Steam Turbine Overspeed Protection Systems
Although a steam turbine overspeed event is rare, it can be catastrophic, and can lead to significant property damage and extremely high repair costs. AEGIS Loss Control considers a steam turbine centrifugal explosion to be the maximum forseeable power plant machinery loss event. A large steam turbine uncontained overspeed event can produce forces approaching those of a large plane crash. A single last-stage turbine blade can exert more than 300,000 pounds of force at synchronous speed.
Fortunately, these events can be prevented with routine inspections, appropriate system maintenance and periodic testing. Regular testing ensures that the protection system is operating properly, which, with improvements in overspeed protection systems, tripping procedures and operator training, have helped reduce the frequency of these events. However, operational and maintenance complacency can outweigh the benefits of the technological advancements, which means the overspeed risk will still exist.
We recognize there is risk involved with conducting full or partial overspeed tests, and that sometimes units run for extended periods between tests. What's more, a number of overspeed incidents have occurred during testing. However, a clear advantage of an electronic overspeed protection system is the ability to functionally test the complete system without subjecting the steam turbine and generator to the forces associated with a full speed trip. This is typically accomplished by lowering the trip point or introducing a signal. An overspeed event requires loss of load, loss of unit control and failure of the overspeed protection system. Many generating station operators have taken to conducting sectionalized overspeed trip system tests while the unit is offline. While that can be helpful, we do not consider a sectionalized test a replacement for a full functional test of the overspeed protection system.
It's crucial to stop the flow of steam to the turbine for a safe shutdown, which includes regular testing of the stop valves. We support OEM recommendations for proscriptive cycling or timed full-stroke tests of stop and intercept valves to ensure their reliable operation. This testing should include the steam extraction non-return valves, which, if faulty, can allow steam to enter the turbine during shutdown.
You can read about our testing cycle recommendations in the Overspeed Protection Systems white paper, which is posted on the AEGIS website. The white paper discusses the following in detail:
For more information on steam turbine overspeed protection systems, please contact Gregg Basnight by e-mail or phone at 201.508.2730, Josh Fleischer by e-mail or phone at 201.508.2637, or your designated AEGIS Loss Control Senior Machinery Professional.
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