Analysis of Power Plant Deaerator Under Transient Turbine Loads

In recent years, mechanical failure of trays and structural damage to steel members inside deaerating sections of power plant deaerators have been encountered during transient load operation of main turbines. These failures have been attributed to numerous causes, but underestimates of differential pressure across the tray stack and insufficient sizing of the pressure equalizers connecting the water storage tank and the deaerating section have been singled out as major factors. Design criteria of the pressure equalizers and tray stack enclosures should take into consideration rapid load reduction of the turbine. Under such mode of operation, pressure decay would produce flashing from the saturated water stored in the tank rushing upward through the pressure equalizers with possible flooding of the deaerating section if the equalizers were improperly sized. Concurrently, the differential pressure across the tray stack would increase by a factor of 10 to 20 times higher than normal maximum load. In such instances, the tray stack designed merely to meet a maximum stable load could not possibly withstand such severe dynamic loading. This paper presents a mathematical analysis of the transient operating behavior of a deaerator, with emphasis placed on sudden load reduction of the turbine. Items such as the quantity of flashing steam from the storage tank, physical properties of vapor and liquid in the deaerator, and the pressure losses through the pressure equalizers and tray stack are analyzed. From the analytical results, criteria of pressure equalizer sizing and tray stack enclosure design are established. A numerical example for design calculation is included for deaerator manufacturers and power plant process system engineers to use as reference for application in design.