Abstract:
A system for separating components of a slurry is disclosed, the system including a housing; a basket for holding at least one shaker screen, the basket movably mounted in the housing; at least one vibrator coupled to the basket; a sump disposed below the basket to collect at least a portion of the slurry passing through the at least one shaker screen; a pressure differential device fluidly connected to the sump for developing a pressure differential across the at least one shaker screen; and a toggling device for toggling the pressure differential across the screen. A system including a degassing chamber fluidly connected to a sump and a pressure differential device, wherein the degassing chamber is disposed between the sump and the pressure differential device, and a fluid conduit fluidly connected to the degassing chamber for recovering a degassed fluid is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/827,567, filed Sep. 29, 2006, and U.S. Provisional Patent Application No. 60/827,542, filed Sep. 29, 2006, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments disclosed herein relate generally to shale shakers and screens for shale shakers. Specifically, embodiments disclosed herein relate to a shale shaker having pulse-vacuum assisted screening. Additionally, embodiments disclosed herein relate to methods and apparatus for removing entrained gases from a slurry. 
     2. Background Art 
     Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. The mud is mixed at the surface and pumped downhole through a bore of the drillstring to the drill bit where it exits through various nozzles and ports, lubricating and cooling the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore. 
     Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) and the density (or its inverse, specific gravity) of the fluid used. Various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture for the type and construction of the formation to be drilled. Increasing the amount of weighting agent solute dissolved in the mud base will generally create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use. 
     Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit stir up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud, and the cutting particulates must be removed before the mud can be recycled. 
     One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker.” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at the top of the shale shaker. As the drilling mud travels down the incline toward the lower end, the fluid falls through the perforations to a reservoir below thereby leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall off the lower end of the shaker table. The above described apparatus is illustrative of one type of shale shaker known to those of ordinary skill in the art. In alternate shale shakers, the top edge of the shaker may be relatively closer to the ground than the lower end. In such shale shakers, the angle of inclination may require the movement of particulates in a generally upward direction. In still other shale shakers, the table may not be angled, thus the vibrating action of the shaker alone may enable particle/fluid separation. Regardless, table inclination and/or design variations of existing shale shakers should not be considered a limitation of the present disclosure. 
     Preferably, the amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it may either return to service in the borehole immediately, be stored for measurement and evaluation, or pass through an additional piece of equipment (e.g., a drying shaker, a centrifuge, or a smaller sized shale shaker) to remove smaller cuttings and/or particulate matter. 
     Screens used with shale shakers are typically emplaced in a generally horizontal fashion on a generally horizontal bed or support within a basket in the shaker. The screens themselves may be flat or nearly flat, corrugated, depressed, or contain raised surfaces. The basket in which the screens are mounted may be inclined towards a discharge end of the shale shaker. The shale shaker imparts a rapidly reciprocating motion to the basket and hence the screens. Material from which particles are to be separated is poured onto a back end of the vibrating screen, flowing toward the discharge end of the basket. Large particles that are unable to move through the screen remain on top of the screen and move toward the discharge end of the basket where they are collected. The smaller particles and fluid flow through the screen and collect in a bed, receptacle, sump, or pan beneath the screen. 
     In some shale shakers a fine screen cloth is used with the vibrating screen. The screen may have two or more overlaying layers of screen cloth or mesh. Layers of cloth or mesh may be bonded together and placed over a support, supports, or a perforated or apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated by vibratory equipment to create a flow of trapped solids on top surfaces of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed. 
     While there are numerous styles and sizes of filter screens, they generally follow similar design. Typically, filter screens include a perforated plate base upon which a wire mesh, or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids therethrough, while the wire mesh overlay defines the largest solid particle capable of passing therethrough. While many perforated plate bases are generally flat or slightly curved in shape, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. In theory, the pyramid-shaped channels provide additional surface area for the fluid-solid separation process to take place, and act to guide solids along their length toward the end of the shale shaker from where they are disposed. 
     The separation of drilling fluid and other solids from drill cuttings using a screen shaker is often incomplete, resulting in wet drill cuttings. As described above, the drilling mud is introduced to the top of the screen and allowed to flow downward through the screen by gravity alone. Often, additional equipment, such as additional screen separators, hydrocyclones, dryers, drying shakers, centrifuges, hydrocyclone shakers, thermal desorption systems, and other equipment, are used to further dry the cuttings and recover drilling fluid. For example, cuttings from a shale shaker may fall onto a rotary vacuum dryer, where the cuttings travel on a circumferentially rotating screen. Air may be used to strip drilling fluid off the cuttings and into the screen, such as by pulling a vacuum from the interior of the rotating screen (for example, the ROTAVAC™ Rotary Vacuum Dryer fluid recovery and cuttings drying system, available from Halliburton). 
     It is desired to improve the rate and efficiency at which shakers remove liquid from cuttings or other solids. To enhance the gravity-driven separation as described above, it is known that increasing the head on the shaker can increase the throughput of fluids through the screen. Increasing the pressure differential through the screen will likewise increase the fluid capacity of the shaker. 
     One example of a shaker with increased pressure differential is disclosed by Hensley et al. in U.S. Patent Application Publication No. 20050183994A1. Hensley et al. disclose an integrated, transportable cutting treatment system, where a pressure differential is developed across the screens to increase the flow rate of drilling mud through the screens. Hensley et al. use an air pump to develop a vacuum beneath the screens to draw mud through the screens. However, applying a continuous vacuum beneath a screen to draw fluid through the screen may result in solids sticking to the screen, hindering the conveyance of solids off the end of the shaker as needed, thereby preventing fluids from being filtered through the screen. 
     There exists a continuing desire for shakers having increased fluid capacity, increased fluid flow-through rates across the screens, and/or improved fluid removal efficiencies. Accordingly, there exists a need for a shaker with increased pressure differential. Preferably, the means used to increase the pressure differential do not substantially hinder the flow of solids across the screen deck. Additionally, there exists a need for a shaker for removing entrained gases from the recovered drilling fluid. 
     SUMMARY 
     A system for separating components of a slurry is disclosed, the system including a housing; a basket for holding at least one shaker screen, the basket movably mounted in the housing; at least one vibrator coupled to the basket; a sump disposed below the basket to collect at least a portion of the slurry passing through the at least one shaker screen; a pressure differential device fluidly connected to the sump for developing a pressure differential across the at least one shaker screen; and a toggling device for toggling the pressure differential across the screen. In some embodiments, the vapor may be degassed within the sump. In other embodiments, the system may include a degassing chamber fluidly connected to the sump and the pressure differential device, wherein the degassing chamber is disposed between the sump and the pressure differential device; and a fluid conduit fluidly connected to the degassing chamber for recovering a degassed fluid. 
     In another aspect, embodiments disclosed herein relate to a method for separating components of a slurry, the method including providing a slurry to a top of a screen and toggling a pressure differential across the screen from static to a vacuum below the screen. In some embodiments, the toggling may include generating at least a partial vacuum below the screen by causing a flow of vapor from a vapor space below the screen, and intermittently interrupting the vacuum by disrupting the flow of vapor. In other embodiments, the partial vacuum may be generated by causing a flow of fluid from a space below the screen, the fluid may be degassed to recover a vapor and a degassed liquid, and the toggling may be performed by intermittently interrupting the vacuum by disrupting the flow of recovered vapor. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a vibratory screen separator useful in embodiments disclosed herein. 
         FIG. 2  is a cross-sectional view of the screen separator of  FIG. 1 . 
         FIG. 3  is a side view of a vibratory screen separator useful in embodiments disclosed herein, where the separator may be fluidly attached to a vacuum system. 
         FIG. 4  is a perspective view of a vibratory screen separator frame and sump useful in embodiments disclosed herein. 
         FIG. 5  is a simplified flow diagram for embodiments of a system for generating a pressure differential across a screen and degassing a fluid according to embodiments disclosed herein. 
         FIG. 6  is a simplified flow diagram for embodiments of a system for generating a pressure differential across a screen and degassing a fluid according to embodiments disclosed herein. 
         FIG. 7  is a perspective view of a vibratory screen separator having a vacuum fume extraction device in accordance with embodiments disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, embodiments disclosed herein relate to a method for separating components of a slurry. As used herein, a slurry refers to a mixture of drilling fluid and drill cuttings. A slurry may be separated using a screen separator having a pressure differential across the screen. In other aspects, embodiments disclosed herein relate to a system for separating components of a slurry. The system may include, in some embodiments, a vibratory screen separator and a pressure differential device or a vacuum generating device. The pressure differential device may additionally provide a driving force to degas the recovered drilling fluid. 
       FIGS. 1 and 2  illustrate one embodiment of a vibratory screen separator. The separator  5  includes a base  10  having four legs  12  and supporting members  14 . Mounted on the four legs  12  are resilient mounts  16 . Each mount  16  includes a spring  18 , a base  20  on each leg  12  and a socket  22  on the separator to receive each spring  18 . Positioned on the base  10  by the resilient mounts  16  is a separator frame or basket  24 . The basket  24 , includes sidewalls  26  and  28  and a back wall  30 . The front side, opposite to the back wall  30 , may be left open. The basket  24  is of sufficient structure to withstand the vibrational loads imposed on the basket  24  in operation. Extending across the interior of the basket  24  between the sidewalls  26  and  28  is a structural tube  32  which may be positioned at roughly the center of mass for further structural strength. 
     Located about the sidewalls  26  and  28  and the back wall  30  is a channel  34 . Located above the channel  34  on the sidewalls  26  and  28  are stops  38  and  40 . The stops cooperate with the channel  34  through its extent along the sidewalls  26  and  28  to form a screen mounting in a first plane. A screen  42  having a screen frame  44  and screen cloth  46  is illustrated positioned in the screen mounting. Resilient members  48  are positioned on the underside of the stops  38  and  40  to help locate, seat and seal the screen frame  44 . Obviously, multiple screens  42  may be employed in any one separator. 
     A sump  50  is located below the screen mounting to receive material passed through the screen  42 . An inlet  52  is positioned at the back wall  30  above the screen mounting. An outlet  54  for material passed through the screen  42  receives material from the sump  50  for discharge. Material not passing through the screen  42  is discharged off the end of the screen  42  and suitably collected. The flow across the screen plane from the inlet  52  toward the outlet  54  defines a linear direction of material travel. Attached to the sides of the basket  24  and specifically to each sidewall  26  and  28  are two rotary eccentric vibrators  56  and  58 . 
     As illustrated in  FIGS. 1 and 2 , sump  50  may be integrally connected to basket  24 . Thus, sump  50  may be referred to as a vibrating sump. In other embodiments, sump  50  may be separate from basket  24 , a stationary sump. 
     A pressure differential device (not shown) may be provided to create a pressure differential between the vapor space above screen  42  and the vapor space between screen  42  and sump  50 . In some embodiments, the pressure differential device may be located internal to sump  50 , such as an air pump (not shown). In other embodiments, the pressure differential device may be located external to sump  50 , such as a vacuum system (not shown). Whether internal or external to sump  50 , the pressure differential device may cause vapor to flow from the vapor space between screen  42  and sump  50  to a point external to sump  50 , such as through outlet  54  or other conduits forming an outlet from sump  50 . 
     The pressure differential device may include, in some embodiments, pumps, blowers, aspirators, ejectors, and the like, and combinations thereof. In various embodiments, the pressure differential may be created by one or more of a positive displacement pump, a momentum transfer pump, or an entrapment pump. 
     Pumps useful in creating the pressure differential or vacuum in some embodiments include reciprocating pumps, centrifugal pumps, vacuum pumps, pneumatic pumps, electric pumps, air pumps, piston pumps, rotary piston pumps, rotary vane pumps, screw pumps, scroll pumps, liquid ring pumps, external vane pumps, Wankel pumps, Toepler pumps, and Venturi vacuum pumps, among others. Blowers useful in creating the pressure differential may include booster pumps, a rotary lobe blower (such as a ROOTS™ blower), and vacuum blowers. Useful ejectors and aspirators may include steam ejectors, water aspirators, or ejectors and aspirators utilizing other motive fluids. In some embodiments, drilling fluid is used as the motive fluid for an ejector or an aspirator. 
     In some embodiments, the pressure differential may be pulsed, toggled, or intermittently interrupted. Toggling or pulsing of the pressure differential, as used herein, refers to the changing of the pressure differential from static (a zero pressure differential across the screen) to at least a partial vacuum below the screen. In some embodiments, the pressure differential may be toggled from static to at least a partial vacuum. In other embodiments, the pressure differential across a screen may be toggled or pulsed from static to a full vacuum below the screen. In some embodiments, the pressure differential may be toggled from static to a pressure differential in the range from about −0.1 to about −1.0 bar, as given by a quantity defined as a pressure below the screen minus a pressure above the screen. In other embodiments, the pressure differential may be toggled from static to a pressure differential in the range from about −0.2 to about −0.7 bar, as given by a quantity defined as a pressure below the screen minus a pressure above the screen; from static to a pressure differential in the range from about −0.2 to about −0.7 bar in other embodiments; and from about −0.3 bar to about −0.6 bar in yet other embodiments. By toggling the pressure between vacuum and static, conveyance of solids across the screen may proceed unhindered, thereby avoiding solids accumulating or sticking on the screen, and thus not preventing fluid flow through the screen. 
     Pulsing or toggling the pressure differential between static and vacuum below the screen, in some embodiments, may be effectuated by a valve disposed between the pressure differential device (pumps, ejectors, etc., as described above) and the screen. Manipulating the valve by opening and/or closing the valve, at least partially, may disrupt the flow of vapor from the sump, thereby affecting the pressure differential. In other embodiments, the toggling device, such as a valve, may be disposed between a vacuum generating device or system and the sump located under one or more of the screens. 
     Valves useful for toggling the pressure differential may include rotary valves, ball valves, globe valves, needle valves, butterfly valves, gate valves, plug valves, diaphragm valves, and piston valves, among others. The valves may be manually operated in some embodiments, or may be remotely actuated valves in other embodiments. 
     In some embodiments of the pulsed-vacuum assisted screening device disclosed herein, the separator may include two or more screens. One or more sumps may be located under the screens such that a pressure differential may be provided across less than all of the two or more shaker screens. In other embodiments, the same or different pressure differentials may be provided across zoned shaker screens. 
       FIG. 3  illustrates one embodiment of a pulsed-vacuum assisted screening device having separate pressure zones, where like numerals represent like parts. Separator  60  may include two or more screens (not shown), correspondingly located above two or more sumps  50  (i.e.,  50 A and  50 B as illustrated). For example, where separator  60  has four screens in series, sump  50 A may be located proximate inlet  52  under the first two screens. Sump  50 B may be located proximate outlet  54 B, under the last two screens (where first and last corresponds to the direction of flow from inlet  52  to outlet  54 B). Sump  50 A may thus create an independent zone from sump  50 B, allowing for operations of the two zones at the same or different pressure differentials. One or more devices may be provided to create a pressure differential across either or both sets of screens. The pressure differential across the screens in either zone may be manipulated to provide additional capacity or to enhance the liquid recovery, resulting in a dryer cutting fraction. 
     To maintain separate pressure differentials, sumps  50 A and  50 B may not be in fluid communication. As such, outlet  54 A may be provided to discharge material passing through the first two screens into sump  50 A. 
     As described above, one or more pressure differential devices may be provided to create a pressure differential between the vapor space above the screens and the vapor space between the screens and sumps  50 A,  50 B. In some embodiments, the pressure differential devices may be located internal to sumps  50 A,  50 B. In other embodiments, the pressure differential devices may be located external to sumps  50 A,  50 B. Whether internal or external to sumps  50 A,  50 B, the pressure differential devices may cause vapor to flow from the vapor space between the screen and sump  50 A,  50 B to a point external to each sump  50 A,  50 B, such as through outlets  54 A,  54 B, or other conduits forming an outlet from sumps  50 A,  50 B. 
     For example, outlet  54 A may be used as a liquid outlet, discharging the drilling fluid and other solids passing through screens. An outlet  56 A may be provided to convey vapor from sump  50 A to create the desired pressure differential. Outlet  56 A, in some embodiments, may be connected to a lobe pump  58  or to one or more vacuum generating devices as described above. The vapor discharge from vacuum generating device  58  may then be vented or further processed, such as through a vapor recovery or incineration system  59 . 
       FIG. 4  illustrates another embodiment of the pulsed-vacuum assisted screening device. The separator may include a base, legs, supporting members, and other component parts as previously illustrated for mounting separator frame  74 . Separator frame  74  may include sidewalls  76 ,  78  and a back wall  80 . The front side  81 , opposite to the back wall  80 , may be left open. The frame  74  may be of sufficient structure to withstand the vibrational loads imposed on the frame  74 . Extending across the interior of the frame  74  between the sidewalls  76 ,  78  may be a structural tube  82  which may be positioned at roughly the center of mass, for further structural strength. 
     Located about the sidewalls  76  and  78  and the back wall  80  is a channel (not shown). Located above the channel on the sidewalls  76  and  78  are stops  88  and  90 . The stops cooperate with the channel (not shown) through its extent along the sidewalls  76  and  78  to form a screen mounting in a first plane. A screen  92  is illustrated positioned in the screen mounting. Multiple screens  92  may be employed in any one separator. 
     One or more shaker screens  92  may be installed in, or secured to, the shale shaker frame  74  with a wedge block  94 . The screen  92  is placed on a support rail (not shown) and positioned underneath a stationary wedge guide  88  (stop  88 ). The wedge block  90  (stop  90 ) is then pounded into position so as to secure the screens  92  to frame  74 . One of ordinary skill in the art will appreciate that the operator often chooses to use a combination of a hammer and a suitable piece of wood in contact with the wedge block  90  to deliver sufficient force to fully tighten the wedge block  90 . As shown in  FIG. 4 , the wedge block  90  may also include a hammer surface  94  to aid in installation (as by pounding on surface  94   a ) and removal (as by pounding on surface  94   b ). 
     An inlet (not shown) may be located proximate back wall  80 . The solids may then travel on top of the screens toward front side  81 . As illustrated, the separator has four screens  92 . The drilling mud may be deposited on the first screen  92 A. 
     A sump  96  may be provided under the first two screens  92 A,  92 B. As illustrated, sump  96  may be integrally formed with frame  74 . One or more pressure differential devices, as described above, may be provided to generate a pressure differential across screens  92 A,  92 B. An outlet  98  may be provided to convey vapor from sump  96  to create the desired pressure differential. 
     Referring now to  FIG. 5 , a simplified flow diagram for embodiments of a system for generating a pressure differential across a screen and degassing a fluid, according to embodiments disclosed herein, is illustrated. A shaker  100  may include a basket  102 , shaker screen  104 , and sump  106 , as described above. A drilling fluid  108  to be separated, such as a mixture of drilling mud and drill cuttings, may be fed to inlet end A of the shaker  100 . Drill cuttings separated from drilling fluid  108  may be recovered at outlet end B. The drilling mud  110  separated from the drill cuttings may be collected in sump  106 . 
     To generate the desired intermittent pressure differential across screen  104 , the vapor space  112  of sump  106  may be fluidly connected via flow line  113  to a valve  114  and a pressure differential device  116 , as described above. To prevent liquids from entering flow line  113  and pressure differential device  116 , vacuum system inlet  118  may be disposed vertically downward, may include a cover  120 , such as to direct fluid away from inlet  118 , or may include other safety devices to prevent fluid from entering the vapor system. 
     Vapors recovered via pressure differential device  116  may be flared, vented, or recovered via flow line  122 . Fluids  110  may be recovered from sump  106  via flow line  124 , and in some embodiments may be directed to a mud tank for further processing and/or recycled to the mud system. 
     In the embodiment illustrated in  FIG. 5 , the fluid  110  collecting in sump  106  during the separations may be degassed or partially degassed by the vacuum or partial vacuum generated by pressure differential device  116 . Operation of pressure differential device  116  results in at least a partial vacuum in sump  106 , and may provide a driving force for gases that may be dissolved or entrained in the fluid  110  to be separated therefrom. 
     If necessary, a vent  126  may be provided to aid in pressure control of sump  106  or to provide means to avoid under-pressure of sump  106 , where vent  126  may include pressure relief valves and other devices known in the art to provide flow in limited circumstances. 
     Referring now to  FIG. 6 , a simplified flow diagram for embodiments of a system for generating a pressure differential across a screen and degassing the recovered fluid, according to embodiments disclosed herein, is illustrated. A shaker  200  may include a basket  202 , shaker screen  204 , and sump  206 , as described above. A drilling fluid  208  to be separated, such as a mixture of drilling mud and drill cuttings, may be fed to inlet end A of the shaker  200 . Drill cuttings separated from drilling fluid  208  may be recovered at outlet end B. The drilling mud  210  separated from the drill cuttings may be collected in sump  206 . 
     To generate the desired intermittent pressure differential across screen  204 , sump  206  may be fluidly connected via flow line  213  to a degassing chamber  212 , valve  214 , and a pressure differential device  216 , as described above. Generation of the intermittent pressure differential across screen  204  results in both liquids and vapors being pulled from sump  206  to degassing chamber  212 . The vapors collecting in degassing chamber  212  may be recovered via flow line  217 , and may be flared, vented, or otherwise recovered via flow line  222 . Fluids  210  collecting in degassing chamber  212  may be recovered via flow line  224 , and in some embodiments may be directed to a mud tank for further processing and/or recycle to the mud system. If necessary, a vent  226  may be provided to aid in pressure control of degassing chamber  212 , where vent  226  may include pressure relief valves and other devices known in the art to provide flow in limited circumstances. 
     The fluid  210  collecting in degassing chamber  212  during the separation process may be degassed or partially degassed by the vacuum or partial vacuum generated by the pressure differential device  216 . Operation of pressure differential device  216  results in at least a partial vacuum in degassing chamber  212 , and may provide a driving force for gases that may be dissolved or entrained to be separated from the fluid  210 . Such degassing that may occur in embodiments described herein may allow for a simplified mud tank system, where vents and other degassing equipment may not be necessary. 
     As described above, shaker systems described herein may include a pressure differential device or vacuum generating device to generate an intermittent pressure differential across a shaker screen. The vacuum generated by the pressure differential device may provide an additional driving force for separating fluids from drill cuttings, and may additionally remove vapors and entrained gases from the filtered drilling fluid. 
     Referring now to  FIG. 7 , a perspective view of a vibratory separator  500  in accordance with an embodiment of the present disclosure is shown. In this embodiment, vibratory separator  500  includes a housing  502 , a drilling fluid inlet end  504 , an outlet end  506 , and a plurality shaker screens  508 . In the embodiment shown, the plurality of shaker screens  508  are assembled in a multi-tier configuration. By vertically stacking multiple shaker screens  508 , the footprint of vibratory separator  500  is decreased, thereby providing equivalent separating potential while requiring less space. In vibratory separators  500  using vertically stacked shaker screens  508 , the size of the apertures in the screens may be varied according to each tier. As drilling fluid begins to flow from a top tier of vibratory separator  500 , the screen assembly apertures may be substantially greater in size than the apertures of lower screen assemblies. To prevent drilling fluid from falling on lower disposed shaker screens assemblies  508 , a series of flowback pans  522  may be located under shaker screens  508 . Flowback pans  522  may be directed to deposit drilling fluid into a sump  510 , thereby allowing drilling fluid to be substantially cleaner at each level of processing. 
     In this embodiment, vibratory separator  500  also includes a fume hood/outlet  516  connected to housing  502 . In one embodiment, the fume hood/outlet  516  may include a vacuum system that extracts vapors into the vent hood/outlet  516 . As drilling fluid enters vibratory shaker  500  through inlet end  504 , the drilling fluid falls onto shaker screen  508  and is conveyed from inlet end  504  to outlet end  506  using vibratory motion and pulse-assisted screening devices as described above. As the drilling fluid is conveyed, vapors, including potentially hazardous gases, for example, hydrogen sulfide (“H 2 S”), entrained in the drilling fluid may be present. 
     In this embodiment, fume hood/outlet  516  is configured to extract vapors from the drilling fluid as it flows across the shaker screens  508 . As vapors and fumes are released in a generally upward direction from the drilling fluid, fume hood/outlet  516  may pull the vapors and fumes inward, thereby trapping the potentially hazardous fumes and/or vapors. Those having ordinary skill in the art will appreciate that by using a fume hood, potentially noxious odors/hazardous conditions may be avoided by directing the flow of air into the hood. Once directed into the hood, a number of subsequent steps may be performed to further treat or vent the trapped gases. 
     One of ordinary skill in the art will appreciate that fume hood/outlet  516  may be turned on during any step of the separation process including during normal separation, during cleaning, or substantially continuously. Thus, embodiments including fume hood/outlets  516  may provide for a vibratory separator  500  that is substantially enclosed, thereby preventing the escape of hazardous materials and/or vapors into the drilling work space. 
     In one embodiment, vibratory separator  500  may further include one or more pressure differential devices (not shown) to create a pressure differential between a vapor space  512  above and between the screens  508  and sump  510 . In some embodiments, the pressure differential devices (not shown) may be located internal to sump  510 . In other embodiments, the pressure differential devices (not shown) may be located external to sump  510 . Whether internal or external to sump  510 , the pressure differential devices (not shown) may cause vapor to flow from the vapor space between the screens  508  and sump  510  to a point external to sump  510 , such as through outlets  54 A,  54 B (shown in  FIG. 3 ). Fume hood/outlet  516  may be in fluid connection with outlets  54 A,  54 B to pull vapors that flow through such outlets  54 A,  54 B, thereby trapping the potentially hazardous fumes and/or vapors. Vapors and fumes trapped in fume hood/outlet  516  may be treated and/or safely removed from vibratory shaker  500  thereafter. 
     In other embodiments, vibratory separator  500  may include one or more pressure differential devices (not shown) to create a pressure differential between a vapor space  512  above the screens  508  and flowback pans  522 . In some embodiments, the pressure differential devices (not shown) may be located internal to flowback pans  522 . In other embodiments, the pressure differential devices (not shown) may be located external to flowback pans  522 . Whether internal or external to flowback pans  522 , the pressure differential devices (not shown) may cause vapor to flow from the vapor space between the screens  508  and flowback pans  522  to a point external to flowback pans  522 , such as through outlets  54 A,  54 B (shown in  FIG. 3 ). Fume hood/outlet  516  may be in fluid connection with outlets  54 A,  54 B to pull vapors that flow through such outlets  54 A,  54 B, thereby trapping the potentially hazardous flames and/or vapors. Vapors and flames trapped in fume hood/outlet  516  may be treated and/or safely removed from vibratory shaker  500  thereafter. 
     In one embodiment, after the drilling fluid passes through shaker screens  508 , the drilling fluid may be directed to a containment area where the drilling fluid may be degassed to remove remaining entrained gases. Degassing the drilling fluid may be performed by any method known in the art. For example, mechanical degassers and aeration devices may used, as disclosed in co-pending Application Nos. 60/776,331 and 60/776,372, assigned to the assignee of the present application, and incorporated herein by reference in their entireties. A mechanical degasser may exert a centrifugal force on the drilling fluid. The centrifugal force of the mechanical degasser multiplies the force acting on the entrained gas (e.g., H 2 S) to increase buoyancy of the gas, thereby releasing entrained gas from the drilling fluid. The increase in buoyancy of the gas accelerates the bubble-rise velocity of the gas velocity, and as the gas bubbles rise toward the surface, they escape the drilling fluid. One of ordinary skill in the art will appreciate that any device known in the art that exerts a centrifugal force on the fluid, thereby reducing the amount of entrained or dissolved gases in the process fluid, may be used in place of a mechanical degasser. 
     Advantageously, embodiments disclosed herein may provide shakers having increased fluid capacity, increased fluid flow-through rates across the screens, and/or improved fluid removal efficiencies. Also, advantageously, embodiments disclosed herein may provide shakers with reduced hazardous vapors in vapor spaces. Finally, embodiments disclosed herein may provide shakers that more efficiently separate entrained gases from drilling fluids. 
     While the present disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure as described herein. Accordingly, the scope of the present disclosure should be limited only by the attached claims.