Solving the problem of turbine speed fluctuations!

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The steam-driven Claus fan was shut down for maintenance two months ago due to issues with the steam turbine’s rotational speed. The rated speed of the turbine is 5360; it is normal for the actual speed to drop below 3500. When the speed exceeds 3500, fluctuations occur regardless of whether the turbine is running without load or under load, with fluctuations of around 10 revolutions. Correspondingly, the secondary oil pressure and the opening degree of the air valves also fluctuate. The fluctuations are irregular; the time intervals between them range from a few minutes to several tens of minutes. Meanwhile, the magnitude of each speed fluctuation varies from two or three revolutions to over 10 revolutions. At first, we suspected that water might have entered the oil system or that the oil quality had become emulsified; indeed, it was found that a considerable amount of water had entered the oil system and that the oil quality was slightly emulsified. Eventually, the entire oil was replaced. After running the machine without load, it was found that the problem remained unresolved. During this period, there was another minor incident while driving: when the vehicle was running with no load, the air valve responded slowly; even when the secondary oil pressure reached 0.2, the air valve still did not open, and subsequently the instrument panel triggered a shutdown. After switching to manual mode, it was found that the opening of the air valve increased gradually only as the secondary oil pressure continued to rise, and ultimately normal operation was restored. But the rotational speed still fluctuates. The ultimate solution to the problem of slow response of the gas valve is to adjust the adjustment screw at the top of the throttle, thereby modifying the secondary oil pressure that drives the initial movement of the piston. The issue with the modified item has been resolved. Later, in order to resolve the rotational fluctuation issue, the throttle system was cleaned. It was discovered that one of the balancing plug screws for the process holes on the rotating disc at the top of the throttle valve had come loose. Since a similar issue—caused by this screw coming loose—had occurred previously, resulting in rotational fluctuations, the screw was located and the problem was successfully resolved. Therefore, the screw was reinstalled this time. But during the final test drive, it was found that the problem had not been resolved. After that, we decided to divide the problem into two parts, as the factors causing fluctuations in speed could also include mechanical looseness in the fan’s guide vanes, which leads to irregular fluctuations in the air intake volume outside the acceptable range; these fluctuations in turn cause variations in the turbine’s speed. Even the operation of the surge valve could be a contributing factor. Therefore, we disconnected the coupling between the turbine and the fan and tried to restart the system, only to find that the turbine still experienced speed fluctuations when tested alone. Therefore, the fault was attributed to the steam turbine. It’s mainly the control system. That’s the progress so far; what should we do next? Is it really necessary to dismantle the entire control system? Help!