Stainless steel tanks have been a viable solution for fluid storage but are now also being considered for repair and refurbishing options. One way to reduce the incidence of tank-bottom leaks is to construct or reconstruct tanks with a stainless steel bottom. Whether it is a replacement bottom, new construction tank bottom or complete tank, duplex stainless steels offer advantages in properties, performance and cost.
Initial selection will require evaluation of several aspects of the material properties and footprint in the marketplace. These include corrosion and mechanical properties, applicable construction codes, availability, fabricability, and inspection and future maintenance requirements. The final consideration will be cost, and both investment and lifecycle costs must be considered.
Tank corrosion has three components that must be taken into account: internal corrosion caused by the stored fluids, external corrosion caused by local atmospheric conditions and soil corrosion where direct contact with the ground is possible.
Lean duplex materials are acceptable for most environments except for severe industrial settings or aggressive marine conditions. The duplex materials family provides corrosion protection for a full range of soil conditions.
Duplex materials have a significantly higher strength level than their austenitic cousins and many standard construction steels. API design allowables are approximately 40-60 percent higher than austenitic grades. This may allow for thinner bottom and wall design which can reduce both material and fabrication costs. In addition, a harder surface provides better wear protection under fluid-flow conditions.
Duplex materials are included in the API 650 and API 620 tank standards. In addition, most product forms for each grade are included in both ASTM and ASME standards.
Duplex materials still require their own procedures based on their strength and metallurgical characteristics. These characteristics offer some unique advantages for tank construction and repair. For example, the coefficient of thermal expansion being closer to carbon steel allows for easier design and welding of dissimilar metal joints.
Visual inspection should reveal any surface contamination or mechanical damage requiring cleaning or repair. Some weld issues may also be visible. While maintenance due to corrosion or wear is not expected, there is always a possibility for a design or fabrication flaw to result in an area needing repair.
Using a design and cost comparison program to calculate tank investment and lifecycle costs can provide an indication of potential savings for either a new tank project or a bottom repair project.
The results in Figure 1 compare a carbon steel tank to 2202 and 2304 duplex stainless steel bottoms and one wall course with a coated carbon steel bottom and single course based on API 650 design requirements. While the investment costs are similar for each bottom type, there is an added expected lifecycle cost based on extended inspection intervals and minimal required repairs of the coated bottom over time.
A duplex solution exists for each kind of product stored and in every atmosphere and soil condition. They provide equal or better corrosion resistance compared to austenitic grades. They do not require coating, painting or protection to maintain their performance or appearance.
Duplex stainless materials offer 1.5-2 times the strength of comparable austenitic grades. The potential for reducing thicknesses allows for the optimization of both material and fabrication costs. Additional cost savings are achieved because no coating or painting is required. Lifecycle cost is improved due to extended inspection intervals, minimal required repairs and ease of repair, when necessary.
For more information, email John M. Grocki at arc.jmg@gmail.com or call (860) 985-8430.
