Monitoring Thermal Stresses in Horizontal Cylindrical Pressure Elements

Thermal loading may develop in horizontal boiler components - drums, superheater inlet and outlet headers, and steam lines - when steam condenses on their internal surfaces. The problem is typical for start-ups, shutdowns, and emergency conditions. At start-up, condensate occupies the lower region of the component, whereas steam remains in the upper region. Consequently, the circumferential temperature difference in a horizontal pressure part can become very large, in some cases approaching 200 K, because super-heated steam may heat the upper wall while condensate at saturation temperature cools the lower wall. Such non-uniform heating can produce local plastic deformation, impair drainage, and further intensify thermal stresses; deformed superheater headers are a frequent practical example. This study develops an inverse heat conduction procedure for reconstructing the temperature field in the cross-section of a horizontal pressure element. The reconstructed field is then used in a finite element analysis to calculate transient thermal stresses. The component is assumed to be externally insulated, and equally spaced thermocouples are placed inside the wall near the inner surface. Numerical tests are carried out for several numbers of measurement locations distributed over half of the circumference, with detailed results reported for 7, 13, and 19 points. The temperature field between the outer surface and the thermocouple radius is obtained from a direct heat conduction solution. The temperature and heat-flux histories at the thermocouple radius are then used as input for the inverse analysis, which provides both the inner-wall temperature and the local heat transfer coefficient in the steam and condensate zones.