The floor of a black liquor recovery boiler at a mill in central Canada has experienced cracking and delamination of the composite tubing near the spout wall and deformation of the floor panels that is most severe in the vicinity of the spout wail. One possible explanation for the observed damage is impacts of salt cake falling from the convective section onto the floor. In order to determine if such impacts do occur, strain gauges and thermocouples were installed on the boiler floor in areas where cracking and deformation were most frequent. The data obtained from these instruments indicate that brief, sudden temperature fluctuations do occur, and changes in the strain experienced by-the affected tube occur simultaneously. These fluctuations appear to occur less often along the spout wall and more frequently with increasing distance from the wall. The frequency of these temperature fluctuations is insufficient for thermal fatigue to bathe sole cause of the cracking observed on the tubes, but the data are consistent with what might be expected from pieces of falling salt cake.
INTRODUCTION AND BACKGROUND
The 304L/SA21Ocomposite floor of the Babcock& WIICOX2.3MM LbDS/day black liquor recovery boiler at Weyerhaeuser?s Prince Albert, Saskatchewan mill has experienced some cracking and some delamination, all primarily within a meter of the spout wall. The areas where cracking was observed during the 1994-1996 period are shown in Fig. 1,1and it is apparent that essentially all of the cracking is within about 1% meters of the spout wall. In addition, portions of the floor within about 3 mof the spout wall have suffered severe deformation so that the tubes are bent below the normal floor position. This boiler was put into service in 1968, and the lower boiler was rebuilt in 1984 at which time the co-extruded tubes were installed. Consequently, all this damage has occurred within the last 14years. The causes of these problems have not been clearly defined although there has been considerable speculation as to what might be responsible.
In 1996, a two-tube section of the floor was removed because of cracking and delamination and an area with the interconnected type of cracking prompted removal of a separate, single tube; these sections were analyzed at Oak Ridge National Laboratory (ORNL). As will be described in a subsequent section, the floor tube cracking has the appearance of that often seen in floors constructed of composite tubing. Recent studies suggest cracking of this type should be attributed to stress corrosion cracking with the added possibility that temperature 24 These temperature fluctuations have fluctuations may contribute to progression of these floor tube cracks . been documented in other boiler floors by thermocouples measuring the temperature on the membrane, above 54 As part of an effort to characterize the environment of the membrane, in the tube wall or on the tube surface . the floor tubes, including a determination of whether thermal fluctuations occur, and to identify the causes of the floor degradation, the temperature and strain in selected areas of the floor have been measured.
Another problem encountered in this floor is one that has been seen very infrequently in other boiler floors. Between the spout wall and the first floor support beam, about 30floor tubes near the right sidewall (tubes 15- 45) are bent downward. This deformation is extensive enough that many of the tubes have a slope that is negative relative to horizontal where the slope should actually be + 5.70,which is the slope used on this boiler floor. A tube removed from this area has been examined in an effort to determine if the tube experienced any overheating or if there is any evid