Failed Mechanical Overspeed Trip after Turbine Overhaul
A 150 MW extraction-condensing steam turbine has just returned from a major overhaul. During the first roll to synchronous speed, the mechanical overspeed trip is tested by increasing speed with the main stop valve. At 3,300 r/min the trip bolt should strike the trip lever, but the unit continues to 3,360 r/min before the operator trips manually. Post-trip inspection finds the trip lever return spring missing and the linkage binding. The shop had tested the overspeed bolt on a bench at 3,300 r/min and issued a certificate. The unit’s normal speed is 3,600 r/min, and the overspeed trip setpoint is 3,300–3,360 r/min. The reheat intercept valves are hydraulically operated and share the same trip oil header. The grid operator has approved a 6-hour return window. The turbine OEM’s instruction book requires a mechanical overspeed test after any reassembly of the trip linkage and a repeat test at the next outage.
1. Evaluate the overspeed test result and the shop certificate. Is the installed overspeed protection acceptable for return to service?
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No. The shop certificate proves only that the bolt itself lifted on a bench at 3,300 r/min; it does not prove the installed trip linkage, lever, spring, and trip oil system will function. The installed test is the governing test, and it failed: the unit passed the expected trip speed and reached 3,360 r/min before a manual trip. That is outside the required automatic trip band and demonstrates a binding linkage and a missing return spring. The fact that the operator could trip manually is not a substitute for automatic overspeed protection; manual action depends on the operator seeing the speed and reacting in time. I would treat the turbine as having no proven overspeed trip. Return to service requires a complete inspection of the trip linkage, replacement of the missing spring, freedom-of-movement checks, and a repeat mechanical overspeed test with the installed system, witnessed and documented. The bench certificate should be retained but cannot be used as the acceptance basis.
2. Explain the reheat-steam overspeed risk and what additional valve tests you would require before the repeat overspeed test.
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On a reheat unit, closing the main stop valves does not stop all steam flow. Steam stored in the reheater and in the hot reheat line can continue to flow through the intercept valves into the IP turbine and accelerate the rotor. If the intercept valves do not close on trip, the unit can overspeed even with the HP stop valves shut. Because the intercept valves share the trip oil header, a binding linkage or low trip-oil pressure can affect both. Before the repeat overspeed test I would require: functional stroke tests of the HP stop valves, reheat intercept valves, and any extraction non-return valves; verification of trip-oil pressure and dump-valve operation; a test that the intercept valves close within the specified time on a simulated trip; and confirmation that the emergency trip lever and remote trip are independent. The overspeed test should be done with the unit rolling and the steam conditions controlled, not by assuming the bench test covers the system.
3. The grid operator offers a 6-hour return window. What is your decision and what conditions, if any, would you attach?
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I would not return in the 6-hour window. The unit has no demonstrated automatic overspeed trip, and the defect is in the installed linkage, which is exactly the part that must work. A 6-hour window is not enough to inspect, repair, re-rig, and repeat a mechanical overspeed test with proper warm-up and witness. My decision would be to keep the unit off line and tell the grid operator the earliest realistic return is after the repeat test, not before. If management pressed for a conditional return, the only defensible conditions would be: the trip linkage repaired and the missing spring replaced; a successful repeat mechanical overspeed test at the specified setpoint with the installed system; all stop and intercept valve tests complete; and a written OEM/engineering acceptance. Even then I would not accept a return based on the bench certificate. The safety case for overspeed protection is not a schedule trade-off.