Abstract:
A gas compression device for compressing and separating a gas including a liquid phase and a gas phase, includes at least one inlet pipe for the wet gas, at least one compressed gas outlet pipe, a shaft and several compression stages. 
     The device includes at least one compression stage suited for separation of the liquid phase and of the gas phase, one or more pipes designed for discharge of an essentially liquid phase resulting at least partly from the separation performed in a suitable compression stage. 
     The device can be used for wet gas transportation in the petroleum field.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wet gas compressor comprising a first compression stage designed to prevent erosion by liquid droplets at the inlet of the impeller blades and to perform separation of the gas phase and of the liquid phase. 
     The compressor can comprise downstream several stages similar to the first stage or conventional impellers designed for compression of a dry gas. 
     The invention is intended for compression of a wet gas, i.e. a two-phase mixture whose gas and liquid volume flow rate ratio in the input conditions of the device (GLR) is higher than about 20. 
     The invention can find applications for production of a wet gas essentially consisting of hydrocarbons without prior gas processing, as well as in any process in the field of refining or chemistry using a gas compressor preceded by a sieve droplet separator or other. 
     BACKGROUND OF THE INVENTION 
     Various multiphase pump types allow compression of a two-phase mixture. However, rotodynamic type machines are limited to GLR ratios hardly higher than 20, and positive-displacement machines are relatively bulky for compression of a wet gas. 
     It is difficult to use conventional, centrifugal or axial gas compressors to compress a gaseous fluid comprising a liquid phase because of the erosion due to the liquid droplets on the blades of the impellers, of the embrittlement of the blades and of the rotor unbalance resulting therefrom. 
     A first primary separation stage (working under the action of the terrestrial gravity) is therefore more generally used upstream from a gas compressor for rough separation of the gas and of the liquid, then a second, secondary (for example sieve) separation stage is used for finer separation of the droplets contained in the gas. This layout also requires a single-phase pump for transfer of the liquid from the input pressure to the discharge pressure. These equipments are heavy and bulky. 
     The volume of the static separators can be reduced while maintaining the same degree of separation of the liquid droplets and of the gas, by generating high centrifugal forces produced only by means of the energy of the fluid (without external energy supply). This is, for example, the working principle of cyclone separators. 
     The volume of the separators can be reduced further yet, while maintaining the same degree of separation of the liquid droplets and of the gas, by generating very high centrifugal forces produced from an external energy (separator referred to as dynamic separator). It is for example the working principle of the dynamic separator described in the Bertin patent WO-87/03,051. While this separator has the advantage of being relatively compact, it constitutes a second rotating machine when mounted outside the compressor, and it reduces the number of impellers of the compressor by about 30% when mounted inside the compressor. 
     SUMMARY OF THE INVENTION 
     The object of the invention is a wet gas compression device that overcomes the drawbacks of the prior art and notably limits erosion caused by droplets at the stage inlet. 
     The wet gas compressor according to the invention comprises one or more compression stages suited to separate the liquid phase from the gas phase, to limit erosion due to droplets at the stage inlet and to compress at least the gas phase, the liquid phase being pressurized. 
     The invention also consists in associating a compression section suited to separate the liquid phase and the gas phase with a compression section suited for a dry gas. 
     The invention is intended for compression of a wet gas, i.e. a two-phase mixture whose gas and liquid volume flow rate ratio (GLR) in the input conditions of the device is higher than about 20. 
     What is understood to be a &lt;&lt;dry gas &gt;&gt; hereafter is a gas containing liquid droplets whose diameter is below 10 microns and consequently generating only a very low erosion at the level of the impeller blades. 
     The present invention relates to a wet gas compression device, said gas comprising a liquid phase and a gas phase, including: 
     at least one inlet pipe for said wet gas, 
     at least one outlet pipe for the compressed gas, 
     a shaft, 
     several compression stages, 
     at least one compression stage suited to limit erosion and to separate the liquid phase from the gas phase, 
     one or more pipes designed for discharge of an essentially liquid phase resulting at least partly from the separation performed in a suitable compression stage. 
     The device is characterized in that the suitable compression stage comprises for example an inlet line, an impeller and an outlet line, the inlet and outlet lines allowing separation of the liquid phase and of the gas phase. 
     The inlet line comprises for example two substantially rectilinear and parallel walls, the walls being respectively extended by two curved walls having a radius of curvature selected to generate a centrifugal effect and the wall is provided with a means allowing passage of the liquid from the outer wall to the inner wall. 
     The outlet line comprises for example a return channel including at least one collecting channel, and at least one means allowing passage of the liquid into the collecting channel, the means being placed on one of the walls at the diffuser outlet. 
     The device can comprise an inlet line and an outlet line having the aforementioned characteristics. 
     At nominal delivery, the inlet angle α 2  of the impeller blades is approximately equal to the angle α 1  of the relative gas velocity V r,g  and, on the other hand, the angle β of the absolute gas velocity V a,g , with the driving velocity V e , is so determined that the relative velocity of the droplets V r,l , is at least twice as low as that of the gas V r,g . 
     At least one of the suitable compression stages can be the inlet stage of the compression device. 
     The device can comprise at least one means for collecting the essentially liquid phases separated in the suitable compression stage(s), the collection means being connected to the pipes designed for discharge of the essentially liquid phase and to the outlet pipe. 
     The present invention also relates to a wet gas compression system. It is characterized in that it comprises at least one compression device and at least one device allowing partial separation of the liquid phase, upstream from the compression device. 
     Using the compression device or the compression system will be advantageous for production of a wet gas in the petroleum or refining field. 
     The compressor according to the invention notably allows to: 
     reduce the power absorbed and the number of machines in comparison with production by means of rotodynamic multiphase pumps, 
     reduce the bulk and weight in comparison with production by means of positive-displacement multiphase pumps, 
     reduce the number of equipments in comparison with conventional production comprising single-phase separators, compressors and pumps, 
     use a larger number of compression impellers in comparison with a wet gas compressor including a dynamic separator in the compressor, as described in patent WO-87/03,051. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the method according to the invention will be clear from reading the description hereafter of a non limitative embodiment example, with reference to the accompanying drawings wherein: 
     FIG. 1 schematizes the working principle of the wet gas compressor with first stages fulfilling both compression and separation functions, the next stages consisting of conventional rotor and stator elements, 
     FIGS. 2 and 3 schematize radial and axial views of an example of a suitable stage (consisting of the stator inlet channel, an impeller and the stator outlet channel), 
     FIGS. 2A,  2 B,  2 C and  2 D show in detail another embodiment example for the inlet and outlet channels of a stage described in FIG. 2, 
     FIG. 4 schematizes (with a view to droplet erosion limitation) the absolute and relative velocities of the liquid and gas phases at the inlet of the first compression stage(s) also acting as a separator, 
     FIG. 5 schematizes a valiant of FIG. 1 with, upstream, a cyclone separation system allowing to reduce the number of stages acting both as compression and separation stages. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematizes an embodiment example of a compression device designed for wet gas or wet gas compressor. 
     The compression device allowing to raise the pressure of the wet gas comprises a casing  1 , one or more compression stages  2  suited for a wet gas, one or more compression stages  3  suited for a dry gas, and a shaft X. The wet gas compression stages thus define a wet gas compression section followed by a dry gas compression section consisting of the dry gas compression stages. 
     A stage suited for wet gas compression (FIG. 2) comprises for example an inlet channel  30  comprising one or more guide blades, an impeller  35  placed downstream from the inlet line and an outlet line or stator line. The impellers are mounted on shaft X. 
     Wet gas compression stage(s)  2  are suited to separate the liquid phase from the gas phase and simultaneously to compress the gas phase and to pressurize the liquid phase. The inlet line and the outlet channel therefore have specific characteristics allowing to obtain the desired result, certain embodiment examples thereof being illustrated in FIGS. 2,  2 A to  2 D and  3 . 
     Casing  1  is provided with several pipes allowing delivery or discharge of the various fluids as shown in FIG.  1 : 
     a wet gas inlet pipe  4 , 
     a compressed dry gas discharge pipe  5  or discharge pipe (FIGS.  1  &amp;  5 ), 
     a pipe  6  (FIG. 5) acting as a collector for the liquid phase separated at the level of the compression stage(s) suited for the wet gas, 
     one or more pipes  6   i  for discharge of the liquid or of an essentially liquid phase separated at the level of a suited compression stage, flowing for example into collector pipe  6 . 
     The device is possibly provided with a pump or with an ejector  8  for transfer of the liquid phase or of the essentially liquid phase from pipe  6  to discharge pipe  5  of the compressor. An ejector is preferably used in cases where the liquid circulating in pipe  6  contains gas. A pump or an ejector can be used when the liquid circulating in pipe  6  contains no gas. The pump can be driven by shaft X of the compressor by means of a gear drive or any other means known to the man skilled in the art. 
     Shaft X is possibly provided with a means allowing to determine its rotating speed N. 
     The compressor is possibly provided with pressure detectors upstream from the inlet and downstream from the discharge end of the compressor. It can also be equipped with a means allowing to measure the flow of gas, for example located immediately downstream from the discharge end of the compressor for measurement under single-phase flow conditions. 
     The compressor is possibly provided with an antipumping protection known to the man skilled in the art. 
     The wet gas compression device can also be provided upstream with a liquid proportion measuring system so as to allow protection of the compressor in case of a sudden and considerable liquid inflow. 
     All the fluid transfer (pump and ejector), measuring and machine protection (antipumping, liquid proportion measurement) means are known to the man skilled in the art. 
     The first compression stage(s) suited for wet gas are designed so as to limit erosion on the impeller blades by the liquid droplets contained in the gas, by limiting the relative velocity of the droplets in relation to the impeller blades. 
     FIGS. 2 and 3 (radial section in the plane of the impeller) schematize an embodiment example of a first compression stage fulfilling simultaneously the aforementioned erosion limitation, liquid phase and gas phase separation, and compression functions. These figures show how the relative velocity of the droplets is decreased by reducing, at the impeller inlet, their distance to the axis of rotation. 
     The essentially gaseous fluid containing liquid droplets is fed into the first compression stage by means of inlet channel  30  (Figures) delimited by two substantially rectilinear and parallel walls  31  (C-MD),  32  (A-A′). Walls  33  (D′-E) and  34  (A′-B) form an extension of these two walls respectively. Walls  33  and  34  have a radius of curvature &lt;&lt;r&gt;&gt; selected to generate a centrifugal force that will allow separation of the liquid phase and of the gas phase. Wall  31  is provided with a means whose function is to allow passage of the liquid phase to wall  34  as described hereunder. This means can be an extension of wall  31  up to a salient point &lt;&lt;s&gt;&gt; (FIG. 2) or a gutter &lt;&lt;g &gt;&gt; (FIGS. 2A to  12 D) with a shape suited for transfer of the liquid phase from outer wall  33  to inner wall  34 . 
     In the description hereafter, the expression &lt;&lt;inner wall&gt;&gt; ( 34 ,  41 ) refers to the wall of the inlet channel that is closer to shaft X and &lt;&lt;outer wall&gt;&gt; ( 33 ,  40 ) refers to the wall that is farther from this shaft. 
     The wet gas flows through inlet line  30  as described hereafter. 
     The essentially gaseous phase containing liquid droplets is centrifuged in the curved part of the inlet line delimited by walls  33  and  34 , which is contained between points A′ and D′ and E, B. 
     As a result of centrifugation, these liquid droplets settle on curved inner wall  34 . 
     The liquid phase flowing down wall  31  in the form of a liquid film is carried along by the gas phase: 
     to salient point &lt;&lt;s&gt;&gt; (FIG. 2) from which it comes off in the form of droplets prior to being transferred to wall  34 , or 
     in gutter &lt;&lt;g &gt;&gt; (FIGS. 2A to  2 B) through which it flows onto inner wall  34 . 
     The liquid film present on wall  34  comes off at point B as a result of the gap existing between fixed inlet channel  30  and rotating impeller  35  in the form of liquid droplets. 
     These droplets enter impeller  35  located downstream from the inlet channel at the point where the distance to the axis of rotation is the shortest and consequently where the peripheral speed of the impeller is the lowest. 
     Impeller  35  is a conventional radial impeller. During rotation, the liquid and gas phases are centrifuged from inlet FG of the impeller to inlet IH of the stator channel or outlet channel located downstream from impeller  35 . 
     The outlet channel comprises a diffuser  36 , a curved channel  37  and a return diaphragm  38 . 
     Curved channel  37  is suited for separation of the liquid phase and of the gas phase. It comprises a collecting channel  39  and a means as described before, for example a salient point &lt;&lt;s&gt;&gt; (FIG. 2) or a gutter &lt;&lt;g&gt;&gt; (FIGS. 2C to  2 D), positioned at the level of wall  41 , for example at the diffuser outlet, allowing passage of the liquid phase into collecting channel  39 . 
     The gas phase and the liquid phase flow as follows at the level of the outlet channel: 
     the liquid phase dispersed in the gas phase is centrifuged at the outlet of diffuser  36  in the axial plane in the direction of collecting channel  39 . As a result of the movement of the gas in the radial plane, the liquid undergoes a tangential movement in channel  39  in the direction of rotation of the impeller. This rotating movement in the axial plane allows the liquid to remain in collecting channel  39 , 
     the gas phase of lower density continues to flow through radial return diaphragm  38  towards the second compression stage, 
     the liquid flowing partly on the walls of the diffuser: 
     for wall  40 , directly after flowing down over the length thereof, and 
     for wall  41 , after coming off of the liquid in the form of droplets at salient point &lt;&lt;s&gt;&gt; (FIG.  2 ), or after flowing through gutter &lt;&lt;g&gt;&gt; (FIGS. 2C,  2 D), flows into collecting channel  39 . 
     Collecting channel  39  is provided with one or more pipes  42   j  (FIG. 3) connected to pipe  6   i . These pipes are for example equipped with means allowing to control the flow of liquid to be discharged, such as a plate  43  provided with one or more orifices  44 . Orifices  44  are oversized so as to provide total discharge of the liquid as well as partial discharge of the gas in order to prevent obstruction of channel  37  by the liquid phase. 
     Such a compression stage can be designed to prevent, downstream from the stator outlet channel, carry-over of droplets with a diameter above about 10 microns and consequently to allow the use, downstream, of impellers suited for compression of a dry gas. However, a second stage similar to the first stage can be used downstream from the first so as to improve dehydration of the gas. 
     FIG. 4 shows, at point G and in the direction of the absolute velocity of the gas V a,g , the outlet of the stator blades  46  and the inlet of the rotor blades  45  (of the impeller) of a compression stage suited for separation of the phases and for limitation of the erosion caused by the droplets at the inlet of the rotor blades. 
     FIG. 4 shows the triangle of velocities relative to the gas phase, at the inlet of the blades of impeller  35  at point G (FIG.  2 ). The inlet angle α 2  of rotor blades  45  is so selected that the angle α 1  of the relative velocity with the blades has a minimum value (or even zero at nominal delivery) so as to minimize incidence losses. The triangle of velocities relative to the liquid phase is not shown at point G, the most part of the liquid flowing down a cylindrical envelope of a radius corresponding to that of point F (FIG.  2 ). 
     FIG. 4 shows the triangle of velocities relative to the gas phase (solid line) and that relative to the liquid phase (dotted line), at the inlet of blades  45  of the impeller at point F (FIG.  2 ). The inlet angle β of rotor blades  45  is so selected that the angle of the relative velocity with the blades has a minimum value (or zero at nominal delivery) so as to minimize incidence losses. The angle of the absolute gas velocity V a,g  with the driving velocity V e  is so selected that the relative velocity V r,l  of the droplets is considerably lower than that of the gas, V r,g  (by half for example). Reduction of the droplets velocity is facilitated by the driving velocity reduction at point F in relation to point G (in the ratio of the distances to the axis of rotation). 
     The local relative velocity V r,l  of the droplets in relation to the impeller blades is determined by the absolute velocity V a,g  of the gas phase, the slippage between the gas phase and the droplets, the orientation of the absolute velocity of flow and driving velocity V e . 
     The value of the slippage between the phases can be obtained by means of correlations or more precisely from a two-phase three-dimensional calculation code, to both methods being known to the man skilled in the art. 
     The allowable velocity of impact of the droplets on blades  45  is determined according to the diameter of the droplets, to the material that constitutes the blades or the material deposited on the material constituting the impeller blades, and to the rate of erosion that should not be exceeded. The acceptable rate of erosion is a data that is specified according to the minimum production time and to the conditions of maintenance of the machine. 
     FIG. 5 shows another embodiment variant where separation is not entirely performed by the stages fulfilling both compression and separation functions, but partly upstream from the compressor with, for example, a cyclone separator  60 . 
     In this example, the wet gas is fed into separator  60  by means of pipe  61 . At the outlet of separator  60 , the gas having a lesser degree of moisture than that entering pipe  61  is discharged through pipe  62  to the compressor inlet, whereas the liquid is discharged through pipe  63  to collector  64 . 
     At the outlet of the first stage serving both as a compression and a separation stage, the mixture consisting of liquid and gas is discharged through pipe  6 i to a separator  65 , whereas the gas containing droplets of very small diameter is sent to the stages located downstream and suited for compression of a gas containing droplets whose diameter does not exceed 10 microns. 
     At the outlet of separator  65 , the gas is discharged through pipe  66  at the level of pipe  62  to the compressor inlet, whereas the liquid is discharged through pipe  67  to collector  64 . 
     The device is possibly provided with a pump or an ejector  68  allowing transfer of the liquid phase from collector  64  to discharge pipe  5  of the compressor. 
     Separators  60  and  65  are possibly provided with liquid level detectors, level control valves situated in pipes  67  and  63 , and a level control system operating these control valves. 
     A comparison with multiphase pumping production is given in the tables hereunder. 
     The results have been obtained with the following comparison basis 
     molecular mass of the gas: 25 
     compression ratio (discharge and suction pressure ratio): 3 
     inlet temperature: 40° C. 
     The tables hereunder give the number of impellers and of pump barrels required in the case of multiphase pumping production, the number of impellers and of compressors required under the above-mentioned conditions for the compression system according to the invention being 7 and 1 respectively. 
     Case GLR=40 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Suction pressure-MPa 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                 Number of multiphase impellers 
                 43 
                 50 
                 54 
                 57 
               
               
                   
                 Number of multiphase pumps 
                 3 
                 4 
                 4 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     Case GLR=100 
     
       
         
               
               
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Suction pressure-MPa 
                 1 
                 2 
                 3 
                 4 
               
               
                   
                 Number of multiphase impellers 
                 57 
                 64 
                 66 
                 68 
               
               
                   
                 Number of multiphase pumps 
                 4 
                 5 
                 5 
                 5 
               
               
                   
                   
               
             
          
         
       
     
     These tables show that the number of multiphase pumps increases both with the GLR and with the suction pressure, whereas the device according to the invention to consists of a single gas compressor in the previous example.