Evaluation of the Thermal Shock Induced to a Steam Header During Shutdown Conditions

It is a practice of some utilities to pressurize (hydro) a boiler shortly after the beginning of an outage to check for the presence of boiler tube leaks. If the boiler is drained during shutdown in order to expedite cooling for personnel entry, then the boiler must be filled with relatively cold water in order to perform the hydro. There is a concern that as the cold water contacts the hot metal of the main steam header there is a risk that the thermal shock could cause a crack to form. Numerical and finite element analyses were used to determine the amount of thermal stress induced when varying temperatures of water are applied to the inside surface of a typical main steam header at elevated temperature. The results of the analyses were used to assess the risk of quench cracking and crack propagation.