Handbook of Turbomachinery Second Edition Revised - Ventech!

can reveal nozzle and/or blade erosion or fouling. Measurement of the
turbine section efficiency by an enthalpy drop test is the most simple and
useful test of turbine stages operating in the superheated steam region; for
accurate results, one needs to have accurate measurements of the pressures
and temperatures at the inlet and outlet of the stage [13–16].
Enthalpy drops for the HP and IP sections of the turbine are fixed by
known temperatures and pressures, whereas that for the LP is not. Until
recently there has been no means to measure the wetness of the interstage
and exhaust steam, and thus LP turbine power and efficiency could only be
inferred indirectly from a heat balance calculation. However, wetness probes
have now been developed that can measure the distribution of wetness at the
interstage and turbine exhaust planes. These data, when combined with
aerodynamic data from pitot tube measurements, can quantify the LP
discharge mass flow, flow-weighted total/static pressure, and turbine
exhaust state point [17]. From the state point, the thermodynamic efficiency
of the last stage can be determined. This allows for baseline data to be
established in efficiency or flow path improvement projects [17–19].
Steam Cycles—Practice
Components of a power plant can be divided into heat sources (the boiler or
steam generator) and the turbine cycle. The turbine cycle includes the
turbine, generator, condenser, pumps, and feedwater heaters.
Fossil Power Plant Cycles—Illustrative Example
Figure16showsatypicalturbine–heatercycleinafossilfuelpowerplant.In
this example the turbine has high-pressure (HP), intermediate-pressure (IP),
and low-pressure (LP) sections. Also shown are a typical feedwater heater
arrangement, the condenser, and the auxiliary turbine. Figure 17 shows the
superposition of the steam cycle for this unit on an outline of the Mollier
diagram. A few pressure curves are shown as dotted lines. The saturation
line is shown as a solid line and located approximately in the middle of the
figure. The expansions through various cycles are shown with solid lines.
The lines for expansion through fossil fuel HP, IP, and LP turbines are in
the upper part of the figure. Typical efficiencies for each of these turbine
sections and typical nuclear reheat and nonreheat cycles are also shown in
Fig. 17.
Note that unlike the idealized regenerative Rankine cycle, typical fossil
and nuclear unit cycles are not isentropic. This is evidenced by the slight
slope toward increasing entropy during the expansions through the turbine
Copyright © 2003 Marcel Dekker, Inc.