Inner and Outer Surface Cracking of the Recovery Boiler Floor Tubes, 2000 Engineering Conference Proceedings

Pekka Saarinen, Hannu Hänninen, Johanna Tuiremo, Kalle Salmi--The cracking of AISI 304/304L type composite tubes has been a serious problem in recovery boiler floors. Basically there have been two types of cracking; transverse and branching. A General assumption has been that branching cracking is caused by stress corrosion cracking and transverse cracking is caused by thermal fatigue. It is assumed that the branching cracks propagate through the stainless steel cladding and stop at the interface of the cladding and the base material or they change their direction of propagation along the interface. The transverse cracks are assumed to propagate through the interface into the base material. In few cases cracks have also been found on the inner surface of the tubes. In those cases there have been transverse cracks in the tube crown inside the furnace.

In a recovery boiler, both transverse and branching cracks were found in the 304L-type composite floor tubes near the right wall. Based on previous experience of Kvaerner Pulping, the presence of cracks also on the inner surface of the tubes was suspected. This was confirmed in closer examination after the floor tubes had been removed. The motive for a thorough investigation was to gain further understanding of composite floor tube cracking and use this information to find out the safe way to use 304L composite tubes. I.e., to evaluate from the appearance of the cracks whether it is necessary to remove the tube or is the tube repairable.