Fossil Plant BFPT and FDFT
PKG 5.1 Overspeed Trip Upgrades
A frustrating paradox of feedpump and forced draft fan turbine operation is the lack of reliability of the mechanical flyweight assembly within the turbine front shaft which initiates an overspeed trip action. Sequential test runs typically yield trip speeds over a span of 50 RPM when successful. The paradox arises when two conflicting risks are present...
(1)
Due to the lack of repeatability of the flyweights, and the dangerous personnel safety and destruction of uncontrolled and unlimited overspeed the system need be routinely tested with multiple full overspeed runs under precautionary conditions.
(2)
According to insurance claim records 90% of turbine blade failures occur during overspeed trip tests.
This leaves the choice of frequent testing with increased blade damage risk events or infrequent testing with increased OST failure and total destruction risk. Not a good choice. But, there is a solution in the form of the LCC Model 470 Digital Speed Monitor which enables an alternate overspeed test procedure which is fully accepted by equipment insurance underwriters and reduces risks to near zero. Here’s how it works...
1.
The Model 470 has three speed probes targeting a toothed timing wheel on the turbine shaft. Each probe reports the tooth-pass frequency to an independent monitor module (DSMA, DSMB, DSMC).
2.
If any two of the three monitor modules resolve a turbine speed in excess of the overspeed trip setpoint, hard-wired logic initiates a turbine trip.
3.
System validation testing is performed in four steps:
A.
A keyswitch is used to divert each monitor module’s input signal one at a time from the speed probe signal to the variable frequency output of a resident wave generator.
B.
A frequency equivalent to the overspeed setpoint is applied to the monitor module (only) under test from the built-in generator and that particular module is validated for the trip. This is repeated for each module, again, one at a time.
C.
A temporary overspeed trip setpoint at a low speed (well under the overspeed setpoint) is entered for all three monitor modules.
D.
Finally, the turbine is operated up to the temporary overspeed setting and a full turbine trip is validated.
This alternate procedure verifies the full overspeed trip setpoint on all modules, yet physically trip tests the turbine at a low stress much slower speed thus reducing test damage risk.
An additional Model 470 feature is the Zero Speed Detection with control status timed output. This feature starts the turbine turning gear motor upon shaft stop after coast downs. Preventing rotor thermal bowing due to rotor resting at a fixed position while hot.
For a full description of features of the LCC Model 470 Digital Speed monitor click below to view the product brochure.