Page 354 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Steam Turbines and Turboexpanders 333

Condensing Turbine Advantages. The condensing turbine has some ad-
vantages over the back-pressure turbine.

It requires less change in live steam for different loads and so control
is easier.
It requires less live steam as the drop in heat energy is larger.
It only affects one steam level for a change in power.
Extraction and Induction Turbines. Either of the basic types of turbine is in
use with one or more intermediate nozzles for extraction or induction of
Steam.
The normal criterion used in operating an extraction turbine is whether
15-20 percent of the power required can be generated by the extracted
steam. Various plant operating conditions are taken into account in this
assessment and not just the design conditions.
Extractionhduction turbines afford the following advantages :

Process steam requirements at two or more levels can be satisfied
without having to provide boilers at different pressures, or al-
ternatively having to throttle steam, without obtaining useful work,
i.e., the power from the extraction steam is obtained cheaply.
A process steam level can be controlled and maintained by the steam
turbine.
An over-all steam balance can be achieved more readily.
Optimization of process steam and power demand.
Allows flexibility under various plant conditions.

Extraction and induction turbines are slightly less reliable because:

Disturbances are caused in the steam at the intermediate nozzle;
therefore, blade vibrations can be excited.
The possibility of starving part of the turbine of steam exists, result-
ing in overheating due to windage.
Extra valves, etc., are required, if the intermediate pressure is con-
trolled.
A longer turbine shaft is required to allow for the extra nozzle: etc.,
thereby increasing the bearing span resulting in a more flexible shaft.
This could produce difficulties with critical speeds.

The efficiencies of these turbines are about five points lower than those
of basic turbines.