Abstract:
An apparatus for removing fines from the bottom of a tank includes an extraction device having an internal chamber along with a vacuum arrangement which, when activated, lowers the pressure within the chamber. An inlet structure adjacent the bottom of the extraction device is adapted to be juxtaposed against fines accumulated on the bottom of the tank, the inlet device acting, when the vacuum is activated, to move fines toward the internal chamber by superimposing on the moving fines an inwardly convergent motion. A cable is provided for lowering the extraction device toward the bottom of the tank. The apparatus includes a distributor above the extraction device. The distributor defines a quiet zone above the device for separation of fines from the fluid.

Description:
This application claims the benefit of Provisional Application Ser. No. 60/129,629, filed Apr. 16, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for the evacuation of settled solid materials from the base of field storage tanks used primarily in the oil industry or from vessels employed in chemical downstream processing of heavy crude oil. More specifically, the present invention relates to a de-sanding method and arrangement which utilizes an apparatus for the formation and evacuation of slurries and preferably a storage tank for recovering the contaminated water. The water is re-used in the process and subsequently treated for minimizing consumption and the risk of pollution. 
     BACKGROUND OF THE INVENTION 
     A method for removing fines, such as sand from a crude oil tank is described in U.S. Pat. No. 5,582,652 issued to Robertson et al. The method discloses a process of directing a fluid into the bottom of a tank by means of a coiled pipe to stir up in an uncontrolled manner the fines and create thereby a slurry that can be evacuated from the tank. The second step consists of drawing the slurry through a drain pipe and pumping it into the bed of a truck where the sand is retained for disposal and the liquid is re-circulated. 
     The method is ineffective in removing the sand from the periphery of the tank due to the fact that the drainpipe is placed transversely across the tank and can reach only a limited area in the vicinity of the pipe. Thus the drainpipe cannot take advantage of the special construction of the inlet line that is designed to distribute uniformly the liquid in the lower region of the tank. Moreover, uniform distribution of the fluidizing liquid cannot be achieved throughout the tank as intended. The diameter of the orifices is the same along the pipe and most of the liquid will tend to exit at one end of the pipe i.e. the central portion of the coil. The supply line orifices are quite small being subjected to intensive erosion that tends to enlarge them and reduce their effectiveness in creating the slurry. At the same time the orifices in the supply line form eddies that tend to emulsify the oil globules circulating through the pipe with the water. The emulsified oil cannot be readily separated within the truck bed contaminating the water employed for slurry creation. This has an undesirable effect on the sand that is recovered from the bottom of the tank and poses a disposal problem both for the water and the sand. The contaminated water that needs to be replaced in order to continue the de-sanding process may be in short supply in certain locations. The slurry also contains fines such as clay that cannot be separated in the bed of the truck because their relative density is very close to the density of water. Consequently the fines in the form of clay also contaminate the water compounding the problem created by the emulsified oil and requiring costly replacement of water. 
     At the same time no provision is made to isolate the crude oil from the vicinity of the drainpipe and a certain amount of oil is entrained with the slurry resulting in wastage of crude oil and contamination of the water used for slurry formation. The line that supplies liquid for stirring up the fines is prone to clogging and, in order to eliminate the clogging, the de-sanding process must be interrupted with undue downtime. 
     Prior art methods for removing the fines from crude oil tanks have made no attempt to deposit the fines in a manner that is advantageous for evacuation. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of an aspect of the present invention to provide an effective method for introducing the crude oil into the holding tank in a fashion that facilitates the sedimentation of the fines mostly in the central region of the tank from where they can be readily removed. This may be achieved in two steps by means of reversing the flow of crude oil in a distributor that absorbs the impact of the fluid entering the tank and then forming a vortex in the lower part of the distributor that is specially designed for this purpose. The distributor is part of a bell shaped apparatus and promotes the flow of liquid in a circular motion that causes the fines to migrate towards the eye of the vortex and settle in the central area of the tank. 
     It is also an object of an aspect of the invention to provide a method for circulating the fluidizing liquid through the lower portion of the tank without entraining oil in the slurry suction line. The fines are removed by means of a bell shaped device that is lowered onto the surface of the fines. The bell is then flooded with water that displaces the oil from the area under the bell and provides an optimal medium for slurry formation. In addition to isolating the oil from the fluidizing area, the bell also ensures a quiet zone above the de-sanding region preventing the oil from being re-entrained into the bell. 
     It is also the object of an aspect of the invention to provide an apparatus for fluidizing and removing the sand from the central region of the tank with minimal disturbance and no emulsification. The apparatus comprises an impeller shaped extraction device that causes a swirling motion of the liquid in its vicinity. The swirling motion causes the sand around the apparatus to migrate towards the extraction device. A specially designed inlet at the lower part of the extraction device allows the sand from the area directly below the apparatus to be removed through the suction action of the apparatus. A modified jet pump placed within the central portion of the extraction device forms a vacuum that is uniformly distributed in the lower portion of the de-sanding apparatus. The slurry is then discharged from the tank for processing in a centrifuge. 
     It is an object of another aspect of the invention to provide a method for removing the fines at a constant rate for preventing turbulence and the overloading of the system. The displacement of the fines causes the slurry extraction device to descend due to gravity and enables it to gradually reach lower areas of the sand sediments. The sand deposit therefore needs agitation only at its surface whereas lower layers remain undisturbed. Consequently the jets employed in removing the upper layer of sand may be relatively weak further minimizing sand disturbance. Meanwhile the sand layers are removed through the downward motion of the de-sanding apparatus that descends due to its own weight. The displacement of an upper layer of sand allows the apparatus to reach the layer immediately below. Due to the fact that the weight of the bell and the flow rate of the fluidizing liquid are constant the slurry concentration also remains relatively constant during the de-sanding process with beneficial effects on the pump and the de-watering device. 
     It is also an object of an aspect of the invention to provide a method for causing the sand to migrate towards the centre of the tank with negligible agitation of the fluid. This is accomplished by means of distributing uniformly the fluidizing liquid throughout the bottom of the de-sanding apparatus and directing the flow towards the central region of the tank by means of specially designed flow-diverting baffles. The fluidizing liquid has a gentle sweeping action and the sand migrates with minimal agitation and at a constant rate for removal. The de-sanding apparatus tends to form a cavity in the central area of the bell that facilitates the migration of sand from the periphery of the tank. 
     It is also an object of an aspect of the invention to provide means of fluidizing the fines on the periphery of the tank, the means being resistive to clogging and not causing eddies with inevitable emulsification. The lower portion of de-sanding apparatus has a circular distribution chamber with equally spaced outlet ports. The outlet ports are in fluid communication with nozzle shaped check valves made from elastomers. The duckbill feature of the valve allows the process of free flow in one direction and provides reliable backflow prevention. The nozzles cannot become plugged and undue downtime due to clogs is avoided during the de-sanding process. The nozzle shaped check valve creates a jet that directs the sand towards the centre of the de-sanding apparatus for disposal. The apparatus is placed onto the sand surface slowly by means of a counterweight. This minimizes the impact and prevents the nozzles from being buried in the sand. Consequently the flow through the nozzles remains constant and so does the strength of the jets generated by the nozzles. This ensures an effective sweeping action of the jets without undue disturbance of the sand layers. Moreover, if the nozzles do get buried inadvertently, the pressure exerted by the sand reduces the aperture of the nozzles. Consequently the velocity of the liquid exiting the nozzles is increased and the liquid has an enhanced force for removing the sand from the vicinity of the nozzles. It can be seen that the nozzles are capable of freeing themselves from sand by generating stronger jets when they become buried inadvertently and then readjusting the strength of the jets for optimal performance. 
     It is also an object of an aspect of the invention to provide an effective method for separating the water from the sand prior to loading the sand into a truck or depositing it for subsequent disposal. A centrifuge well known in prior art may be employed for de-watering the sand and re-circulating the water to the crude oil tank for resuming the de-sanding process. The centrifuge is very effective in separating the water from sand and most of the water returns to the crude oil tank. 
     It is also an object of an aspect of the invention to provide a method for minimizing the water necessary for the de-sanding process. After fluidizing the sand and being separated from the slurry by the centrifuge, the water is recirculated and undergoes a gravity separation process in the area under the de-sanding apparatus where it releases the oil that may contaminate the fluidizing water. The oil-water separation is assisted by the coalescing effect of the perforated plate of the de-sanding apparatus. The oil then reaches the quiet zone above the de-sanding apparatus and is prevented from being re-entrained in the fluidizing water. The resulting relatively clean water can then be effectively reused for slurry formation. 
     An object of another aspect of the invention is to provide means for effectively removing the clay from the crude oil tank and water storage tank. The fines that have a relative density very close to that of the fluidizing water cannot be removed during the de-sanding process. However, upon completion of the de-sanding sequence, the water is returned to a water storage tank where it is subjected to gravity separation. Due to sufficient retention time, the fines slowly deposit themselves in the lower region of the tank. The buildup of clay is monitored by means of an optical sensor that initiates a sequence for the automatic removal of clay. Sand that may be transferred from the crude oil tank to the water holding tank is removed in a similar manner when sand accumulation is detected by means of a radioactive sensor. 
     It is still another object of an aspect of the invention to provide a system formed by a crude oil tank placed in series with another crude oil tank. The system is designed to pass the crude oil through the de-sanding tank for gravity separation of fines. Due to sufficient retention time most of the fines are retained in the crude oil tank for removal through an automatic de-sanding process. The crude oil is then directed to adjacent crude oil tanks for storage. It should be appreciated that there is a negligible amount of fines reaching other tanks in series with the de-sanding tank. The latter do not need a de-sanding system such as the one disclosed above resulting in important savings in terms of equipment costs. The fines that may be carried to other tanks can be removed during the scheduled maintenance of the crude oil tanks. 
     The present invention contemplates, in a preferred aspect, an electrical control to initiate, monitor and end automatically the de-sanding process in order to ensure the recovery of the fluidizing liquid, the removal of fines from the water storage tank, and automatic replenishing of the water tank. 
     More particularly, this invention provides an apparatus for removing fines from the bottom inside of a tank, comprising: 
     an extraction device having 
     wall means defining an internal chamber, 
     vacuum means within the chamber which, when activated, lowers the pressure within the chamber, 
     an inlet device adjacent the bottom of the extraction device, the inlet device being adapted to be adjacent fines accumulated on the bottom of the tank, the inlet device acting, when the vacuum means is activated, to move fines toward the internal chamber by superimposing on the moving fines an inwardly convergent motion, and 
     operating means for 1) lowering the extraction device toward fines accumulated on the bottom of the tank, and 2) activating the vacuum means. 
     Further, this invention provides an apparatus for removing fines from the bottom inside of a tank, comprising: 
     an extraction device adapted to be adjacent fines accumulated on the bottom of the tank, and to remove the fines along a pathway by creating a vacuum effect; 
     a distributor within the tank and vertically aligned above the extraction device, the distributor having: 
     a distributor wall means defining an enclosed distributor space, 
     ducting conduit means for ducting fluid to the enclosed distributor space, said ducting conduit means having at its downstream extremity, within said enclosed distributor space, an outwardly flared portion the effect of which is to decrease the speed with which the fluid enters the enclosed distributor space; 
     a vortex generating means by which fluid in the distributor is discharged therefrom in a swirling motion to facilitate the separation of fines from the fluid. 
     Finally, this invention provides a method of removing fines from the bottom inside of a tank, comprising the steps: 
     providing an extraction device employing the jet pump effect to create a vacuum which, when the device is adjacent the fines, draws the fines into a discharge pathway that directs them out of the tank; 
     suspending the extraction device above the bottom of the tank, 
     lowering the extraction device toward and into contact with the fines, and 
     energizing the extraction device to remove the fines from the tank. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various embodiments of the present invention are illustrated in the appended drawings wherein: 
     FIG. 1 is an elevational section through the de-sanding apparatus according to the invention; 
     FIG. 2 is a sectional view taken along line  1 — 1  of FIG. 1 showing the vortex-forming device of the distributor; 
     FIG. 3 is an elevational section through extraction device. 
     FIG. 4 is a sectional view along line  3 — 3  of FIG. 3 showing the configuration of the inlet device of the extraction device; 
     FIG. 5 is a sectional view along line  2 — 2  of FIG. 1 showing the impeller shaped baffles, the nozzle distribution chamber and the non-clogging nozzles of the de-sanding apparatus according to the invention; 
     FIG. 5 a  is a partly elevational section through the distribution chamber; 
     FIG. 6 is an elevational section through the water storage tank  160  according to the invention; 
     FIGS. 7A through 7F shows the configuration of the de-sanding apparatus of FIG. 1 during various phases of operation; 
     FIGS. 8 a  and  8   b  shows a control circuit for the de-sanding apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The operation of the invention will be described in the context of removing sand from the crude oil tank. However the method and apparatus of the invention may be used to remove other phases having different densities as will be apparent from the following explanation. 
     As shown in FIG. 1 crude oil contaminated with sand enters the tank through inlet valve  100  and pipe  156 . Inlet pipe  156  is in fluid communication with distributor  21 . Valve  101   a  isolates tank  10  from conduit  156 . Conduit  156  has a flexible portion  156   a,  which allows the free movement of the de-sanding apparatus  20  in a vertical path. Conduit  156  also enters distributor  21  being bent upwards to direct the flow of the liquid to the top of distributor  21  and has an enlarged end  156   b  that reduces the velocity of the fluid. The oil flow is reversed in distributor  21  which is generally vertically disposed and has a cylindrical wall  22 , a circular top  23  and a vortex generator  26  at the bottom of the distributor  21 . Top  23  of distributor  21  may be flat or convex and is welded to the cylindrical sidewall  22  absorbing the impact of the liquid that enters distributor  21  prior to deflecting it downwards. 
     Referring to FIG. 2, the bottom plate  27  is also welded to cylindrical wall  22  and partly encloses distributor  21  which is in fluid communication with the lower portion of the de-sanding apparatus  20  through circular aperture  30 . The diameter of cylindrical wall  22  is in the order of magnitude of one tenth of the diameter of tank  10  and the height of the wall  22  is preferably about one fifth of the height of tank  10 . The liquid flows downwards and reaches the lower part of the distributor  21  where it is imparted a rotary motion by vortex generator  26 . The upper portion of vortex generator  26  is formed by a circular plate  28  with a diameter that is approximately one third smaller than the diameter of the distributor  21 . A plurality of curved blades  29  converges towards the centre of distributor  21  being welded to plate  28 . Plate  28  and blades  29  are welded to circular plate  27  that encloses the vortex generator  21 . The liquid enters the vortex generator  26  through the annular space  27   a  and exits through aperture  30 . The resulting swirl facilitates the separation of sand from oil and the sedimentation of sand in the central region of tank  10 . Further gravity separation takes place in the lower portion of the de-sanding apparatus  20  that comprises a frusto-conical perforated plate  31  shown in FIG. 1 that has a diameter slightly smaller than the diameter of the tank  10 . The upper portion of the perforated plate  31  is welded to the bottom of the cylindrical wall  22  of distributor  21 . Plate  31  has two orifices for allowing conduits  150  and  141  to be connected to extraction device  50  so that extraction device  50  can move relatively to extraction apparatus  20 . The flow of liquid is reversed in the lower portion of the perforated plate  31 , thus being directed upwardly. Perforated plate  31  permits the liquid to move into a quiet zone  31   a  above the de-sanding apparatus  20 . The quiet zone produced by perforated plate  31  facilitates further gravitational separation of sand from oil. Sand falls through the perforations of plate  31  and deposits at the bottom of tank  10  in the space enclosed by de-sanding apparatus  20 . 
     Clean crude oil leaves tank  10  through overflow pipe  157  and outlet valve  157  and it is stored in adjacent crude oil tanks (not shown). Conduit  157  has its open end in the upper middle portion of tank  10  so that water from tank  160  cannot accidentally reach other tanks in series with tank  10  through conduit  157 . Sediments deposited towards the periphery of tank  10  are displaced towards the centre of tank  10  for evacuation by means of jets generated by a plurality of nozzles  38 . 
     As shown in FIGS. 5 and 5 a  nozzles  38  are generally equally spaced and placed laterally along distribution chamber  32  at the bottom of de-sanding apparatus  20 . Nozzles  38  are secured to pipes  37  and are made from elastomeric material. In effect nozzles  38  are check valves of duckbill design well known in prior art allowing the free flow of liquid in one direction and providing reliable backflow prevention. 
     Nozzles  38  provide generally horizontal jets that displace the sand between blades  39  towards the centre of tank  10  being supplied with liquid under pressure from distribution chamber  32 . Distribution chamber  32  is in fluid communication with nozzles  38  through pipes  37  and distributes uniformly the fluidizing liquid to nozzles  38 . Distribution chamber  32  is also in fluid communication with pump  120 . 
     Distribution chamber  32  has an outer cylindrical wall  33  placed concentrically with inner wall  34 . The height of outer wall  33  and inner wall  34  is in the order of magnitude of one tenth of the height of frusto-conical plate  31 . The upper portion of distribution chamber  32  is enclosed by annular plate  35  whereas annular plate  36  encloses the bottom of chamber  32 . Plate  35  and plate  36  are welded to outer cylindrical wall  33  and inner cylindrical wall  34  making distribution chamber  32  a generally enclosed space. Blade  39  has a generally rectangular shape with an end welded to inner wall  34  of distribution chamber  32  and the other end extending radially towards the centre of the tank  160  as shown in FIG.  5  and FIG. 5 a.  Blade  39  is curved and the free end of bade  39  converges towards the extraction device  50  conveying the slurry for evacuation. 
     Extraction device  50  shown in FIG. 3 is a modified jet pump with an outer casing formed by cylinder  51 . The upper end of cylinder  51  is welded to circular plate  52 , the latter closing extraction device  50  at its upper end. The lower part of cylinder  51  comprises a specially designed inlet device  53  for admitting the slurry into the extraction device  50 . 
     As shown in FIG. 4, inlet device  53  has a circular plate  54  at the upper portion and a circular plate  55  at the lower portion with an aperture  56  for admitting sand into the extraction device  53  from the region immediately below the inlet device  53 . Rectangular blades  57  are curved and converge towards the aperture  56  being welded between upper plate  54  and lower plate  55 . Blades  57  direct the slurry into inlet device  53  forming a vortex around extraction device  50  that assists the sand for migrating from the periphery of tank  10  towards the extraction device  50 . A modified jet pump shown in FIG. 3 is adapted for removing the slurry from the central portion of tank  10 . First wall means defines a first frusto-conical compartment  59  in fluid communication along conduit  141  with a source of liquid under pressure. Compartment  59  is closed at the bottom by the circular plate  54  of inlet device  53  and is located concentrically within the cylinder  51 . The frusto-conical compartment  59  extends upwardly with the narrower portion at the top connected to nozzle  60 . Nozzle  60  accelerates the pressurized fluid flowing through frusto-conical compartment  59 , creating a jet pump effect within cylinder  51  for extracting the slurry through inlet device  53  and the lower part of cylinder  51 . 
     Second wall means defines a second frusto-conical compartment  61  generally symmetrical with compartment  59 , and having the narrow end  62  directed downward. The lower end of compartment  61  is in fluid communication with the interior of the extraction apparatus  50  through a narrow portion  62  placed concentrically with respect to nozzle  60  and in the immediate vicinity of nozzle  60 . The upper end of pipe  61  is welded to plate  52  of extraction device  50  and is connected to conduit  150  through an aperture  63 . Slurry extracted through the jet pump effect by the extraction device  50  from the bottom of tank  10  enters the space enclosed by cylinder  51  through inlet device  53  and is discharged into compartment  61 . From compartment  61  the slurry exits the extraction apparatus through conduit  150 . 
     As shown in FIG. 1 de-sanding apparatus  20  is suspended by means of cable  70  and pulleys  71 . One end of the cable  70  is attached to de-sanding apparatus  20  and the other end of cable  70  is attached to counterweight  75 . The mass of counterweight  75  is slightly smaller than the mass of de-sanding apparatus  20  in order to minimize the torque on motor  77  and the impact when de-sanding apparatus  20  lands on the bottom of tank  10 . Cable  70  is wound around drum  73  which turns around shaft  75  being driven by motor  77  or gravity. Motor  77  is coupled to drum  73  by means of pneumatic brake  76  which is operated by compressed air. Motor  77  is engaged to drum shaft  75  when compressed air is supplied via three-way valve  102  to the pneumatic brake  76 . Conversely, motor  77  is disengaged when the air supply is interrupted and pneumatic brake  76  is vented to the atmosphere through conduit  162 . Metal disc  72  secured to cable  70  moves vertically and operates limits switches  180  and  181  to ensure the automatic operation of the lifting arrangement described above. 
     Referring to FIG. 6, the fluidizing liquid is stored in water storage tank  160 , which is preferably cylindrical but may have a rectangular or square base. In the central area at the bottom of tank  160 , extraction device  170  performs various functions as will be described in detail in the operation of the system. Extraction device  170  is identical with extraction device  50  and operates in various modes for sand removal from tank  10 , clay removal from tank  10  and transfer of liquid from tank  10  to tank  160 . 
     As shown in FIG. 7A pump  120 , preferably a positive displacement type, is connected to de-sanding apparatus  20  in tank  10  or extraction device  170  in tank  160  on the discharge side. Pump  120  is also connected to extraction device  50 , the lower portion of tank  160  and centrifuge  130  on the suction side. Pump  120  is connected in various pipe configurations in order to perform various functions. Referring to FIG. 7B, for flooding tank  10 , pump  120  draws water from tank  160  via conduit  155 , valve  108 , conduit  145 , valve  107 , and conduit  144 . Pump  120  delivers the water to the lower section of extraction device  50  through conduit  143 , valve  106 , conduit  142 , valve  105 , conduit  141  and flexible conduit  141   a.  During the de-sanding process of tank  10  (FIG. 7B) liquid under pressure is supplied by pump  120  to extraction device  50  through conduit  143 , valve  106 , conduit  142 , valve  105 , conduit  141  and the flexible portion  141   a  of conduit  141 . Conduit  141  is connected to conduit  140  for delivering pressurized liquid from pump  120  to distribution chamber  32  of de-sanding apparatus  20 . 
     Conduit  141  has a flange  141   b  welded to it in the vicinity of perforated plate  31  of de-sanding apparatus  20 . When de-sanding apparatus  50  is hoisted by motor  77 , extraction device  50  is also lifted as perforated plate  31  encounters flange  141   b.  Conversely, when de-sanding apparatus  50  is lowered by the action of gravity, extraction device  50  can land on sand at a more elevated position than the bottom of de-sanding apparatus  50  as the perforated plate encounters no obstruction. Therefore extraction device  50  does not interfere with the descending movement of de-sanding apparatus  20  which can land on sand at a lower position than extraction device  50 . This arrangement ensures that both distribution chamber  32  and extraction device  50  settle on the surface of the sand when sand forms a mound in the central region of tank  10 , as shown in FIG.  7 C. 
     Slurry exits de-sanding apparatus through conduit  150  with flexible portion  150   a,  flows through valve  104  and is directed by valve  103  through conduit  149  to centrifuge  130  for processing. Sand-free liquid returns to pump  120  through conduit  146 , valve  108 , conduit  145 , valve  107  and conduit  144 . 
     For removing the water from tank  10  upon completion of the de-sanding process (FIG. 7D) pump  120  draws water from the bottom of tank  10  through the lower section of extraction device  50 , conduit  141 , valve  105 , conduit  153 , valve  107  and conduit  144 . Pump  120  then discharges the liquid to tank  160  via conduit  143 , valve  106 , conduit  154  and extraction device  170 . 
     For removing clay from tank  160  (FIG. 7E) pump  120  draws water from tank  160  through conduit  155 , valve  108 , conduit  145 , valve  107  and conduit  144 . The liquid is then pumped to extraction device  170  through conduit  143 , valve  106  and conduit  154 . Clay containing water is evacuated by extraction device  170  and delivered to centrifuge  130  through pipe  152  , valve  103  and conduit  149 . Sand-free water that contains clay exits centrifuge  130  and is pumped into a well via conduit  148  and valve  109 . 
     For sand removal from tank  160  (FIG.  7 F), pump  120  draws liquid from the lower portion of tank  160  through conduit  155 , valve  108 , conduit  145 , valve  107  and conduit  144 . Pump  120  delivers the liquid under pressure to extraction device  170  though conduit  143 , valve  106  and conduit  154 . Slurry is then drawn into extraction device  170  and pumped to centrifuge  130  via conduit  152 , valve  103  and conduit  149 . After the system is primed the liquid returns from centrifuge  130  to pump  120  through conduit  147 , valve  109 , conduit  146 , valve  108 , conduit  145 , valve  107  and conduit  144  whereas conduit  155  is isolated from pump  120 . The upper portion of tank  10  is in fluid communication with crude oil tank  160  through conduit  159  which ensures the free flow of liquid between tank  160  and crude oil tank  10  in both directions. 
     As shown in FIGS. 8A and 8B the de-sanding apparatus includes a control circuit for appropriately sequencing the process of de-sanding for both the crude oil tank and water storage tank and also clay removal from the water tank. The control circuit includes sand detectors  187  and sand detector  183  that may be operable preferably by means of radioactive indicators but also sonically, optically etc. to monitor the accumulation of sand in tank  160  and respectively tank  10 . Sand sensors  187  and  183  provide a suitable signal to the control circuit for initiating the de-sanding cycles in tank  160  and tank  10  respectively. 
     Sand detectors  187  and  183  are installed at the lower portion of tank  160  and respectively tank  10  through nipples welded to the wall  181  of tank  160  and tank  10 . 
     Sand detector  183  detects the build up of sand in tank  10  as the sand obstructs the beam emitted by sand detector  183 . Then sand detector  183  initiates the flooding sequence of tank  10 . Sand detector  183  operates in conjunction with oil sensor  182  which is operable preferably by means of a capacitance indicator. Sand detector  183  is placed at a level below de-sanding apparatus  20  when de-sanding apparatus  20  is in stand-by position. Oil probe  182  detects the presence of water when the lower region of tank  10  is flooded with water and starts the de-sanding process via oil probe relay OPR 2 . 
     Oil probe  182  operates in conjunction with sand content meter  185  which is a sensor preferably operated by means of a radioactive indicator. Sand content meter  185  monitors the concentration of sand in the slurry and selects the proper operating mode terminating the de-sanding process via timer SCMT. At the completion of the de-sanding process the concentration of sand in the slurry drops below a preset level. Timer SCMT prevents transitory conditions from adversely affecting the operation of sand content meter  185 . Sand content meter  185  activates timer SCMT that ends the de-sanding cycle and initiates the water removal cycle with approximately 120 seconds delay. Limit switch relay LSR is activated by means of upper limit switch  180  and lower limit switch  181  which may be operated by means of optical, mechanical or magnetic means through plate  72 . Limit switch relay LSR responds to sensors  181  and  180  and actuates various controllable devices to set the position of the de-sanding apparatus that is best suited for a particular operation cycle. Timer SCMT operates in conjunction with oil probe  184  which is installed at the bottom of tank  10  and is identical with oil probe  182 . At the completion of the water removal cycle oil probe  184  detects the presence of oil and shuts down the unit by actuating the related devices. Oil probe  182  then places tank  10  in the stand-by mode shown in FIG. 7A until the sand accumulation triggers another cycle. Turbidity detector  188  installed on the wall of tank  160  is preferably operable by means of an optic indicator and monitors the accumulation of clay in tank  160 . When the clay sediments build up in the lower portion of tank  160  their presence is detected by sensor  188  which activates various controllable devices which are best suited for the removal of clay from tank  160 . 
     The clay removal cycle is selectable to be of a duration from 0 to 240 sec. Turbidity detector  188  incorporates a timer (not shown in FIG. 8) that is of an on-type delay being activated at the onset of the clay removal sequence. The timer begins to count a fixed timing interval at the start of the clay evacuation cycle and terminates the cycle by de-activating the related devices when it finishes counting. 
     Oil probes  190  and  189  mounted on tank  160  are identical with oil probes  182  and  184  and monitor the layer of oil formed in the upper portion of tank  160 . When oil is detected by oil probe  189  relay OPR 1  actuates the related devices in order to transfer the oil accumulated in tank  160  into tank  10 . Relay OPR 1  ends the oil transfer when water reaches oil probe  190 . Oil probes  190  and  189  operate in conjunction with level switch  191  and level switch  192 . 
     Level switches  191  and  192  may be operable optically, magnetically, sonically, electronically etc. to provide a signal to the control unit and activate the appropriate devices when the liquid level in tank  160  exceeds or drops below a preset level. All sensors described above may be threaded into the vertical wall of tank  160  and respectively tank  10  through couplings welded to said walls. 
     In accordance with a preferred embodiment of the invention, the system may be operated as follows: 
     Tank  10  is initially filled with clean crude oil. As shown in FIG. 7A crude oil containing sand is admitted into tank  10  through conduit  156  and valve  101  which is manually opened by means of push-button PB (FIG.  8 B). The liquid flows through the flexible section  156   a  of conduit  156  and enters the distributor  21  that directs the flow upwards. The liquid exits the enlarged end  156   b  having a reduced velocity and it is deflected downwards by the top of distributor  21  which absorbs the impact of the liquid jet. At the same time the flow is uniformly distributed towards the lower portion of the distributor  20 . The liquid then enters the annular opening  27   a  in vortex generator  26  and exits through the aperture  30  in a rotary motion (FIG.  2 ). The swirl formed by vortex generator  26  promotes the migration of sand towards the eye of the vortex and its sedimentation in the central area below the distributor  21 . Most of the sand is collected at the bottom of the tank  10  around extraction apparatus  50  from where the sand can be readily removed. The flow of liquid is then reversed and is directed through the perforated plate  31  of de-sanding apparatus  20 . The perforations of plate  31  distribute the flow of liquid uniformly and create a quiet zone above the de-sanding apparatus  20  for separation by gravity. At the same time perforated plate  31  allows the descent of sand separated from oil in the upper portion of tank  10  and its sedimentation at the bottom of tank  10  for subsequent disposal. Crude oil is then transferred to other storage tanks connected in series with tank  10  through conduit  157  and valve  100 . 
     The layer of sand collected at the bottom of tank  10  grows in thickness and obstructs the radioactive beam emitted by sand detector  183 . Sand detector  183  is activated to energize pump  120  and valve  107 . As shown in FIG. 7B pump  120  draws water from tank  160  via conduit  155 , valve  108 , conduit  145 , valve  107  and conduit  144 . Pump  120  floods the lower portion of tank  10  through conduit  143 , valve  106  conduit  142 , valve  105 , conduit  141 , flexible conduit  141   a  and the lower compartment  59  of extraction device  50 . The oil is displaced from the bottom of the tank  10  towards an area relatively isolated from de-sanding apparatus  20  in order to preclude it from interfering with the de-sanding cycle that follows. 
     The oil-water interface moves upwards until it reaches oil probe  182 . The signal provided by sensor  182  configures the control circuit for initiating the de-sanding cycle. 
     Referring to FIG.  7 C and FIGS. 8A and 8B oil probe  182  energizes oil probe relay OPR 2  that activates valve  102  in order to interrupt the compressed air supply to the pneumatic brake  76  and vent conduit  161  to the atmosphere. Because the pneumatic brake  76  is de-activated drum  73  is released and de-sanding apparatus  20  descends to the bottom of the tank  10  by gravity. The flexible conduits  156   a,    150   a  and  141   a  allow the free movement of de-sanding apparatus  20 . Metal disc  72  secured to the cable  70  moves upwardly as the de-sanding apparatus descends. Metal disc  72  operates the upper limit switch  180  which closes its internal contact in order to prepare the hoisting motor  77  for the next mode of operation. 
     Oil probe relay OPR 2  also activates valve  108  via pressure switch  186  which closes its contact when liquid discharged by centrifuge  130  builds up pressure in conduit  146 . Valve  108  connects pump  120  with centrifuge  130  through conduit  147 , conduit  146 , valve  108 , conduit  145 , valve  107  and conduit  144 . Oil sensor  182  simultaneously energizes valve  104  and centrifuge  130 . Pneumatic timer CFT of contactor CF activates pump  120  with approximately 1 min. delay in order to allow centrifuge  130  to reach the required speed before pump  120  is activated. Then pump  120  is energized and draws liquid from centrifuge  130  delivering the liquid under pressure through conduit  143 , valve  106 , conduit  142 , valve  105 , conduit  141 , flexible conduit  141   a  and conduit  140  to the distribution chamber  32  of de-sanding apparatus  20  and through conduit  141   a  to extraction device  50 . 
     From distribution chamber  32  the liquid exits through nozzles  38  which provide a slanted horizontal jet pattern at relatively low pressure. The jets are impinging on blades  39  that direct the flow towards the sand extraction device  50 . The sand deposited at the bottom of tank  10  is agitated and fluidized by the jets emerging from nozzles  38 . As sand becomes suspended in the liquid, the flow of liquid between blades  39  directs the slurry towards the extraction device  50 . In the vicinity of extraction device  50  a swirl is formed by the circular pattern of flow created by specially designed inlet  53 . The slurry migration is assisted both by the radial displacement of the liquid and the centripetal forces created within the vortex. At the same time a depression is created below the inlet  53  of extraction device  50 . This cavity promotes the migration of sand from the periphery of de-sanding apparatus  20  towards the central area for disposal. Thus sand removed from the cavity causes the layers of sand in the vicinity of the cavity to collapse towards the core of the cavity from where they are extracted through the inlet of the extraction device  50 . 
     The liquid under perforated plate  31  of de-sanding apparatus  20  undergoes oil-water separation and small oil droplets coalesce in contact with perforated plate  31 . Larger oil globules have enhanced buoyancy and rise above de-sanding apparatus  20  joining the oil in tank  10 . This results in cleaner liquid and sand and enhanced performance of de-sanding apparatus  20 . Liquid under pressure is also directed to extraction device  50  via conduit  141   a.  The liquid enters inlet  53  of extraction device  50  and travels through the pressure nozzle  60 . Suction created by the high-velocity liquid within cylinder  51  draws the slurry up through the inlet  53  and discharges it through the compartment  61 . Discharge of liquid (through conduit  150   a,  conduit  150 , valve  104 , conduit  151 , valve  103 , and conduit  149  to centrifuge  130 ) occurs at a pressure greater than the suction stream but lower than the operating liquid in conduit  141   a.  Sand separated from liquid in centrifuge  130  is dumped into a truck  131  or deposited in a specially designated area for subsequent disposal. Liquid free of sand is then re-circulated for continuing the de-sanding process in tank  10 . 
     The evacuation of sand from the bottom of the crude oil tank  10  causes the concentration of the slurry to be gradually reduced. Sand content meter  185  is energized by the oil probe relay OPR 2  at the beginning of the de-sanding process of tank  10  and monitors continually the sand concentration in the slurry. Sand content meter  185  energizes sand content meter timer SCMT when the amount of sand in the slurry drops below the preset limit. SCMT then times out and configures the control circuit for the water removal cycle. 
     As shown in FIG.  7 D and FIGS. 8A and 8B sand content meter timer SCMT de-energizes valves  108  to isolate pump  120  from centrifuge  130 . Timer SCMT also de-energizes valve  107  and centrifuge  130  and energizes valve  105  to connect the suction side of pump  120  to extraction device  50 . Sand content timer SCMT maintains pump  120  energized in and activates valve  106  order to transfer the liquid into tank  160 . From the lower portion of tank  10  the liquid is drawn through extraction device  50 , conduit  141   a,  conduit  141 , valve  105 , conduit  153 , valve  107  and conduit  144 . Pump  120  discharges the liquid to tank  160  via conduit  143 , valve  106  conduit  154  and de-sanding apparatus  170 . As the water level in tank  160  increases, the oil is displaced from the top of tank  160  and flows back into tank  10  through conduit  159  for recovery. SCMT simultaneously energizes limit switch relay LSR that in turn activates hoisting motor  77 . As drum  73  is rotated by motor  77  cable  70  raises de-sanding apparatus towards its stand-by position. Metal disc  72  on the cable  70  descends and upper limit switch  180  opens its internal contact. When metal disc  72  reaches the lower limit switch  181  the internal contact of switch  181  opens and relay LSR is de-energized. LSR deactivates motor  77  and de-energizes valve  102  that rotates in order to supply compressed air to pneumatic brake  76 . Drum  73  is immobilized by pneumatic brake  76  and de-sanding apparatus  20  remains suspended within tank  10 . 
     At the completion of the water removal cycle the oil-water interface operates oil sensor  184  that de-energizes oil probe relay OPR 2 . Relay OPR 2  causes pump  120  to stop. Valves  106  and  105  are also de-energized and return to their initial position shown in FIG.  7 A. 
     Water recovered in tank  160  from tank  10  contains a certain amount of fines, particularly clay, that cannot be removed by centrifuge  130 . Due to ample residence time in tank  160  the fines settle at the bottom and form a layer whose thickness grows gradually. Referring to FIG.  7 E and FIG. 8 turbidity detector  188  is operated when the layer of fines reaches sensor  188  and configures the control circuit for the clay disposal cycle. 
     Sensor  188  energizes valves  107  and valve  106 . The suction side of pump  120  is connected to water storage tank  160  through conduit  155 , valve  108 , conduit  145 , valve  107  and conduit  144 . Sensor  188  also energizes valve  103  and valve  109  connecting extraction device  170  to centrifuge  130  through conduit  152 , valve  103  and conduit  149 . The discharge side of pump  120  is put in fluid communication with extraction apparatus  170  through conduit  143 , valve  106  and conduit  154 . Clay sensor  188  actuates centrifuge  130  and pump  120  with 1-minute delay via timer CFT. The liquid is circulated as shown by arrows in FIG.  7 E. Liquid containing clay and a small amount of sand settled in the vicinity of the extraction device  170  is drawn into extraction apparatus  170  due to the venturi effect and delivered to centrifuge  130  which extracts and recovers the sand. The liquid exiting centrifuge  130  is then pumped through conduit  147 , valve  109  and conduit  148  into a well for disposal. 
     A timer (not shown in FIGS. 8A and 8B) incorporated within clay detector  188  begins a counting sequence at the onset of the clay disposal cycle. At the end of the time interval determined by the timer, detector  188  is operated and causes pump  120 , centrifuge  130  and valves  107 , 106 , 103  and respectively  109  to be de-energized. 
     A certain amount of water is removed from tank  160  during the clay removal cycle and tank  10  requires replenishing. Level switch  192  is operated when the liquid level in tank  160  drops and oil probe relay OPR 1  is energized. 
     Relay OPR 1  energizes valve  110  which opens and supplies water under pressure to tank  160 . At the same time valve  101  opens to allow the transfer of liquid from tank  10  to other crude oil tanks (not shown). The level of liquid in tank  160  rises and liquid flows from tank  160  to tank  10  through conduit  159 . Excess liquid from tank  10  is transferred through valve  101  and conduit  156  to other crude oil tanks. 
     When water reaches oil probe  190  the replenishing cycle of tank  160  is completed sensor  190  causes valves  110  and V 101  to be de-energized. Level switch  191  prevents the level in tank  160  and tank  10  from rising above the level at which switch  191  is placed. In case the level of liquid exceeds the predetermined level due a malfunction malfunctions of probe  190  switch  191  opens its contact and de-energizes valve V 1  interrupting the supply of water under pressure to tank  160 . 
     Small amounts of sand may be present in the liquid transferred from tank  10  to tank  160  during the during the water removal cycle. Sand is deposited at the bottom of tank  160  in the vicinity of the extraction device  170  and it is normally evacuated during the clay removal cycle or during the flooding mode tank  10 . 
     In case the crude oil contains only sand there are no other fines transferred to tank  160 . Turbidity detector  188  will not be operated and a sand detector  187  is required for initiating a sand disposal cycle from tank  160 . Referring to FIG.  7 F and FIG. 8, sand detector  187  operates in the same fashion as sand detector  183  when it detects the presence of sand in its vicinity. Sand detector  187  causes valve  107  to be energized. Valve  107  connects pump  120  to tank  160  through conduit  155 , valve  108 , conduit  145  and conduit  144 . Valves  106  and  103  are also energized connecting pump  120  to extraction device  170  and centrifuge  130 . Contactor CF is energized and starts centrifuge  130 . At the same time contactor CF actuates pneumatic timer CFT which energizes pump  120  with 1 min. delay. Pump  120  circulates the liquid through extraction device  170  which removes the slurry from tank  160  and delivers the slurry to the centrifuge  130 . 
     Initially there is no liquid pressure in conduit  146  that connects centrifuge  130  to valve  108  and pump  120  is primed with liquid from tank  160 . Liquid exiting centrifuge  130  builds up a certain pressure in conduit  146  and actuates pressure switch  186  that energizes valve  108  to direct the liquid to pump  120 . The suction side of pump  120  is connected to centrifuge  130 . Liquid is then circulated as shown in FIG.  7 F. At the completion of the de-sanding process in tank  160 , sand content meter  187  energizes timer SCMT that begins a counting sequence. When timer SCMT times out pump  120 , centrifuge  130  and valves  108 ,  107 ,  106  and  103  are de-energized. 
     Excess oil is removed from tank  160  by means of probe  189 , which is operated when it detects the presence of oil at its level. Sensor  189  energizes oil probe relay OPR 1  which activates valve  110  and valve  101  for transferring the oil from tank  160  to tank  10  via conduit  159 . 
     When the oil-water interface reaches the oil sensor  190  relay OPR 1  is de-energized and valves  110  and  101  are de-activated. The supply of water under pressure is interrupted and tank  10  is isolated from other crude oil tanks. Valve  101  may then be opened manually for supplying oil to tank  10 . Low liquid level in tank  160  is detected by level switch  192  that energizes oil probe relay OPR 1  in order to activate valve  110 . Liquid under pressure is supplied to tank  160  and the level rises until water reaches oil probe  190  that de-energizes valve  110  and interrupts the flow of water to tank  160 . Level switch  191  detects high level in tank  160  and overrides level switch  189  to prevent inadvertent supply of water to tank  160  in case level switch  192  malfunctions. 
     In summary, a eferred aspect of the invention includes 
     The method of de-sanding the crude oil tank in several steps as follows: 
     a) The step of introducing the oil into the crude oil tank by reversing the flow of crude oil in a distributor that absorbs the impact of the fluid entering the tank and then forming a vortex in the lower part of the distributor in order to cause the fines to migrate towards the eye of the vortex and deposit in the central area of the crude oil tank. 
     b) The step of passing the crude oil through a perforated plate towards the upper portion of the crude oil tank in order to achieve further separation by gravity in a quiet zone above the perforated plate of the de-sanding apparatus. 
     c) The step of transferring the crude oil after removing the fines to adjacent crude oil tanks in order to minimize the deposits and maintenance in said tanks. 
     d) The step of flooding the crude oil with fluidizing liquid prior to the de-sanding process in order to displace the oil above the de-sanding apparatus and minimize the contamination of the recovered sand with oil. 
     e) The step of flooding the crude oil tank with water to reduce the viscosity of the medium below the de-sanding apparatus and thus to facilitate the migration of sand towards the central portion of the tank for extraction. 
     f) The step of lowering de-sanding apparatus onto the sand deposited at the bottom of the crude oil tank prior to de-sanding the said tank in order to place the nozzles and the extraction device onto the surface of the tank for minimizing agitation and emulsification of oil during the de-sanding process, assisting the sand to migrate towards the centre of the tank, creating a depression at the bottom of said tank that causes layers of sand in the vicinity of the said depression to collapse and further migrate towards the centre. 
     g) The step of assisting the separation of oil from water during the de-sanding process in order to minimize the oil contamination of sand by means of coalescing the oil droplets as they adhere to the perforated plate of the de-sanding apparatus and form larger globules that have enhanced buoyancy. 
     h) The step of lifting the de-sanding apparatus upon completion of the de-sanding process in order to prevent the nozzles and the extraction device from being buried in the sand when the unit is on stand-by. 
     i) The step of recovering the water for reuse and therefore minimize the water consumption upon completion of the de-sanding process. 
     j) The step of removing fines other than sand from the water storage tank for ensuring a relatively clean fluidizing liquid. 
     k) The step of removing sand that may accumulate in the water storage tank. 
     l) The step of replenishing the water storage tank upon completion of evacuation of fines from said tank. 
     m) The step of removing oil that may accumulate at the upper portion of the water storage tank. 
     2) The method of automatically removing the fines from tank  10  by means of an automated process disclosed herein. 
     Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing the spirit of the invention or the scope of the appended claims.