A TURBINE STAGE

The present invention relates generally to turbo machinery, including both turbines and compressors, and more particularly, to stator shrouds and blades therein.

The invention can be applicable to any type of turbines with different working medium such as gas, steam, water, etc., the turbines where there is a stator and a rotor with blades .

As it is known, blades in turbo machinery, such as turbines and compressors, are rotating airfoil-shaped components designed to convert energy from a working medium such as gas, steam, or water, etc. into mechanical work via the rotation of a rotor. There is generally a minimum physical radial clearance requirement between the tip of the blade and a stator for safety and other concerns. This radial clearance, however, also allows the escape of the some of the working medium without performing useful work. Performance of a turbine and a compressor thus can be enhanced by decreasing the radial clearance and sealing the outer edge of the blade to prevent the working medium from escaping into the gap.

In an effort to maintain a high degree of efficiency, manufacturers of turbine machineries have strived to maintain the closest possible radial clearance between the blade tip and the surrounding stator structure, which comprises a stator shroud, since any working medium which may pass there between represents a loss of energy to the system.

If a system were to operate only under steady-state conditions, it would be a simple matter to establish the desired close radial clearance relationship between the rotor blade tip and the stator shroud to obtain the greatest possible efficiency without allowing frictional interference between the elements. However, in reality, all turbine engines must initially be brought from a standstill condition up to the steady-state speed, and then eventually decelerate to the standstill condition. This transitional operation is not compatible with the ideal low radial clearance condition just described since the variation in rotor speed also causes a variation in the size. Further, as the turbine engine is brought up to speed from a standstill position, the temperature of the working medium passing there through is increased proportionately, thereby exposing both the blades and the stator shroud to variable temperature conditions. These conditions cause thermal growth of the two structures, and if the two structures have different thermal coefficients of expansion, which is generally true, then there is also the occurrence of relative thermal expansion between the elements. Characteristically, a rotor with blades on it is necessarily a large mass element which allows it to rotate at very high speeds, thereby inherently yielding a very slow thermal response (high thermal inertia) . On the other hand, the stator shroud is a stationary element and preferably has a high thermal response (low thermal inertia) to allow the thermal deformation of the stator shroud during periods of acceleration to accommodate the deformation of the rotor during those periods.

In many turbine engine applications, there is a requirement to operate at variable steady-state speeds, and to transit between those speeds as desired in the regular course of operation. As mentioned hereinbefore, a primary concern is to maintain the minimum radial clearance between the stator and the rotor of the engine while preventing any mechanical interaction. Otherwise blades, in particularly blade tips, and respective stator shroud suffer and can be damaged: the blade tip cuts into the stator shroud surface, the friction and rubbing between the said components appear. The said friction and rubbing are very considerable and leads to the loss of materials either stator shroud or blade tip, in some cases - both of them. The result is the components geometry distortion. The main consequence of this blade tip and stator shroud hard interaction is the radial clearance increase for all the operation modes and the turbine efficiency output decrease. It occurs since the stator shroud fixing to the stator is rigid. FIG 1 shows portion of a conventional turbine stage (prior art) that may be used in the turbine or otherwise. The turbine stage 1, having the axis 2, comprises a rotor disc 3, a blade 4 with a blade tip 5, a stator 6 and a stator shroud 7 with a stator shroud outer surface 8. The blade 4 including the blade tip 5 and the stator shroud outer surface 8 are affected by the working medium 9.

Typically, the stator shroud 7 is formed by a plurality of arcuate stator shroud segments combined into an annular assembly, and attached to an inner peripheral wall of the stator 6 by intermediate components 10. The stator shroud 7 is rigidly fixed to the stator 6.

Surrounding the blades 4 is an annular stator shroud 7 fixedly joined to a surrounding stator 6 casing. The stator shroud 7 is suspended closely atop the blade tips 5 for providing a small gap or radial clearance 11 there between. The radial clearance 11 should be as small as possible to provide an effective medium seal thereat during operation for minimizing the amount of working medium 9 leakage there through for maximizing efficiency of operation of the engine.

However, due to differential thermal expansion and contraction of the blades 4 and surrounding stator shroud 7, the blade tips 5 occasionally rub against the stator shroud outer surface 8 causing abrasion wear.

Such mechanical contacts of the blade tip 5 and the stator shroud outer surface 8 results in the loss of the material either the stator shroud 7 or the blade tip 5 of the blade 4, in some cases both of them and their geometry failure. Therefore the efficiency output of the turbo machinery in whole decreases. On the other hand the radial clearance 11 should be kept as small as possible within all operational modes of the turbo machinery.

Efforts have been made to actively control the spacing between rotating engine blade tips and static stator shroud segments so as to try to maintain the target minimal spacing while avoiding blade -stator shroud contact. One such active control is to utilize heat to move the stator shroud (US. No. 4, 928, 240) .

Other know solution is a flexible stator shroud in a turbine engine which can be deflected toward blade tips by means of pressurized gas introduced into a chamber adjacent to the stator shroud. Solutions to the problem which involve selectively moving the blades or blade tips toward and away from the stator shroud are also disclosed in the prior art. Examples of such solutions are found in U.S. Pat. No. 5,203,673 granted Apr. 20, 1993 to D. H. Evans; and in U.S. Pat. No. 5,263,816 granted Nov. 23, 1993. These patents disclose mechanical and magnetic active controls respectively. A problem with prior art active clearance systems is their response time and complexity.

Other prior art solutions to maintaining radial clearance include the use of abradable stator shroud systems. A problem with an abradable stator shroud system is that it is not adjustable once the stator shroud is abraded. Once set, the radial clearance depends solely on the thermal response of the rotor and the casing.

Accordingly, it is desired to provide mobility of a stator shroud in a turbine stage for keeping the radial clearance between the blade tips and respective stator shroud as small as possible, but on the other hand for avoiding damage of the blade tips and the respective stator shroud.

The object is solved by a system of providing mobility of a stator shroud in a turbine stage as defined in claim 1.

Consequently, the present invention provides a system of providing mobility of a stator shroud in a turbine stage, wherein the turbine stage comprises a plurality of blades that are connected to a rotor disc, a stator, and at least one stator shroud.

The at least one stator shroud comprises a plurality of stator shroud circumference segments in the circumference direction. Each stator shroud circumference segment is connected to the stator in such way that there is at least one internal cavity between the stator and each stator shroud circumference segment. Each stator shroud circumference segment is adapted to move in radial direction. The system comprises at least one group of elastic components wherein each elastic component is arranged in the at least one internal cavity between the stator and the respective stator shroud circumference segment. Each elastic component is adapted

to allow moving radially outwards the respective stator shroud circumference segment in case at least one blade gets in contact with the respective stator shroud circumference part , and

to move the respective stator shroud circumference part radially inwards in case the blade is not in contact with the respective stator shroud circumference segment.

The present invention is based on the insight that the fixation of the stator shroud to the stator is not rigid, and the stator shroud has elastic components implemented in the cavity between the stator and the stator shroud. Therefore the stator shroud is mobile.

So as soon as the blade tip gets contact with the respective stator shroud, the last one moves radially outwards. It means that there is a contact between the stator shroud and the blade tip, but such contact is without loading and pressure from both parts - either the stator shroud or the blade. Therefore there is no loss of materials from both parts and no geometry failure of the both parts occurs.

On the other hand as soon as the tip blade is not in contact with the respective stator shroud, the elastic element pushes the stator shroud radially inwards till the contact, but still no loading and pressure for both parts between the stator shroud and the blade tip appears.

So during all operational modes there is contact between the stator shroud and the blade tip, but without any losses of materials of the blade and the stator shroud, and without any damages to these parts. Therefore the minimal possible radial clearance between the stator shroud and the blade is provided, and therefore there is no leak of working medium. Thus, the present invention is proposed to provide a new system of providing mobility of a stator shroud in a turbine stage .

Further embodiments of the present invention are subject of the further sub-claims and of the following description, referring to the drawings .

In a possible embodiment of the system the elastic components of the group are arranged in the at least one internal cavity in such way that the elastic components are evenly distributed along the circumference direction of the respective stator shroud circumference segment.

This feature allows providing uniform radial movement of the respective stator shroud circumference segment along the circumference direction of the stator shroud.

In other possible embodiment of the system there is only one elastic component arranged in the at least one internal cavity between the stator and the respective stator shroud circumference segment. This can be desirable for some turbine stages with small stator shroud circumference segments.

In other possible embodiment of the system the stator and each stator shroud circumference segment have special arrangements that limit the movement of the respective stator shroud circumference segment inwards and outwards.

This feature allows controlling the movement of the stator shroud circumference segments and also such special arrangements can be required within assembling the turbine stage .

In enhanced embodiment of the system the stator and each stator shroud circumference segment have seals arranged in such way that there is no leakage of medium into or out of the at least one internal cavity between the stator and the respective stator shroud circumference segment.

It prevents leaking working medium into the internal cavity and leaking cooling agents, if any, out the internal cavity. In enhanced embodiment of the system at least one elastic component of the group of the elastic components is a spring or bellows.

For a more complete understanding of the present invention and advantages thereof, reference is now made to the following description taken in accompanying drawings. The invention is explained in more detail below using exemplary embodiments which are specified in the schematic figures of the drawings, in which:

Fig. 1 schematically illustrates portion of a conventional turbine stage (prior art) ;

Fig. 2 schematically illustrates a system of providing mobility of a stator shroud in a turbine stage in accordance with the present invention;

Fig. 3 schematically illustrates a system of providing mobility of a stator shroud in a turbine stage in accordance with the present invention (III-III view) ;

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 practice without these specific details.

FIG 2 and FIG 3 (III-III view) illustrate a system 12 of providing mobility of a stator shroud 7 in a turbine stage 1.

The turbine stage 1 comprises a plurality of blades 4 that are connected to a rotor disc 3, a stator 6, and at least one stator shroud 7. The at least one stator shroud 7 comprises a plurality of stator shroud circumference segments 13 in the circumference direction.

Each stator shroud circumference segment 13 is connected to the stator 6 in such way that there is at least one internal cavity 14 between the stator 6 and each stator shroud circumference segment 13. There can be more internal cavities 14 formed between the stator 7 and particular stator shroud circumference segment 13. Each stator shroud circumference segment 13 is adapted to move in radial direction.

The rotor disc 3 rotates around the axis 2 of rotation. Moving in radial direction refers to the moving towards and outwards of the axis 2 along a radius that is a line from the center of the turbine stage 1 to the outer surface perpendicular to the axis 2.

The system 12 comprises at least one group of elastic components 15 wherein each elastic component 15 is arranged in the at least one internal cavity 14 between the stator 6 and the respective stator shroud circumference segment 13.

Each elastic component 15 is adapted

to allow moving radially outwards the respective stator shroud circumference segment 13 in case at least one blade 4 gets in contact with the respective stator shroud circumference segment 13, and

to move the respective stator shroud circumference segment 13 radially inwards in case the blade 4 is not in contact with the respective stator shroud circumference segment 13.

The elastic component 15 can be a spring, a bellow or sylphon, or any other component that has compliance and elasticity in the radial direction.

It is preferable to distribute elastic components 15 evenly along the circumference direction of the respective stator shroud circumference segment 13 to provide even movement of the stator shroud 7 inwards and outwards.

The less stator shroud circumference segments 13 that are used, and therefore the bigger each stator shroud circumference segment 13 is in the circumference direction, the more elastic components 15 should be inside in the internal cavity 14 between the stator 6 and the stator shroud circumference segment 13 to provide even movement of the stator shroud circumference segment 13 along circumference direction. And vise versa the more stator shroud circumference segments 13 the stator shroud 7 are formed of, the less elastic components 15 should be inside in the internal cavity 14 between the stator 6 and the stator shroud circumference segment 13. So the only one elastic component 15 can be arranged in the at least one internal cavity 14 between the stator 6 and the respective stator shroud circumference segment 13.

There are generally eight or more of these stator shroud circumference segments 13. The number of the stator shroud circumference segment 13 of the stator shroud 7, the type and number of elastic components 15 per each internal cavity 14 and their characteristics should be defined by experts.

In enhanced embodiment of the system 12, the stator 6 and each stator shroud circumference segment 13 have special arrangements 16 that limit the movement of the respective stator shroud circumference segment 13 inwards and outwards.

In preferable case the stator 6 and each stator shroud circumference segment 13 have seals 17 arranged in such way that there is no leakage of medium into and out of the at least one internal cavity 14 between the stator 6 and the respective stator shroud circumference segment 13. In other words, there is no leakage of working medium 9 into the at least one internal cavity 14, and there is no leakage of cooling agent, if any, out of the at least one internal cavity 14.

Such sealing 17 should be especially arranged in the area where the stator shroud circumference segment 13 is connected to the stator 7 and in the area of the special arrangements 16 in such a way that during the movement of the stator shroud circumference segment 13 there are no leakage of medium as well.

The systems 12 works as following: the stator shroud 7 that is formed of a plurality of circumferentially adjoining, segments 13, is mounted atop the blades 4. The size of the blades 4 and the stator shroud 7 can vary under different conditions (temperature, vibration, etc) during different operational modes of the turbine.

Having the system 12 implemented into the turbine stage 1 as soon as at least one blade 4 gets in touch with the stator shroud circumference segment 13, the stator shroud circumference segment 13 moves radially outwards since the stator shroud circumference segment 13 is not fixed rigid and since the elastic components 15 allow such movement. The respective stator shroud circumference segment 13 moves radially outwards till the state when there is no pressure / no push from the side of the blade 4 on it.

Therefore the stator shroud 7 and the blade 4 can be still in the contact, but such contact without mutual pressure from each component does not harm the blade tip 5 of the blade 4 and the stator shroud 7 as well. Consequently the radial clearance 11 between the stator shroud 7 and the blade tips 5 is minimal .

In case due to, for example, decreasing of the temperature of the working medium 9, the size in radial length of blade 4 decreases, and the blade 4 is not in contact with the respective stator shroud circumference segment 13, the elastic components 15 move the respective stator shroud circumference segment 13 radially inwards. The elastic components 15 move the respective stator shroud circumference segment 13 radially inwards till the state when the respective stator shroud circumference segment 13 and the at least one blade 4 get in contact, but without any pressure / any push from the side of the blade 4.

Consequently the radial clearance 11 between the stator shroud 7 and the blades 4 is minimal again.

The stator shroud circumference segments 13 and the stator 6 have special arrangements 16, for example hooks as it is shown on FIG 2, that limit the movement of the respective stator shroud circumference segment 13 inwards and outwards. Such special arrangements 16 prevent the stator shroud circumference segments 13 from separation from the stator 6. Also such special arrangements 16 are required during assembling the turbine stage 1. Type of the special arrangements 16, their sizes, etc. are defined by experts.

So having such elastic components 15 inside the internal cavity 14 between the stator 6 and the stator shroud circumference segment 13 together with absence of the rigid fixation the stator shroud circumference segments 13 to the stator 6 allows providing mobility of the stator shroud 7 in the turbine stage 1, and consequently, providing minimal radial clearance 11 between the blades 4 and the stator shroud 8 during different operational modes of the turbine.

While the invention has been illustrated and described in detail with the help of preferred embodiment, the invention is not limited to the disclosed examples. Other variations can be deducted by those skilled in the art without leaving the scope of protection of the claimed invention.

Reference numerals

1 - turbine stage

2 - axis

3 - rotor disc

4 - blade

5 - blade tip

6 - stator

7 - stator shroud

8 - stator shroud outer surface

9 - working medium

10 - intermediate components

11 - radial clearance

12 - system

13 - stator shroud circumference segment

14 - internal cavity

15 - elastic component

16 - special arrangements

17 - seals