Method for operating a screw pump, control unit for a screw pump, screw pump, software program product

Abstract

A method for operating a screw pump (14), wherein the screw pump (14) comprises a first screw (31) and a second screw (32). A thread of the first screw (31) engages with a thread of the second screw (32), such that the mutual engagement of the threads forms working chambers whose volume decreases from a suction side (20) of the screw pump (14) to a pressure side (21) of the screw pump (14). The screws (31, 32) are set into non-contact rotation. Within a tolerance range (25) of the non-contact rotation, the relative rotational position of the first screw (31) and the second screw (32) is intentionally changed in order to remove a deposit from a surface of the first screw (31) and a surface of the second screw (32). The invention also relates to a control unit for a screw pump, a screw pump, and a computer program.

Description

[0001] The invention relates to a screw pump, a control unit for a screw pump, a screw pump and a software program product.

[0002] Screw pumps can be used to create a vacuum. The space to be evacuated is connected to a suction side of the pump, allowing the pump to draw gas from the space. The gas is compressed inside the pump and released at a higher pressure on a pressure side.

[0003] Screw pumps can be used in applications where gases contaminated with foreign substances are conveyed. These foreign substances may already be present when the gas enters the pump or may be generated within the pump during the compression process. Examples include coating technology systems. If a coating process is carried out under vacuum conditions, residual gas from the vacuum environment usually also draws in elements of the coating material. These foreign substances can accumulate inside the screw pump, potentially impairing its operational reliability.

[0004] The invention is based on the objective of presenting a screw pump, a control unit for a screw pump, a screw pump, and a software program product, with which the aforementioned disadvantages are reduced. This objective is achieved by the features of the independent claims. Advantageous embodiments are specified in the dependent claims.

[0005] In a method according to the invention for operating a screw pump, the screw pump comprises a first screw and a second screw. A thread of the first screw engages with a thread of the second screw, so that the mutual engagement of the threads forms working chambers whose volume decreases from a suction side of the screw pump to a pressure side. The screws are set into non-contact rotation. Within a tolerance range of the non-contact rotation, the rotational position of the first screw relative to the rotational position of the second screw is systematically changed in order to remove a deposit from a surface of the first screw and a surface of the second screw.

[0006] Screw pumps are generally operated so that the screws rotate synchronously with each other. This ensures that mechanical contact between the screws is avoided. Such an operating condition of the screw pump is referred to as contactless. With respect to the rotational position of the first screw relative to the second screw (mutual rotational position), there is a tolerance range within which the screw pump can operate contactless. It is common practice when operating a screw pump to maintain the relative rotational position of the two screws approximately in the middle of this tolerance range. Deviations from this midpoint of the tolerance range are permissible in both directions without jeopardizing the contactless operating condition, i.e., without any mechanical contact between the two screws occurring.Operating the screw pump in the middle position of the tolerance range is generally beneficial for the operational reliability of the screw pump.

[0007] The invention recognizes that the operational reliability of a screw pump can be impaired by deposits that accumulate on the screw surfaces during operation. These deposits reduce the tolerance gaps between the screws, thereby also narrowing the tolerance range for the rotational position between the screws. While a reduction in the tolerance range is not immediately problematic during normal operation, it has been found that the deposits can create a situation where, after a temporary shutdown, the pump cannot be restarted. The deposits adhere so firmly to each other that the resistance cannot be overcome by the pump's drive force.This leads to a failure of the screw pump and potentially to a failure of the entire system in which the screw pump is used.

[0008] The invention proposes a planned variation in the relative rotational position of the screws within the tolerance range during operation of the screw pump. This planned variation alters the height of the tolerance gap between the first and second screws. Deposits within this tolerance gap come into contact with each other and are abraded without any mechanical contact between the substrate of the first screw pump and the substrate of the second screw pump. In this way, the screw pump can be maintained in a state in which it can be readily restarted after a period of inactivity.

[0009] The tolerance range reaches its limit when mechanical contact occurs between the substrate of the first screw pump and the substrate of the second screw pump. Deposits on the screws do not change the size of the tolerance range.

[0010] The operation of a screw pump according to the invention requires synchronization of the relative rotational positions of the first and second screws. This synchronization can be achieved by a mechanical coupling between the first and second screws. The first and second screws can be coupled, for example, by a gearbox or a toothed belt. Alternatively, the relative rotational positions of the first and second screws can be achieved by suitable control of a drive motor for the first screw and a drive motor for the second screw. For this purpose, the screw can include a first rotational angle sensor for the rotational position of the first screw and a second rotational angle sensor for the rotational position of the second screw. Measured values ​​from the first rotational angle sensor and measured values ​​from the second rotational angle sensor can be supplied as input to a control unit.The control unit can process the measured values ​​to generate control commands that actuate the drive motor of the first screw and the drive motor of the second screw. Depending on current requirements, the control unit can be configured to maintain a constant relative rotational position of the first and second screws, or to vary this position within a tolerance range. It is also possible to switch between a standard mode with a constant relative rotational position and a cleaning mode with changing relative rotational positions.

[0011] The first angle sensor can, for example, be designed to determine the rotational position of the first screw by scanning a toothed disc connected to the first screw. The same can apply to the second angle sensor. A first toothed disc connected to the first screw and a second toothed disc connected to the second screw can be designed as an auxiliary gear, which maintains the relative rotational position of the first and second screws within the tolerance range if synchronization by the control unit fails.

[0012] In embodiments where the first screw and the second screw are mechanically synchronized, a first shaft carrying the first screw and a second shaft carrying the second screw typically have a precisely defined rotational position relative to each other. To utilize the method according to the invention, the screw pump can include an adjustment mechanism with which the rotational position of the first screw can be adjusted relative to the rotational position of the first shaft. The adjustment mechanism can include an actuator that can be controlled by a control unit. The actuator can be actuated, for example, electrically, magnetically, hydraulically, or pneumatically.

[0013] The control unit can include a standard mode in which the relative rotational position of the first and second screws is set to a standard position. This standard position can be a fixed rotational position within the tolerance range. In one embodiment, the standard position corresponds to the midpoint of the tolerance range. Depending on the design of the screw pump, the adjustment of the relative rotational position can be achieved by directly controlling the drive motors of the first and second screws or by controlling an adjustment mechanism located between the first screw and the first shaft.

[0014] The control unit can include a cleaning mode in addition to or as an alternative to the standard mode. In this cleaning mode, the relative rotational position of the first and second screws is varied within the tolerance range. The screw pump can be controlled to change the relative rotational position of the first and second screws towards one end of the tolerance range. This change can be stopped before reaching the first end of the tolerance range. In a subsequent phase of the cleaning mode, the relative rotational position can be changed towards a second end of the tolerance range, opposite the first end. This sequence can be repeated multiple times.

[0015] A monitoring device may be provided that is designed to monitor the operating status of the screw pump. For this purpose, the monitoring device may, for example, process measured values ​​recorded at the screw pump or other data concerning the screw pump's operating parameters. The processed parameters may include information about the rotational speed of the first screw and / or the rotational speed of the second screw. Additionally or alternatively, the processed parameters may include information about the torque with which the first screw is driven and / or information about the torque with which the second screw is driven.

[0016] The monitoring device can be designed to monitor, in cleaning mode, whether an observed state variable changes as the system approaches the first or second end of the tolerance range. If this occurs, it can be interpreted as an indication that mechanical contact has taken place between a deposit on the first screw and a deposit on the second screw. Mechanical contact is generally accompanied by an increase in the internal resistance of the screw pump, which can manifest itself as a change in rotational speed and / or torque.

[0017] A change in a state variable detected by the monitoring device can be transmitted to the control unit and processed there as an input. The control unit can, for example, be designed to stop adjusting the relative rotational position of the first and second screws as soon as a change in a state variable occurs. The current relative rotational position can be maintained, allowing deposits to be worn away. Alternatively, the relative rotational position of the first and second screws can be moved back towards the center position of the tolerance range and then slowly moved again towards the first end of the tolerance range, allowing deposits to be worn away with less internal resistance and without a change in a state variable of the screw pump.This procedure can be continued in one or more iterations until the first end of the tolerance range is almost reached. "Almost reached" means that, relative to the angular difference between the center position of the tolerance range and the first end of the tolerance range, the angular difference between the current rotational position and the first end of the tolerance range is no more than 50%, preferably no more than 20%, and more preferably no more than 10%. The procedure is preferably carried out in such a way as to avoid completely reaching the first end of the tolerance range and thus preventing contact between the substrate of the first screw and the substrate of the second screw.

[0018] The control unit can be designed to execute a corresponding process towards a second end of the tolerance range. This second end of the tolerance range is opposite to the first end of the tolerance range.

[0019] The screw pump can be operated in standard mode for part of its operating period and in cleaning mode for another part. The method can be carried out such that operation in standard mode accounts for at least 50%, preferably at least 80%, and more preferably at least 90% of the screw pump's operating period. Operating period is defined as the time during which the screw pump operates continuously. For example, the screw pump can be operated in cleaning mode once per hour or once per day. Switching between standard mode and cleaning mode can be done while the screw pump is running. It is also possible to carry out the method such that the screw pump is operated continuously in cleaning mode.The middle position of the tolerance range is then simply a relative rotational position of the first screw and the second screw, which is repeatedly traversed at time intervals during the continuous adjustment process of the rotational position.

[0020] The tolerance range can, for example, extend over an angular range between 0.4° and 3°, preferably between 0.5° and 1.5°. The tolerance range can be measured by keeping the rotational position of the first screw constant and measuring the angular difference over which the second screw can be rotated between the first and second ends of the tolerance range.

[0021] The limit of the tolerance range is determined by the section of the screw pump where mechanical contact first occurs between the substrate of the first screw and the substrate of the second screw. The screw pump can be designed such that the axial gap height, measured in absolute terms, on the suction side of the screw pump is greater than the axial gap height on the pressure side. The suction side is defined as the section of the first and second screws adjacent to the inlet port of the screw pump. The pressure side is defined as the section of the first and second screws adjacent to the outlet port of the screw pump. The suction side can extend over at least 10%, preferably at least 20%, and more preferably at least 50% of the length of the first and second screws.The pressure side can extend over at least 10%, preferably at least 20%, and more preferably at least 50% of the length of the first screw and the second screw.

[0022] The screw pump can be designed such that the thread pitch of the first and second screws on the discharge side is smaller than on the suction side. A smaller axial gap on the discharge side does not necessarily mean that the tolerance range of the relative rotational position of the first and second screws is also limited by the discharge side. In a preferred embodiment, the screw pump is designed such that the axial gap between the first and second screws on the suction side is larger than the axial gap between the first and second screws on the discharge side, and that the tolerance range is limited by the suction side of the first and second screws. Such a design has a beneficial effect on the operational reliability of the screw pump.Due to the larger thread pitch on the suction side, the operation of the screw pump is not affected by a brief contact between the first and second screws on the suction side. Such contact acts like a gearbox, so that the first and second screws automatically return to their tolerance range. A corresponding contact on the pressure side, however, would lead to an immediate blockage of the screw pump due to the smaller thread pitch there.

[0023] The following considerations underlie this. If the screw pump has larger leakage gaps on the suction side, this facilitates the flow of the pumped gas into the pump because the gas can spread through the leakage gap between the working chambers. This improves the gas flow into the pump, particularly in the initial phase of an evacuation process when the aspirated gas is still under high pressure. Conversely, the leakage gaps on the discharge side of the screw pump should be as small as possible because any leakage between the working chambers negatively impacts the pump's efficiency. These considerations lead to a screw pump design where the axial gap height is smaller on the discharge side than on the suction side.

[0024] On the other hand, the two screws on the pressure side are self-locking, meaning that the screw pump locks up when the two screws on the pressure side come into contact with each other. Therefore, the difference between the axial gap height on the pressure side and the axial gap height on the suction side should be sufficiently small in the pump according to the invention so that the screws on the suction side come into contact with each other first. In other words, the tolerance range should be limited by the suction side so that the screws can touch without self-locking. The fact that the tolerance range can be limited by the suction side despite the larger axial gap height there is possible because the thread pitch is greater than on the pressure side. The method according to the invention can be applied to a screw pump whose screws have a self-locking section and a non-self-locking section.The self-locking section can be located on the pressure side, the non-self-locking section can be located on the suction side.

[0025] There are applications for a pump according to the invention where the growth of deposits on the pressure side progresses faster than on the suction side. This is particularly the case when the conditions for deposit growth are only created by the pressure increase within the pump. The method according to the invention can be advantageously applied especially in such applications because it makes it possible to remove the unwanted deposits on the pressure side before they cause pump failure due to self-locking.

[0026] The invention also relates to a control unit for a screw pump, wherein the control unit is designed to set the screws of the screw pump into non-contact rotation and, within a tolerance range of the non-contact rotation, to systematically change the relative rotational position of the first screw and the second screw in order to remove a deposit from a surface of the first screw and a surface of the second screw. The invention further relates to a screw pump, wherein the screw pump comprises a first screw and a second screw. A thread of the first screw engages with a thread of the second screw, such that the mutual engagement of the threads forms working chambers whose volume decreases from a suction side of the screw pump to a pressure side of the screw pump. The screw pump comprises a control unit designed to actuate the screws of the screw pump.

[0027] The invention further relates to a computer program product or a set of computer program products comprising program parts which, when loaded into a computer or into interconnected computers connected to a control unit according to the invention, are designed to carry out the method according to the invention.

[0028] The disclosure includes further developments of the method with features described in connection with the control unit or screw pump according to the invention.

[0029] The invention is described below by way of example with reference to the accompanying drawings, using an advantageous embodiment as an example. The drawings show: Fig. 1: a perspective view, partially cut away, of a screw pump according to the invention; Fig. 2: a section of the pump from Fig. 1 in enlarged view; Fig. 3: the view from Fig. 2 in another state of the pump; Figs. 4-7: schematic representations of details of a screw pump according to the invention; Fig. 8: a block diagram of a screw pump according to the invention; Fig. 9: the view according to Fig. 6 in an alternative embodiment of the invention.

[0030] One in Fig. 1 The screw pump 14 shown comprises a first screw 31 and a second screw 32, which are housed in a pump casing 15. The first screw 31 is visible along its entire length due to the pump casing 15 not being fully shown, while the second screw 32 is largely obscured by the pump casing 15. The first screw 31 and the second screw 32 are in engagement with each other, meaning that the thread projections of the first screw 31 engage in the recess between two thread projections of the second screw 32, and vice versa.

[0031] The pump comprises a control and drive unit 16, in which an electronically controlled drive motor 33, 34 is arranged for each of the screws 31, 32. The control of the drive motors 33, 34 is configured so that the two screws 31, 32 operate without contact, i.e., the threaded projections of the screws 31, 32 do not come into mechanical contact with each other. As an additional safety measure, each of the two screws 31, 32 is equipped with a toothed washer 35, 36. The toothed washers 35, 36 engage with each other and create a positive coupling of the two screws 31, 32 in the event that the electronic synchronization of the screws 31, 32 fails.

[0032] Each screw 31, 32 is equipped with two threads 19, so that the pump has a total of four threads 19. The threads 19 extend from a suction side 20 in the center of the screws 31, 32 to a pressure side 21 at the outer ends of the screws 31, 32, see Fig. 2, 3 The two threads 19 of a screw 31, 32 are aligned such that aspirated gas is compressed from the suction side 20 to the pressure side 21.

[0033] The threads 19 are designed such that a larger volume is enclosed between two adjacent thread projections in the suction side 20 area than in the pressure side 21 area. The volume of the working chambers, which corresponds to the volume enclosed between the thread projections, decreases from the suction side to the pressure side, so that gas contained in the working chamber is compressed on its way from the suction side to the pressure side.

[0034] The pump housing 15 is provided with an inlet opening 24, which is arranged to provide access to the suction sides 20 of all four threads 19. To allow a large volume flow into the pump, the inlet opening 24 has a large cross-section. In the exemplary embodiment, the cross-sectional area of ​​the inlet opening 24 is larger than the circular contour defined by a screw 31, 32.

[0035] In Fig. 4, 5 The following are schematically depicted in enlarged form details of the screw pump 14. A threaded projection of the first screw 31, which is rotatably mounted about a first axis 37, engages with a threaded projection of the screw 32, which is rotatably mounted about a second axis 38. Parallel to the direction of the axes 37, 38, there is an axial gap with a gap height 39 between the threaded projection of the first screw 31 and the threaded projection of the second screw 32. The gap height 39 on the suction side 20 is greater than the gap height 39 on the pressure side 21.

[0036] The screw pump 14 has a tolerance range 25 within which the relative rotational position of the first screw 31 and the second screw 32 can be adjusted without there being any mechanical contact between the first screw 31 and the second screw 32, see Fig. 6, 7 In Fig. 7 A central position 26 of the tolerance range 25 is shown. Starting from the central position 26, the rotational position of the first screw 31 can be adjusted equally in both directions relative to the rotational position of the second screw 32 before contact occurs between the two screws 31 and 32. There is a first adjustment range 27 extending from the central position 26 to a first end of the tolerance range 25, and a second adjustment range 28 extending from the central position 26 to a second end of the tolerance range 25 opposite the first end.

[0037] The tolerance range 25, which extends over an angular range through which the first screw 31 and the second screw 32 can be rotated relative to each other, acts in the axial direction between the thread projections of the first screw 31 and the second screw 32. Rotating the screws 31 and 32 relative to each other thus changes the gap height 39 between the first screw 31 and the second screw 32. At one end of the tolerance range, two thread projections of the screws 31 and 32 abut each other in the axial direction.

[0038] The screw pump 14 is designed such that mechanical contact occurs on the suction side 20, while the thread projections on the pressure side 21 still maintain a gap between them. Although the gap height 39 on the suction side 20 is greater than the gap height 39 on the pressure side 21 when the screw pump is in the neutral position 26, the larger thread pitch on the suction side 20 allows for a design of the screws 31, 32 that defines the two ends of the tolerance range 25 through the suction side 20. This design has a beneficial effect on the operational reliability of the screw pump 14, because mechanical contact in an area with a large thread pitch automatically returns the screw pump 14 to the tolerance range 25.If the first mechanical contact were to occur in an area with a small thread pitch, this would very likely lead to a blockage of the screw pump 14 due to the self-locking design of the screws in this section.

[0039] When the screw pump 14 is operated in a standard mode, the relative rotational position of the first screw 31 and the second screw 32 is set to the center position 26. The first adjustment range 27 to the first end of the tolerance range 25 is the same as the second adjustment range 28 to the second end of the tolerance range 25. According to the invention, it is proposed to operate the screw pump 14 in a cleaning mode such that the center position 26 is intentionally left, which leads to a reduction in the gap height 39 between the first screw 31 and the second screw 32. In such an operating condition, deposits that have formed on the surfaces of the screws 31 and 32 can be worn away by mutual friction between them.

[0040] According to Fig. 8 The screw pump 14 includes a control unit 43 that controls the interaction of its components. Specifically, the control unit 43 generates control signals that actuate the drive motors 33 and 34 of the screw pump 14. The screw pump 14 includes a first rotation angle sensor 41 for the angular position of the first toothed disc 35 and a second rotation angle sensor 42 for the angular position of the second toothed disc 36. The measured values ​​from the rotation angle sensors 41 and 42 are supplied to the control unit 43 as input variables. The control unit 43 processes the measured values ​​and, in standard mode, actuates the drive motors 33 and 34 such that the relative rotational position of the first screw 31 and the second screw 32 corresponds to the center position 26.

[0041] After a prolonged period of operation in standard mode, the control unit 43 enters a cleaning mode in which the drive motors 33, 34 are controlled such that the relative rotational position of the screws 31, 32 deviates from the center position 26. In the first part of a maintenance procedure, the rotational position is changed in the direction of the first adjustment range 27. The adjustment of the rotational position continues until the first end of the tolerance range 25 is almost reached. Subsequently, the relative rotational position is moved back to the center position 26. A second part of the maintenance procedure then follows, in which the relative rotational position is adjusted in the direction of the second adjustment range 28 until the second end of the tolerance range 25 is almost reached.

[0042] The screw pump 14 includes a monitoring device 44 that monitors the operating parameters of the screw pump 14, namely the rotational speed of the screws 31, 32 and the torque with which the screws 31, 32 are driven. These parameters remain within fixed ranges when the screw pump is operated in normal mode. An increase in the internal friction of the screw pump 14 affects these parameters and is detected by the monitoring device 44.

[0043] In cleaning mode, the screw pump 14 is deliberately brought into an operating state in which deposits on the first screw 31 are expected to come into contact with deposits on the second screw 32. If a condition occurs in which the monitoring device 44 registers a change in the state variables, a message is sent to the control unit 43. The control unit 43 can react by maintaining the relative rotational position of the screws 31 and 32 until the state variables of the screw pump 14 are back within the normal range. This is the case when the deposits have been sufficiently worn away so that there is no longer any increased resistance between the screws 31 and 32. The control unit 43 can continue the maintenance process by further adjusting the relative rotational position of the screws 31 and 32 towards the end of the tolerance range 25 in order to remove further portions of the deposits.This prevents the screw pump 14 from being overloaded during cleaning mode. If the monitoring device 44 detects that the load on the screw pump 14 is increasing too much, it can send a signal to the control unit 43 to shut down the screw pump 14.

[0044] In Fig. 9An alternative embodiment of a screw pump 14 is schematically illustrated. In this embodiment, a shaft 46 of the first screw 31 is mechanically coupled to the shaft 47 of the second screw 32. The relative rotational position of the shafts 46 and 47 is thus determined by mechanical coupling. In order to make use of the method according to the invention, the first screw 31 is equipped with an adjustment mechanism 45 with which the rotational position 46 of the first screw 31 relative to the shaft 46 can be adjusted. The adjustment mechanism 45 is electrically actuated and is controlled by control signals from the control unit 43 when the screw pump 14 is operated in cleaning mode.

Claims

1. A method for operating a screw pump (14), wherein the screw pump (14) comprises a first screw (31) and a second screw (32), wherein a thread of the first screw (31) engages with a thread of the second screw (32), such that working chambers are formed by the mutual engagement of the threads, the volume of which decreases from a suction side (20) of the screw pump (14) to a pressure side (21) of the screw pump (14), wherein the screws (31, 32) are set into a non-contact rotation and wherein, within a tolerance range (25) of the non-contact rotation, the relative rotational position of the first screw (31) and the second screw (32) is systematically changed in order to remove a deposit from a surface of the first screw (31) and a surface of the second screw (32).

2. Method according to claim 1, wherein the first screw (31) is mechanically coupled to the second screw (32).

3. Method according to claim 1 or 2, wherein the rotational position of the first screw (31) and the rotational position of the second screw (32) is detected by rotation angle sensors (41, 42).

4. Method according to claim 3, wherein the relative rotational position of the first screw (31) and the second screw (32) is synchronized based on measured values ​​from the rotation angle sensors (41, 42).

5. Method according to claim 1 or 2, comprising an adjusting mechanism (45) between the first screw (31) and a shaft (46) supporting the first screw (31) to change the rotational position of the first screw (31) relative to the shaft (46).

6. Method according to one of claims 1 to 5, comprising a control unit (43) for controlling the interaction of components of the screw pump (14).

7. Method according to claim 6, wherein the control unit (43) is designed to maintain the relative rotational position of the first screw (31) and the second screw (32) in a central position (26) of the tolerance range (25) in a standard mode.

8. Method according to claim 6 or 7, wherein the control unit (43) is designed to vary the relative rotational position of the first screw (31) and the second screw (32) within the tolerance range (25) in a cleaning mode.

9. Method according to claim 8, wherein the control unit (43) is designed to change the relative rotational position of the first screw (31) and the second screw (32) in a first section of a maintenance operation in the direction of a first end of the tolerance range (25) and in a second section of the maintenance operation to change the relative rotational position of the first screw (31) and the second screw (32) in the direction of a second end of the tolerance range (25).

10. Method according to claim 8 or 9, comprising a monitoring device (44), wherein the monitoring device (44) is designed to monitor state variables of the screw pump (14) and to influence the progress of a maintenance process depending on changes in the state variables during the maintenance process.

11. Method according to any one of claims 1 to 10, wherein an axial gap between the first screw (31) and the second screw (32) on the suction side (20) has a greater gap height (39) than on the pressure side (21) and wherein the tolerance range (25) is limited by the suction side (20).

12. Control unit for a screw pump (14), wherein the control unit is designed to set the screws (31, 32) into a non-contact rotation and to systematically change the relative rotational position of the first screw (31) and the second screw (32) within a tolerance range (25) of the non-contact rotation in order to remove a deposit from a surface of the first screw (31) and a surface of the second screw (32).

13. Screw pump, wherein the screw pump (14) comprises a first screw (31) and a second screw (32), wherein a thread of the first screw (31) engages with a thread of the second screw (32), so that working chambers are formed by the mutual engagement of the threads, the volume of which decreases from a suction side (20) of the screw pump (14) to a pressure side (21) of the screw pump (14), further comprising a control unit according to claim 12 for controlling the screws (31, 32) of the screw pump.

14. Computer program product or set of computer program products, comprising program parts which, when loaded into a computer or into interconnected computers connected to a control unit according to claim 12, are designed to carry out the method according to any one of claims 1 to 11.

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