TPi October 2012

The case story One of Turkey’s largest steel producers wanted to reduce its energy consumption in one of its annealing furnaces in the rolling mill section. It also wanted to maintain and prolong the service life of furnace recuperators and simultaneously increase steel production. To make this possible, the recuperators, essential for recovering outgoing heat from the furnaces, had to be able to run at higher temperatures. The maintenance department of the steel mill was, in 2002, in contact with engineers from Sandvik Materials Technology regarding a solution for their problems. Technical presentations and discussions were initiated and the problems and the service parameters were presented (see Table 1). The main problem of the tubes was the relatively short lifetime, caused mainly by oxidation. The existing tube material was a common, low-alloyed carbon steel, St358, with a lifetime of 2-3 years. One cause was the fuel used in the furnace: 50 per cent coal and 50 per cent fuel oils. Another problem and cause for the short lifetime was the relatively short distance between the furnace exhaust and the recuperator; this led to very high temperature of the flue gases entering the recuperator, which in turn attacked the recuperator tubes more aggressively. The third distinctive problem the customer faced was regional electrical cut-offs, which left hot air trapped in the pipes, sometimes for long periods, causing a faster oxidation of the tubes. The challenge was to find a steel grade suitable for these higher temperatures in this corrosive environment, keep the high efficiency and prolong the lifetime of the recuperator tubes. After consulting with engineers at Sandvik R&D together with the customer’s maintenance manager, material grade Sandvik 253 MA [3] was specified as the best solution. Conclusion and summary The Sandvik 253 MA tubes were installed in 2003, and were still in use as of 2011. The new tubes have so far showed a three times longer lifetime, running without any problems, and the last inspection was made at the beginning of 2011. This means that the customer has not only saved initial tube cost for three complete bundles of recuperator tubes of the old material, but also saved cost for maintenance, replacement and tubing cost, standstill cost and loss of production cost. On top of this, the new tubes showed a higher efficiency in production, and a higher resistance to the temperatures, which resulted in higher preheating temperature of the combustion air. This resulted in major fuel and cost savings for the company. The forecast from the customer was to save approximately €400,000 per year in energy cost for four furnaces in its two rolling mills. In addition, the company saved time-consuming processes of documentation and bureaucratic paperwork for quotation and order processing.

Figure 2 : Typical corrosion caused by nitridation/oxidation at high temperatures

References

[1] www.enercon.gov.pk [2] www.pcra.org [3] www.smt.sandvik.com

Johan Wallin is the global technical marketing specialist responsible for high temperature products at Sandvik Materials Technology. He has been responsible for the technical marketing for Sandvik’s special segments products in the Middle East and North Africa for the last four years. He is in close contact with end-users regarding corrosion-resistant, high-temperature alloys for different applications. Tommy Sand is a research and development engineer at Sandvik Materials Technology. He conducts research and development activities related to stainless steel for high- temperature corrosion applications at the Sandvik R&D centre in Sweden.

Sandvik Materials Technology – Sweden www.smt.sandvik.com

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Tube Products International October 2012

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