28.02.2013 Views

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

Handbook of Turbomachinery Second Edition Revised - Ventech!

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

an additional electronic overspeed trip is provided through a ‘‘toothed<br />

wheel’’ and proximity probe mounted in the front standard or pedestal.<br />

Low-Bearing-Oil Pressure Trip<br />

This device typically consists <strong>of</strong> a spring-loaded diaphragm exposed to<br />

bearing oil pressure. Excessively low-bearing-oil pressure can cause overheating<br />

or scoring <strong>of</strong> journal bearings. A diaphragm is connected to a dump<br />

relay through a linkage mechanism. If the bearing pressure falls below a<br />

preset valve <strong>of</strong> 5 to 6 psig, it will release the oil from the overspeed trip valve<br />

through an intermediate relay. Other OEM design configurations utilize a<br />

dual system <strong>of</strong> hydraulic pressure switches for the low-bearing-oil pressure<br />

switch.<br />

Low Vacuum Trip<br />

The low vacuum trip protects the turbine from operation without adequate<br />

condenser vacuum. Low condenser vacuum can cause ‘‘stall-flutter’’ <strong>of</strong> the<br />

last row <strong>of</strong> rotating turbine blades, particularly at low operating loads. This<br />

device typically utilizes a pressure-responsive bellow, which is exposed to the<br />

turbine exhaust pressure. When the exhaust pressure drops below a preset<br />

value, adjustable between 18 and 25 in. <strong>of</strong> mercury, it causes the trip relay to<br />

release the overspeed trip valve. A means <strong>of</strong> latching the vacuum trip is<br />

provided, which when engaged prevents the device from operating during<br />

startup when the exhaust vacuum is less than the normal trip setting.<br />

However, it will function with the latch engaged should the exhaust pressure<br />

increase to 2 1 = 2 to 3 1 = 2 psig during the starting cycle. The latch will fall out <strong>of</strong><br />

engagement when the exhaust vacuum has reached a value <strong>of</strong> 20 to 28 in. <strong>of</strong><br />

mercury and will thereafter trip at the normal value <strong>of</strong> 18 to 25 in. <strong>of</strong><br />

mercury, unless relatched. This function can also be accomplished through<br />

the use <strong>of</strong> a dual system <strong>of</strong> hydraulic pressure switches.<br />

Emergency Solenoid Trip<br />

A protective trip is provided that, when energized, operates through the<br />

same mechanism as the vacuum trip device. The solenoid trip feature<br />

permits remote tripping from the control room or by means <strong>of</strong> protective<br />

relays.<br />

Copyright © 2003 Marcel Dekker, Inc.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!