Wet gas compressor and method

Abstract

A centrifugal compressor for processing a wet gas. The centrifugal compressor includes: a casing; and least one compressor stage comprising at least one impeller rotatingly arranged in the casing and provided with an impeller hub and a plurality of impeller blades, each impeller blade having a suction side and a pressure side. The at least one compressor stage comprises at least one droplet breaking arrangement configured for promoting breakup of liquid droplets flowing through the compressor stage.

Claims

1. A centrifugal compressor for processing a wet gas comprising a liquid phase and a gaseous phase, the centrifugal compressor comprising: a casing; at least one compressor stage comprising at least one impeller rotatingly arranged in the casing and provided with an impeller hub and a plurality of impeller blades, each impeller blade having a suction side and a pressure side; wherein the at least one compressor stage comprises at least one droplet breaking arrangement configured for promoting breakup of liquid droplets flowing through the compressor stage, the least one droplet breaking arrangement comprises droplet diverters arranged on the pressure side of the impeller blades, the droplet diverters imparting to liquid droplets moving along the pressure side of the impeller blades a speed component directed transversely to a main flow speed direction of the wet gas flow across the impeller, and the impeller hub comprises a plurality of grooves disposed thereon between consecutive impeller blades, the grooves being configured to direct the liquid droplets towards the pressure side of each respective impeller blade.

2. The centrifugal compressor according to claim 1, wherein the at least one droplet breaking arrangement is configured to alter a speed of the liquid phase with respect to a speed of the gaseous phase in the wet gas flowing through the at least one compressor stage.

3. The centrifugal compressor according to claim 1, wherein the at least one droplet breaking arrangement is configured to modify the speed direction of the liquid phase with respect to the speed direction of the gaseous phase.

4. The centrifugal compressor according to claim 1, wherein the droplet diverters are arranged at least along a radial extension of the impeller blades, between an impeller inlet and an impeller outlet.

5. The centrifugal compressor according to claim 1, wherein the droplet diverters are arranged at least at an impeller-outlet end of the impeller blades.

6. The centrifugal compressor according to claim 1, wherein the at least one droplet breaking arrangement comprises a variable impeller outer diameter.

7. The centrifugal compressor according to claim 6, wherein each impeller blade has a root portion, a tip portion and a trailing edge at an outlet of the impeller, the trailing edge being inclined radially inwardly from the tip portion to the root portion.

8. The centrifugal compressor according to claim 6, wherein: the impeller comprises an impeller shroud; the impeller shroud has a diameter larger than a diameter of the impeller hub; and the impeller blades have a trailing edge extending from an outer shroud edge to an outer hub edge, the trailing edge of the impeller blades being inclined towards an impeller axis from the impeller shroud to the impeller hub.

9. The centrifugal compressor according to claim 1, further comprising a plurality of compressor stages, each compressor stage comprising a respective impeller, wherein the at least one compressor stage is comprised of the droplet breaking arrangement is the most upstream one of the plurality of compressor stages.

10. The centrifugal compressor according to claim 9, wherein the impeller of the most upstream compressor stage has a diameter larger than the subsequent compressor stages.

11. The centrifugal compressor according to claim 1, further comprising a plurality of stator axial blades and a plurality of rotor axial blades arranged at an inlet of the impeller of the at least one compressor stage.

12. The centrifugal compressor according to claim 11, wherein the stator axial blades are arranged downstream of the rotor axial blades with respect to a direction of flow of the wet gas.

13. The centrifugal compressor according to claim 1, wherein upstream of the at least one compressor stage a vaned swirled inlet plenum is arranged.

14. The centrifugal compressor according to claim 1, wherein at the inlet of the at least one compressor stage a wet-gas flow swirling arrangement is provided, configured to generate a swirl in the wet-gas flow at an inlet of the compressor stage.

15. The centrifugal compressor according to claim 14, wherein the wet-gas flow swirling arrangement comprises a tangential wet-gas flow inlet.

16. The centrifugal compressor according to claim 1, further comprising a speed control system configured to control a rotation speed of the centrifugal compressor as a function of an amount of the liquid phase in a wet-gas flow delivered through the centrifugal compressor.

17. The centrifugal compressor according to claim 16, wherein the speed control system comprises a two-phase flow meter, configured for detecting the amount of liquid phase in a wet-gas flow delivered to the centrifugal compressor, and a controller configured for controlling the rotation speed of the centrifugal compressor based on the detected amount of liquid phase in the wet-gas flow.

18. The centrifugal compressor according to claim 17, wherein the controller is arranged for controlling the speed of a variable-speed electric motor driving the centrifugal compressor.

19. The centrifugal compressor according to claim 16, wherein the speed control system comprise a device for detecting a parameter which is a function of a torque applied to a compressor shaft, and a controller configured for controlling the rotation speed of the centrifugal compressor based on the parameter.

20. The centrifugal compressor according to claim 1, wherein the impeller blades have a trailing edge forming a first discharge angle on the pressure side of the blade and a second discharge angle on the suction side of the blade, the first discharge angle and the second discharge angle being different from one another.

21. A method of operating a centrifugal compressor for processing a wet gas, the method comprising: processing a wet-gas flow containing a liquid phase and a gaseous phase in at least one compressor stage comprising an impeller rotatingly arranged in a compressor casing, the impeller comprising an impeller hub and a plurality of impeller blades, each impeller blade comprising a suction side and a pressure side; directing liquid phase droplets towards the pressure side of each respective impeller blade by a plurality of grooves disposed on the impeller hub and between consecutive impeller blades; and breaking the liquid phase droplets flowing through the impeller by imparting to the liquid phase droplets moving along the pressure side of the impeller blades a speed component directed transversely to a main flow speed direction of the wet-gas flow across the impeller.

22. The method according to claim 21, further comprising altering a speed of the liquid phase with respect to a speed of the gaseous phase in the wet-gas flow being processed in the compressor stage.

23. The method of claim 21, further comprising modifying the speed direction of the liquid phase with respect to the speed direction of the gaseous phase.

24. The method of claim 21, further comprising generating a swirl in the wet-gas flow at an inlet of the impeller.

25. The method of claim 21, further comprising breaking up liquid droplets at an inlet of the impeller.

26. The method of claim 21, further comprising providing a vaned swirled inlet plenum at an inlet of the at least one compressor stage and generate a vorticity in the wet-gas flow processed in the compressor stage.

27. The method of claim 21, further comprising modulating a rotation speed of the compressor as a function of the amount of liquid phase in the wet-gas flow, reducing the rotation speed when the amount of liquid phase increases.

28. A centrifugal compressor for processing a wet gas comprising a liquid phase and a gaseous phase, the centrifugal compressor comprising: a casing; at least one compressor stage comprising at least one impeller rotatingly arranged in the casing and provided with an impeller hub and a plurality of impeller blades, each impeller blade having a suction side and a pressure side; wherein the at least one compressor stage comprises at least one droplet breaking arrangement configured for promoting breakup of liquid droplets flowing through the compressor stage, and the droplet breaking arrangement comprises a plurality of intermediate auxiliary blades, positioned between consecutive impeller blades, the intermediate auxiliary blades extending between an impeller inlet and a position between the impeller inlet and an impeller outlet, the intermediate auxiliary blades being shorter than the impeller blades.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

(2) FIG. 1 illustrates a schematic representation of a compressor arrangement according to the prior art, including a scrubber as describe here above;

(3) FIG. 2 illustrates a perspective cut-out view of a representative prior art centrifugal compressor as describe here above;

(4) FIG. 3 illustrates a simplified cross-section of the compressor of FIG. 2;

(5) FIG. 4 diagrammatically represents the principle of operation of some of the embodiments disclosed herein;

(6) FIG. 5 diagrammatically illustrates the break up process of large liquid droplets according to an embodiment of the invention;

(7) FIGS. 6 and 7 diagrammatically illustrate the way in which the liquid phase accumulates in a centrifugal compressor impeller in a cross-section and in a front view according to line VII-VII of FIG. 6, respectively;

(8) FIGS. 8, 9, 10, and 11 schematically illustrate embodiments of droplets breaking up arrangements;

(9) FIG. 12 illustrates a front view of a compressor impeller provided with grooves for promoting the collection of a liquid phase along the pressure side of the impeller blades according to an embodiment of the invention;

(10) FIG. 13 illustrates a schematic cross-section of two sequentially arranged stages in a centrifugal compressor according to one embodiment of the subject matter disclosed herein;

(11) FIGS. 14A and 14B illustrate a cross-section and a front view, according to line XIV-XIV, of an axial stator and rotor blade arrangement at the inlet of a compressor stage, according to one embodiment of the subject matter disclosed herein;

(12) FIGS. 15A and 15B show a schematic vector representation of the inlet wet-gas flow speeds and the effect of a swirl generation arrangement on the flow speed according to an embodiment of the invention;

(13) FIGS. 16 and 17 illustrate embodiments of swirl generating arrangements at the inlet of a compressor stage, or upstream of said inlet, e.g. at the inlet plenum;

(14) FIG. 18 illustrates a block diagram of a system for controlling the rotational speed of the compressor as a function of the amount of liquid phase in the wet-gas flow processed by the compressor according to an embodiment of the invention;

(15) FIG. 19 illustrates a diagram of rotation speed vs. liquid content;

(16) FIG. 20 illustrates a block diagram of a further embodiment of a system for controlling the rotational speed of the compressor as a function of the amount of the liquid phase in the wet gas flow;

(17) FIG. 21 illustrates a diagram of the rotational speed vs. torque in the system of FIG. 20.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(18) The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

(19) Reference throughout the specification to one embodiment or an embodiment or some embodiments means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase in one embodiment or in an embodiment or in some embodiments in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

(20) FIG. 4 schematically illustrates the principle underlying the operation of some of the embodiments described in the present disclosure. In FIG. 4 a compressor impeller for a centrifugal compressor is schematically illustrated. Reference number 100 designates the impeller as a whole. In this schematic representation the impeller 100 is a shrouded impeller. The shrouded impeller 100 comprised an impeller hub 103, an impeller shroud 105 forming an impeller eye 107, and blades 109 arranged between the impeller hub 103 and the impeller shroud 105. 111 indicates the impeller inlet and 113 indicates the impeller outlet, i.e. the impeller discharge. In other embodiments the impeller can be open, i.e. not provided with a shroud.

(21) The wet-gas flow entering the impeller inlet 111 contains droplets D as diagrammatically shown in FIG. 4. The droplets D represent the liquid phase of the wet gas. Reference V1 indicates the speed vector of the liquid phase, i.e. of the droplets D entering the impeller 100. Vg indicates the speed of the gaseous phase of the wet gas. Due to the higher inertia of the liquid phase, the speed V1 is usually slightly less than the speed Vg. When the gas flow enters the impeller 100, the speed difference increases due to the different inertia of the liquid phase and gas phase, respectively.

(22) The speed difference between the two phases is used to provoke or promote break-up of the liquid droplets and reduce the volume of each droplet, so that their potential erosion effect on the components of the compressor is substantially reduced. FIG. 4 schematically shows that at the impeller discharge side the difference between the liquid phase speed V1 and the gaseous phase speed Vg is strongly increased. Due to this speed difference, the droplets forming the liquid phase are broken-up as shown schematically by the smaller dimension of the outlet droplets (labeled d) vis--vis the inlet droplets D.

(23) FIG. 5 schematically illustrates possible mechanisms of droplet break up induced by the speed difference. On the right hand side of FIG. 5 three possible break up mechanisms are pictorially illustrated. The first break up mechanism is indicated as bag break up. The gaseous flow impacts a larger droplet D and deforms it like a bag as indicated in DX until the bag finally bursts forming a plurality of smaller droplets d.

(24) The second break up mechanism is indicated as stripping break up. The gaseous flow impacts the larger droplet D and flows there through stripping smaller droplets d out of the larger droplet D.

(25) The third breaking up mechanism, indicated as catastrophic break up. The gaseous flow impacts a larger droplet D and causes the latter to blow up into a plurality of smaller droplets d.

(26) According to some embodiments, at least the first impeller, i.e. the impeller of the first compressor stage (or the sole impeller, in case of one-stage compressor), is designed such as to improve or increase the droplet break up in the impeller, so that the dimension of the droplets flowing through the compressor is sufficiently small to avoid or limit erosive phenomena of the mechanical components of the compressor. In order to increase the droplet break up effect, measures are taken to modify or alter the speed of the liquid phase. It shall be understood that more than one impeller of the same multistage compressor can be designed to increase the droplet break up.

(27) FIG. 6 illustrates a schematic section along a plane containing the impeller axis. A single impeller blade 109 is illustrated in FIG. 6. The impeller blade 109 has a leading edge, or inlet edge 109A and a trailing edge, or outlet edge 109B. The impeller blade 100 develops from a root portion 103R, where the impeller blade 100 merges with the hub 103, towards a tip portion 109T. When the impeller 100 is a shrouded impeller, the tip portion 109T of the Due to the higher inertia of the liquid phase with respect to the gaseous phase, the liquid phase tends to accumulate in the area indicated with LH, on front surface of the hub 103, i.e. the surface of the hub 103 from which the blades 109 project.

(28) FIG. 7 illustrates a front view of the impeller 100, according to line VI-VI in FIG. 6. Each impeller blade 109 is schematically represented as a simple line, but it shall be understood that in actual facts the blades have a thickness, not represented in FIG. 7.

(29) In FIG. 7 the pressure side and the suction side of the impeller blades 109 are indicated as 109P and 109S, respectively. Due to the higher inertia of the liquid phase with respect to the gaseous phase, the liquid phase tends to accumulate in LB on the pressure side 109P of each impeller blade 109.

(30) The speed of the wet gas is not the same in the entire cross-section of a vane defined between two subsequent impeller blades 109. The gaseous phase has a higher speed and the liquid phase as a lower speed. In actual fact the flow speed is variable along the height of the vane and along the width of said vane, as indicated by the speed vectors schematically represented in FIGS. 6 and 7. The speed gradually diminishes moving from the tip region 109T towards the root region 109R when viewing the impeller in the cross-section of FIG. 6. Moreover, the speed reduces when moving from the suction side to the pressure side viewing the impeller in the front view of FIG. 7.

(31) The speed difference between the liquid phase and the gaseous phase is exploited to promote droplet break up. In order to have a sufficient break up effect on the droplets present in the wet-gas flow, a droplet breaking arrangement is provided in at least the first impeller of the centrifugal compressor. The droplet breaking arrangement can have different configurations and be based on different phenomena. Some possible droplet breaking arrangements will be disclosed here below. Each arrangement described and illustrated in the drawings adopts one out of several possible features and measures to promote droplet break up. As will become apparent from the following description and as those skilled in the art of compressor designing will understand, two or more of the simple droplet breaking arrangements disclosed herein can be combined to form a more complex and possibly more efficient droplet breaking arrangement.

(32) FIG. 8 schematically illustrates a first embodiment of a droplet breaking arrangement according to the present disclosure. FIG. 8 represents a front view according to the axis direction of the impeller 100. The impeller 100 comprises impeller blades 109. According to this embodiment, the outlet or trailing edge portion of each impeller blade 109 is shaped such that the outlet angle, i.e. the discharge angle on the pressure side 109P of the impeller blade 109 is different from the discharge angle on the suction side 109S. The discharge angle is defined as the angle formed between the radial direction and the direction tangent to the trailing or discharge edge of the blade 109. In FIG. 8 the discharge angle on the pressure side of the blades 109 is indicated as P and the discharge angle on the suction side of the blades 109 is indicated as S. The two angles are different from one another. The discharge angle represents the direction of the speed vector of the wet gas flowing out of the impeller 100. Consequently the mainly gaseous flow exiting along the suction side 109S of the impeller blade 109 has a speed Vg which differs in module and direction from the speed V1 of the liquid phase, which collects along the pressure side 109P of the blade 109. The module and direction differences between the two vector speeds enhance the break up effect on the liquid droplets.

(33) A different embodiment of a droplet breaking engagement is shown in FIG. 9. Here an impeller blade 100 is again shown in a front view. At least some, but possibly all, of the impeller blades 109 are provided with droplet diverters 120. These diverters can be in the form of projections extending from the respective impeller blades 109. Since, for the reasons discussed above, the liquid phase tends to accumulate on the pressure side 109P of the impeller blades 109, the droplet diverters 120 are arranged on the pressure side 109P of each impeller blade 109. As shown by way of example in FIG. 9, one or more droplet diverters 120 can be provided along the pressure side 109P of the impeller blades 109.

(34) When the droplets moving along the pressure side 100P of the impeller blade 109 impact against a droplet diverter 120, they are diverted from the pressure side 109P towards the center of the respective vane of the impeller 100. The speed module and speed direction of the droplets is modified. The droplets are caused to move transversely to the speed direction of the gaseous phase in the vane between the two consecutive impeller blades 109. The speed difference (module and direction) between the gaseous phase and the liquid phase causes droplet break up.

(35) A further embodiment of a droplet breaking arrangement is schematically shown in FIG. 10, which illustrates an impeller 109 in a section along a plane containing the axis A-A of the impeller 100. The radius RH of the impeller hub 103 in this embodiment is smaller than the radius RS of the impeller shroud 105. If the impeller 100 is not shrouded, i.e. if no impeller shroud 105 is provided, the radius RS will represent the largest radius of the impeller blade 109, i.e. the radial dimension of the radially outmost point or tip of the discharge or trailing edge 109B of the blade 109.

(36) The speed of the working medium flowing through the impeller 100 is determined by the speed of the impeller. The larger the impeller radius, the larger the discharge speed of the working medium. Since in the embodiment of FIG. 10 the radial dimension of the impeller 100 varies from the impeller hub to the impeller shroud, also the speed of the working medium at the impeller discharge side will vary from the impeller hub to the impeller shroud. More specifically, the speed of the working medium at the impeller discharge on the hub side will be smaller than the speed of the working medium at the impeller discharge in the area of the shroud. Since the liquid phase will tend to accumulate on the hub side, this difference in the radial dimension will provoke a speed difference between the liquid phase (speed V1) and the gaseous phase (speed Vg), the gaseous phase being accelerated to a substantially higher speed than the liquid phase. This speed difference provokes or enhances the droplet break up.

(37) FIG. 11 illustrates a further embodiment of a droplet breaking arrangement. FIG. 11 illustrates a front view of the impeller 100 provided with a plurality of impeller blades 109. The impeller blades 109 extend from the impeller inlet 111 to the impeller outlet 113. Between each pair of sequentially arranged impeller blades 109 at least one intermediate auxiliary blade 122 is provided. Each intermediate auxiliary blade 122 is shorter than the impeller blades 109. This means that the intermediate auxiliary blades 122 develop from the impeller inlet 111 to an intermediate position along the vane between the respective impeller blades 109, without reaching the impeller outlet 113. Liquid droplets or a liquid film collecting on the pressure side of the intermediate auxiliary blade 122 will be mixed in the main flow of the working medium provoking droplet break up as soon as said liquid phase moving along the pressure side of the intermediate auxiliary blades 122 reaches the trailing edge 122B of the respective intermediate auxiliary blade 122.

(38) It shall be understood that the four embodiments of droplet breaking arrangements described in connection with FIGS. 8 to 11 can be combined one with the other. For example, the arrangement of FIG. 8, based on a modification of the discharge angle so that the pressure side and the suction side of each blade have differing discharging angles, can be combined with the use of droplet diverters along the development of the impeller blades 109. The radial dimension difference between impeller hub and impeller shroud as disclosed with reference to FIG. 10 can also be combined with either one or the other or both of the arrangements of FIGS. 8 and 9 and in all said three arrangements intermediate auxiliary blades 122 can be additionally provided.

(39) In order to increase the efficiency of the droplet breaking arrangement illustrated in FIG. 8 it would be useful to collect the largest possible amount of liquid phase on the pressure side of the impeller blades 109. In FIG. 12 a possible embodiment of the impeller 100 is illustrated, which improves the behavior of the impeller in that respect. On the hub side of the impeller 100, i.e. along the surface of the impeller hub 103 facing towards the inlet side of the impeller, grooves 125 are provided. These grooves develop generally from the inlet toward the outlet of the impeller 100 and are inclined with respect to the radial direction so that they will end along the pressure side of the respective impeller blades 109. Droplets collecting on the hub side of the impeller 100 will thus be guided by the grooves 125 towards the pressure side 109P of the impeller blades 109 and collect thereon, where the most effective droplet break up arrangement can be provide, reducing the amount of liquid phase moving along the hub side surface of the impeller 100.

(40) FIG. 13 illustrates an embodiment in which two subsequently arranged compressor stages 130, 131, are designed with different radial dimensions. The first compressor stage 130 comprises a first impeller 100X and the second compressor stage 131 comprises a second impeller 100Y. The first impeller 100X has a radial dimension R1, greater than the radial dimension R2 of the second impeller 100Y of the second compressor stage 131. The two impellers rotate at the same angular speed, since they are supported on the same shaft. However, the peripheral speed at the outlet of the first impeller 100X is higher than the speed at the outlet of the second impeller 100Y due to the larger diameter of the first impeller with respect to the second impeller. Since droplet breaking up is mainly performed in the first compressor stage, designing the first compressor stage with a larger diameter will increase the efficiency of the droplet breakup. In fact, the speed difference between the liquid phase and the gaseous phase will be increased with increasing speed of the working fluid flowing through the compressor.

(41) Use of a larger first compressor stage can be combined with one or more of the droplet breaking arrangements disclosed above.

(42) In order to prevent the formation of a liquid layer at the inlet of the first compressor stage, according to possible embodiments an axial blade arrangement can be provided at the inlet of the first compressor stage. Such an embodiment is schematically shown in FIGS. 14A and 14B. Reference 100 again indicates the impeller of the first compressor stage. In front of the impeller inlet a set of stator blades 131 are arranged, fixed to the compressor casing 133. Upstream of the stator blades 131, with respect to the speed of the working fluid, a set of rotor blades 135 are arrange, said rotor blades 135 being constrained to the shaft 137 supporting the compressor impeller 100. FIG. 14B illustrates a front view according to line XIV-XIV of the set or rotor blades 135. The liquid droplets entering the compressor are mechanically broken up by the co-action of the stator blades 131 and the rotor blades 135. This breaking effect upstream of the first impeller can be useful to reduce the erosive effect of the droplets on the impeller eye and/or on the leading edge of the impeller blades of the first compressor impeller.

(43) According to a further embodiment of the subject matter disclosed herein, the erosion of the impeller eye in the first compressor stage due to the presence of liquid droplets in the working fluid can be reduced by acting upon the wet gas speed at the inlet of the first impeller. FIG. 15A illustrates diagrammatically the vector speeds of the impeller (speed U1) and of the wet-gas flow (C1). The vector W1 represents the relative speed of the wet gas with respect to the impeller. The higher the relative speed, the higher is the erosive effect of the liquid droplets on the surfaces of the impeller, specifically the impeller eye and/or the leading edges of the impeller blades.

(44) By introducing a swirl effect in the wet gas entering the impeller, the relative speed between the wet gas and the impeller will be reduced. This is shown schematically in FIG. 15B, where the same reference numbers are used to indicate the same speed vectors as in FIG. 15A. U1 again represents the speed vector of the impeller, C1 represents the speed vector of the incoming wet gas and W1 is the speed vector representing the speed of the wet gas relative to the impeller. By introducing a swirl component in the wet gas speed, represented by the vector S, the relative speed between the wet gas and the impeller, and therefore the erosive effect on the impeller, are reduced.

(45) This swirl effect can be introduced by using a tangential inlet as schematically illustrated in FIG. 16. The gas enters the first compressor stage with a speed direction which is non-orthogonal to the speed of the impeller, i.e. in a non-axial direction. This rotational motion is imparted by the spirally-shaped inlet channel 140 along which the wet gas is delivered into the first compressor stage.

(46) FIG. 17 illustrates a cross-section along a plane containing the axis of the compressor, of a different arrangement for generating a swirl effect in the wet gas flow. In this embodiment, an inlet duct 150 is provided upstream of the first compressor stage 130 where the first impeller 100 is arranged. An arrangement of fixed blades 152 is provided in the inlet duct 150. The fixed blades 152 are inclined so that a tangential speed component will be imparted to the wet gas entering the compressor stage 130.

(47) The erosion effect of the liquid phase contained in the wet gas increases with increasing compressor speed, i.e. the higher the compressor rotational speed, the higher is the risk of erosive damages caused by liquid droplets in the working fluid.

(48) According to further embodiment, in order to reduce the erosion effect of possible liquid droplets present in the wet-gas flow, the speed of the compressor is controlled such that the rotational speed of the impellers is reduced when the amount of liquid phase in the wet-gas flow increases.

(49) FIG. 18 illustrates a block diagram of a first embodiment of a system for controlling the compressor rotary speed as a function of the liquid content in the working fluid delivered to the compressor. In the schematic representation of FIG. 18 the compressor is indicated 200 as a whole. The compressor is driven into rotation by a mover, for example an electric motor 121. The electric motor 201 can be an electronically controlled, variable speed motor. A speed controller 211 can be provided for controlling the rotational speed of the electric motor 201 and of the compressor 200. A driving shaft 203 connects the electric motor 201 to the compressor 200. The wet gas is fed through an inlet duct 205. Along the duct 205 a two-phase flow meter 207 can be arranged. The two-phase flow meter 207 generates a signal which provides information on the amount of liquid phase flowing there through. The signal generated by the flow meter 207 is delivered (line 209) to the speed controller 211. The speed controller 211 in turn controls the speed of the motor 201 by reducing the rotational speed of the motor, and thus the rotational speed of the compressor 200, when the amount of liquid phase in the wet-gas flow delivered to the compressor 200 increases.

(50) FIG. 19 schematically illustrates a diagram of the angular speed of the compressor (on the vertical axis) as a function of the liquid phase amount (Lq) in the working fluid, which amount is reported on the horizontal axis. The rotational speed of the compressor is reduced when the liquid amount increases. In the schematic example of FIG. 19 the rotational speed of the compressor 200 varies in a continuous, non-linear manner. Different control functions can be used, for example a stepwise variation of the rotational speed rather than a continuous variation can be envisaged. Additionally, the inclination of the curve can be different and can be for example linear.

(51) FIG. 20 illustrates a block diagram of a different system for providing a speed control for the compressor, as a function of a parameter which is linked to the amount of liquid in the wet-gas flow delivered to the compressor. The same reference numbers indicate the same or equivalent parts as in FIG. 18. In this embodiment the amount of liquid is determined indirectly. The system is based on the recognition that the liquid phase present in the wet gas increases the torque which must be applied to the compressor rotor to maintain it into rotation. Therefore, an increasing amount of liquid phase in the wet-gas flow will increase the power needed to drive the compressor 200.

(52) The system shown in FIG. 20 is based on detection of the torque required to drive the compressor 200 into rotation. A torque meter 213 detects the torque applied by the motor 201 to the compressor shaft and the torque measured by the torque meter 213 is provided as an input signal to the speed controller 211. The signal can be conditioned before being delivered to the speed controller 211, if required. FIG. 21 illustrates the compressor rotational speed (on the vertical axis) as a function of the torque detected by the torque meter 213, reported on the horizontal axis (T). The rotational speed is controlled such as to be reduced when the measured torque increases, such increased torque being caused by an increased amount of liquid phase present in the wet gas delivered to the compressor 200.

(53) The control can be continuums as shown in FIG. 21 or step wise. The inclination and the shape of the curve can be different from the one shown in FIG. 21, for example a linear curve can be used.

(54) In further embodiments (not shown) different parameters can be used to control the rotational speed of the compressor as a direct or indirect function of the amount of liquid phase in the wet-gas flow. For example the current absorbed by the electric motor 201 can be used as a parameter, which is proportional to the torque required to drive the compressor into rotation, said torque being in turn proportional to the amount of liquid phase in the wet gas flow.

(55) In general terms, the speed of the compressor is controlled so as to decrease the speed if an increasing amount of liquid in the two-phase flow is detected. In some embodiments, a threshold can be provided, representing a limit amount of liquid in the wet gas processed by the compressor. If the threshold is not exceeded, the compressor will be driven at a standard speed. If the amount of liquid (directly or indirectly measured) exceeds the threshold, the speed can be modulated, i.e. decreased gradually, as a function of the detected parameter linked to the amount of liquid in the working fluid.

(56) While the disclosed embodiments of the subject matter described herein have been shown in the drawings and fully described above with particularity and detail in connection with several exemplary embodiments, it will be apparent to those of ordinary skill in the art that many modifications, changes, and omissions are possible without materially departing from the novel teachings, the principles and concepts set forth herein, and advantages of the subject matter recited in the appended claims. Hence, the proper scope of the disclosed innovations should be determined only by the broadest interpretation of the appended claims so as to encompass all such modifications, changes, and omissions. In addition, the order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments.