Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
  • F04D27/00—Control, e.g. regulation, of pumps, pumping installations or pumping systems specially adapted for elastic fluids
  • F04D27/001—Testing thereof; Determination or simulation of flow characteristics; Stall or surge detection, e.g. condition monitoring
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
  • F04D27/00—Control, e.g. regulation, of pumps, pumping installations or pumping systems specially adapted for elastic fluids
  • F04D27/02—Surge control

Definitions

• the present invention is concerned with a method for controlling a compressor in a compression system, and a compression system implementing the method.
• a compression system implementing the method.
• Centrifugal gas compressors are widely used in industrial applications of gas compression systems such as gas lifting, gas processing, and pipeline transport. These large rotating machines are almost always mission critical and a compressor fault leads to a production delay or loss, and with inevitably large economic losses. As a consequence high availability is a much desired feature for gas compression systems.
• the operation of gas compressors is normally limited by several constraints such as minimum speed, maximum speed, choke limit and surge limit. Although most of the limits can be handled rather well, the surge limit is quite challenging to enforce in the presence of unexpected process disturbances.
• Compressor surge occurs when the developed compressor head drops below the network resistance and it leads to an unstable operating mode, where severe fluctuations of flow and pressure are observed up to the point of flow reversal with possible adverse effects such as vibration, overheating and mechanical damage to the system components. This damage may occur not only to bearings and seals but also to the compressor blades and even the piping arrangement around the compressor. Surge events may be violent enough to damage a compressor system in just a few cycles. As a consequence anti-surge control systems are strict safety requirements for compressor protection systems so as to ensure the avoidance of surge. Anti-surge control systems rely on special valves that can recycle or blow-off the compressed gas to reduce the system resistance and ensure forward
• VSD variable-speed drive
• Measurements of process variables are normally made to monitor and control the compressor, as well as to avoid a surge
• Temperature and pressure measurements on both the suction and discharge sides of the compressor in addition to other process variables such as compressor flow measurement are used by the anti-surge control system to determine the current distance to surge and in case of a disturbance, if cold, hot or both recycle valves need to be opened and to what degree. These measurements are critical to provide surge protection to the compressor system.
• the aim of the present invention is to remedy one or more of the above mentioned problems.
• This and other aims are obtained according to a first aspect of the invention by a method for controlling at least one compressor in a gas compression system, the compressor being driven by an electric motor powered by a drive, the method further comprising: obtaining measurements of one or more process variables for the compressor and/or
• a gas compression system comprising at least one compressor driven by an electric motor powered by a drive and controlled according to the method of the first aspect is disclosed, the system comprising at least one compressor driven by an electric motor powered by a drive, the system further comprising:
• sensors mounted in the gas compression system for measurements for one or more process variables for the compressor and/or compression system
• system further comprises : computer or processor hardware and computer program software which on execution implements the functions of :
• the validation block being further arranged for validating the measurement of at least one process variable
• control means for generating at least one control signal input for controlling the compressor.
• a computer program, and a computer program recorded on a computer-readable medium for controlling at least one compressor driven by an electric motor powered by a drive in a gas compression system according to another aspect of the invention .
• the invention enables a gas compression system to be operated with high availability.
• the authors have made some use of the electrical signals, but only to a limited extent such as for generating a static map for an operating point or a static map for comparison with a stored surge map, or for estimating a torque on the shaft or determining other operating conditions compared to a torque map .
• the present invention disclose a method and system in which values of electrical parameters are used to provide an estimation and/or validation of current measurements of process variables such that an estimated value for a specific process variable can be substituted when a missing or bad quality sensor signal is detected, thus allowing operations to be continued as planned.
• By calculating estimates for process variables based in part on electrical parameters compressor operations may be continued in the absence of a sensor signal, or in the presence of a sensor signal of a predetermined (low) quality.
• the use of such estimated process variable values allows production to continue as planned and under continuing monitoring and control, thereby maintaining production as planned, and so increasing availability and reliability of the gas compressor.
• this arrangement and system can be used for gas compressors such as for natural gas, air compressors, CO2 compressors, nitrogen compressors and all other types of gas compressors.
• gas compressors such as for natural gas, air compressors, CO2 compressors, nitrogen compressors and all other types of gas compressors.
• the compression system needs to be run by an electrical variable speed drive.
• the electrical variable speed drive may be a frequency converter, an inverter, a variable frequency drive or other type of power converter.
• the presented method can be implemented and executed on a fast drive control system, where all fast
• any feature of the first aspect may be applied to the second aspect and the third aspect, wherever appropriate.
• any advantage of the first aspect may equally apply to the second aspect, and/or the third aspect, respectively, and vice versa.
• Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.
• Figure 1 shows a schematic block diagram of apparatus and control functions in a gas compressor system controlled by a method according to an embodiment of the invention
• Figure 2 shows a schematic block diagram of a model used for control purposes in the method in the invention of Fig. 1 and in particular a model for calculating or estimating a value for a process variable according to an embodiment
• Figure 3 shows schematically a graph of a process variable used for control purposes in the invention of Fig. 1 which graph in particular is of measurements of a process variable and
• Figure 4 shows a schematic block diagram of apparatus in a gas compressor system according to a known general arrangement
• Figure 5 shows a schematic diagram of the invention of Fig. 1 and in particular apparatus and control functions in the method and gas compression system according to a preferred embodiment
• Figure 6 shows a schematic diagram of the invention of Fig. 1 comprising in particular a flowchart of steps of the method according to an embodiment
• Figure 7 schematically shows a data carrier with computer program code, in the form of a CD-ROM disc, for performing the steps of the method of the invention of Fig.l, according to an embodiment
• Figure 8 shows a display unit used to display a result of the method of Figure 1, in particular displaying the result on a user interface of the display unit, according to an embodiment.
• Figure 4 shows schematically apparatus in a gas compressor system process according to a known general arrangement.
• the figure summarizes important elements in a gas compression system driven by an electrical variable speed drive 102.
• the electrical drive is powered by the grid and drives an electric motor 103.
• a compressor 113 is coupled to the motor by a shaft. Natural gas coming from the inlet header 110 passes through the suction valve 111 and the cooler. The gas is then compressed in the centrifugal gas compressor 113 and either discharged through the outlet valve 114 to the outlet header 115; or recirculated through one or both recycle paths.
• the gas flows through the hot recycle path and the hot recycle valve 112 back to the inlet of the compressor.
• the cold recycle path and the cold recycle valve are used in case of cold recycle. Temperature and pressure measurements taken on both the suction and discharge sides of the compressor, in addition to a
• compressor flow measurement are used by an anti-surge control system to determine a current distance to surge and, in case of a disturbance, if cold, hot or both recycle valves need to be opened, and to what degree, in order to avoid a surge condition. These process measurements are critical to providing surge protection to the compressor system according to general
• an embodiment of the present invention is to use the electrical variable-speed drive as an additional sensor providing electrical parameter data that is used in order to estimate certain process variables, for example the compressor flow. Given the electrical parameters, and other available process measurements, estimates are calculated and substituted for the actual process
• FIG. 1 shows a schematic diagram of apparatus and control functions in a gas compressor system according to an embodiment of the invention.
• FIG. 1 shows an overview in which is shown some apparatus which it has in common with the apparatus shown in the known arrangement of Figure 4 (Prior art), namely: an electrical variable speed drive 2, powering an electric motor 3, which drives a compressor 13 via a drive shaft.
• Fig. 1 also shows a gas inlet valve 11, hot recycle valve 12, and gas outlet valve 14.
• control functions comprised in an
• control functions comprise, namely: Data Acquisition Block 32, Estimation of Process Variables Block 34; Fault Detection Block 22, Validation of Process Signals Block 40, Value Replacement Block 42; Compressor Control 44'.
• electrical signals are collected at sampling rates of preferably between 0.1-litis from the electrical drive 2 (and/or for some electrical parameters sampled from the electric motor) .
• the electrical signals may include: (a) DC voltage, (b) DC current, (c) grid voltages, (d) grid currents, (e) motor voltages, (f) motor currents, (g) phase-to-phase and phase-to-ground voltages for individual phases, (h) another derived or estimated quantity based on the electrical parameters.
• the electrical variable speed drive may, for example, be a variable frequency drive, a form of voltage frequency converter, an inverter or other type of power converter. Commercially available power converters and frequency converters are normally arranged with power electronic circuits and functions with which electrical parameters of the drive may be obtained or sampled in a straightforward manner.
• An electrical variable speed drive is normally also arranged with means for measuring or detecting electrical parameters such as phase-to-phase voltages and phase-to-ground voltages.
• process variable signals are acquired at a sampling rate of preferably between 50-100ms from a controller or the PLC of the process and are transmitted to the data acquisition block 32 through the fault detection block 22.
• Process variable signals may comprise: (i) suction pressure, (ii) suction temperature, (iii) discharge pressure, (iv) discharge temperature, (v) compressor flow and (vi) suction or discharge flow. Note that together with the process signals also the quality of the process variable signals is sent to the data acquisition block. With the information from process variables and electrical signals the estimation of the process variables is carried out.
• the Value Replacement Block 42 replaces the process variable measurements by their estimated values in case of sensor failure.
• the replaced values are then used by the compressor control system to generate the inputs to the plant.
• the inputs to the plant can include a control signal input or setpoint to any of: (i) suction valve position, (ii) discharge valve position, (iii) hot recycle valve position, (iv) cold recycle valve position, (v) reference - e.g. such as for motor torque, for the electrical drive 2.
• the estimations are employed to replace a measurement in one or more feedback signals.
• An important feature of present invention is the ability to estimate process variables from the process side on the basis of measurements of electrical signals from the drive and motor side.
• estimation of the compressor flow is described in this section.
• the compressor flow can be calculated when the load torque is available because those variables are tightly coupled. In steady-state conditions the load torque is balanced by the motor torque and thus it can be replaced by an estimation of the motor torque. When this is repeated for various operating points an appropriate mapping function can be derived. Unfortunately, such a relationship is only valid for steady-state operating conditions because during transient conditions the load torque is obviously not equal to motor torque. However, this disadvantage can be overcome by
• Fig. 2 shows a model for calculating or estimating a value for a process variable such as load torque of the compressor according to an embodiment.
• the diagram shows from left to right a
• comparator 47 which compares speed estimated from the drive signals with speed calculated from the Rotating mass model; and also forwards a difference signal to a Load Torque Estimator 48. Output from the Load Torque Estimator is then fed forward to both a Flow calculation 49 and fed back as Load Torque to a second comparator 45, left hand side. In the second comparator 45 the Load Torque is compared to an estimate of Drive Torque based on the motor drive signals. The difference from that comparison is then fed forward to the Rotating mass model 46. A flow amount is calculated 49 which in this example is an
• the estimation of a value for compressor flow q es t is based on the relation between load torque derived in steady-state, where the torque is calculated using an estimator block 48. Its goal is to balance the estimated speed derived from the rotating mass model and the estimated (or measured) speed provided by the compressor drive, in other words by electrical variable speed drive 2.
• the main assumption is that the rotating mass model and the real compressor are driven by the same variable, namely the motor torque. Therefore, correct balancing of the model leads to the estimation of load torque even in steady-state.
• the Load Torque Estimator 48 can be implemented as proportional-integral-derivative controller (PID controller) , which receives as an input the error derived as the difference of the rotating speed of compressor and its model.
• PID controller proportional-integral-derivative controller
• an inverse model may be used for modelling and/or control.
• the model formulation is reliable because it depends only on the shaft inertia parameter which is stable even over a long time hori zon .
• the calculated flow estimation q es t can be used for various purposes - for example it can be compared with the output signal from flow sensors to check if the process side measurements are still valid and do not deviate above acceptable limits as discussed below. In such a situation, the control system may select the estimation as the input signal instead of a
• est for eg compressor flow has been found to be less than 2%.
• the fault detection block tunnels the process signals through to the data acquisition block together with a good quality signal for each individual channel.
• the process side process variables 21
• electrical measurements 31 from eg electrical variable speed drive 2
• the estimation of process variables is carried out and the resulting estimations are transmitted to a
• Validation Block 40 The validation block, checks for example if one particular measurement is in a reasonable range of the estimated signal. For example, if ⁇ z est — z meas ⁇ ⁇ TH
• Zest is an estimated process variable
• Zmeas is a measured process variable
• TH is a threshold value or setpoint
• shut-down phase operation including a maintenance state or manual control.
• operation including a maintenance state or manual control.
• the Value Replacement Block is triggered and the fault detection block is notified.
• the control feedback is performed using the one or more estimated values for selected process variables based on electrical signals.
• Sensor failures are typically detected by the fault detection block 22.
• a flow sensor for measuring compressor flow failed.
• the flow signal is detected to be faulty and a bad quality signal is generated in the fault detection block.
• good quality signals are generated. All process signals and quality signals are sent to the data acquisition block 32, which discards the bad quality signals.
• the process side measurements are not uniquely determined by the electrical measurements. Given which signal is not available different estimation logics are used. The estimates are then given to the validation block, which is not able to validate the faulty signal and therefore directly triggers the Value Replacement Block for the specific signal (in the present example the compressor flow measurement) .
• the compressor controller receives the replaced value and continues to provide process control and anti-surge control also in this faulty situation. At the same time an alarm is sent to the operator and higher automation levels to inform about the failure in the system. (See information on an example of a User Interface 64 and a Visual Indicator 65, 66 described below in relation to Fig. 8.) As a reaction, different actions can be triggered automatically or by the operators in order to preserve safe operation and meet safety requirements.
• actions for example, in the form of a control input signal can include starting of a timer to limit the duration in which the faulty process signal will be replaced with the estimated value for process control and anti-surge control purposes, followed by a confirmation requirement by the operators to reset the timer to continue using the estimated value instead of the faulty process signal or to shut-down the process.
• the system can be prepared for a controlled shutdown instead of an abrupt shut-down via a predetermined sequence of automated actions such as the gradual unloading of the compressor system with the faulty process signal within a limited time during which the faulty process signal will be replaced with the estimated value again for process control and anti-surge control purposes.
• the latter case will provide a significant advantage in cases, where multiple compressor trains are involved and where an abrupt shut-down of one unit may lead to a cascaded shut-down of other units due to transient effects.
• FIG. 5 a preferred embodiment of the invention is shown schematically.
• An electrical variable speed drive 2 is powered by the grid 1, which drives a motor 3.
• a compressor 13 is coupled to the motor 3 by a shaft. Natural gas coming from the inlet header 10 passes through the suction valve 11. The gas is then compressed in the compressor 13 and either discharged through an outlet valve 14 to an outlet header line 15 or recirculated through a hot recycle path 16. Alternatively, or as well, gas can be re-circulated through a cold recycle path 17 by opening the cold recycle valve 18. In case of recycling via the hot recycle path, the gas flows through the recycle path 16 and a recycle valve 12 back to the inlet side of the compressor.
• valves are manipulated using their dedicated control signals: the outlet valve control signal 51, the hot recycle valve control signal 52 and the inlet valve control signal 53.
• a similar dedicated control input signal (not shown) may be used to operate the cold recycle valve 18 of the cold recycle path 17. This description of this preferred embodiment assumes a separation between a compressor and process controller 20 and a controller 30 of the electrical variable speed drive. This is a typical arrangement encountered in current industry practice.
• the fault detection block 22 After receiving process variable signals 21 from the process side such as from temperature, pressure and flow sensors, the fault detection block 22 detects if a fault is present or not. This information together with the process signal values for the various process variables is transmitted 24 to the data
• This block acquires electrical signals 31 from the electrical variable speed drive 2 and passes all signals 33 to a process variable estimation block 34.
• This block executes the main algorithm described in the previous section and communicates the results to the Validation Block 40 implemented preferably by the compressor and process controller 20.
• the Validation Block 40 also receives the process signals and the status signals 25 from the fault detection block 22 and is able to validate if the process signals are in agreement with the estimated variables 35. This information (26 and 41) is forwarded to the fault detection block 22 and to the Value Replacement Block 42. If a fault was detected, the Value
• Replacement Block 42 replaces the measured values by the estimations and sends signal 43 (comprising an estimate for a value of one or more process variables) for feedback control to the compressor and process controller 44 or/and anti-surge controller 44.
• One or more actions may be arranged to be carried out automatically as a control action by means of a control signal input, or setpoint, as a result of an estimated failure of a sensor or a detection of a fault.
• the control system may send 43 as a replacement value an estimation value for a failed sensor for a period of time according to a
• the control system may send 43 as a replacement value an estimated value for a failed sensor if a fault of a sensor measurement is present for more than a predetermined, relatively small number of samples or relatively short period of time, a time measured in seconds or minutes rather than hours or longer.
• the control actions may comprise: starting of a timer to limit the duration in which the faulty process signal will be replaced with the estimated value for process control and anti-surge control purposes, followed by a confirmation requirement by the operators to reset the timer to continue using the estimated value instead of the faulty process signal; or else to shut-down the process according to one or more procedures for a shutdown.
• the system can be prepared for a controlled shut-down instead of an abrupt shut-down via a predetermined sequence of automated actions such as the gradual unloading of the compressor system with the faulty process signal within a limited time during which the faulty process signal will be replaced with the estimated value again for process control and anti-surge control purposes.
• Such automatic control actions are specified in the control system in one or more predetermined rules for type of fault and operating conditions, with or without threshold values, for at least one process variable that would result in an automatic control action in the form of a control signal input such as those described herein.
• One or more proposed control actions dependent, for example, on a sensor failure type can be generated and displayed or otherwise presented to an operator for approval and execution or not; preferably together with summary information about the detected sensor failure.
• Figure 8 is a diagram showing a user interface or UI .
• the UI in this example displays a process graphic 64 of a part of the gas compression system. Process graphics are a widely used method to display monitoring and/or control information for an industrial process that is being monitored and/or controlled.
• Figure 8 shows a display unit 63 or workstation, a process graphic 64, a first visual indicator 65 and a second visual indicator 66.
• a fault in a sensor reading has been detected this event is signaled in the control system used to monitor and control the gas compression system.
• a first visual indicator such as a coloured light or symbol 65 may be displayed as a still or blinking graphic close to the schematic position of a faulty sensor in the compression system.
• a second type of visual indicator such as a tab or bar 66
• the detected fault may generate a notice, a fault, an event and/or an alarm.
• the detected fault is normally included in an alarm and event list (not shown) where it may be seen by the operator or other user(s) and subsequently marked to record that the event or alarm has been: acknowledged; un-acknowledged; shelved; is active; has been given a selected priority level; or other state as appropriate.
• the examples described above are of a graphic type of user interface UI .
• a detected fault or detected probable fault of a sensor will in any event be included in the alarm and event list of the control system, which is commonly provided in a text or tabular format.
• a detected fault may be displayed in graphic or text format.
• a detected fault may be displayed without a special visual indicator intended to inform a user at first glance that the fault has been detected by comparison of a measurement with an estimate as described herein.
• a visual indicator may include additional information to convey that the detected fault was detected by comparison of a measurement value and an estimated value. It is also possible that a specific visual indicator is displayed to show a user at first glance, that the fault, alarm or event has been detected by a comparison of measured with estimated values.
• the detected fault is arranged such additional information about the fault, including that the detected fault has been generated by a comparison of measured value with an estimated value, may be retrieved and displayed by the control system on receiving user input. This may be a user input such as a mouse- over or a mouse click on the first or the second visual
• a visual representation eg symbol or faceplate of the process object in question, such as a sensor.
• the measured process variables 21 are directly transmitted 23 to the process and/or anti-surge controller (P and/or SC) 44 after the signals were validated through the signals 26 in the Validation Block 40.
• the anti-surge controller 44 manipulates the recycle valve using control signal 52 and the process controller 44 manipulates the inlet and outlet valves using the control signals 51 and 53.
• the reference for the drive is manipulated using control signal 54. This reference is given to the speed and torque controller (S & TC) 36 of the driver controller 30 and is used to manipulate the electrical variable speed drive 2 with a control signal input 37.
• This arrangement can be used for gas compressors such as for natural gas, air compressors, CO2 compressors, nitrogen
• compressors and all other types of gas compressors.
• the only limitation is, that the compression system needs to be run by an electrical variable speed drive.
• the presented method can be implemented and executed on a fast drive control system, where all fast electrical signals are available for utilization or alternatively on a dedicated platform which is communicating through fiber-optics or other high-speed communication means to the drive control.
• FIG. 5 is a preferred embodiment and as such, only an example and not limited to other cases with missing equipment or additional elements such as blow-off paths, additional valves, tanks, coolers, scrubbers, etc .
• Figure 6 is a diagram which comprises a flowchart showing steps for carrying out the method according to an embodiment.
• Figure 6 shows a plurality of control functions or blocks which carry out the following steps: 81 Fault detection block 22 (FDB) detects whether a fault is present or not after receiving process signals 21 from the process ;
• FDB Fault detection block 22
• Data acquisition block passes 33 signals 21 from drive, process signals 21 and fault present-or-not information from 40 to process variable estimation block 34 (PVEB) ;
• Process variable estimation block executes main algorithm and communicates result to Validation Block 40 (VB) ;
• Validation Block validates if process signals and status signals 25 from detection block are in agreement with estimated variables, and sends result 26 to the fault detection block 22 and 41 to the Value Replacement Block (VRB) ;
• VRB Value Replacement Block
• the Value Replacement Block (VRB) 42 replaces the measured values in the process signals and sends signals 43 for feedback control to the process controller and/or anti-surge controller 44.
• Another embodiment discloses in a first aspect a method for controlling at least one compressor (13) in a gas compression system (60), the compressor being driven by an electric motor (3) powered by a drive (2), the method further comprising:
• the method comprises the further steps of: calculating, in a control unit (20) arranged with computer or processor hardware and computer program software by means of an estimator block (34), an estimation (35) of the one or more process variables, comparing with a Validation Block (40) the estimation of at least one process variable with a measurement of the process variable, validating with the Validation Block (40, 22) the measurement of the at least one process variable, or else replacing it with an estimated value (41) of the at least one process variable, and generating in the control unit (20) or with a control means (44) at least one control signal input (51-54; 37) to control the operation of the compressor.
• the embodiment discloses in a second aspect a gas compression system (60) comprising at least one compressor (13) driven by an electric motor (3) powered by a drive (2), the system
• sensors mounted in the gas compression system for measurements for one or more process variables (21) for the compressor and/or compression system, and
• control unit (20) or two or more control units distributed partly in the drive control system and partly in the compressor control system or anti surge system, arranged therein with computer or processor hardware and computer program software which on execution implements the functions of:
• a Validation Block (40) for comparing the estimation of the one or more process variables with a measurement of the process variable
• the Validation Block (40, 22) being further arranged for validating the measurement of at least one process variable, or else replacing it with an estimated value (41) of the at least one process variable, and with
• control means (44) or the control unit (20) being further arranged for generating at least one control signal input (51- 54) for controlling the compressor.
• the methods of controlling a compressor or compressor system as described in this specification may be carried out by a computer application comprising computer program code or software code which, when loaded in a processor or computer, causes the computer or processor to carry out the method steps described relative to Figures 1, 5 and 6.
• the methods and control may be carried out by a computer application comprising computer program code or software code which, when loaded in a processor or computer, causes the computer or processor to carry out the method steps described relative to Figures 1, 5 and 6.
• compressor system 60 may be carried out by processing digital functions, algorithms and/or computer programs and/or by analogue components or analogue circuits or by a combination of both digital and analogue functions .
• One or more integrated or distributed units of hardware or configurable hardware such as a Field-Programmable Gate Array (FPGA) or other types of processors including a Complex Programmable Logic Device (CPLD) or an Application Specific Integrated Circuit (ASIC) may be used.
• the or each processor may be arranged with a memory storage unit of a process system control unit, a surge controller (44), process controller (44) control unit (20) or a PLC (programmable Logic Controller) or other system part thereof, or may as well run in a local or central control system in a local or distributed computerised control system.
• FPGA Field-Programmable Gate Array
• CPLD Complex Programmable Logic Device
• ASIC Application Specific Integrated Circuit
• the or each processor may be arranged with a memory storage unit of a process system control unit, a surge controller (
• a part of the program may be stored in a processor and/or also in a ROM, RAM, PROM, EPROM or EEPROM chip or similar memory means.
• the program in part or in whole may also be stored on, or in, other suitable computer readable medium such as a magnetic disk, such as a CD (compact disc) or a DVD (digital versatile disc) , hard disk, magneto-optical memory storage means, in volatile memory, in flash memory, as firmware, stored on a data server or on one or more arrays of data servers.
• Figure 7 shows such a data carrier 72 with computer program code 73 stored on it, in the form of a CD-ROM or DVD disc, for performing the steps of the method of the invention of Figs. 1, 5 or Fig. 6, according to an embodiment.

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• Control Of Positive-Displacement Pumps (AREA)
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