Steam Turbine Rating

As with the motor driver, the steam turbine must be matched to the compressor. Also, a turbine rating of 110% of the maximum power required by the compressor should be specified. This should be at the compressor's normal speed point. The turbine speed should include a maximum continuous speed 105% of the normal compressor speed. API Standards 611 and 612 cover general purpose and special purpose steam turbines [7, 8],

A few notes on the application of steam turbines, which are frequently overlooked, should be mentioned. One which is misunderstood more than any other is the exhaust pressure, particularly on a condensing turbine. If the condenser cooling medium fluctuates, say summer to winter, careful consideration must be given to the normal value of the condenser pressure. For most efficient operation, the value realized most of the time should be specified. If a lower value can be achieved with unusually cold coolant, then this value should also be given. The latter should be stressed as not being a long-term operating point, but given primarily to

Figure 7-17. Multivatve steam turbine with the upper half removed. (Courtesy of G.E Industrial & Power Systems)

use in evaluating the stress on the latter stages blading. A change from 4 in Hg to 2 in Hg doubles the specific volume, which doubles the exhaust volume raising velocities considerably. If the turbine vendor understands the situation, steps can be taken at the time of design to keep a few cold days from causing the operator to have to take corrective action to prevent blade damage.

Conversely, letting the vendor design his exhaust end to the lower value will cause him to grossly oversize, at extra cost and ultimately at reduced efficiency during the normal period.

Concerning condensing turbines, be sure to obtain an adequately sized vacuum breaker for the condenser. There are no words to describe the agony of watching a turbine or the driven compressor tear itself up.

because the train was tripped and it continued to rotate with condenser vacuum that couldn't be broken.

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