EP3298248B1

EP3298248B1 - Turbine for organic rankine cycles having improved centering between casing and shaft tube member

Info

Publication number: EP3298248B1
Application number: EP16729376.0A
Authority: EP
Inventors: Roberto Bini; Mario Gaia
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2015-05-19
Filing date: 2016-05-16
Publication date: 2020-01-15
Anticipated expiration: 2036-05-16
Also published as: WO2016185361A1; EP3298248A1

Description

Technical field

The present invention relates to the field of turbines for gas and steam expansion, in particular for expansion of a working fluid in an organic Rankine cycle (ORC), and specifically concerns the improvements to the overall structure of a radial or axial or mixed type turbine, in one or more stages. More particularly, the invention relates to an improved centering between the casing of the turbine and the tube member containing the turbine shaft.

Background art

As known, the expansion turbines for gas and steam of the type considered here essentially comprise a fixed body or casing having an inlet passage and an outlet passage for the working fluid, at least a first stator, steadily connected to the turbine casing, and any eventual subsequent stators respectively of a first and any eventual subsequent stages of the turbine, also steadily connected to the turbine casing, a turbine shaft, which rotates around an axis and carrying at least a first rotor and any subsequent rotors associated respectively to the first stator and any subsequent stators. To take proper advantage of heat sources at a not very high temperature or having medium-low flow rates, it is necessary to utilize working fluids that enjoy certain properties, different from water ones, that well fit said sources.
In the context of the present invention, reference is made, as is frequently customary in the field of turbines, to an axisymmetric coordinate system in which a generic plane on which lie the axis of rotation of the turbine is called the meridian plane. The direction perpendicular to the axis of the machine and lying in the meridian plan is defined radial direction.
The expression "tangential direction in a point of a meridian plane" identifies the direction orthogonal to the meridian plane and orthogonal to a radial direction passing through the point.
A direction parallel to the axis of rotation of the machine is defined axial direction.
More in detail, an axial stage of a turbine includes an array of stator blades and a corresponding array of rotor blades, respectively upstream and downstream relative to the flow direction of the working fluid; in turn, the flow predominantly occurs due to the axial component of the velocity within the flow itself. A radial stage of a turbine includes an array of stator blades and a corresponding array of rotor blades, respectively upstream and downstream relative to the flow direction of the working fluid; in turn, the flow predominantly occurs due to the radial component of the velocity within the flow itself.
A very important aspect in the design of a turbine is to provide a good shaft centering, in other words the centering of the housing that contains it, with respect to the turbine casing. In this way, the correct clearances between the moving rotoric elements and fixed statoric element are ensured, even in presence of differential strains between the two groups, which normally work at different temperatures. On the contrary, too high or not uniform clearances between the two parts can reduce the performance of the machine so as to affect the function of the labyrinth seals internal to the machine.
According to known technique, the connection between the turbine casing shaft and the shaft housing is made by means of a flange, which has a plurality of fixed holes distributed along the circumference.
During normal working conditions, the turbine casing reaches high temperatures due to contact with the organic working fluid, which flows through the turbine in vapor phase at high temperature. The shaft of the turbine and its housing are at a lower temperature, as cooled by the lubricating fluid which is necessary to ensure the proper functioning of bearings and seals. Consequently, the turbine casing expands outward more than the shaft housing.
Among the known solutions that are adopted to connect the shaft to the turbine casing, so that the two components remain centered between them, the followings can be mentioned:

a) a specific design of the turbine casing (Fig. 1),
b) the use of dowel pins (Fig. 2),
c) the connection by means of frontal toothing, for example "Hirth" joint.

However, each of these solutions has drawbacks. In particular, in the solution shown in Fig. 1, the inner portion 11 of the casing 10 forces the tube member 20 of the shaft 15 to follow the displacements due to thermal expansion, creating a remarkable state of tension and deformation on the whole shaft supporting housing. Evidently, a high stress state on the shaft assembly and a not perfect centering can also influence negatively the reliability of the bearings.
The use of the dowel pins 30 ', distributed along the centering circumference in corresponding centering holes 30 (Fig. 2a, 2b, 2c) between the casing 10 and the tube member 20, enables a good connection at ambient temperature, but presents the same drawbacks of the previous solution when the turbine reaches higher temperatures, i.e. the induction of an important stress and deformation state on the shaft supporting housing.
As known, in case of connection by means of "Hirth joint, the teeth are distributed on a circular crown and the axis of the teeth point toward the center of the circular crown. This realization has to be done on both sides of the connection or, alternatively, a pair "Hirth" joints must be permanently connected to each of the two parts (turbine casing and shaft housing). The particular shape of the teeth prevents any relative movement except for a uniform radial expansion, that does not affect the centering. Therefore, this solution is very accurate but is of complex construction and, accordingly, expensive.
The system shown in the following Fig. 3, 4a and 4b allows to avoid the above mentioned problems; such a system has already been described in the European patent EP 2 422 050 B1 . In this case, elements 30" do not serve for the centering, but are fixing screws of the shaft tube member to the turbine casing. Therefore, there is a clearance between the fixing screws 30 'and the holes 30. The system also includes a plurality of centering tabs 40 steadily connected to one of the two parts to be connected (in the example, the casing 10 of the turbine), which leave free to move only in the radial direction the other connected part, thanks to the presence of radially oriented slots 41, made in the other part (the shaft tube member 20 in the example). Between the slot and the centering tab 40 there is a radial clearance 42 that allows the radial movement. The minimum number of tabs is three at 120°. In the example of Fig. 3 the tabs are four, installed at 90° to one another.
The centering tabs 40 positioned at 0° and 180° allow vertical displacements, while the tabs positioned at 90° and 270° allow horizontal displacements. In this way, a displacement due to thermal effects, uniform with respect to the X axis of the machine, is allowed, but the shaft remains coaxial with the turbine casing. Even with this solution, however, difficulties related to the industrial feasibility remain: in fact, the realization of these centering holes with a very tight tolerance is difficult, either if made on the turbine casing or on the shaft tube member.
WO 2012/143799 discloses another example of a turbine for expanding a working fluid in an organic Rankine cycle.
Therefore, there is a need to obtain an innovative turbine for organic Rankine cycle systems able to overcome the drawbacks of the known turbines.

Invention summary

Subject of the present invention is therefore a gas and steam expansion turbine, in particular for expansion of a working fluid in an organic Rankine cycle, which presents an innovative centering system between the turbine casing and the shaft tube member, made by means of a plurality of centering tabs housed in corresponding centering bushings, in turn accommodated in corresponding seats formed respectively in the casing and in the tube member, as in the enclosed independent claim.
Further embodiments of the invention, preferred and/or particularly advantageous, are described according to the characteristics as in the enclosed dependent claims.

Brief description of the drawings

The invention will be now described by reference to the enclosed drawings, which show some non-limitative embodiments, namely:

Figure 1 is a detail of a centering system between the turbine casing and the shaft tube member, according to a first embodiment based on prior art,
Figures 2a, 2b and 2c show the detail of a centering system between turbine casing and shaft tube member utilizing a plurality of dowel pins according to an alternative embodiment based on prior art,
Figure 3 is a front view of a centering system between turbine casing and shaft tube member utilizing centering slots according to a further known embodiment,
Figures 4a and 4b show two details of the centering system of Fig. 3,
Figure 5 is a schematized front view of a turbine for ORC system,
Figures 6a, 6b and 6c show in a front view and in two details the centering system between turbine casing and shaft tube member according to the present invention,
Figures 7a, 7b and 7c show a first embodiment of the centering between the centering tabs and the centering bushings of the system of Fig. 6,
Figures 8a, 8b, 8c and 8d show a second embodiment of the centering between the centering tabs and the centering bushings of the system of Fig. 6,
Figure 9 show a first embodiment of the seats of the centering bushings of the previous figures,
Figures 10a and 10b show two preferred embodiments of the seats of the centering bushings of the previous figures.

Detailed description

The following description relates to a turbine in which the transport of mass from the inlet to the outlet of the fluid path where the expansion occurs is mainly due to the axial component of the fluid velocity and is referred to as axial turbine, or mainly due to the radial component of the velocity and is referred to as a radial turbine, or still is of mixed type. In particular in Fig. 5 is schematically shown in a front view a turbine 100 that comprises a casing 10, at least a first stator (not shown in the figure), steadily connected to the turbine casing, a turbine shaft 15, rotating around an axis X and carrying at least a first rotor (not shown in the figure) and contained in a housing or tube member 20. The tube member 20 and consequently the shaft 15 are centered with respect to the turbine casing via centering means 30 which will be described below.
According to an embodiment of the invention shown in figures 6a, 6b and 6c, both the casing 10 and the tube member 20 are provided with circular seats 60, 60' arranged along the centering circumference and angularly equidistant. For example, the seats can be in number of three, at 120° from each other or in number of four, at 90° one from the other or even in a higher number. The seats 60, 60' accommodate corresponding centering bushings 50, 50'. According to convenience, alternatively bushings 50 or bushings 50' (as in the example in the figure) are provided with a slot 41, radially oriented. The centering system further comprises a plurality of centering tabs 40, each steadily connected (for example, by interference or bonding) to a centering bushing (in the example, to the bushing 50, accommodated in the turbine housing 10), said centering tabs leaving the tube member 20 free to move only in the radial direction, thanks to the presence of the slots 41 oriented radially, realized in the other set of centering bushings (the bushings 50' housed in the tube member 20, in the example in Fig. 6). In fact, between the slot 41 and the centering tab 40 there is a radial clearance 42 that allows the radial movement, as well as a small tangential clearance, namely a small clearance between the width of the tab 40 and the width of the seat of the bushing 50', so as to allow the relative displacement between the two parts in the radial direction. This clearance should be very small (of the order of hundredths of millimeter) since a larger clearance would affect the accuracy of the centering retention. In the case of four centering seats, the tabs located at 0° and 180° allow vertical displacements, while the tabs located at 90° and 270° allow horizontal displacements. In this way, a displacement due to thermal effects, radial and uniform with respect to the X axis of the machine, is allowed, but the shaft remains coaxial with the turbine casing.
The advantages of this solution compared to the known solutions, but in particular with respect to the solution that foresees the realization of the slots 41 directly on the casing 10 or on the tube member 20, are important: the centering system is highly precise; the realization of circular holes in the casing and in the tube member (instead of the realization of the centering slots) is much more precise and more simple; being small in size, differently from the casing 10 and the tube memeber 20, the centering bushings can be obtained from high-hardness material (for example, hardened steel) and realized with very small tolerances between the housing diameter in the seats 60, 60' and the slots 41 for the centering tabs 40.
The centering bushings 50, 50' are provided with one or more grooves 51, in order to allow their precise positioning in the seats 60, 60' and maintain the bushings properly aligned. This groove will engage with a special system fixed to the tube member 20, for example with a plate 70' or ring that is fixed to the tube member by means of the screws 30".
Figures 7a, 7b and 7c show a first embodiment of the centering between the centering tabs and the centering bushing. According to this example, the tab 40 is forced into the bushing 50, housed in the casing 10 of the turbine 100, and is free to perform radial movements inside the slot 41 of the bushing 50 ', the one housed in the shaft tube member 20.
Figures 8a to 8d show a second embodiment of the centering between the centering tabs and the centering bushing. According to this example, the bushing 50' is provided with a centering slot 41, having dimensions A and B, being B> A, wherein B being the dimension that allows radial movement to the tab. The tab 50' is formed in a single piece with the bushing housed in the tube member 20 and is provided with a protrusion 40' which is housed in the slot 41 of the bushing 50' and has a radial dimension C <B. The dimension A' must be slightly inferior to A to allow the radial relative sliding of the two parts, but not too much smaller for not introducing a clearance that frustrates the maintenance of centering. Of course, it is also possible to realize the dual case with integrated tongue into the bushing of the casing.
The seats 60, 60' of the centering bushings can be realized, using different modes. A first embodiment is the one shown in Figure 9. As can be seen, the two seats 60, 60', respectively in the casing 10 and in the shaft tube member 20, are two circular through-holes and facing one another. This solution, although simple to implement because it does not involve changes to the flange portions between casing and tube member, would present risks of leakage of the working fluid, through the inevitable gaps that are created between the casing/tube member and the respective centering bushings. Therefore, in this solution the bushing 50 should be provided with a sealing system (not shown in Fig. 9).
On the contrary, the two embodiments shown in Fig. 10 are not subject to any kind of leakage of the fluid. In fact, the embodiment of Fig. 10a provides that the seat 60 of the bushing 50 of the casing 10 is a blind hole. However, the configuration of Fig. 10b requires the presence of a casing appendix 12, substantially annular shaped, within which the seat 60 is realized in the form of through hole. Therefore, both configurations achieve a perfect sealing of the working fluid with respect to the seat 60.
To improve the tolerances, it is possible to realize the final diameters of the seats 60, 60' only after the casing and the tube member have been coupled together by means of fixing screws 30 ", for example by milling or boring.
Summarizing, the proposed solution allows to realize and maintain a good centering between the turbine shaft, with its housing, and the turbine casing even in the presence of high thermal gradients. Therefore, the present invention improves the centering process between the two parties, said centering resulting more reliable of the already known solutions, as well as cheap and easy to implement.
Other than the embodiments of the invention, as above disclosed, it is to be understood that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples and are not intended to limit the scope, applicability, or configuration in any way. Rather, the foregoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and their legal equivalents.

Claims

Turbine (100) for expanding a working fluid in an organic Rankine cycle, comprising a casing (10), inside of which at least a statoric group is accommodated, a turbine shaft (15) contained inside a tube member (20) and carrying at least a rotoric group, contained within the casing (10), wherein said tube member (20) and said casing (10) are each other axially pushed and radially centered, said turbine being characterized in that
- the centering between the tube member (20) and the casing (10) is made by means of a plurality of centering tabs (40, 50") housed in corresponding centering bushings (50,50'), in turn accommodated in corresponding seats (60, 60') formed in the casing (10) and in the tube member (20), and in that

- said tabs allow a relative displacement in the radial direction but not in the tangential direction of the tube member (20) with respect to the casing (10), and one of said centering bushings (50, 50') is provided with a slot (41), radially oriented for the centering tab (40,50").
Turbine (100) according to claim 1, wherein said seats (60, 60'), said bushing (50), mounted on the casing (10) and the corresponding bushing (50') mounted on the tube member (20) are arranged angularly equidistant along the centering circumference.
Turbine (100) according to claim 1 or 2, where said seat (60) of the centering bushing (50) of the casing (10) is a blind hole.
Turbine (100) according to any of the preceding claims, where said seat (60) of the centering bushing (50) of the casing (10) is a through hole realized within a casing appendix (12), having a substantially annular shape.
Turbine (100) according to any of the preceding claims, wherein said centering bushings (50, 50') are provided with one or more grooves (51), that allow their correct orientation in the seats (60, 60') and said grooves are in turn hold in place by a suitable system for position keeping.
Turbine (100) according to claim 5 wherein said system for position keeping comprises a plate (70'), blocked by suitable fastening means (30 ").
Turbine (100) according to claim 5 or 6, wherein said centering tab (40) is steadily connected to one of the two centering bushings (50, 50') and free to move in the radial direction inside the slot (41), available in the other of the two centering bushings.
Turbine (100) according to any of claims 5 to 7, wherein said centering tab (50") is formed in one piece with the bushing housed in the tube member (20) or in the casing (10) and is provided with a protrusion (40') housed in the slot (41) of the centering bushing (50') housed in the casing (10) or in the tube member (20).
Turbine (100) according to any of the preceding claims, wherein the centering bushings (50, 50') and the tabs (40, 50") are made of a high hardness material or of a base material subjected to a surface treatment, suitable to increase the hardness of the base material.
Turbine (100) according to any of the preceding claims, wherein the clearance between the width of the centering tab (40, 50 ") and the width of the seat of the centering bushing (50, 50') is smaller than 0.02 mm.