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Handbook of Turbomachinery Second Edition Revised - Ventech!

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908 arc nozzle chambers feed each half <strong>of</strong> the control stage. The control<br />

valves are so arranged that the valves open in pairs and feed chambers that<br />

are opposite in direction (one chamber <strong>of</strong> each stage) and diametrically<br />

opposite. An equal force on the opposite wheel balances the torque force on<br />

one control stage wheel. The only dynamic reaction that remains on the<br />

spindle is the torque produced by the diametrically opposite forces on the<br />

individual control stages.<br />

Protective Trip and Supervisory Systems<br />

The main steam valves are under the control <strong>of</strong> autostop oil, which is<br />

produced by latching the overspeed trip valve. The trip devices operate<br />

directly to rapidly close the throttle valves and reheat stop valves. The<br />

governing valves and interceptor valves are hydraulically interlocked with<br />

the protective trip devices and remain closed whenever the turbine is tripped.<br />

The protective trip devices normally include an overspeed trip, a lowbearing-oil<br />

pressure trip, a solenoid trip, a thrust-bearing trip, and a low<br />

vacuum trip. The protective devices are included in a separate assembly but<br />

connected hydraulically to the overspeed trip valve through the trip relay.<br />

Overspeed Trip Mechanism<br />

This mechanism consists <strong>of</strong> an eccentric weight or ring mounted at the end<br />

<strong>of</strong> the turbine shaft, which is balanced in position by a spring until the speed<br />

reaches approximately 111% <strong>of</strong> normal operating speed. Its centrifugal force<br />

then overcomes the holding spring and the weight or ring flies out striking a<br />

trigger, which trips the overspeed trip valve releasing the autostop pressure<br />

to drain.<br />

The autostop control pressure is connected to a governing emergency<br />

trip valve, which releases the control oil pressure. Therefore, all valves<br />

capable <strong>of</strong> admitting steam into the turbine will close. An air pilot valve<br />

used to monitor the extraction nonreturn valves is also triggered from the<br />

autostop pressure. The autostop is also tripped when any one <strong>of</strong> the<br />

protective devices, such as the low-bearing oil, low vacuum, solenoid, thrustbearing<br />

trip, etc. are actuated.<br />

Provision is made for testing the overspeed trip mechanism without<br />

actually overspeeding the turbine. If the overspeed trip mechanism reset<br />

lever is held to prevent the trip valve from opening, it can be tested without<br />

taking the unit <strong>of</strong>f the line or removing load. This is accomplished by<br />

admitting oil under pressure to the chamber beneath the trip weight and<br />

noting the pressure required to move the weight outward. On EHC systems,<br />

Copyright © 2003 Marcel Dekker, Inc.

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