6. Over Speed Trip
The turbine is aquipped with an overspeed trip which prevents overspeed of the turbine by tripping the emergency stop valve and shutting off the supply of the steam to the turbine.
If the turbine speed increases by abnormal condition as much as predetermined value (about 115% rated speed), the over-speed operates and shuts the emergency stop valve through the emergency mechanism linkage.
It is an eccentric weight type and is equipped in the main oil pump end of the gear shaft.
The center of the gravity of the weight is slightly offset from the axis of ratation and is normally held in place by the compression of the spring.
When the turbine is operating at normal speed, the lever is kept in its normal position.
But when the turbine reaches the predetermined speed (about 115% rated speed), the eccentric weight due to centrifugal force, overcomes the spring compression and moves outward, and strikes the lever. The valve moves down-ward, supply oil to trip cylinder is cut off and be drained.
Supply oil press., from overspeed trip decreases, The oil piston moves up-watd and it rotates the trip lever through the trip pin.
The latch pin and spring seat hand wheel, spindle, coupling, pilot disc and disc are moved together by the spring.
Consequently, the emergency stop valve is shut down.
The tripping speed is tested at the factory and should not be changed unless absoulutely necessary.
When needed, loose sets screw then turn the adjust screw clocwise to increase the set speed or counter clockwise to decrease the set speed.
If should be taken that to lock the set screw after adjusting of the overspeed trip.
If the overspeed trip has been operated, pull the red reset knop before the emrgency stop valve is reset.
When manual trip is needed, push the trip knob the emergency stop valve is tripped.
7. Excess Back Pressure Trip
The turbine is provided with an excess pressure trip device to prevent trouble due to excessive back pressure rise.
When the turbine back pressure rises, the steam sentinel valve begins to emit steam.
When it rises further up the operating pressure, the pressure switch acts and actuates the solenoid valve provided in the hydraulic circuit led to trip cylinder of emergency stop valve, due to which oil pressure is shut off and discharged to drain.
Therefore, in the same manner as when the overspeed trip device acts, the trip mechanism of emergency stop valve, closes the emergency stop valve to shut off steam inflow.
The operating pressure of the excess back pressure trip device can be adjusted as required by turning the adjust screw of pressure switch.
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