원전 1차기기 냉각해수계통 유량조절밸브 침식 개선에 대한 고찰

Abstract

The function of Nuclear Service Cooling Water(NSCW) system is for supplying cooling water(sea water) to the Component Cooling Water Heat Exchangers, Essential Chillers and Emergency Diesel Generator Chillers. In the NSCW system, the Flow Control Valve(FCV) is installed to prevent an overflow of the system. As metal loss damage(erosion) inside the FCV due to the cavitation and sea water corrosion occurred frequently, the material of valve disc was changed from AL-Bronze to Monel in 1996. After changing the material, the erosion by sea water corrosion was clearly improved. But the erosion induced by cavitation is still observed. System overflow is prevented by a FCV(18 inch), and an orifice is provided at the downstream of the heat exchanger to avoid cavitation due to excessive pressure drop through a FCV, and to meet the flow rate through each flow line. The FCV is of a butterfly type, which is commonly used for modulating in the opening angle range of 20°∼70°, because in general it has an equal percentage nature in this range. To calculate the pressure drop through the FCV, Flow-Series, a system sizing computer program, was used. The pressure loss charged by the FCV was calculated based on the conditions of low low water level and high high water level, and it was evaluated to determine whether or not cavitation occurred at this pressure drop. The result shows that cavitation occurs at the pressure drop where the valve should be charged, in a manner dependent on the sea water flow rate and sea water level. Damage of the valve disc and the body shows typical erosion, and it has a uniform direction. This phenomenon is caused by the fact that wear occurs on the metal surface due to the serious turbulent flow of the fluid, and the flow velocity varies more rapidly as the FCV operates in a state of partial opening at around 65% during operation. It is assumed that the mixed coarse sand presents a more serious environmental factor in terms of the likelihood of erosion. Although the present valve material, Monel, has superior corrosion resistance and relatively excellent erosion resistance against sea water, the erosion speed increases rapidly when solid particles such as sand are included in the fluid. To address this weakness, it is desirable to apply super stainless steel(SR50A), which has excellent corrosion and erosion resistance, as the valve material. SR50A has excellent corrosion resistance against sea water to the same extent as Monel, and its mechanical characteristics are much more superior than those of Monel and Al-Bronze. The plan suggested is that the valve size be increased to 24 inch to mitigate the high velocity through the valve, while the required head be decreased so as not to generate cavitation at the valve, and the orifices installed at downstream of the heat exchanger and the cooler of each flow path additionally charge the reduced head. This study describes the reasons for valve damage and the plan to address these reasons by reviewing the design data of the NSCW system and the FCV, analyzing the operating condition, and evaluating to determine whether or not cavitation occurs at the pressure drop that the valve charges. The results indicate that it is desirable to replace valve material with SR50A, which can endure in sea water that includes coarse sands and which has excellent corrosion and erosion resistance, to increase the valve size to 24 inch from 18 inch to mitigate the high flow velocity, and finally to decrease △P of the valve by increasing △P that the orifice charges at the upstream of the valve.