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Storage Tanks
Correct problems to prevent API tank failures

By ERIKA HENDERSON/Pittsburg Tank & Tower

Tank failures can occur from many external and internal elements: defective ventilation, leaks, the environment, natural disasters and operator’s error. Most deficiencies can be found during an American Petroleum Institute inspection. Structural, safety and coating conditions are inspected and evaluated in accordance with API 653 standards created for tank inspections, repairs, alterations and reconstruction of all tanks. Tanks constructed prior to API codes could contain many deficiencies and great potential risk that require more frequent inspections and closer monitoring. Not just anyone can inspect to API 653 standards. A reputable tank company trained to inspect and test API tanks should be hired to perform these services, and a written report containing a tank overview with test results, photographs and deficiencies should be given to the tank owner. All information collected during the inspections should be maintained by the tank owner and used to further understand the individual characteristics and history of the tank. Safety concerns and deficiencies found during the inspection must be addressed timely. However, the deficiencies that could result in a tank failure must be addressed and corrected immediately.

 Ventilation

All tanks must be adequately vented to prevent the development of a vacuum or pressure buildup. If vents are not sized appropriately, or if they have some element of deficiency, they may not function properly, resulting in tank failures. Vents can also become clogged from environmental factors and should be checked regularly. If the inspection report shows any deficiency with the ventilation system, this should be repaired immediately. Operator’s error can also occur with the ventilation resulting in a tank failure whether a tank is rapidly emptied or overfilled. Rapidly emptying a tank can produce a vacuum, collapsing the shell or roof. Overfilling or filling too fast can lead to pressure buildup, leaks, spills or roof damage.

 Leaks

Great effort has been put into the design of API tanks to prevent leaks, but leaks still occur. Leak detection and secondary containment is crucial when dealing with hazardous materials that can result in disastrous fires, environmental damage or great financial loss. Leaks can occur anywhere and are often the result of corrosion, failed coatings, poor maintenance, cracked welds or settlement. 

 Corrosion

The most vulnerable area for leaks and cracked welds is the tank bottom plates. These plates are subjected to internal corrosion and bottom side corrosion. Bottom side corrosion can occur more rapidly if rainwater reaches and ponds under or around the tank often. When water gets under the tank, the foundation can settle and floor weld seams can crack. 

Extreme corrosion can also be found on the roof where consistent exposure to sunlight and ponding occur. The underside of the roof and internal floating roofs are also susceptible to heavy corrosion where vapors and condensation collect. Roof replacements are common when this type of extreme corrosion takes over, and aluminum geodesic dome roofs are often used to replace the more costly carbon steel cone roofs.

 Metal Loss

The thickness of the steel should be a serious concern depending on the ultrasonic readings taken during the inspection. If the thickness of the steel has deteriorated to unsafe levels, the tank’s structural integrity could be compromised. Large-scale metal loss often occurs in concentrated areas or pits. Deep pitting and corrosion can occur rapidly in tanks contaminated with water, acids and soluble salts. The temperature of the product and the humidity level can accelerate this rate of corrosion. Deep pitting can be found beneath the thermal insulation and may not be detected unless the thermal insulation is removed. Deep pits and excessive thinning metal plates that could result in leaks or jeopardize the structural integrity of the tank should be repaired immediately. 

 Environment

Lightning, wind, hurricanes and earthquakes can cause catastrophic damage to API tanks resulting in fires, explosions, toxic releases and water pollution. Tanks should be grounded, contain the proper amount of freeboard and be designed appropriately for high winds. Storage tanks hit by a direct lightning strike can ignite flammable vapors or blow off the roof. High winds cannot only blow the roof off, but also collapse or overturn an empty tank if it is not properly anchored to the foundation .Ground motion from earth-quakes creates liquid sloshing that moves independent of the tank structure. Sloshing puts stress on the tank walls and can cause a rupture or damage the roof if the proper amount of freeboard is not applied. If the inspection report shows deficiencies in any of these areas, repairs should be made immediately.

 Equipment Failure

Valves that do not function properly can create a tank failure as well. Tanks should be equipped with proper valves designed to close automatically if the inlet pipe malfunctions. Operating, shutoff and check valves should also function properly for protection against back flow. Floating roof’s seal system must remain in proper working order when the liquid level is raised or lowered. Liquid level alarms, high-level interlocks and all control panels must function properly to prevent overfilling, or a tank failure could occur.   

 Safety Measures

Even though there are numerous tank failure elements that surround API tanks, most tank failures can be avoided if these deficiencies are found and corrected quickly. API tank repairs should only be performed by a trained and reputable tank company. Poor workmanship can contribute to tank failures just as easily as any other element; safety and quality control must be top priority with companies hired to perform these services. Hazard reduction measures including proper hot-work procedures and a plan of emergency action must be established. Material Safety Data Sheets (MSDS) for the particular chemical the tank contains must be reviewed, and all safety measures must be applied accordingly. All OSHA, NFPA and API standards must be reviewed and understood before any work is performed. If these measures are not taken, a tank failure could still result. Contact Erika Henderson at 270-826-9000 ext. 228 or send questions to sales@watertank.com.

 
 

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