Introduction
Stainless steel materials offer future benefits. They reduce maintenance and lower lifecycle costs. High-strength duplex stainless steels are cost-effective for initial investments. This case study shows why 2507 super duplex stainless steel is cheaper than weathering steel. It focuses on a factory exhaust chimney replacement project.
Background
In 2017, a sintering plant’s exhaust chimney partially collapsed in severe weather. The plant primarily produces sintered iron ore powder as raw material for nearby blast furnaces. The massive chimney, approximately 60 meters tall and 3.7 meters in diameter, featured a double-layer design with a 6mm thick weathering steel inner wall, an insulation layer, and a galvanized steel outer layer. This design relied on the insulation layer to keep the chimney temperature above the dew point to avoid corrosive condensation on the inner surface. However, under the harsh conditions of a coastal industrial facility, the insulation began to deteriorate. Pre-collapse inspections showed significant wall thickness reduction in areas where the insulation had aged.
Standing Out in Competition
Due to the chimney snapping in two, the plant needed a quick and effective replacement plan that significantly improved corrosion resistance. Industeel assisted with corrosion testing and provided technical support for potential replacement solutions, considering several key operational and environmental parameters. The plant, located near the ocean, experiences significant annual temperature variations and frequent strong winds.
The plant operates around the clock, with an average exhaust gas temperature of 150°C. The gas’s acid dew point is 110°C, with a highly acidic pH of 1, meaning highly corrosive condensate forms when temperatures drop below 110°C. Chloride content in the chimney’s particles far exceeded what weathering steel could withstand.
The 60-meter tall chimney was challenging to inspect and maintain. Its three-layer design, while protective, made internal layer inspections difficult. The plant evaluated various stronger corrosion-resistant replacement designs since the chimney’s lifespan fell short of the expected 20 years. The table below summarizes the relative costs, construction time, and design elements of each option.
The two stainless steel solutions (options 2 and 3) that could withstand condensate corrosion were the best combination of simplicity, replacement speed, and lifespan. Super duplex stainless steel emerged as the lowest-cost solution due to its higher strength and lower alloy cost.
| Feature | Option 1 | Option 2 | Option 3 | Option 4 | Option 5 |
| Material and Design | Double layer, weathering steel exterior, inner protective liner | Single layer, self-supporting, 2507 super duplex stainless steel | Single layer, self-supporting, 904L austenitic stainless steel | Double layer, 316Ti stainless steel exterior, inner protective liner | Reinforced concrete structure exterior, firebrick inner liner |
| Insulation | Yes | No | No | Yes | No |
| Corrosion Protection | Thick exterior shell (added corrosion allowance), outer liner | Not needed | Not needed | Thick exterior shell (added corrosion allowance), outer liner | Not needed |
| Foundation | New | Existing | Existing | New | New |
| Delivery Time | 9 months | 7 months | 7 months | 9 months | On-site construction: 12 months |
| Expected Lifespan | 20 years | 30 years | 30 years | 20 years | 50 years |
| Cost of Chimney and Foundation | Reference value = 100 | 88 | 100 | 100 | 128 |
Using Less Material for a Longer Lifespan
Based on the costs, time, and expected performance of the proposed designs, option 2 was chosen to replace the chimney, using a single-layer 2507 super duplex stainless steel construction. This duplex stainless steel contains about 3.5% molybdenum and 0.25% nitrogen, with chromium, molybdenum, and nitrogen combining to provide excellent resistance to localized chloride corrosion.
This design eliminated insulation maintenance costs and potentially extended inspection intervals. Additionally, the single-layer design made wall thickness loss easier to detect. The study found that the 2507 duplex stainless steel alternative had the lowest investment cost and, when considering total lifecycle costs, saved more than other options.
The replacement chimney was constructed using 6mm to 10mm thick cold-formed stainless steel plates, welded into large rings and transported to the site for installation. Each ring was lifted by a crane and placed on top of the previous one, resembling a large circular tower. The replacement chimney was completed and operational by the summer of 2019.
Since installation, the chimney has operated without issues, expected to provide low-maintenance service for at least 30 years. This case study illustrates how the high strength and excellent corrosion resistance of duplex stainless steel not only reduce weight but also provide immediate cost savings. Stainless steel is one of the most recycled materials on Earth, so part of the initial investment can be recovered as scrap value at the end of its lifespan. When considering lifespan and maintenance needs, seemingly luxurious materials can prove to be the most cost-effective choices.
Conclusion
In conclusion, using 2507 super duplex stainless steel for the chimney replacement was both economical and efficient. Its superior strength and corrosion resistance reduced maintenance needs, leading to significant cost savings throughout the chimney’s lifecycle. This case study highlights the value of investing in high-quality materials like super duplex stainless steel, which offer long-term benefits and durability in harsh environments.