Stainless Steel vs Carbon Steel Tanks & Vessels

It was once commonly thought that sourcing a Stainless Steel (SS) tank was double or triple the cost of a Carbon Steel (CS) painted tank. Not so. Expertise with ensuring the proper material selection, along with A&G’s method of fabrication, means that our stainless steel vessels can be cheaper at the outset, as well as over the life cycle of the vessel.

Many of our customers have reaped the benefits of engaging A&G to provide a SS vessel to suit individual processing needs.

Material – CS v SS

Let’s first take a look at the composition of the two materials. SS and CS contain the same elements of iron and carbon, however SS is made by adding a minimum of 10.5% chromium. The SS then forms a protective chromium oxide layer protecting the surface from corrosion. Adding amounts of alloying elements like molybdenum, nickel, manganese, titanium and others determines the type of grade and it’s suitability to corrosion resistance, strength, hardness and heat resistance. SS material grades have advanced in recent years, offering better choices to specific industry needs. LDX2101 for example, provides superior strength and corrosion resistance while maintaining comparable costs to more traditional grades such as 304 and 316.

Tank Thicknesses & Weight

Although SS & CS tank fabrication practices are traditionally similar, welding durations on CS are typically increased, particularly with coded designs such as API650, AS1210 & AS1692. Consumables for SS tanks are slightly higher, however, this can be offset by the overall reduction in tank weight due to the higher strength of SS. Below is a weight comparison of a 1,500,000L storage tank, 12.5m Dia. x 12.5m high shell, material thicknesses as per API650 design code, including a corrosion allowance of 0.25mm for SS and 3mm CS.

Table 1: Weight Comparison of 1,500,000L API650 Storage Tank.

Material Thickness (mm) 316SS (kg) Thickness (mm) LDX SS (kg) Thickness (mm) Carbon Steel (kg)
Top 6 7,995 5 6,645 6 10,205
Shell 5, 6, 8 24,565 5 20,069 6, 8, 10 30,249
Floor 5 6,024 5 6,024 8 9,250
Total Weight   38,584   23,738   46,142
Fabrication Cost Benefits

Continuing with the 1,500,000L tank example, we see the additional costs associated with surface treatment on CS tanks, whereas no costs are incurred with the use of SS. With properly selected types and grades, the material is resistant to tank contents and the external environment; painting will never be required!

Table 2: Based on 810m2 surface area @ $240 m2

Stainless Steel Carbon Steel
Grit Blasting

Primer & Paint

Scaffolding

EWP Lifts

Not Applicable $194,400 AUD
Other considerations
  • Fabrication of CS tanks requires preheating and post heating for welds. Post welding treatment and RT (radiography testing) inspection are also subjected to CS tanks. Dependent on the design parameters this is not required for SS tanks.
  • Grit blasting for paint preparation causes airborne particles with potential to contaminate surrounding properties and foul equipment.
  • Grit blasting and painting requires personnel to work at heights. This necessity is eliminated with SS tanks as there is no need to paint. This reduces the potential of falls and WHS issues.
  • On-Site duration: fabrication of an A&G stainless steel tank is generally much quicker than a CS tank, in particular with field erected vessels.
  • Environmental impact of painting outdoors is eliminated with SS.
Maintenance

Costs for painting maintenance and rework are indicative of scheduled cycles and vary depending on type of product stored and facility location. Tanks near coastal environment and storing aggressive materials may experience higher maintenance costs. The following is a general guide only, as depending on the individual application and process, costs can vary greatly. Not listed below, however of significant consideration, is the ‘Out of Service Costs’ for CS tanks due to tank unavailability during the requisite maintenance works.

Table 2: Based on 810m2 surface area @ $240 m2

304 SS 316 SS LDX 2101 SAF 2205 Carbon Steel
Maintenance 10-12 Years Ø Ø Ø Ø $279,000
Maintenance 20-22 Years Ø Ø Ø Ø $335,280
Maintenance 30 Years Ø Ø Ø Ø $402,350
Total Cost Ø Ø Ø Ø $1,016,630
Cost of Ownership
  • Initial costs of SS tanks can be lower than CS.
  • Lack of ongoing maintenance cost for the life of the SS tank, and no loss of production.
  • SS tanks are aesthetically more pleasing at a facility, as opposed to discoloured, rusting, painted CS tanks.
Conclusion:

It is clear that the common belief that stainless steel is cost prohibitive is in many cases unfounded. SS tanks offer significant cost benefits across a many industries, not only at the initial capital outlay phase, but also over the full life of the vessel.
If you require a vessel fabricated for your storage or processing needs, talk to A&G about how the use of Stainless Streel can provide you with a superior product, and a safer and more environmentally friendly project, at a very attractive price.

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