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FIELD OF INVENTION 
     The present invention relates in general to hydraulic fracturing, and in particular to systems and methods for controlling silica dust during the handling of frac sand. 
     BACKGROUND OF INVENTION 
     Hydraulic fracturing (“fracing”) is a well known technique for releasing oil and natural gas from underground reservoirs within rock formations having a limited permeability. For example, fracing is often used to release oil and natural gas, such as natural gas or oil, from shale formations. 
     Fracing is a well completion technique performed after the drilling of the wellbore, which in the case of releasing natural gas from shale, is commonly a horizontal wellbore, although occasionally the wellbore is vertical. Fracing fluid, which is primarily water and chemicals that form a viscous gel, is pumped into the well to create fractures within the surrounding rock. The viscous gel carries a “proppant” into the fractures, such that when the pumping stops, the fractures remain substantially open and allow the oil and natural gas to escape into the wellbore. 
     One typical proppant is “frac sand.” Frac sand is normally high purity silica sand with grains having a size and shape capable of resisting the crushing forces applied during the closing of the fractures when the hydraulic force provided by the pumping is removed. However, given that frac sand contains a high proportion of silica, the loading, transportation, and unloading of frac sand presents significant safety challenges. 
     The United States Occupational Safety and Health Administration (“OSHA”) lists silica as a carcinogen. In particular, the exposure and inhalation of silica dust has been linked to silicosis, which is an irreversible lung disorder characterized by inflammation and scarring of the upper lobes of the lungs. The best, and perhaps only way, to reduce or eliminate the threat of silicosis is to carefully control worker exposure to silica dust. 
     OSHA lists a number of different ways to limit worker exposure to silica dust, including limiting worker time at a worksite, limiting the number of workers at a worksite, watering roads and other worksite areas, enclosing points where silica dust is released, and requiring workers to wear respirators. These techniques do not, at least on their own, provide a complete solution to the problem of controlling silica dust. Furthermore, these existing techniques, while commendable, are nonetheless burdensome, time-consuming, inefficient, and impractical. 
     SUMMARY OF INVENTION 
     According to one representative embodiment of the principles of the present invention, a system is disclosed for controlling silica dust generated during the transfer of frac sand from a storage container through a conveyor system and includes a system of conduits having a plurality of inlets for collecting silica dust generated at selected points along the conveyor system. An air system pneumatically coupled to the system of conduits generates a negative pressure at each of the inlets to induce the collection of silica dust at the selected points along the conveyor, including container access ports, belt-to-belt drops, and belt-to-blender drops. 
     The present inventive principles advantageously provide for efficient and flexible systems and methods for collecting the silica dust generated during the offload of frac sand from one or more trailers or other storage facility at a fracing worksite. In particular, silica dust may be collected, as needed, at the base of the conveyor integral to each trailer (“trailer conveyor”), the point of discharge from each trailer conveyor to an associated portable conveyor system, at points along the portable conveyor system, and from within the trailer itself. The application of these principles improves the efficiency and flexibility of the frac sand offloading process by allowing increased worker time at the worksite and/or for more workers to be present at the worksite at one time, reducing the need for watering of worksite areas and the enclosure of points where silica dust is released, reducing the need for respirator wear, and decreasing the amount of silica dust intake by the engines of nearby vehicles and equipment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective diagram of a representative frac sand transportation and unloading system including a frac sand silica dust control system according to a preferred embodiment of the principles of the present invention; 
         FIG. 2  is a plan view diagram of the frac sand transportation and unloading system of  FIG. 1 , which emphasizes the airflow paths through the frac sand silica dust control system; 
         FIG. 3  is a plan view diagram of the frac sand transportation and unloading system of  FIG. 1 , which generally indicates the locations of particular structures of the frac sand silica dust control subsystem shown in more detail in  FIGS. 4-6 ; 
         FIG. 4A  is a diagram showing in further detail the pneumatic connections between the inlets of the silica dust control unit and the manifolds of  FIG. 1 ; 
         FIG. 4B  is a diagram showing in further detail the direct airflow path between the silica dust control unit and the silica dust control conduit subsystem servicing one selected trailer of  FIG. 1 ; 
         FIG. 4C  is a diagram showing in further detail the pneumatic connection between a selected manifold and the silica dust control conduit subsystem serving another selected trailer of  FIG. 1 ; 
         FIG. 4D  is a diagram showing in further detail the pneumatic connections between a selected manifold and the silica dust capture hose controlling silica dust generated during the operation of a corresponding trailer discharge conveyor shown in  FIG. 1 ; 
         FIG. 4E  is a diagram showing in further detail the pneumatic connections between a selected manifold and the silica dust capture hoses controlling silica dust generated by the system discharge conveyor of  FIG. 1 ; 
         FIG. 5A  is a diagram showing in further detail a selected silica dust capture hose controlling silica dust generated by the discharge of frac sand from the tank of representative trailer to the base of the corresponding trailer discharge conveyor shown in  FIG. 1 ; 
         FIG. 5B  is a diagram showing in further detail a selected silica dust capture hose controlling silica dust generated by the discharge of frac sand from the outlet of a corresponding representative trailer conveyor to the lateral transfer conveyor section of  FIG. 1 ; 
         FIG. 5C  is a diagram showing the hoses controlling silica dust generated during the movement of sand by the upwardly angled conveyor section of  FIG. 1  to a point above the bin of the blender of  FIG. 1 , along with the silica dust capture hose controlling silica dust generated during the discharge of sand into the blender bin from the conveyor section spout; 
         FIG. 6A  is a diagram showing in further detail the pneumatic connections of the silica dust control conduit subsystem of a representative one of the trailers of  FIG. 1 ; 
         FIG. 6B  is a diagram showing in further detail one of the T-fittings interconnecting the air conduits of the silica dust control conduit subsystem shown in  FIG. 6A ; 
         FIG. 6C  is a diagram showing a one of the end fittings terminating the air conduits of the silica dust control conduit subsystem shown in  FIG. 6A ; 
         FIG. 6D  is a diagram showing the four-way fitting interconnecting the air conduits of the silica dust control subsystem of one particular trailer with the silica dust control unit, as shown in  FIG. 4B ; 
         FIG. 7A  is a diagram showing an alternate embodiment of the principles of the present invention in which a cover is provided over portions of the representative frac sand transportation and unloading system of  FIG. 1  for containing silica dust generated during movement of sand through the system; 
         FIG. 7B  is a conceptual diagram providing a first detailed view of a representative embodiment of the cover shown in  FIG. 7A ; and 
         FIG. 7C  is a conceptual diagram providing a second detailed view of the representative embodiment of the cover shown in  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The principles of the present invention and their advantages are best understood by referring to the illustrated embodiment depicted in  FIGS. 1-7  of the drawings, in which like numbers designate like parts. 
       FIG. 1  is a diagram of an exemplary frac sand transportation, storage, and unloading system  100  including a frac sand silica dust control system according to a preferred embodiment of the principles of the present invention. System  100  is also shown in the plan views of  FIGS. 2 and 3 , with  FIG. 2  emphasizing the air flow paths of the silica dust control system and  FIG. 3  generally showing the locations of particular features of the silica dust control system shown in further detail in  FIGS. 4-6 . 
     Generally, system  100  is assembled at a hydraulic fracturing worksite and is used to offload frac sand transported to the worksite from a frac sand supplier via trailers and offloaded into a blender. The blender mixes the sand with the water and chemicals to form the fracing fluid. Given the significantly large amounts of frac sand that are typically required during typical hydraulic fracturing operations, a substantial amount of potentially hazardous silica dust is commonly generated during conventional trailer offloading operations. The principles of the present invention advantageously provide for the control of frac sand produced silica dust, which consequently improves personnel safety, helps reduce the need for respirators and other burdensome safety equipment, and allows personnel to work longer and more efficiently at the worksite. 
     In the illustrated embodiment of system  100  shown in  FIGS. 1 ,  2 , and  3 , four (4) conventional sand storage trailers  101   a - 101   d  are shown at a fracing worksite. While four (4) trailers  101  are shown as an example, the actual number of sand storage trailers  101  utilized in any particular embodiment or configuration of system  100  may vary based on the needs and restrictions at the worksite. The size and configuration of system  100  in any given worksite application will depend on such factors as the amount of sand that must be offloaded, the speed at which sand must be offloaded, and the size and capabilities of the offloading conveyor system. In the illustrated embodiment of system  100 , each trailer  101  includes a retractable trailer discharge conveyor (transfer belt)  102   a - 102   d , which receives sand from the compartments of the trailer internal tank via a lateral transfer belt running underneath the trailer tank (not shown). Trailers  101  are, for example, Sand King 3000/4000 frac sand trailers from Convey-All Industries, Inc., although there are a number of other commercially available sand storage trailers known in the art. It should also be recognized that the principles of the present invention are also applicable to embodiments of system  100  in which sand is stored and discharged from other types of fixed and transportable storage systems, such as tanks, silos, compartmented vehicles, and so on. 
     Each trailer discharge conveyor  102   a - 102   d  discharges sand to a conventional transportable conveyor system, for example, Unibelt conveyor system from Convey-All Industries, Inc., which includes a continuous transfer belt running through a lateral conveyor section  103  and a upwardly angled discharge conveyor section  105 . During typical offloading operations, one or more randomly selected trailers  101  discharge sand to the lateral conveyor section  103  at a given time. 
     Sand being discharged by each trailer discharge conveyor  102   a - 102   d  falls through slots  104  and onto lateral conveyor section  103 . Lateral conveyor section  103  then carries the sand to upwardly angled discharge conveyor section  105 , which discharges the sand to a bin of a blender truck  119  ( FIGS. 3 and 5C ), which mixes the sand with water and chemicals in quantities needed for the formulation of the particular fracing fluid being used. 
     The amount of sand being transferred at any one time in system  100  can be substantial. For example, a Convey-All Unibelt conveyor can nominally transfer and discharge 22,000 pounds per minute of sand from trailers  101   a - 101   d . The generation of a corresponding substantial amount of fine silica dust is a natural consequence of this transfer and discharge process. 
     According to the principles of the present invention, silica dust generated during the offloading of trailers  101   a - 101   d  is collected by suction at selected points around system  100  most susceptible to the generation and discharge of silica dust. In the preferred embodiment, silica dust is collected: (1) within the compartments of the tanks of trailers  101   a - 101   d ; (2) at the base of each trailer discharge conveyor  102   a - 102   d , near the point at which sand is received from the trailer lateral conveyor and the trailer tanks; (3) at the point sand is discharged from trailer discharge conveyors  102   a - 102   d  through slots  104  and onto lateral conveyor section  103 ; (4) at multiple points along upwardly-angled discharge conveyor section  105 ; and (5) near the point sand is discharged from the spout of discharge conveyor  105  in to the bin of blender  119 . It should be noted that in alternate embodiments, silica dust may be collected at additional points, or even fewer points, within system  100 , as required. 
     The silica dust control function of system  100  is driven by a silica dust control unit  106 , which draws silica dust-bearing air collected at points across the system though a pair of large manifolds  107  and  108 . In the illustrated embodiment of system  100 , silica dust control unit  106  also draws silica dust-bearing air directly from trailer  101   d  through flexible hosing  109 , although this is not a strict requirement of the principles of the present invention. Silica dust control unit  106 , which may include a baghouse and/or cyclone, separates the silica dust from the air and discharges substantially silica dust-free air into the surrounding environment. One exemplary silica dust control unit, suitable for use as silica dust control unit  106  of system  100 , is an ETI Cyclone 20 DC system, available from Entech Industries, which includes multiple twenty-inch (20″) inlets and produces a nominal airflow of 20000 cubic feet per minute (cfm). 
     Silica dust control unit  106  establishes airflow in the direction shown by arrows in  FIG. 2 . In the preferred embodiment, two intake ports of silica dust control unit  106  are pneumatically connected with manifolds  107  and  108 , which run along corresponding sides of lateral conveyor section  103 , and one intake port of silica dust control unit  106  is directly pneumatically connected to trailer  101   d  through flexible hosing  109 . 
     Silica dust generated in each of the compartments of trailers  101   a - 101   d  is collected through a corresponding set of fittings  110   a - 110   f  and hoses  111   a - 111   e . In the illustrated embodiment of system  100 , the compartments of trailers  101   a - 101   c  are pneumatically coupled to manifold  107  through flexible hosing  113   a - 113   c . For trailer  101   d , one fitting  110  is replaced with a four-way fitting  112 , which directly pneumatically couples the compartments of trailer  101   d  with silica dust control unit  106 . 
     Flexible hoses  114   a - 114   c , which tap manifold  107 , and the flexible hose  114   d , which taps manifold  108 , collect silica dust at the bases of each trailer discharge conveyor  102   a - 102   d . Flexible hoses  115   a - 115   d , which tap manifold  108 , collect silica dust at the discharge points of trailer discharge conveyor  102   a - 102   d  into slots  104   a - 104   c  of lateral conveyor section  103 . Flexible hoses  116   a - 116   d , which tap manifold  108 , collect silica dust moving up upwardly angled discharge conveyor section  105 . It should be noted that the pneumatic paths between silica dust collection hoses  113 ,  114 ,  115 , and  116  and silica dust control unit  106  may vary between embodiments of system  100 . In the preferred embodiment of system  100  shown in  FIG. 1 , the tapping point, as well as the manifold  107  or  108  being tapped, minimizes the lengths of manifolds  107  and  108  and silica dust collection hoses  113 ,  114 ,  115 , and  116 . Generally, so long as sufficient suction is available at a given silica dust collection point, the manifold  107  or  108  tapped, the point on the manifold  107  or  108  tapped the corresponding flexible hose, or both, may be varied. 
     A flexible hose  117 , which taps manifold  107 , captures silica dust generated by the discharge of sand from upwardly angled discharge conveyor  105  into the bin of blender  119 . (While flexible hose  117  taps manifold  107 , in alternate embodiments flexible hose  117  may tap manifold  108 ). 
     Manifolds  107  and  108  include a number of straight sections  120  and bent or curved sections  121  and are preferably constructed as tubes or pipes of rigid metal, such as aluminum. Rigid metal embodiments provide durability, particularly when manifolds  107  and  108  sit on or close to the ground and/or are exposed to contact by personnel or to other structures within system  100 . However, in alternate embodiments, manifolds  107  and  108  may be constructed, either in whole or in part, from sections of semi-rigid conduit or flexible (corrugated) hose. For example, semi-rigid conduit or flexible hose may be used in sections  121  of manifolds  107  and  108  that must be bent to provide a path around, over, or under, other structures in system  100 . 
     Preferably, manifolds  107  and  108  are each constructed in multiple straight sections  120  and multiple bent or curved sections  121 , which are clamped together using conventional clamps. This preferred construction allows manifolds  107  and  108  to be efficiently assembled and disassembled at the worksite, allows the most direct paths to be taken to silica dust control unit  106 , and allows the overall system of conduits to be adapted to different configurations of system  100  (e.g., different types and number of trailers  101 , different transportable conveyor systems, different surface conditions). 
     Additionally, the diameters of the various sections of manifolds  107  and  108  may increase or decrease, depending on the airflow provided by the given silica dust control unit  106 . The diameters of manifolds  107  and  108  are determined by a number of factors, including the intake diameters of silica dust control unit  106 , the airflow produced by silica dust control unit  106 , and the amount of suction needed at the silica dust collection points. Similarly, the diameters of silica dust collection hoses  113 ,  114 ,  115 , and  116  will depend on factors such as the airflow available from silica dust control unit  106 , the diameters of manifolds  107  and  108 , and the amount of suction required at a given hose inlet. In one typical embodiment of system  100 , manifolds  107  and  108  have a nominal diameter of twenty inches (20″) and silica dust collection hoses  113 ,  114 ,  115 , and  116  are nominally within the range of six to sixteen inches (6″-16″) in diameter. In other words, the principals of the present invention advantageously allow for variations in the components and configuration of system  100 . 
     It should be recognized that the transportable conveyor system, including lateral conveyor section  103  and discharge conveyor section  105 , is not always required. In this case, one or more trailer discharge conveyors  102  discharge sand directly from the corresponding trailers  101  into the bin of blender  119 . In embodiments of system  100  that do not utilize the transportable conveyor system, only a corresponding number of flexible hoses  114  and  115  are required for collecting silica dust at the base and outlet of each trailer discharge conveyor  102  discharging to blender  119 . (Along with the desired connections for removing dust within the trailers  101  themselves.) Advantageously, only single manifold  107  or  108  may be required in these embodiments. 
       FIG. 4A  is a more detailed diagram showing the pneumatic connections between manifolds  107  and  108  and silica dust control unit  106 .  FIG. 4B  shows the direct pneumatic connection between trailer  101   d  and silica dust control unit  106  through flexible hose  109  in further detail. 
       FIGS. 4C-4E  illustrate representative tapping points between the heavier rigid sections  120  of manifolds  107  and  108  and selected flexible hoses utilized in system  100 . In particular,  FIG. 4C  shows a representative pneumatic connection between manifold  107  and hose  113   c  collecting silica dust from the tank compartments of trailer  101   c .  FIG. 4D  shows representative pneumatic connections between manifold  108  and hose  114   d , which collects silica dust generated at the base of trailer discharge conveyor  102   d , and hoses  115   c  and  115   d , which collect silica dust generated at corresponding outlets of trailer discharge conveyors  102   c  and  102   d .  FIG. 4E  shows representative pneumatic connections between manifold  108  and hoses  116   a - 116   d  collecting silica dust generated by discharge conveyor section  105 . 
     As well known in the art, numerous techniques are commonly utilized for connecting flexible hose with a rigid conduit or pipe, many of which are suitable for use in system  100 . In the illustrated embodiment shown in  FIGS. 4C-4D , an aperture is tapped through the wall of the given manifold  107  or  108  and the lower periphery of a fitting (e.g., aluminum or steel pipe)  401  is attached, for example, by welding or brazing. The lower section of a coupling  402  is attached to the upper periphery of fitting  401 , for example by welding or brazing. The tubular upper section  403  of coupling  402  is received with the periphery of the corresponding hose, which is then clamped in place by one or more conventional clamps  404 . When necessary, an extension or elbow (not shown) may be provided between upper section  403  of coupling  402  and the corresponding hose. Similarly, a reduction coupling (see  FIG. 5A , designator  501 ) may be provided between upper section  403  and coupling  402 , as required to transition to the selected hose diameter. 
     In the preferred embodiment shown in  FIGS. 4C-4E , each coupling  402  includes a slide gate, which provides for air flow control between the given silica dust capture hose  113 ,  114 ,  115 , and  116  and the corresponding manifold  107  or  108 . In addition to allowing control of the amount of suction produced at the capture hose inlet, these slide gates also allow any unused taps to manifolds  107  and  108  to be completely shut off, particularly when a hose is not connected to coupling  402 . 
       FIG. 5A  depicts in further detail representative silica dust collection hose  114   b  collecting silica dust generated at the base of trailer discharge conveyor  102   b . Hose  114   b  pneumatically couples with manifold  107  through a reduction coupling  501 . The inlet end of hose  114   b , which includes an optional nozzle or shroud  502 , is disposed proximate the point where the lateral conveyor of trailer  101   b  discharges sand to the base of trailer discharge conveyor  102   b . Silica dust generated during sand transfer is captured by the suction created by silica dust control unit  106  at the discharge end of hose  114   b  and carried through manifold  107  to silica dust control unit  106  to be filtered from the air. Silica dust collection hoses  114   a ,  114   b ,  114   c , and  114   d , which respectively collect silica dust generated at the bases of trailer discharge conveyors  102   a ,  102   b ,  102   c  and  102   d , are similar in configuration and operation. 
       FIG. 5B  depicts in further detail representative silica dust collection hose  115   b  collecting silica dust generated during the discharge of sand from trailer discharge conveyor  102   b  into lateral conveyor section  103 . In the illustrated embodiment, trailer discharge conveyor  102   b  discharges through a section of flexible hose (conduit)  503  into the corresponding slot  104  of lateral conveyor section  103 . The inlet  504  of silica dust collection hose  115   b  is disposed proximate the outlet of flexible hose  503 . The suction produced by silica dust control unit  106  gathers silica dust generated during the transfer of sand, which in turns moves to silica dust control unit  106  for filtering through manifold  108 . The configuration and operation of silica dust collection hoses  115   a ,  115   b ,  115   c , and  115   d , which respectively collect silica dust from the discharge points of trailer conveyors  102   a ,  102   b ,  102   c , and  102   d  into lateral conveyor section  103  are similar. 
     Silica dust collection hoses  116   a - 116   d , and the suction generated by silica dust control unit  106 , collect silica dust generated by the lifting and discharge of sand by discharge conveyor section  105 . As shown in  FIG. 5C , silica dust collection hoses  116   a - 116   d  extend from apertures through the body of discharge conveyor section  105  at selected spaced-apart points. During operation, silica dust generated as sand moves upwards towards the outlet spout is removed through silica dust collection hoses  116   a - 116   d  and manifold  108  for filtering by silica dust control unit  106 . 
       FIG. 5C  also one possible configuration for flexible  117  with respect to the spout of upwardly angled conveyor  105 . Generally, the intake end of flexible hose  117  is located near the discharge point of the spout of conveyor  105  and creates an updraft, which captures silica dust generated as sand falls into the bin of blender  119 . The actual attachment point of flexible hose  117  to the spout of conveyor  105 , as well as the proximity of the intake end of hose  117  to the blender bin, may vary in actual practice of system  100 . 
     As discussed above, silica dust generated in the compartments of the tanks of trailers  101   a - 101   d  is collected by a set of fittings  110  and hoses  111 .  FIGS. 6A-6C  depict this subsystem in further detail, using trailer  101   a  as an example. 
     Each trailer  101  includes a set of inspection hatches  601  through the trailer roof. In the illustrated embodiment, trailers  101  include two rows of hatches  601  that run along opposing sides of the trailer roof. (In other embodiments of trailers  101 , the number and location of inspection hatches  601  may differ. For example, some commercially available sand storage trailers utilize a single row of inspection hatches that run along the centerline of the trailer roof.) 
     In addition,  FIG. 6A  shows optional skirts  610 , which run along each side of the depicted trailer  101 . Skirts  610 , which are preferably constructed from a durable flexible material, such as heavy plastic or canvas, contain silica dust generated by the movement of sand through the lateral conveyor that runs underneath the trailer tank. 
     In the preferred embodiment of system  100 , silica dust collection is performed using the hatches  601  running along one side of the trailer tank, although in alternate embodiments silica dust collection could be performed using the hatches running down both sides of the trailer tank. For a given compartment, the regular hatch  602  is pulled back and replaced with corresponding cover  603  attached an associated fitting  110  ( FIGS. 6B-6D ). 
       FIG. 6B  shows in further detail an example of a T-shaped (three-way) fitting  110   e  interfacing with corresponding hoses  111   d  and  111   e .  FIG. 6C  shows an example of a elbow (two-way) fitting  110   a  and the final section of hose  111   a  in the trailer silica dust subsystem. The remaining connections between the given trailer  101  and fittings  110  and  111  are similar. The four-way fitting  112  used to connect trailer  101   d  and silica dust control unit  106  through hose  109  is shown in detail in  FIG. 6D . In each case, fittings  110  include well-known transitions and clamps to connect to hoses  111 . Similar to the taps shown in  FIGS. 4C-4E , each fitting, such as T-shaped (three-way) fitting  110   e , elbow fitting  110   a , and four-way fitting  112 , includes a slide gate for controlling airflow between the space within the given trailer  101  and manifold  107 . 
       FIGS. 7A-7C  illustrate an enhancement to system  100 , which includes a flexible cover system  700  for containing the silica dust generated during the movement of sand through the system. Preferably, flexible cover system  700  extends over the discharge ends of trailer discharge conveyors  102   a - 102   d , the length of lateral conveyor section  103 , and the length of upwardly angled discharge conveyor section  105 . (In alternate embodiments, flexible cover system  700  may only cover portions of system  100 , as necessary to effectively control silica dust.) 
     In the preferred embodiment, flexible cover system  700  is constructed as separate sections  701   a - 701   c  and  702 , as shown in  FIGS. 7B and 7C . Sections  701   a - 701   c  cover corresponding portions of lateral conveyor section  103  and section  702  covers upwardly angled discharge conveyor section  105 . Boots  703  are provided to allow insertion of corresponding flexible capture hoses  115  and  116  into the underlying silica dust containment spaces when cover system  700  is deployed. Boots  704  extend over the ends of trailer discharge conveyors  102   a - 102   d.    
     Section  702  also includes a lateral extension  705  for covering the spout of upwardly angled discharge conveyor section  105 . A boot  707  provides for the insertion of flexible hose  117  into extension  702  for fastening on or near the outlet of the discharge spout of conveyor  105 . 
     Flexible cover system  700  is preferably constructed of canvas, heavy plastic, or other flexible material that is durable, relatively easy to deploy and remove, and transportable. Preferably, the surfaces of the selected material are impervious to frac sand, as well as able to withstand the normal wear and tear expected at a fracing worksite. When deployed, sections  701  and  702  are attached to each other with areas of Velcro  706  or similar attachment system, which minimizes the escape of silica dust at the seams between the sections. 
     In sum, the principles of the present invention provide for the efficient capture and removal of silica dust generated during the offloading of frac sand at a worksite. Silica dust removal is performed near, but not limited to, substantial sources of hazardous silica dust, including at trailer to trailer conveyor sand transfer point, each point of transfer from the trailer discharge conveyors and the lateral site conveyor, and points along the lifting/discharge conveyor. The embodiments of the inventive principles are scalable, and can be applied to any discharging system serving single or multiple frac sand storage trailers and can be implemented with various commercially available cyclone/baghouse silica dust removal systems. Moreover, the configuration and construction of these embodiments are also variable, allowing silica dust control to be effectively implemented under widely varying worksite conditions. 
     Although the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed might be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     It is therefore contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.

Summary:
A system for controlling silica dust generated during the transfer of frac sand from a storage container through a conveyor system includes a system of conduits having a plurality of inlets for collecting silica dust generated at selected points along the conveyor system. An air system pneumatically coupled to the system of conduits generates a negative pressure at each of the inlets to induce the collection of silica dust at the selected points along the conveyor.