A previously-developed procedure applies to evaporator section of bottom-supported heat-recovery steam generators (HRSG): The procedure features methods to use on pressure internal parts designed according to Section I, as required by the Boiler & Pressure Vessel (B & PV) Code. It uses ASME Power Piping Code B31.1, which is for external piping, to evaluate additional concerns on pressure parts that have already met Section I. This mainly because other sections of B & PV code do not cover flexibility analysis: Procedure includes classical methods for the static analysis of the lower header and a computer-modeled flexibility analysis. They investigate the stability of the harp high-temperature components, which may be critical because of their longitudinal dimensions, thermal expansions and loading: Preliminary stress evaluation showed that Italian pressure formula is largely conservative.

Methods’ application to a worked example using both ASME and Italian pressure formulae shows now that the pressure contribution is the greatest. Also, the maximum stress found out on the header in sustained case appears consistent with that from numerical model, though bending contribution the greatest; Sustained case appears less critical than thermal. So, the design should be safe from risks of static instability due to longitudinal pressure stress: deformation occasionally observed in field on the lower headers may be correctly imputed to thermal expansion. Herein, a bottom-supported HRSG double-width unity of combined cycle power plant is considered as the worked example.

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