over a large area but may be localized. The rate of corrosion is determined by taking ultrasonic thickness measurements and comparing them to the original tank design specification. The lowest shell plates are in the critical zone and allowed to have a loss of 0.1in. The strength of the plate will not be affected by isolated pitting.

      Tank failure due to brittle fractures is another area of assessment. The highest incidence of failure is shortly after erection or after the first filling during cold weather. If a tank withstands hydrostatic testing under cold conditions then the risk of future brittle fractures are minimal.  Tank shells thicker than 0.5in, and operating at a temperature of less than 60 degrees Fahrenheit, have a very low possibility of failure when designed to the API-650 standard.

      Tank shell penetrations can corrode from either the inside or outside of the tank. If a shell nozzle is in need of repair then adding reinforcing plates is recommended. Welding must be done on both sides of the new plate when access is available.  The addition of reinforcing plates may also be required if there is a change in the maximum fill height of the tank. When installing or replacing a shell penetration, a 24-hour hydrostatic test is required.

      Tank Foundations

      Excessive foundation settlement of storage tanks can affect the integrity of tank shells and bottoms. Therefore, monitoring the settlement behavior of tanks is a recognized practice to assess the integrity of tank bottoms. Settlement, erosion, and cracking of concrete are the principal causes of foundation deterioration. Concrete damage is caused by:

      • Groundwater
      • Expansion by frost
      • Calcining (loss of hydration)
      • Sulfate attack (due to sulfates in groundwater)
      • Temperature Cracks (due to extended high temperatures)

      This photo shows the backfilling of a replacement ring wall foundation.

        Photo of crew backfilling an aboveground storage tank foundation

        Evaluation of tank foundations must also include the anchor bolts. Settlement in tank foundations can cause excessive cracking of the concrete in the area where the anchor bolts were embedded.  

        If a foundation is installed on a true horizontal plane, but not on a ring wall, then the shell level must be within ±1/8 inch in any 10 feet of circumference and within ± 1/2 inch in the total circumference measured from the average elevation. When installed on a true horizontal plane with a concrete ring wall then the shell level must be within ± I/8 inch in any 30 feet of the circumference and within ± 1/4 inch in the total circumference measured from the average elevation.

        Tank RoofsPhoto showing uniform corrosion of a steel beam roof support for an aboveground tank

        Roof plates corroded to an average thickness of less than 0.09 inch in any 100 square/inch area or roof plates with any holes through the roof plate must be repaired or replaced. Insulated tanks must have enough insulation removed to determine the condition of the roof. Roof support columns in supported roof tanks must be inspected as part of the roof structure. The dimensional tolerances for structural framing must be reasonably true to line and surface.

        Corrosion on the lower third of supported cone rood columns is the most common area to fail. Roof supports can be repaired if the new sections meet the stress levels in Section 3.10.1 of API Standard 650. Repairs to self-supporting roofs require a nominal thickness of 3/16 inch or the plate thickness specified in API-650. Emergency vent devices and roof to shell welds must also meet the API-650 requirements. 

        A Case Study of a Leaking Aboveground Storage Tank

        A fiber optic cable manufacturer in New Hampshire discovers a leak has occurred from a chemical supply tank connected in series with 3 other storage tanks. The tank is heated to keep the chemical in a fluid state and has to be taken offline to repair it. The hardened compound now presents an impediment to performing a proper inspection of the leaking tank. In order to examine the compromised area, the tank must be taken off line and the compound removed from the tank bottom using jackhammers and chisels. An API-653 inspection can now be performed on the tank to determine the cause of the leak and recommendations for the repair or replacement of the storage tank.  

        Tank Repair Costs Versus Tank Replacement Costs

        Tank Repair Costs

        An approximate cost to repair the 15,000-gallon aboveground tank would be $24,000 dollars. If the proposed alterations, repairs and restoration meet API-653 then you can estimate the repair of your aboveground storage tank. This repair would include sand blasting and removal of any paint and surface corrosion. The areas affected by the corrosion would be re-inspected to confirm that the thickness remains within tolerance.  The internal and external surfaces of the tank would be prepared, primed, and coated.  The restoration of the coating is required to prevent corrosion from reoccurring on the surfaces of the tank.  The repaired tank would now meet the intent, guidelines, and requirements of API 653 and would be fit for continued service and operation.  The repair time in this example would be 4 days and because the tank would be bypassed there would be no downtime for the manufacturer.

        Technician is monitoring the flow of sandblast medium 

        Tank Replacement Cost

        The cost to replace the 15,000-gallon aboveground tank would be approximately $80,000 dollars. A crane would be required to open a portion of the roof for access, remove the leaking tank and install the new tank. A vertical tank, built to a customer’s specification, could take up to 6 weeks to build and deliver. Hydrostatic testing is required when a tank has undergone major repairs or reconstruction. A full 24-hour hydrostatic test would be performed, unless an engineer with experience in API 650 design reviews the repairs. The tank in this example is insulated and heated to keep the petroleum product fluid so the installation time is double that of a standard storage tank. 

        Besides cost, the biggest issue facing a production facility is downtime of a manufacturing line. Additional costs resulting from manufacturing line downtime are not factored into the replacement price. This fiber optic cable manufacturer could incur a $30,000 loss while the cable jacketing line is shut down. 

        The Benefits of New Tank Construction

        Photo of a water storage tank used in fire protection

        Most modern designs provide 110% containment of the primary tank. As a construction material, steel is strong, affordable, reliable and environmentally friendly. Steel’s unique combination of properties and characteristics enable it to achieve performance levels required in today's storage tanks. In double-wall tank designs two thinner walls of steel act as a single structural unit. This reduces the cost to build the tank and provides a port for continuous monitoring. 

        Double-wall above ground fuel storage tanks provide a UL-142 listed, time tested fluid storage solution. Some of the benefits are:

        • Rust and corrosion resistant.
        • Performs in even the most aggressive environments.
        • Resistant to rain penetration, flood damage, and impact.
        • Can be tightness tested on site
        • Can be monitored via interstitial space for leak detection

        Bolted Flat-panel Tanks

        A bolted water tank built by Tarsco

        Flat-panel tanks offer performance, reliability and ease of installation. Epoxy powder coating techniques result in superior coverage to ensure long term service in the field.  Using the latest sealant to replace all strip gaskets, bolted flat-panel designs dramatically reduces the possibility of bolted-panel seam leaks. Here are some features/benefits of bolted steel tanks:

        • Interchangeable panels make it easy to expand tank size or replace damaged panels
        • Custom engineered to fit local building codes including wind, seismic and snow loads
        • Flat-panel design provides a smooth interior wall surface
        • Backed by a 30-year warranty
        Tank Repair and Replacement Prevention

        Performing a tank inspection service at 3 year intervals will help identify weakened welds and thinning metal, and provide estimates on tank life. Routine maintenance can prevent tank corrosion and deterioration. Fuel storage tanks require water monitoring and removal to ensure high quality fuel. Exterior coatings on tanks should be maintained because rust or corrosion can weaken a tank’s structure. Grounding straps and sacrificial anodes should be checked because they can prevent corrosion. Tank vents are needed to regulate internal pressures and should be inspected to make sure they aren’t blocked and are operating properly.