Modeling Studies to Predict Stresses in Composite Floor Tubes of Black Liquor Recovery Boilers

The results of finite element modeling studies to analyze the composite floor tubes of recovery boilers used by the pulp and paper industry are presented. Thermal and mechanical analyses have been carried out to examine the changes in the stresses during a normal operating cycle. Three different materials for the clad layer have been studied—304L stainless steel and alloys 625 and 825. The results show that the coefficient of thermal expansion and the strength of the clad layer play a significant role in the stress variation due to the heating and cooling associated with an operating cycle, with implications for the development of cracks in the tubes due to stress corrosion cracking. The analyses also show that axial stress in the carbon steel layer is compressive at the fireside tube crown and tensile in the membrane, which provides a possible explanation for the commonly observed differences in crack propagation in tubes and membranes.