WO2017059897A1

WO2017059897A1 - High availability compressor for a gas compression system

Info

Publication number: WO2017059897A1
Application number: PCT/EP2015/073127
Authority: WO
Inventors: Andrea CORTINOVIS; Daniel Lewandowski; Mehmet Mercangoez; Piotr Lipnicki
Original assignee: Abb Schweiz Ag
Priority date: 2015-10-07
Filing date: 2015-10-07
Publication date: 2017-04-13
Also published as: EP3359820B1; AU2015411533B2; EP3359820A1; AU2015411533A1

Abstract

A method is disclosed 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 comprises obtaining measurements of one or more process variables (21) for the compressor and/or compression system from sensors mounted in the compressor or the gas compression system and obtaining a value of at least one electrical parameter (31) from the drive (2) and/or the electric motor (3). Further an estimation (35) of at least one process variable is calculated and compared with a measurement of the process variable (q). The measurement is then either validated or else replaced with the estimated value (q _est) of the process variable. A computer program for carrying out the method and a compressor and compression systems employing the method are also disclosed.

Description

High availability compressor for a gas compression system TECHNICAL FIELD.

The present invention is concerned with a method for controlling a compressor in a compression system, and a compression system implementing the method. In particular it is concerned with monitoring a compressor in a compression system in which the compressor is driven by a variable speed electric motor and controlling the compressor to achieve high availability and reliability .

TECHNICAL BACKGROUND

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

compressor flow.

Electrical variable-speed drive (VSD) equipped compressors are becoming more common in the oil and gas industry and these systems are expected to gain more market share in the future, mainly because of higher efficiencies and a reduced need for maintenance. Moreover, new legislations in various countries on CO2 emissions will further encourage the adoption of electrical drives. As a final point, for subsea operation electrical driven machines are the most preferred choice.

In patent publication US2010/0198480, entitled Method for detecting a rotating stall fault in a compressor fed by an inverter, and assigned to Siemens AG, 2010 for example, the motor current and motor speed are used to create an estimation of the torque. This value is compared to the nominal value at this operating point in order to detect if rotating stall has occurred so that a rotating stall condition can be detected without requiring a sensor in the compressor. In US2009/ 0252617 (Al), entitled Method for operation of a compressor supplied by a power converter, assigned to Siemens AG, 2009, the compressor speed and active currents of the drive are used to calculate the actual operating point, which is then compared to a surge map and torque map. These static maps are used for anti-surge control. In WO 2010/114786 (Al) entitled Compressor Surge

Control System and Method, assigned to GE Automation Systems LLC. 2010, the torque and speed or power and speed of the electrical motor are used to generate a 2D steady-state map and intended to be compared with a stored surge map and incorporated into the anti-surge and process control of the compressor.

Measurements of process variables are normally made to monitor and control the compressor, as well as to avoid a surge

condition. 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.

If one or more sensors or transmitters fail, then an alarm is generated and a fallback mode is initiated, many of which modes will assume the worst case reading for the missing transmitter and result in the maximum opening of the recycle valves. In the case of persisting sensor failures, the result is often a shutdown leading to reduced production and availability of the compressor plant. SUMMARY OF THE INVENTION

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

compression system from sensors mounted in the compressor or the gas compression system and obtaining a value of at least one electrical parameter from the drive and/or the electric motor, wherein the method comprises the further steps of:

calculating, with computer or processor hardware and computer program software an estimation of the one or more process variables ,

comparing the estimation of at least one process variable with a measurement of the process variable,

validating the measurement of the at least one process variable, or else replacing it with the estimated value of the at least one process variable, and

generating at least one control signal input to control the operation of the compressor.

According to a second aspect of the invention 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, and

means for obtaining a value of at least one electrical parameter from the drive and/or electric motor, wherein the system further comprises : computer or processor hardware and computer program software which on execution implements the functions of :

calculating an estimation of the one or more process variables; a validation block for comparing the estimation of the one or more process variables with a measurement of the process variable ,

the validation block being further arranged for validating the measurement of at least one process variable,

or else replacing it with an estimated value of the at least one process variable, and

arranged with 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 is disclosed 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. In the documents named in the Technical Background, 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 .

In contrast, 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.

Advantages include that 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. The only limitation is that the compression system needs to be run by an electrical variable speed drive. For example 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

electrical signals are available for utilization or

alternatively on a dedicated platform which is arranged for communications through fiber-optics or other high-speed

communication means to the variable speed drive control.

It is to be noted that any feature of the first aspect may be applied to the second aspect and the third aspect, wherever appropriate. Likewise, 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.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and system of the present invention may be had by reference to the following detailed description when taken in conjunction with the

accompanying drawings wherein:

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

estimated values for the process variable shown together against time; Figure 4 (Prior art) 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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully

hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention.

Figure 4 (Prior art) 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. In case of hot recycling, the gas flows through the hot recycle path and the hot recycle valve 112 back to the inlet of the compressor. Similarly, 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

practice. If one or more sensors or transmitters fail, then an alarm is generated and a fallback mode is initiated, many of which modes according to the prior art will assume the worst case reading for the missing sensor or transmitter signal and result in the maximum opening of the recycle valves and, in the case of persisting sensor failures, in a shut-down eventually leading to reduced availability of the compressor plant.

As noted in the Summary above a basic idea of 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

measurements in the case where a sensor failure is detected. To this end, it is necessary to establish the relationships or physical laws, which relate the electrical signals and the process variables. In the patent publication documents mentioned in the Technical Background above, the authors have made use of electrical signals, but only to a limited extent - e.g. in order to generate a static map or to estimate torque on the compressor shaft or to determine an operating point of a compressor. In contrast, the present invention shows, amongst other things, how to actively or dynamically estimate process variables and use the estimates to increase availability and reliability of a gas compression system. Figure 1 shows a schematic diagram of apparatus and control functions in a gas compressor system according to an embodiment of the invention. The figure 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. In addition control functions comprised in an

embodiment of the present invention are also shown in a

simplified block diagram form, for the purposes of visual clarity, which 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'. In the overview of the invention shown in Figure 1, 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. Equally well, traditional measuring instruments such as current transformers and voltage meters may be used for one or more of the electrical parameters such as input current or voltage and/or output current or voltage. 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.

Additionally, 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.

These estimations of values for the process variables are given back to the Validation Block 40, which uses them to either validate the process measurements or to replace measured variables under abnormal conditions. In normal conditions the validation succeeds and the measured signals are used for control. On the other hand, if abnormal conditions were

detected, 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. In the following, methods for the estimation of process variables are discussed in more detail. Moreover, more information is given about the algorithm under normal conditions, e.g. estimations are used for

validation of the measurements, and under sensor failure conditions, e.g. the estimations are employed to replace a measurement in one or more feedback signals.

Estimation of Process variables 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. As an example, 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

introducing a dynamic estimation of load torque on the basis of a rotating mass dynamic model according to an embodiment of the invention. The principle of the dynamic estimation is presented in Figure 2. 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

compressor motor drive 2' (similar to 2 in Fig.l) and a Rotating mass model 46 which both feed values forward to a first

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

(estimated) value for compressor flow, q_est- The estimation of a value for compressor flow q_est 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. In the basic form, 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. Alternatively 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_est 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

measurement that has been detected to be faulty. In some cases only the flow estimation may be used for control purposes.

Experiments which have been conducted have shown for example that a flow control goal based only on the flow estimation on the basis of electrical signals is possible as shown in

Figure 3. The error in flow stabilization, the error or

difference between actual measured value shown by line q and estimated values, _est for eg compressor flow has been found to be less than 2%.

Normal Conditions

Under normal conditions, the fault detection block tunnels the process signals through to the data acquisition block together with a good quality signal for each individual channel. As shown in Fig. 1, after receiving the process side (process variables 21) and 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

where

Zest is an estimated process variable,

Zmeas is a measured process variable, and

TH is a threshold value or setpoint;

then the measurement is correct and a corresponding signal is sent to the fault detection block. Other rules may be used as well as or instead of the above exemplary rule to specify a condition for replacing a measured value with an estimated value. For example different rules may be configured for various operating states such as: normal operation, start-up phase;

shut-down phase; operation including a maintenance state or manual control. In the case where a threshold has been exceeded the Value Replacement Block is triggered and the fault detection block is notified. In this case, the control feedback is performed using the one or more estimated values for selected process variables based on electrical signals.

Sensor Failure

Sensor failures are typically detected by the fault detection block 22. For illustration, let us assume that a flow sensor for measuring compressor flow failed. In this case the flow signal is detected to be faulty and a bad quality signal is generated in the fault detection block. For the other process signals, 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. In the estimation of the process variables only good quality signals are used. Clearly, there is a limit for how many signals can fail at the same time due to the fact that 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. These 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. Alternatively in safety critical cases 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.

In Figure 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. All 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.

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

acquisition block 32 of the controller 30 of the electrical variable speed drive 2. This block also 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. For example, 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

predetermined rule; depending, for example, on which process measurement is faulty, or on which control loop for which the faulty process measurement is used. 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. Alternatively or as well in safety critical cases 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.

Preferably 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. When 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. Thus 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. Alternatively, or as well, a second type of visual indicator such as a tab or bar 66

displayed with a colour or a visual pattern may be displayed representing a detected fault. The detected fault may generate a notice, a fault, an event and/or an alarm. In any event, 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. Alternatively 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.

Regardless of which type of visual indicator is used, in any event 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

indicator and/or on a visual representation eg symbol or faceplate, of the process object in question, such as a sensor.

Normal operations

On the other hand, in normal operation, 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. Finally, 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. At the same time 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.

Similarly, the arrangement shown in Figure 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 ;

82 Data acquisition block (DAB) passes 33 signals 21 from drive, process signals 21 and fault present-or-not information from 40 to process variable estimation block 34 (PVEB) ;

83 Process variable estimation block executes main algorithm and communicates result to Validation Block 40 (VB) ;

84 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) ;

85 if no fault is present as determined in the Validation Block 40 then measured process signals are transmitted 23 preferably directly from fault detection block 22 to the process and/or anti surge controller 44 (PC +/or SC) ;

86 if a fault is present as determined in the Validation Block 40 then 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.

In Figure 6 process side measurements and electrical side inputs such as 31, 21, are shown indicated with alternate long dash, short dash dotted lines. Control signal inputs such as 51-54 and 37 are shown with dotted lines. Reference numbers for apparatus or control functions correspond to the numbering of similar elements in Figures 5, 1. As described above, in this description it has been assumed that there is a separation between control functions carried out by a compressor and process controller 20; and those carried out by a drive controller 30 of the electrical variable speed drive, as is common practice. However it is within the scope of the claims that one or more control functions described here in relation to the compressor and process controller 20 unit may be implemented in one or more parts of the motor and drive side such as in the drive controller 30; and vice-versa.

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:

obtaining measurements of one or more process variables (21) for the compressor and/or compression system from sensors mounted in the compressor or the gas compression system and obtaining a value of at least one electrical parameter (31) from the drive (2) and/or the electric motor (3), wherein 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 further compris ing :

sensors mounted in the gas compression system for measurements for one or more process variables (21) for the compressor and/or compression system, and

means for obtaining a value of at least one electrical parameter (31) from the drive (2) and/or electric motor (3), wherein the system further comprises:

a 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:

an estimator block (34) for calculating an estimation (35) of the one or more process variables;

a Validation Block (40) for comparing the estimation of the one or more process variables with a measurement of the process variable and

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

a 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

functions of the 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. 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. The computer programs described may also be arranged in part as a distributed application capable of running on several different computers or computer systems at more or less the same time. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Sweden or any other country.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different

dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. It should be noted that while the above describes exemplifying embodiments of the invention, there are several variations and modifications which may be made to the disclosed high availability compressor method and system without departing from the scope of the present invention as defined in the appended claims.

Claims

1. 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:

obtaining measurements of one or more process variables (21) for the compressor and/or compression system from sensors mounted in the compressor or the gas compression system and

obtaining a value of at least one electrical parameter (31) from the drive (2) and/or the electric motor (3),

wherein the method comprises the further steps of:

calculating, with computer or processor hardware and computer program software an estimation (35) of the one or more process variables ,

comparing the estimation of at least one process variable with a measurement of the process variable (q)

validating the measurement of the at least one process variable, or else replacing it with the estimation value ( qr_est) of the at least one process variable,

and generating at least one control signal input (51-54) to control the operation of the compressor.

2. A method according claim 1, wherein the one or more process variables (21) sensed in the compressor or the gas compression system comprise at least one from the group of: (i) suction pressure, (ii) suction temperature, (iii) discharge pressure, (iv) discharge temperature, (v) compressor flow (q) , (vi) suction or discharge flow.

3. A method according to claim 1, wherein the electrical parameter is/are at least one of the group of: (a) DC voltage, (b) DC current, (c) grid voltage, (d) grid current, (e) motor voltage, (f) motor current, (g) phase to phase and phase to ground voltages for individual phases, (h) another derived or estimated quantity based on the electrical parameters.

4. A method according to any previous claim comprising the further step of generating in the control unit (20) at least one control signal input (51-54; 37) for controlling respectively at least one device from the group of: output valve (14), hot recycle valve (12), cold recycle valve (18), input valve (11), drive (2) .

5. A method according to any previous claim wherein at least one of the steps of, calculating, comparing, validating, generating are carried out in a block (22, 40, 42,) of the control unit (20) and/or carried out in a block (34) of the drive controller (30) .

6. A method according to claim 1 or 5 comprising at least one further step from the group of:

identifying in a fault detection block (22) if a fault is present or not;

sending (43) as a replacement value an estimated value for a variable which would normally be generated by a failed sensor for a period of time according to a predetermined rule; sending (43) as a replacement value an estimated value for the variable normally generated by a failed sensor if a fault of a sensor measurement is present for more than a predetermined number of samples or period of time;

comparing in a process variable Validation Block (40) if a fault is present generating and sending from the Validation

Replacement Block (42) the estimated value ( qr_est) of the process variable for replacement in generation of the at least one control signal input.

7. A method according to claim 1 or 6 comprising the further step of generating a notice or alarm about an estimated failure detected for a sensor measuring a process variable from the group of: (i) suction pressure, (ii) suction temperature, (iii) discharge pressure, (iv) discharge temperature, (v) compressor flow (q) and (vi) suction flow or discharge flow.

8. A method according to claim 6 comprising the further step of automatically generating at least one control action or control signal input based on the notice or alarm about an estimated failure identified for a sensor.

9. A method according to claim 1 or 6, comprising the further step of generating in a control means or process controller (44) a reference for the drive (2) in a control signal input (54) and sending it to a speed and torque controller (36) of the drive.

10. A method according to claim 1 or 6, comprising the further step of generating in a control means or process controller (44) a control signal input (51-54, 37) to influence a throughput of gas produced by the compressor (13) or compression system (60) .

11. A method according to claim 1 or any other previous claim, comprising the further steps of determining, with a process controller and/or a surge controller (44) block comprised in the control unit, that the compressor is approaching a surge condition and controlling the compressor (13) or the gas compression system (60) to avoid the surge condition.

12. A gas compression system (60) comprising at least one compressor (13) driven by an electric motor (3) powered by a drive (2), the system further comprising

computer or processor hardware and computer program software which on execution implements the functions of:

calculating an estimation (35) of the one or more process variables ;

a Validation Block (40) for comparing the estimation of the one or more process variables with a measurement of the process variable (q)

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 ( qr_est) of the at least one process variable, and arranged with

control means (44) for generating at least one control signal input (51-54; 37) for controlling the compressor.

13. A system according to claim 12, wherein the compressor or the gas compression system comprises at least one sensor for measuring at least one parameter from the group of: (i) suction pressure, (ii) suction temperature, (iii) discharge pressure, (iv) discharge temperature, (v) compressor flow (q) , (vi) suction flow or discharge flow.

14. A system according to claim 12, wherein the drive and/or motor is arranged for supplying one or more values for at least one of the electrical parameters: (a) DC voltage, (b) DC current, (c) grid voltage, (d) grid current, (e) motor voltage, (f) motor current, (g) phase-to-phase and phase-to-ground voltages for individual phases, (h) another derived or estimated quantity based on the electrical parameters.

15. A system according to claim 12, comprising control means (42) for inserting in an control signal input (51-54, 37) an estimated value (q_est) for the at least one process variable sent (43) to at least one the group of: control unit (20); process controller (44) and/or surge controller; drive controller (30) .

16. A system according to claim 12 or 13, wherein control functions are arranged in the control unit (20) for generating at least one control signal input (51-54; 37) for controlling respectively at least one device from the group of: output valve (14), hot recycle valve (12), cold recycle valve (18), input valve (11) , drive (2) .

17. A system according to claim 12, wherein control functions for a Validation Replacement Block (42) are arranged in the control unit (20) for generating and sending (43) the estimation of the process variable for replacement if comparison in the process variable Validation Block (40) has found a fault is present.

18. A system according to claim 12 or 17, wherein a fault detection block (22) or the control unit (20) is arranged for detecting if a fault is present or not and for carrying out a further step of generating, when a fault has been detected, a notice or alarm about an estimated failure of a sensor measuring a parameter from any of the group of: (i) suction pressure, (ii) suction temperature, (iii) discharge pressure, (iv) discharge temperature, (v) compressor flow (q) and (vi) suction flow or discharge flow.

19. A system according claim 12 or 14, wherein the control unit further comprises a control means or process controller (44) arranged for generating a reference for the drive (2) in an control signal input (54) for sending to a speed and torque controller (36) of the drive.

20. A system according to claim 13 or 15, wherein the control unit further comprises a control means or process controller (44) arranged for generating a control signal input (51-54; 37) to influence a throughput of gas produced by the compressor (13) or compression system (60) .

21. A system according to claim 12, wherein the control unit (20) comprises a process controller and/or a surge controller (44) block arranged for determining that the compressor is approaching a surge condition and controlling the compressor (13) or the gas compression system (60) to avoid the surge condition .

22. A system according to claim 19, wherein the control unit (20) further comprises an anti-surge controller (44) arranged for manipulating at least one recycle valve (12, 18) by means of a signal (52) and the process controller (44) which is arranged for manipulating an inlet and/or outlet valve respectively by means of at least one control signal input (53, 51) .

23. A system for increasing an availability of a gas compression system (60) comprising at least one compressor (13) driven by an electric motor (3) powered by a drive (2), the system further comprising

means for obtaining a value of at least one electrical parameter (31) from the drive (2) and/or electric motor (3), wherein the system further comprises

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 q_est (43) of the at least one process variable, and with

a control unit (20) which is arranged for generating at least one control signal input (51-54; 37) controlling the compressor to operate according to a predetermined setpoint using an estimated value for a process variable based on the at least one electrical parameter (s) .