INVERSE AND DIRECT TRANSFER-FUNCTIONS FOR THE FATIGUE FOLLOW-UP OF PIPING SYSTEMS SUBMITTED TO STRATIFICATION

In this paper we outline a methodology to assess the fatigue induced i n piping systems submitted to thermal stratification. More specificall y. the transformation from the measured outer wall temperature time hi stories to stress time histories in any point of the line is treated. By means of inverse transfer functions, the fluid temperature distribu tion is calculated from the outside wall temperatures measured in a li mited number of temperature sections. Using direct transfer functions, the local stresses due to stratification may be determined as well as the pipe free curvatures and the pipe free axial strains. Using a fin ite beam element model of the line, the global response of the line (i n terms of displacements or stresses) due to the applied curvatures, a xial strains, end point displacements, internal pressure and possible contacts with the pipe environment may be determined. The method is il lustrated for the surge lines of the Doel 2 and Doel 4 nuclear power p lants. An excellent correlation is found between measured and calculat ed displacements. Typical stress time histories are shown for a plant cool down.