LCC Quick Notes, Digital Redundant Overspeed Trip Considerations
Digital Redundant Overspeed Trip upgrades replacing mechanical "bolt, spring, and lever" OEM turbine overspeed trip hardware provide many advantages and can reap quick paybacks. There are pitfalls, however, and care must be taken in specifying a sound replacement.
(1) OLD STYLE MECHANICAL OVERSPEED TRIP BOLTS AND TESTING
Most smaller mechanical drive steam turbines were designed with centrifugal operating mechanical overspeed trip mechanisms which when properly functioning moved levers to dump hydraulic valves on turbine autostop oil headers thus initiating a trip when the rotational speed reached the safety limit. Unfortunately problems like oil contamination, bent levers, hammered strike pins, and looseness can cause these devices to fail.
A further disadvantage of the mechanical bolt systems is that the turbine needs to be physically brought to the overspeed trip speed to test or confirm the setpoint. For years the very act of testing mechanical overspeed trip devices has been the number one cause of turbine failures.
(2) REPLACEMENT WITH REDUNDANT 2-of-3 DETECTION DIGITAL PROBES and MONITORS
A three channel system with 2-of-3 voting/auctioning with each channel having an independent speed probe and monitor will nicely supply the one RPM detection accuracy and 15 millisecond response time needed for turbine overspeed trip. Advantages of upgrading to electronic trip are:
■ Much better repeatability in trip setpoint, improving from approximately +/-50 RPM to +/-1 RPM.
■ Redundancy prevents single component failures from defeating safety function, while monitoring detects first failures.
■ Monitor tracking memory supplies trip speed data for insurance reports.
■ Trip setpoint test speeds may be lowered temporarily for test runs, thus not stressing turbine rotors, blades.
■ The Zero Speed Detection system can be replaced in the same upgrade with a combination monitor system.
(3) KEY SPECIFICATION DETAILS
3.1 Timing Wheels... If the turbine does not have a square-cut toothed timing wheel it is far better to install one than to use an existing spur gear as a speed target. Spur and power gears produce distorted speed pulse profiles which can become marginal in some situations. Also, the width of the timing wheel needs to account the total thrust travel of the turbine so that routine thrust positioning does not miss-target probes.
3.2 Speed Probes... If Zero Speed Detection Monitoring is to be included, the speed probes must be active style (rather than magnetic) in order to detect down to slow speed tooth pass speeds. Active probes also benefit overspeed systems by having fault detection at any speed, not just 200 RPM+.
3.3 Timing Gear Mounting... Care must be taken to securely mount timing gears to the turbine rotor. Turbine stub shafts should be avoided due to coupling failures potentially eliminating all speed channels at once.
3.4 Speed Probe Mounting... Probes must be securely mounted in accurate alignment to target toothed timing wheels. Probe brackets must not resonate with operating turbine speeds. Provision should be made to simplify probe gapping, even while the turbine is spinning.
3.5 Active Probe Transmitters... When using active speed probes, each should have a corresponding transmitter when wiring runs are greater than 25 ft. to the monitors. Amplified signals should be RS-422 or RS-485.
3.6 Test Signal Injection... Although dropping the test speed is accepted by insurance carriers, many then want to see the individual speed channels each verified as initiating at the full overspeed setpoint, thus "closing the loop" on calibrations. A Test Signal Injection capability is needed to meet this criteria which connects simulated speed signals from a built-in square wave generator to the channel in test only with isolated trip output. Without this feature a cumbersome method of attaching an external signal generator and manual disconnections is needed.
3.7 Zero Speed Engagement Delay... The Zero Speed Monitor circuits should have a built-in adjustable engagement delay period to allow the turning gear (Bendix device) a second or two to come to speed and engage the turning gear pinion on the bull gear. Without this delay feature the initial speed sensing will drop out the engagement immediately and prevent turning gear operation.
3.8 High Speed Alarm... A good feature is an additional speed setpoint programmed as a High Speed Alarm. This is used as a warning to operators when the turbine speed is approaching the overspeed setpoint and can often prevent an accidental speed excursion and turbine shutdown.
Please follow the links to view specific LCC solutions in overspeed and zero speed redundant digital monitors.
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