API 653 key terms

One horizontal ring of steel plates that forms part of the cylindrical side of a tank. Courses are stacked and welded to build the shell and are numbered from the bottom upward.

The ring of bottom plates that lies under and around the inside edge of the shell. It is usually thicker than the rest of the bottom and carries shell loads into the foundation.

A sharp outward bend or kink in the shell at a horizontal weld seam, caused by weld shrinkage. It is measured with a weld reinforcement gauge and limited because it concentrates stress.

A flat or dished spot in the shell beside a horizontal weld seam, forming a band of distortion around the circumference. Like peaking, it is measured and limited to control shell stress.

Deviation of the shell from a true circle, taken as the difference between the largest and smallest radius at a given level. Excessive values make fit-up difficult and concentrate stresses.

A shell thickness calculation in which each course is designed for the liquid head one foot above its bottom edge. It is the standard approach for most tanks and is used to compute minimum acceptable thickness.

A shell thickness calculation used for large-diameter tanks in which the design point for each course is chosen to give the least required thickness. It accounts for the stiffening effect of lower courses.

Downward movement of the shell caused by foundation compression or soil failure. Levels are taken at points around the circumference and compared with limits for uniform, planar-tilt, and out-of-plane settlement.

A nondestructive method that sends high-frequency sound into steel and reads the echo return to determine remaining wall thickness. It is the main tool for mapping corrosion in shells, bottoms, and roofs.

A bottom inspection method that magnetizes the floor and uses sensors to find where flux leaks out at areas of metal loss or pitting. Scanning is done from inside an empty tank.

A leak test in which a soapy solution is applied to a weld, a clear box is sealed over it, and air is drawn out. Bubbles under the box reveal a through-thickness leak.

The speed at which metal thickness is lost, found by dividing the difference between two thickness readings taken on known dates by the elapsed time. It is normally reported in mils or millimeters per year.

The smallest wall thickness a shell course, bottom plate, or roof may have while stresses stay within allowable limits for the design liquid level and product. It sets the trigger point for repair decisions.

The estimated time before a measured thickness falls to the minimum acceptable value, calculated by dividing the thickness above the minimum by the corrosion rate.

The longest time allowed before the next internal or external inspection. It is set from corrosion rate, remaining life, and service conditions, and there is a ceiling on how far it can be extended.

A planning method that ranks tanks and components by the likelihood and consequence of failure, then directs inspection effort where risk is highest instead of applying fixed intervals to every tank.

A documented evaluation by an inspector or engineer of whether a tank with known corrosion, cracks, or distortion can safely remain in service until the next scheduled inspection.

Metal loss on the underside of the tank bottom caused by moisture, soil chemistry, and bacteria. It cannot be seen from inside and is controlled by liners, release barriers, and cathodic protection.

Filling a tank with water to a stated level and holding it to prove structural adequacy and tightness. It is required after major repairs, alterations, and reconstruction before the tank returns to service.

The region where shell meets bottom: the shell plate within about three inches above the corner weld, that weld, and the adjoining bottom plate. It gets extra inspection and restricts certain repairs.

Work that restores a tank or component to sound condition without changing its size, design, or service. Replacing a corroded plate or fixing a roof are typical examples.

A change that affects the tank's physical configuration or design basis, such as raising shell height, changing diameter, adding a large opening, or switching to a more severe product. It needs engineering review.

Dismantling a tank and erecting it again in whole or in major part, including new bottom, shell, or roof. The rebuilt tank must meet current design rules and be tested before service.

A replacement plate cut to fit an area removed from shell, roof, or bottom and joined with full-penetration welds. It restores both strength and tightness where corrosion or cracking has taken metal.

A plate laid over a damaged area and welded around its edge without cutting out the original plate. Size, location, and thickness are restricted, and it is not allowed over the shell-to-bottom weld.

Adding layers of weld metal to a thinned surface to restore thickness. It is permitted only in limited areas and depths, using qualified procedures, because it can create hard zones and cracking.

Cutting or drilling an opening into a tank or pipe that is still in service, using a special machine and valve so product does not escape. It needs a written procedure and strict safety controls.

Putting a tank into a product, temperature, or duty different from its original design. This triggers review of materials, thickness, vents, and testing before the tank is used again.

A large-capacity vent that opens to relieve internal pressure when a tank is exposed to fire, protecting the shell and roof from overpressure. It is sized for the expected heat input.

Alarms, level devices, and procedures that stop a tank from being filled above its safe limit. They keep product from overflowing into dikes, drains, or the environment.

Buried anodes or an impressed current that push a small electrical current onto the underside of a steel bottom, blocking the electrochemical reaction that causes soil-side corrosion.

Going into a tank, pit, or similar enclosure with limited entry and exit and poor natural air movement. It requires a permit, atmosphere testing, ventilation, an attendant, and rescue provisions.

A written authorization to weld, cut, grind, or do other spark-producing work in a tank area after the space has been cleaned, tested for flammable gas, and isolated from product.

The highest product level a tank may be filled to, established from shell thickness limits, freeboard, and overfill protection so that design stresses and containment are not exceeded.

The condition of a tank that has been emptied, cleaned, isolated, and taken out of product service, usually so it can be entered for inspection, repair, or permanent closure.

Permanently ending a tank's service, either by cleaning and filling it with inert material or by removing it, with site restoration and final records completed as required for out-of-service tanks.

Moving a tank to a new site by dismantling, transporting, and re-erecting it. The rebuilt tank is treated as reconstructed, so it needs current design review and a hydrostatic test.

Keeping design, inspection, repair, and test records for each tank so its condition and history can be traced. Records support setting inspection intervals and proving compliance over the tank's life.

A metal plate on the tank recording the design standard, nominal capacity, materials, and design conditions as built. Inspectors use it to confirm the original design basis.