Control of a liquid ring pump

Abstract

A control system comprising: a liquid ring pump (10); a motor (12) configured to drive the liquid ring pump (10); and a controller (20) configured to: determine a current within the motor (12); determine a speed of the motor (12); calculate a value of a function (F) that is a function of the determined current within the motor (12) and the determined speed of the motor (12); and output one or more control signals based on the calculated value of the function (F).

Description

Technical Field

[0001] This invention relates to the control of liquid ring pumps. Background Technology

[0002] Liquid ring pumps are a known type of pump commonly used commercially as vacuum pumps and gas compressors. A liquid ring pump typically includes a housing having a chamber therein, a shaft extending into the chamber, an impeller mounted on the shaft, and a drive system (such as a motor) operably connected to the shaft to drive it. The impeller and shaft are eccentrically positioned within the chamber of the liquid ring pump.

[0003] During operation, the chamber is partially filled with an operating fluid (also known as a service fluid). As the drive system drives the shaft and impeller, a liquid ring forms on the inner wall of the chamber, thereby providing a seal that isolates separate volumes between adjacent impeller blades. The impeller and shaft are positioned eccentrically relative to the liquid ring, which causes a cyclical change in the volume enclosed between adjacent impeller blades and the liquid ring.

[0004] In the portion of the chamber where the liquid ring is further away from the shaft, there is a larger volume between adjacent impeller blades, resulting in lower pressure. This allows the portion of the liquid ring further away from the shaft to act as the intake zone. In the portion of the chamber where the liquid ring is closer to the shaft, there is a smaller volume between adjacent impeller blades, resulting in higher pressure. This allows the portion of the liquid ring closer to the shaft to act as the exhaust zone.

[0005] Examples of liquid ring pumps include single-stage liquid ring pumps and multi-stage liquid ring pumps. A single-stage liquid ring pump involves using only a single chamber and impeller. A multi-stage liquid ring pump (e.g., a two-stage liquid ring pump) involves using multiple chambers and impellers connected in series. Summary of the Invention

[0006] The inventors have recognized that in certain situations, such as after a period of downtime, a liquid ring pump can be started in a "dry" or "dry-running" state. In such a dry state, the liquid ring pump contains a lower-than-desirable level of operating fluid. Therefore, a significant amount of heat can be generated within the liquid ring pump, potentially damaging its components. The inventors have also recognized that the mechanical seals of liquid ring pumps are often susceptible to damage due to the heat generated during dry-running.

[0007] The inventors have further recognized the need to provide a method for controlling a liquid ring pump in a manner that prevents, reduces, or limits its operation in a dry state.

[0008] The inventors have further recognized that when a liquid ring pump operates in its dry state, the current in the motor driving the pump (i.e., the current in the motor wiring, such as the current in the motor's stator windings) is lower than the current under normal operating conditions. However, it is often impossible to detect whether a liquid ring pump is operating in its dry state using only this current measurement, because the current in a motor operating at low speed will also tend to be low, even if the driven liquid ring pump is not operating in its dry state. The inventors have further recognized that the dry state of a liquid ring pump can be determined based on some function (such as a ratio) of the current in the motor driving the pump and the speed of that motor.

[0009] In one aspect, a control system is provided, comprising: a liquid ring pump; a motor configured to drive the liquid ring pump; and a controller configured to: determine a current within the motor; determine a speed of the motor; calculate the value of a function which is a function of the determined current within the motor and the determined speed of the motor; and output one or more control signals based on the calculated value of the function.

[0010] The control system may also include an alarm module configured to output audible and / or visual alarms. A first control signal of the one or more control signals may be used to control the operation of the alarm module. A second control signal of the one or more control signals may be used to control the operation of the motor.

[0011] The function can be the ratio between a given current in the motor and a given speed of the motor.

[0012] The controller may also be configured to: compare a calculated value of a function with a threshold; and output one or more control signals based on the comparison. The determined current may be a value in amperes. The motor speed may be a value in revolutions per minute. The threshold may be a value greater than or equal to 0.015. The threshold may be approximately 0.02.

[0013] The control system may also include an alarm module configured to output audible and / or visual alarms. The controller may also be configured to output a first control signal to control the alarm module to output audible and / or visual alarms in response to a calculated value of a determining function being less than or equal to a threshold.

[0014] The controller may also be configured to output a second control signal to the motor to stop the motor from driving the liquid ring pump in response to a calculated value of the determined function being less than or equal to a threshold. The controller may also be configured to output a second control signal to the motor in response to a value of the function being less than or equal to the threshold within a predefined time period. This predefined time period may be in the range of 2 to 5 seconds, for example, about 3 seconds.

[0015] The controller can be configured to output a third control signal to the motor in response to a calculated value of a deterministic function being greater than a threshold, thereby controlling the motor drive (e.g., continuing to drive) the liquid ring pump.

[0016] The control system may also include: a pump configured to pump operating liquid into a liquid ring pump; and an additional motor configured to drive the pump. One of the one or more control signals may be used to control the operation of the pump.

[0017] The controller may include a frequency converter.

[0018] In another aspect, a method for a control system is provided. The system includes a liquid ring pump, a motor configured to drive the liquid ring pump, and a controller. The method includes: determining a current within the motor by the controller; determining a speed of the motor by the controller; calculating a value for a function that is a function of the determined current within the motor and the determined speed of the motor by the controller; and outputting one or more control signals by the controller based on the calculated value of the function.

[0019] In another aspect, a program or programs are provided, arranged such that, when executed by a computer system or one or more processors, the program or programs cause the computer system or the one or more processors to: determine a current in a motor connected to the computer system or the one or more processors, the motor being configured to drive a liquid ring pump; determine a speed of the motor; calculate the value of a function that is a function of the determined current in the motor and the determined speed of the motor; and output one or more control signals based on the calculated value of the function.

[0020] In another aspect, a machine-readable storage medium is provided that stores a program according to the preceding aspect or at least one of the plurality of programs. Attached Figure Description

[0021] Figure 1 This is a schematic diagram (not to scale) showing a vacuum system;

[0022] Figure 2 This is a schematic diagram of a liquid ring pump (not to scale); and

[0023] Figure 3 It is a process flow diagram that shows some steps of the control process implemented by the vacuum system. Detailed Implementation

[0024] Figure 1 This is a schematic diagram (not to scale) showing vacuum system 2. Vacuum system 2 is connected to facility 4 such that, in operation, vacuum system 2 establishes a vacuum or low-pressure environment at facility 4 by drawing gas (e.g., air) from facility 4.

[0025] In this embodiment, the vacuum system 2 includes a check valve 6, a liquid ring pump 10, a motor 12, a separator 14, a pump system 16, a controller 20, and an alarm module 22.

[0026] Facility 4 is connected to the inlet of liquid ring pump 10 via a suction or vacuum line or pipe 28.

[0027] Check valve 6 is installed on suction line 28. Check valve 6 is located between facility 4 and liquid ring pump 10.

[0028] The check valve 6 is configured to allow fluid (e.g., gas, such as air) to flow from facility 4 to liquid ring pump 10 and to prevent or stop fluid from flowing in the opposite direction (i.e., from liquid ring pump 10 to facility 4).

[0029] In this embodiment, the liquid ring pump 10 is a single-stage liquid ring pump.

[0030] The gas inlet of the liquid ring pump 10 is connected to the suction line 28. The gas outlet of the liquid ring pump 10 is connected to the discharge line or pipe 30. The liquid ring pump 10 is connected to the pump system 16 via the first operating liquid line 32. The liquid ring pump 10 is configured to receive operating liquid from the pump system 16 via the first operating liquid line 32. The liquid ring pump 10 is driven by the motor 12.

[0031] Figure 2 This is a schematic diagram (not to scale) of a cross-section of an example liquid ring pump 10. (In the description...) Figure 2 Following the liquid ring pump 10 shown, the remainder of the vacuum system 2 will be described in more detail later below.

[0032] Figure 2 The liquid ring pump 10 illustrated includes a housing 100 defining a generally cylindrical chamber 102, a shaft 104 extending into the chamber 102, and an impeller 106 fixedly mounted to the shaft 104. A gas inlet 108 of the liquid ring pump 10 (connected to a suction line 28) is fluidly connected to an air inlet of the chamber 102. A gas outlet of the liquid ring pump 10 (…) Figure 2 (Not shown) The gas output is fluidly connected to chamber 102.

[0033] During operation of the liquid ring pump 10, the operating fluid is received in the chamber 102 via the first operating fluid line 32. The shaft 104 is rotated by the motor 12, thereby causing the impeller 106 to rotate within the chamber 102. As the impeller 106 rotates, the operating fluid (not shown) in the chamber 102 is forced against the wall of the chamber 102, thereby forming a liquid ring that seals and isolates individual volumes between adjacent impeller blades. Furthermore, gas (such as air) is drawn into the chamber 102 from the suction line 28 via the gas inlet 108 and the air inlet of the chamber 102. This gas flows into the volume formed between adjacent blades of the impeller 106. The rotation of the impeller 106 causes a decrease in the size of said volume. As the gas moves from the air inlet of the chamber 102 to the gas outlet of the chamber 102, the rotation of the impeller 106 compresses the gas contained within the volume, and at the gas outlet, the compressed gas leaves the chamber 102. The compressed gas leaving chamber 102 then exits the liquid ring pump via gas outlet and discharge line 30.

[0034] Now back Figure 1 As described, discharge line 30 is connected between the gas outlet of liquid ring pump 10 and the inlet of separator 14. Separator 14 is connected to liquid ring pump 10 via discharge line 30, such that discharge fluid (i.e., compressed gas, which may be accompanied by or include water droplets and / or vapor) is received by separator 14.

[0035] The separator 14 is configured to separate the discharge fluid received from the liquid ring pump 10 into gas (e.g., air) and operating liquid.

[0036] The gas separated from the received discharge fluid is discharged from the separator 14 and the vacuum system 2 via the system outlet pipe 34.

[0037] The separator 14 includes an operating liquid inlet through which the operating fluid separated from the received discharge fluid is output from the separator 14 and the vacuum system 2 via a discharge pipe or vent pipe 36.

[0038] In this embodiment, the pump system 16 includes a pump (e.g., a centrifugal pump) and a motor configured to drive the pump. The pump system 16 is configured to pump operating liquid from an operating liquid source 38 via a second operating liquid line 40 and to pump the operating liquid to a liquid ring pump via a first operating liquid line 32.

[0039] The operating liquid source 38 can be any suitable operating liquid source. For example, in an embodiment where the operating liquid is water, the operating liquid source 38 can be a tap water supply, a river, a lake, a water storage tank, etc.

[0040] Controller 20 may include one or more processors. In this embodiment, controller 20 is a proportional-integral (PI) controller. In this embodiment, controller 20 includes a variable frequency drive (VFD) 42. VFD 42 is configured to control the speed of motor 12. VFD 42 may also be configured to control the speed of a motor in pump system 16.

[0041] Controller 20 is connected to motor 12 via its VFD 42 and via a first connection 44, such that control signals for controlling motor 12 can be transmitted from controller 20 to motor 12. The first connection 44 can be any suitable type of connection, including but not limited to wired or fiber optic connections, or wireless connections. Motor 12 is configured to operate according to the control signals it receives from controller 20. (Refer to below.) Figure 3 The control of motor 12 by controller 20 is described in more detail.

[0042] The controller 20 is also connected to the pump system 16 via its VFD 42 and via a second connection 46, such that control signals for controlling the pump system 16 can be sent from the controller 20 to the motor of the pump system 16. The second connection 46 can be any suitable type of connection, including but not limited to wired or fiber optic connections, or wireless connections. The pump system 16 is configured to operate according to the control signals it receives from the controller 20.

[0043] The controller 20 is also connected to the alarm module 22 via a third connection 48, such that control signals for controlling the alarm module 22 can be sent from the controller 20 to the alarm module 22. The third connection 48 can be any suitable type of connection, including but not limited to wired or fiber optic connections, or wireless connections.

[0044] Alarm module 22 is configured to provide or output alarms or notifications to persons and / or other systems (e.g., computer systems) remote from vacuum system 2. Examples of appropriate alarms or notifications include, but are not limited to, audible alarms (such as bells) and visual alarms (such as messages or flashing lights on displays).

[0045] Therefore, an embodiment of vacuum system 2 is provided.

[0046] By constructing or adapting any suitable device (e.g., one or more computers or other processing devices or processors) and / or providing additional modules, an apparatus for implementing the above arrangements and performing the method steps described later below can be provided, including controller 20. The apparatus may include a computer, a network of computers, or one or more processors for implementing instructions and using data, including instructions and data in the form of one or more computer programs stored in or on a machine-readable storage medium such as computer memory, computer disk, ROM, PROM, or any combination of these or other storage media.

[0047] Figure 3 This is a process flow diagram illustrating certain steps of an embodiment of the control process for controlling the operation of the liquid ring pump 10.

[0048] Note that this can be omitted. Figure 3 The process steps depicted in the flowchart and described below may be presented differently from those shown below. Figure 3 The process steps are performed in the order shown. Furthermore, although all process steps have been depicted as discrete, time-sequential steps for convenience and ease of understanding, some of these process steps may actually be performed simultaneously or at least overlap in time to some extent.

[0049] Figure 3 The process can be viewed as an "anti-dry run" process.

[0050] At step s2, starting from the power-off state ("off"), the controller 20 controls the motor 12 to drive the liquid ring pump 10. In other words, the pumping operation of the liquid ring pump begins.

[0051] At step s4, controller 20 determines or measures the current in motor 12 (i.e., within the wiring of motor 12), such as the current in the stator windings of motor 12. Specifically, in this embodiment, VFD 42 determines or measures the current in motor 12. Specifically, in this embodiment, VFD 42 converts the input power of motor 12 from AC to DC and then back to AC to achieve the desired frequency. The output current is determined or measured during this conversion process. The measured current value can be stored in a register on the integrated circuit board of VFD 42.

[0052] The current in motor 12 can be determined or measured when or shortly after the liquid ring pump 10 starts pumping operation (e.g., during a predetermined period of time when motor 12 is started).

[0053] At step s6, the controller 20 determines or measures the speed of the motor 12. Specifically, in this embodiment, the VFD 42 determines or measures the speed of the motor 12. More specifically, the VFD determines the frequency of the output power supplied to the motor 12. The VFD 42 uses the frequency of the output power supplied to the motor 12 to determine the speed of the motor 12. Specifically, in this embodiment, the speed of the motor 12 is determined as:

[0054]

[0055] Where: nN is the speed of motor 12;

[0056] f is the frequency of the output power; and

[0057] P is the number of pole pairs of motor 12.

[0058] The speed of motor 12 can be determined or measured when or shortly after the liquid ring pump 10 begins pumping operation (e.g., during a predetermined period of time when motor 12 is started).

[0059] Preferably, the time point at which the speed of motor 12 is determined or measured is the same as the time point at which the current in motor 12 is determined or measured in step s4.

[0060] At step s8, the controller 20 calculates a function of the current in the motor 12 (determined at step s4) and the speed of the motor (determined at step s6).

[0061] In this embodiment, the controller 20 calculates the ratio of the current to the speed of the motor 12. In other words, the controller 20 calculates a function F, where:

[0062] F = I / s

[0063] in:

[0064] I is the current defined in motor 12, which can be measured in amperes (A); and

[0065] s is the determined speed of motor 12, which can be measured in revolutions per minute (rpm).

[0066] For example, the current in motor 12 can be determined to be 77A, and the speed of motor 12 can be determined to be 2100rpm. Therefore, the value of function F can be 77 / 2100 = 0.037.

[0067] At step s10, the controller 20 compares the determined function value F with a threshold. The first threshold can be any suitable value. The inventors have recognized that the dry operating conditions of the liquid ring pump can correspond to a function value of F = 0.015. Therefore, preferably, the threshold is greater than 0.015. For example, the threshold can be in the range of 0.015-0.030, or more preferably in the range of 0.015-0.025, or even more preferably about 0.020.

[0068] If at step s10, the controller 20 determines that the function value F is less than or equal to the threshold (e.g., if F ≤ 0.02), then the method proceeds to s12.

[0069] However, if at step s10 the controller 20 determines that the function value F is greater than the threshold, the method proceeds to s18. Step s18 will be described in more detail later below.

[0070] At step s12, in response to determining that the function value F is less than or equal to a threshold, the controller 20 determines that the liquid ring pump 10 is operating under dry operating conditions (i.e., there is insufficient operating fluid in the liquid ring pump 10). Therefore, at step s12, the controller 20 controls the alarm module 22 to output an alarm.

[0071] At step s14, under the control of controller 20, alarm module 22 outputs alarms, siren bells, or notifications (such as visual and / or audible alarms) to the human operator of vacuum system 2. This notifies the human operator to take appropriate action. Examples of such actions include, but are not limited to, checking or determining the root cause of the dry operating condition, taking action to eliminate the abnormal element, resetting the error on the controller's display, and restarting the system.

[0072] At step s16, in response to determining that the liquid ring pump 10 has been operating under its dry operating conditions for a period of time greater than or equal to a predefined time period, the controller 20 controls the motor 12 to stop driving the liquid ring pump 10. Therefore, if the liquid ring pump 10 operates for a predefined time period while the function F value is less than a threshold, the liquid ring pump 10 is shut down. Advantageously, this shutdown of the liquid ring pump tends to reduce or limit damage to components of the liquid ring pump 10 (such as the mechanical seal of the liquid ring pump) that can be caused by excessive heat generated due to the liquid ring pump 10 operating in its "dry" state (i.e., where the operating fluid is insufficient).

[0073] The predefined time period can be settable or adjustable, for example, set or adjusted by a human operator. The predefined time period can be any suitable time period. The inventors have recognized that time periods between approximately 2 seconds and 5 seconds, and more preferably approximately 3 seconds, tend to provide a reduction in damage to the components of the liquid ring pump 10.

[0074] After step s16, Figure 3 The process ends, during which the liquid ring pump 10 stops. Subsequently, by restarting the liquid ring pump 10 at step s2, the process can be restarted. Figure 3 The process.

[0075] Now, returning to step s10, when controller 20 determines that the function value F is greater than the threshold, the method proceeds to s18.

[0076] At step s18, the controller 20 determines that the liquid ring pump 10 is not operating under dry operating conditions (i.e., there is sufficient operating fluid in the liquid ring pump 10). Therefore, at step s18, the controller 20 controls the motor 12 to continue driving the liquid ring pump 10. The liquid ring pump 10 can be driven in this manner until it stops, and Figure 3 The process is over.

[0077] Therefore, an embodiment of the control process implemented by the vacuum system 2 is provided.

[0078] Advantageously, the systems and methods described above allow for the control of liquid ring pumps in a manner that reduces or limits their operation under dry conditions (i.e., with insufficient operating fluid). Therefore, the systems and methods described above tend to reduce or limit damage to components of the liquid ring pump, such as its mechanical seals.

[0079] Advantageously, the controller (e.g., VFD) is configured to determine the current within the motor and the motor speed using the VFD's own operating parameters or states. The VFD may be primarily configured for motor speed control. During VFD operation, both the current within the motor and the frequency of the power output by the VFD are measured by firmware / execution software and stored in the VFD's registers. Those values ​​are advantageously available to the controller's processors via existing communication with the VFD. Therefore, the need for additional sensors for measuring either or both of these parameters is often reduced, eliminated, or avoided. The risk of such sensor failure is often reduced or eliminated. Furthermore, design savings are often achieved. Additionally, the requirements for sensor maintenance are often reduced or eliminated. Nevertheless, in some embodiments, one or both of the current within the motor and the motor speed may be measured by sensors. Such sensors may be coupled to the motor and configured to send measured values ​​to the controller.

[0080] In the above embodiments, the vacuum system includes the above-referenced... Figure 1The components described. Specifically, the vacuum system includes a check valve, a liquid ring pump, a motor, a separator, a pumping system, a controller, an alarm module, and connections therebetween. However, in other embodiments, the vacuum system includes components that replace or are not included in addition to those described above. Moreover, in other embodiments, some or all of the components of the vacuum system may be connected together in a suitable manner different from that described above. In some embodiments, multiple liquid ring pumps may be implemented.

[0081] In the above embodiments, the separator outputs the separated operating liquid and separated gas from the system via corresponding output pipes. However, in other embodiments, the separated operating liquid and / or separated gas are not output from the system. For example, in some embodiments, the operating liquid is recirculated from the separator back to the liquid ring pump. Recirculation of the operating liquid advantageously often reduces operating costs and water consumption. In some embodiments, the separator may be omitted.

[0082] In the above embodiments, the liquid ring pump is a single-stage liquid ring pump. However, in other embodiments, the liquid ring pump is a different type of liquid ring pump, such as a multi-stage liquid ring pump.

[0083] In the above embodiments, the operating liquid is water. However, in other embodiments, the operating liquid is a different type of operating liquid.

[0084] In the above embodiments, the controller is a PI controller. However, in other embodiments, the controller is a different type of controller, such as a proportional (P) controller, an integral (I) controller, a derivative (D) controller, a proportional-derivative (PD) controller, a proportional-integral-derivative (PID) controller, or a fuzzy logic controller.

[0085] In the above embodiments, a single controller controls the operation of multiple system components (e.g., motors). However, in other embodiments, multiple controllers may be used, each controlling a corresponding subset of the component group. For example, in some embodiments, each motor may have a dedicated controller.

[0086] In the above embodiments, the function F of the current in the motor wiring and the motor speed is F = I / s. However, in other embodiments, different functions of the current in the motor wiring and the motor speed are implemented. For example, weights may be applied to the determined current and / or motor speed.

[0087] In the above embodiments, alarms and possible shutdowns of the liquid ring pump are performed based on a comparison of the value of function F with a threshold. However, in other embodiments, one or more different actions are performed based on the comparison of the value of function F with a threshold, replacing or supplementing one or both of alarms and shutdowns of the liquid ring pump. For example, in some embodiments, if it is determined based on the comparison of the value of function F with a threshold that the liquid ring pump is operating in its dry state, the pumping system motor can be controlled to regulate or modulate the flow of the operating liquid into the liquid ring pump, for example, to increase the flow of the operating liquid into the liquid ring pump. This allows the liquid ring pump to exit its dry state operation.

[0088] Figure Labels

[0089] 2-Vacuum System

[0090] 4- Facilities

[0091] 6-Check valve

[0092] 10-Liquid Ring Pump

[0093] 12-motor

[0094] 14-Separator

[0095] 16-Pump System

[0096] 20-Controller

[0097] 22-Alarm Module

[0098] 28-Suction line

[0099] 30-Emission Pipeline

[0100] 32-First Operating Liquid Tube

[0101] 34-System Outlet Pipe

[0102] 36-Drain pipe

[0103] 38-Operating Liquid Source

[0104] 40-Second Operating Liquid Tube

[0105] 42-Variable Frequency Drive

[0106] 44-First Connection

[0107] 46-Second Connection

[0108] 100-Shell

[0109] 102-chamber

[0110] 104-axis

[0111] 106-Impeller

[0112] 108-Gas Inlet

[0113] s2-s28-Method Steps

Claims

1. A control system comprising: Liquid ring pump; A motor configured to drive the liquid ring pump; as well as The controller is constructed as follows: Determine the current within the motor; Determine the speed of the motor; Calculate the value of a function that is the ratio of a given current within the motor to a given speed of the motor; and Based on the calculated value of the function, one or more control signals are output to indicate operation in the "dry run" state.

2. The control system according to claim 1, further comprising an alarm module configured to output audible and / or visual alarms, wherein, The first control signal among the one or more control signals is used to control the operation of the alarm module.

3. The control system according to claim 1, wherein, The second control signal among the one or more control signals is used to control the operation of the motor.

4. The control system according to any one of claims 1 to 3, wherein, The controller is also configured to: The calculated value of the function is compared with a threshold; and The one or more control signals are output based on the comparison.

5. The control system according to claim 4, wherein: The determined current is a value of current in amperes; The speed of the motor is a value in revolutions per minute; and The threshold is a value greater than or equal to 0.

015.

6. The control system according to claim 5, wherein, The threshold is equal to 0.

02.

7. The control system according to claim 4, wherein: The control system further includes an alarm module configured to output auditory and / or visual alarms. The controller is further configured to: in response to determining that the calculated value of the function is less than or equal to the threshold, output a first control signal to control the alarm module to output the auditory and / or visual alarm.

8. The control system according to claim 4, wherein: The controller is also configured to: in response to determining that the calculated value of the function is less than or equal to the threshold, output a second control signal to the motor to stop the motor from driving the liquid ring pump.

9. The control system according to claim 8, wherein: The controller is configured to output the second control signal to the motor in response to the calculated value of the function being less than or equal to the threshold within a predefined time period.

10. The control system according to claim 9, wherein, The predefined time period is in the range of 2 to 5 seconds.

11. The control system according to claim 4, wherein: The controller is further configured to: in response to determining that the calculated value of the function is greater than the threshold, output a third control signal to the motor to control the motor to drive the liquid ring pump.

12. The control system according to any one of claims 1 to 3, wherein: The control system further includes: a pump configured to pump an operating liquid into the liquid ring pump; and... An additional motor is configured to drive the pump; and One of the one or more control signals is used to control the operation of the pump.

13. The control system according to any one of claims 1 to 3, wherein, The controller includes a frequency converter.

14. A method for controlling a system, the system comprising a liquid ring pump, a motor configured to drive the liquid ring pump, and a controller, the method comprising: The current within the motor is determined by the controller; The speed of the motor is determined by the controller; The controller calculates the value of a function that is the ratio of a given current in the motor to a given speed of the motor; and The controller outputs one or more control signals indicating operation in a "dry run" state based on the calculated value of the function.

15. A computer program product comprising one or more programs arranged such that, when executed by a computer system or one or more processors, the one or more programs cause the computer system or the one or more processors to: Determine the current in a motor connected to the computer system or one or more processors, the motor being configured to drive a liquid ring pump; Determine the speed of the motor; Calculate the value of a function that is the ratio of a given current within the motor to a given speed of the motor; and Based on the calculated value of the function, one or more control signals are output to indicate operation in the "dry run" state.

16. A machine-readable storage medium storing one or more programs arranged such that, when executed by a computer system or one or more processors, the one or more programs cause the computer system or the one or more processors to: Determine the current in a motor connected to the computer system or one or more processors, the motor being configured to drive a liquid ring pump; Determine the speed of the motor; Calculate the value of a function that is the ratio of a given current within the motor to a given speed of the motor; and Based on the calculated value of the function, one or more control signals are output to indicate operation in the "dry run" state.

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