Abstract:
The invention relates to a method for operating a compressor unit, in particular for underwater operation. The aim of the invention is to minimize the risk of gas hydrate formation during the compression process. To reduce said risk, according to the method components of the compressor unit are supplied with anti-freeze and/or anti-freeze is injected into the flow path of the conveyed medium to be compressed. The invention also discloses a compressor unit, which operates according to said method.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2007/052755, filed Mar. 22, 2007 and claims the benefit thereof. The International Application claims the benefits of european application No. 06006071.2 filed Mar. 24, 2006, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a method for operation of a compressor unit, in particular for underwater operation. The invention also relates to a compressor unit, in particular for underwater operation, comprising a compressor and an electric motor, which compressor unit has a housing with an inlet and an outlet for the pumping medium, with a rotation axis about which a rotor of the compressor unit can rotate. 
     BACKGROUND OF THE INVENTION 
     Recent developments in the field of compressor design have also been concentrated on undersea arrangements of large compressors which are intended to be used for the pumping of natural gases. 
     Because of the particular operating conditions, in particular because of the greatly restricted accessibility both for maintenance purposes and by means of supply lines, the specialists are confronted with major requirements. The relevant environmental regulations forbid any exchange of substances between the equipment to be installed and the surrounding sea water. Furthermore, sea water is an aggressive medium and extreme pressure and temperature conditions can be found at the various depths in the sea. A further requirement is that the equipment should on the one hand have an extremely long life and on the other hand must be designed to be virtually free of maintenance. An additional exacerbating factor is not-inconsiderable contamination of the medium to be pumped which in some cases is chemically aggressive. 
     A compressor unit of the abovementioned type has already been disclosed in international patent application WO 02/099286 A1. With the aim of simplification, without any compromises, in order to reduce the maintenance effort, and of achieving a long life at the same time, this document proposes that the compressor rotor be formed integrally with the motor rotor and be mounted at each of the ends by means of just two radial bearings. 
     In addition, it is known from European patent application EP 1 074 746 B1 for a turbocompressor to be equipped with three radial bearings, with the motor rotor being connected to the compressor rotor by means of a coupling. 
     WO 2005/003512 A1 has already disclosed a compressor unit for under-sea compression, to which an automation unit is connected by means of special connectors which are suitable for under-sea use. In addition GB 370 003 A discloses the injection of an antifreeze during the compression of air. 
     The compression of fluids close to the freezing point may be problematic. When natural gas is being pumped, the development relating to the formation of gas hydrates results in considerable problems. Gas hydrates are inclusion compounds which are similar to ice and in which small gas molecules, for example noble gases and various natural gas components, are surrounded in a cage of water molecules. Hydrate formation must be expected even with small amounts of liquid water and at temperatures of, for example, 10° C. The major gas catastrophe in the year 1988 on the Norwegian North Sea drilling rig Piper Alpha was supposedly due to such hydrate formation. Considerable additional operation costs are also incurred in natural gas pumping as a result of gas hydrate deposits, since they are deposited in pipelines, blocking them. 
     SUMMARY OF INVENTION 
     The invention is based on the object of providing a method for operation of a compressor, and a compressor unit, which very largely minimizes the risk of gas hydrate formation, for example when pumping natural gas under the sea. 
     The invention solves the problem by proposing a method for operation of a compressor unit, and a compressor unit as recited in the claims. The dependent claims, which respectively refer back to them, contain advantageous developments of the invention. 
     One particular advantage of the invention is the reliable protection against hydrate formation, as a result of the injection of the antifreeze. This not only allows protection of susceptible components of the compression unit but also of the entire pumping path, starting from the point at which the pumping medium is injected to the subsequent separation point. The method is also particularly advantageous because separation of undesirable additives is carried out in any case during the course of the chemical treatment of natural gases in a base station which is adjacent to the compressor unit after a pipeline. The resultant operational reliability is expressed both in higher availability of the compressor and in a high degree of safety against blocking hydrate formation in the pipeline which is connected to the compressor unit. 
     The antifreeze can be injected in the intake connecting stub, or directly in the compressor. Application of antifreeze to components of the compressor unit is particularly expedient for the bearings, the electric motor and other moving parts. If there is a particular risk of hydrate formation in the overflow area of individual compressor stages, antifreeze can also expediently be injected here. The primary field of application of the invention is the pumping of natural gas, since the risk of the formation of gas hydrates is relatively high here. 
     In particular, various alcohols make it possible to ensure protection against freezing of the gases. The injection of methyl ethylene glycol is worthwhile both for financial and technical reasons. 
     A somewhat more economic variant of obtaining safety against hydrate formation is to inject antifreeze at the critical points in the compressor unit before the compressor unit is started, in particular at the points mentioned above. One advantageous development of the invention provides that an amount of antifreeze is injected at the sensitive points in the compressor unit before each planned stop of the machine. 
     It is particularly expedient to use the antifreeze both before each start and before each machine stop. In the case of emergency stopping or tripping of the compressor unit, the primary factor of interest is to stop the machine as quickly as possible, so that it may generally not be possible to previously inject the antifreeze. Another possibility is to cause the antifreeze to be injected at the same time that the machine stop is initiated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following text using one specific exemplary embodiment and with reference to the drawings. The illustrated embodiment should be regarded only as an illustration, as an example of the invention. In the FIGURE: 
         FIG. 1  shows a schematic illustration of a longitudinal section through a compressor unit according to the invention and the major adjacent modules, which is operated using the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows, schematically, a section along a compressor unit  1  according to the invention which has, as major components, a motor  2  and a compressor  3  in a gas-tight housing  4 . The housing  4  accommodates the motor  2  and the compressor  3 . The housing  4  is provided with an inlet  6  and an outlet  7  in the area of the junction between the motor  2  and the compressor  3 , with the fluid to be compressed being sucked in through the inlet  6  by means of a suction connecting stub  8 , and with the compressed fluid flowing out through the outlet  7 . 
     The compressor unit  1  is arranged vertically during operation, with a motor rotor of the motor  2  above a compressor rotor  9  of the compressor  3  being combined to form a common shaft  19  which rotates about a common vertical rotation axis  60 . 
     The motor rotor is borne in a first radial bearing  21  at the upper end of the motor rotor. 
     The compressor rotor  9  is borne by means of a second radial bearing  22  in the lower position. 
     An axial bearing  25  is provided at the upper end of the common shaft  19 , that is to say at the upper end of the motor rotor. The radial bearings and the axial bearing operate electromagnetically and are each encapsulated. In this case, the radial bearings extend around the respective bearing point of the shaft  19  in the circumferential direction and in this case are circumferential through 360° and are undivided. 
     The compressor  3  is in the form of a centrifugal compressor and has three compressor stages  11  which are each connected by means of an overflow  33 . The pressure differences which result across the compressor stages  11  ensure that there is a thrust on the compressor rotor  9  which is transmitted on the motor rotor and is directed against the force of gravity from the entire resultant rotor comprising the compressor rotor  9  and the motor rotor, thus resulting in a very high degree of thrust matching during rated operation. This allows the axial bearing  25  to be designed to be comparatively smaller than if the rotation axis  60  were to be arranged horizontally. 
     The electromagnetic bearings  21 ,  22 ,  25  are cooled to the operating temperature by means of a cooling system (not illustrated in detail), with the cooling system providing a tap in an overflow  33  of the compressor  3 . A portion of the pumping medium, which is preferably natural gas, is passed from the tap by means of pipelines through a filter, and is then passed through two separate pipelines to the respective outer bearing points (first radial bearing  21  and second radial bearing  22  as well as the axial bearing  25 ). This cooling by means of the cold pumping medium  80  saves additional supply lines. 
     The motor rotor is surrounded by a stator  16  which has encapsulation such that the aggressive pumping medium  80  does not damage the windings of the stator  16 . The encapsulation is in this case preferably designed such that it can contribute to the full operating pressure. This is also because a separate cooling arrangement is provided for the stator, in which cooling arrangement a dedicated cooling medium circulates. 
     The compressor rotor  9  expediently has a compressor shaft  10  on which the individual compressor stages  11  are mounted. This can preferably be done by means of a thermal shrink fit. An interlock, for example by means of polygons, is likewise possible. Another embodiment provides for different compressor stages  11  to be welded to one another, thus resulting in an integral compressor rotor  9 . 
     The pumping medium  80  or natural gas NG is passed from the natural reservoir first of all into a condensate separator  81 , which separates condensates  82 , including water, from the gaseous phase. The condensates  82  are passed into a condensate line  84 , into which a downstream drain line  95  also opens, which introduces condensates that have been deposited in the compressor unit into the condensate line  84 . The condensates  84  are passed from a condensate pump  85  to a mixing unit  86 , in which they are mixed with the compressed natural gas NG or pumping medium  80 . The resultant mixture is pumped into a pipeline  87  in the direction of a base station  89 . 
     The compressor unit  1  has a system for distribution of antifreeze  73 , comprising distribution lines  94  and injection modules  72 . The antifreeze  73  is pumped from a reservoir tank  92  by means of a metering pump  93  to the various injection modules  72  on the compressor unit  1 . The injection modules  72  locally apply antifreeze to the first radial bearing  21 , to the axial bearing  25 , to the second radial bearing  22  and to the overflows  33 . A further injection module  72  is located on the intake connecting stub  8 , by means of which module the antifreeze  73  is injected directly into the pumping medium  80  which is sucked in. 
     Part of the injected antifreeze  73  is deposited in the compressor unit  1 , to be precise such that it is emitted through a drain  96  (at the “single drain point”) of the compressor unit  1  into the drain line  95 . The rest is pumped together with the compressed natural gas NG through the outlet  7  into the mixing unit  86 . The antifreeze  73 , the natural gas NG and the condensate  82  are pumped to the base station  89  at the earth&#39;s surface through the pipeline  87 . Hydrate formation in the pipeline  87  is precluded because of the antifreeze  73  being carried with it. Before reaching the base station  89 , a further condensate separator  88  ensures that the natural gas NG is dry, with the condensate including the antifreeze  73  being passed to a conditioner  90  in which the antifreeze  73  is separated from the rest of the condensate  82 . The conditioned antifreeze  73  is passed back by means of a return line  91  along the pipeline  87  to the reservoir tank  92 . The closed circuit of the antifreeze  73  ensures protection against hydrate formation on the one hand, and on the other hand compliance with the relevant environmental standards.