Steam turbine

Details Steam turbine Steam turbine

1999-07-14

2002-02-12

Verdier, Christopher (Department: 3745)

Rotary kinetic fluid motors or pumps

Axially opposed working fluid paths to or from runner

Pump impeller means

C415S093000, C415S103000, C415S107000, C415S108000

Reexamination Certificate

active

06345952

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a steam turbine having a turbine shaft directed along a turbine axis and a plurality of turbine stages along the turbine shaft, each turbine stage including a guide-blade structure and a moving-blade configuration axially downstream of the guide-blade structure.
Known steam turbines are classified as action turbines (also called “constant-pressure” turbines) and reaction turbines (also called “excess-pressure” turbines). They have a turbine shaft with moving blades disposed on it and have an inner casing with guide blades disposed between axially spaced moving blades.
In the case of a constant-pressure turbine, the entire energy gradient is converted essentially into kinetic flow energy in the ducts that are narrowed by the guide blades. During the process, the velocity rises and the pressure falls. In the moving blades, the pressure and relative velocity remain essentially constant, being achieved through ducts having a uniform clear width. Because the direction of the relative velocity changes, action forces occur that drive the moving blades and, thus, cause rotation of the turbine shaft. The magnitude of the absolute velocity decreases considerably when the flow passes around the moving blades, resulting in a flow that transfers a large part of its kinetic energy to the moving blades and, therefore, to the turbine shaft.
In the case of an excess-pressure turbine, only part of the energy gradient is converted into kinetic energy when the flow passes through the guide blades. The rest of the energy gradient brings about an increase in relative velocity within the moving-blade ducts formed between the moving blades. Where the blade forces are almost exclusively action forces in the constant-pressure turbine, in an excess-pressure turbine, a greater or lesser fraction resulting from the change in the velocity magnitude is added. The term “excess-pressure” turbine is derived from the pressure difference between the downstream and the upstream side of the moving blade. In an excess-pressure turbine, therefore, a change in the velocity magnitude takes place when the pressure varies.
In a thermal turbo-machine, the percentage apportionment of the isentropic enthalpy gradient in the moving blades to the total isentropic enthalpy gradient by a stage having a guide-blade ring and moving-blade ring is designated as the isentropic reaction degree r. A stage in which the reaction degree r is equal to zero and the greatest enthalpy gradient occurs is designated as a pure constant-pressure stage. In the case of a classic excess-pressure stage, the reaction degree r is equal to 0.5, so that the enthalpy gradient in the guide blades is exactly the same as in the moving blades. For example, a reaction degree of r=0.75 is designated as a strong reaction. In steam-turbine construction practice, the classic excess-pressure stage and the constant-pressure stage are predominantly employed. However, as a rule, the latter has a reaction degree r that is somewhat different from zero.
Furthermore, the terms “chamber turbine” and “drum turbine” are also used. Conventionally, a constant-pressure turbine employs a chamber configuration and an excess-pressure turbine employs a drum configuration. A chamber turbine has a casing that is divided into a plurality of chambers through intermediate floors disposed at an axial distance from one another. A disc-shaped rotor, on the outer periphery of which the moving blades are mounted, runs in each of these chambers, while the guide blades are inserted into the intermediate floors. One advantage of the chamber configuration is that the intermediate floors can be sealed off at their inner edge relative to the turbine shaft in a highly effective manner through labyrinth gaskets. Because the labyrinth gasket diameter is small, the gap cross-sections and, therefore, the gap leakage streams both become small. In known turbines, the configuration is used only in the case of low reaction degrees, that is to say a high stage gradient and, therefore, a small number of stages. The pressure difference on the two sides of a rotor disc is small in the case of a low reaction degree and, in the borderline case, is even zero. An axial thrust exerted on the rotor remains low and can be absorbed by an axial bearing.
In a drum turbine, the moving blades are disposed directly on the periphery of a drum-shaped turbine shaft. The guide blades are inserted either directly into the casing of the steam turbine or into a special guide-blade carrier. The moving blades and guide blades may also be provided with covering strips, to which labyrinth gaskets are attached, so that a sealing gap between the guide and moving blades and the turbine shaft and inner casing, respectively, is sealed off. Because these sealing gaps are located on large radii, at least in the case of the moving blades, the gap leakage streams are at all events considerably greater than in the case of chamber turbines. Due to the higher reaction degree, for example r=0.5, favorable flow paths in the blade ducts and, therefore, high efficiencies are achieved. The axial overall length and the outlay for an individual stage are less than in a chamber turbine, but a larger number of stages is required because the reaction stages process a lower gradient. The axial thrust occurring in the blading is considerable. One possibility for counteracting the axial thrust is to provide a compensating piston, to the front side of which the pressure of the outlet port is applied through a connecting conduit.
A steam turbine of the drum configuration is described in German Published, Prosecuted Patent Application 20 54 465, corresponding to U.S. Pat. No. 3,754,833. A turbine shaft carrying the moving blades and an inner casing surrounding the turbine shaft are disposed in a pot-shaped outer casing. The inner casing carries the guide blades. The inner casing is connected to the outer casing via corresponding bearing and centering points for the absorption of an axial thrust.
German Patent No. 312856 describes an excess-pressure steam turbine with a high reaction degree, a plurality of stage groups being disposed in a casing. Different reaction degrees are achieved in the various turbine stages, the start of the group having a reaction degree well above 0.5 and the end of the group having a degree well below 0.5. Stages located at an axial distance from one another have a different reaction degree in each case. A plurality of turbine stages is combined into partial groups, a plurality of partial groups forming an excess-pressure blade group. In a first excess-pressure blade group, the reaction degree in each partial group increases towards the steam outlet, but the average reaction degree of the partial groups decreases towards the steam outlet. In the second excess-pressure blade group assigned to the steam outlet, the reaction degree in each partial group decreases towards the steam outlet. The average reaction degree has a local maximum.
An excess-pressure steam turbine having the drum configuration is specified in German Patent No. 880307. The steam turbine is configured in such a way that, with the exception of the last stage, the reaction degree of the preceding stages increases continuously towards the evaporation region and is well above 0.5. Only in the last stage does the reaction degree fall to a value below 0.5.
A configuration of partial turbines connected fluidically to one another is described in U.S. Pat. No. 1,622,805 to Pape. This patent sought to achieve a higher degree of freedom in the configuration of steam turbines. The embodiments illustrated therein show a high-pressure steam turbine of the chamber configuration in the region of the highest steam pressure. In the same casing, there follows, at a lower steam pressure, a partial-turbine region that is of the drum configuration and has a reaction stage. A following low-pressure partial is of the double-flow configuration.
SUMMARY OF THE INVENTION
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