Thermal constraint considerations in design of a heat recovery boiler

Heat recovery boilers are typically designed to conform to the rules of the ASME Boiler and Pressure Vessel Code, Section I on Power Boilers. While the unfired pressure vessel design rules of Section VIII of the ASME Code have evolved to accommodate a wide range of vessels and loading conditions (including system thermal loads), Section I of the Code remains characteristically simple, specifying explicit rules only for primary loading (pressure mainly). By and large, the combination of boilermaker experience and Section I rules makes for robust boilers. However, departure from conventional design, although in conformance to the explicit rules of Section I, can result in premature failures. This paper describes one such design of a heat recovery boiler in a hardwood Kraft pulp mill, wherein system thermal stresses induced on superheater tube-to-tube tie welds were high enough to result in weld toe cracking and early tube leaks. A finite element stress analysis of the design showed that operational thermal stresses due to constraint imposed by the tie welds were in excess of the tube material yield strength and above a level that would be considered acceptable for design. The analysis highlights the need for considering effects of thermal constraint in designs that otherwise meet the basic Code requirements for boilers.