Page 176 - Steam Turbines Design, Applications, and Rerating

Governors and Control Systems 155

The pressure limit line is the limit line for exhaust flow and is the
upper terminus for the extraction lines. To the right of this line,
exhaust flow exceeds the capacity of the turbine exhaust, and no more
steam can make its exit by that route without an extraction stage pres-
sure increase.
The operating limits of the extraction turbine are between the total
extraction line and the pressure limit line, with the size of the turbine
inlet as the upper limit, and the zero extraction line as the lower limit
(exhaust size). Inside of these limits the turbine is free to perform and
satisfy demands for extracted steam. And if these limiting conditions
are fully understood and considered in the original design, a most sat-
isfactory system, capable of the necessary flexibility, will result. (For
additional information on extraction diagrams, see sec. 15.3.)
Modern extraction control systems consist of electronic devices and
interfaces, as depicted in Figs. 8.11 and 8.12. Note that the schematic
of Fig. 8.11 does not provide extraction pressure control; Fig. 8.12 incor-
porates this feature.
Recall that the extraction valves are an integral part of the turbine
and are not valves in the process line. These valves limit turbine
exhaust end flow. When closed, maximum extraction flow occurs and
when open, maximum turbine exhaust end flow results.