This invention relates to an improved multiple-shell turbine casing for high pressures and temperatures, consisting of a multi-stage inner casing in the form of a guide blade carrier and split in an axial plane, and an outer casing also split in an axial plane.
It is required in general of casings for steam turbines, depending on the turbine construction, that they should withstand high pressures and temperatures with only slight deformation and with a uniform as possible expansion of the casing and rotor so that favorable small blade and seal clearances can be maintained under all operating conditions.
With high pressures and temperatures, the wall thickness of single-shell turbine casings is too great to meet these requirements satisfactorily because the thermal stresses cannot be accommodated, or only with great difficulty. Also, the thick walls constitute a further possible source of faults because the risk of defects in the casting is greatly increased. The casings of steam turbines for high pressures and temperatures are therefore built in the form of a multiple-shell structure. The most common form is a double-shell construction consisting of an inner casing which forms the stationary guide blade carrier, and an outer casing which is at a reduced stage pressure or exhaust-steam pressure and at a lower temperature.
As a result of the transition to steam turbines of even greater unit capacity, due in particular to the rapid development of nuclear power stations, the multiple-shell construction has become unable to satisfy the requirements referred to above. The greater volumes of steam to be processed by each turbine unit demanded increased casing dimensions, while the greater number of stages required per unit resulted in higher stresses which in turn led to a further increase in wall thickness.
This, however, gives rise to individual casing shells with very thick walls which present serious casting problems and hinder the course of technical progress.
Attempts have been made to split the casing shells and then weld them together after casting, but this method does not provide a satisfactory solution. Welding did not reduce the time needed to manufacture the whole turbine casing, but rather increased it, although the risk of a casting reject was slightly lessened. However, the danger of cavities and gas bubbles when using casting steel alloys, for example, 10 CrMo910 to DIN 17006, cannot be eliminated. Furthermore, when alloyed casting steels are used, welding large wall thicknesses of these materials presents serious technical difficulties, and in many cases is even impossible because the weldability of the steel can no longer be guaranteed.
The principal object of the present invention is to avoid all the disadvantages mentioned and to make possible turbine casings which satisfy the stated requirements for high pressures and high temperatures, and in so doing keep non-uniform deformation forces of the casing away from the guide blade carrier. In accordance with the invention, this object is achieved in that an axially divided inner casing which constitutes the guide blade carrier is supported by axially spaced load-bearing rings anchored to a foundation and which also extend through and support an axially divided sheet metal outer casing.
The invention is further distinguished by the fact that the outer casing is provided with at least one thermally variable lead-through for the steam inlet pipe to the inner casing and the steam inlet pipe is rigidly connected to the inner casing.
A further feature of the invention is distinguished by the fact that the outer casing is fitted with brace supports which transmit forces between the endplates of the casing and the load-bearing rings.
The advantages of the construction according to the invention for the high-pressure inner casing, and in many instances also the high-temperature intermediate-pressure casing, of a steam turbine reside in the fact that manufacture is simplified and that any occurring deformations can more easily be accommodated. Also, the wall thickness of the casing parts is reduced, resulting in an appreciable saving in raw material.
More particularly, manufacture is simplified by using sheet-metal i.e., rolled metallic sheet for the outer casing as distinguished from castings. This is unaffected by foundry capacity and stresses can more easily be allowed for. Furthermore, by utilizing sheet metal, no attention need be paid to exact matching of the wall thicknesses, for example, between the cylinder endplate and mantle of the outer casing, or at least less attention than with castings, because there are no dangerous casting stresses.
A preferred embodiment of a turbine casing according to the invention is shown schematically in the accompanying drawings wherein:
FIG. 1 is a vertical central longitudinal section through the turbine structure;
FIG. 2 is a section in the axial dividing plane A--A of FIG. 1, and
FIG. 3 is a cross-section taken at line B--B of FIG. 1.
In FIG. 1 a stationary, guide blade carrier, subsequently denoted as an inner casing 11, is shown somewhat schematically as enclosing a bladed rotor 2 mounted on shaft 26. Inner casing 11 rests within and is supported by axially spaced load-bearing rings 6 by means of projections 12 on the casing shown in FIG. 2, such that forces resulting from the dead weight of inner casing 11, reactions of the piping and the torque of rotation are transmitted via keyways 13 and the fixed-point supports 14 visible in FIG. 2 to the load-bearing rings 6.
The outer casing 3 of the double shell turbine is fabricated from sheet metal parts attached to the load-bearing rings 6 and consisting of welded together endplates 8, bleed-chamber mantle 9, exhaust-steam mantles 10 and a rectangular frame 7 which extends round the axial dividing plane as shown in FIG. 2.
Packing glands 22 for shaft 26 are provided at the pass-through holes provided in endplates 8 and rectangular frame 7 and shaft 26 is supported by free-standing bearing pedestals, which are not shown. Accurate longitudinal guidance of the outer casing is provided by guides 17 in endplates 8. These correspond with guideways in the bearing pedestals.
Stiffening pieces 27 are fixed inside outer casing 3 to endplates 8. These act as deflector plates for the exhaust steam and, together with brace supports 21 also fixed to endplates 8, serve to counteract deformation of the endplate, particularly in the vicinity of the shaft-aperture, i.e., packing glands 22. The outer ends of brace supports 21 are secured to the load-bearing rings 6 so that axial forces exerted by the steam are transmitted from outer casing 3 to the load-bearing rings 6 located in axially spaced radial planes. Brace supports 21 are also arranged in such manner as to produce a centering effect. It is therefore advantageous to make brace supports 21 adjustable in length, as indicated by coupling sleeves 29.
The bleed-chamber mantle 9 fitted between the two axially spaced load-bearing rings 6 is provided with a thermally variable lead-through 20 for steam inlet pipe 1, consisting essentially of a bellows 30 located between the outer casing 3 and steam inlet 1 which is rigidly connected to the inner casing 11. In the drawing, the lead-through 20 is shown only schematically and other specific arrangements different from the one depicted are possible.
Intermediate webs 19 are connected to load-bearing rings 6 by way of flexible seals 18, not further depicted, such that bleed steam can flow through slot 28 into bleed chamber 4 and leave through bleed-steam stub pipe 25 located in mantle 9. The bleed steam then flows, for example, to a preheater.
The greater part of the steam entering inner casing 11 through steam inlet 1 expands, imparting energy to rotor 2, and then flows into exhaust-steam chamber 5, from where it passes in the case of a nuclear power station, for example, through exhaust stub pipe 24 to a water separator, and/or an intermediate heater. The arrows denote the direction of steam flow within the casing during operation.
The rectangular frame 7 provided on the outer, sheet metal casing 3 in the axial dividing plane A--A is rigidly connected to the load-bearing rings 6, and is interrupted on the axis of rotation by the packing glands 22, which are also set rigidly in frame 7 and endplate 8. Adjacent to load-bearing rings 6 is a fixed-point support 14 which transmits the axial forces of inner casing 11 to load-bearing rings 6. The same purpose is served by projections 12, which form the connection between inner casing 11 and load-bearing ring 6, and also transmit the peripheral forces resulting from the torque of rotation and piping reaction forces.
Feet 15, let into or set into foundation 16, engage the outer circumference of load-bearing rings 6. The main purpose of these feet is to transfer the loads of inner casing 11 and outer casing 3 to foundation 16.
FIG. 3 shows a cross-section at line B--B of FIG. 1. There it will be seen that load-bearing rings 6 are provided with a fixed part 23 which engages a slot 31 in the inner casing 11 and thus, together with keyways 13 between the inner and outer casings, ensures that a central position is maintained.
The dual casing structure in accordance with the invention is thus seen to be comprised of an outer casing constructed as two all-welded longitudinal halves which are joined together along an axially dividing horizontal plane, each half consisting of a rectangular frame 7, 7', axially spaced semi-circular load bearing ring halves 6, 6', a semi-cylindrical bleed-chamber mantle 9, 9', a pair of semi-cylindrical exhaust steam mantles 10, 10', and a pair of semi-circular end plates 8, 8'. The inner casing which constitutes the guide blade carrier surrounding the turbine rotor is likewise structured as two longitudinal halves 11, 11' joined together along the same axially dividing horizontal plane is supported by the load bearing rings, and the load-bearing rings are seated upon the foundation and thus carry the entire load of the dual casing structure.
It follows from the preceding description that with a dual shell turbine casing built in accordance with the invention, the load-bearing rings 6 together with frame 7 and endplates 8 of the outer sheet metal casing, and brace supports 21 form the supporting framework of the outer casing 3, while the sheet metal parts that form the mantles 9 and 10 are subjected only to the forces resulting from the internal pressure, i.e., have the function of an outer enclosure.
In this way it is possible to support or arrange the inner casing 11 so that it can expand freely, and expansion of the inner casing is not opposed by forces originating from the outer casing 3.