Solution & Implementation
The project involved evaluating the extent of the 24-inch feedwater discharge isolation valve damage, designing a repair solution, and implementing the solution quickly while the plant remained in operation. This valve, while non-safety related, performs the function of isolating feedwater flow to the reactor during shutdown and lowpower service conditions.
A MAJOR CHALLENGE: The 24-inch feedwater valve experienced a stemdisc separation, was stuck in the closed position and is positioned upside down 30 feet in the air.
Traditional repair options, including removing valve internals and valve replacements, would have required a plant shutdown to implement. Furthermore, on-line repair options, without protection or isolation from the pressure boundary, presented unacceptable industrial safety hazards due to the system characteristics (high temperature, high pressure, complex rigging, etc.). The feedwater system utilizes a dual loop flow path with each loop being able to maintain 65 percent reactor power. Once the valve was opened, the plant could ramp up over the following day to 100 percent power.
Before the actual work was performed, Furmanite designed a mock-up of the valve and the fixture was tested to ensure the required force could be applied safely and that rigging and access could be assessed in a controlled manner.
Then the team moved to the actual valve location. To solve the issue, Furmanite designed a specialty fixture to allow insertion of two push rods through the body of the valve, applying 300,000-lbs of force on the valve gate. The first step in the procedure was to hot tap two holes in the valve body perfectly aligned with either side of the valve gate. There was 3/32-inch clearance between the stellite seat and the edge of each push rod. This was performed using a pre-drill operation and a specially designed series of drill bits to eliminate the introduction of foreign material in the system.
Once the pre-drill operation was complete and a remaining half-inch wall was left before breakthrough into the live system, the fixture was mounted to the valve with a customized alignment system that allowed the previously drilled holes to align with the three-inch valves and launch a mechanism that acted as a pressure boundary. Prior to breaching, a pilot hole was drilled into the remaining valve body material to depressurize the valve cavity. Once the valve cavity was depressurized and once foreign material (FM) recovery methods were finalized (based on whether the cavity was pressurized or not), the final drilling operation could then be executed.
The breakthrough was carried out with a drill type that would create chips smaller than the critical size identified by Energy Northwest and would also allow for extraction via the nitrogen inject and flushing method. Once this was complete, bolt-tensioning equipment was used to apply even force on a large steel plate known as the “dog bone” because of its configuration. The force applied to the “dog bone” was transferred to two push rods inserted in the hot tapped holes. The force was slowly applied while simultaneously heating the valve seat to 450 degrees through the use of heat treatment equipment. When the valve seat expanded and sufficient force was applied, the valve gate was guided to the fully open position.