The Challenge

IMI CCI’s nuclear customer in Canada is currently refurbishing six of their eight CANDU type 779-817 MW reactors. One of the unique issues is tube wall thinning in the high-pressure heat exchangers. As the tube walls become thinner in a shell and tube heat exchanger design, the outlet temperature of the main feedwater increases. Over time, the slight temperature rise will shorten the life of the steam generator. As the customer plans the long-term replacement of the heat exchangers, a feedwater bypass line will be added to slightly reduce the heat exchanger outlet temperature and prolong the life of the steam generator. To modulate the bypass flow, a robust control valve is required. The valve would modulate flow with a very low pressure drop. Since the bypass piping is temporary and the valve stem will be horizontal, weight is a key factor in valve selection criteria.