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Eight year reference provides solution to tube cracking problems in recovery boilers

Sandvik Coromant UK : 10 January, 2005  (Company News)
Cracking in water tubes in black liquor recovery boilers, especially in boiler floors and lower waterwalls, has been the source of unscheduled downtime for many years.
Now, these problems may be over or, at least, substantially reduced as tube material specialists, Sandvik Materials Technology reveal an installation reference for stainless/carbon steel composite tubes which show no signs of cracking after eight years in service.

Composite or co-extruded tubes, comprising a 304L stainless steel outer layer and an SA210 Grade A1 carbon steel inner layer, have been around for over 30 years, but it was only in the early 90's that it became apparent that cracking in the 304L layer, particularly in floor tubes was a widespread problem. From over 100 installations surveyed, as many as one third of all boilers had cracking problems, some becoming evident as soon as six months after start-up.

Analysis of failed TP304L composite floor tubes confirms that the cause of failure is either the effect of stress corrosion cracking, thermal fatigue or the synergistic effect of both. It was found that stress corrosion cracking (SCC) requires that tensile stresses are present in the stainless steel tube surface at the same time as the tube is exposed to the corrosive boiler environment. Thermal fatigue is the result of cyclical stress variations due to cyclical variations in temperature. In floor tubes, it is concluded that thermal fatigue is caused by process variations in smelt chemistry leading to a higher viscosity smelt, higher char bed temperatures and differences in coefficients of thermal expansion between the stainless and carbon steel elements of the carbon tube.

As early as 1988 Sandvik Materials Technology identified the need for an improved composite tube for BLRB floors which would provide improved and high resistance to SCC and better resistance against thermal fatigue. The result is a 38% nickel stainless steel outer - designated Sandvik Sanicro 38 (UNS N08825) and an SA210 A1 (Sandvik 4L7) inner.

The improved thermal fatigue properties of Sandvik Sanicro 38/Sandvik 4L7 is because the thermal expansion coefficient of Sanicro 38 is closer to Sandvik 4L7 (SA210 AI) than is 304L. The more closely matching of thermal expansion coefficients between the stainless and carbon layers, the less are the thermal stresses between the two components.

Having been alloyed with 38% nickel, there is much higher resistance to chloride and sodium hydroxide induced corrosion and, therefore, a substantially reduced corrosion rate.

Several thousand meters of the new composite have been supplied worldwide. The first complete floor installation was made in 1996 at the Metsa Rauma Mill in Finland - the source of Sandvik's application reference. 2700 metres of Sandvik Sanicro 38/Sandvik 4L7 were installed and with regular inspections, no cracking has been found after eight years in service.

The plant has a dry solids capacity of 3000 tonnes/24 hours and produces a steam flow of 188kg/s (934.000 1b/h) at 92 bar (1334 psig) and 490C (914F).

Says Siw Asberg, Product Manager for composite tubes at Sandvik Materials Technology, ' Sandvik Sanicro 38 composite tubes have been used in over 60 floor installations in total, worldwide and includes nine complete floor installations - four in Finland, three in US and two in Canada. Black liquor production processes are evolving continually, and just as the 304L/carbon composite tube facilitated process development, the solution to the floor cracking problem with the new composite is now becoming the industry standard, not only for floors but also the lower parts of the boiler including walls and airports. The pulp and paper market is expanding and the current trend is to increase production capacity. This trend means bigger black liquor recovery units with higher temperatures and pressures. The combination of more severe working conditions, increases in dry solid content and more closed systems, in turn places new demands on tube materials, which need to be addressed.'

'There are, of course, alternatives to composite tubes for use in BLRB floor, wall and superheater applications, but we believe that none of these give the design and fabrication flexibility of our purpose designed composite tubes.'

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