THE EVOLUTION OF SUCCESSFUL ELIMINATION OF ELECTRO-CORROSION IN MECHANICAL SEALS FOR REACTOR- AND BOILER FEED WATER PUMPS HANDLING ULTRA-PURE WATER

摘要

Since the introduction of mechanical seals in nuclear boiling water reactor (BWR) feed pumps operating with high drive ratings, keeping mechanical seal reliability high became a challenge. A problem also encountered in fossil feed pump applications using similar electrically low conductive feed water, referred to as Combined Water Treated (CWT) feed water operation. After a short number of operating hours, rotating seal faces made from various kinds of silicon- or tungsten carbide materials show severe forms of edge chipping and crater type damages, while the opposing carbon graphite seal faces, may display a significant loss of seal face binder material, which inevitably results in the formation of uncontrolled radial flow channels across the seal face gap. A unique phenomenon referred to as 'seal face generated electro corrosion' (EC). This tutorial discusses the pragmatic approach, which was applied to investigate what major contributing factors play a role in the development of seal face generated electro corrosion and how it can be avoided. While satisfactorily results are achieved for applications with a water purity down to 0,5 μS/cm using more traditional seal face materials, research shows that for the very purest of water qualities (i.e. electrical conductivities 0,055 < X < 0,5 μS/cm), a new development step is required to suppress seal face generated electro corrosion. All developments are centered around the specific nature of these applications under real world conditions, whereby resilience and performance predictability have been at the very forefront.