The Pressurizer Safety Valves (PSVs) at Diablo Canyon Power Plant were manufactured by Crosby and are model 6M6 HB-86-BP valves. The valves have intermittingly experienced leakage as the units are heated up and pressurized to normal operating conditions. When leakage occurs, a time-consuming de-pressurization and re-pressurization process is implemented that causes significant delays in the return to power after an outage. Following the elimination of the upstream water seal, leakage has occurred on individual valves during all but one start-up.
After recent PSV leakage, instrumentation was installed to obtain information during the subsequent unit heat up and pressurization. The instruments included thermocouples on the inlet pipe, the discharge pipe, and the valve. Data was collected during a unit start-up in which two of the three installed valves leaked.
Piping and valve analyses were performed using the data to modify the valve installation procedures, to modify the process for accommodating piping thermal growth, and to modify the pressurization rate during start-up. Upon implementation of these changes, none of the PSVs leaked during the subsequent unit heat up and pressurization, saving more than 24 critical path hours.
The data, analyses, and modifications will be described along with the analyses used to support an increase in the pressurization rate for unit start-up.
