Page 374 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 359

Of course to really generate power we superheat
the steam in the boiler. That allows us to use a more ef-
ficient turbine that extracts more energy from the steam.
In large power generation equipment the steam is piped
off the turbine and back to the boiler to be reheated be-
fore continuing its trip through the turbine. The reason
we use reheat is the superheat necessary to prevent
condensation on a full path through the turbine would
be so high that the superheater tubes and pipes would
melt, we simply don’t have metal that could take those
high temperatures. By reheating we can boost the tem-
perature at an intermediate stage in the turbine to about
the same temperature as the steam at the turbine inlet
without requiring more exotic metals in the superheater
Figure 10-100. Impulse and reaction stages of turbines and piping. The heat exchange surface in the boiler that
does this is called the reheater and it requires special
style turbines for power generation have twenty or more consideration in the startup and operation of a boiler
stages in the high pressure turbine and another 18 or that’s equipped with one.
more in the low pressure turbine(s). As the steam pres- The turbine arrangement that will probably be-
sure drops from stage to stage it expands. If you check come more common with the development of ‘distrib-
the volume of a pound of steam in the steam tables uted generation’ is the ‘topping turbine’ or ‘high back
you’ll notice that the volume of steam increases rather pressure turbine’ that will generate electric power. All
dramatically. The manufacturer of the turbine either has the generated steam passes through the turbine for use
to make the latter stages of the turbine much larger or in the facility. The steam will be produced at high pres-
provide for bleeding off some of the steam. sure (600 to 900 psig being the most common) and su-
It’s typical for a large power generating turbine perheated, then dropped through the power generation
to have at least two bleeds. High pressure bleed steam turbine to generate power while dropping to pressures
is regularly used for feedwater heating between the you’re operating at now, the level required in the facility
deaerator and the economizer or boiler. Some high pres- served by the boiler plant.
sure bleed steam is at a pressure high enough to be used The few things you need to know about turbines is
as the steam supply for auxiliary turbines. Intermediate that their lubrication is critical and the torque of a turbine
pressure steam can be used in the deaerator or for other is at maximum when it’s not rotating and decreases as
purposes. Low pressure bleed steam can be used for speed increases. Most power generation turbines have
plant services such as building heating and reheating pressure lubrication; the oil is supplied to the bearings
condensate after it leaves the condensers. See the utility under pressure. The oil feed can be from a pump directly,
cycle (Figure 1-8). in some cases one attached to the turbine, or from a head
Steam turbines, and engines, extract energy from tank set well above the turbine which is constantly re-
the steam without condensing it. That’s very important filled from the sump by pumps. You may recall being in a
because the turbine would be severely damaged by power plant and seeing a viewport in some piping where
droplets of condensate hammering the turbine blades. you can see oil splashing through; that’s the overflow
The energy that’s removed from the steam to generate from a head tank. As long as you see oil spilling down
power is only enough to reduce the superheat. that overflow you know there’s lubricating pressure for
Despite what some people think, a turbine that the turbine. If you don’t see it you have a short period of
runs on saturated steam is only extracting superheat. time in which to get that turbine shut down.
The steam contains the same amount of energy after it As for the torque business; you don’t want to dam-
passes through the first nozzles of a turbine as it did at age the turbine. Spinning open a steam valve on an idle
the inlet of the turbine. Since the pressure in the steam turbine inlet can result in so much torque at the first stag-
is lower the steam has to be superheated. As long as the es that the plate holding the turbine blades or the shaft
turbine doesn’t draw too much energy from the steam it can be bent enough to cause the blades to hit with serious
will still be superheated (a little bit) at the outlet of the damage. The rapid acceleration of the turbine from zero
turbine. speed can result in serious over-speed conditions. Just