Modeling Refractory Stiffness for Piping Flexibility Analysis

Refractory is commonly used in pipes for cold wall design. Generally, to analyze refractory lined piping, a piping stress analysis is done using commercially available piping flexibility analysis software. In the stress analysis, the refractory weight and stiffnesses are included in the piping model. From the piping analysis, the calculated sustained and expansion stresses are compared with ASME B31.3 [1] allowable stresses and the forces and moments from the analysis are used in the design of attached equipment (vessel nozzle, valves, expansion joints etc.) piping restraints and supports. In this paper it will be shown that for a piping stress analysis, using the combined stiffness of the refractory and pipe material on the straight section of the pipe is satisfactory but when it is used on the bends, it will result in un-conservative resultant forces and moments from thermal expansion. To obtain satisfactory resultant forces and moments, the calculated refractory stiffnesses on the bends should be increased in the piping model. The piping stress analysis results were verified with finite element results and past experimental work.