A METHOD FOR APPLYING AUTOMATIC TEMPERATURE CONTROL TO THE TRANSIENT FINITE ELEMENT ANALYSIS OF A PRESSURE VESSEL UNDERGOING POSTWELD HEAT TREATMENT

摘要

For complex finite element problems it is often desirable to prescribe boundary conditions that are difficult to quantify. The analysis of a pressure vessel undergoing postweld heat treatment (PWHT) is an example of such a problem. The PWHT process is governed by Code rules, but the temperature and gradient requirements they impose are not sufficient to precisely describe the complete vessel temperature profile. The imposition of such a profile in the analysis results in uncertainty and errors. A suitable but difficult approach is to specify heater power instead of temperatures, letting the solver determine the temperature profile. Unfortunately, the individual heater power levels necessary to meet the Code requirements are usually not known in advance. Determining the power levels necessary is particularly difficult if a transient solution is required. A means of actively controlling the heaters during the FEA solution is requirement for this approach. A simple and adaptive control algorithm was incorporated into the FEA solver via its scripting capability. Heat flux boundary conditions (heater power) were applied instead of transient temperature boundary conditions. Heater power levels were optimized to achieve predetermined time/temperature goals as the solution proceeded. The algorithm described was successfully applied to a pressure vessel PWHT with 14 zones of control. The approach may be adapted to other problems and boundary conditions.