Method for modernization of steam turbine (options) and modernized steam turbine

The invention is intended for upgrading steam turbines

The first upgrade of a steam turbine with a reactive stage design was made in order to form a second turbine with a substantially active stage design using components common with the first turbine. In order to upgrade a new steam path in the first turbine, the upper outer and inner casings and the rotor of the first turbine are removed, leaving the lower outer casing. The lower supporting section is mounted on the lower outer casing. The lower inner casing, forming part of the new steam path, is mounted in the lower supporting ring. A rotor is mounted, forming part of the new steam path. The upper inner casing is bolted to the lower inner casing, surrounding the rotor, and the upper supporting section is bolted to the lower supporting section. Finally, the upper outer casing is bolted to the lower outer casing. Therefore, a new steam path of reduced diameter is retrofitted inside the original turbine using the outer casing of the original turbine. Such methods and device will increase the efficiency of the retrofitted turbine. 3 n. and 6 c.p. f-ly, 10 ill.

Background of the invention

The present invention relates to a device and methods for retrofitting steam turbines and to retrofitted steam turbines. In particular, the present invention relates to methods for replacing a large diameter steam path, such as a primarily reactive stage design, with a smaller diameter steam path, such as a primarily active stage design, while certain portions of the components, including the outer casing of the original turbine, remain in the retrofitted turbine.

Two different steam path designs predominate in steam turbine technology. In reaction stage turbine designs, a portion, for example about 50% of the stage pressure drop occurs on the rotating blades, increasing the steam velocity and transferring energy to the blades by reaction and momentum transfer. In active stage turbine designs, theoretically all of the stage pressure drop is converted into nozzle velocity. No pressure drop occurs on the rotating blades, which change the direction of the steam and absorb energy by momentum transfer.

The wheel and diaphragm mechanical designs are typical of active stage steam path designs, while the drum type design characterizes reaction stage steam path designs. It must be appreciated, however, that the active stage design can be operated with either a wheel and diaphragm design or a drum type design. Significant improvements in the design and efficiency of steam turbines have resulted in an increase in the root reaction of the active stage design without a significant increase in the stage reaction (see, for example, U.S. Patents Nos. 3,592,557 and 6,305,901). Thus, the increased efficiency of the steam turbine occurs with an increased reaction of the active stage design, but the magnitude of the reaction is significantly less than in the reactive stage design. There are significant differences in the dimensions and design of the steam path with this improved active stage design compared to the steam path with the reactive stage design. For example, the improved active stage design combined with a smaller root diameter and blade length results in the corresponding dimensions used in the reactive stage design being an order of magnitude, approximately 50%, smaller. Thus, the improved active stage design of the steam path has an inner casing of a much smaller diameter than the corresponding inner casing diameter of the reactive stage design of the steam path. The active steam path stage design typically has a smaller outer casing diameter. Despite these differences in size and design, it is necessary to upgrade steam turbines having existing types of reactive steam path stage with an improved active steam path stage design to create a modernized turbine with higher efficiency.

Brief Description of the Invention

According to a preferred embodiment of the present invention, methods are provided for upgrading a larger diameter steam path, such as one that is typical of a reactive stage steam path design, to a smaller diameter steam path, such as one that is characterized by an improved and more efficient active stage steam path design. It should be appreciated that while the smaller diameter rotor and inner casing that are characteristic of an improved active stage steam path design replace the corresponding inner portions of the reactive stage steam path design, there remains a need to utilize the outer casing of the existing turbine with the steam path of the improved active stage design, as well as other components. That is, simply replacing the steam path of the reactive stage design with the steam path of the improved active stage design would require an awkward inner casing design with long, thick support lugs in order to accommodate the larger outer casing of the existing turbine. Thick projections will be difficult to cast and may result in excessive thermal stresses during heating and cooling of the upgraded steam path. Accordingly, the present invention provides a coupling device between the replaced steam path with an improved active stage design and the outer casing of the turbine, which was previously the casing of the steam path with a reactive stage design. The coupling device also makes it possible to maintain axial, vertical and radial placement, while maintaining the thickness of the inner casing to a minimum in order to avoid thermal stresses during transient processes.

In order to upgrade the steam path with a reactive stage design by means of a steam path with an active stage design in accordance with a preferred embodiment of the present invention, the inner casing and rotor of the reactive stage design are removed and replaced with the inner casing and rotor of the improved active stage design. Due to the gap between the outer casing of the original turbine and the inner casing of the replaced steam path with the active stage design, a coupling device or bridge part is provided between the new inner casing and the old outer casing. In particular, halves of the bearing section or rings are inserted between the new inner casing and the original outer casing and make it possible to reduce the diameter of the steam path to fit into the outer casing of a turbine that previously had a larger diameter steam path.

In a preferred embodiment according to the present invention, there is provided a method for upgrading a first steam turbine having an outer casing including a pair of upper and lower halves of the outer casing and a first steam path of a first diameter in a portion defined by a first inner casing and a first rotor in order to create a modernized second steam turbine, comprising the steps of: (a) removing the upper half of the outer casing, the first inner casing and the first rotor from the lower half of the outer casing of the first turbine, (b) inserting a lower bearing section into the lower half of the outer casing, (c) providing a second rotor and a second inner casing in a portion defining a second steam path of a second diameter smaller than the first diameter of the first steam path, (d) placing the lower half of the inner casing of the second inner casing inside the lower bearing section, (e) placing the second rotor in the lower half of the inner casing of the second inner casing; (f) placing the upper half of the inner casing of the second inner casing around the second rotor; (g) placing the upper bearing section around the upper half of the inner casing of the second inner casing; and (h) attaching the upper half of the outer casing to the lower half of the outer casing of the first turbine, thereby creating a modernized second steam turbine having a reduced diameter second steam path. Wherein the first turbine comprises a dual steam flow path having a central inlet for steam passing axially in opposite directions through removable first and second separate, axially separated, turbine sections of the first turbine. Moreover, in the case where the upper and lower bearing sections comprise halves of bearing sections, respectively, the method according to the invention includes attaching the upper half of the bearing section and the lower half of the bearing section to each other.

In a further preferred design according to the present invention, there is provided a method for upgrading a first steam turbine having a first steam path with a substantially reactive stage structure in order to create a second turbine having a second steam path with a substantially active stage structure, comprising the steps of: (a) removing a first inner casing and a first rotor forming part of the first steam path with a substantially reactive stage of the first turbine, from the outer casing of the first turbine structure; and (b) arranging in the outer casing of the first turbine a steam path having a structure of the active stage of the second turbine, including a second inner casing and a second rotor, wherein said supporting section is arranged between the second inner casing and the outer casing of the first turbine in order to bridge the gap therebetween.

In a further preferred embodiment according to the present invention, a modernized turbine is provided, comprising an inner casing surrounding the rotor and defining a steam path, an outer casing surrounding the inner casing and the rotor; and a structural part-bridge between the outer and inner casings in order to bridge the gap between the casings. Moreover, the turbine comprises a path for a double steam flow, having a central inlet for steam and a pair of turbine sections separated by a gap along the axis on opposite sides of the said inlet, wherein half of the upper bearing section includes a pair of upper bearing halves arranged with a gap along the axis, mainly radially fixed to the corresponding sections of the turbine, and the lower half of the lower bearing section includes a pair of lower bearing halves arranged with gaps along the axis, mainly radially fixed to the corresponding sections of the turbine.