Ultrasound measurements can determine the rate of corrosion. - Photos provided by Pittsburg Tank & Tower

Ultrasound measurements can determine the rate of corrosion.

Photos provided by Pittsburg Tank & Tower

American Petroleum Institute (API) created the API 650, 651 and 653 to provide standards for the safe storage of petroleum products. These standards were designed to protect employees, the public and the environment. However, it is the tank owner’s responsibility to put these standards into place. API tanks must be inspected regularly, and accurate information must be obtained to prolong the life of the tank safely. Researching and studying the characteristics of the product stored is essential, and could be lifesaving. The Material Safety Data Sheets (MSDS) can provide this information, and should be referred to before any type of inspections or repairs are performed. Serious injury or death could occur if the inspector has not received the proper information needed. Ethanol, for example, reacts violently and explodes with several products. It cannot be mixed or stored with other fuels. Many precautions are to be taken when inspecting a tank with such highly explosive and hazardous contents. A reputable tank company with proper training should be hired to perform the inspections. This training requires ten hours of OSHA, Red Cross First Aid, CPA and BBP.  Pulmonary function test, medical evaluations and respirator fit test should be performed on employees before entering the tanks. Tank owners should have a rescue team available no more than fifteen minutes from the site. The rescue team should be properly equipped and trained. 

After all safety precautions have been addressed, the features of the tank and information needed should be determined. Several features of an API tank need to be inspected regularly, and a written report should be provided to the owner after each inspection. The report should include a detailed evaluation with photographs, recommendations of needed repairs, code updates, and a detailed cost estimate for each item.  All aspects of the tank should be inspected for structural, safety, and coating conditions in accordance with API standards.  Vents, screens, manways, overflows, ladders and drain valves should also be inspected to prevent unauthorized access. The tank foundation, soil, and settlement conditions should be monitored. Any tank distortions and all operating conditions such as filling and emptying rates need to be noted. All visual maintenance checks should be documented with corrosion monitoring, and type of leak detection system used, such as double bottom liners or liners under tank bottoms with leak detection pipes. The cathodic protection system used, stress levels and UT readings from previous inspections can help give a more accurate overall present condition. Other documents received during construction should also remain with the tank owner for the life of the tank. These documents include the design of the tank, loadings, metal characteristics, weld testing, and location of the tank. All information is useful during an inspection, and it can provide the inspector with a complete picture to the life of the tank.

A pressurized LPG tank. -

A pressurized LPG tank.

After determining the information needed, the next step is to choose which inspection should be performed. There are three primary types of inspections that should be performed regularly.

  1. Routine In-Service Inspection

This inspection can be performed by someone at the facility, but it should be performed on a set, routine basis with each part of the tank being visibly inspected with a check sheet in hand. The general foundation condition should be noted and any signs of settlement. Any evidence of leaks, corrosion, or damage should be listed and reported. Drains, secondary containment, plates, shell, roof, nozzles, attachments, ladders stairs, and platforms should all be inspected and the overall condition of the tank noted. These routine in-service inspections help determine the scope of the formal inspections.

  1. Formal External In-Service Inspection

This inspection should be performed by an authorized inspector at least every five years as stated in API 653 6.3.2. The inspector should use the information provided by the routine in-service checklist with their checklist that includes corrosion, distortions, leaks, and damage. External ultrasonic thickness measurements of the shell can be taken to help determine the integrity of the shell and a rate of corrosion. Creating a matrix chart to locate specific areas that have been tested before can provide a more accurate picture than random testing. Annual or biennial inspections may be needed if the rate of corrosion seems higher than normal. Often higher than normal corrosion occurs when a tank is not adequately protected or standing water and debris has remained for an extended period.  Corrosion can also occur under the insulation and on the underside of floor plates. Nozzles attached to the roof, shell and floor should also be included in this corrosion search.

  1. Formal Out-of-Service Internal Inspection

This inspection requires draining and cleaning the tank to achieve a safe working environment for accurate readings. The internal visual inspection requires the inspector to do more extensive investigations for corrosion/erosion, thinning/cracking, and other damage that may have occurred to the tank. The floor of the tank is given a detailed inspection including UT floor readings.

If the tank is experiencing higher than normal corrosion rates, a professional tank company can also perform a risk analysis. A risk analysis can help determine the frequencies and types of tank inspections needed to safely maintain the tank. The analysis considers the tank’s age and whether it has a secondary containment or cathodic protection. Deterioration often occurs first on the tank bottom. Therefore, the structure and condition must be monitored carefully to prevent the risk of leakage, a catastrophic environmental disaster and great financial loss.  History and knowledge of corrosion areas will help decide the extent of ultra-thickness data needed to accurately determine a safe timeline for inspections. The chance of failure and the damage that could result in that failure are weighed. Depending on the variables at stake inspections may need to be more frequent than originally recommended.

These inspections and tests help ensure tanks are operating safely, but inspections do require time and discipline to be perform correctly. A reputable tank company trained to inspect and test API tanks should be hired to perform these services, and the tank owner should maintain all information collected during these inspections to completely understand the characteristics and history of the tank.