Patent Document

TECHNICAL FIELD 
     This disclosure relates generally to process control systems and more specifically to slug mitigation by increasing available surge capacity. 
     BACKGROUND 
     Processing facilities, such as manufacturing plants, chemical plants and oil refineries, typically are managed using process control systems. Valves, pumps, motors, heating/cooling devices, and other industrial equipment typically perform actions needed to process materials in the processing facilities. Among other functions, the process control systems often control industrial operation in the processing facilities. 
     A very common problem encountered in many industries, such as the oil production and petrochemical industry, is compensating for variations in the flow rate of fluids (liquid and/or gases) coming into a particular processing unit. Such disturbances are usually common and ordinary events in the routine operation of the process. 
     As an example, in oil production processes, production from oil wells includes a mixture of oil, water and gas that flows from the wells to surface vessels, such as separators. The separation of phases can cause an undesirable operating condition when production flow becomes discontinuous with periods of large volume of liquid phase followed by periods of predominately gaseous phase. This phenomenon, referred to as “slugging,” causes the liquid flow into the separator to swing significantly. Slugging can lead to unstable operation of the process equipment downstream from the separators, which can shut down the production platform and result in a significant economic loss to the oil producing company. 
     Microprocessor-based proportional, integral and derivative (PID) controllers are commonly used for level control to reduce variations in the flow supplied to a downstream process. However, PID algorithms run by PID controllers often have two significant limitations. First, PID algorithms are typically unable to address non-linearities. Second, PID algorithms often cannot be used to specify high and low limits for liquid levels explicitly. Moreover, if the inlet flow has a large noise component, such as due to an upstream process that is noisy, control using a PID algorithm becomes increasingly ineffective. 
     SUMMARY 
     This disclosure provides a system and method for slug mitigation. 
     In a first embodiment, an apparatus includes a housing having a volume and an inlet. The inlet is configured to receive a fluid that includes a gas and a liquid. The apparatus also includes a baffle configured to partition the volume into a first portion and a second portion. The baffle extends from a base of the housing. The first portion is configured to receive the liquid and separate the liquid into a first part and a second part. The second portion is configured to receive the second part of the liquid from the first portion. The apparatus further includes a controller configured to regulate an amount of the second part of the liquid in the second portion such that a level of the second part of the liquid is higher than a height that the baffle extends from the base of the housing. 
     In a second embodiment, a method includes receiving a fluid from an upstream process at a separator. The separator includes a baffle that separates a first portion and a second portion. The method also includes separating the fluid into two or more parts and storing at least a portion of the first part of the fluid in the second portion of the separator. The method further includes maintaining a level of the first part of the fluid stored in the second portion in a range above the baffle. 
     In a third embodiment, a system includes a central processing unit and a computer readable medium electronically coupled to the central processing unit. The computer readable medium includes a control program that uses a vessel with a baffle that separates the vessel into at least two portions. The control program causes the central processing unit to regulate a level of a first part of a fluid stored in at least one portion of the vessel in a range above the baffle. 
     Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of this disclosure, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates an example system for processing a fluid through a separator according to this disclosure; 
         FIG. 2  illustrates an example vessel according to this disclosure; 
         FIG. 3  illustrates an example system configuration for processing a fluid through a separator according to this disclosure; 
         FIGS. 4A through 4D  illustrate an example separator according to this disclosure; and 
         FIG. 5  illustrates an example process for controlling surge capacity of a separator vessel according to this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 through 5 , discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the invention may be implemented in any type of suitably arranged device or system. Also, it will be understood that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some elements in the figures may be exaggerated relative to other elements to help improve the understanding of various embodiments described in this patent document. 
       FIG. 1  illustrates an example system  100  for processing a fluid through a separator according to this disclosure. The embodiment of the system  100  shown in  FIG. 1  is for illustration only. Other systems could be used without departing from the scope of this disclosure. 
     In this example, the system  100  includes one or more upstream processes  105  that provide an outlet flow of at least one fluid. The fluid includes a gaseous portion and a liquid portion. The liquid portion of the fluid may fluctuate significantly and therefore be “noisy.” The outlet flow from the upstream process  105  is coupled via one or more vessels  110  to an inlet flow of one or more downstream processes  115 . The downstream process  115  can be, for example, a separator vessel having equipment such as valves, pumps and motors. The downstream process  115  can also be a distillation tower or a furnace. 
     Each vessel  110  may be coupled to a non-linear level controller (NLLC)  120 . The NLLC  120  can be a non-linear level controller as described in U.S. patent application Ser. No. 11/859,432, which is hereby incorporated by reference in its entirety. In this example, the NLLC  120  includes a central processing unit (CPU)  125 , as well as associated memory. The memory can be any computer readable medium, for example, the memory can be any electronic, magnetic, electromagnetic, optical, electro-optical, electro-mechanical, and/or other physical device that can contain, store, communicate, propagate, or transmit a computer program, software, firmware, or data for use by the microprocessor or other computer-related system or method. The NLLC  120  stores or receives a plurality of inputs, such as a setpoint (SP), a high level (HL), a low level (LL), and tuning constants (T 1  and T 2 ), as well as current level (inventory) data from the vessel  110 . The NLLC  120  also executes a control algorithm and adjusts system flows, including a flow of the fluid to the downstream process  115  via a flow controller (FC)  130 , which adjusts the outlet flow from the vessel  110  together with a control valve  135 . 
       FIG. 2  illustrates an example vessel  110  according to this disclosure. The embodiment of the vessel  110  shown in  FIG. 2  is for illustration only. Other vessels could be used without departing from the scope of this disclosure. 
     In this example, the vessel  110  includes a liquid shown reaching a current level  205 . The vessel  110  is associated with several parameters such as a low level limit  210 , a setpoint  215 , and a high level limit  220 . The desired level is shown as the setpoint  215 . The operating capacity of the vessel  110  is specified in terms of the low limit  210  and the high level limit  220 , outside of which operation is not desirable. Low limit  210 , setpoint  215  and high level limit  220 , together with the current level  205 , define two differential parameters—a setpoint deviation (SD)  225  and a capacity deviation (CD)  230 . To keep the liquid level at the setpoint  215  would mean that the outlet flow follows fluctuations in the inlet flow. As shown in  FIG. 2 , the current level  205  is above the setpoint  215 . However, the current level  205  can also be at or below the setpoint  215 . 
     The CD  230  is a measure of discrepancy in the current level  205  and a prevailing limit, which is generally either the low limit  210  or the high limit  220 . Although  FIG. 2  shows the prevailing limit as being the high limit  220 , the prevailing limit can also be the low limit  210  such that the CD  230  is a measure of the discrepancy between the actual inventory (the current level  205 ) and the low limit  210 . The prevailing limit can be detected by the NLLC  120  based on, for instance, the sign of the time derivative of the liquid level in the vessel  110 . For example, if the level in the vessel  110  is increasing, the sign of the derivative is positive, and the prevailing limit can be the high limit  120 . Conversely, if the level is decreasing, the sign of the derivative is negative, and the prevailing limit can be the low limit  210 . The CD  230  is thus measured in terms of the volume (available or excess) from the current level  205  to generally reach the prevailing limit. 
     The SD  225  is a measure of discrepancy of the current level  205  from the setpoint  215 . The SD  225  can be measured in terms of the volume (available or excess) from the current level  205  to generally reach the setpoint  215 . 
     A first time (T 1 ) to reduce the CD  230  by a first specified percentage (such as 100%/exhaust) and a second time (T 2 ) to reduce the SD  225  by a second specified percentage (such as 100%/exhaust) are generally provided. For example, values for T 1  and T 2  can be derived from conventional empirical process tuning tests. In addition, the tuning constants (T 1  and T 2 ) can be derived from observations of plant operating data, vessel dimensions, the nature and magnitude of disturbances experienced based on actual plant operating data, and control objectives defined by engineers or other plant personnel. T 2  may generally be greater than T 1 . It may also be possible to determine T 1  and T 2  through modeling. In some embodiments, the specified percentages can both be 100%, T 1 =15 minutes, and T 2 =60 minutes. 
       FIG. 3  illustrates an example system configuration for processing a fluid through a separator according to this disclosure. The embodiment of the system  100  shown in  FIG. 3  is for illustration only. Other systems configurations could be used without departing from the scope of this disclosure. 
     In this example, the system  100  includes a separator  305  coupled to a compressor  310  and other downstream equipment  320 . The separator  305  receives, via an inlet, fluid from one or more upstream processes. The fluid can include a gaseous (G) portion and a liquid (L) portion. The separator  305  separates the fluid into a gaseous portion and a liquid portion. The gaseous portion can be delivered to the compressor (C)  310  coupled to a first outlet to the separator  305 . The liquid portion can be delivered via a second outlet of the separator  305  to additional process equipment  320  in the downstream process  115 . 
     In some embodiments, the separator  305  can receive fluid produced as part of an oil production process from a number of oil wells in the upstream process  105 . The fluid produced from the oil production process includes a mixture of oil, water, and gas. The separator  305  separates the oil and water in the liquid phase from the gas and provides the gas to the compressor  305 . The oil and water are drawn out of the separator  305  and delivered to the downstream equipment  320 . 
     The compressor  310  can be damaged if liquid-phase material is processed through the compressor  310 . Therefore, the separator  305  separates the liquid from the gas and provides the gas to the compressor  310  as the gas is separated from the liquid. However, conventional separators  305  cannot compensate for slugging where, for example, no gas or no liquid is received by the separator  305 . When no gas is received by the separator  305 , the separator  305  may deliver liquid, such as oil, water or both, to the compressor  310 . As a particular example, this can occur if all of the gas is drained from the separator  305 . This can damage the compressor  310 . 
     In addition, the separator  305  may be unable to deliver liquid to the downstream equipment  320  if only gas is received for a period of time. For example, the separator  305  may receive only gas for thirty seconds. During that period, the liquids may be fully drained from the separator  305 . Therefore, the separator  305  does not deliver either water or oil to the downstream equipment  320 , which may cause damage to one or more pieces of the downstream equipment  320 . 
       FIGS. 4A through 4D  illustrate an example separator  305  according to this disclosure. The embodiments of the separator  305  shown in  FIGS. 4A through 4D  are for illustration only. Other separators could be used without departing from the scope of this disclosure. 
     In this example, the separator  305  includes a vessel volume contained by a housing  405 . In some embodiments, the separator  305  includes a cylinder housing that is placed horizontally or vertically. The separator&#39;s vessel volume is partitioned into two parts by a baffle  410  positioned vertically and attached to a lower half of the housing  405 . An oil and water portion  415  forms on a first side  417  of the baffle  410 , and an oil-only portion  420  forms on a second side  422  of the baffle  410 . The oil and water portion  415  can be significantly larger than the oil-only portion  420 . For example, the size of the oil and water portion  415  can be ten times larger than the oil-only portion  420 . The larger oil and water portion  415  includes enough volume to enable the oil to separate from the water. 
     Production from the wells flows into the oil and water portion  415  via an inlet  425 . In the oil and water portion  415 , separation between gas and oil and water occurs. Gas flows into an overhead portion  430 . The oil and water settle into the oil and water portion  415 , where the oil and water separate such that the oil rises to the top and the water settles to the bottom of the oil and gas portion  415 . Since (i) the oil has a low density compared to water and (ii) the oil includes non-polar organic compounds (while water is a polar compound), the oil does not substantially mix with water and floats on the water&#39;s surface. Therefore, an immiscible liquid mixture of oil and water is received from the production of wells. 
     The gas is drawn out of the separator  305  via a first outlet  435 , while water is drawn out of the separator  305  via a second outlet  440 . The water is drawn out of the separator  305  at a rate such that the water level in the oil and water portion  415  remains below the height of the baffle  410 . As the water and oil separate in the oil and water portion  415 , the oil settles on top of the water. Therefore, the immiscible liquid mixture of oil and water separate into a light phase  418  and a heavy phase  419 . The heavy phase  419  settles to the bottom of the vessel volume, and the light phase  418  settles on top of the heavy phase  419 . The water is drawn out at a rate that enables at least a portion of the oil on the surface of the water to flow over the top of the baffle  410  into the oil-only portion  420 . The oil is drawn out of the separator via a third outlet  445 . 
     The separator  305  includes or is otherwise associated with an NLLC  450 . The NLLC  450  can include similar structure and functionality as the NLLC  120  and, as such, can include a central processing unit (CPU)  455  and associated memory. The NLLC  450  stores or receives inputs such as setpoint, high level, low level, tuning constants, and current level (inventory) data for oil in portions  415  and  420  of the separator  305 . The NLLC  450  executes a control algorithm and adjusts flow out of the outlet  445  to the downstream equipment via flow controller  460   c  and control valve  465   c . Other control valves  465   a - 465   b  and respective flow controllers  460   a - 460   b  adjust the outlet flows from the separator  305  under control of one or more additional controllers (not shown). 
     In conventional separator systems, oil is drawn at a rate such that the oil level is maintained at a fixed level, such as level L 2 , below the baffle height. The conventional separator systems are unable to maintain the oil level above the baffle&#39;s height because a mathematical discontinuity occurs when the oil level exceeds the baffle height. 
     In accordance with this disclosure, the NLLC  450  executes an optimal surge control process to control the oil level in the separator  305  such that the oil level can be controlled in a range above the baffle  410 . The NLLC  450  also addresses any non-linear behavior in the operation of horizontal and vertical separators. In particular, the NLLC  450  can maintain the oil at level L 1  (shown in  FIG. 4D ) instead of L 2  (shown in  FIG. 4C ). The NLLC  450  can maintain the oil at level L 1  by using a lower limit that is established above or below the baffle height. In addition, the NLLC  450  can use an established upper limit and setpoint. In some embodiments, the lower limit is at or above level L 1 . 
     Maintaining the oil level above the baffle  410  can increase or maximize the use of the available surge capacity in the separator  305  and thus provide an opportunity to stabilize the oil flow from the separator  305 . The available surge capacity can equal the volume of the oil-only portion  420  between the baffle&#39;s height and level L 2  plus the volume of the entire separator vessel between the level L 1  and the baffle&#39;s height. For example, when the separator  305  receives only gas for a period of time, such as for thirty seconds, the oil can continue to be drawn out of the separator at the same rate to “ride-out” the gas-only flow because of the increased surge capacity of the oil created by maintaining the oil at level L 1  instead of L 2 . 
     In some embodiments, the NLLC  450  calculates an unforced time (referred to as T*) to reduce the CD by the first specified percentage. For example, if the first specified percentage is 100%, T* is thus defined as the time to completely exhaust the CD corresponding to a volume of the fluid comprising liquid if no changes to the flows are made by the controller (e.g., the status quo). Thus, the conditions for calculating the unforced time can be based on the most recent measurement of the current level  205  that is stored in the memory associated with the NLLC  450  and/or the change in the current level  205  since the last execution of the NLLC process. 
       FIG. 5  illustrates an example process  500  for controlling surge capacity of a separator vessel according to this disclosure. In this particular example, the process  500  is configured to exhaust the majority of CD in T 1  minutes and/or exhaust the majority of SD in T 2  minutes. Non-linear control is based on the differential parameters CD and SD and the relative speed of the flow disturbance, which controls surge capacity for the separator  305 . 
     In block  505 , SD and CD as described above are calculated. In block  510 , T* can be calculated or otherwise provided, such as stored from an earlier computation. In block  515 , T* is then compared to a time constant T 1 . 
     If T*≦T 1 , the variation/disturbance in inlet flow is relatively fast. The NLLC  450  computes a first minimum change in the outlet flow (for outlet  445  from the separator  305 ) to reduce CD by the first specified percentage in a time T 1 . The minimum change is calculated and implemented as a flow change in block  520 . 
     If T*&gt;T 1 , the variation/disturbance in inlet flow is relatively slow. The NLLC  450  computes a second minimum change in the outlet flow (for outlet  445  from the separator  305 ) to reduce or exhaust SD by the second specified percentage in a time T 2 . The minimum change is calculated and implemented as a flow change in block  525 . 
     Accordingly, the NLLC  450  is configured to perform an optimal surge control process to maintain the oil at a level sufficient to provide a surge capacity. The NLLC  450  can determine the current “inventory” level of the oil and access a previous level stored in memory. If the current level is outside a limit and worsening, the NLLC  450  computes a change to the flow rate through the outlet  445  to keep the oil level from getting worse. If the unforced time to exhaust CD (T*) is less than a specified time (T 1 ), the NLLC  450  computes a capacity change. If the unforced time to exhaust CD (T*) is greater than the specified time (T 1 ), the NLLC  450  computes a setpoint change. The flow change can be implemented using a flow control parameter. The NLLC  450  can control the output flow setpoint of the flow controller  460   c  that is coupled to the control valve  465   c  to implement the first minimum flow change (when T*≦T 1 ) or the second minimum flow change (when T*&gt;T 1 ). 
     Although the figures above illustrate specific systems, structures, and processes, various changes may be made to the figures. For example, various components in the systems and structures can be combined, omitted, further subdivided, or moved according to particular needs. Also, while shown as a series of steps, various steps in  FIG. 5  could overlap, occur in parallel, or occur multiple times. 
     It may be advantageous to set forth definitions of certain words and phrases used throughout this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. 
     While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims.

Technology Category: 7