T H E U N I V E R S I T Y O F T U L S A THE GRADUATE SCHOOL DESIGN AND PERFORMANCE OF MULTIPHASE DISTRIBUTION MANIFOLD. Angel R.

Transcription

1 T H E U N I V E R S I T Y O F T U S A THE RADUATE SCHOO DESIN AND PERFORMANCE OF MUTIPHASE DISTRIBUTION MANIFOD by Angel R. Bustamante A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in the Discipline of Mechanical Engineering The raduate School The University of Tulsa 2003

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3 ABSTRACT Bustamante, Angel R. (Master of Science in Mechanical Engineering) Design and Performance of Multiphase Distribution Manifold (76 pp.- Chapter VI) Directed by Dr. Ram Mohan, Dr. Shoubo Wang and Dr. Ovadia Shoham. (150 words) A novel multiphase distribution manifold is studied experimentally and theoretically. The distribution manifold is elevated, with slug dampers and CCs attached downstream, as an integrated system. The objective of the novel manifold system is to gather production from several inlet wells, and to ensure equal split of gas and liquid phases for downstream equipment. Over 200 experimental runs were conducted, evaluating eight different inlet well configurations. Individual inlet well gas and liquid flow rates and liquid and gas split ratios were measured. Data analysis revealed that the capability of the distribution manifold system is to ensure fairly equal split ratios downstream, for different inlet flow conditions. A mechanistic model is developed, based on the Hardy-Cross method, capable of predicting the downstream liquid and gas split ratios. ood agreement is observed between the model predictions and the experimental data for all tested inlet flow conditions, showing errors between 3% to 15%. iii

4 ACKNOWEDMENTS I am extremely grateful to Dr. Ram Mohan for his unconditional support; to Dr. Ovadia Shoham for his continous help at any time; to Dr. Shoubo Wang and Dr. uis omez for their support throughout this investigation, and, more than that, for being my friends. I also wish to thank Dr. Brenton Mcaury, member of the Thesis Committee, for his suggestions and comments. I am also indebted to Dr. ene Kouba from ChevronTexaco, for initiating this project and for serving on my Thesis Committee. I want to thank the following persons and entities for their support and guidance during my study and research: PDVSA, for giving me the chance to come to TU and support me, enabling me to accomplish this achievement. Mr. Carlos Torres, my office mate and friend, for his help throughout my research, especially while developing the mechanistic model. Ms. Judy Teal for her help, support and encouragement throughout my stay at TU. TUSTP graduate students and member companies for their valuable comments, cooperation and friendship during this project. The U.S. Department of Energy (DOE) for supporting this project. To all my friends who share with me this important period of my life. iv

5 DEDICATION This work is dedicated to my lovely wife Nivia for her support, encouragement and love. I would also like to dedicate this work to my parents Miguel and Juana, my family, and especially my youngest brother Miguel. v

6 TABE OF CONTENTS APPROVA PAE ABSTRACT ACKNOWEDMENTS DEDICATION TABE OF CONTENTS IST OF FIURES ii iii iv v vi viii I. INTRODUCTION 1 II. ITERATURE REVIEW Splitting at Impacting Tee Two-Phase Flow in T-Junctions Slug Flow Manifolds Flow Conditioning 8 III. EXPERIMENTA PRORAM Experimental Facility Metering and Storage Sections Modular Test Section Instrumentation, Control and Data Acquisition System Multiphase Distribution Manifold/Slug Damper /CC System Experimental Setup 19 vi

7 Parameter Definitions Test Procedure Test Matrix Experimental Results iquid Carry-Over Operational Envelope of Integrated System Manifold Operational Envelope for iquid Carry-Over iquid and as Split Ratios in Distribution Manifold Resistance Coefficient of Manifold Transient Performance of Multiphase Distribution Manifold 36 IV. MODE DEVEOPMENT Manifold Design Model Manifold Sizing as and iquid Outlets sizing Manifold / Slug Damper / CC System Performance Model Calculation of liquid height at node a Calculation of Head osses between nodes 54 V. SIMUATIONS AND RESUTS iquid Split Ratio as Split Ratio 64 VI. CONCUSIONS AND RECOMMENDATIONS 68 NOMENCATURE 72 REFERENCES 74 vii

8 IST OF FIURES Figure 1.1. CC Separator 2 Figure 1.2. Multiphase Distribution Manifold Schematic 3 Figure 3.1. Schematic of Experimental Facility 10 Figure 3.2. Tanks, Pumping Station and Metering Section 12 Figure 3.3. Schematic of Manifold / Slug Damper / CC System 15 Figure 3.4. iquid and as Inlet Flow Meters 16 Figure 3.5. Multiphase Distribution Manifold Schematic 17 Figure 3.6. Slug Damper Manifold 18 Figure 3.7. CC Separator Schematic 19 Figure 3.8. Flow Configurations in Inlet Wells 23 Figure 3.9. Operational Envelope for CO of Manifold / Slug Damper / CC System 26 Figure Operational Envelope of the Distribution Manifold 28 Figure iquid Split Ratio in the Distribution Manifold 30 Figure as Split Ratio in the Distribution Manifold 31 Figure Resistance Coefficient of iquid Outlet vs. Reynolds Number 34 Figure Conditions for Determination of Resistance Coefficient 35 Figure Transient Performance of Manifold Under No as Flow 37 Figure Transient Performance of Manifold Under ow as Flow 38 Figure Transient Performance of Manifold Under Moderate as Flow 39 Figure 4.1. Stationary Finite Wave on as-iquid Interface 41 viii

9 Figure 4.2. Manifold Sizing-Criterion 1 44 Figure 4.3. Manifold Sizing-Criterion 2 45 Figure 4.4. iquid and as Split Calculations 45 Figure 4.5. Variables Involved in iquid Outlet Sizing 46 Figure 4.6. Variables Involved in Manifold Performance Model 50 Figure 4.7. Simplified Piping and Nodes Model 52 Figure 5.1. Comparison between Model Predictions and Experimental Data for iquid Split Ratio in Flow Configuration I 60 Figure 5.2. Comparison between Model Predictions and Experimental Data for iquid Split Ratio in Flow Configuration III 61 Figure 5.3. Comparison between Model Predictions and Experimental Data for iquid Split Ratio in Flow Configuration VI 62 Figure 5.4. Comparison between Model Predictions and Experimental Data for iquid Split Ratio in Flow Configuration IV 63 Figure 5.5. Comparison between Model Predictions and Experimental Data for as Split Ratio in Flow Configuration I 64 Figure 5.6. Comparison between Model Predictions and Experimental Data for as Split Ratio in Flow Configuration III 65 Figure 5.7. Comparison between Model Predictions and Experimental Data for as Split Ratio in Flow Configuration VI 66 Figure 5.8. Comparison between Model Predictions and Experimental Data for as Split Ratio in Flow Configuration IV 67 ix

10 CHAPTER I INTRODUCTION During the past decade the petroleum industry has been forced to seek less expensive and more efficient alternatives to conventional gravity-based separators. Compact separation systems represent a key factor in the reduction of costs of oil and gas production and handling. The so-called as-iquid Cylindrical Cyclone (CC 1 ) is a successful example of how a simple idea can provide a solution to the above-mentioned need of the petroleum industry. The CC is a compact, cheap and lightweight separator that requires almost no maintenance, is easy to operate and construct and has a small footprint. Thus, the CC is an economically attractive alternative to the big, heavy and expensive conventional gravity-based separator. The CC, shown schematically in Figure 1.1, is simply a vertical piece of pipe with a downward inclined tangential inlet and two outlets, one at the top for gas and one at the bottom for liquid. An important feature of the CC is that it does not have moving parts or any internal devices, which make it easy to operate and maintain. The inclined tangential inlet provides a swirling motion in the CC, whereby due to gravitational and centrifugal forces the gas and liquid phases are separated. The liquid is forced towards the wall and leaves the CC from the bottom outlet, whereas the gas moves to the center of the cylinder and exits from the top. One of the problems with the operation of CC is its low residence time. This feature causes operational problems when flow fluctuations occur. To solve this problem, 1 CC as-iquid Cylindrical Cyclone-copyright, The University of Tulsa, 1994

11 2 the CC can be equipped with a robust control system that maintains a constant liquid level inside the separator. However, this solution causes problems downstream of the CC because large outlet liquid flow rate fluctuations may occur during the production of large slugs, much higher than the average liquid flow rate. To improve the performance of the separation equipment some inlet flow conditioning devices can provide an effective and cheap solution by smoothening the flow fluctuations. Reinoso (2002) analyzed one such device, namely the Slug Damper. This device not only protects downstream equipment but also extends the operational envelope of CC. When a terrain slug hits the damper, the larger slug is dampened, providing fairly constant flow rate into the downstream CC. Figure 1.1. CC Separator This study presents the Multiphase Distribution Manifold, which can be used in conjunction with the slug damper and the CC. This integrated system minimizes downstream operational problems, by dampening large flow variations and distributing the liquid and gas flow equally. The proposed distribution manifold, shown schematically

12 3 in Figure 1.2, is a simple horizontal pipe section, into which different wells are connected. As can be seen, upper gas outlets and lower liquid outlets are provided. This novel device can function as a pre-separator upstream of compact separators. Also, the distribution manifold attempts to provide equal liquid and gas split for downstream processing facilities, regardless of the variation of flow pattern and flow rates in the different inlet wells. Another important feature of the proposed distribution manifold, when working in conjunction with a slug damper, is the increment of the damping capacity of the system, due to the available additional volume to dampen large slugs before entering the CCs. as Outlets Distribution Manifold Inlet Wells iquid Outlets Figure 1.2. Multiphase Distribution Manifold Schematic Research goals and objectives: The main goal of this study is to acquire experimental data in order to determine the liquid and gas split ratios in the distribution manifold system for different flow conditions in each inlet well and different flow

13 4 configurations inside the manifold. This study also intends to identify the mechanisms involved in the distribution manifold performance, so as to enable the development of a mechanistic model for design and performance prediction purposes. The specific objectives of this study are as follows: Design, construct and install a multiphase distribution manifold facility. Instrument the multiphase distribution manifold facility to enable measurement and monitoring of the outlet liquid and gas flow split. Acquire experimental data of liquid and gas split ratios for different flow rates in each inlet well and different flow configurations in the manifold. Identify and characterize the mechanisms involved in the liquid and gas split process in the distribution manifold to shed light on the complex hydrodynamic behavior of the distribution manifold. Develop a mechanistic model as a tool to design a distribution manifold that aims at equal liquid and gas flow rates in each downstream separator. The mechanistic model can also be used to evaluate the performance of the distribution manifold when its geometry is given. The next chapter encompasses a review of the literature relevant to this study. In Chapter III, the experimental investigation is presented, which includes description of the facilities, experimental program and experimental results. Chapter IV presents the developed mechanistic model, and Chapter V shows a comparative study between the model predictions and the experimental data. Conclusions of this investigation are summarized in Chapter VI along with some recommendations for future work. This is followed by a list of nomenclature and references in separate sections.

14 CHAPTER II ITERATURE REVIEW The proposed Multiphase Distribution Manifold system configuration is a relatively new device for flow conditioning upstream of compact separators. There are few literature references related to the performance of individual components of the manifold system, but almost none related to the design and evaluation of the distribution manifold as a whole. This chapter presents an overview of pertinent literature studies, such as two-phase flow splitting at an impacting tee, two-phase flows in tee junctions and manifolds, slug flow and flow conditioning devices. 2.1 Splitting at Impacting Tee Because of the geometry of the distribution manifold, two-phase flow coming from the inlet wells impinges the opposite manifold wall, causing splitting of both liquid and gas in an impacting configuration. Hong and riston (1995) developed a model to predict the liquid and gas splitting in impacting tees. The experimental facilities used in their study included a splitting tee of the same diameter as the upstream pipe, and also downstream separators to measure the liquid and gas that each branch carried. For the majority of flow conditions tested by Hong and riston, the flow pattern upstream of the tee was stratified flow. Results from this experimental study cannot be used in the analysis of the proposed distribution manifold. This is mainly due to the fact that different flow patterns may exist in the different inlet wells connected to the

15 6 distribution manifold. Also, several impacting tees are found in a distribution manifold, where the flow splitting in each tee may affect the flow behavior of others. Wang and Shoji (2002) conducted an experimental study to evaluate the fluctuating characteristics of two-phase flow splitting at a vertical impacting tee. This study focused mainly on churn flow, which exhibits the strongest flow fluctuations as compared to other flow patterns, such as bubble flow or annular flow. The main tube of the impacting tee was vertical and the two branches were horizontal. Data included the effect of the extraction flow ratios and the upstream superficial gas velocities on the flow fluctuations. 2.2 Two-Phase Flow in T-Junctions For single-phase flows, the present state of knowledge is sufficiently advanced to enable the majority of cases of flow in junctions to be designed. In the case of two-phase flow, however, the number of variables is much larger; in addition there are complicating factors in the distribution and mixing of the phases. The problem is particularly acute for dividing junctions, whereby either phase can pass preferentially into the side branch of the junction. In any calculations involving junctions, the flows are governed not only by what occurs at the junction itself but also by the flow resistances at different elements of the entire system. Azzopardi (1992) showed that the liquid and gas flow split and the pressure drop in a tee junction are affected by the angle between the main tube and the side arm, the ratio of side arm to main tube diameters and the degree of rounding of the corner. Because gravity can cause stratification, it is also necessary to specify the angle

16 7 between the main tube and the vertical, and the angle between the side arm and the main tube. The effect of the orientation of the side-arm in a tee junction, when the flow in the main tube (horizontal) is stratified, was studied by Reimann and Khan (1984). They found that initially only one phase is extracted, namely, the gas, when the side-arm is vertical upwards, and liquid when side arm is vertical downwards. When the side-arm was at the bottom of the main tube, Reimann and Khan observed that the free surface of the liquid above the side-arm became depressed with some gas entrained into the sidearm. Taitel and Dukler (1987) analyzed the hydrodynamics near the exit of a pipe carrying gas and liquid in horizontal stratified flow. This work demonstrated the effect of pipe length on the stratified to non-stratified transition boundary. For low-viscosity fluids, pipe length effects are unimportant but for high-viscosity liquids the transition from stratified flow can be profoundly influenced by the pipe length. 2.3 Slug Flow An important additional feature of the Distribution Manifold is its capacity to minimize the effect of slug flow produced from wells, by providing damping time and functioning as a pre-separator. Dukler and Hubbard (1975) developed the fundamental and pioneering mechanistic model of slug flow. Based on their observations, Dukler and Hubbard defined an idealized slug unit and suggested a mechanism for the flow. The developed model is capable of predicting the slug hydrodynamic flow behavior, including the length, holdup, velocity and pressure distributions.

17 8 Taitel and Barnea (1990) presented a comprehensive analysis of slug flow. It aimed to extend the scope of slug flow modeling into a unified model for horizontal, inclined and vertical upward flow. For the study of the hydrodynamics of the film, several approaches were proposed, including rigorous, equilibrium film and open channel flow approaches. Also, two methods to predict the pressure drop were described, namely, a global force balance on a slug unit and a force balance on the liquid slug only. A comprehensive discussion on closure relationship is also presented. 2.4 Manifolds Manifold can be defined as more than one tee junction connected together to a main pipe. Miller (1971), Collier (1976) and Coney (1980) conducted experiments on manifolds. Miller (1971) noted that for single-phase flow, interaction between junctions occurs if the interjunction distance is less than three main tube diameters. No equivalent information was obtained for two-phase flow. However, some kind of interaction between junctions for multiphase flow is expected, too. Collier (1976) analyzed a horizontal system of four tubes linked by inlet and outlet headers. Void fractions and pressure drop were used to determine the flow rates and qualities in each branch of the manifold. It was found that the manifold results were consistent with the measurements taken utilizing equivalent set of single junctions. 2.5 Flow Conditioning Sarica et al. (1990) presented a mechanistic model for the prediction of the required dimensions of a finger storage slug catcher. The approach is based on the effect

18 9 of the finger pipe diameter and inclination angle on the transition boundary between slug flow and stratified flow. Based on this model, the required length and optimal downward inclination angle of the fingers can be determined. Ramirez (2000) presented experimental data on slug dissipation in helical pipes. The data depict the effect of different operating conditions and helical pipe geometry on slug dissipation. The data showed the effects of helix diameter, pitch angle and number of circular turns on slug dissipation phenomenon. Reinoso (2002) conducted a study on a novel device known as a slug damper. In this work, experimental data were acquired in order to determine the slug propagation in the slug damper and the corresponding outlet liquid flow rate, and to identify the mechanisms involved in the slug damper performance. A mechanistic model was developed for design purposes.

19 CHAPTER III EXPERIMENTA PRORAM 3.1 Experimental Facility An experimental flow loop has been constructed in the College of Engineering and Natural Sciences Research Building, located in the North Campus of The University of Tulsa. This indoor facility enables year around data acquisition and simultaneous testing of different compact separation equipment. This chapter includes details of the experimental facility, data acquisition procedure and representative experimental results. Figure 3.1 shows a schematic of the experimental facility. The oil-water-air threephase flow facility is a fully instrumented state-of the-art, two-inch flow loop, enabling testing of single separation equipment or combined separation systems. The three-phase flow loop consists of a metering and storage section and a modular test section. Following is a brief description of both sections. Air Filter Micromotion CC/CC 3-Phase Separator HC Water Tank Micromotion HPS Oil Tank Micromotion Distribution Manifold Figure 3.1. Schematic of Experimental Facility

20 Metering and Storage Sections Air is supplied from a compressor and is stored in a high-pressure gas tank. The air flows through a one-inch metering section, consisting of Micromotion mass flow meter, pressure regulator and control valve. The liquid phases (water and oil) are pumped from the respective storage tanks (400 gallons each), and are metered with two sets of Micromotion mass flow meters, pressure regulators and control valves. The pumping station, shown in Figure 3.2, consists of a set of two pumps (10 HP and 25 HP equipped with motor speed controllers) for each liquid-phase. Each set of pumps has an automatic re-circulating system to avoid the occurrence of high pressure in the discharge line. The liquid and gas phases can be either mixed at a tee junction and sent to the test section or, in the case of the test section of the multiphase distribution manifold, the two phases can flow separately through two 2 independent lines up to the test section. Downstream of the test sections, the gas, oil-rich and water-rich streams flow through three Micromotion net oil computers to measure the outlet gas flow rate, and the flow rate and water-cut of the two liquid streams. The three streams then flow into a threephase conventional horizontal separator (36-inch diameter and 10 feet long), where the air is vented to the atmosphere and the separated oil and water flow back to their respective storage tanks. A technical grade white mineral oil type, Tufflo 6016, with a specific gravity of and a viscosity of 27 cp (@ 75 F) is used as the experimental oil along with tap water. For this research, only water and air were used in the experimental program.

21 12 Figure 3.2. Tanks, Pumping Station and Metering Section Modular Test Section The metered three-phase mixture coming from the metering section can flow into any of the six different test stations. This flexibility enables the testing of single separation equipment, such as a CC, CC 2, iquid-iquid Hydrocyclone (HC), Horizontal Pipe Separator (HPS 3 ), Multiphase Distribution Manifold or conventional separators, or any combination of these, in parallel or series, forming a compact separation system. Each test section is briefly described following. CC/CC. This facility allows conducting experiments in a rudimentary compact separation system using a iquid-iquid Cylindrical Cyclone (CC) 2 CC iquid-iquid Cylindrical Cyclone-copyright, The University of Tulsa, HPS Horizontal Pipe Separator-copyright, The University of Tulsa, 2001

22 13 downstream of a CC. These two separators working in series perform as a three-phase separator. Horizontal Pipe Separator (HPS). This facility allows conducting experiments using a horizontal pipe separator to analyze the separation process between two immiscible liquid phases (oil continuous). Hydrocyclone (HC). This facility allows the conduction of experiments to separate droplets of any liquid dispersed into another continuous liquid-phase. The main condition to run this facility is that there must exist a difference in density between both, continuous and dispersed phases. Multiphase Distribution Manifold. This facility allows conducting experiments to evaluate the gas and liquid split in a manifold for different flow configurations and different flow conditions of the inlet wells. Downstream this manifold there are two instrumented CCs enabling metering the gas and liquid phases flowing into each separator, and subsequently, liquid and gas split ratios of the manifold can be determined Instrumentation, Control and Data Acquisition System Control valves placed along the flow loop control the flow into the test sections. The flow loop is also equipped with several temperature sensors and pressure transducers for measurement of the in-situ temperature and pressure conditions. All output signals from the sensors, transducers, and metering devices are collected at a central panel. A state-of-the art-data acquisition system, built using abview, is used to both control the flow into and out of the loop and to acquire data from the analog signals transmitted by the instrumentation.

23 Multiphase Distribution Manifold/SlugDamper/CC System The multiphase distribution manifold system, shown schematically in Fig. 3.3, consists of four sections, namely, inlet wells, manifold, slug dampers, and down stream CCs. Detailed description of each section is given below. Inlet Well Section. The inlet wells are connected to the manifold from the liquid and gas supply flow lines by two sets of four tees. Thus, four separate inlet gas lines and four separate liquid lines are available, respectively. The flow rates in each of the 8 inlet lines are measured using 8 rotameters. The inlet well section is shown in Figure 3.4. The four inlet wells are simulated by combining pairs of single-phase liquid and single-phase gas lines, resulting in 4 two-phase inlet wells, which are connected to the manifold. Manifold Section. Figure 3.5 shows an isometric view of the distribution manifold (dimensions are in inches). The manifold is an 8 feet long, 3- inch ID horizontal pipe section. The four inlet wells are connected horizontally to the manifold with equal spacing of 2 ft. The manifold has two 3 upper gas exits at the top and two 3 lower liquid exits at the bottom, with a spacing of 4 ft. from each other. The upper and lower exits each consist of a vertical section, 1 ft. long, which are radially opposed. The upper exits are connected to the upper legs of the 2 slug dampers and the lower exits are connected to the lower legs of the slug dampers. During operation, the production from individual wells flows into the distribution manifold where a pre-separation of the phases occurs, whereby the liquid goes to the lower exits and the gas goes to the upper exits.

24 15 as Outlet Vortex Meter Vortex Meter Distribution Manifold iquid ine WE 4 WE 3 WE 2 WE 1 as ine CC # 2 CC # 1 Slug Damper Rotameter iquid Outlets to Micromotion Figure 3.3. Schematic of Manifold / Slug Damper / CC System

25 16 When slugs or any increase in the liquid or gas flow rates from any well are introduced into the manifold, they are dissipated and, therefore, the effect of these slugs or perturbations on the system is minimized. It is important to note that because of the geometry of the gas and liquid outlets and the way the manifold is designed, it must be installed in an elevated position in such a way that upper and lower legs of the manifold could be connected to the slug damper inlets. Doing so will allow the liquid and gas to flow towards the CCs. Figure 3.4. iquid and as Inlet Flow Meters

26 17 Figure 3.5. Multiphase Distribution Manifold Schematic (dimensions in inches) Slug Damper Section. As shown in Figure 3.6, the slug damper consists of two large diameter legs located one above the other. The two legs are 7.5 ft. long and 3- inch diameter, where the lower leg is inclined downward at 1.4 while the upper leg is inclined upward at 1.8. The vertical distance between the two legs at the end of them is 28 inches. These two legs are connected to the CC, resembling a long dual CC inlet. In this system there are two slug dampers connecting the distribution manifold with 2 downstream CCs. These 2 slug damper units allow the liquid and gas to run independently from the manifold to the separators. The main operational mechanism of the slug damper is a segmented orifice located in the lower leg, just upstream of the CC. The orifice is open at the bottom and

27 18 closed at the top. When a slug hits the damper, due to the flow restriction provided by the segmented orifice in the lower leg, the slug is damped, accumulating in the lower leg, and fairly constant liquid flow rate enters the CC through the orifice. Note that in addition, the distribution manifold provides an additional damping capacity upstream of the slug dampers. Figure 3.6. Slug Damper Manifold CC Section. The experimental facility includes 2 identical CCs, connected to the 2 slug damper units. The CC, shown schematically in Figure 3.7, is a 6 feet high, 3-inch ID vertical pipe, with dual inlets. The lower inlet of the CC is connected to the lower leg of the slug damper. The CC inlet slot area is 25% of the inlet full bore

28 19 cross sectional area and is connected tangentially to the vertical pipe. The upper inlet, as outlet which is connected to the upper leg of the slug damper, is a full bore pipe, connected Upper inlet (as) ower inlet (iquid) 1 foot below the top of the vertical pipe. The 2-inch ID CC gas outlet is located radially at the top of the vertical CC body. The 2-inch ID CC liquid outlet is connected tangentially at the bottom of the vertical CC section. iquid and gas flow rates from each CC are measured iquid outlet 0.5 Figure 3.7. CC Separator Schematic separately and then recombined before entering the 3-phase separator. iquid streams are measured using a Micromotion mass flow meter and gas streams are measured using Vortex shedding meter. 3.2 Experimental Setup Over two hundred experimental test runs were carried out in this study to quantify the performance of the distribution manifold, for different flow conditions and flow patterns occurring in the inlet wells. These experimental tests include variations in gas and liquid flow rates of each inlet well.

29 Parameter Definitions Following is a description of the parameters used to determine the performance of the multiphase distribution manifold. Superficial as Velocity: The superficial gas velocity is defined as the in-situ total volumetric gas flow rate in the manifold divided by the total cross sectional area of the manifold. Combining the equation of state and the definition of gas superficial velocity yields the value of this variable, as follows: V S m = 3.056* (3.1) ρ * d 2 p where, VS is the superficial gas velocity, in ft/s. m is the gas mass flow rate, in lb/min. ρ is the density of the gas, in lb/ft 3. d P is the pipe diameter, in inches. Superficial iquid Velocity: Similarly, the superficial liquid velocity is defined as total liquid flow rate entering the manifold divided by the cross sectional area of the manifold. Following is the equation used to determine the superficial liquid velocity. V S m = 3.056* (3.2) ρ * d 2 p where, VS is the superficial liquid velocity, in ft/s. m is the liquid mass flow rate, in lb/min. ρ is the density of the liquid, in lb/ft 3.

30 21 d P is the pipe diameter, in inches. iquid Split Ratio: This is the ratio between the liquid flow rate at the exit of one CC to the total liquid flow rate entering the manifold. This ratio is given in percentage, as follows. where, q, CC % SR = * 100 (3.3) q, TOTA SR is the liquid split ratio, in %. q, CC is the liquid flow rate at the exit of any CC, ft 3 /min. q, TOTA is the total liquid flow rate entering the manifold, in ft 3 /min. as Split Ratio: Similarly, the gas split ratio is the ratio between the gas flow rate at the exit of one CC to the total gas flow rate entering the manifold, given in percentage, as follows. where, q, CC SR = *100% (3.4) q, TOTA SR is the gas split ratio, in % q, CC is the gas flow rate at the exit of any CC, in ft 3 /min. q, TOTA is the total gas flow rate entering the manifold, in ft 3 /min. as Volume Fraction (VF): The VF is the volumetric fraction of gas flow rate in the manifold. It is given by the ratio between the superficial gas velocity and the mixture velocity inside the manifold.

31 22 iquid Carry-Over (CO): Operational condition for a given superficial gas velocity and superficial liquid velocity, where some liquid is carried into the exit gas stream. In the case of the manifold, liquid carry-over is defined when some liquid appears in the horizontal section of either of the two upper gas legs of the manifold Test Procedure The general procedure followed for conducting experiments is given below: 1. Start the liquid pump and the air compressor. 2. Fix the liquid and gas flow rates utilizing the flow control system, control valve/micromotion, using abview program. 3. Measure the individual liquid and gas flow rates of the four inlet wells. 4. Measure the liquid and gas flow rates at the liquid and gas exits of the 2 down stream CCs. 5. Measure pressure and temperature of fluids in the system. The detailed procedure and the purpose of each experiment are described in section 3.3 of this chapter Test Matrix The following data acquisition matrix was selected in order to study the performance of the distribution manifold and the corresponding gas and liquid split ratios.

32 23 Flow Configurations: Eight different flow configurations of the inlet wells were analyzed. These flow configurations are shown schematically in Figure 3.8. In this figure, designates liquid dominant well and designates that the well is gas dominant. The eighth case, which is not shown in this figure, corresponds to equal inlet gas and liquid flow conditions in all the wells. CASE I CASE V CASE II CASE VI CASE III CASE VII CASE IV Vsg: 10.5 fts/s to 30.5 ft/s Vsl: 1.0 ft/s to 2.75 ft/s Figure 3.8. Flow Configurations in Inlet Wells Operating Flow Conditions: Air and water were used in this study. The ranges of superficial velocities inside the manifold are: Superficial gas velocity: ft/s. Superficial liquid velocity: ft/s.

33 Experimental Results In this section, detailed experimental results are presented. The results include: Operational envelope of Manifold/Slug Damper/CC integrated system for liquid carry-over. Operational envelope of the Manifold itself for liquid carry-over. iquid and gas split ratios in the Manifold. iquid discharge resistance coefficient of the Manifold. Transient performance of the Manifold/Slug Damper/CC system under liquid flow rate surges at the inlet. When plotting the CO envelope of either the manifold or the total system, the superficial liquid velocity, V sl, is plotted in the vertical axis and the superficial gas velocity, V sg, is plotted in the horizontal axis. When plotting the liquid or gas split ratios, the gas volume fraction, VF, in the distribution manifold is plotted in the horizontal axis, and either the gas or liquid split ratio is plotted in the vertical axis iquid Carry-Over Operational Envelope of Manifold / Slug Damper / CC Integrated System During normal operation, it is desirable to avoid liquid carry-over in either of the two CCs, in order to maintain high separation efficiency. The operational envelope of the Manifold/Slug Damper/CC System for iquid Carry-Over is the onset of liquid carry-over in either of the 2 downstream CCs. Operational envelopes can be presented for either the CC or for the entire system to characterize their respective capacities to ensure no liquid carry-over.

34 25 In the existing facility, it is possible to measure not only the total liquid and gas flow rates entering the system, but also the gas and liquid flow rates into each CC. Therefore the envelope of a single CC can be determined. Once the envelope for a single CC is plotted, the operational envelope of the combined two identical CCs operating in parallel can be plotted. So, the theoretical maximum capacity of the system is twice the capacity of a single CC, if equal split from the manifold is provided. Several experiments were conducted, with different inlet wells configurations, to determine the operational envelopes for liquid carry-over of a single CC, two parallel CCs and the envelope of the entire system (Manifold / Slug Damper / CCs), under different flow conditions of the inlet wells. The procedure followed to determine these operational envelopes is given below: 1. Choose the flow configuration of the inlet wells, as shown in Figure 3.8, and adjust the flow through each well in such a way that in the adjustment process the flow configuration is maintained, namely, either liquid or gas dominant or equal flow. 2. Starting with high gas flow, fix the gas flow rate and increase the liquid flow rate until liquid carry-over can be observed in either of the down stream CCs. 3. Repeat step 2 with a lower gas flow rate. Figure 3.9 shows the operational envelopes of the entire system under different inlet well flow configurations. In this figure, the lower curve represents the envelope for a single CC. The upper curve represents the operational envelope of two CCs operating in parallel under equal flow conditions.

35 26 For flow configurations I, III, VI and VII, the wells that are liquid dominant or gas dominant are placed in one end of the manifold, creating an uneven split. For these cases, as shown in the Figure 3.9, for higher superficial gas velocities, the capacity of the system is higher than the capacity of two parallel CCs. However, for lower superficial gas velocities, the capacity of the system is reduced significantly. For cases I and VI, the superficial gas velocity separating the 2 different capacity behaviors is around 26 ft/s. For case III, this superficial gas velocity is around 23 ft/s and for case VII it is around 29 ft/s. The explanation of the phenomenon is presented next. Vsl (ft/s) Single CC Manifold/Slug Damper/CC's Manifold / Slug Damper iquid Carry-Over 2 Parallel CC's Single CC Double CC Case I Case II Case III Case VI Case VII Case VIII Equal Flow Vsg (ft/s) Figure 3.9. Operational Envelope for CO of Manifold / Slug Damper / CC System Due to the uneven split generated at the Manifold, one CC can operate in one edge of the individual CC envelope (low gas flow rate / high liquid flow rate) and the

36 27 other CC will operate on the opposite edge of the envelope (high gas flow rate / low liquid flow rate). This uneven split can explain the fact that running the system under uneven split and superficial gas velocities higher than 30 ft/s, the capacity of the system can be higher as compared to the capacity of two CCs working in parallel. For flow configuration II the envelope shows that as the superficial gas velocity increases up to 30 ft/s, the capacity of the system approaches the theoretical capacity of two parallel CCs operating under equal split conditions. ow superficial gas velocities in this case greatly affect the capacity of the system, as can be seen in Figure 3.9. Cases IV, V, which are not shown, and case VIII provide equal liquid and gas flow rate in both CCs. Theoretically, the envelope for these cases should run parallel to the envelope for two parallel CCs. The experimental data validate this hypothesis. Due to the fact that these experiments were conducted under steady state conditions and the level inside the CCs was almost constant and high enough from the liquid outlet, no gas carry under was observed while conducting the experiments. It is also important to mention that the capacity of the compressor limited the amount of experimental data points that could be obtained. It was not possible to go beyond a total superficial gas velocity of 30 ft/s Manifold Operational Envelope for iquid Carry-Over As defined before, the liquid carry-over for the manifold is defined as the locus of all pairs of superficial liquid and superficial gas velocities that cause liquid carry-over in the horizontal pipe of either of the two upper gas legs of the manifold. To obtain these operational envelopes for CO, the same procedure utilized to determine the envelope of

37 28 the entire system was followed, and eight different envelopes were obtained for each of the different inlet flow configuration, as shown in Figure It can be seen that even though there are differences in the envelopes for the different configurations, the behavior of the manifold is fairly constant for superficial gas velocities ranging from 18 to 25 ft/s, and superficial liquid velocities ranging from 1.5 to 2.0 ft/s. Manifold Operational Envelope for iquid Carry-Over Vsl (ft/s) Case I Case II Case III Case IV Case V Case VI 0.5 Case VII Case VIII Equal Flow Vsg (ft/s) Figure Operational Envelope of the Distribution Manifold By comparison of Figures 3.9 and 3.10, the operational envelopes for liquid carryover of the manifold for all the different flow configurations always fall below the envelope of the entire system. For this reason, it is decided to use the envelope of the manifold as a design criterion of the system to avoid liquid carry-over in the compact separators. This will serve as a conservative approach for the design of the system. The

38 29 fact that the envelope of the manifold is chosen for the design and performance of the entire system, and the fact that some liquid carry-over in the manifold does not necessarily mean liquid carry-over in the downstream CCs, allows the designer to establish a safety factor permitting slugs or sudden increment of gas or liquid flow rates to enter the manifold without affecting the performance of the entire system iquid and as Split Ratios in Distribution Manifold The liquid and gas split ratios in the distribution manifold are two of the most important parameters to be determined experimentally. To obtain the liquid and gas split ratios in the manifold and how they are affected by variables such as the superficial gas velocity, the superficial liquid velocity and inlet well flow configurations, the same eight different cases, as presented in Figure 3.8, were analyzed. The procedure is described below: 1. Choose a flow configuration for the inlet wells, according to Figure 3.8, and adjust the rotameters in such a way that as total flow from the metering section is changed in the adjustment process, flow through the individual wells change in the same proportion, so that the desired flow configuration remains the same. 2. Choose operational points that do not give liquid carry-over in the CCs, starting from high gas flow rate and low liquid flow rate to low gas flow and high liquid flow rate. For each operational point selected, individual liquid and gas flow rates in both CCs were measured. As the total flow rate into

39 30 the system is known, the liquid and gas split ratios can be calculated for each separator. The liquid and gas split ratios are plotted as a function of the gas volume fraction (VF) inside the manifold. Two different plots are obtained, which are described in the following section. iquid Split Ratio: Figure 3.11 shows the liquid split ratio vs. the gas volume fraction. It can be seen that in flow configurations I, III and VI, the liquid split ratio is greatly affected by the VF. For these cases, as VF increases, the liquid split goes far from 50%, which is the desirable value. In cases IV, V and VIII, theoretically the liquid split ratio, as expected, is 50% because these flow configurations give equal gas and liquid flow rate along the distribution manifold. Cases II and VII show a little deviation with respect to 50% liquid split ratio value. iquid Split ( CC# 1 over Total Flow) vs. VF iquid Split Case I Case II Case III Case IV Case V Case VI Case VII Case VIII Equal Flow VF Figure iquid Split Ratio in the Distribution Manifold

40 31 According to Figure 3.11 and the explanation given, cases I, III and VI should be avoided. On the other hand, flow configurations similar to cases IV, V and VIII, which give liquid split very close to 50 %, should be pursued. In cases where most of the wells are liquid dominant or gas dominant, like cases II and VII, the liquid split ratio is acceptable because it is close to 50%, regardless of the VF. as Split Ratio: Figure 3.12 shows how the measured gas split ratio is related to the VF. As can be seen, the gas split ratio in flow configurations I, III and VI is greatly affected by the VF. For these cases, as VF decreases, the gas split ratio goes away from 50%, which as mentioned before, is the desirable value. It also can be seen that all other flow configurations, especially cases IV, V and VIII give a gas split ratio fairly close to 50%, regardless of the VF. Cases II and VII give a gas split ratio that has a little deviation from the preferred 50% as VF decreases. as Split ( CC# 1 over Total Flow) vs. VF as Split Case I 0.30 Case II Case III 0.20 Case IV Case V 0.10 Case VI Case VII Case VIII Equal Flow VF Figure as Split Ratio in the Distribution Manifold

41 32 Comparing Figure 3.11 with Figure 3.12, it can be observed that cases I, III and VI should be avoided while designing a distribution manifold, as for these cases the liquid and gas split ratios are very sensitive to the VF inside the manifold. In these cases, low VF values promote equal liquid split but also promote uneven gas split. Also cases IV, V and VIII are the desirable flow configurations because the liquid and gas split ratios are around 50%, regardless of VF. Even though in cases II and VII liquid and gas split ratios show a little deviation from 50%, these flow configurations are acceptable Resistance Coefficient of Manifold (K l ) An important factor, which affects the liquid flow rate through the lower outlet legs of the manifold, and in turn the capacity of the distribution manifold, is the resistance of the outlet liquid leg. A high resistance to liquid flow will mean an increase of the liquid level inside the manifold, resulting in lower capacity of the system. This is also due to the fact that as the liquid level inside the manifold increases, the chance of having liquid carry-over increases. To enable prediction of liquid flow rate in each lower leg of the manifold, an experiment was conducted to obtain an equation to relate the Resistance Coefficient (K l ) of this liquid outlet to the Reynolds Number in the liquid legs. The procedure followed in this experiment was to flow only liquid into the manifold, under even and uneven split, measure the liquid level close to each liquid outlet, and obtain the corresponding liquid flow rate through each leg from the CC outlet measurement.

42 33 Once these measurements were obtained, Bernoulli s equation was applied to the liquid flow in each leg to relate the potential energy available (expressed in terms of the level inside the manifold) to the velocity of the liquid in the liquid leg. Bernoulli s equation applied to any stream line on the liquid surface level in the manifold gives: where, is obtained, 2 2 Kl * V 1 * 1 V g h =. (3.5) 2 2 h is average liquid height in the manifold in the liquid outlet, in m. V1 is the velocity of the liquid in the liquid leg, in m/s. Kl is the resistance coefficient of the liquid leg. Manipulating Equation 3.5 to obtain an equation for K l, the following expression 2* g * h K l = 1 (3.6) 2 V 1 Reynolds number in the liquid leg is calculated using the following equation: where, ρ l * V1 * d Re = (3.7) µ ρl is the density of the liquid, in kg/m 3. V1 is the velocity of the liquid in the liquid leg, in m/s. d is the diameter of the liquid leg, in m. µ is the viscosity of the liquid, in cp.

43 34 Once Resistance Coefficient and Reynolds number are calculated, a plot is generated to relate these two variables, as given in Figure This plot shows that as Reynolds number increases, the Resistance Coefficient tends to approach a constant value, as discussed next. This experiment was conducted in absence of gas flow, for this reason, the effect of the gas or the effect of bubbles in this resistance coefficient was not determined. Further analysis and experimentation would be required to determine the resistance coefficient if the ratio of areas in the splitting tee is different than one and also if the gas flow is considered important. K l vs. Reynolds Kl ,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 Reynolds Number Figure Resistance Coefficient for iquid Outlet vs. Reynolds Number For low Reynolds numbers in the liquid leg, smaller than 30,000, the liquid flow through the outlet is basically by free drainage, i.e., the connection manifold-liquid leg is not totally covered with liquid. This phenomenon can be seen in sketch (a) of Figure

44 For Reynolds numbers bigger than 30,000, the liquid outlet of the distribution manifold is totally flooded and the resistance coefficient tends to be constant and have a value around 5. Sketch (b) of Figure 3.14 shows the level inside the manifold when the inlet is totally flooded. WE WE WE WE IQ IQ (a) (b) Figure Conditions for Determination of Resistance Coefficient Results obtained in this study for the Resistance Coefficient are different from the values found in the literature. Miller (1971) conducted some experiments for dividing flow in 12 and 8 inch-id tees, and reported a value around 2.0. Collier (1976) also conducted experiments for standard tees and reported values around 1.3, regardless of the diameter. Crane Corporation (1981) presents a Resistance Coefficient of 0.78 for a pipe entrance that better represents a tee. The difference between the values found in the literature and the experimental results obtained are due to the fact that the geometry and the way the liquid flow towards