JPH06221108A - Improving method of thermal efficiency of low-pressure steam turbine and supporter of exhaust flow guide body - Google Patents

Abstract

PURPOSE: To provide a method and a device for improving efficiency of a low pressure stage of a steam turbine by replacing an existing final moving blade line by a large diameter moving blade line in decomposition work in a casing of the low pressure steam turbine. CONSTITUTION: An existing exhaust current guide is removed, and a mounting ring 56 is mounted on an inner cylinder 38 of a turbine. By using the mounting ring 56, an exhaust current guide 54 for replacement having a large diameter to be compatible with a large diameter moving blade line can be installed. In decomposition work, for reducing latent possibility of stress-corrosion cracking at a boundary part between a disc and a rotor, a rotor having a shrinkage- fit blade disc can be replaced by a monoblock type rotor having an integral blade disc.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to low pressure steam turbines, and more particularly to a method and apparatus for improving the thermal efficiency of low pressure steam turbines by increasing the annular exhaust area of the final blades in the low pressure steam turbines. is there.

[0002]

DESCRIPTION OF THE PRIOR ART For the last few years, electric power plants have focused on increasing their availability and efficiency over the replacement of existing steam turbine power plants.

In a nuclear power plant, generally,
Very large low pressure turbines are used that rotate at relatively low speeds. Many such turbines were originally constructed in the era when there was no ability to manufacture a rotor having an integral blade support disk or wheel as part of the rotor. In this type of early installation, the rotor
It had an assembly structure with a number of disks that were shrink fitted onto the rotor shaft. This type of rotor structure is
It was susceptible to stress corrosion cracking in the shrink-fitted disc holes, especially in the keyways of the disc. The U.S. Nuclear Regulatory Commission has established special disk inspection requirements for electric utilities using turbines with shrink-fitted disk rotors to prevent the possibility of turbine failure due to stress corrosion cracking. Imposing. This disk inspection condition has the potential to add costs associated with turbine maintenance and adversely affect turbine availability.

Recent advances in steel melting and forging techniques have made it practically possible to manufacture turbine rotors from large, unitary forgings without the need for shrink fit discs. Such forgings can be manufactured large enough to allow the disk or turbine blade support to be machined into the forging. This forged rotor is
Generally referred to as a "monoblock" rotor, the disk holes and keyways that were susceptible to stress corrosion cracking have been removed. As such, the U.S. Nuclear Regulatory Commission no longer requires a disk inspection for such turbines or rotors. These attributes of the monoblock rotor increase the availability of the turbine and reduce its maintenance costs, making the monoblock rotor an attractive structure for electric utilities. When replacing shrink-fit rotors with monoblock rotors, electric utilities are given the opportunity to replace blade rows of low pressure turbines with blades that have significantly improved blade reliability and thermal efficiency. One blade row modification that can provide improved thermal efficiency is to increase the annular exhaust area of the final blade row in a low pressure steam turbine. On the other hand, one way to increase the annular exhaust area is to increase the length or diameter of the final row of blades.
However, such modifications typically require increasing the diameter of the outer exhaust flow guide connected to the inner barrel of the steam turbine to accommodate the large diameter blade row. In general, the exhaust flow guide is fixed to the exhaust side end of the inner cylinder by a bolt. Therefore, simply bolting another exhaust flow guide member to the same position on the inner cylinder cannot provide a sufficient diameter for accommodating the final rotor blade row having a large diameter. Therefore, some measure must be taken to increase the diameter of the exhaust flow guide to accommodate the large diameter final blade row.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus that overcomes the above and other drawbacks of the prior art.

A more specific object of the present invention is to provide a method and apparatus for installing a final row of blades in a low pressure steam turbine having a larger diameter than the row of blades installed in the original turbine. To provide.

Yet another object of the present invention is to provide a method and apparatus for enlarging the exhaust annulus opening of the inner casing of a low pressure steam turbine.

The above and other objects are to provide a plurality of circumferentially spaced bores formed along an inner diameter of the low pressure steam turbine so as to be aligned with a plurality of screw holes formed in a coupling end surface of an inner cylinder of the low pressure steam turbine. This is accomplished by a method and apparatus for supporting a large diameter exhaust flow guide on the inner tube of a low pressure steam turbine using a mounting ring having a first opening. The attachment ring is attached to the coupling end surface of the inner cylinder with a bolt. A countersink is formed in the opening through the mounting ring so that the bolt head fits within the mounting ring. Along the outer diameter of the mounting ring,
A plurality of circumferentially spaced second openings are formed, where the outer diameter is greater than the inner diameter. Then, the large-diameter exhaust flow guide was bolted to the mounting ring using the opening along the outer diameter, and the replacement exhaust flow guide was bolted directly to the inner cylinder. Have a larger annular exhaust area than the body. As a result, the large-diameter exhaust flow guide body that accommodates the final row of blades having a larger diameter is provided adjacent to the exhaust-side end of the inner cylinder.

In accordance with another aspect of the invention, the turbine is disassembled and the existing rotor with the shrink-fitted disk and blades attached to it is removed from the turbine inner barrel and the original rotor removed. A method is proposed to improve the performance of existing low pressure steam turbines operating in nuclear power plants by installing a monoblock rotor in the turbine instead of. Replacement vanes are mounted on the monoblock rotor integral disk so as to have at least a final blade row including blades having a larger diameter than the blades removed from the final blade row during disassembly of the low pressure turbine. Next, a mounting ring is attached to the exhaust side end of the inner cylinder of the turbine adjacent to the final blade row, and then the large-diameter exhaust flow guide body is annular with a diameter sufficient to be compatible with the large diameter of the final blade row. It is connected to the mounting ring so as to have an exhaust area.

In order to obtain a clear understanding of the present invention,
A preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a conventional rotor for use with a low pressure steam turbine, such as that used in a nuclear power plant, is shown in partial cross-sectional view. Although only the upper half of the turbine rotor is shown in the figure, it should be understood that the lower half of the turbine rotor has the same structure. The rotor indicated by reference numeral 10 is formed as an integral shaft 10A having a plurality of stepped portions generally indicated by reference numeral 12. The wing support disk or wheel 14 includes different stepped portions 12 of the rotor shaft.
Has been pressed into. When the rotor shaft is arranged in the inner cylinder of the low-pressure steam turbine, the blades 16 are installed around the outer circumference of the rotor disk 14. FIG. 2 shows a stepped portion 1 having a shaft 10A and a rotor disk 14 press-fitted on the shaft.
One of the two is shown. Key groove 18 is shaft 10A
And the boss 20 of the rotor disk. A key 22 is inserted into this keyway to prevent the rotor disk from freely rotating around the rotor axis. This type of rotor structure was susceptible to stress corrosion cracking in the shrink or shrink fit disc holes, particularly at the keyways of the disc. Because of the potential for stress corrosion cracking, the U.S. Nuclear Regulatory Commission has set out to reduce the possibility of stress corrosion cracking leading to brittle fracture.
It imposes special disk inspection conditions on the related electric utilities.

Referring now to FIG. 3, there is shown a low pressure turbine rotor 24 having a monoblock construction which avoids the problems of stress corrosion cracking associated with disks shrink fitted to the rotor shaft. In this monoblock configuration, the disk 14 is integrally formed with the rotor shaft 10A and then machined to allow attachment of the wings 16. Because it is desirable to replace the old rotor shaft 10A with the shrink-fitted disc 14 with a new monoblock rotor, many electric utilities will use a monoblock rotor with an integrated low pressure steam turbine. It has been upgraded. With this replacement, the annular exhaust area of the final rotor blade row is increased, and thus the thermal efficiency of the low-pressure steam turbine can be improved. In Figure 4,
A graph of back pressure versus thermal efficiency in a low pressure steam turbine is shown. In this figure, curve 30 shows a typical characteristic curve for a certain annular exhaust area. Curve 30
The size of the annular exhaust area indicated by is, for example, a dimensional characteristic of a nuclear powered low pressure steam turbine using a shrink-fitted disk array. When the annular exhaust area is increased by 5%, the thermal efficiency vs. back pressure curve becomes the characteristic curve shown by the straight line 32. As is clear from this figure, significant efficiency can be achieved at low exhaust pressures.

As previously mentioned, one method of improving thermal efficiency by increasing the annular exhaust area of the final blade requires the use of a large diameter blade row. FIG. 5 is a diagram showing a typical connection of the exhaust flow guide 34 to the end 36 of the inner cylinder 38 of the low pressure steam turbine. The inner cylinder 38 has a plurality of circumferentially-spaced and threaded openings 40, which correspond to the openings 44 formed on the flange 44 of the exhaust flow guide 34. Aligned with the opening 42. Corresponding bolts 46 penetrate the opening 42 and are screwed into the inner cylinder 38 to fix the exhaust flow guide body to the inner cylinder. The bolt 46 is usually provided with a stop washer 48 to prevent the bolt from loosening. The final row of blades typically shown by a single blade 50 has the outer tip of each blade 50
It is dimensioned to slightly avoid contact with the inner surface of the exhaust flow guide 34. A non-rotating nozzle 52 is attached to the inner cylinder at a location adjacent the end 36. It will be appreciated that the annular exhaust area of the exhaust flow guide 34 must be increased at the point of attachment to the end 36 of the inner tube 38 to accommodate the large diameter final blade row. In connection with this, unfortunately, due to the bolt arrangement passing through the flange 44 of the exhaust flow guide, it is not possible to attach and utilize the large diameter exhaust flow guide directly to the inner cylinder 38. For this reason, it is desirable to provide a method and apparatus for attaching a large diameter exhaust flow guide to an inner cylinder.

Referring to FIG. 6, one method for attaching the larger diameter exhaust flow guide 54 to the inner barrel 38 using the attachment ring 56 is shown. The mounting ring 5
6 includes a plurality of first openings 58 circumferentially spaced from each other so as to be aligned with the screw holes 40 formed in the inner cylinder 38.
Are arranged along the inner diameter of the mounting ring 56, and the mounting ring 56 is countersunk to allow the head of the bolt 62 to be received below the outer surface 64 of the mounting ring. Alternatively, it has a counterbore 60. Further, the attachment ring is formed with a plurality of second openings (connection means) 66 circumferentially spaced along an outer diameter larger than the inner diameter with which the openings 58 are aligned. These openings 66 are preferably threaded to receive a plurality of bolts (connection means) 68. Exhaust flow guide 5
4 also includes a plurality of openings 70 that are circumferentially spaced so as to be aligned with the openings 66 when it is installed in abutting relationship with the mounting ring 56. The mounting ring 56 is provided for fixing the inner ring with the screw hole 40 existing in the inner tube.
To use. By inserting the bolt 62 into the mounting ring 56 through the counterbore, the bolt does not interfere with the positioning of the exhaust flow guide body 54. The mounting ring allows a larger exhaust flow guide 54 to be installed to provide additional space for the larger diameter final blade row in the low pressure steam turbine. Although not shown in FIG. 6, the mounting ring 56 can be formed as two 180 degree fan members to facilitate fixation to the inner barrel 38. The bolt 62 may be a bolt having a head covered with a bag socket made of stainless steel to enhance corrosion fatigue resistance. Advantageously, these bolts have rolling threads and are prestressed to increase fatigue life. Bolts 68 are also made of prestressed or PS stainless steel material and have rolling threads, but are preferably hexagonal headed bolts.

FIG. 7 shows another construction for attaching the attachment ring 72 to the inner cylinder 38. In this configuration, the mounting ring 72 surrounds the outer surface of the inner cylinder at a location adjacent the end 36. A bead weld is formed on the outer surface of the inner cylinder at the boundary with the edge of the attachment ring 72, as indicated by reference numeral 74. The mounting ring 72 also includes a plurality of circumferentially spaced threaded openings (connecting means) 76 for receiving hexagonal headed stainless steel bolts (connecting means) 68.
Is provided. Also in this embodiment, the exhaust flow guide 54 has the attachment ring 72 in the manner described in connection with FIG.
Attached to. This method is an alternative method of attaching a mounting ring to the inner cylinder of a low pressure turbine, but requires on-site welding, and the heat of welding may cause some distortion in the inner cylinder. Therefore, this alternative method of using welding is shown in FIG.
It is thought that it will be structurally inferior to the bolt fixing structure shown in. In addition, additional in-situ grinding may be required to eliminate the distortions that would be expected to result from welding the mounting ring 72 to the inner barrel. It is believed that the flange surface of the mounting ring would have to be ground in-situ, after which the welded mounting ring bolt holes would have to be drilled and threaded. Furthermore, because the latter structure cannot be stress relieved after welding, residual stresses can reduce the fatigue strength of the structure. In contrast, the configuration shown in FIG. 6 allows the bolt holes to be drilled and threaded prior to assembling the mounting ring to the inner barrel.

While the present invention has been described above with reference to presently preferred embodiments, other modifications and variations will occur to those skilled in the art. Therefore, it is added that the present invention is not limited to the particular embodiments described herein.

[Brief description of drawings]

FIG. 1 is a simplified cross-sectional view showing the upper half of a low pressure turbine rotor having a shrink-fitted disc typically used in the prior art.

FIG. 2 is an enlarged view of a rotor shaft with a single shrink fit disc according to the prior art.

FIG. 3 is a cross-sectional view of a monoblock rotor currently used in a low pressure steam turbine.

FIG. 4 is a graph illustrating turbine efficiency as a function of exhaust back pressure for two differently sized annular exhaust areas.

FIG. 5 is a view showing one form in which an exhaust flow guide body is attached to an inner cylinder of a low pressure steam turbine according to the prior art.

FIG. 6 is a partial cross-sectional view of a mounting ring connected to an inner tube of a steam turbine for mounting a large diameter exhaust flow guide according to the present invention.

FIG. 7 shows an alternative method of attaching a mounting ring to the inner cylinder for connecting a large diameter exhaust flow guide.

[Explanation of symbols]

 36 End of Inner Cylinder 38 Inner Cylinder 40 Screw Hole Provided at Joining End Surface of Inner Cylinder 50 Blade 54 Large Diameter Exhaust Flow Guide 56 Mounting Ring 58 First Opening Along Inner Diameter 62 Bolt 66 Second Along Outer Diameter Opening 68 Bolt (connecting means) 70 Opening formed in large-diameter exhaust flow guide 72 Mounting ring 76 Opening (connecting means)

Claims (2)

[Claims] 1. A method for improving the thermal efficiency of a low pressure steam turbine by increasing the annular exhaust area defined by the final row of blades and the exhaust flow guide body, the first exhaust flow guide body being the first exhaust flow guide body of the steam turbine. Removing from the state of attachment to the inner cylinder, removing the final blade row, replacing each blade of the final blade row with another blade having a long blade length, at the exhaust side end of the inner cylinder, the exhaust side Attaching a mounting ring having a plurality of circumferentially spaced connecting means displaced radially outward from the end, at the connecting means having a large inner diameter corresponding to the diameter of the final row of blades. A method of improving the thermal efficiency of a low pressure steam turbine comprising the steps of connecting a radial exhaust flow guide to the mounting ring. 2. A mounting ring having a plurality of first openings circumferentially spaced along an inner diameter for supporting a large diameter exhaust flow guide in an inner cylinder of a low pressure steam turbine, said first ring comprising: 1 opening is substantially parallel to the axis of the mounting ring and is alignable with a corresponding plurality of screw holes formed in the coupling end face of the inner cylinder, the mounting ring being It can be attached to the inner cylinder by a plurality of corresponding bolts that pass through the attachment ring and are screwed into the inner cylinder, and the attachment ring is circumferentially spaced along the outer diameter of the same attachment ring. A plurality of second openings aligned substantially parallel to the axis of the mounting ring, the outer diameter being larger than the inner diameter, and the second opening along the outer diameter being Corresponding plurality formed on the large-diameter exhaust flow guide installed on the inner cylinder Corresponding to the plurality of openings formed in one of the attachment ring and the exhaust flow guide through the other of the attachment ring and the exhaust flow guide. A support device for an exhaust flow guide of a low-pressure steam turbine, which is threaded so as to be screwed with a plurality of bolts.