THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. 345 E. 47th St., New York, N.Y

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published 95-GT-260 m in an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not failing within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright 1995 by ASME All Rights Reserved Printed in U.S.A. AN ADVANCED CONTROL AND MONITORING SYSTEM FOR TURBOMACHINERY Wilfried Blotenberg Compressor and Turbine Division MAN Gutehoffnungshutte AG Oberhausen Germany ABSTRACT An advanced digital control and monitoring system for turbomachinery is introduced. Important aspects for the design of anti-surge control systems, such as the influence of the controller program execution time on the performance, are investigated. The influence of the gas composition (molecular weight) on the compressor surge line is demonstrated. Control laws are discussed. The design basis of an ultra-high speed antisurge and speed control system is shown and the data-logging features are demonstrated on the log of a compressor surge. The paper closes describing some dynamic process simulation being performed using the controller hardware and software for process simulations. Nomenclature Formula coefficients C Constant Oh Compressor head dt Time increment e Control error KP Controller gain k Isentropic exponent p Pressure Ap Pressure difference R Gas constant T Temperature TN Controller reset time T S Scan time t Time u Controller output ^r Volumetric flow rate xd Control Deviation, distance between operating point and control line y Controller Output z Compression factor Subscripts 1 Suction side 2 Discharge side INTRODUCTION Anti-surge and speed controls are the most important protection systems for turbomachines. The design of these control systems has changed dramatically in the last two decades. In the middle of the seventies, the first electronic control systems, based on analog systems, were introduced to the market. Micro-processor based systems followed in the eighties. The market for microprocessor based anti-surge and speed control systems has witnessed dramatic developments the last years. 8-bit processors are no longer used, 16 bit processors are the basic standard and 32-bit processors are becoming more and more popular. The processing speed increased from 4 MHz at the beginning of the 80s to over 50 MHz. A second improvement was the processor technology. Beside the standard microprocessors, digital signal processors have been developed for high-speed processing. State-of-the-art signal processors operate with 50 MHz, 40 MIPS, and are able to effect one multiplication and one addition of 16 bit values in one stroke. This allows ultra-high speed control systems. Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8, 1995

2 CONTROL LAWS FOR ANTI-SURGE CONTROL In a multi-stage compressor, surge may be initiated by any of the stages. It depends on the design of the individual impellers and the matching of the impellers to each other which stage will actually initiate the surge. Different stages may initiate surge in different areas of the performance map. This can lead to a surge line which may contain bends. The design of the anti-surge control has to consider this. A better approach than using any sophisticated (but not fully correct in multi-stage applications) control law for calculating the surge line is to measure the surge line and plot it in a performance map showing compressor head versus suction volumetric flow rate. This performance map is almost independent of changes in the suction conditions such as inlet pressure, inlet temperature and gas composition (molecular weight). During commissioning, the compressor is exposed to real surges at different speeds and/or guide vane positions, and the surge points are noted and the surge line corrected, if required. 4e 46 m 3e y and V=k R-z C Apt Tl [2] P1 where C is a calibration factor containing all non-variables, such as orifice data, standard density etc. The polytropic head or isentropic head may be used instead of the adiabatic head. The head is calculated in the controller software; the setpoint for the anti-surge control (minimum acceptable flow) is determined from the head and compared with the actual suction volumetric flow as per [2]. This deviation is input to the anti-surge controller. This method is widely used by many compressor manufacturers. API 612 asks for a 10 % safety distance between surge line and control line. This distance is referred to the flow at the surge line, not to the compressor design flow or the flow range of the flow instrument. As the surge flow is higher with higher compressor heads, the absolute distance between surge line and control line increases with increasing head. Modern analog or fast-response digital control systems with program execution times of a few milliseconds (below 10 ms) will have a much smaller overshoot over the control line than this 10%, even under very severe process upsets (fast closing of a discharge valve). The 10% safety margin includes a safety distance to compensate for transmitter drift, errors of the instruments used during surge line detection, and changes in surge line location, due to changes in gas composition or compressor fouling. 30 ire 0' ; E 'I Fig. 1: Performance map of a 12 stage axial flow compressor A typical compressor performance map is shown in Fig. 1. The surge line is steeper in the lower section and flatter in the upper section. An anti-surge control line is located to the right of the surge line, following the shape of the surge line. The antisurge control line determines the minimum acceptable inlet flow to the compressor. For anti-surge control, the adiabatic head Ah (instead of dicharge pressure or Ap across the compressor) and suction volumetric flow rate V are used as input signals to the antisurge control algorithm. They are calculated according to the formulas r k-1 = k R z T,^ P2 k 1 [1] k-1 L i ) PID CONTROL ALGORITHMS Digital controller often use a P1I) algorithm, where the actual controller output is determined from the controller output at the last program execution and a correction factor which depends on the actual controller error. The controller output u(t) is calculated according to the formula r T u(t)=u(t Ts ) KPIe(t) e(t Ts )+ TN e(t) J [3] Anti-surge controller normallyoperate with the blow-off valves fully closed and u(t) being 100 %. A change in control error e will change the controller output u(t) whenever the rate of change between two calculation steps exceeds the control error e, divided by the reset time TN. As a consequence, a noisy control error, caused by turbulences in the inlet flow measurement, may inhibit a fully closed blowoff valve even with the operating point being some 10 % to the right of the control line. Fig. 2 shows a plot of a typical compressor operation with a control algorithm of this design. The blow-off valve oscillates by some 3 % despite the compressor operating some 10 % away from the control line. 2

3 A must for anti-surge controllers is to allow fully closed valves irrespective of noise in the flow signal. A deadbeat function is required to suppress the control error whenever the compressor operates in the safe portion of the performance map. Any derivate function must be interlocked during steady-state compressor operation. circle shows the same operating point; however, head and flow are calculated using k and R parameters for a molecular weight of 19 instead of 23. Both operating points are located on the same surge line. The error in head calculation completely compensates the error in flow calculation despite great errors in flow and head themselves. 1.e00 Controller output EEO Control error Fig. 2: Compressor at steady-state operation 10% away from the control line with the anti-surge controller responding to noisy flow signals VARIABLE GAS COMPOSITION (MOLECULAR WEIGHT) Process gas compressors are often operated with different gas compositions. It is widely accepted that a performance map showing compressor head versus suction volumetric flow rate is independent of the gas composition. Formulas to calculate adiabatic head and flow are given above as [1] and [2]. Formulas for isentropic or polytropic head are similar. Both formulas above contain the gas constant R and the isentropic exponent k, which both depend on the gas composition. In many applications, the gas composition is unknown and thus R and k are unknown. Consequently, head and flow cannot be calculated. Some authors claim that a performance map showing head versus flow or values derived from these is independent of the gas composition. This is more or less correct. In many applications, it is acceptable to neglect the influence of R and k when only the surge line (and not the compressor performance curves) need to be determined. If default values for R and k are used to determine the surge line and the control line, both head and flow will be calculated wrong if the actual gas composition differs from the default values. However, the error in flow and head will move the calculated operating point in the same direction, making the surge point move to the upper right corner or the lower left comer of the performance map. A typical example of a 4-stage compressor stage group for gas lift is shown in Fig. 3. The cross shows the actual location of the compressor operating point. The Fig. 3: Actual operating point for molecular weight 23 and calculated operating point for gas data with molecular weight 19 (third stage group) The error in head and flow parameters will depend only on the error in the values of R and k as described in formulas [1] and [2]. Therefore if the surge line is flatter, or steeper than the displacement between the actual and calculated operating points, then the calculation of surge margin will be in error. Therefore, it is apparent that self compensation, as demonstrated in figure 2, applies only for a certain slope of the surge line. For a flatter slope of the surge line, the calculated surge point moves to the right of the actual surge line with decreasing gas constant R. CE01] m n m W I SSO CE011 -> SUCTION VOLUME (M3/H) Fig. 4: Actual operating point for molecular weight 23 and calculated operating point for gas data with molecular weight 19 (second stage group)

4 m m w EE e -Eee e-4ee Isl_06 3SO.00 SS-Be CE013 -> SUCTION VOLUME (M3/H) Fig. 5: Actual operating point for molecular weight 23 and calculated operating point for gas data with molecular weight 19 (second stage group) Fig. 4 shows the second stage group of the same compressor shown in Fig. 3. A small deviation between the circle point and the actual surge line is shown in the lower section of the map where the slope is steep. Fig. 5 shows an operating point in the upper section of the performance map of the same compressor stage. In this case, the flatter surge line results in the stage curve being calculated some 7% to the right of the actual surge line. To compensate for this, R and k need to be known. The most common way is to determine these figures using a gas analyzer. The author has developed a control strategy to determine R and k from the compressor geometry. The compressor performance maps are stored in the controller memory, and actually measured parameters for inlet and outlet pressures and temperatures, flow, speed and guide vane position are compared with the performance maps. This software package is available for the anti-surge control system described below. The same package can be used to determine compressor fouling. The actual compressor performance is compared with the performance of a clean compressor, and the deviation indicated. INFLUENCE OF THE PROCESSOR SCAN TIME ON THE CONTROL PERFORMANCE Digital controls have many advantages over analog controllers. They are free of drift, free-programmable, and allow the inclusion of sophisticated calculation and control laws. Their disadvantage to analog systems is that digital controllers operate in sequence whereas analog controllers give an immediate response to any input change. A digital controller receives a set of input data, then starts to calculate the control law, and after that generates a command (controller output) signal. The next sequence is started by reading a new set of input values. Any change in input signals which occurs during a program execution cycle is ignored till a new set of input data is read. The scan time (or program execution time) varies with the processor design. DCS systems have scan times between 250 milliseconds and I second, PLC-based systems may be as fast as 100 milliseconds, and some single-loop industrial controllers are available at 30 milliseconds. For most industrial process control loops, any of the above controllers are adequate. The scan time does normally not influence the performance of a pressure, flow or level control loop. Requirements are however different for machinery protection systems such as anti-surge control, speed control or overspeed protection. These protection systems must respond not only to normal process fluctuations but also to total process upsets. An anti-surge controller has to deal with a 100 % change in compressor flow (caused e. g. by a process trip valve closure), with such a response that the compressor is kept away from the surge line which may be located as close as 10 % (referred to the actual flow) away from the actual operating flow. Control errors of more than 5 % of control range may lead to a surge. A speed controller has to cope with a generator full load shedding and avoid rotor overspeed. This means that these controllers need to cover even these conditions with a minimum overshoot over the control line, as only this guarantees the required wide operating range. The importance of the program execution time on the performance of an anti-surge controller shall be explained using a small model. Two identical compressors, A and B, are installed in the same process. Both follow identical control laws with the only deviation being the program execution time (controller scan time). Unit A has a program execution time (time from change of input to change of output) of I millisecond, unit B has 30 milliseconds. The anti-surge control valve stroke time is 1.5 seconds for full stroke. Both units are exposed to a process upset which requires the anti-surge control system to open the control valve with fastest speed. On first guess, it is hard to believe that a scan time below 30 milliseconds should have any influence on the controller performance, if the actuated valve needs 1.5 seconds or 50 scan cycles for the full stroke. One would expect that the influence is almost negligible. But this is not true. It will take at least one program execution cycle between change of input to change of output. Under worst conditions, it may take two program cycles, if one cycle just started prior to the change of input data. The valve of unit A will thus start to travel 1 to 2 milliseconds after the incident. The valve of unit B will be delayed by 30 to 60 milliseconds. During 60 milliseconds, the valve of unit A has opened already 4 % of its stroke. As these valves are normally designed with a linear characteristic and a service factor of 30% or more, 4% stroke corresponds to 5.2% in flow. Unit A will blow-off up to 5.2% more flow than unit B. This means that the operating point of 4

5 unit B approaches the surge line up to 5.2% closer. l To get the same degree of compressor protection from both units, unit B requires an up to 5% higher safety distance between control line and surge line, caused only by the 30 millisecond scan time controller. In addition to this, the scan time limits the maximum selectable controller gain. Controller gain is the most important tuning parameter for a closed-loop anti-surge control as can be seen from the formula y=kp xdl 1 +TN dtj [4] When the operating point is moving towards the surge line, xd is getting smaller and the controller output will be proportional to the change in xd, multiplied by the gain KP. The integral action has almost no effect during the first seconds of a fast upset. The influence of the scan time on the selectable gain has been investigated by Blotenberg in If the scan time is increased from 1 millisecond to 30 milliseconds, the gain needs to be reduced by 60%. This may result in a 60% wider overshoot over the control line. With increasing scan times, the gain needs to be reduced further. A 250-millisecond scan time requires a gain reduction to only 7.4% of the value, to which an immediate response controller ca be tuned. This is easy to understand. If a scan time of i.e. 5 seconds applies, it is well known from experience that a gain of as low as KP = 1 will lead to high oscillations between fully open and fully closed valve, as the controller will not respond to any change of the blow-off valve position until a new scan cycle starts. Fig. 6 shows the track of a compressor operating point after a process upset where a discharge shutoff valve closed in 5 seconds. The records were taken from a dynamic computer U a N N W L U Millisecond 30 Millisecond Millisecond Control line e Flow U3 Fig. 6: Track of the operating point for control systems with different scan times when a discharge valve is closed in 5 seconds 1 As unit Ahas a scan time of 1 millisecond, its dead time is 1 to 2 milliseconds, which corresponds to 0.17% flow.. simulation, described by Blotenberg. The controller scan time was varied between 0 and 250 milliseconds. The measured influence of the scan time is not as high as stated above, because the tests did not require the full dynamics of the blowoff valve. These tracks however prove the statements made above. The commercial impact shall be discussed on the basis of a compressor with 15 MW design power. If the power demand with operation on the anti-surge control line is 10 MW, the additional power required for unit B with a 5 % higher flow at the anti-surge control line, is 500 kw. If this compressor is operated for 10 % of its annual operating time in the vicinity of the surge line, this corresponds to a total additional power requirement of 800 h 500 kw = 400,000 kwh. With 0.1 $ per kwh, this will lead to a total annual saving of 40,000 $ when a high-speed control system with 1 millisecond scan time is used. For longer periods of operation in the vicinity of surge, the figures are even higher. The control system described below operates with 1 millisecond program execution time. These statements will still be doubted, and compressor units might be named which are protected by an anti-surge control as part of a DCS system with 500 milliseconds or even I second scan time, without having been exposed to any surge incident. Several of these units have operated for many years without any problem. This operation experience is said to prove that an antisurge control with 1 second scan time is adequate. The author knows several car drivers which always drive with high speed and only 10 m safety distance to the car ahead of them. Many of these have never been involved in a traffic accident. They assume from this experience that 10 m safety distance is an adequate safety distance, even with 150 km per hour speed. Everyone knows that these drivers just had good luck and never have been in a critical situation. Otherwise, severe damage would have occurred. The same applies to antisurge control with DCS systems. Simply good luck has prevented these compressors from being damaged by a surge. CONTROL SYSTEM DESIGN Our company has been using Digital Signal Processor technology (DSP) since 1986 for their digital control systems. These processors are able to implement a compressor anti-surge control system including PID control, manual and auto mode, auto start, flow calculator, dynamic control lines, variable gain, safety line, adaptive filter, transmitter monitoring and controller feedback to the valve position with a program execution time (time from change of input to change of output) of less than one millisecond. This allows quasi-analog controls. The control system is based on a two-processor architecture. A signal processor is used for high-speed controls (anti-surge, speed, critical logics,...). An industrial personal computer is 5

6 built into the controller as a second processor. This PC serves as programmer and monitor during the tuning phase. During normal operation, this PC operates as video-screen based machinery monitoring, event recording, and data logging system. It contains all features of modem DCS systems, including alarm manager and trend recording. In addition, this system can display the compressor operating point in the performance map. I- ii Fig. 7: Advanced control and monitoring system REDUNDANCY Redundancy is a very popular feature, often included in control system specifications. The purpose of this requirement is to increase the availability of the system and avoid machine trips caused by a control system failure. A control system consists of a) local sensors, b) cables from the sensors to the controller, c) controller input cards, d) central units like CPU, power supply, etc., e) controller output cards f) cables from the controller to the actuators, g) actuators, valves, etc. The highest and most effective levels of redundancy require redundancy of all components. Redundant control valves and actuators are very expensive as not only the valves are affected but also the piping arangement. To duplicate guide vane actuators on a compressor is even more complicated. Thus, requirement g) is deleted in almost every application. Local sensors and cables can be provided redundant, however high costs are involved for this. Thus many customers cannot afford this and delete these as well. In most applications, redundancy is limited to the control room mounted controller unit. Some specifications even ask for redundant CPUs only. A look at the statistics shows that between 80 and 95% of all faults are caused by field elements such as sensors, cables or actuators. Only the remaining 5 to 20% are affected by the control room equipment. Redundancy for control room equipment can only reduce this 5 to 20% risk. The main risk remains unchanged. Zender reports that a CPU redundancy on PLC systems will increase the MTBF by no more than 1.5 to 15%, depending on the number of inputs. The author has investigated 21 digital control systems of the author's company which are in industrial use. A total of over 216,000 operating hours was recorded, which is equivalent to 25 years of operation. In this period, no faults of central units (CPU, power supply, bus) have been reported. Five input/output cards were reported faulty. All faults happened in only 1 of 8 locations. Two of these faults were reported to have caused a machine trip. Factory investigations showed that one of the two "faulty" cards showed no damage at all. The trip must have been caused by another problem. The other card was an input/outputcard which is part of the INTEL-based redundant emergency control loop (see below), which was used on this unit. The control system is available with dual or triple redundancy for CPU only, for CPU and power supply, and for CPU, power supply and all input/output boards, as specified. The described control system allows another level of redundancy to take advantage of its controller architecture. The two processors, the DSP and the INTEL/PC-type, are fully independent of each other. The INTEL can be equipped with its own input and output cards, thus allowing use of the INTEL as a back-up system for the DSP. The performance speed of the INTEL is much slower than that of the DSP (program execution time between 30 and 100 milliseconds depending on the load of this unit). But with an increased safety distance to the limits (surge line or overspeed trip), the INTEL can serve as a full emergency control system with only very few additional components. As both processors are of different design and different manufacture, even systematic fabrication or programming errors, which may occur in both redundant systems if they are of identical design and from the same manufacturing series, will be avoided with this DSP design. 6

7 EVENT RECORDING AND DATA LOGGING WITH SURGECORDER A high-speed event recording function called SurgeCorder is included as a standard in the anti-surge controller introduced here. The track of the operating point is digitally stored on a hard disk built into the controller. A set of data is stored every 50 milliseconds. SurgeCorder stores the last six minutes of operation. If a surge occurs, the system continues to record for another minute. Then, this file is closed and a new file is opened. With the SurgeCorder in operation, a time frame of 5 minutes prior to the surge and 1 minute after is available for later evaluation. The data are stored with 12 bit width which is equivalent to a tolerance of 0.05%. This high-speed time base with a set of data for every 50 milliseconds allows a detailed evaluation of any surge incident. Figs. 8 and 9 show the replay of a compressor surge in two different modes. v CE-1] L 7 N iu a _e00 L U- 3-DOD..., 2.00e,, Flow Fig. 8: x/y-replay of logged data from a compressor surge _e00 Flow All data, collected with SurgeCorder, can be sent via diskette or data link to the engineering head office of the compressor manufacturer for evaluation. This evaluation allows to determine precisely the surge point within the compressor performance map. It can be checked whether the compressor surged on the known surge line, determined during initial startup, or whether the compressor surged away from this line. If the surge happened away from the original surge line, the shift of the surge line can be determined from the measured data. The use of SurgeCorder is not limited to recording the compressor surge points. Any value, input value, calculated value or output value can be stored at a freely selectable time rate. Even the hard disk capacity is no limitation, as the plug-in hard disk can be easily exchanged. AUTOMATIC CONTROLLER TUNING WITH SURGETUNER An additional software tool, called SurgeTuner, is available. This software is able to determine automatically the location of the surge point in the compressor performance map. This software is the basis for making the anti-surge control an intelligent self-tuning control system. SurgeTuner determines whether the surge points noted are located on the known surge line or away from it. If the surge point is located on the surge line, the safety distance between surge line and control line is not adequate and needs to be increased. If the new surge point is located to the right of the known surge line, the surge line has moved (i.e. by fouling, transmitter drift, etc.) and the shape of the surge line, stored in the control system, needs to be adjusted accordingly. This is effected by SurgeTuner as well. The new shape of the surge control line is determined and loaded into the controller software, the software downloaded to the controller, and the program started. PROGRAMMING OF THE CONTROLLER WITH TURVIS TURVIS (TURbolog ViSual) is the programming language of the digital control system described. The origin of TURVIS is a simulation language. It consists of a basket of functional blocks for different control and logic functions. Pressure Time in ser- Fig. 9: Time-based replay of logged data from a compressor surge Fig. 10: Block diagram of a speed control loop 7

8 Turvis Ver. MG 6.1 DSPU-LPVEL File : ASMEO1.RUN Current Level: 1 Filc Edit Run/Stop Setup Info/Help Pr Title:Speed Controller Co ^HEGIN 100,AIR, ;Speed Channel 1 Parameter 1110,SIN, ;Speed Channel 2 NPI 1120,MAX, ;Actual Speed 130,THR,120 ;Speed below min li Kr : ,00T,130 ;Speed below min Tn : ,T1,110 ;Speed above max Min : ,000,150 ;Speed above max Max : , AIN, ;Speed Setpoint 210,DIN, ;Trip 300,SUM, ;Control Deviation 310,CON, 320,14PI, ;Controller 330,ACT,320 ;Controller Output END Fig. 11: Program listing The application programmer only needs to link blocks from the basket to a control diagram and enter it into the controller. On-line help is also available as self-diagnostics for programming errors. An example of a simplified speed control loop is shown in Fig. 10. The program listing is shown Fig. 11. For graphic display on the VDU, a number of graphic blocks such as bar graph, trend recorder, digital voltmeter, LED, etc. are available. Flow diagrams and mimic charts may be created with any graphic editor and can be loaded into the system. LITERATURE Blotenberg, W., 1988 "Ein Beitrag zur digitalen Pumpschutzregelung von Turbokompressoren," Schriftenreihe Heft 31 des Lehrstuhls fir Regelungssysteme and Steuerungstechnik der Ruhr-Universittit Bochum Schmitt, K., and Blotenberg, W., 1992 "Modernisation and Upgrading of MAN GHH Turbo Compressors and Their Control Systems," Turbomachinery Maintenance Newsletter 5/92 Zender, P., 1994 "Sicherheitsgerichtete SPS in Anlagen mit Gefahrdungspotential", Automatisierungstechnische Praxis 36 (1994)12 Blotenberg, W., 1993, "A Model for the Dynamic Simulation of a Two-Shaft Industrial Gas Turbine with Dry Low NOx Combustor," ASME paper 93-GT-355 DYNAMIC PROCESS SIMULATIONS Due to its origin, TURVIS is well suited not only for controls, but it is also capable of doing dynamic process simulations. A widely used method is to run a performance test of a control system by loading the control program to the DSP processor of the contract controller and load a dynamic simulation model to the INTEL/PC-processor. Thus a full functional test of the entire system is possible long before the machine unit is actually built. The control strategy does not need to be converted into a simulation model but is running with the original software and hardware in closed-loop simulation. Beside different studies to develop advanced anti-surge and speed/extraction pressure control algorithms, a dynamic response study of a twin-shaft gas turbine with Dry Low NOx combustor was performed with TURVIS. A complete offshore gas lift compressor application was investigated, consisting of 3 parallel compressors with 3 stage groups each. The entire system of 9 stage groups, 18 control loops and all pipes, vessels and coolers was simulated and tested dynamically, using only the contract anti-surge controller hardware. Currently, a simulation study is performed for an FCC power recovery train with the aim to pretune all controllers, especially those which have to maintain the regenerator discharge pressure when the expander inlet valves close in 0.6 seconds due to a generator load shedding. For this study, only the contract control system will be used as well.