Editor's Note: This article first appeared in the Winter 2019 issue of IFW magazine.
Oil changes, teeth cleanings, mowing lawns, updating software – these are all forms of maintenance regularly performed. Changing the oil every 5,000 miles or so helps avoid engine failure. Visiting the dentist for a checkup biannually prevents cavities. Mowing the lawn keeps the grass cut and makes it a less hospitable environment for bugs and mosquitoes to breed. Updating a computer with the latest software typically fixes bugs that slow it down. All these forms of maintenance are completed to keep operations running smoothly.
Storage tanks must also be maintained, whether they contain oil, chemicals or water. This column will address how to maintain API storage tanks.
No matter what business you are in, it’s good to keep records to refer to or make comparisons. With tanks, original blueprints and drawings should be kept as reference points. These documents can be used to compare notes and measurements such as steel thickness.
After their monthly checks, operators should keep comprehensive inspection records on file for reference when the tanks are inspected. These will come in handy, and in some cases are required, when repairs need to be done or when it comes time for professional internal or external inspections.
API 653 recommends that a professional inspector perform external inspections every five years and internal inspections every 10 years. Owners or operators should also visually check the tanks on a monthly basis. If anything is amiss, a more detailed inspection and repairs are recommended. Tanks that have internal containment liners should be checked weekly.
Rafters, columns, girders, and bases – basically anything that supports a fixed roof – should be inspected for soundness (4.2.2). Any roof support found to be corroded or damaged should be repaired or replaced as necessary.
Factors that influence brittle fracture include plate thickness, the acting tensile stress on the tank shell, the overloading effect of a water test, the minimum design metal temperatures of the tank shell, the crack-like defects present, and the notch toughness of the shell plate material (de Wit, 1990). Inadequate draining that results in high localized stresses in the bottom plates can also cause the tank bottom to leak or fail (4.4.2).
Metal can break down, especially if it isn’t well maintained. Corrosion is one of the surest signs that a metal is weakening. Reddish brown streaks or spots are indicative of corrosion. If left unchecked, this can lead to tank failure. During their monthly inspections, operators should look for any signs of corrosion or weld deterioration.
Roof plates that show obvious signs of corrosion or have holes in them should either be repaired or replaced, depending on the severity. The same holds true if there are any cracks or punctures in the roof plates.
The lower part of a tank shell has the thickest plates to prove adequate support. If there are visible signs of deterioration like heavy rusting the plates should be repaired or replaced before winter arrives. Many tanks fail during the winter when it’s colder and the air pressure changes.
Little problems can snowball into major ones if the operator fails to maintain the tank. Pitting, or localized metal deterioration should be evaluated carefully. Pitting is thought of as more of a threat to a steel structure’s integrity than uniform corrosion because it’s harder to detect, predict and design against, according to the National Association of Corrosion Engineers.
However, localized pitting is generally not considered too much of a risk to a tank’s shell unless they are situated closely together or are larger pits. Like any problem, if it’s not fixed, it will get worse. If the pitting is unaddressed, eventually, the tank will fail, and oil, chemicals or gasses will leak and trigger an environmental hazard.
Because API tanks generally store chemicals toxic to the environment, it’s important to ensure the tank is in proper working order before it’s put into service.
Leaking water is a lot less hazardous to the environment than oil or a chemical. So, a hydrostatic test should be completed before a tank is put into service. A test should also be completed any time a tank is repaired or modified. This helps ensure there are no leaks, cracks or irregularities with the tank or foundation. The tank should be filled with water for at least a 24-hour period. If there’s an issue with the tank, it will become apparent pretty quickly during the hydrostatic test.
Once a tank is in service, operators should never overfill the tank. A good rule of thumb is to limit the fill height of the tank to less than 75 percent during the winter (de Witt, 1990).
Insulation and Seasonal Changes
Tanks are influenced by changes in temperature, expanding during warmer weather and contracting during colder weather. If a tank fails, history and research have shown that it will often fail in the winter when the temperatures are at their lowest.
Repeated exposure to moisture can cause steel to corrode over time unless the steel is protected. Insulation can help protect the tank from exposure. It can also help prevent brittle fractures. Insulating a tank’s shell will help keep the temperature of the tank more consistent with whatever chemical product is stored in the tank. Removable insulation panels are efficient and effective. Not only do the panels insulate manholes and pumps, but since they are removable, the panels are easy to take out and perform internal inspections.
FOUNDATIONS & RELEVELING
Concrete foundations can crack, erode, settle and deteriorate over time. This can occur due to natural conditions like frost or the presence of underground water. Chemicals and acids coming into contact with the concrete can also cause a breakdown. Cracking around an anchor bolt can indicate structural weaknesses like foundation settlement or a tank overpressure uplift condition (4.5.3).
Some preventative measures include fully water-testing the tank, repairing serious corrosion, releveling the tank, and avoiding shock loads on the tank shell.
If the tank’s foundation has extensive settlement or is generally deemed unsound, a tank owner might opt to have the tank releveled. “When floating-roof tank bases have experienced differential settlement, the roofs can bind and seals may not be effective or may be damaged. Releveling can often cause the tank to return to a round shape. Tanks that have buckled due to settlement or tanks that have been constructed with initial out-of-roundness are usually not susceptible to much improvement by releveling” (Philip E. Myers, Aboveground Storage Tanks, Releveling Tanks chapter, 1997, McGraw-Hill Professional).
If a tank bottom is replaced, owners should consider installing a leak detection system that channels any leak in the tank bottom to a place where it can be more easily observed outside the tank (4.4.5).
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