How Effective Is Your Steam Turbine Sequential Tripping Protection?
Recent AEGIS evaluations of steam turbine generators have highlighted concerns about the effectiveness of sequential tripping protection. As the preferred normal shutdown method, sequential tripping is critical in preventing a steam turbine centrifugal explosion. Although infrequent, this type of destructive event represents the worst-case maximum foreseeable power plant loss event.
To prevent steam turbine generator overspeed events and associated collateral damage, the following should occur, in sequence:
Sequential Tripping Validation in 5 Minutes
During a typical shutdown of the turbine generator, securing steam flow to the turbine and the subsequent opening of the generator circuit breaker should result in an immediate and continual reduction of turbine speed.
Create time series plots of turbine generator speed (RPM) at 1-second intervals starting 1 minute before the generator circuit breaker opens and ending 1 minute after the generator circuit breaker opens. These plots should be done for both the most recent normal turbine generator shutdown and the most recent turbine trip.
Expected Observation
Each speed data point should have a value equal to or less than the previous data point value.
Deviation from the expected observation criteria should result in an engineering disposition prior to restarting the turbine generator, as shown in the following illustration.
Monitoring turbine coast-down speed should be standard operating practice for every shutdown.
Note: A simultaneous steam turbine and generator trip will occur on most generator faults. A trip initiated from a generator fault should not be prevented or delayed by sequential tripping.
Want to learn more?
AEGIS Loss Control will be hosting a webinar on Steam Turbine Generator Sequential Tripping and Overspeed Protection on September 14 at 2:00 pm EDT. Power plant operations and engineering personnel responsible for the safe, reliable operation of steam turbines are encouraged to attend, along with risk management employees who would like to get a better understanding of these essential protection schemes. Click here for more information or to register for this critical safety webinar.
For further information on the subject matter in this Quick Tip, please contact:
Stephane Jacquemond
Senior Machinery Professional
AEGIS Insurance Services, Inc.
1 Meadowlands Plaza
East Rutherford, NJ 07073
StephaneJacquemond@aegislimited.com
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