LCC Quick Notes, Steam Turbine Digital Over Speed Protection
The days of relying upon centrifugal bolts to mechanically trip turbines on repeated over speed test runs are over. Over speed testing of trip bolts has been the number one turbine failure condition, and it makes no sense to stress the blades when unnecessary. There are, however, several key points to consider in specifying a digital over speed protection system:
◘ Redundancy is mandatory, 2-out-of-3 auctioned logic should be applied to separate sets of probes and monitors.
◘ Power Feeds should likewise be redundant.
◘ Self-Test Signal Injection Systems are a must to realize an effective low stress test procedure.
◘ Speed probe installations must be carefully engineered to avoid rotor thrust, thermal expansion, and bracket resonance problems.
◘ Systems providing parallel Zero Speed Detection are much more useful, and can likewise replace aging mechanical-hydraulic zero speed components.
◘ Systems that provide recorded logs of highest speed reached simplify the test qualifications.
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