Clearance leakage. A 100% efficiency cannot be
obtained because of friction in the blading and clearance between the
stationary and rotating parts, and because the nozzle angle cannot be zero
degrees.
Axial clearance increases in the stages further from the
thrust bearing to satisfy the need to maintain a minimum clearance at extreme
operating conditions when the differential expansion between the light rotor
and heavy casing is at its worst.
To reduce this leakage, radial spillbands are used. These
thin, metal-strip seals may be attached to the diaphragm or casing and extend
close to the shroud bands covering the rotating blades.
This clearance can be kept quite close (0.020 to 0.060 in),
and axial changes in the rotor position do not affect the clearance since the
important on reaction stages with small blade heights because of the pressure
drop across the moving blades.
Nozzle leakage. Leakage around the nozzles between
the bore of the blade ring or nozzle diaphragm and the drum or rotor must be
kept to a minimum.
This leakage is controlled through the use of a metallic
labyrinth packing which consists of a single ring with multiple teeth arranged
to change the direction of the steam as well as to minimize the leakage area.
Labyrinth packings are also used at the shaft ends to step
the pressure down at the high-pressure end and to seal the shaft at the vacuum
end.
Rotation loss. Rotation of the rotor consist of
losses due to the rotation of the disks, the blades, and shrouds. Partial-arc
impulse stages have a greater windage loss within the idle buckets. Rotation
losses vary directly with the steam density, the fifth power of the pitch
diameter, and the third power of the rpm.
In general, the windage loss amounts to less than 1% of
stage output at normal rated output. At no-load conditions, windage loss for
noncondensing turbines approximates 1.5% of the rating per 100 lb/in2 exhaust
pressure, and on condensing units approximates from 0.4% to 1.0% of the rating
at 1.5 inHg (abs) exhaust pressure.
Carryover loss. A carryover loss (about 3%) occurs on
certain stages when the kinetic energy of the steam leaving the rotating blades
cannot be recovered by the following stage because of a difference in stage
diameters or a large axial space between adjacent stages.
Typically, this happens in control stages and in the last
stages of noncondensing sections. The last stages of condensing turbines have
the largest carryover losses (normally referred to as exhaust loss) because of
the large variations in exhaust volumetric flow with exhaust pressure and the
large variation of stage pressure ratio with load.
Stages preceding the last operate with essentially a
constant pressure ratio down to very low loads and consequently can be designed
for peak efficiency at a wide range of loads.
Leaving loss. Condensing turbines are frequently
“frame sized” by last-stage blade height. It is sometimes economical to size
the unit with exhaust loss equal to 5% deterioration in overall turbine
performance at the design point (valves wide-open throttle flow and 1.5 inHg
[abs] exhaust pressure) when the normal expected exhaust pressure will be
higher or the unit will be operating at part load for a large part of the time.
Nozzle end loss, partial arc. Control stages and
partial-arc impulse stages are subject to end losses at the interface of the
active and inactive portions of the blading as the stagnant steam within the
idle bucket passages enters the active arc of nozzles and must be accelerated.
There is also a greater turbulence in the steam jet at both
ends of the active arc. In partial-arc impulse stages, the increase in
efficiency due to larger blade heights (aspect ratio) is partially offset by
increased rotation and end losses, and there is an optimum to this
proportioning beyond which there is an overall loss.
Supersaturation and moisture loss. Moisture in the
steam causes supersaturation and moisture losses in the stage. The acceleration
of the moisture particles is less than that of the steam, causing a momentum
loss as the steam strikes the particles.
The moisture particles enter the moving blades (buckets) at
a negative velocity relative to the blades, resulting in a braking force on the
back of the blades. Supersaturation is a temporary state of supercooling as the
steam is rapidly expanded from a superheated state to the wet region before any
condensation has begun.
The density is greater than when in equilibrium, resulting
in a lower velocity as the steam leaves the nozzle. As soon as some
condensation occurs at approximately 3.5% moisture, according to Yellot, a
state of equilibrium is almost instantly achieved and supersaturation ceases.
