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Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

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water content can cause corrosive attack <strong>of</strong> lubrication system components,<br />

resulting in rust spots, corrosion pits, or rapid corrosive removal <strong>of</strong> bearing<br />

surfaces.<br />

STEAM TURBINE CONTROL SYSTEMS<br />

In the past, turbine control technology had to satisfy the demands <strong>of</strong> a few<br />

isolated turbine generators. This was accomplished by the application <strong>of</strong> a<br />

relatively simple mechanical control system. Even when generators were<br />

interconnected to larger power pools, this system was the only means <strong>of</strong><br />

controlling speed and load <strong>of</strong> steam turbines. The earliest steam turbine<br />

control systems were purely mechanical with a ‘‘fly ball’’ governor<br />

controlling speed through a series <strong>of</strong> levers, cams, and springs [124–125].<br />

Since large steam turbines are essentially designed for constant-speed<br />

service, the operating governors are designed primarily for speed response.<br />

The governor weights in operation rotate with the main shaft, and<br />

centrifugal force causes the weights to move away from the shaft upon<br />

increase <strong>of</strong> shaft speed (Fig. 63). These weights are restrained by ahelical<br />

spring and revolve around the spindle, causing the link attached to the<br />

horizontal arm overhead to be moved either up or down. An increase in<br />

speed will cause the weight to move outward and change the position <strong>of</strong> the<br />

overhead arm so as to push the relay valve downward. Oil is used as a fluid<br />

medium under pressure to move the pilot or relay cylinder pistons. When<br />

the piston has assumed a new position under the action <strong>of</strong> oil admitted<br />

through the relay valve, a horizontal link moved by the pilot or relaycylinder<br />

piston rods through linkages connected back to the relay pilot or<br />

valve repositions the latter in a neutral or mid-position.<br />

Mechanical Hydraulic Control Systems<br />

As the sizes <strong>of</strong> steam turbines have grown in capacity, so have the throttle,<br />

stop, and control valves, which require larger motive forces to operate the<br />

control system quickly and reliably. Fundamentally, all mechanical<br />

hydraulic controls are operated utilizing the lubricating oil supplied by the<br />

shaft-mounted main oil pump. This hydraulic motive oil pressure is used to<br />

obtain the necessary force to actuate the servomotor pistons. The same highpressure<br />

oil is orificed and regulated by various controllers to obtain the<br />

lower pressures necessary to monitor the position <strong>of</strong> the servomotors, by<br />

means <strong>of</strong> automatic stop and oil control pressures (Fig. 63).<br />

Copyright © 2003 Marcel Dekker, Inc.

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