Ion Exchange and Mechanical Purification of Fire-Resistant Phosphate Ester Fluids Used in Steam-Turbine Control Systems

摘要

Steam turbines at nuclear stations have electro-hydraulic control (EHC) systems that use a phosphate ester-based fire-resistant fluid. This fluid undergoes degradation in service via hydrolytic, oxidative, and thermal mechanisms that are influenced by system design and operating conditions. Past experience (OPEX) has shown that the condition of the fire-resistant fluid in service is critical for station safety and nuclear regulatory authorities; therefore, chemistry control of this fluid is included as a part of a station's operating license. The typical industry approach to maintaining fluid quality within specification is to continuously circulate a portion of the fluid through an adsorbent solid to remove degradation products. Since the late 1980s, ion-exchange treatment has become one of the most effective purification processes. However, there are now several different resin types available that can interact with the fluid in different ways, and the optimum process for resin treatment of phosphate esters has still to be identified. In fact, it will probably be necessary to have several different options depending on the operating conditions. EHC fluid purification is not limited to acidity control. It is also important to keep the fluid clean and dry if it is to operate efficiently and offer a long service life. Mechanical techniques are, therefore, needed to complement and maintain the activity of the resin treatment. For example, resin fouling by particulates can reduce its activity and this may require improved filtration. The main objective of this paper is to present the initial results of a new comparison of resin behavior intended to improve performance of the ion-exchange treatment at CANDU (Canada Deuterium Uranium) nuclear stations. Also included are the results of early investigations into different techniques for drying the fluid and for removing small particles arising from fluid degradation. The paper will additionally provide a brief description of the design requirements of the steam-turbine electro-hydraulic control system, together with an explanation of the degradation mechanisms of phosphate esters, the products of degradation, and their impact on fluid life and performance. An introduction to the principal factors affecting the efficiency of different ion-exchange treatments follows, and the paper concludes with a discussion on the work required before a final resin selection can be made.