Modeling studies of a composite tube floor in block liquor recovery boilers

Stress-strain analysis of a composite tube floor in black liquor recovery boilers used a commercially available finite element (FE) code. The analysis examined the stress state in an SS304L/SA210 composite tube floor at the normal operating cycle of a boiler and the stresses that develop from temperature excursions during I operation. Measurement of residual stresses from tube manufacturing used neutron diffraction. With the residual stresses from assembly welding computed by finite elements, the residual stress measurements were incorporated into the FE models developed for analysis of in-service conditions. Results show high compressive stresses in the overlay material that exceed the yield stress at normal service conditions. They significantly exceed the yield stress during a temperature excursion and become tensile after returning to normal service conditions. A parametric study calculated stresses in the floor tubes as a function of the yield stress and the coefficient of thermal expansion of the overlay material. The analysis provided a generalized solution of stresses in the composite tubes for many overlay materials. Using these results, a recommendation for overlay materials with the most suitable mechanical properties is possible. Application: For mechanical behavior, SS304L is not a suitable overlay material, but alloy 625 is satisfactory.