The Effect of Heat-and-Mass Transfer and Flow Hydrodynamics on the Flow Accelerated Corrosion Rate in Evaporators of Combined-Cycle Unit Heat-Recovery Steam Generators

Flow accelerated corrosion (FAC) is considered one of the major causes of damage to heat-recovery steam generators (HRSG) of combined-cycle units. In the last few decades, a great number of papers devoted to this problem have been published in many countries around the world. The results of investigations were analyzed as a rule, on the basis of the flow-averaged thermochemical parameters of the flow, while the features of the near-wall liquid film determining the FAC rate were not taken into account. The indicators in the near-wall region depends on the heat-and-mass transfer conditions, flow hydrodynamics, and the specific of chemicals used to correct the water chemistry (WC). For ammonia and oxygen water treatment chemistries, an equilibrium model is proposed and its validity for HRSG evaporators is substantiated. The rate of the exchange of gas reagents between steam and water is determined by the distribution coefficient K_d. To simplify calculations, simple approximating correlations of K_d for ammonia and oxygen is recommended. For the water chemistry with solid reagents, a diffusion model has been developed to calculate mass transfer on the basis of data on heat transfer in steam-generating tubes. The concentration of reagents near the wall is determined by both the turbulent transport of the liquid between the flow core and the near-wall layer and the coefficient of reagent distribution between the phases. Simple approximating correlations of distribution coefficients for Na₃PO₄, NaOH, and 90H Turb helamine are proposed. An approximate correlation between the hydraulic resistance coefficient K_h and the geometric parameter K_с of the FAC process has been established. The hydrodynamic fundamentals of the standard HRSG structural members have been studied in more detail than FAC. To estimate K_с, one can use the known value of K_h and the correlation between them. However, this correlation should be used with care since it has been obtained for certain conditions of unambiguity (i.e., specific construction material, water chemistry, flow history, etc.). Under other conditions, this correlation will change somewhat; therefore, it can only be used for rough estimation.