Tank integrity investigation.

Application: Explains the fundamentals of storage-tank integrity, including how tanks work, how tanks fail, and when an engineering assessment is required.

The integrity of a storage tank must be reassessed when its service conditions change, when making repairs and alterations, or when indicated by measurements of tank wall thickness.

Study objective

This report explains how tanks work, bow they fail, and when engineering reassessment is required. A review of actual mill cases shows that reassessment of tank integrity can increase capacity, save money, and reduce risk.

Storage tanks

How they work. Most large liquid storage tanks are cylindrical, aboveground, flat-bottom, welded steel storage tanks. They are typically designed and constructed to the standards of API (American Petroleum Institute) 650 or API 620. The cylindrical sidewall is a membrane designed to resist the hydrostatic pressure generated by liquid in the tank. Because the pressure increases with the depth of the liquid, the sidewall is often thinner at the top and thicker at the bottom to resist the hydrostatic forces efficiently. The bottom of a flat-bottom tank is also a membrane, but its primary function is to prevent the tank contents from leaking into the ground. A membrane bottom requires uniform support by the tank foundation. Anchorage may be required to secure the sidewalls against internal pressure, wind, or earthquake.

How they fail. Tanks can fail by leaking or breaking (collapse or rupture of the tank). Catastrophic failure of storage tanks is rare, but the consequences can be severe.

A leak can be a mechanical or a structural failure, and it can be inconsequential or catastrophic, depending on the magnitude of the leak and the contents of the tank. Common causes of leaks in storage tanks are under-bottom pitting corrosion, excessive foundation settlement, gasket failure, and mechanical damage to flange faces.

A break is a structural failure that can range from minor tank damage to catastrophic failure resulting in damage to surrounding equipment and loss of life. Common causes of failure include excessive vacuum, excessive internal pressure, foundation failure, brittle fracture, corrosion, change of service, and defects introduced by repairs and alterations. Uplift forces due to internal pressure can pull anchors out of concrete foundations and break anchor-to-sidewall attachment welds. If the anchors fail to resist the internal pressure, the cornerweld-to-sidewall weld can rupture, spilling the contents of the tank and turning the tank into a rocket, as seen in Fig. 1.

[FIGURE 1 OMITTED]

Assessing tank integrity

The purpose of tank integrity assessment is to reduce the risk of tank failure. Tank integrity assessment is necessary when service conditions change, when making repairs and alterations, or when indicated by thickness measurements.

Existing codes and standards define good engineering practice for common cases. However, for cases not directly addressed by codes and standards, good engineering practice may require additional inspection or analysis. A detailed stress analysis by finite element can determine discontinuity stresses. Additional nondestructive examination (NDE) methods can be used to detect stress corrosion cracking (SCC). Material testing may be required to determine strength and toughness. Fracture mechanics can help in determining whether observed defects can be tolerated.

Spot thickness measurements are a necessary and inexpensive way to monitor integrity of tanks. However, thickness measurements may not detect preferential corrosion, SCC, or pitting. The most effective approach to the task of assessing tank integrity combines the engineer's understanding of tank behavior and the user's knowledge of the tank's service history.

Wade is a consulting engineer with Washington Group International, Inc., Rust Engineers, 100 Corporate Parkway, Birmingham, AL 35242. Address correspondence to Wade by email at [e-mail]