Main oil pump hydraulic geometry was the root cause of numerous false turbine/generator overspeed trips.
Troubleshooting rotating equipment failures and improving reliability is a process of elimination. The process of elimination should strip away the layers of possible contributors until the true root cause can be identified and corrected. Rarely does an opportunity arises that allows for taking incremental steps to solve a problem.
In the case study below, the symptom was addressed, and the solution provided manageable results, which meant that forced plant outages were stopped. However, it still required a plant shutdown every 12 to 18 months to replace the unreliable but improved component.
This was a manageable process until five years later, when forced outages began to occur again. This reoccurrence required a re-examination of the failures and, ultimately, led to finding the root cause of the problem.
The Initial Solution
A Southern Company plant has four 818-megawatt electrical plants with identical General Electric (GE) turbine generator units. In November 1990, Unit 1 had a forced outage due to a false trip of the overspeed governor during normal operation. The same failure and forced outage occurred on Unit 4 in August 1993 and again on Unit 1 in November 1993. In October 1993 and April 1994, the overspeed trip mechanisms were inspected on Unit 2 and 3 respectively. In each case, severe wear was detected in the bushing and spindle areas, and the overspeed trip mechanisms were replaced.
The original failures seemed to be related to excessive overspeed guide bushing wear. It was thought that either the overspeed guide bushings wore and placed undue stresses on the spindle threads causing them to fail, or the trip ring became unsupported and allowed contact with the mechanical trip linkage.
While the source, or root cause, for the wear was not identified, GE addressed the excessive bushing wear by upgrading the material to a more durable “Chemloy,” a Teflon-based material with 5 percent Mo52 and 15 percent glass fiber. The material upgrade was made on all four units in 1994 and provided some improved life of the overspeed trip device, but significant bushing wear still occurred. The overspeed trip devices were placed on a schedule to be inspected/replaced every 12 to 18 months, and the forced outages were eliminated until January 1997.
Reoccurrence of the Problem
In January 1997, Unit 4 had a forced outage due to a false overspeed trip. This occurred three months after a scheduled maintenance outage in which the turbine overspeed trip device had been replaced. The failed overspeed speed device had been rebuilt by a company other than GE, and the quality of that rebuild was brought into question.
A new overspeed trip device rebuilt by GE was installed and carefully inspected for quality. Then 13 months later, another false overspeed trip occurred on Unit 4. After this failure, the decision was made to measure Unit 4's and Unit 1's control rotor vibration and compare the results. The vibration data revealed 10 times greater vibration amplitudes at six times the running speed on Unit 4's control rotor than on Unit 1's.
The overspeed trip device was located on the end of the control rotor stub shaft. The supporting sleeve was screwed on the end of the control rotor stub shaft and was encircled by the ring assembly. The ring ran concentric with the shaft but was dynamically unbalanced, and the weight of the moving parts was distributed so that their center of gravity lay about 0.220 inches from the center of rotation in the tripping direction of the ring.
The centrifugal force of the ring assembly, due to this unbalance, was counteracted by the force of a compressed helical spring. When the speed increased, this centrifugal force overcame the spring's force. The ring moved out and struck the trip finger of the mechanical trip linkage.
Attached to the front of the overspeed trip device was the spur gear, which served three functions:
• A coupling for the quill shaft to the permanent magnet generator (PMG)
• A balance point for the control rotor
• An oil channel for the oil trip test feature
In an effort to determine the cause of the vibration, a detailed review of the Unit 4 1996 outage report was performed. The review revealed that the steady bearing and overspeed governor had been replaced without the necessary testing to determine if the components were assembled within specifications.