Description of Event:
The powerhouse operator observed the upper left strut on one of the dam’s radial spill was bent.
Actions Taken:
The operator called the Mechanical Engineer, to discuss the observed abnormality, and distributed pictures of the abnormal structure. The operator also discussed the abnormal structure with the operations supervisor, and was advised to tag the machinery with an ABNORMAL TAG and to put the control panel located at the dam beside the gate hoist motor into the, OFF position. The operator reported the ABNORMAL CONDITION to system control. The gate was removed from service and tagged DO NOT OPERATE. The Maintenance bulkhead was installed as soon as possible afterwards, to allow complete inspection and testing of the gate in a safe condition.
Method & Findings:
All elements of the gate were visually inspected for damage by a High Angle Rope access team which found the three elements on the top left side were the only ones found to be deformed. Dry friction testing was conducted to determine if the trunnion friction was the problem, but in a dry condition the trunnion loads are very low and no useful friction values resulted from this process.
A root cause analysis was conducted to determine the cause for the bending. It was found that the excessive friction in the left trunnion caused the upper left strut to partially buckle and deform other elements also. The trunnions bushings design is problematic in that there is a lack of flow of grease to ¾ of the trunnion. Due to this design, water is able to migrate onto the shaft and corrode on the operating surface. As this corrosion grows, it consumes the operating clearance resulting in binding, and produces a rough surface for the bushing to operate on. Both trunnion bushings had turned in their hub and yoke assembly, with the left viewed downstream being worse.
Supporting Materials:
Corrective Actions:
1. Three main strut elements on the left side were straightened using hydraulic jacks and strong backs to mechanically straighten the elements to visually straight.
2. The original grease lubricated bronze trunnion bushings were removed and replaced with a modern greaseless bushing design operating on a stainless steel sleeve interference fit onto the trunnion pin. To access this bushing 36 original rivets were removed per side and the hub and yoke assembly main bore was opened up to allow installation of the new greaseless bushing. Body fit bolts replaced the removed rivets. Also a PTFE dust seal was added on both ends of the pin to exclude foreign material.
3. After the trunnions were fully reassembled a dry gate friction test was conducted and frictions were unmeasurable because of the low force values from the gate being in the dry. A full gate opening test was conducted to verify that no binding existed. A wet gate friction test was conducted to confirm the installed operating condition was acceptable.
Sequence & Completion Dates for Actions Listed:
Lessons Learned:
The differential friction imposed by the trunnion has the ability to transfer forces to the upper less rigid elements. Further analysis is warranted on these gates to evaluate non-symmetric load conditions.
Recommendations:
Although the motions of both sides of these type gates is the same, the condition and related friction can be different, resulting in non-symmetric load scenarios.