Abstract:
The invention relates to a wet gas compression system comprising a compact flow conditioner ( 21 ), intended to be placed below sea level in close vicinity to a well head or on a dry installation, said flow conditioner ( 21 ) being intended to receive a multi-phase flow through a supply pipe ( 11 ) from a sub sea well for further transport of such hydrocarbons to a multi-phase receiving plant, and where preferably avoid sand accumulation or remove as much sand as possible from the multi-phase flow, the gas (G) and the liquid (L) being separated in the flow conditioner ( 21 ) whereupon the separated gas (G) and liquid (L) are re-assembled and enters a multi-phase meter ( 46 ) prior to boosting by means of a compressor ( 22 ). In the combined multi-phase pump and compressor unit ( 22 ), as an integrated unit, comprises a combined multi-phase pump and compressor unit ( 22 ) functioning on the centrifugal principle, used for trans-porting liquid and gas from a flow conditioner ( 21 ) to a remote multi-phase receiving plant.

Description:
THE TECHNICAL FIELD 
       [0001]    The present invention relates to a system for wet gas compression, comprising a compact flow conditioner, a multi-phase flow meter and a downstream multi-phase compressor, preferably of the centrifugal compressor type, designed to be installed below sea level in the vicinity of a well head or on a dry installation, such as a platform or an onshore plant, the flow conditioner being designed to be supplied with multi-phase flow of hydrocarbons from a sub sea well, convey and preferably avoid accumulation or remove as much sand from said multi phase flow as possible. 
       BACKGROUND FOR THE INVENTION 
       [0002]    Future sub sea installations will require equipment for increasing the pressure in the well flow in order to achieve optimum exploitation of the reservoir. Use of machines which increases the pressure, contribute to a reduction of the down hole pressure in the well. This will then lead to an accelerating production from the reservoir, providing a possibility for maintaining a stable flow regime through the well casing, so that formation of fluid plugs is avoided. Prior art solutions comprise use of pumps for pumping liquids (water and raw oil, etc.), and mixing of liquid and gas where the liquid represents more than 5 volume %, while compressors which are able to pump wet gas, are under development and testing. Today, compressors have limited capacity, and the increase in pressure and power are at maximum limited to a few megawatts. Hence, there is a need for development of compressor systems which may handle large volumes of gas having in part substantial pressure differences and with power up to several tens of megawatts. 
         [0003]    The challenges to be met in this respect are amongst others transfer of large effect volumes below sea level; handling of sand, water, oil/condensate, and gas; together with possible pollution, such as production chemicals, hydrate inhibitors, pollutions from the reservoir; and uneven distribution of such matter over the life span of the field; liquid plugs during the start-up phase and transients, etc. 
         [0004]    Solutions exit for such systems. All the systems have a common denominator, namely their dependence of the functioning of a number of components, having to work together in order to obtain the required system functionality. Many of these prior art components are not qualified for use in connection with offshore exploitation of oil. 
         [0005]    GB 2 264 147 discloses a booster arrangement for boosting multi-phase fluids from a reservoir in a formation to a processing plant, where the boosting arrangement is placed in a flow line between the reservoir and the processing plant. The arrangement comprises a separation vessel for separation of liquid/gas, where said separation vessel has an inlet for supplying a mixture of oil and gas prior to further separate transport of the gas and the liquid. Further, the boosting arrangement comprises a motor driven pump, designed to lift the liquid fraction out of the scrubber and further to a jet pump, while the separated gas is allowed to flow through a separate pipe to said jet pump. From the jet pump, the mixed gas and liquid is then compressed to a processing plant at a substantially higher pressure than the pressure at the inlet to the separation vessel. 
       SUMMARY OF THE INVENTION 
       [0006]    The flow conditioner is designed for receiving a multi-phase flow of mainly hydrocarbons from one or more sub sea wells, to transport and secure an even flow of gas and liquid to the wet gas compressor and preferably to avoid accumulation or remove as much sand as possible from said multi-phase flow. The presence of a well flow liquid through the entire compressor shall prevent formation of deposits, increase the pressure conditions in the machine, secure cooling of the gas during the compression stage and reduce erosion, since the velocity energy from possible particles is absorbed by the liquid film wetting the entire surface of the compression circuit. 
         [0007]    An object of the present invention is to be able to handle large volumes of gas and accompanying smaller volumes of liquid, at partly substantial pressure differences between said two fluids. 
         [0008]    Another object of the invention is to increase available power of the system by more than tens of megawatts. 
         [0009]    A still further object of the invention is to reduce the number of critical components in the process system on the sea bed, and to make critical components more robust by introducing new technological elements. Such critical components or back-up functions are: anti-surge control valve, 
         [0010]    handling of the separation vessel liquid, 
         [0011]    pump, 
         [0012]    sand handling, 
         [0013]    cooler, 
         [0014]    volume measurements, and 
         [0015]    control system. 
         [0016]    A still further object of the invention is to improve the existing systems. 
         [0017]    The compressor remains a vital part of the system, handling the pressure increase in the gas as its primary function. The compressor is designed to be robust with respect to gas/liquid flow conditioning, redundancy, several levels of barriers against failure and simplified auxiliary systems. 
         [0018]    The compressor is installed in the vicinity of the sub sea production wells and shall deliver output to a single exit pipeline. 
         [0019]    The objects of the present invention are achieved by a solution as further defined in the characterizing part of the independent claim. 
         [0020]    Several embodiments of the invention are defined by the dependent patent claims. 
         [0021]    According to the invention, a combined pump and compressor unit for transportation of gas and liquid from the flow conditioner to a multi-phase receiving unit is provided, such combined pump and compressor unit forming an integral part of the flow conditioner. The pump and compressor unit comprises one or more impellers functioning on the centrifugal principle and will in the following be denoted as the wet gas compressor. Such unit shall be in position to pressurize a well flow comprising of gas, liquid and particles. The wet gas compressor may be powered by a turbine, but is preferably powered by an electromotor integrated within the same pressure casing as the compressor, where process gas or the gas from the well flow is used for cooling the electromotor and the bearings. The hot gas used for cooling the electromotor may be transferred to places where there is a need for heating. This may in particular be relevant for the regulating valves in the system, such as for example the anti-surge valve, in order to prevent formation of hydrates or ice in valves which normally are closed. 
         [0022]    An alternative embodiment of the wet gas compressor is to have a rotating and/or static separator for collecting the liquid in a rotating annulus, so that the liquid is given velocity energy which is transformed into pressure energy in a static system, such as a pitot, and that the pressurized liquid is fed outside and past the compressor part of the unit, and thereupon mixed again with the gas downstream of the unit. 
         [0023]    The flow conditioner may preferably include a built-in unit in the form of a liquid separator and a slug catcher upstream of the combined compressor and pump unit. Further, the flow conditioner may be oblong with its longitudinal length in the fluid flow direction. If there is a need for cooling the gas prior to the compressor inlet, the flow conditioner may also include a cooler. 
         [0024]    The function of such flow conditioner may be based on different principles. A technical solution is based on the feature that gas and liquid may be sucked up through separate ducts and mixed just upstream of the wet gas compressor. The liquid is sucked up and distributed in the gas flow by means of the venturi principle, where such effect preferably may be obtained by means of an constriction in the inlet pipe to the impeller, just upstream of the impeller, so that an increase of gas velocity may give sufficient under pressure, securing that the liquid is sucked up from the flow conditioner. Gas and liquid will thus form an approximate homogeneous mixture before reaching the first impeller. Corresponding functions may also be secured by using a flow conditioner where the liquid is separated out in a horizontal tank and where an increasing liquid height in the tank will secure more flow of liquid in the gas, since the flow area of the liquid is given by the holes in a vertically arranged perforated dividing wall. The mixing of gas and liquid as such will then be done in the flow conditioner and there will be a need for passing the gas and the liquid through a system for multiphase flow metering defining the volumes of gas and liquid passing through the inlet of the wet gas compressor. In addition to conventional control of anti-surge, such multiphase flow metering device must also secure slug control when the liquid increases substantially or is pulsating, this being detected by the multiphase meter, and a regulation valve is then opened (anti-surge valve) in order to secure recirculation of gas from the outlet back to the inlet of the wet gas compressor. If required, the control system secures that the revolutions per minute of the wet gas compressor is lowered. 
         [0025]    The most essential advantage of the present invention is that liquid and gas is given increased pressure in one and the same unit. Thus, there is no need of conventional gas/liquid separation and the liquid pump may be omitted. A compression system may hence be made substantially simpler and may be produced at a substantially lower cost. 
     
    
     
       SHORT DESCRIPTION OF THE DRAWINGS 
         [0026]    A preferred embodiment of the invention shall in the following be described in further detail referring to the drawings, where: 
           [0027]      FIG. 1  shows schematically a diagram of a sub sea system according to the prior art; 
           [0028]      FIG. 2  shows schematically a diagram of a sub sea system including a flow conditioner according to the present invention, based on the venturi principle; 
           [0029]      FIG. 3   a  shows schematically in further detail a unit according to the invention; 
           [0030]      FIG. 3   b  shows in enlarged scale the featured indicated within the ring A in  FIG. 3   a;    
           [0031]      FIG. 4  shows schematically a detail of an alternative embodiment of a wet gas compressor according to the present invention; 
           [0032]      FIG. 5  shows a generic sub sea system according to the present invention, where a multiphase meter is used for measuring the volume of gas and liquids at the inlet of the wet gas compressor, thus providing data used in a conventional anti-surge control system, and a recirculation loop (anti-surge line) and where the flow conditioner is based on separation the gas and liquid and providing a controlled re-entrainment of the liquid into the gas within the tank; 
           [0033]      FIG. 6  shows a detailed sub sea system according to the present invention where the wet gas compressor is powered by an electromotor and where the process gas is used for preventing formation of hydrate and ice downstream of the anti-surge valve; and 
           [0034]      FIG. 7  shows in a more detail a schematic disclosure of the flow conditioner used in the system shown in  FIGS. 5 and 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]      FIG. 1  shows schematically a system diagram of sub sea compressor system  10  according to a prior art solution. According to the prior art solution the system comprises a supply line  11  where the well flow either may flow naturally due to an excess pressure in the well through the ordinary pipe line  41 , when the valves  49  and  51  are closed, while the valves  52  and  54  are open, or through the compressor system when the valves  49  and  51  are open and the valves  52  and  54  are closed. 
         [0036]    When the well flow is fed into the compressor system  10 , the well flow is fed to a liquid scrubber or separator  12 , where gas and liquid/particles are separated. Up front of the inlet to the liquid separator  12 , a cooler  13  is arranged, cooling the well flow down from typically 70° C. to typically 20° C. before the well flow enters the liquid separator  12 . The cooler  13  reduces the temperature of the well flow so that liquid is separated out and the portion of liquid is increased. This reduction of mass flow of gas which is fed into the compressor  17  reduces the power requirement in the compressor  17 . The cooler  13  may in principle be placed upstream of the compressor  17 , as shown in  FIG. 1 . A corresponding cooler may possibly also in principle be placed downstream of the compressor  17 , thereby securing a temperature lower than the limiting temperature in the pipe line. 
         [0037]    The liquid separated out in the separator  12  is then fed through a liquid volume metering device  54  and into the pump  15 . The metering device  54  may alternatively be arranged upstream of the pump  15 . Further, the liquid from the pump  15  is returned back to the separator  12  in desired volume by regulating a valve  50 . Said circulation of liquid secures a larger operational range (larger liquid volumes) through the pump  15 . 
         [0038]    The gas separated out in the separator  12  is fed into a volume metering device  53  and then into the compressor  17 . The compressor  17  increases the pressure in the gas from typically 40 bar to typically 120 bar. Downstream of the outlet from the compressor  17  a recirculation loop is arranged, feeding the gas through a cooler  55  and back to upstream of the separator  12  when the valve (anti-surge valve  19 ) is opened. The cooler  55  may optionally be integrated in the inlet cooler  13  by feeding re-circulated gas back upstream of the inlet cooler  13 . Said re-circulation of gas increases the operational range of the compressor  17 , and ensure that the volume of gas through the compressor  17  is sufficient during trip and subsequent closing of the machine. The pressure increase in the liquid by means of the pump  15  corresponds to the pressure increase in the gas through the compressor  17 . 
         [0039]    The gas coming from the compressor  17  is then fed through a reflux valve  57 , while the liquid coming from the pump  15  goes through a non-return valve  58 . Gas from the compressor  17  and liquid from the pump  15  are mixed in a Y-joint  59 . The well flow goes further in the pipeline  20 , bringing the well flow to a multiphase receiving plant (not shown). When required, a post-cooler (not shown) may be incorporated. 
         [0040]      FIG. 2  shows a corresponding system according to the present invention. According to this solution, a multiphase flow from a well (not shown), including possible sand, is flowing through a supply line  11  into a flow conditioner  21  where the fluid flow from the well is stabilized by separating the liquid and the gas in said flow conditioner  21 . The liquid is taken from the bottom of the flow conditioner  21  through an outlet pipe  24 , while the gas is taken out at the top of the flow conditioner through an outlet pipe  23 . As a consequence of such solution an outlet pipe  16  with two separate pipes  23 , 24  formed as an integral gas/liquid pipe  16  in the form of separate pipes for gas and liquid, is connected to a combined pump and compressor  22 . The purpose of the combined pump and compressor unit  22  is to increase the pressure both in the gas and the liquid for further transport to a multiphase plant (not shown). This may be done, as indicated in  FIG. 3 , where gas and liquid is intended to be uniformly distributed and fed to a wet gas compressor  22  producing pressure increases in the gas and the liquid through same flow duct/impeller. Alternatively, this may be obtained as indicated in  FIG. 4 , where gas and liquid are separated at the inlet to the machine and where the gas fraction is fed to a standard gas compressor, while the separated liquid is given sufficient rotational energy so that the liquid may be transported out of the liquid chamber  44  with sufficient pressure to meet the pressure in the gas fraction at the exit from the compressor unit. 
         [0041]    The outlet pipe  16  is in the form of a gas pipe  23  communicating with the upper, gas filled part of the flow conditioner  21 , while an inner liquid pipe  24 , having smaller diameter than the outlet pipe  16   b , communicates with the lower, liquid filled part of the flow conditioner  21 . The gas pipe  23  ends as shown in  FIG. 3  in the inlet pipe of the compressor  22 . The inner liquid pipe  24  exits in a spray nozzle  23 ′, designed to distribute the liquid evenly into the gas. The gas pipe  23  is connected to the inlet flange on the compressor  22 . The liquid spray nozzle  23  is arranged at the inlet flange, close to the impeller  35  of the compressor. From the combined pump/compressor  22  the multiphase flow is exported through a pipe  20  to a multiphase receiving unit (not shown). The outlet pipe from the combined pump and compressor unit  22  is shown in  FIG. 2  and  FIG. 4 . 
         [0042]    From the bottom of the flow conditioner  21 , a second outlet pipe  25  for removal of sand is arranged, if required. When sand is to be removed, the combined compressor/pump unit  22  is preferably shut down. The pipe may for this purpose be equipped with a suitable valve  26 . The pipe is connected in such way that if it is required to empty sand from the flow conditioner  21 , the compressor is stopped, the valve (not shown) in the line  20  is closed and the valve  26  is opened while the pressure in the receiving plant is reduced. 
         [0043]    In the same manner as shown for the prior art shown in  FIG. 1 , a cooler  13  is incorporated upstream of the flow conditioner  21 . The purpose and temperatures are in essence corresponding to the purpose and temperatures for the prior art solution according to  FIG. 1 . 
         [0044]    As shown in  FIG. 2  an anti-surge valve may now be superfluous. A possible elimination of the anti-surge valve depends on the flow resistance characteristics of the pipeline and the characteristics of the compressor, and must be suitably adapted in each single case. The compressor characteristics have from recently performed analyses and tests shown to change for compressors which operate with two phases and because of internal re-circulation for motor cooling gas, so that the need for anti-surge flow rate is reduced. 
         [0045]    The flow conditioner  21  according to the present invention may preferably be oblong in the direction of flow with a cross sectional area larger than that of the supply pipe  11 , thus also contributing to enhanced separation of gas G and liquid L, and enhanced separation of possible sand in the flow. 
         [0046]    The lowest point in the compressor may preferably be the compressor outlet and/or inlet. This secures simple draining of the compressor  22 . 
         [0047]      FIG. 3   a  shows schematically details of the flow conditioner  21  according to the present invention, where gas G and liquid L firstly are separated in the separator  21  upstream of the impeller  35  of the unit. The liquid L is sucked up and delivered through the inlet pipe  24 , which at its one end is provided with a constriction or a spray nozzle  23 . The liquid L is distributed as evenly as possible in the gas flow G by means of the venturi principle, caused by the constriction in the supply line  36  of the gas pipe. As shown, the flow conditioner  21  may be oblong. At one end of the flow conditioner an inlet pipe  27  is arranged, connected to the supply line  11 . At this end a lead plate  28  is arranged in order to direct the fluid flow entering the flow conditioner  21  towards its bottom area. In the flow conditioner  21 , the liquid L and sand will flow down towards the bottom of the unit  21  due to gravity and reduction in flow velocity within the flow conditioner  21 , caused by the increased flow area, while the gas G remains in the upper part. Suitable, robust, insides  29  may be installed internally in the flow conditioner  21 . This is an arrangement which increases the separation efficiency and evens out the liquid/gas flow. An important aspect is that said insides  29  preferably also may comprise a cooler, allowing omission of a cooler placed outside the flow conditioner  21 , upstream of said flow conditioner  21 . 
         [0048]    According to the invention gas G is fed from the flow conditioner  21  to the combined pump and compressor unit  22  through an outlet pipe  23 , while the liquid L is sucked up through a pipe  24 . The gas G and the liquid L is simultaneously presses/pumped further to a multiphase receiving plant (not shown). 
         [0049]    The robust insides internally in the flow conditioner  21  may be in the form of a unit which optimizing slug levelling and forms basis for effective separation of liquid L and gas G, so the that liquid L and sand in a proper manner may be directed towards the bottom of the pipe. 
         [0050]    Collected sand may periodically be removed from the flow conditioner  21  by means of an output pipe  25  and suitable valve  26 . 
         [0051]    An alternative for the use of a cooler  13 , or as an addition, the compressor  22  may be installed at a distance from the well(s), forming sufficient surface area of the inlet pipe to achieve the necessary cooling of the fluid in the pipe by means of the surrounding sea water. This depends on a possible need for protection layer on the pipe and pipe dimension (need for trenching). 
         [0052]    If process requirements or regularity require more than one compressor  22 , then such compressors may be arranged in parallel or in series. If they are arranged in series, it may be possible to construct both compressors  22  so that the system characteristic always will be to the right of the surge line. Both compressors may still be a backup for each other. The need of the function of the anti-surge valve  19  will then diminish completely or partly. If it should be necessary to consider removing the need of an anti-surge valve  19 , this will mean that a start up of the compressor may be done subsequent to more or less pressure equalizing of the pipe line. Surge detection, i.e. the lower limit for the stable flow rate of the compressor, is implemented so that by detection of too low flow rate, the compressor is closed down in order to avoid damage from mechanical vibrations. In order to protect the compressor during suddenly, unintentional down closing, necessary protective valve securing quick pressure equalizing between the inlet and outlet of the compressors may be considered. 
         [0053]    The liquid L and particles may be transported out by means of the compressor  22  and a constriction  36  in the inlet pipe to the compressor  22  is arranged, so that liquid L is sucked up and evenly distributed to the compressor inlet. 
         [0054]      FIG. 3   b  shows in an enlarged scale the outlet end of the flow conditioner  21 , marked A in  FIG. 3   a . As shown in  FIG. 3   b  the gas G is fed from the conditioner  21  into a funnel shaped constriction  36  which leads to one or more impellers  35  which is brought to rotate by means of a motor  30 . Due to the funnel shaped constriction  36  and the shape of the opening in the impeller  35 , and also due to the rotation of the impeller  35 , the liquid is in addition sucked up through the supply pipe  24  and exit through the liquid spray nozzle  23 , formed of a constriction at the end of the supply pipe  24 . In the impeller  35  the mixture of liquid L and gas G is radially fed out through the diffuser  38  and out into an annulus  39  surrounding the impeller. From the annulus  39  the multiphase flow is forced out at a very high pressure through a pipeline (not shown) to a multiphase receiving station (not shown). At the end of the impeller  35  facing the funnel shaped constriction  35 , a seal  40  is arranged preventing unintended leakage of gas/liquid. Mechanical means such as bearings for the impeller  35 , suspension means of the supply pipe  24  etc. are not shown. The motor  30  and the compressor  22  may preferably be directly connected to each other and mounted in a common pressure vessel  37 , avoiding rotating seals towards the environment. The motor  30  may be powered by electricity, hydraulics or the like. 
         [0055]      FIG. 4  shows an embodiment where the liquid L is fed to a 0′th step comprising a spinning element  32 , hurling the liquid L out towards the periphery of the constricted pipe  36  and further to a rotating chamber  44 . Upstream of the rotating chamber  44  spinning elements  32  may be arranged, said spinning element either may be in the form of a stationary or rotating separator. The separating spinning element  32  separates the liquid L and the gas G, the gas G being brought to move ahead to the impeller  35  and the annulus  39  via a diffuser  38 , while the liquid L is brought to flow through the inlet  34  to the rotating chamber  44 . The inlet to the rotating chamber  44  may be provided both with internally arranged mean  32  for separation of the liquid phase with particles from the gas phase, and an annulus shaped supply duct  34  for transport of liquid in to the rotating chamber  44 . The liquid L in the rotating chamber  44  is pressed out of the rotating chamber  44  through the opening  45  in the combined outlet pipe/pitot tube  43 . The opening  45  is placed in such way that the opening is arranged in the section of the rotating chamber  44  being filled with liquid L. The exit pipe  43  for the liquid from the rotating chamber  44  is in fluid communication with the outlet  42  from the annulus  39  of the compressor. The purpose is to separate liquid L from the gas G just in front of the gas impeller  35  and to make the liquid rotate, i.e. to give the liquid L sufficient kinetic energy so that the kinetic energy may be recovered in a diffuser or a pitot tube and transform such energy into pressure energy. The connection between the rotating chamber  35  and the stationary unit  36  is provided with sealing means  40  allowing relative movement between the two parts  35 , 36 . For such solution the pressurized liquid L will bypass the compressor unit  35 , whereupon gas G and liquid L is re-mixed together downstream of the unit. 
         [0056]    As for the embodiment shown in  FIG. 3 , the annulus  29  according to the present invention is also provided with a diffuser  38 , arranged downstream of the exit from the impeller  35 . 
         [0057]    The rotating liquid chamber  44  will be selfregulating in that when liquid is increasingly filled into the liquid chamber  44 , the pressure at the liquid collection point will increase, thus forcing the liquid towards the compressor outlet. In such manner an increase in the liquid volume will also increase the pump capacity, so that the liquid level in the flow conditioner  21  is kept within acceptable limits. 
         [0058]    According to this embodiment the rotating chamber  44  rotates together with the impeller  35 . 
         [0059]      FIG. 5  shows a corresponding sub sea system  10  according to the invention. A well flow consisting of gas, liquid and particles arrives trough the pipe line  11 , of which a natural flow from the well is secured when the valve  13  is open and the valve  49  and  51  are closed. Production from the well may be increased by letting the flow from well flow in the sub sea system  10  by opening the valve  49  and the valve  51 , while the valve  13  is closed. Upstream of the inlet to the flow conditioner  21  a cooler  13  is arranged, cooling the well flow down from typically 70° C. to typically 40° C. The cooler  13  reduces the temperature in the well flow so that liquid is separated out and the liquid portion is increased. This increase in liquid volume may in certain cases result in increased effect consumption in the wet gas compressor  22 , so that the cooler  13  in such cases must be moved down-stream of the wet gas compressor  22  in order to secure temperatures lower than the limiting temperature of the pipeline. The cooler  13  may in principle be based on natural convection cooling from the surrounding sea water or based on forced convection. A multi-phase flow meter  46  is located between the wet gas compressor  22  and flow conditioner  21 . The multiphase flow meter  46  measures the volume of gas and liquid flowing into the wet gas compressor  22 . At substantial liquid rates or pulsating supply of fluid, this may be detected by the multiphase flow meter  46 , so that the regulating valve  19 , (the anti-surge valve) opens, securing increased volume of gas and a stable flow regime inside the machine. A gas output unit  47  downstream of the compressor secures that a very small volume of liquid circulates back to the wet gas compressor  22  through the recirculation loop  18 . Alternatively, a cooler  48  may be included in the recirculation loop  18 , so that it may be possible to operate the wet gas compressor, while the valves  49  and  51  are closed, i.e. no supply of well flow to the sub sea system  10 . It will also be possible to eliminate the cooler  48  by placing the recirculation loop  18  upstream of the cooler  13 . According to the present invention the wet gas compressor  22  functions as a combined pump and compressor so that the sub sea system  10  shown in  FIG. 5  is simplified compared to the conventional system described in  FIG. 1 . The wet gas compressor  22  shown in  FIG. 5  comprises one or more impellers based on the centrifugal principle, set to rotate by an integrated powering unit, such as for example a turbine or an electromotor. The presence of liquid through the wet gas compressor  22  may change the operation window (surge line) of the wet gas compressor  22  and it will be important to continuously monitor possible low vibration frequencies, less than the running frequency of wet gas compressor shaft, by applying a Fast Fourier Transform analysis of the vibration signal from the rotor, which also may be measured by means of an accelerometer on the exterior of the machine housing. In such way the sub-synchronous level of vibration (frequency of vibration lower than the frequency of rotation) may be used to open the control valve  19  in order to secure increased flow of gas at the inlet of the wet gas compressor  22 . Further, the presence of liquid at the inlet of the wet gas compressor  22  will increase the pressure ratio across the machine as a consequence of increased bulk density of the fluid. Erosion from particles is reduced since the liquid wets the rotating surfaces and prevents direct impact between the particles and the impeller. Still further, the liquid will distribute evenly in radial direction through an impeller based on the centrifugal principle, while the liquid at the same time is transferred into small droplets which easily may be transported by the gas flow. Such small droplets will at the same time secure a large interface area (surface area of contact) between the gas and the liquid so that the gas effectively may be cooled by the liquid during compression through the wet gas compressor  22 . Such cooling of the gas during compression will reduce the power requirements while the outlet temperature from the wet gas compressor  22  at the same time will be lower than for a conventional compres-sor. A formation of a surface layer in the compressor  17  will normally be experienced in a conventional compressor system shown in  FIG. 1 , caused by small volumes of liquid arriving with gas containing particles which adheres to the inner surfaces of the compressor  17  when the liquid is evaporated as a consequence of increased temperature across the compressor  17 . In a wet gas compressor  22  shown in  FIG. 5 , the volume of liquid will be significant and normally being in the range of 1-5 volume percentage at the inlet. This will secure that liquid is present across the entire machine, thus eliminating formation of a surface layer. 
         [0060]    A reflux valve  60  is placed downstream of the wet gas compressor  22 , preventing backflow of gas and liquid into the wet gas compressor  22 . The pressurized well flow is then directed back to the pipe line  20  through the opened valve  51  for further transport to a suitable receiving plant (not shown). 
         [0061]      FIG. 6  shows a sub sea plant  10  according to the present invention, based on the main components shown in  FIG. 5 , but shown in further detail. A well flow comprising gas, liquid and particles is directed into the sub sea plant  10  through the pipeline  11  and the main valve  49 , and then flowing through the pipe  61  which may be horizontal, but preferably slightly inclined so that a flow back towards the main line  11  is catered for during standstill. A vertical pipe  62  extends from the top of the horizontal pipe  61  and goes to a constriction  63  which preferably may be represented by an orifice plate or a valve. A minor part of the gas at the top of the horizontal pipe  61  will flow into the vertical pipe  62 , while the major part of well flow will continue to the flow conditioner  21  due to less flow resistance, and then to be mixed with the gas coming from the vertical pipe  62  downstream of the flow conditioner  21 . 
         [0062]    The flow conditioner  21  in  FIG. 6  is disclosed in more detail in  FIG. 7 . The pipe  61  leads to the flow conditioner  21 , which preferably is in the form of a cylindrical, elongated tank. The velocity of the gas is substantially reduced due to the increased area of flow together with use of a wall  64 , securing that liquid and particles are allowed to settle in the tank  21 . The bottom  65  of the flow conditioner  21  may be inclined downwards towards the outlet pipe  66  in order to secure that particles are not accumulated inside the tank  21 , alternatively the entire flow conditioner  21  may be inclined correspondingly with respect to a horizontal plane, thus meeting said function of the bottom  65 . Liquid and particles separated out in the tank  21  will meet a perforated wall  67  shown in more detail in the section A-A′ in  FIG. 7 , provided with a large number of small holes  69  through which the liquid will flow and then subsequently re-mix with the gas upstream of the outlet pipe  66 . Between the bottom of the flow conditioner  21  and the perforated plate  67  an opening  68  as shown in  FIG. 7  is arranged, intended to secure that sand and other particles do not separate out and accumulate or build-up in the tank  21 , but is forced out together with the liquid through the outlet pipe  66 . The function of the flow conditioner  21  is secured in that a quick change in liquid volume at the inlet pipe  61  in  FIG. 6  will be smoothened out due to a change in liquid level inside the tank  21 . As the level increases inside the flow conditioner  21  the liquid will flow through more and more holes  69  in the perforated wall  67 , thereby increasing the supply of liquid to the outlet pipe  66 . 
         [0063]    Gas and liquid coming from the vertical pipe  62  and the flow conditioner  21  in  FIG. 6  then flow through a vertical multi-phase flow meter  46 , metering the flow rates for gas and liquid. A wet gas compressor  22  in  FIG. 6  (horizontal in the Figure, but may have any orientation) which comprises one or more impeller based on the centrifugal principle, driven by an electromotor forming part of the wet gas compressor  22 , receives the well flow from a vertical pipe  70  from its bottom side. The pressure increases then in the well flow through the wet gas compressor  22  and is then fed into a vertical pipe  71  arranged towards the bottom side of the wet gas compressor  22 . The purpose of a vertical inlet pipe  70  is to secure good drainage of liquid from the wet gas compressor  22  during a stop, and correspondingly from the multi-phase flow meter  46  and the flow conditioner  21  with associated pipe system through the orifice plate  63  and down into the pipe  61 , ending into the main pipe  11 . In the same manner the liquid may also be drained out from the exit side of the wet gas compressor  22  during stop so that liquid from the outlet pipe  71 , the cooler  13 , gas exit unit  47 , reflux valve  60 , and valve  51  with associated pipes is flowing in a natural manner back to the main pipe  20 . The gas exit unit  47  secures that very small volumes of liquid are re-circulated back upstream of the multi-phase flow meter  46 . Such re-circulation loop  18  is normally used for increasing the volume of gas flow through the wet gas compressor  22  during stop or start of the wet gas compressor  22 , but also in situations where the multi-phase flow meter  46  detects unusually high level of liquid or possibly an unstable pulsating liquid rate. The regulating valve  19  will also open if the appearing vibration frequencies are lower than the running frequency of the wet gas compressor shaft, which could indicate that re-circulation of gas occurs in one or more of the stationeries or rotating parts inside the wet gas compressor  22 . According to prior art technology, process gas is used for cooling the electromotor and the bearings and is supplied from the wet gas compressor  22  in order to secure an over-pressure in these parts compared to the pressure at the inlet of the wet gas compressor  22 . Such cooling gas extracted from the wet gas compressor  22  may contain liquids and particles since the wet gas compressor  22  is boosting an unprocessed well stream mixture. Such particles being magnetic may deposit and accumulate inside the electromotor and in and on the bearings. It is therefore proposed to use an arrangement where permanent magnetic elements are incorporated into the pipe wall or by incorporating a separate chamber in order to collect such magnetic particles prior to feeding the process gas into the area of the electromotor and the bearings. In this manner deposits of magnetic particles in the electro-motor or the bearings used in the wet gas compressor  22  are avoided. The hot gas which has been used to cool the electromotor may be fed from the electromotor in a pipe  72  through a reflux valve  73  and into the pipe downstream of the regulating valve  19  (the anti-surge valve) in order to secure that formation of hydrates or ice are avoided during normal operation when the regulation valve is closed. Optionally the hot gas may be fed in to a heating jacket surrounding the regulation valve  15  in order to heat up the entire valve  15 , if necessary, prior to feeding the hot gas in downstream of the regulation valve  15 . The pressurized well flow will thus be sent from the sub sea plant  10  via the main pipe line  20  to a suitable receiving plant (not shown).