Improved nozzle ring for radial steam turbine

Abstract

 A structure of one or more improved nozzle vanes of an improved nozzle ring of a steam turbine is disclosed. In an aspect of the present invention, each of the one or more nozzle vanes includes an effective surface of the improved nozzle vane, at least one grove and a bolting surface in the improved nozzle vane. The bolting surface in each of the one or more improved nozzle vane is used for connecting the improved nozzle ring with at least one section of the steam turbine.

Description

[0001] The present invention relates to an improved nozzle ring for radial turbine and more particularly, to an improved nozzle ring with leak proof nozzle vanes for radial flow steam turbines.

[0002] From last few decades there is a continuously increasing demand of power for various residential and industrial applications. Natural sources of power, like wind, water etc., are considered as promising solution for fulfilling the increasing demand of power. A turbine is a well known mechanical device that extracts power from a flowing substance, like water, wind etc., and converts it into a useable form of power. There are various types of turbines like steam turbines, gas turbines, water turbine, wind turbine, and so on. The turbines are further classified based on the turbine mechanical structure, casing type, operating principle and like.

[0003] Steam turbine is an important class of turbines. The steam turbines extract thermal energy from steam and convert it to mechanical energy. The operation of the steam turbine, in principle is, when high temperature and high pressure steam passes through a series of rotor blades of the steam turbine, the rotor blades absorb the energy from the steam and start rotating. The steam turbines are of two types i.e. axial flow turbine and radial flow turbine. In axial flow steam turbines, the direction of steam flow is in parallel to the rotation shaft of the turbine. The axial flow steam turbines can up-scaled easily hence are suitable for large scale power plants. On the other hand, the radial flow steam turbines utilize the steam pressure difference along the radial direction i.e. perpendicular to the rotation shaft. The radial flow steam turbines are suitable for the small scale type applications and are relatively high efficiency turbines.

[0004] The steam flowing through the steam turbines has three components of energy i.e. kinetic energy due to velocity of steam, pressure energy due to steam pressure and thermal energy due to temperature of the steam. The steam turbines convert all three energy components of the steam into mechanical energy. For maintaining a high efficiency of the steam turbines, it is desirable to preserve the three forms of energy of the steam till the steam reaches to the rotor blades. In other words, all kind of losses of the three energy components of the steam should be avoided.

[0005] A major loss of the energy of the steam is occurred due to leakage of the steam from different sections of the steam turbine. The leakage of the steam results in loss of pressure of the steam which in turns negatively affected the efficiency of the steam turbine. Specifically, in a single stage radial flow steam turbine a nozzle ring is bolted to turbine casing. In spite of providing heavy bolting at the nozzle ring and the turbine casing junction, the contact pressure is insufficient to stop the steam from leaking, because the bolt pretension relaxes during operation. Once the steam leaks, it passes through the bolted surface of nozzle vanes and casing flange of the steam turbine. The leaked steam turns wet due to expansion in the nozzle and causes stress-assisted corrosion (SAC) which leads to the pitting of the bolted surfaces of the nozzle vanes and the casing flange of the steam turbine. The pitting zones of the bolted surfaces initiate formation of cracks at the pitted surfaces or edges which with time may lead to severe failures like breaking of nozzle tongues of the steam turbine nozzle ring during field operation.

[0006] To avoid leakage of the steam, an increase in the contact pressure between the nozzle ring and the turbine casing is required. In order to achieve high contact pressure bolting been provided between the nozzle ring and casing flange through every single nozzle vane of the nozzle ring in accordance to the state of the art. But during field operation bolt pretension tends to relax due to creep with time which results in lose bolts. The lose bolts leads to a significant drop in the contact pressure at the junction which subsequently leads to leakage of the steam in the steam turbine.

[0007] From the above description it is clearly evident that for maintaining a high efficiency of a steam turbine it is desirable to avoid loss in the pressure energy of the steam in the steam turbine. In addition to this, leakage of the steam from the nozzles of the nozzle ring bolted with the turbine casing in a radial flow steam turbine leads to the pitting of the nozzle vanes and casing flange which may result in a complete failure of the steam turbine during field operation.

[0008] From the foregoing description it is evident that there is a strong need of an improved nozzle ring for a steam turbine in order to achieve high contact pressure at the bolting plane, with the same magnitude of bolt pretension force.

[0009] It is therefore an object of the present invention to provide an improved nozzle ring for a steam turbine which is capable of achieving high contact pressure at the bolting surface and avoid the leakage of steam from the improved nozzle ring and turbine casing junction of a radial flow steam turbine.

[0010] The object is achieved by providing an improved nozzle ring, for a steam turbine, having one or more improved nozzle vanes according to claim 1.

[0011] In an aspect of the present invention, structure of one or more improved nozzle vanes of an improved nozzle ring of a steam turbine is disclosed. In accordance to the disclosed aspect of the present invention each of the one or more nozzle vanes includes an effective surface of the improved nozzle vane, at least one grove and a bolting surface in the improved nozzle vane. The bolting surface in each of the one or more improved nozzle vane is used for connecting the improved nozzle ring with at least one section of the steam turbine wherein the at least one section of the steam turbine is the casing of the steam turbine.

[0012] Accordingly, the present invention provides a structure of one or more improved nozzle vanes of an improved nozzle ring used in a steam turbine.

[0013] The present invention is further described hereinafter with reference to illustrated embodiments shown in the accompanying drawings, in which:

FIG 1

illustrates a radial flow steam turbine,

FIG 2

illustrates the nozzle ring in accordance with the radial flow steam turbine of FIG 1,

FIG 3

illustrates nozzle vanes in accordance with the nozzle ring of FIG 2, and

FIG 4

illustrates a part of an improved nozzle ring as an embodiment of the present invention.

[0014] Various embodiments are described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident that such embodiments may be practiced without these specific details.

[0015] Referring to FIG 1, a radial flow steam turbine 100 is illustrated. The radial flow steam turbine 100 includes a turbine casing 102 and a nozzle ring 104. The nozzle ring 104 is bolted to the turbine casing 102 through one or more bolts 106. The turbine casing 102 and the nozzle ring 104 has to be bolted in a way that the steam inside the turbine 100 should not leak through one or more nozzle vanes, not shown in FIG 1, of the nozzle ring 104. The objective of the invention is to modify the one or more nozzle vanes of the nozzle ring 104 to avoid leakage of the steam from the turbine casing 102 and the nozzle ring 104 junction. Shape of the one or more nozzle vanes in accordance with the state of the art and one or more modified nozzle vanes of a modified nozzle ring are described in the subsequent figures.

[0016] FIG 2 illustrates a detailed view of the nozzle ring 104 of the radial flow steam turbine 100 shown in FIG 1. The nozzle ring 104 is a ring structure with one or more nozzle vanes 202. Bolting is provided using one or more bolts, not shown in figures, between the nozzle ring 104 and the turbine casing 102 through each nozzle vane of the one or more nozzle vanes 202. It is recommended that the one or more bolts should be capable of exerting enough pressure on the nozzle ring 104 and the turbine casing 102 junction so that the leakage of steam from the steam turbine 100 can be avoided. But during continues operation of the radial flow steam turbine 100 the one or more bolts start losing the grip in the one or more nozzle vanes 202 of the radial turbine 104 with time. A result of lose one or more bolts the steam start leaking from the nozzle ring 104 and the turbine casing 102 junction which leads to different kind of tear and wear, like pitting, of the one or more nozzle vanes 202 and/or the nozzle ring 104 which shorten the life of the steam turbine 100. The pitting of the one or more nozzle vanes 202 and/or the nozzle ring 104 also leads to frequent maintenance requirements which increase the operating cost of the steam turbine 100. In addition to this, the leakage of the steam from the steam turbine 100 reduces the pressure of steam flowing inside the steam turbine 100 hence making the turbine 100 less efficient. FIG 3 illustrates a detailed structure of one or more nozzle vanes 202 of the nozzle ring 104 shown in FIG 2. For connecting the nozzle ring 104 with the turbine casing 102 the one or more bolts, not shown in figures, bolts the nozzle ring 104 with the turbine casing 102 through one or more bolting surfaces 302 of the one or more nozzle vanes 202, shown in FIG 3. The contact pressure created by each bolt of the one or more bolts is inversely proportional to the surface area of contact which is equivalent to the area of vane surface 304 of a nozzle vane of the one or more nozzle vanes 202 as shown in FIG 3. Effective pressure can be calculated from the following equation EQU 1:

[0017] In the above equation, P1 is an effective contact pressure exerted by the bolt, F is pretension force of the bolt, and A1 is the area of the vane surface 304 of a nozzle vane 202. The objective of the present invention is to increase the effective contact pressure, while keeping a constant pretension force of the bolt, to avoid the leakage of the steam from the steam turbine 100 which is achieved by improving the structure of the one or more nozzle vanes 202.

[0018] FIG 4 illustrates an improved nozzle ring 400 having one or more improved nozzle vanes 402 as an embodiment of the present invention. Only a part of the improved nozzle ring 400 is shown in FIG 4 for illustration purpose. A person ordinarily skilled in the art will appreciate that the improved nozzle ring 400 will be a complete circular nozzle ring, equivalent to the nozzle ring 104 shown in FIG 2, with improved nozzle vanes 202. Each one or more improved nozzle vanes 402 of the improved nozzle ring 400 includes an effective vane surface 404, a grove 406 and a bolting surface 408. The improved nozzle ring 400 will connect with the turbine casing 102 of the steam turbine 100, shown in FIG 1, by using the one or more bolts, not shown in figures. The one or more bolts passes through the bolting surfaces 408 of the one or more improved nozzle vanes 402. The bolting surfaces 408 of the one or more improved nozzle vanes 402 are formed between the groves 406 of the one or more improved nozzle vanes 402. Due to the formation of the groves 406 within the one or more improved nozzle vanes 402 the effective surface area of contact i.e. area of the effective vane surface 404 reduces significantly. The reduction in the area of contact i.e. area of the effective vane surface 404 an effective contact pressure exerted by a bolt of the one or more bolts increases as the contact area is inversely proportional to the effective contact pressure exerted by the bolt. The effective pressure for the one or more improved nozzle vanes 402, shown in FIG 4, can be calculated from the following equation EQU 2:

[0019] Where P2 is the effective contact pressure exerted by the bolt for the one or more improved nozzle vanes 402, F is pretension force of the bolt, and A2 is the area of the effective vane surface 404 of the one or more nozzle vanes 402, shown in FIG 4.

[0020] In an exemplary embodiment of the present invention, assuming the area A1 of the vane surface 304 of a nozzle vane 202, shown in FIG 3, is three times to the area A2 of the effective vane surface 404 of the one or more nozzle vanes 402 of FIG 4. The relation between surface area A1 and surface area A2 can be defined from the following equation EQU 3:

[0021] Now putting the value of equation EQU 3 in equation EQU 2, following equation EQU 4 will come:

[0022] If we compare the equations EQU 1 and EQU 4, we get following equation EQU 5:

[0023] From the equation EQU 5 it is clearly evident that the effective contact pressure P2 exerted by the one or more bolts connected through the one or more improved nozzle vanes 402 of FIG 4, is three times of the effective contact pressure P1 exerted by the one or more bolts connected through the one or more nozzle vanes 202, as shown in FIG 3. The exemplary embodiment described above is for demonstration purposes, in other embodiments of the present invention the effective contact pressure P2 exerted by the one or more bolts connected through the one or more improved nozzle vanes can be more than or less than three times of the effective contact pressure P1 exerted by the one or more bolts connected through the one or more conventional nozzle vanes 202 as illustrated in FIG 3.

[0024] It is evident from the foregoing description of the present invention that the invention provides an improved nozzle ring having one or more improved nozzle vanes for a radial flow steam turbine in order to achieve high contact pressure at the bolting plane, with the same magnitude of bolt pretension force.

[0025] Due to an enhanced contact pressure between the improved nozzle ring and the steam turbine casing, the leakage of steam reduced significantly during field operation. Due to reduction in the steam leakage the pitting of the bolted surface of the improved nozzle vanes of the improved nozzle ring is also avoided that increase the life of the steam turbine and also significantly reduces the maintenance cost for the steam turbine during field operation.

[0026] Another advantage of using the improved nozzle ring in the steam turbine, as disclosed in the present invention, is the loss of the steam pressure i.e. pressure energy within the steam turbine reduced as only a negligible amount of or no steam leaks through the improved nozzle vanes of the improved nozzle ring which leads to an enhancement in an overall efficiency of the steam turbine.

[0027] While the present invention has been described in detail with reference to certain embodiments, it should be appreciated that the present invention is not limited to those embodiments. In view of the present disclosure, many modifications and variations would present themselves, to those of skill in the art without departing from the scope of various embodiments of the present invention, as described herein. The scope of the present invention is, therefore, indicated by the following claims rather than by the foregoing description. All changes, modifications, and variations coming within the meaning and range of equivalency of the claims are to be considered within their scope.

Claims

1. An improved nozzle ring (400) of a steam turbine (100) having one or more improved nozzle vanes (402), at least one improved nozzle vane of the one or more improved nozzle vanes (402) includes:

- an effective vane surface (404);

- at least one grove (406); and

- at least one bolting surface (408) wherein at least one bolt utilises the at least one bolting surface (408) to connect the improved nozzle ring (400) with at least one section of the steam turbine (100).

2. The one or more improved nozzle vanes (402) according to claim 1, wherein at least one section is a turbine casing (102) of the steam turbine (100).

Drawing