A Complete Piping Analysis With Thermal Stratification Loads

The U. S. Nuclear Regulatory Commission (USNRC) had issued Bulletins 88-08, 88-11, 89-90, and 93-38 to address the concerns and problems due to thermal stratification loading during the life span of normal plant operation. The thermal stratification condition typically will cause pipe to bow in on a long horizontal segment. These conditions have not been commonly considered in piping design. However, the additional thermal cyclic stresses and loads due to these conditions could lead to the fatigue damage of the piping components and the failures of pipe supports. Analyzing the effects of thermal stratification loads can be very cumbersome if it is not a built-in functionality of the analysis program. Thus in response to the recent increase in such cases we have incorporated this feature in our piping stress computer program. A stress engineer can now define the thermal stratification conditions easily and the program will compute the pipe stresses and pipe support loads automatically as one of the load cases. The program then combines the thermal stratification load cases with other load cases as required in accordance with the load histogram to determine the cumulative fatigue damage of the piping system. The thermal cyclic stresses are evaluated in accordance with the design rules of Nuclear Class 1 piping components provided in NB-3650 of ASME section III Code. This paper presents the method, modeling and validation for implementing the functionality of analyzing thermal stratification loads in a computer program, as well as an application on an actual piping system as an illustration.