EP3186487A1

EP3186487A1 - Seal arrangement in a turbine and method for confining the operating fluid

Info

Publication number: EP3186487A1
Application number: EP15766648.8A
Authority: EP
Inventors: Roberto Bini; Mario Gaia
Original assignee: Turboden SpA
Current assignee: Turboden SpA
Priority date: 2014-08-28
Filing date: 2015-08-26
Publication date: 2017-07-05
Also published as: WO2016030845A1

Abstract

A turbine (1) of an Organic Rankine Cycle ORC, comprising a shaft (4) supported by bearings (8) and a plurality of mechanical seals (10, 11, 18, 21) arranged around the shaft for confining the operating fluid expanding in the turbine, is described. The seal are arranged so that to define and preserve the insulation of four chambers arranged in succession along and around the shaft. A first chamber (101) is among the turbine expansion stages and the second chamber (102), named buffer chamber; a fourth chamber (104) is the nearest to the bearings and the third chamber (103) is in-between the second and the fourth chamber. In the buffer chamber a barrier fluid (302) is fed and, advantageously, it is the same organic operating fluid fed to the turbine. In this way the confinement of the operating fluid in the turbine and the respective non-contamination are guaranteed. A corresponding method for confining the operating fluid in a turbine is also provided.

Description

"SEAL ARRANGEMENT IN A TURBINE AND METHOD FOR

CONFINING THE OPERATING FLUID"

***

Field of the invention

The present invention refers to a seal arrangement in a turbine operating in a

Rankine cycle with organic operating fluid (ORC), and to a method for confining the operating fluid in the turbine.

The invention can be applied also to feeding pump used in the Rankine cycle ORC plants.

Background of the invention

The acronym ORC "Organic Rankine cycle" , as everyone knows, identifies the thermodynamic cycles of Rankine type that use an organic operating fluid preferably provided with high molecular mass, much higher than that of the water vapor used in most of the Rankine power cycles.

In plants exploiting this thermodynamic cycle biomass is often used to generate the heat necessary for vaporizing the organic operating fluid, or waste heats of industrial processes. The operating fluid is expanded in a turbine to which an electric generator is usually connected for producing electric power.

In most of the Rankine cycle ORC plants, the organic operating fluid must necessarily remain confined in the plant, in order to avoid atmosphere contaminations. On the other hand air must not entry the thermodynamic cycle, as the organic operating fluid would be subject to oxidation and corrosion phenomena favored by oxygen and, furthermore, the humidity in the air would pollute the operating fluid.

In this sense, the confinement of the organic operating fluid must prevent both the leaks thereof in the surrounding environment and the air input into the plant.

Typically, criticalities arise at interfaces among stationary parts and rotating shafts of the turbine. At such interfaces effective seals for confining the turbine are difficult to be made.

A number of technical solutions have been suggested.

Figure 1 shows a classic solution according to the known art: the turbine 1 and the generator 2 are coupled directly and isolated inside a casing 3. The shaft 4 of the turbine 1 and the generator 2 both rotate in the same volume defined by the volute 3, in which there is the operating fluid. The shaft 4 of the turbine does not cross the casing 3 and, therefore, the risk of operating fluid leaks is confined to stationary seals only. Electric power produced by the generator is transmitted to the outside through convenient electric connectors 5 constrained to the volute 3, these being obviously fluid-tight, to which corresponding cables can be connected. This solution suffers from the drawback of exposing the electric generator to the operating fluid. As the insulation of electric windings of the generator 2 are continuously in contact with the operating fluid, in the long run it can be damaged and impaired.

Figure 2 shows an evolution of the previous solution, still according to the known art. The stator part and the rotor part of the generator are kept fluidically separated by a cylindrical septum 6, called liner, and gaskets 7.

In both solutions shown in figures 1 and 2, the adoption of a specifically designed and sized generator 2 is provided. This entails higher costs and complications with respect to adoption of a standard generator available on the market that, among other things, on the average is more reliable.

Moreover, also the bearings 8 (schematized) supporting the shaft 4 are exposed to the operating fluid, therefore the latter having to act also as lubricating and cooling fluid. The operating fluid is discharged through convenient portholes. As an alternative to this solution, magnetic, radial and axial bearings have been proposed.

Another drawback is that the operating fluid is present in the gap between the stator and the rotor of the electric generator 2; independently of the latter being in liquid or vapor phase, high fluid-dynamic losses arise, certainly greater than what happens if the operating fluid is in gaseous phase of a gas with low molecular mass, such as the air surrounding the rotor of a conventional generator. If the liner 6 is present, the respective bulk forces the gap to be again large, this inevitably leading to not obtaining the maximum electrical efficiency possible for the generator, other conditions unchanged.

Moreover, in the volume inside the casing 3, saturated by the operating fluid, positioning the instruments, lights and indicators is impeded, both because of the potential damage to the instruments themselves and the need of crossing the sealed casing with the connecting elements.

Because of the described drawbacks, in the Rankine cycle ORC plants having medium or great size with some hundreds of kW up to 10 MW, oil-lubricated bearings for supporting the turbine shaft, and a convenient arrangement of the fluidic seals to achieve the confinement of the operating fluid in the plant, are used. This solution allows adopting electric generator of standard type, and a reduction gear can be introduced between the turbine and the generator and, therefore, the revolution number of turbine and generator can be optimized.

Over the years a lot of arrangements of fluidic seals have been proposed in order to achieve the confinement of the process fluid, particularly in chemical plants and in the oil & gas field. A lot of these arrangements are described in the norm ANSI/ API Std. 682 and Std. 617.

Figure 3 shows one of the arrangements provided by the norm: it is an arrangement named "Double seal" or "Tandem seal" of "back to back" type, particularly recommended when a possible leak of operating fluid in the environment cannot be accepted. The back part of the seals 10 and 11 abuts against corresponding countercheck elements 12 and 13, i.e. the seals are pushed in the opposite direction. The seals 10 and 11 and the corresponding countercheck elements 12 and 13 reciprocally move due to the rotary movement of the shaft. It is an arrangement providing for an intermediate chamber 9 between the bearings supporting the turbine shaft and the zone where the operating fluid expands. As far as the "Double seal" case, being the most effective solution for assuring the confinement, in the intermediate chamber 9 definable buffer chamber, the pressure of a sealing fluid, definable barrier fluid, is kept greater with respect to the pressure of the operating fluid in the zone adjacent the turbine. Typically, oil or water is used as barrier fluid.

Figure 4 shows another arrangement provided in the norm, this time of the "face to face " type, the seals being pushed one against the other. The seals 10, 11 slide axially in order to move in abutment at the respective front face against only one ring 14 provided in the seals themselves, on which the countercheck elements 12 and 13 are provided.

Figures 5, 5a and 5b are schematic views in axially symmetrical section of corresponding double-sealed arrangements, which are used in conventional Rankine and not-organic ORC cycle turbines, particularly adapted for being used where the rotation speed of the shaft at the slide surfaces is high, greater than 10 m/s.

In particular, the solution shown in figure 5 is of "back to back" type with the seals 10 and 11 pushed in opposite directions by corresponding springs 15 and 16 towards the countercheck elements 12 and 13. Obviously, the seal is realized at the interface SI and S2 between, respectively, the seal 10 and the countercheck element 12 and between the seal 11 and the countercheck element 13.

A barrier liquid is fed through a feeding duct A, which is then drained by several output ducts B and C, in case also through the interfaces SI and S2 if the seal is not perfect. For example, through the duct C the flow of the mixture containing the possible flow rate of the barrier fluid able to cross the interface SI and part of the lubricating oil initially fed to the bearing 8, are drained. The same operating fluid expanding in the turbine is fed to D.

Figure 5a shows a embodiment equivalent to that shown in figure 5, the difference being that the springs 15 and 16 have been replaced with metal bellows 15' and 16', which are more resistant against high temperatures and the abrasive action applied by the fluid polluted with solid substances, for example particulate.

Figure 5b is a embodiment substantially identical to that shown in figure 5, but provided with an additional sleeve 17 connected to the stationary portion of the turbine and provided with helical grooves generating an effect of fluid dynamic pumping. The viscous friction of the fluid fed between the seals 10 and 11 exerts an action pumping onto the fluid itself, in the way defined by the tilt of the helical grooves of the sleeve 17. Thanks to the pumping effect, the barrier fluid is thrown against the base of the countercheck element 12 in the form of jet, as denoted by the arrow in figure.

In some embodiments the feeding of a minimal and controlled flow rate of barrier fluid is provided, in order to keep the faces separated from the seals and, therefore, to avoid the relative wear. Solutions offered by the known art do not assure the effective confinement of the operating fluid in case in which the fluid is organic, as occurring in Rankine ORC cycles, and the turbine rotates at very high speed, i.e. typically at speeds higher than 10 m/s next to the slide surfaces.

Furthermore, adopting barrier fluids such as oil or water is a problem in

Rankine ORC cycles as these fluids, in presence of a leak flow towards the ORC process, contribute to thermal degradation of the organic operating fluid, aid the sediment accumulation and can interfere with the Rankine ORC cycle when they are generously used.

Object and Summary of the Invention

It is an object of the present invention to provide a Rankine cycle ORC turbine provided with an optimal seal arrangement in order to achieve the effective confinement of the operating fluid and to guarantee the contamination of the operating fluid is avoided.

It is another object of the present invention to provide a method for effectively confining the operating fluid of a turbine in a Rankine cycle ORC and avoiding the possible contamination thereof.

Therefore the present invention, in a first aspect thereof, relates to a turbine according to claim 1 of an organic Rankine cycle ORC.

In particular, the turbine comprises a shaft supported by bearings and a plurality of mechanical seals arranged around the shaft for confining the operating fluid expanding in the turbine. The seal are arranged so that to define and preserve the insulation of four chambers arranged in succession along and around the shaft.

A first chamber is among the turbine expansion stages and the second chamber, named buffer chamber; a fourth chamber is the nearest to the bearings and the third chamber is in-between the second and the fourth chamber.

In the buffer chamber a barrier fluid is fed and, advantageously, it is the same organic operating fluid fed to the turbine. In this way the confinement of the operating fluid in the turbine and the respective non-contamination are guaranteed.

Further preferred features of the present invention are described in the dependent claims.

In its second aspect the present invention concerns a method according to claim 9 for confining the operating fluid in a turbine working in an Organic Rankine Cycle ORC and preventing leaks into the surrounding environment.

Additional preferred steps are described in claims 10-15.

Brief description of the drawings

Further details of the invention will be evident anyway from the following description course made with reference to the attached drawings, in which:

figure 1 is a schematic view in axially symmetrical section of a sealed solution, according to the known art, between the turbine and the generator;

figure 2 is a schematic view in axially symmetrical section of another sealed arrangement according to the known art;

figure 3 is a schematic view of an arrangement of seals according to norm ANSI/API;

figure 4 is a schematic view of another arrangement of seals according to norm ANSI/API;

figure 5 is a schematic view in axially symmetrical section of an arrangement of seals in a turbine, according to the known art;

figure 5a is a schematic view in axially symmetrical section of a embodiment of the arrangement of seals shown in figure 5;

figure 5b is a schematic view in axially symmetrical section of a embodiment of the arrangement of seals shown in figure 5;

figure 6 is a schematic view, partially in axially symmetrical section, of a first arrangement of seals according to the present invention;

figure 7 is a schematic view, partially in axially symmetrical section, of an apparatus comprising a turbine provided with the arrangement of seals shown in figure 6;

figure 8 is a schematic view, partially in axially symmetrical section, of a second arrangement of seals according to the present invention;

figure 9A is a schematic view, partially in axially symmetrical section, of an apparatus comprising a turbine provided with the arrangement of seals shown in figure 8;

- figure 9 is a principle scheme of the seal arrangement according to the present invention;

- figures 10 and 11 are schemes of plants associated with the arrangement of seals according to the present invention.

Detailed Description of the Invention

Figures l-5b refer to double-sealed solutions according to the known art, in a "back to back " arrangement, and the respective description is given at the beginning of the text.

Referring to figure 6, a scheme is shown referring to the present invention: a turbine portion, in an axially symmetrical section, is provided with fluid seals 10 and 1 1 and with the corresponding countercheck elements 12 and 13, as in the scheme shown in figure 5. The scheme of the seals is of "back to back" type, but in general the present invention can be implemented also with seals having "face to face" or "face to back" arrangements, which are not shown.

It has to be mentioned that they extend circumferentially around the shaft and are coaxial thereto.

Differently from the solution of the known art shown in figure 5, the seals formed by the elements 10 and 12 and by the elements 11 and 13 cooperate with at least one labyrinth-shaped ring 18 to define four adjacent chambers (not only three anymore), i.e. chambers arranged one after another in the longitudinal direction along the shaft 4 of the turbine. Obviously, they are annular chambers extending circumferentially around the shaft 4.

A first chamber is depicted with the numeral 101 and is adjacent to the turbine part where expansion stages are provided, being denoted also with "process side". Therefore it is the side having the highest temperature. With the letter D is depicted the fluid adduction duct to the chamber 101.

A second chamber, defined buffer chamber 102, is adjacent to the first chamber 101, at the side opposite to the turbine stages, towards the bearings 8. The first chamber 101 is separated from the second chamber 102 by the seal 11. The buffer chamber 102 is fed through the duct A; the return duct is depicted with B.

A third chamber 103 is adjacent the buffer chamber 102, at the opposite side with respect to the first chamber 101, towards the bearings 8. The third chamber 103 is separated from the buffer chamber 102 by the seal 10. A draining duct, i.e. a drainage, is denoted with F.

The fourth chamber 104 is among the bearings 8 and the third chamber 103, from which it is separated by the ring 18 defining a labyrinth. A draining duct is denoted with C.

Obviously, the turbine 1 is provided with a plurality of ducts A, B, C, D, F circumferentially arranged to operate on the whole respective chamber.

Another difference with respect to the known art is that the barrier fluid fed into the buffer chamber 102 is the same organic operating fluid expanding in the turbine 1.

The bearings 8 are lubricated by injectors 19 spraying a lubricant.

The feed of the barrier fluid is carried out by an apparatus 300, now described in detail.

The feeding apparatus 300 comprises a vessel 301 in which there is the pressurized barrier fluid 302. The pressurization can be obtained, for example, by feeding an inert gas such as nitrogen into the upper volume 303 of the vessel, above the open surface of the barrier fluid, through the line 309, or by prearranging an elastomeric bag always in the upper volume 303, the bag being inflatable with a fluid in turn pressurized.

The pressure of the barrier fluid 302 has to be sufficient to assure the good functioning of the seal 1 1 operating at the highest temperatures among all seals, without significantly producing fluid vapor at the interface S2.

According to the present invention the following pressure conditions are provided in the chambers 101-104.

The pressure p2 of the barrier fluid 302 in the buffer chamber 102 must be higher than the pressure pi in the first chamber 101, so that the leak flow through the faces of the seal 11 does not correspond to a leak of operating fluid from the turbine stages through the interface S2, but at worst a flow of barrier fluid 302 can be established, and therefore of operating fluid, towards the turbine stages. This should not be a damage for the turbine because, as mentioned, the barrier fluid 302 and the operating fluid are the same fluid and contamination is not possible. If the flow rate of the barrier fluid 302 should succeed in crossing the seal 11, it is simply mixed together with the operating fluid flowing through the turbine. The mixing will happen indifferently in every process point, depending on the connection point between the chamber 101 and the process itself.

Furthermore, also the pressure p3 in the third chamber 103 must be lower than the pressure p2 in the buffer chamber 102. Therefore, in case of malfunction of the seal 10, at worst a flow of barrier fluid 302 can be established from the buffer chamber 102 towards the third chamber 103 through the interface S2. Advantageously, if in the third chamber 103 the barrier fluid 302 should be present, it is polluted by the lubricant used for the bearings 8 only modestly, since the labyrinth-shaped ring 18 acts as a rough fluidic seal.

In the embodiment shown in figure 6, the pressure p3 in the third chamber must be more or less equal to the pressure p4 in the fourth chamber. As will be described hereinafter, in the second embodiment this condition will be different.

In other words, for the embodiment shown in figure 6 and the arrangements equivalent thereto, the following conditions must be contemporaneously fulfilled:

p2 > pl,

p2 > p3,

p3 ^ p4.

Preferably, p2 = pi + n, where n is comprised between 1 bar and 3 bars. Anyway, the pressure p2 must not be lower than 1 bar than the vapor pressure of the barrier fluid 302, at the feed temperature.

The feeding apparatus 300 is a closed circuit comprising a delivery line 304 of the barrier fluid 302 to the buffer chamber 102 and a corresponding return line 305 along which a cooling unit 306 and a circulation pump 307 are provided. The latter can also not be present if a pumping member, as depicted with numeral 17 in figure 5b, having a sufficient predominance and flow rate is provided.

A level controller LT, i.e. a sensor detecting the level of the barrier fluid 302, controls the replenishment of the barrier fluid 302 through the replenishment line 308. A sensor T of the temperature of the barrier fluid 302, a pressure sensor P at the upper volume 303 and a flow rate sensor FT of the barrier fluid 302 sent into the buffer chamber 102, are further provided.

Preferably, the barrier fluid is fed into the first chamber 101 also through the feeding duct D, here however the mass throughput is little, lower than one hundredth of the flow rate of the operating fluid expanding in the turbine at full power. The feed of the operating fluid in D is operated by outside means, herein not described in detail. A higher flow rate would subtract an excessive heat amount from the thermodynamic cycle ORC, such a heat not being available for use in the regenerator.

Preferably, the operating fluid fed in D is in turn withdrawn from a point of the ORC cycle at a temperature lower than the temperature set up for the shaft part contacting the rotors. The fed operating fluid can be in vapor state, or biphase liquid/vapor. This allows reducing the temperature in the zone of the seal 11 and avoiding accumulation of possible products abrading the same seal.

Figure 7 shows the solution according to figure 6, integrated in a plant.

The line 410 connects the drain F to a collecting vessel 411 for the accumulation of the fluid present in the third chamber 103, which could comprise the fluid possibly leaked through the seal 10 and condensed, and/or lubricant leaked through the labyrinth- shaped ring 18 and/or a little flow rate of inert gas coming from the line 409 and passed through the labyrinth-shaped ring 18. From the vessel 411 a line 412 venting to atmosphere opens, provided with oil-separating and purifying filters 413, for example of the activated-carbon type. With LT a level indicator and with FT a flow rate meter are depicted.

The vessel 411 is periodically emptied through the valve 414.

The line 412 is further provided with a valve 419 having the purpose of controlling the outputting flow of inert gas in relation to a flow value FT measured and transmitted to the control system. Figure 8 shows a second embodiment of the present invention similar to the previous one, but with the difference that the second seal, dividing the chambers 103 and 104, is made differently and doesn't have the labyrinth-shaped ring 18.

In particular, in this embodiment a floating ring is depicted with the numeral 20, i.e. sliding on the shaft 4 of the turbine. The seal 10 is moved to abutment against the floating ring 20, at a side. At the opposite side there is a third seal 21, in turn sliding axially to move in abutment against the floating ring 21. The spring or bellow pushing the seal 21 is calibrated so that to apply a thrust greatly lower than the thrust applied by the respective bellow/ spring onto the seal 10.

In this second embodiment, conditions concerning the pressures in the chambers 101-104 are the following:

p2 > pl,

p2 > p3,

p3 < p4.

Clearly, with respect to the first embodiment shown in figure 6 the seal 21 allows, together with the floating ring 20, maintaining the pressure p3 lower than the pressure p4. In this circumstance if the seal 21 should become less effective, at worst a flow of lubricant from the chamber 104 - where there are the bearings 8 - to the chamber 103 should be established. Every possible contamination of the lubricant is therefore avoided.

Preferably, p4 is higher than p3 of at least 0.2 bar.

Moreover, the third seal 21 allows maintaining in the third chamber 103 a pressure greatly lower than the pressure in the vessel 401 of the apparatus 400, this fact not generating a significant gas flow from the bearings 8 towards the third chamber 103. This is true also when the turbine 1 is not operating, also for long times, for example for maintenance.

Apart from that, the two solutions of figure 6 and 8 are equivalent.

Also this second embodiment can be implemented by arranging the seals 10, 1 1 and 21 in different "face to face" and/or "back to back" arrangements, and various face/back combinations. The main thing is the subdivision of the chambers 101-104 and the maintaining of pressure conditions.

Figure 9 A shows the solution according to figure 8, integrated in a plant. With the numeral reference 400 it is generically denoted the apparatus for lubricating and treating the polluted lubricating oil returning from the lines C.

The treating apparatus 400 comprises a vessel 401 for collecting the polluted lubricating oil suctioned by the pump 402 that send it to a treating unit 403, for example a unit performing the fractional distillation in a separation tray column, or a unit according to the known art. There are two lines exiting from the treating unit 403 : the return line 405 of the barrier fluid 302, i.e. the operating fluid of the Rankine cycle ORC, which is separated from the lubricating oil, and the return line 404 of the lubricating oil separated from the barrier fluid 302.

The line 404 returns the lubricating oil to the vessel 401. With the numeral reference 406 a drain pipe of undesired fractions, residuals of the treatment, is denoted.

The field technician must take care of designing the treating unit 403 to obtain both the barrier fluid 302 and the lubricating oil with a purity level sufficient for the condenser of the Rankine cycle ORC and the apparatus 400.

The apparatus 400 is maintained over-pressurized with respect to the surrounding atmosphere, preferably at a pressure comprised between 10 and 1000 Pascal above the atmospheric pressure. This can be achieved by connecting a source 407 of a preferably inert gas to the vessel 401, for example nitrogen; a regulator 408 intervenes in real time so that overpressure can be maintained. In this way the risk of forming an explosive environment in the treating apparatus 400 and in the environment of the bearings 8 of the turbine 1 is avoided, particularly in case of inflammable operating fluid.

Convenient pressure sensors PS, provided with the transmitter PT, and level sensors LS, provided with the transmitter LT, generate signals processed by the plant control unit, for example based on the PLC technology.

With numeral reference 409 a balancing line is depicted, fluidically communicating the upper zone of the environment of the bearings 8 with the upper volume of the vessel 401. As a matter of fact, figure 9 shows a principle scheme of the present invention, independently from the fact that the seals are arranged "face to face" or "back to back". Therefore, in summary, according to the present invention three seals defining four annular chambers in axial succession are provided. Pressure conditions in the chambers and the treating of fluids are described above.

Figure 10 shows a plant 500 replenishing the content of operating fluid/ barrier fluid 302 in the vessel 301 shown in figures 6 and 8. The plant 500 withdraws a flow rate of organic operating fluid from the ORC cycle through the line 501. The withdrawal must be carried out in a point in which the operating fluid is in the liquid state, at low temperature and preferably with a low content of contaminants which are, for example, water, particulate, dissolved gas, etc.

A withdrawal is carried out by a positive displacement pump 502, which has also the role of dosing pump manually or electrically operated and protected by a filter 503 (20-50 micrometers). A fine filter 504 (2-10 micrometers) downstream of the pump 502 and, in case (also in not shown in figure), other oil separator filters, a drying unit, a deacidifi cation unit, etc., are provided so that to clean and make as inert as possible the barrier fluid 302 then fed to the seals. Withdrawal points are then provided on the vessel 301, for the periodical tests of the fluid 302 contained therein.

Figure 11 shows a plant 600 for treating the fluid withdrawn from the above described drainages F, and particularly adapted in presence of a great amount of drained operating fluid. In this circumstance putting again the conveniently purified operating fluid into the Rankine cycle ORC is advantageous and, preferably, at the condenser downstream of the turbine 1, for example at the conventional withdrawal point of incondensable elements. In this way the possible fraction of incondensable elements is more easily separated.

The plant 600 comprises a sloped duct 601 receiving the fluid to be treated from drainages F and feeding it to a compressor 602. Here the fluid is compressed to a pressure higher than the atmospheric pressure, typically at 1.5 - 10 bars of absolute pressure, and sent to a vessel 603 for collecting and treating the compressed fluid. A control unit adjusts the temperature of the fluid in the vessel 603 through a heating element 604. In the vessel 603 there are also provided an oil separator filter and/or a demister and a relief valve 605 from which the operating fluid purified from the lubricant or other contaminants is injected again in the Rankine cycle ORC, preferably at the condenser. A container 606 for collecting the recovered lubricant is present downstream of the vessel 603.

Alternatively to the herein described treatment, the plant 600 can be designed to carry out the column separation according to the known art.

In a not claimed embodiment, and for which the Applicant reserves the right of filling a divisional Patent Application, an inert gas, for example nitrogen, is fed to the third chamber 103 at a pressure p3>p2. In this way, the possible gas leak through the seals 10, 18 or 21 causes the accumulation of nitrogen in the apparatus 400 and in the barrier fluid 302, anyway in a manageable way. In this circumstance the treating of the barrier fluid by the apparatus 400 is not needed; the nitrogen can be simply vented into the atmosphere and the part circulating with the operating fluid in the Rankine cycle ORC is withdrawn from the condenser. On the other hand, the seal 10 remains cool and can be lubricated with oil mist as the bearings. Therefore, in this not claims embodiment the pressure conditions are: P2 > pi, p3 > p2, p3 > p4.

Claims

1. A turbine (1) of an Organic Rankine Cycle ORC, comprising a shaft (4) supported by bearings (8) and a plurality of seals (10, 11, 18, 21) arranged around the shaft (4) for confining the operating fluid expanding in the turbine, wherein the seals are arranged so that to define and preserve the insulation of a first chamber (101), a second chamber (102) named buffer chamber, a third chamber (103) and a fourth chamber (104), in which the first chamber (101) is among the turbine expansion stages and the buffer chamber (102), the fourth chamber (104) is the nearest to the bearings (8) and the third chamber (103) is between the fourth chamber (104) and the buffer chamber (102), and in which a barrier fluid (302) is fed into the buffer chamber (102), characterized in that the barrier fluid (302) is the same organic operating fluid fed to the turbine so that to simultaneously guarantee the confinement of the operating fluid in the turbine and the respective non-contamination.

2. Turbine (1) according to claim 1, wherein the pressure p2 of the barrier fluid (302) in the buffer chamber (102) is higher than the pressure pi in the first chamber (101) and than the pressure p3 in the third chamber (103), and wherein the pressure p3 in the third chamber (103) is almost equal to or lower than the pressure p4 in the fourth chamber (104), i.e. the following conditions are simultaneously fulfilled:

p2 > pl,

p2 > p3,

p3 < p4.

3. Turbine (1) according to claim 1 or claim 2, wherein the pressure p2 is at least equal to:

p2 = pl + n,

where n is comprised between 1 bar and 3 bars.

4. Turbine (1) according to any one of preceding claims 1-3, wherein the pressure p2 is higher of at least 1 bar than the vapour pressure of the barrier fluid (302).

5. Turbine (1) according to any one of preceding claims 1-4, wherein the first chamber (101) is fed with an operating fluid having a mass throughput lower than one hundredth of the flow rate of the operating fluid expanding in the turbine (1) at full power.

6. Turbine (1) according to any one of preceding claims 1-5, wherein the bearings (8) are lubricated and the lubricant, eventually leaked through the seal (18, 21) present between the third chamber (103) and the fourth chamber (104), is withdrawn from the third chamber (103) together with the possible barrier fluid (302) present therein and sent to an outer treating apparatus (411).

7. ORC Rankine cycle plant comprising a turbine (1) according to any one of the preceding claims and a feeding apparatus (300) to feed a barrier fluid (302) to the buffer chamber (102), wherein the apparatus (300) comprises a vessel (301) for collecting the barrier fluid (302), a line (304) connecting the vessel (301) to the adduction (A) of the buffer chamber (102) and a return line (305) connected to the outlet (B) of the buffer chamber (102) so that the barrier fluid (302) can return to the vessel (301), wherein the barrier fluid (302) in the vessel (301) is alternately pressurized through:

- an inert fluid, for example nitrogen, fed as pressurized into the upper volume (303) of the vessel (301), or

- an expandable watertight bag arranged in the upper volume (303) of the vessel (301) and inflatable by an inert gas.

8. Plant according to claim 7, wherein the bearings (8) of the turbine (1) are lubricated and the lubricant, in case leaked through the seal (18, 21) present between the third chamber (103) and the fourth chamber (104), is withdrawn from the fourth chamber (104) together with the possible barrier fluid (302) present therein and sent to an outer treating apparatus (400), further comprising such an apparatus (400) for treating the lubricant, wherein the treating apparatus (400) in turn comprises:

- a vessel (401) for collecting the lubricant and a treating unit (403), for example of the type of fractional distillation in a separation tray column, or equivalent, where the treating unit (403) separates the lubricant from the barrier fluid (302) and in which the lubricant and the barrier fluid (302) both purified are reused, and

- a vessel (411) for collecting the barrier fluid (302) and the lubricant which are eventually present in the third chamber (103), the vessel (411) being connected in turn to an line (412) venting to atmosphere that is provided with oil- separating and purifying filters (413), for example of the activated-carbon type, wherein the vessel (411) is intended for being periodically drained.

9. A method for confining the operating fluid in a turbine working in an Organic Rankine Cycle ORC and preventing leaks into the surrounding environment, where the turbine (1) comprises a shaft (4) supported by bearings (8) and a plurality of seals (10, 11, 18 or 21) arranged around the shaft (4) for confining the operating fluid expanding in the turbine, comprising the steps of:

- arranging the seals so that to define and preserve the insulation of a first chamber (101), a second chamber (102), named buffer chamber, a third chamber (103) and a fourth chamber (104), wherein the first chamber (101) is among the turbine expansion stages and the buffer chamber (102), the fourth chamber (104) is the nearest to the bearings (8) and the third chamber (103) is between the fourth chamber (104) and the buffer chamber (102),

- feeding a barrier fluid (302) into the buffer chamber (102),

characterized in that the barrier fluid (302) is the same organic operating fluid fed to the turbine (1).

10. Method according to the claim 9, comprising the further steps of:

- maintaining the pressure p2 of the barrier fluid (302) in the buffer chamber (102) higher than the pressure pi in the first chamber (101) and than the pressure p3 in the third chamber (103), and maintaining the pressure p3 in the third chamber (103) almost equal to or lower than the pressure p4 in the fourth chamber (104), so that the following conditions are simultaneously fulfilled:

p2 > pl,

p2 > p3,

p3 < p4.

11. Method according to claim 9 or claim 10, comprising the step of maintaining the pressure p2 is at least equal to:

p2 = pl + n,

where n is comprised between 1 bar and 3 bars.

12. Method according to any one of preceding claims 9-11, comprising the step of maintaining the pressure p2 higher at least 1 bar than the vapour pressure of the barrier fluid (302).

13. Method according to any one of preceding claims 9-12, wherein the pressure of the barrier fluid (302) is adjusted by collecting the fluid itself in a collecting vessel (301) either pressurized through:

14. Method according to any one of preceding claims 9-13, comprising the step of feeding the first chamber (101) with a flow rate of operating fluid having a mass throughput lower than one hundredth of the flow rate of the operating fluid expanding in the turbine at full power.

15. Method according to any one of preceding claims 9-14, comprising the steps of:

- lubricating the bearings (8) of the turbine through a lubricant;

- withdrawing the lubricant and the barrier fluid (302), possibly present therein, from the third chamber (103) and sending them to an outer treating apparatus (400) for the respective recycling separation.