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6 Steam Turbines
so-called half speed: 1500 rpm in a 50 Hz system. Rotor rows in the HP-IP part have
cover bands, composed by blade heads. In the HP-IP part, the degree of reaction is
very low at the hub, but it increases towards the periphery (Sect. 6.9.2 discusses the
increase of the degree of reaction with radius). So, saying that the HP and IP parts
are of impulse type means that the degree of reaction is very low at the hub, but the
degree of reaction is not very low on the average radius. In the LP-parts of the turbine, the degree of reaction is also low at the hub, but it is near to 90 % at the casing
in the last stage (see Exercise 6.10.5).
The LP part of Fig. 6.27 is from a steam turbine of a nuclear power plant. The
general appearance of an LP part of a coal-fired plant is similar, but the diameters
are smaller and the rotational speed is 3000 rpm in a 50 Hz system. In the last stage
of the LP turbine in Fig. 6.27, rotor blades have reinforcing elements, touching each
other (so-called snubbers). These elements are intended to increase the stiffness of
the rotor (higher eigenfrequencies), but cause aerodynamic losses. The snubbers
are necessary in the example because the blades are very long. Last stage blades in
LP parts of coal-fired power stations are shorter and made with large chord at the
root, making reinforcements unnecessary. The last two LP part stages have no cover
bands. The blades are very long. Thus, the clearance between the rotor and the casing is, relatively seen, very small. Clearance loss is relatively low. Moreover, cover
bands cannot be applied to the last stages for reasons of strength. The blade tip
speed is at the highest level there. A cover band would unacceptably contribute to
the centrifugal force. In the example shown, the blade speed at the circumference of
the last stage is about 530 m/s. Rotors in the LP part of steam turbines for a nuclear
power station may be made by discs shrunk on a shaft (1500 rpm in a 50 Hz system). With coal-fired plants, the construction is typically by welding forged parts, as
for HP and IP parts (3000 rpm in a 50 Hz system). Casings are made from welded
steel plates. Blades are mounted with axial slots in disc type rotors. With the drum
type rotor of Fig. 6.27, the blades of the first four stages are mounted with tangential
slots. The slots are axial with the last two stages (curved for the last stage).
Figure 6.28 represents the impulse type VHP module (very high pressure) of
a large steam turbine for a coal-fired power station with ultra-supercritical steam
conditions. The machine has two reheats. Steam conditions are 285 bar and
580 °C/580 °C/580 °C. The machine has a very-high-pressure part (VHP), a highpressure part combined with an intermediate-pressure part, a double-flow intermediate-pressure part and two LP parts, each with two flows. In the VHP part, steam
expands from 285 to 78 bar. The VHP section has an inner and an outer casing, both
with horizontal splits. The figure shows the nozzles incorporated in diaphragms.
At low degree of reaction, pressure drop occurs for the largest part across the stator. It is then advantageous to reduce the diameter of the hub sealing of the stator
vane ring. For very high steam density, lower clearance losses may compensate the
intrinsically somewhat lower internal efficiency, compared to reaction stages. As a
result, the overall efficiency of an impulse type turbine part may even be higher for
high steam density.
6 Steam Turbines
so-called half speed: 1500 rpm in a 50 Hz system. Rotor rows in the HP-IP part have
cover bands, composed by blade heads. In the HP-IP part, the degree of reaction is
very low at the hub, but it increases towards the periphery (Sect. 6.9.2 discusses the
increase of the degree of reaction with radius). So, saying that the HP and IP parts
are of impulse type means that the degree of reaction is very low at the hub, but the
degree of reaction is not very low on the average radius. In the LP-parts of the turbine, the degree of reaction is also low at the hub, but it is near to 90 % at the casing
in the last stage (see Exercise 6.10.5).
The LP part of Fig. 6.27 is from a steam turbine of a nuclear power plant. The
general appearance of an LP part of a coal-fired plant is similar, but the diameters
are smaller and the rotational speed is 3000 rpm in a 50 Hz system. In the last stage
of the LP turbine in Fig. 6.27, rotor blades have reinforcing elements, touching each
other (so-called snubbers). These elements are intended to increase the stiffness of
the rotor (higher eigenfrequencies), but cause aerodynamic losses. The snubbers
are necessary in the example because the blades are very long. Last stage blades in
LP parts of coal-fired power stations are shorter and made with large chord at the
root, making reinforcements unnecessary. The last two LP part stages have no cover
bands. The blades are very long. Thus, the clearance between the rotor and the casing is, relatively seen, very small. Clearance loss is relatively low. Moreover, cover
bands cannot be applied to the last stages for reasons of strength. The blade tip
speed is at the highest level there. A cover band would unacceptably contribute to
the centrifugal force. In the example shown, the blade speed at the circumference of
the last stage is about 530 m/s. Rotors in the LP part of steam turbines for a nuclear
power station may be made by discs shrunk on a shaft (1500 rpm in a 50 Hz system). With coal-fired plants, the construction is typically by welding forged parts, as
for HP and IP parts (3000 rpm in a 50 Hz system). Casings are made from welded
steel plates. Blades are mounted with axial slots in disc type rotors. With the drum
type rotor of Fig. 6.27, the blades of the first four stages are mounted with tangential
slots. The slots are axial with the last two stages (curved for the last stage).
Figure 6.28 represents the impulse type VHP module (very high pressure) of
a large steam turbine for a coal-fired power station with ultra-supercritical steam
conditions. The machine has two reheats. Steam conditions are 285 bar and
580 °C/580 °C/580 °C. The machine has a very-high-pressure part (VHP), a highpressure part combined with an intermediate-pressure part, a double-flow intermediate-pressure part and two LP parts, each with two flows. In the VHP part, steam
expands from 285 to 78 bar. The VHP section has an inner and an outer casing, both
with horizontal splits. The figure shows the nozzles incorporated in diaphragms.
At low degree of reaction, pressure drop occurs for the largest part across the stator. It is then advantageous to reduce the diameter of the hub sealing of the stator
vane ring. For very high steam density, lower clearance losses may compensate the
intrinsically somewhat lower internal efficiency, compared to reaction stages. As a
result, the overall efficiency of an impulse type turbine part may even be higher for
high steam density.