Loads of the continuous kraft digester have been determined during the start-up of the digester house. Loading was caused by the pressure of the proof test, the start-up pressures of the digester and, finally, the normal working pressure. The apparent threshold stress level in the base metal was greater than that achieved during the normal continuous cooking process but the level in the weld of the impregnation zone were exceeded due to the superposition of the tensile residual stresses. This two- axial tension is considered as the precondition for stress corrosion cracking (SCC), which was confirmed by fractography studies. The results showed, that SCC in the impregnation zone is possible only in the welds during the normal continuous cooking process. During the refill and blow phase of the digester the measured loading stress changes corresponded to the stresses of the proof test and increased the risk for SCC. Some procedures to avoid or minimize SCC are discussed.
Continuous digesters, constructed of carbon steel, were originally believed to be relatively free of corrosion problems. For two decades, corrosion was of relatively little concern in continuous digesters, and visual inspection was considered sufficient to ensure the integrity of these vessels. Then in September, 1980, the sudden failure of the digester at Pine Hill, Alabama occurred due to a form of stress corrosion cracking (SCC) called caustic SCC1.This disaster resulted in greatly increased activity in research of caustic SCC leading to attention of inspection, which still is very wide also in Scandinavia. A survey revealed that nearly 75% of all continuous digesters had cracks presumed to be SCC3.
Research is currently underway to uncover some of the factors which contribute to cracking, such as wood species, liquor composition, and digester operation4 but the only significant correlation noted to date for the tendency toward cracking is the stress distribution of the digesters, especially in the welds. The total stress is composed of the additive combination of the factors such as stresses due to pressure, dead weight, external loads, stress concentration due to geometric discontinuities and residual stresses from fabrication and/or thermal treatment. It is significant to note that usual pressure vessel stress calculations take into account only the first three factors above. In usual calculations, a safety factor is added to take into account the other contributions to stress.
This paper will discuss the contribution of loading stresses to the caustic SCC problem in a continuous digester, where cracks were found in the impregnation zone. The loading was caused by the pressure of the proof test, the start-up pressures of the digester and, finally, the normal working pressure. The effects of the indicated residual stresses in the weld will be combined to loading stresses. Some aspects of the loading stress changes especially during the start-up procedures affecting to the SCC will also be discussed.
