Description of Event:
There have been several hydropower facilities that have lost control of the governor during normal operation or while attempting to bring the unit online due to the governor actuator tank float failing unexpectedly and breaking off the valve actuator arm. The tank float is located inside the governor oil actuator tank and Drawing 1 illustrates the typical configuration.
If the governor actuator tank float breaks off during operation, the ability to control the unit will be lost, and the unit will typically have to be shut down by performing an unbalanced closure of the head gate. If the governor actuator tank float breaks off while a unit is shutdown, it will result in the inability to control the governor and the unit will not be able to be brought online.
Actions Taken:
Please use Photographs 1-6 and Schematic 1 as references for the following discussion. If the float breaks off during operation, the float valve (Part 5 of Schematic 1) will close and immediately stop the flow of oil and pressure from the actuator tank. The hydraulic system will be hydraulically locked, and the gates will be fixed in their current position with no control. If the unit is running, will result in the governor no longer being able to maintain frequency or maintain load properly. The only way to shut down the unit after this occurs, will be to close a valve upstream of the unit or perform an unbalanced closure of the head gate. Depending on the scenario, the unit could go into an overspeed condition prior to action being taken to shut down the unit. If the unit is stopped and watered up the pressure will bleed off allowing the system to relax allowing the unit to creep. Also, if the float has a crack which propagates slowly, or if there is a flaw in the weld repair, the float may fill with oil over time, which will also render the governor inoperable.
Method & Findings:
Supporting Materials:
Corrective Actions:
Sequence & Completion Dates for Actions Listed:
Lessons Learned:
Recommendations:
Create an inventory of affected equipment to track inspections, results, and repairs. Perform actuator tank float inspection of all affected units during the next unit outage and at every subsequent 5-year actuator tank inspection. During the inspection, remove the float and arm and inspect the float and arm connection by non-destructive examination methods using either dye penetrant or magnetic particle testing. If there are any indications noted during the non-destructive examination, the areas with indications should be repaired or the float assembly should be replaced entirely.