Rotary kinetic fluid motors or pumps – Including heat insulation or exchange means – Working fluid on at least one side of heat exchange wall
Reexamination Certificate
2000-05-03
2001-11-13
Lopez, F. Daniel (Department: 3745)
Rotary kinetic fluid motors or pumps
Including heat insulation or exchange means
Working fluid on at least one side of heat exchange wall
Reexamination Certificate
active
06315520
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a turbine casing having a multilayer casing wall with a pressure-tight, heat-insulating intermediate layer between an inner layer sealing off a pressure space and a force-transmitting outer layer. The invention also relates to a method of manufacturing such a casing. The term turbine casing refers in particular to the outer casing of a high-pressure steam turbine.
In a turbine working as a prime mover, potential energy of a flowing working medium such as, for example, gas or steam, is converted into mechanical work. To that end, the turbine includes a rotor wheel and a fixed guide wheel as essential elements. Thus, steam serving as a flow medium in a steam turbine is expanded until it condenses to perform work. In that case, the structure of the steam turbine is determined in particular by steam states, i.e. steam pressure and steam temperature.
Especially high steam states are aimed at due to the desire for an efficiency of a steam turbine which is as high as possible. In a high-pressure turbine, an increase in the live-steam pressure, e.g. to 300 bar, and in the live-steam temperature, e.g. to 600° C., requires a material selection corresponding to the temperature effect and a turbine-casing wall thickness corresponding to the stress due to the internal pressure prevailing at high temperature. In that case, it should be taken into account that the admissible stresses markedly decrease with increasing component temperature. A correspondingly larger wall thickness would therefore be necessary to absorb the pressure forces with the turbine casing.
The casing parts that are required for high steam states and are made of temperature-resistant materials having a large wall thickness result in considerable material costs, in view of the high costs for such materials. However, ease of manufacture is also an aspect which proves to be an obstacle to an increase in the wall thickness, in particular the castability of the alloys at the requisite wall thicknesses. Further aspects to be taken into account are the operating behavior of the turbine with regard to the start-up and shutdown times influenced by the heating-up and cooling-down behavior of the casing parts and the handling due to the mass, which increases with the wall thickness. It should also be taken into account that, in the turbine casing, not only does the wall thickness increase with increasing pressure, but the strength of the conventional materials also decreases with increasing temperature.
In order to provide the heat insulation of an outer casing of a high-pressure turbine, it is known from German Published, Non-Prosecuted Patent Application DE 195 35 227 A1 to place an insulating layer or course on the inside of the outer casing, and to provide the inside of the insulating layer with a lining. In that casing wall having a multilayer construction, a castable ceramic or a castable lightweight refractory concrete is provided as an insulating layer. However, a disadvantage of that construction is that the hardened insulating layer tends to fracture as a result of operationally induced thermal stresses. That may lead in an undesirable manner to the formation of cracks in the adjacent layers, in particular in the outer layer.
Furthermore, it is known from Austrian Patent 381 367 B to provide a metallic insulating body, in particular in the form of metal fibers, in the steam space of a steam turbine. Since the insulating body provided in the steam space comes into direct contact with the steam serving as an insulating medium, on one hand the metal parts must be sufficiently large in order not to be entrained by the steam flow, which would lead to the destruction of the turbine. On the other hand, a sufficiently loose bulk fill of the metal parts is required so that the steam can flow at least more or less unimpeded through the insulating body. Sufficient heat insulation and pressure tightness is not achieved with such a metallic insulating body.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a multilayer turbine casing for a high-pressure turbine and an especially suitable method of manufacturing a multi-walled turbine casing for a high-pressure turbine, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which the turbine casing enables high steam states, i.e. a high pressure and a high temperature of a flow medium, to be realized, through the use of an especially suitable, pressure-tight, heat-insulating intermediate layer.
With the foregoing and other objects in view there is provided, in accordance with the invention, a turbine casing, in particular an outer casing of a high-pressure turbine, defining a pressure space, comprising a multilayer or three-layer casing wall having an inner layer sealing off the pressure space, a force-transmitting outer layer, and a pressure-tight, heat-insulating intermediate layer formed of non-metallic bulk material, preferably sand, and disposed between the inner layer and the outer layer.
In this case, the use of sand as a bulk material for the intermediate layer is especially expedient. The intermediate layer, in an especially advantageous manner, then performs the function of a heat-insulating layer or course, over the thickness or radial extent of which a reduction in the temperature (temperature gradient) is effected. The intermediate layer absorbs the pressure forces of the inner layer and transmits them. It is therefore both pressure-resistant and temperature-resistant, but has no sealing function. In this case, it is advantageous for the thermal conductivity to be as low as possible, since the thermal conductivity determines the thickness of the insulating layer and the heat flow. At the same time, when sand is used as an intermediate layer, it is essential that this sand, in contrast to a solid material such as a metallic material, for example, achieves relatively good heat insulation and adapts itself especially effectively to conditions, with regard to the requisite shape.
As compared with a ceramic material, which is rigid in the hardened state, or with a castable lightweight refractory concrete, and with masonry, the risk of an incipient crack in the adjacent layers is avoided when using sand as an insulating layer, since no stress concentrations with the sudden appearance of a fracture in the insulating layer can occur in the intermediate layer.
The inner layer of the casing wall, which faces the flow medium and is directly exposed to the latter, merely performs the function of sealing off the pressure space and separates the medium from the further layers. To this end, a small wall thickness in relation to the entire thickness of the wall is required, since this inner layer is supported on the outer layers and only has to transmit the existing internal pressure to the latter. The inner layer is preferably made of a temperature-resistant and extensible material, since this inner layer has to follow the mechanically and thermally induced expansions of the other layers. High-temperature chromium steel or cast steel, preferably 10% chromium steel having a ferritic/bainitic mixed structure, is therefore expediently used as the material for the inner layer.
The outer layer serves to absorb the pressures which are transmitted by the insulating layer of the intermediate layer and which result from the medium pressure and it bears the forces produced by the internal pressure in the turbine casing. The outer layer therefore applies a force opposing the pressure force of the medium. A ferritic/bainitic mixed structure is likewise expediently used as the material for the outer layer.
However, since the load-bearing outer layer has a temperature which is markedly below the medium temperature, due to the inner heat-insulating layer in the form of the intermediate layer composed of a bulk fill, a cost-effective material (spheroidal-graphite cast iron or cast steel) having a comparatively small or low tem
Feldmuller Andreas
Haje Detlef
Greenberg Laurence A.
Lerner Herbert L.
Lopez F. Daniel
McAleenan James M
Siemens Aktiengesellschaft
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