He jishu (Nov 2022)
Distribution of pH values in a model simulating the secondary side of a steam generator
Abstract
BackgroundThe corrosion of the secondary circuit has remained a challenging problem influencing the security and efficiency of a nuclear power plant. In an actual operation, an amount of alkalizer is usually added into the secondary circuit water to adjust its pH value to alleviate the corrosion of the pipelines. However, the very complex real working condition results in a significantly inhomogeneous distribution of the pH values, and the enclosed nature of the secondary circuit have frustrated various efforts to control the precise pH values at some key sites inside the circuit. So far, pH values around such key locations have been roughly estimated by either external simulating experiments or by using patented commercial software of foreign companies. However, it is difficult for such external simulations to take into account the various important working conditions.PurposeThis study aims to develop a method without specific assumptions or uncontrolled approximations to calculate the distribution of pH values in the secondary circuit under its working conditions.MethodsFirstly, the complex and repeated structure inside the steam generator was simplified and simulated by using one direct tube model. Except for the length, the other geometrical dimensions, support plates as well as the tube material etc. were chosen to be as similar as possible to the actual ones. Then, all temperature-dependent equilibriums involving H+ in water solution of the secondary circuit were considered with additional consideration of the equilibriums of the alkalizers in the gas and liquid phases. As an essentially field quantity defined on the space of the secondary circuit, these pH values together with other relevant parameters, such as temperature, fluid velocity etc. were depended on the position coordinates, and were calculated by using the finite element method coded in the COMSOL package. Finally, the boundary conditions of flowing rate and temperature of the water at the inlet were set as 1.0 m·s-1 and 543.2 K, respectively, and the pressure at the outlet was set as ~6.8 MPa, a stepwise linear heat flux model was used to simulate the thermal energy transfer from the primary side to the secondary one. The bubbly-flow model was used to simulate the actual steam-water fluid in the secondary side, which was assumed to be in a steady state working condition.ResultsThe calculated pH field under the working conditions shows clearly an inhomogeneous distribution, e.g. ΔpH = ~ -0.6 from z = 0 to 3.8 m, due to the influences of the tube support plates, the temperature and the heat transfer, etc. The investigations on the ammonia/ethanolamine (ETA) binary alkalizer with different total concentrations of various NH3/ETA molar ratios show a better enhancing effect of ETA over ammonia for the pH value (ΔpH>~0.14), and reveal a saturation effect (molar ratio NH3:ETA≤ ~1:4).ConclusionsThe distribution of the pH values in the realistic working conditions can be calculated without resorting to empirical formulae and uncontrolled approximations. The developed method and the calculated results provide valuable information for solving the corrosion problem in the secondary circuit. The method and the model can be extended to simulate the more realistic conditions of a nuclear power plant.
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