Abstract:
The invention relates to an eccentric screw pump and a method for its operation. Unlike the previously known prior art, where changes to the internal geometry of the pump stator caused by wear were always remedied with radially acting clamping measures, the invention is based on bringing this about by a change in the length of the lining.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority of German patent application No. 10 2005 042 559.3 filed on Sep. 8, 2005, the content of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a method and a device for the operation of an eccentric screw pump wherein the internal dimensions of the stator are adapted to the circumstances arising during the operation. 
     BACKGROUND OF THE INVENTION 
     There emerges from DE 1303705 an eccentric screw pump whose useful life is to be extended. For this purpose, a pump design is provided which comprises a stator housing conical on the inside and a lining conical on the outside. If wear occurs on the lining that leads to an enlargement of the internal cross-section of the lining, the two conical parts, the stator housing and the lining, are shifted towards one another in the longitudinal direction. The lining is placed radially under pressure as a result of this relative movement, no change in the length of the lining of the stator taking place. The position of the lining is brought about by the transfer of compensating discs from the position in front of a flange into a position behind the flange. 
     DD 279043 A1 shows a stator structure of an eccentric screw pump which, as in DE 1303705, also comprises conically shaped parts, referred to here as sleeve and stator. The reduction in the internal diameter of the stator takes place by the shifting of the parts towards one another. This shifting process is initiated by a tensioning nut, with which a thrust piece shifts the stator into a sleeve. 
     DE 1553126 discloses in  FIG. 4  the design of a rotor, which is made up of an internal and external polygonal sleeve and a polygonal lining. 
     A rotor designed screw-shaped on the outside can be found in DE 19821065. The stator sleeve and the lining are joined binder-free. 
     A longitudinally split stator sleeve is shown in  FIG. 4  of DE 10042335. Two levers are shown as a closure, said levers entering with the second half of the sleeve element into a keyed connection. 
     In several examples of embodiment, DE 1 204 072 A1 shows the stator of an eccentric screw pump with the adjacent device parts of a storage container and an outlet pipe. The multi-part cylindrical stator housing is connected via screw connections to this storage container and to the outlet pipe. The distance between the storage container and also between the outlet pipe can be changed by various measures. This distance is changed either directly in the region of the storage container and the outlet pipe or in the central region between the two stator sleeve parts. As a result of the axial change in the distance between the storage container and the outlet pipe, an annular cap reduces the axial length of the stator lining from one or from both sides of the stator. Since, in all the examples of embodiment, the stator lining is clamped in the middle of its longitudinal extension between individual stator parts, a uniform distribution of the material of the stator lining does not take place in the region of the internal cross-section. Moreover, it emerges from all the examples of embodiment that each change in the length of the stator lining is accompanied by a change in the overall length of the pump. 
     The problem according to the invention consists in making it possible to adapt the pump to the most varied operating conditions without changing the pump length and only with a small assembly outlay. 
     SUMMARY OF THE INVENTION 
     The problem according to the invention is solved with the features stated here and below. 
     During the operation of an eccentric screw pump, account must be taken of the most varied phases which influence the mode of operation or the design of the pump parts actively involved in the delivery. By way of example, it can be stated that the pump naturally gives rise to different pump reactions when conveying slightly viscous to highly viscous products with or without abrasive particles, this becoming apparent during the start-up phase and in the normal pump operation. 
     In order to be able to respond to reactions of the pump, such as a drop in the delivery pressure, dry running, temperature increase or blockage, provision is made according to the invention to change the internal cross-section of the stator by shortening or lengthening the stator lining. For this purpose, the elastomer stator lining is subjected to an axial tensile or compressive action. 
     Abnormalities are most frequently detected due to the drop in the delivery pressure or the increase in the power consumption of the drive motor. Depending on how fast a reaction must take place, the adaptation of the internal dimensions of the stator lining can take place mechanically or electrically/electronically. It has been shown that the interaction of rotor and stator can be controlled or corrected not only by radial deformation of the stator lining, but also, according to the invention, by axial shortening (compression) or lengthening (extension). Different measures are required for the shortening of the stator length, whereby a shortening both of the length of the lining and of the stator sleeve can be understood. A shortening of the length of the elastic stator lining can be achieved by a reduction in the distance between the contact surfaces of the lining on the pump housing and on the pressure flange. 
     If it is only in the installed state that the stator and thus the lining of the stator acquire the internal dimensions provided for the operation of the pump due to the desired axial compression, the assembly of the pump is facilitated. This results from the fact that the stator with a larger rest or initial internal geometry can be pushed more easily over the already assembled rotor. 
     With a suitable design of the stator and its lining, the start-up behaviour with the finish-mounted pump can also be influenced. For this purpose, provision is made to stretch the elastic stator lining. The elastic material of the lining thereby reduces the pressure on the rotor and thus facilitates the start-up behaviour by lowering the breakaway torque. 
     In a basic design for the axial shortening of the lining, its initially available distance between the pump housing or a part thereof and a pump end piece is shortened. According to the invention, one or more inserts in the form of rings are provided here. In the examples of embodiment, it is necessary for the stator sleeve and the stator lining to comprise separate parts. For the purpose of uniformly distributing the pressure or tensile force applied at the end over the whole stator length, the stator sleeve and the stator lining have contact surfaces running parallel to the longitudinal axis of the pump. Only in this way is a homogeneous cross-sectional reduction or increase possible, since no blockages are thereby created. 
     In order to make the adjustment of the pretensioning of the stator lining on the rotor more easily manageable, use may also be made of an adjusting ring controllable from the outside of the pump instead of the aforementioned insertion ring. 
     With this embodiment, the adjusting ring can be incorporated both in an end connection piece and also in the pump housing. The adjusting ring is axially mobile and, insofar as a fluid is used, provided with seals. If an electrical adjustment unit is used, the applied or generated tensioning between the adjusting ring and the lining is sufficient as a sealing force. By using a mobile adjusting ring, the stator lining can also be loaded or relieved of load by the supply and removal of a pressure medium during the pump operation. The adjusting ring, which is also referred to as adjusting spectacles on account of the cross-sectional shape of the duplex stator, can thus be the actuator for a control that responds to various operating parameters, such as the delivery pressure or the pump temperature. If the control detects an increase in the temperature, which is accompanied by an expansion of the elastomer, the pressure on the adjusting ring drops and the pretensioning on the rotor is reduced. 
     Since the stator lining and the stator sleeve are separate parts and the rotor transmits forces onto the stator lining, the latter itself tends to twist. This twisting must however be avoided in order to maintain the pump function. According to the invention, the stator lining and the stator sleeve are therefore not formed round, but polygonal at the contact surfaces. Rigid positioning can of course also be achieved by other surface shapes, such as a groove shape, a wedge shape or a wave shape. 
     Since the stator sleeve and the stator lining are separate parts, the lining can be rapidly replaced when necessary. For this purpose, provision is made according to the invention to form the stator sleeve from a profile with a longitudinal slot. A closure rail tensions and holds the profile stable. Without the closure rail, the two profile longitudinal sides open out from one another, the insertion and removal of the stator lining being greatly facilitated. The closure rail fits into the profile level on the inside of the stator sleeve. On the outside, the closure rail enters into a keyed connection with the longitudinal sides of the stator sleeve. 
     In order to increase the torsional reliability, the closure rail could of course also extend inwards, the lining then having to have a corresponding groove. 
     In order to simplify the method of production of the stator sleeve, the latter comprises a one-part or multi-part extruded profile in the longitudinal or transverse form. The stabilisation of the stator, which is dependent on the delivery pressure, is also taken into account by the selection of different materials in production. Various plastics as well as metals are therefore provided as materials for the stator sleeve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures described below show examples of embodiment of the invention: 
         FIG. 1  is a partial section of an eccentric screw pump; 
         FIG. 2  is a partial section of an eccentric screw pump; 
         FIG. 3  is a partial section of an eccentric screw pump; 
         FIG. 4  is a partial section of a stator and eccentric screw pump; 
         FIG. 5  is a stator sleeve; and 
         FIG. 6  is a stator lining. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a typical arrangement of a stator  10  in an eccentric screw pump. Stator  10  is clamped between a pressure flange  12  and pump housing  14 . Tightening screws can be provided as clamping elements. The distance between pump housing  14  and pressure flange  12  is determined by the length of stator sleeve  16 . As long as the stator sleeve and stator lining  18  are not installed between pump housing  14  and pressure flange  12 , the two parts can be displaced axially towards one another. In the installed state, however, the stator lining is limited at both ends by a stop  20 ,  22 . The stop comprises an annular end face, on the pressure flange or the pump housing. The length of the stator lining shown in  FIG. 1  does not correspond to the length in the uninstalled state, but is already compressed slightly and accordingly is axially shortened. The length of the stator lining in  FIG. 1  corresponds to the new state of the pump in the as-delivered condition. In this operational state, the ends of the stator lining are pretensioned only to such an extent that they give rise to a certain sealing function between delivery chamber  24  and the external atmosphere. 
     An axial change in the stator length caused by the operation, in particular the length of the stator lining, is shown in  FIG. 2 . An axial shortening has occurred here, for example, on the right-hand side of the stator lining. The shortening has arisen on account of a spacer ring  26 , which sits in the region of the pressure flange between stop  20  and the complementary end face of the stator lining. The elastic material of the stator lining, which is pushed back by the spacer ring, is distributed over its whole internal surface. A larger internal surface thus arises, which leads to increased pressure on the rotor, which is not shown. This measure is taken when the delivery pressure diminishes in the region of pressure flange  12 , which allows the conclusion that there is wear on the internal surface of the stator lining (referred to in the following as lining). 
     A further possibility for changing the internal geometry of the stator lining is shown in  FIGS. 3 and 4 . The essential difference with this design is that a mobile adjusting ring  28  is used here. Adjusting ring  28  can be operated externally without assembly work on the pressure flange or the pump housing. For this purpose, the adjusting ring is provided with one or more adjusting screws, which can be operated from the surface of the pump. Apart from this mechanical variant, a hydraulic drive  31  can of course also be provided for the axial deformation of the stator lining. The hydraulic fluid passes via line  30  into annular chamber  32 . The annular chamber is bounded by seals  34 ,  36  both in the direction of lining  18  and also on the product-carrying side. 
     The hydraulic pressure in the annular chamber can be controlled by a manually operated piston screw or automatically via a hydraulic system. The hydraulic system or an electrical device enable the operation of adjusting ring  28 , depending on what pressure or temperature values are prevailing in the pump region. As can be seen from  FIG. 3 , annular chamber  32  is bounded by adjusting ring  28  and an end face  38  on the pressure flange. 
     If adjusting ring  28  lies against end face  38 , the stator lining is only under a small amount of pretensioning. The more hydraulic fluid is pressed into the annular chamber, the more the lining is compressed and the smaller the internal dimensions become. If, during lengthy pumping, the distance by which the lining is compressed is not sufficient, this can be remedied by the shortening of the stator sleeve, whereby individual elements, e.g. annular elements, have to be removed. 
       FIG. 5  and  FIG. 6  show lining  18  and stator sleeve  16 , two separate components, which are not joined together over the whole area even during operation. The torsion-resistant arrangement of the lining in the stator sleeve takes place solely by positive locking by means of the polygonal internal and external shape of these elements. For the purpose of easier removal of the lining, the stator sleeve is provided with a longitudinal slot. The two longitudinal edges  42 ,  44  of the stator sleeve form with closure rail  46  a keyed connection. Closure rail  46  ends level at the inside of the stator sleeve. Although the stator sleeve is shown in one piece in  FIG. 5 , it can comprise several longitudinal or transverse parts. The important thing is that the diameter or the longitudinal slot of the stator sleeve without the closure rail is larger in order to facilitate the insertion or removal of the lining.