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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 peripheral speed 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 a seal arrangement in order to achieve the effective confinement of the operating fluid and avoid the contamination thereof in every operative condition.

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 possible contaminations.

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

The turbine comprises a shaft supported by bearings and a plurality of mechanical seals arranged around the shaft, along the entire circumference thereof, for confining the operating fluid expanding in the turbine.

The seals are arranged so that to define and preserve the insulation of three consecutive chambers along the shaft of the turbine: a first chamber, a second buffer chamber and a third chamber.

The first chamber is between the expansion stages of the turbine and the buffer chamber, and the third chamber is between the bearings and the buffer chamber. A barrier fluid is fed into the buffer chamber.

Advantageously, the barrier fluid is the same organic operating fluid fed to the turbine. In this way, the effective confinement of the operating fluid and the guarantee of avoiding the contamination thereof are achieved. 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 an 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 scheme of a treating plant associated with the arrangement of seals according to the present invention; figure 9 is a scheme of another treating plant 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 fluidic seals 10 and 11 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 known art, the barrier fluid fed into the buffer chamber 102 is the same organic operating fluid expanding in the turbine.

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

The 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 pressure p2 of the barrier fluid 302 in the buffer chamber 102 must be higher than the pressure pi in the adjacent chamber 101 nearest the turbine and also higher than the pressure p3 in the adjacent chamber 103 nearest the bearings 8. In other words, the following conditions must be simultaneously fulfilled:

p2 > pl

p2 > p3.

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 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 ring, as depicted in figure 5b, having a sufficient predominance is provided.

A level controller LT, i.e. a sensor detecting the level of the barrier fluid 302 by an appropriate logic control unit, 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 preferably 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 (302) is fed into the first chamber 101 also through a feeding duct D, with a little mass throughput smaller than one hundredth of the flow rate of the operating fluid expanding in the turbine at full power, in case of liquid phase feed, in order to avoid the fed fluid from affecting banefully the process taking place in the ORC cycle. The feed of the barrier fluid in D is operated by outside means, herein not described in detail. A higher flow rate of barrier fluid would subtract an excessive heat amount from the expanding fluid, such a heat not being available for use in the regenerator.

Also if the seal 11 would not be particularly effective (considering that in a mechanical seal there is usually a little leak flow) and a not-permissible part of the barrier fluid would cross the interface S2, this should not lead to a turbine damage since, as mentioned, the barrier fluid and the operating fluid are the same fluid and contamination is not possible. If the flow rate of the barrier fluid should succeed in crossing the seal 11, it is simply mixed together with the operating fluid doing the cycle. In case wherein the seal 10 is not effective, a flow rate of barrier fluids could move into the chamber 103 where there is also the oil used for lubricating the bearings 8.

In this circumstance, according to the present invention a treating device to treat the lubricating oil polluted by the barrier fluid is provided. Such a device is described referring to figure 7.

More in detail, the figure 7 shows the solution relating to figure 6, integrated in a plant provided with a turbine 1, equipped with a shaft 4 supported by the bearings 8. With the numeral reference 400 it is generically denoted the apparatus for lubricating and treating the polluted lubricating oil returning from the line C.

The 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.

Preferably, the collecting vessel 401 is kept pressurized through an apposite feeding line of pressurized gas, for example nitrogen or another inert gas. Such a line can be in turn fed by cylinders or other means. By maintaining the pressure of the collecting vessel 401, the risk of forming a virtually dangerous atmosphere in the vessel 401 itself is avoided, when the operating fluid is inflammable and in presence of leakage of operating fluid from the chamber 102 to the chamber 103, and from here to the collecting vessel 401. Another advantage of this solution is the reduction of oxidation phenomena of the lubricating oil. Figure 8 shows another example of treating plant 600 of the fluid withdrawn from drainages of the turbine 1. It is a particularly recommended plant in presence of a significant 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 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.

The same technical solution herein presented can be used also in arrangements of seals of "face to face" type, also if not shown. Furthermore, metal bellows can be used in place of the springs.

In another embodiment, the bearings 8 are lubricated with the operating fluid additivated with a suspension of solid lubricant, i.e. additivated with microparticles of a solid having good lubricating and antiscufff features between metal surfaces. For example, polytetrafluoroethylene PTFE particles are adapted for this purpose. A grain size adapted for PTFE particles is, for example, the following: the diameter of less than 10% of the particles is comprised between 0.2 and 1 micron, the diameter of 99% of the particles is shorter than 10 microns. The proportion by mass of particles with respect to the total content of the lubrication circuit is preferably comprised in the range 3% - 20%.

Also in this embodiment the collecting vessel 401 is preferably pressurized through an apposite feeding line of pressurized gas, for example nitrogen or another inert gas. Such a line can be in turn fed by cylinders or other means. By maintaining the pressure of the collecting vessel 401, the risk of forming a virtually dangerous atmosphere in the vessel 401 itself is avoided, when the operating fluid is inflammable. Another advantage of this solution is the reduction of oxidation phenomena in the operating fluid contained in the apparatus 400, which is in part sent to the turbine stages for the expansion.

In this way in the lubrication circuit there is substantially the presence of the operating fluid, the inert gas for pressurizing the vessel 401 and the solid lubricant in particles in suspension in the operating fluid.

Figure 9 shows an alternative embodiment of the lubricating and treating apparatus 400 provided with a system 700 for separating the particles of solid lubricant.

The leakage of small amounts of operating fluid from the chamber 102 to the chamber 103, and therefore from the collecting vessel 401, can be predicted. Therefore, a certain amount of fluid has to be extracted from the vessel 401 to balance this incoming leakage flow-rate and avoid the consequent level decrease in the vessel 401. The extracted fluid can be treated on the outside. Alternatively, the extracted fluid can be filtered in a treating and filtering unit 703 fed by a pump 702 and provided with a return line 704 for the fluid added of particles towards the vessel 401. The filtered and treated fluid is sent through the line 705 to the ORC process, preferably to the condenser 1000. On the line 705 a valve 709 is provided and controlled in relation to the level transmitted by the instrument LT, which unloads to the ORC process the convenient fluid amount to maintain the level in the vessel 401 inside values adapted for a well pump functioning.

One or more stirrers can be added in the vessel 401 to guarantee a uniform suspension of particles in the vessel 401 itself, and then to guarantee a proper lubricating power of the operating fluid sent to the bearings 8.

The feeding circuit of the bearings 8 can be provided with filters for intercepting possible foreign bodies and particles that are excessively big, but it has to allow the passage of particles of solid lubricant without clogging risks.

When the vessel 401 is pressurized with inert gas, the pressure in the environment of the bearings 8 is higher than the ambient pressure on the outside. An effective solution to avoid the leak of operating fluid to the outside is to provide the shaft 4 with a seal, for example a mechanical seal 903, and to arrange on the shaft 4, more on the outside, a second seal 900 preferably of the labyrinth type; the environment 901 between the two seals 900 and 903 is kept slightly depressed by an aspirator connected to the duct 902. The suctioned fluid is sent to a vent towards the atmosphere or to the apparatus 400.

The separation of particles of solid lubricant in the unit 703 can happen by simply filtering through fabric or felt, or else porous sintered product, in case with the aid of a valve play to achieve a sporadic reverse scavenging of the filter. Alternatively, a cyclonic static or rotating centrifugal separator can be used.

The apparatus 400, in case provided with the system 700 for separating the particles of solid lubricant if these particles are used in substitution of the lubricating oil, if necessary can be substituted or supplemented with a treating unit made according to the scheme of figure 8, but connected to the aeriform fluid above the open surface of the liquid contained in the vessel 401. In this way, the treatment is applied to a gas mixture and a possible operating fluid, with a modest concentration of oil droplets or particles of suspended solid lubricant. In this way the treatment can be more effective than starting from a mixture of oil and operating fluid having principally oil, or from a liquid strongly added with particles. In the treatment of an aeriform fluid, the pressure of inert gas in the vessel 401 can be maintained slightly sub-atmospheric (1-5 mm of water column), so that to avoid every leak of oil, in case polluted with operating fluid, to the outside.