Abstract
Several tubes in NPP#3 feedwater heater were found broken and thinned. In order to resolve tube and end-plate thinning problems, Taipower provides many corrective actions such as operation water level increase and end-plate thickness increase. The objective of this research is to assess the validity of the corrective action – end-plate thickness increase.We speculate that two-phase mixture from vapor-condensation zone and subcooled drain-water from draining zone meet somewhere inside the length of the end-plate gap and is referred as an “interface front”. This interface front tends to move toward the draining zone because of the difference in pressure across the length of the end-plate gap. On the other hand, two-phase mixture in the end-plate gap will condense and the interface front tends to move toward the vapor-condensation zone because of added water mass from condensation. We propose that the main reason for broken/thinned end-plate and tubes is that end-plate gap can not provide enough water sealing function to prevent two-phase mixture in vapor-condensation zone to flash into low pressure draining zone. If we can keep the position of the interface front inside the end-plate gap, we can assure that no water flash will take place and tube erosion can be minimized.Based on the knowledge of flow pressure drop through porous media and basic film condensation/dropwise condensation mechanisms, our model indicates that increasing end-plate thickness can stall the movement of the front interface toward draining zone. At the same time, it also provides more heat transfer area and condensation rate. On the other hand, operation water level changes can affect vapor-condensation zone mixture moisture contents. Added moisture contents from water level increase tends to move the front interface toward vapor -condensation zone.