slug detection as a tool for predictive control of glcc(r)...

** Currently with Par1 GLCC - Gas-Liquidof Tulsa, 1994

Proceedings of ETCE 2001 Engineering Technology Conference on Energy

February 5-7, 2001, Houston, TX

ETCE2001-17136

SLUG DETECTION AS A TOOL FOR PREDICTIVE CONTROL OF GLCC COMPACT SEPARATORS

Shankar Earni**, Shoubo Wang, Ram S. Mohan and Ovadia Shoham

Mechanical and Petroleum Engg. Departments The University of Tulsa

Tulsa, OK - 74104 Jack D. Marrelli, Texaco, Inc.

Humble, TX - 77338

ABSTRACT Current design and performance of the GLCC©1 separator is dependent on the prediction of the upstream inlet flow conditions based on available models. It is expected that early detection of terrain slugging (slug length, slug velocity and holdup) and controlling the liquid level in the GLCC using feed forward mechanism can improve the operational range of GLCC, by decreasing the gas carry under and liquid carry over, and thereby decreasing the control valve dynamics. The conventional feedback control loops can seldom achieve perfect control considering the impact of huge slugs that are keeping the output of the process continuously away from desired set point value. The reason is simple: a feedback controller reacts only after it has detected a deviation in the value of the level from the set point. Unlike the feedback systems, a feed forward control configuration measures the disturbance directly and takes control action to negate the effect of the disturbance on liquid level in the GLCC. Therefore, feed forward control system has the theoretical potential for perfect control if the slug detection and characterization are accurate.

A strategy for GLCC predictive control has been proposed which integrates the feedback and feed forward loops to compensate for error due to modeling and slug characterization. A model has been developed for predictive control system design and simulated in MATLAB-Simulink. Experimental results obtained demonstrate that the proposed strategy is a viable approach for GLCC predictive control.

ametric Technology Corporation, Cincinnati, OH Cylindrical Cyclone - copyright, The University

INTRODUCTION Compact separators such as GLCC have proven to provide considerable impact in improving the optimization and productivity of the petroleum industry (Marrelli et al.1, 2000). There is an increasing need to develop appropriate control strategies, design tools and simulators for the GLCC to improve the performance under slug flow conditions, as its residence time is very small. Slug flow can be mitigated by slug catchers or by some flow conditioning techniques. Another way to handle such flow fluctuations in gas and liquid flow rates and to avoid unplanned shutdowns of the production system is an appropriate control strategy of downstream process equipment. Performance of the conventional feedback control system by manipulating the upstream control valve, in order to maintain the level in the GLCC, is not completely satisfactory during the onset of severe slugs. A feedback controller reacts only after it has detected a deviation in the value of the output from the desired set point. So it makes perfect sense if one can estimate the disturbance that is coming into the system and provide suitable control action in order to negate the effect of the disturbance. This is the main principle of the feed forward control system.

Several studies were conducted to characterize the slug in the two-phase flow. Andreussi et al.2 (1993) characterized the slug using a pair of conductance probes made of two ring electrodes mounted flushed to the pipe wall. Dhulesia et al.3 (1995) developed a method to detect slugs in multiphase flow line based on the analysis of vibrations (acoustic principles) of pipeline structures caused by the flow of fluid. This information in the control room allowed the operator to take appropriate control actions on the downstream facilities depending upon the

1 Copyright © 2001 by ASME

slug length. Barnea et al.4 (1980) successfully used conductance probes to identify two-phase flow patterns in horizontal, near horizontal and upward flows in air-water systems. This study provided the knowledge required to construct and implement the conductance probes used to measure slug dissipation for the current investigation. Wang5 (1997) and Mohan et al.6 (1998) have developed a steady-state model and dynamic model for GLCC control and performed a sensitivity analysis and control system design. As a continuation of this work, Wang et al.7,8 (2000, 2000a) conducted detailed experimental investigations to evaluate the improvement in the GLCC operational envelope for liquid carry-over with the integrated level and pressure control system, for a wide range of flow conditions. Their detailed experimental studies demonstrated that by using control systems, the GLCC operational envelope for liquid carry-over could be increased by three folds in the high liquid flow and gas flow regions. The optimal control strategy of Wang9 (2000) has the advantages of handling large flow variations with minimum pressure drop across the GLCC. Payne et al.10 (1996) has developed a feed forward slug flow mitigation control system. This technique involves liquid slug flow control by manipulating the control valve, which is placed in the flow-line upstream of gas-liquid separator (conventional), based upon the signal from the slug detector, a device for measuring the presence and the volume of the slug in the flow line. The objective of current work is to develop a feed forward control system to improve the performance of a GLCC under severe slugging conditions and integrate it with the feedback control system. Towards this objective a mathematical model has been developed to conduct the control system design. Subsequently dynamic simulations are performed to evaluate the design. The developed control strategies are implemented in a real GLCC system and detailed experimental investigations are conducted to evaluate the effectiveness of the control strategies. In this study, the features of the GLCC dynamic model, the control system design and the dynamic simulators are developed using MATLAB/Simulink software for evaluation of several different GLCC control philosophies for two-phase flow metering loop and bulk separation applications. A predictive control strategy, integrating feed forward and feedback control schemes is proposed for real time control of GLCC. This strategy has the capability of maintaining the liquid level close to set point, in spite of large flow fluctuations. This augments the existing control system to realize the control objectives more effectively, thereby increasing the operational efficiency of the GLCC, and providing the petroleum industry with another effective tool for slug mitigation. PREDICTIVE LIQUID LEVEL CONTROL SYSTEM Figure 1 shows the schematics of a GLCC equipped with control systems. The control systems can be broadly classified into feedback and feed forward control systems.

The feedback control loops could be liquid level control by LCV and pressure control by GCV. In the LCV liquid level

control loop, the liquid level sensor (such as a differential pressure transducer) measures the liquid level and sends the signal to the liquid loop controller. The controller (LC1) sends the actuating signal to drive the liquid control valve so as to manipulate the liquid outflow and control the liquid level. The GLCC feedback control system is discussed in more detail by Wang9 (2000).

The main concept of GLCC feed forward control is liquid flow rate control. In the liquid flow feed forward control loop, liquid slug is characterized and measured by suitable sensors upstream of the GLCC and the signal is sent to the feed forward liquid control loop controller. The output signal from the controller drives the liquid control valve to match the liquid outflow rate to the inflow rate (disturbance), so that the liquid level in the GLCC can be maintained around the set point. Any time lag between the inflow and outflow will affect the level in the GLCC. The level will then either rise or fall, depending on the direction of the flow rate change, causing the level controller to adjust the outflow proportionally. This control system always returns level to the set point, with an offset equal to the error between the actual flow and measured flow from slug characterization.

The only serious drawback of the feed forward technique is its dependency on plant accuracy and accuracy of disturbance characterization. To provide perfect feed forward control, a system must model the plant exactly; otherwise whatever error may exist in positioning the manipulated variable causes offset. Errors may arise from several sources, namely, inaccuracies in the measurements of load and manipulated variables, errors in calculations, failure of the model to represent the characterization of the plant adequately and exclusion of significant load components from the feed forward system. Ongoing improvements in slug detection and use of sophisticated data acquisition tools have been helpful in reducing these errors. The feed forward system cannot be all-inclusive. Some load components are so slight, or invariant or ill defined that their inclusion is not warranted. All these errors contribute to considerable offset. If this offset is large, this can upset the stability of the system. To counter this offset, some means must be provided for recalibration while the system is operating. In general, this can be done by adding a feedback control loop in order to take care of the small deviations from the set point, which result from the errors.

Feed forward and feedback controllers can be combined in several different ways. One possible configuration for feed forward-feedback control is to add the outputs of the feed forward and feedback controllers together and send the resulting signal to the final control element. This is the configuration implemented in this study as shown in Fig.2. The primary advantage of this configuration is that the feed forward controller does not affect the stability of the feedback control loop. In general, since feed forward control is warranted only on the most demanding and most difficult situations, integral is the only useful feedback mode. The feedback controller should have the same control modes as it would without feed forward


control, but the settings should not be as restrictive. The feed forward control system then positions the manipulated variable so that the error in the controlled variable disappears. If acted upon by closely set feedback, the correct position will be altered, producing another disturbance that prolongs the settling time of the system. To this extent, the settings of the feedback controller, regardless of what modes have been selected, should be relaxed.

An adaptive control system can be defined as having the ability to change its parameters in accordance with the changing flow conditions. A feed forward control system, by itself, can only generate an output relative to known and measurable inflow conditions, as prescribed by the given control algorithm. Some factors relating to the process may be unknown and variable, for example the slug characteristics, may change while flowing from the slug detector to the GLCC. For optimum performance, the feed forward control system should be supplied with information regarding these unknowns. A feedback controller, on the other hand, is geared to solve for unknown variable, as it knows the resultant of the known. So the inclusion of feedback control system in a feed forward loop actually adapts the feed forward loop to unmeasured changes in the process. This mutual adaptation is a further indication how perfectly feed forward and feedback complement each other if designed properly. Feed forward is fast, intelligent, and responsive but also inaccurate; feedback is slow but accurate and is capable of regulation for unknown load conditions. CONTROLLER DESIGN AND DYNAMIC SIMULATION The linear transfer functions of the system are a stepwise mathematical description of the sub-system behavior and are always stated in Laplace domain. The deviation variable used below is defined as the deviation of a variable from its steady state or set point value. The derivation of the transfer function is in the order of block numbers given in Fig.2.

The blocks, which constitute the feedback control loop, are Block1: This is the transfer function of the liquid level controller. Block2: This is the transfer function describing the pneumatic line and actuator delay. Block3: This is the transfer function for the LCV dynamics and characteristics. Block4: The Laplace transformation of the relationship between the liquid level and the GLCC liquid volume Block5: This is the transfer function representing the liquid level sensor/transmitter.

The feedback model is discussed in more detail by Wang9 (2000). Brief procedure of the feedback controller design is given below. Feedback Controller Design. There are several design tools for feedback control system design and analysis. Most common techniques are the root locus and the frequency domain methods. The following is the frequency domain approach that is followed to design the feedback controller. PID means

Proportional, Integral, and Derivative. The integral term is most

effective at low frequencies, the proportional term at moderate frequencies and the derivative term at higher frequencies. These frequencies are relative to the bandwidth of the process.

To achieve our design specifications: 1. Start by choosing both the zeros of the PID-Controller

one decade below the crossover frequency required. 2. A program has been written to calculate the magnitude

and phase at different frequencies. The magnitude and phase plots for closed loop transfer function are plotted for different cases in Fig 3.

3. From Fig. 3 one can find that with increase in the distance of the zeros of the PID-Compensator from the imaginary axis, the magnitude peak increases indicating that there is a decrease in the damping ratio. But a decrease in the damping ratio increases the percentage overshoot or number of overshoots and increases the settling time. On the other hand, with a decrease in the distance of the zeros of the PID-Compensator, the peak decreases indicating that there is an increase in the damping ratio. An increase in the damping ratio decreases the percentage overshoot and decreases the settling time. So, the controller settings can be optimized between the percentage overshoot and settling time. As the phase margin becomes larger, the amount and number of overshoots diminish.

4. Placing the zeros of the PID-Compensator very close to the origin increases the steady- state error, which is not acceptable.

5. The design has been optimized by running several trials of the program with different locations of the zeros of the PID-Compensator.

Feed forward Controller Design. The feedback controller can be designed using the conventional design tools like root-locus and frequency domain techniques, whereas the feed forward controller cannot be designed similarly as the feed forward control loop is an open loop (Stephanopoulos11, 1984). To illustrate how dynamic models can be used to design a feed forward control system, consider the block diagram shown in Fig. 2, which contains a single disturbance. Here the objective is to keep the liquid level in the GLCC constant. The disturbance is the inflow to the GLCC in the form of a slug. The time delay for the slug to travel from the slug detector to the GLCC is given by,

Slug theofVelocity GLCC odetector t slug ebetween th Distance

=dτ �.(1)

The transportation process can be assumed as first-order transfer function as it is a pure time lag function. The final form of block 6 is given by,

vld

d G1s

1)s(G+

≅τ

��.��.. (2)


where,

sD

G 1vl =

The manipulated variable is the outflow which can be

controlled by the liquid control valve, whose transfer function is given by Glcv and the pneumatic line transfer function is given by Gpl. The incoming slug is characterized with a sensor whose transfer function is given by Gm2. The information is sent to the pneumatic line and the liquid control valve through a feed forward controller Gc2, to accommodate the slug. The error encountered due to slug characterization is taken care of by feedback controller Gc1. The liquid level set point is specified for both the feedback and feed forward controllers. Gsp is the set point tracker. From Fig. 2, the control loop output is given by,

( ) ( ) ( ) ( ) ( )sdsGsmsGsG)s(y dvllcv ∗+∗∗= ��.(3)

where,

)d*Gy*G(*G*G)y*Gy(*G*Gm 2mspsp2cpl1msp1cpl −+−= ..(4) Substituting equation (4) in (3) gives,

To analyze the stability of the closed-loop system, one can consider the closed-loop transfer function in Eq. (5). Setting the denominator equal to zero gives the characteristic equation,

The roots of the characteristic equation completely determine the stability of the closed-loop system. Since the expression shown above is not a function of any of the feed forward control transfer functions. So the feed forward controller has no effect on the stability of the feedback control system. This is a desirable situation, which allows the feedback and feed forward controllers to be tuned individually.

The slug detector (Block 7) is represented by the liquid flow rate transmitter gain (4-20 mA) and is given by,

)()420(

minmaxQ

QQe ∆−×

−−=∆ ��(7)

)5.....(..........d)G*G*G*G*G(1

)G*G*G*G*G(G

yG*G*G*G*(G1

)G*G(G*G*G*Gy

1m1cvllcvpl

2m2cvllcvpld

spm1)c1vllcvpl

spc2c1vllcvpl

+

−+

+

+=

)6..(....................0)G*G*G*G*G(1 1m1cvllcvpl =+

Taking the Laplace transform results in,

( )( ) minmax

2)420(

QQsQseGm −

−−=∆∆= ��.(8)

The set point tracker (Block 8) is specified in the controller

design. The control mechanism should be capable of making the process output track exactly any changes in the set point (i.e., y = ysp). This implies that the coefficient of ysp in Eq. (5) should be equal to 1 and the set point tracker transfer function is given by,

The feed forward controller transfer function (Block 9)

needs to be designed for the system. The controller should be capable of completely eliminating the impact of a slug on the liquid level. This implies that the coefficient of d in Eq. (5) should be zero and the feed forward controller transfer function is given by,

Control System Simulation. Based on the linear model and the designed controller, a simulator for the liquid level control system using liquid control valve is built in MATLAB/Simulink. The outputs from the simulator are the liquid flow rate, liquid level and control valve position. The inflow disturbance (unit slug) is 0.7 cft/s (10773 bbl/d) with a slug velocity of 11 ft/sec. The liquid level in the GLCC with feedback control varies from ± 6 ft. The outflow rate overshoots to 0.72 cft/s and settles down in about 16 seconds. If the same disturbance 0.7 cft/s (10773 bbl/d) is introduced to the integrated feedback and feed forward control system, the liquid level in GLCC with feedback control varies between ± 0.25 ft. No overshoot is observed in this case. The control valve dynamics are also negligible when compared to dynamics associated with feedback control system alone. Details of the simulator and simulation results are described in Earni12 (2001). EXPERIMENTAL PROGRAM The experimental setup consists of a standard two-phase air-water flow metering loop, a slug generator, a GLCC test section and a slug detection system. Slug lengths varying from 20-800 pipe diameters can be generated using the slug generator. The slug generator consists of a 9-gallon metallic tank with a level indicator. Three pneumatic 2-in ball valves are connected to this tank. The GLCC body is a 3-in transparent PVC pipe with a modular 3-in inclined aluminum tangential inlet. The inlet slot area is 25% of the cross sectional area of the inlet pipe. The

)10.....(..............................).........G*G*G*G

G(G m2vllcvpl

dc2 =

)9(......................................................................G

GGd

m2sp =


total height of the GLCC is 7 ft, which is divided by the inlet into the lower liquid section and the upper gas section. The liquid leg is a 2-in gray PVC pipe with a liquid control valve. To measure the liquid level, the GLCC is equipped with a differential pressure transducer. The gas leg is also a 2-in gray PVC pipe with a gas control valve and an absolute pressure transducer to measure the GLCC pressure. The outlet section is constructed to either recombine the gas and liquid legs (metering loop application) or to separate the gas and liquid streams (bulk separation application) corresponding to the application. The recombination point is 6 inch below the inlet plane, which helps to self-regulate the liquid level of the GLCC for operating without any control system. For full separation configuration, the liquid and the gas outlets are separated and a suitable control system should be used for proper operation.

A slug detection section, which is 22-ft away from the GLCC, is set up comprising of two conductance probes (C1 and C2), as shown in Fig. 4. There is provision to alter the distance between the two probes. More precise slug detection is accomplished when a minimum distance of 8.5 pipe diameters separates the two probes. Conductance probes are used to characterize and measure the length of the slug at upstream of the GLCC. The voltage from the probes is scaled from 0-10 volts. In other words, when no liquid is in contact with probe, or liquid does not bridge the negative end simultaneously, 0 volt is measured.

The slug detection can be classified by the following stages: • Transmission of the current to the conductance probes and

receiving the voltage signals from the probes. Current is sent to the probes and the voltage across the probe is measured, which is a function of the wetted area of the probe.

• Conditioning (amplification and anti-aliasing filtering) of signals from the probes. The voltage signals are scaled from 0-10 volts. It means that some events (related to small gas bubbles or small liquid droplets) have to be eliminated from the original signal. In order to filter out these spurious signals they are sent through a median filter.

• A/D Conversion: The filtered signals are digitized based upon a threshold voltage, i.e. if the voltage from the sensor is greater than or equal to the threshold voltage, in order to eliminate small slugs. The effect of small slugs is handled by feedback control system alone.

• Signal Processing and Analysis: The digital signals are then processed to obtain the slug velocity and length.

• Slug Characterization: Transmission of the processed information to the feed forward control system.

Three different control strategies are configured and tested to evaluate the performance of each strategy under different flow conditions and with different length of dumped slugs. It may be noted that for the different control strategies that were tested, the pressure is always feedback controlled by GCV and

the control configuration varied only for level control. These strategies are feedback level control by LCV, feed forward level control by LCV, integrated feedback and feed forward level control by LCV. Two sample cases of flow conditions are described below as typical examples, to evaluate the effectiveness of the integrated feed forward and feedback control system. Case 1 � Evaluation of the system response for a unit severe slug superimposed on a normal stratified flow: A unit liquid slug is introduced to the system using the slug generator. The initial liquid superficial velocity is 38.0Vsl = ft/s, and the gas superficial velocity is 84.3Vsg = ft/s, and with the slug generator, adjusting the timer appropriately, a unit slug of 12-ft length is dumped. In this process the time responses of the slug detector probes, liquid level and LCV position are measured by the appropriate sensors. The data are acquired and plotted for different control configurations (feedback control alone, feed forward control and integrated feedback feed forward control systems) as shown in Fig. 5. In the case of the feedback control alone the liquid level overshoot is around 54 inches and level settles to around the set point (25 inch ± 5 inch) after 6.5 seconds. The control valve and liquid level dynamics are also estimated by calculating the RMS value. The slug associated liquid level RMS value is around 17 and RMS value of the LCV dynamics is 15. When one considers feed forward control loop alone, the liquid level in the GLCC shoots up to about 48 inches and settles at a different position. It is obvious that the feed forward control system alone cannot bring the level to the set point level, as it does not have the information about the set point. However, with integrated feedback and feed forward control system, the liquid level overshoot is only 38 inches and the liquid level is brought back to the set point level in only about 5 seconds. The liquid level RMS value is around 7.33 and RMS value of the LCV dynamics is 11. It may also be noted that the liquid control valve saturates at full open position for a shorter period of time for integrated control loop compared to the feedback control alone.

Case 2 � Evaluation of the system response for a unit severe slug superimposed on a normal slug flow: The other flow condition for which the predictive control strategy is tested is the normal slug flow condition. A long slug, ranging from 20-25 ft, is dumped into the system, which is already operating under slugging conditions. The results are shown in Fig. 6. A unit slug of length 25 ft is dumped into the system, which is operating at

41.0Vsl = ft/s, and 61.6Vsg = ft/s. The system performance is evaluated for feedback control alone and integrated feedback-feed forward control system. In the case of feedback control system the maximum liquid level overshoot in the GLCC is around 115 inches whereas it is only 100 inches with the integrated feedback-feed forward control system. The settling time for the liquid level to come back to the set point level is 36 seconds for the former, while it is only around 17 seconds for


the latter. The liquid level dynamics are evaluated by calculating the RMS value of the liquid level readings associated with the slug, which is around 42 for without feed forward case and for the case with feed forward control system it is only 32. Similarly, the total control valve dynamics are also evaluated based on its RMS value, which is calculated to be 17 for the feedback control system, and it is 11 for the integrated feedback-feed forward control system.

Table 1 is a summary of all the operational conditions for which the control strategy is tested. CONCLUSIONS When severe slugs are encountered, the feed forward control system takes the lead by manipulating and timing the control valve position appropriately to negate the effect of the slug. The feedback control system takes care of small variations in the flow and maintaining the set point liquid level. The conclusions from this investigation are summarized below. 1. A mathematical model is developed for GLCC predictive

liquid level control system. 2. As feed forward control is an open loop system, the

conventional design methods cannot be used. Hence, an analytical method of control system design is adopted for designing the feed forward controller. Addition of a feed forward control loop does not affect the stability of the existing feedback control system. From the analytical design, it can be noted that the feed forward controller settings are a function of the slug velocity. Even though for a perfect feed forward control, different settings have to be deployed for different slug velocities, the controller settings designed for one particular slug velocity can be used for a range of velocities with satisfactory performance.

3. Integrated feedback and feed forward liquid level control system simulators were developed using MATLAB/Simulink software. Detailed dynamic simulations show that the predictive liquid level control system can handle long slugs without affecting the operational efficiency of GLCC by maintaining the liquid level close to the set point level, which is otherwise not possible with a feedback control system alone.

4. A state-of-the-art experimental facility was designed and constructed to study the GLCC performance for the different control strategies. Detailed experimental studies demonstrate that slug detection has to be exact for perfect feed forward control. However, good performance can be obtained with feed forward control system if it is integrated with the feedback control system to compensate for the error associated with the slug detection. Maintaining the liquid level close to the set point level reduces the liquid carry over (LCO) and gas carry under (GCU) considerably. The control valve dynamics are also reduced enhancing the control valve life. However, if there is a significant error in slug characterization, it will lead to

larger control valve dynamics in order to achieve good level control. Thus the experimental results obtained demonstrate that the proposed strategy is a viable approach for GLCC predictive control.

NOMENCLATURE d = flow disturbance D1 = constant from GLCC geometry e = error signal G = sub-system transfer function GCV= gas control valve LC = liquid level controller LCV= liquid control valve PC = pressure controller Q = volumetric flow rate ( /sft3 ) s = Laplace variable V = velocity (ft/s) y = output

Greek Letters τ = time constant (s)

∆ = incremental deviation Subscripts

c = controller d = time delay FB = Feedback Control FF = feed forward Control lcv = liquid control valve

max = maximum min = minimum

m = sensors pl = pneumatic line s = slug sg = superficial gas sl = superficial liquid sp = set point vl = volume to level

ACKNOWLEDGMENTS The authors wish to thank Tulsa University Separation Technology Projects (TUSTP) member companies for supporting this project. The authors also acknowledge the National Petroleum Technology Office (NPTO), U.S. Department of Energy for the Grant: DE-FG22-97BC15024. REFERENCES 1. J.D. Marrelli, M. Tallet, B. Yocum, D. Dunbar, R. S. Mohan and

O.Shoham, "Methods for Optimal Matching of Separation and Metering Facilities for Performance, Cost, and Size: Practical Examples from Duri Area 10 Expansion" ETCE00-ER-10165, proceedings of the ETCE 2000 Conference of ASME Petroleum Division of ASME Petroleum Division, February 14-17, 2000.


2. Andreussi, P., Bendiksen, K.H., and Nydal, O.J.: � Void Distribution in Slug Flow,� International Journal of Multiphase Flow Vol. 19, No 5, pp. 817-828, 1993.

3. Dhulesia, H., Bernicot, M and Romanet, T.: �Field Installation of an Acoustic Slug-Detection System,� presented at the SPE Annual Technical Conference and Exhibition, Dallas, October 22-25, 1995.

4. Barnea, D., Shoham, O. Taitel, Y. and Dukler, A. E.: "Flow Pattern Characterization for Two-Phase Flow by Electrical Conductance Probe," Int. J. Multiphase Flow, v. 6, 387-397, 1980.

5. Wang, S.: �Control System Analysis of Gas-Liquid Cylindrical Cyclone Separators,� M.S. Thesis, The University of Tulsa, 1997.

6. Mohan, R., Wang, S., Shoham, O. and Kouba, G.: �Design and Performance of Passive Control System for Gas-Liquid Cylindrical Cyclone Separators,� ASME J. Energy Resources Technology, v. 120(1), pp. 49-55, March 1998.

7. Wang, S., Mohan, R.S., Shoham, O., Marrelli, J. D. and Kouba, G.E.: "Performance Improvement of Gas Liquid Cylindrical Cyclone Separators Using Integrated Liquid Level and Pressure Control Systems," ETCE00-ER-035, proceedings of the ASME Energy Sources Technology Conference and Exhibition, ETCE '00, New Orleans, Louisiana, Feb. 14-17, 2000.

8. Wang, S., Mohan, R., Shoham, O., Marrelli, J. and Kouba, G.: "Control System Simulators for Gas-Liquid Cylindrical Cyclone Separators," ETCE00-ER-036, proceedings of the ASME Energy Sources Technology Conference and Exhibition, ETCE '00, New Orleans, February 14-17, 2000a.

9. Wang, S.: �Dynamic Simulation, Experimental Investigation And Control System Design Of Gas-Liquid Cylindrical Cyclone Separators,� Ph.D. Dissertation, The University of Tulsa, 2000.

10. Payne, R.L., Huff, R. E., Ogren, W.E.: U.S. Patent 5544672, � Slug flow mitigation control system and method, August, 1996.

11. Stephanopoulos, G.: �Chemical Process Control�, PTR Prentice Hall International Series in the Physical and Chemical Engineering Sciences, 1984.

12. Earni, B.S.: �Predictive Control of Gas-Liquid Cylindrical Cyclone (GLCC) Separators,� M.S. Thesis, The University of Tulsa, 2001.


Fig. 2 � Integrated Feed forward and feedback Level Control Loop

Multiphase Flow

LC1

Level Transducer

GCV

PC Pressure Controller

Pressure Transducer

FF2

LC2

FF1

Level Controller 2

Slug Detector

Gas Stream

Level Controller 1

Liquid Stream

Fig. 1 � Schematic of GLCC Control Systems

1 2 3 4

5

6

7 8

9

+ ++

Slug Detector Sensor

Set point Tracker

FeedforwardController

E1(s)

E2(s)

y(s)

C2(s)

C1(s) C(s) m(s)

Gsp(s) Gm2(s)

Gc2 (s)

Gpl(s) Glcv(s)Gc1(s)

Gm1(s)

Setpointysp(s)

Setpointysp(s)

Gvl(s)

Liquid Level Sensor

+ _

_

+

Gd(s)

SLUG

_

Pneumatic Line

LCVFeedback Controller

d(s)

+ ++

Slug Detector Sensor

Set point Tracker

FeedforwardController

E1(s)

E2(s)

y(s)

C2(s)

C1(s) C(s) m(s)

Gsp(s) Gm2(s)

Gc2 (s)

Gpl(s) Glcv(s)Gc1(s)

Gm1(s)

Setpointysp(s)

Setpointysp(s)

Gvl(s)

Liquid Level Sensor

+ _

_

+

Gd(s)

SLUG

_

+

Gd(s)

SLUG

_

Pneumatic Line

LCVFeedback Controller

d(s)


-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0.10 1.00

Frequency(Hz)

Mag

nitu

de(d

B)z1=z2=wc/10

z1=z2=wc/9

z1=z2=wc/8

z1=z2=wc/7

z1=z2=wc/6

z1=z2=wc/5

z1=z2=wc/15

z1=0.4;z2=0.5

z1=0.4;z2=0.3

z1=0.4;z2=0.2

Fig. 3 � Closed Loop Magnitude Plot for Different Compensator Zero Locations

D

Vs

D Conductance

Probes

Vs

C1 C2D

Vs

D Conductance

Probes

VsVs

C1 C2

Fig. 4 � Schematic of the Slug Detection Zone with Two Conductances Probes (C1, C2)


Fig. 5a � System Response for Feedback Control System (Vsg=3.84ft/s, Vsl=0.38 ft/s, Slug Length=12 ft)

Fig. 5b � System Response for the Integrated Feed forward and Feedback Control Systems (Vsg=3.84ft/s, Vsl=0.38 ft/s, Slug Length=12 ft)

0

20

40

60

80

100

120

1700 1750 1800 1850 1900 1950 2000

Time Step (0.05 sec)

Flow

Cha

ract

eris

tics

Liquid Level in GLCC

Conduct1

LCV

Max Liquid Level = 54 inchesSettling Time = 6.4 secRise Time = 2.05 secLiquid Level Dynamics(RMS) = 16.97LCV Dynamics(RMS) = 14.83

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glcc2 diff pressure Conduct2

LCV FF

Max Liquid Level = 38.12 inchesSettling Time = 5 secRise Time = 2 secLiquid Level Dynamics(RMS) = 7.33LCV Dynamics(RMS) = 11.07


Fig. 6a � System Response for Feedback Control System (Vsg=6.61ft/s, Vsl=0.41 ft/s, Slug Length=25 ft)

Fig. 6b � System Response for the Integrated Feed forward and Feedback Control Systems (Vsg=6.61ft/s, Vsl=0.41 ft/s, Slug Length=25 ft)

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Liquid Level LCV-TotFF C1C2 FB


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Liquid Level LCV-TotFF C1C2 FB



S.No Vsg Vsl Slug ft/sec ft/sec Dumped ( Liq Lvl Settling Time Liq Lev LCV Dynamics Liq Lvl Settling Time Liq Lev LCV Dynamics

(Ft) " of H20 (Rise Time) RMS RMS " of H20 (Rise Time) RMS RMS

1 3.84 0.3 12 54 6.4(2.05) 16.97 14.83 38.12 5(2.6) 7.33 11.072 5.54 0.3 19 83 14.5(3.75) 32.58 14.57 52 7.85(1.5) 14.82 9.823 3.65 0.4 21 106 26(3.2) 33.54 13.46 61 6.2(2.92) 17.03 9.8

18 98 23.65(3.1) 30.22 15.67 53 5.75(2.3) 15.05 9.817 96 28.5(3.75) 29.32 14.32 50 7.86(2.65) 16.68 10.1

4 6.61 0.41 15 79 21.15(2.75) 26.2 8.67 56 7.2(2.95) 16.93 13.9225 115 35.8(3.85) 41.89 16.83 100 17.3(2.8) 32.66 10.85

5 6.61 0.83 25 109 50(3.8) 44.56 14.52 109 26(4.8) 31.98 12.3812 74 22.2(3.65) 27.32 16.53 52 5.9(2.1) 16.98 12.17

6 7.27 0.55 22 97 23.6(2.45) 36.57 14.28 67.5 12.6(1.75) 18.29 10.2321 85 22.4(4.6) 36.78 14.13 62.5 14.35(1.9) 15.08 13.76

7 8.4 0.55 19 87 15.2(2.6) 30.25 8.69 78 5.3(1.7) 24.65 12.2721 93 22(2.32) 33.12 8.71 86 9.4(2.3) 25.32 15.92

8 11.1 0.55 19 110 24.1(2.5) 47.35 9.44 98 7.15(1.5) 32.29 11.45

Feedback Control Feedbackforward Control

Table 1: Summary of the Results for Feedback Control System and Integrated Feedback & Feed forward Control Systems


slug detection as a tool for predictive control of glcc(r)...

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