The Life Expectancy Of A Steel Tank

The life expectancy of most assets, including storage tanks, is originally based upon non-engineering factors.  Typically, many organizations use accounting asset depreciation life as the initial design factor.  However, no organization expects to replace their tank assets when fully depreciated. In fact, they plan  to maintain the tank as long as it is still cost effective. Most tanks are expected to last 40, 60, 80 or more years.

The question is, “How long can a welded steel tank be expected to last”?  With a good engineering design, competent construction contractor, professionally applied coating system, along with regular maintenance and qualified inspection, a steel tank can last almost indefinitely.

Some of the common factors that affect the longevity of a steel structure are:

►Original Design Specifications

The original design of the structure plays a significant role in determining it’s life expectancy. A poorly designed structure is more difficult to upgrade and maintain, leading to premature failure. Examples of design flaws, in terms of life expectancy are:

·         Channel support columns

·         Foundation design

·         Roof design

·        Stitch welding

·         Hidden surfaces under support column base

·         Hidden surfaces under internal pipe supports

Inspections cannot be performed, and coating cannot be applied, in inaccessible areas. Without proper inspection, the owner will never identify trouble until it becomes a major problem. Additionally, hidden surfaces can trap sediment and deposits. Sediment and deposits have a tendency to promote corrosion, which  will lead to failure.

►Tank Construction and Inspection

No matter how well the design and specifications were prepared, an inadequate field contractor, along with unqualified inspection, will adversely impact the tank’s life expectancy.  If the field construction contractor fails to follow the specifications and design, or fails to use proper work practices, the tank will be adversely effected. Qualified construction inspection can identify most of these errors before they become problems. However the inspector must be familiar with the tanks design, welding procedures, tank construction techniques, and proper tank construction practices to be completely effective.

Some examples of potential problems include:

·        Poor weld quality

·        Incorrect welding material

·        Sharp corners or edges

·        Incorrect construction procedures

►Coating System

The protective coating is a crucial factor in tank’s service life, especially for water storage tanks. A coating specialist should be consulted to identify the correct coating system to be applied.  The coating system must be compatible with the environment, either inside or outside of the tank. If properly designed, selected and applied, most protective coatings for steel should last 10 to 15 years.

A few factors that can shorten the life expectancy of the steel coating system are:

·        Incorrect surface preparation

·        Inadequate coating thickness

·        Sharp edges

·        Using coating past its designed shelf life

·        Using coating past its designed pot life

►Routine Inspection

Both API and AWWA recommend that a certified inspector should inspect steel storage tanks at least every 5 years. The inspector will monitor items such as corrosion on the roof, shell, floor and accessories. They will also monitor the foundation settlement, as well as the coating, for early signs of problems Through comparison with previous inspection data, failure predictions can be made and corrective action taken to avoid failures.

Conclusion

With proper design, construction, coating and maintenance, a welded steel storage tank should last indefinitely. One significant factor is the utilization of qualified inspection at every stage of the process. Investing, through the use of qualified inspectors, will insure proper construction, coating, maintenance and operation.

As the old saying goes, "An ounce of prevention, is worth a pound of cure".