Abstract:
An automatically skimmable tank for removing oil from water. A water inlet with a 90 degree elbow that is turned sideways and points slight upward causes flow to first impinge on the wall of the tank then flow upward helically around the tank&#39;s internal diameter (ID) to maximize retention time so that oil separates from the water. At the tank&#39;s upper fluid level, the water flows to the center of the tank, then spirals downward to the bottom and under the incoming fluid flow before exiting via the water outlet. Oil is removed from the tank via an upwardly extending oil outlet riser provided with two horizontal anti-vortex plates. A level transmitter senses the oil-gas and the oil-water interfaces and a programmed PLC activates an oil valve and a low shear oil transfer pump to remove oil from the tank when the oil layer surrounds the anti-vortex plates.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is a skimmer tank for skimming oil from water contained within the tank as it is being used as a water collection or water storage tank. The tank seeks to maximize retention time within the tank and to provide for automatic skimming of oil from the top of the tank without the need for any moving parts within the tank. 
     2. Description of the Related Art 
     Current oil skimmer tanks have moving parts that tend to cause problems. Also, most skimmer tanks require operator activation of oil removal from the tanks which can result in overflow of oil from the tanks. Additionally, because of the design of the tanks, the flow path of water through the tanks short circuits so that the retention time within the tank is less than optimum. Less retention time results in less effective separation of oil from the water, with loss of valuable oil and with production of waste water containing more oil-contamination which must be disposed. The more oil-contamination in the waste water creates a shortened life for waste water disposal wells and increases the cost of disposal. 
     The present invention addresses these problems by providing a skimmer tank that has no moving parts, is not operator activated and is designed to maximize retention time within the tank, resulting in better oil separation and recovery and cleaner water to be disposed. 
     SUMMARY OF THE INVENTION 
     The goal of the design of this water collection/storage tank is to 1) maximize retention time by redirecting the inlet flow to minimize that flow through the normal path of least resistance, and 2) to provide for the automatic skimming of oil from the top of the water collected/stored water layer below. 
     To achieve the first goal of maximizing retention time, the inlet flow stream is redirected through an extension of the inlet nozzle into a 90 degree long radius elbow which prevents the inlet flow from flowing directly through the tank to the outlet nozzle. 
     The 90 degree long radius nozzle is turned so the flow is counterclockwise in applications above the earth&#39;s equator, and clockwise in applications below the earth&#39;s equator. This is done to mimic the earth&#39;s rotation so natural flow tendencies are maintained rather than opposed. This compliments the natural rotation of the inlet fluid stream, thereby increasing the likelihood of increased retention time. 
     The 90 degree long radius elbow is positioned so the inlet flow is directed at an angle slightly less that than parallel to the inside diameter of the tank so the inlet fluid is forced to contact and impinge upon the internal diameter (ID) of the tank wall, thereby using the tank wall as an impingement baffle so that a portion of the oil entrained in the inlet water stream can impinge and attach to the tank wall ID. This impingement augments oil-water separation as droplets of oil collected on the surface of the tank wall tend to stay attached to the tank wall ID, wet its surface area, and eventually wick upward to disengage into and become a part of the oil layer above. 
     The 90 degree long radius elbow is fixed to the inlet nozzle turned upwards at a 15 degree angle. This upturn directs the inlet flow stream slightly upward in an upwardly rising helical flow pattern up and around the inside diameter of the tank. This helical flow pattern prevents the inlet stream from taking the path of least resistance directly from the inlet to the outlet of the tank, and instead causes the inlet flow stream to circulate in an upward rising spiraling helix flow path directed to the top of the fluid level in the tank at all times, thereby increasing the retention time of the inlet flow stream as the velocity of the stream is slowed by the distribution of the fluid across the outer 50% of the tank&#39;s horizontal cross section which contains approximately 80% of the tank&#39;s total volume, and thereby the majority of the potential retention time. As the fluid flow rate slows smaller and smaller droplets of oil naturally separate consistent with Stokes&#39; Law of gravity separation. At the top of the fluid level oil-water separation has reached its maximum. 
     Then, as the helical flow path nears the top of the liquid level in the tank the momentum of the upward flowing fluid redirects the flow path of the fluid toward the center or middle 50% of the tank which represents only about 20% of the total tank volume. As the flow concentrates toward the middle of the tank near the top of the fluid level the radius of rotating helix begins to collapse on itself in a cyclonic, downward flowing decreasing radius flow path through the smaller retentive area of the tank in the center of the tank. As the radius of the now downward flowing stream diminishes, the flow stream begins to accelerate and a reducing radius helical flow path downward toward the middle of the tank is established. 
     At this point any separation that was going to occur by virtue of impingement on the tank walls and by virtue of Stokes&#39; Law ceases, and the accelerating water stream moves down to near the bottom of the tank, across the tank bottom under the inlet flow path toward the outlet nozzle, and leaves the tank via the water outlet nozzle. The water outlet nozzle is located on the opposite side of the tank from the water inlet nozzle. 
     The result is for an oil layer to accumulate on top of the water, regardless of the tank&#39;s fluid level. As more water flows through the tank more oil accumulates, building an ever-thicker or deeper layer of oil above the water. 
     Since the throughput rate of the inlet water stream in most process plants is rarely 100% consistent, the overall level in tanks used for this purpose is ever changing. The challenge therefore is to allow the liquid levels to rise and fall consistent with the incoming fluid loading, while removing the more valuable oil layer as frequently as circumstances allow. 
     To achieve the second goal of automatically skimming and removing the oil from the tank, a dedicated oil withdrawal system is installed within the tank. This is a piping system designed to collect oil and prevent vortexing. To accomplish this function the oil outlet nozzle is extended through the tank wall and turned upward. This nozzle is normally one or two pipe diameters oversized to minimize pressure drop and assure the low velocity extraction of a horizontal layer of oil above the water to prevent vortexing of either gas above the oil or water below the oil layer. 
     In order to augment the nozzle velocity consideration two horizontal anti-vortex plates are added to the top of the outlet nozzle riser. These horizontal plates are each approximately 24 inches in diameter and are separated by 4 inches using four pieces of thin strap material aimed parallel to the flow path between the plates. The bottom plate is perforated in its center to allow the outlet riser nozzle to penetrate it though a similarly sized hole. Therefore, oil flowing out of the tank must flow horizontally between the two anti-vortex plates, into the hole in the bottom plate, down through the outlet nozzle riser, turn horizontally to exit the tank with the oil outlet. 
     With the flow of oil now controlled to prevent vortexing, the skimming function is focused on the features that automate it. 
     Automating the skimming function is the key to maximizing oil recovery and preventing tank overflow and resulting oil losses. Automating this skimming function also takes the human element out of the skimming process, freeing up operation personnel to perform other tasks which may require less precision. 
     The automation system necessary to accomplish automatic skimming employs 1) a sophisticated level transmitter, 2) an electrically actuated valve, 3) a low shear oil transfer pump, and 4) a dedicated software algorithm. 
     The level transmitter used here is a device that electronically detects both the oil-gas interface, i.e. the tank liquid level, and the oil-water interface below. This can be a dual channel guided wave radar transmitter in applications where the oil layer has little or no entrained emulsion, or a combination transmitter with guided wave radar to detect the oil-gas interface and a capacitance feature to detect the emulsion-water interface. In either case the detection of these two interfaces is the key. 
     Once the level transmitter is calibrated to detect the two key interfaces, it is then tasked to do so, interfacing with a programmable logic controller (or PLC) programmed to trigger the automatic electrically actuated valve and its associated low shear transfer pump. A dedicated automation software algorithm is installed in the PLC which allows the automatic valve to open and the pump to start only when two specific conditions are met simultaneously. These conditions are 1) the oil-gas interface is above the elevation of the upper anti-vortex plate on the oil outlet riser, and 2) the oil/emulsion-water interface is below the lower anti-vortex plate on the outlet oil nozzle riser. When both of these two conditions are met, the automatic valve is opened and the low shear pump is started and oil is pumped out of the skimmer tank and directly into oil storage tanks. When either or both of these conditions are not met, the PLC prevents the valve from opening if the valve is closed and prevents the low shear pump from starting, or closes the valve if the valve is open and stops the low shear pump. 
     The low shear pump by definition prevents the shearing of oil and water droplets by its design, thus minimizing the re-emulsification of oil and water. This allows all freely separable water to separate from the oil according to Stokes&#39; Law when it reaches the oil storage tanks, rendering the oil emulsion free. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is cut away elevation view of a highly retentive automatically skimmable tank that is constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is an enlarged isometric view of the oil outlet nozzle riser and its associated two horizontal anti-vortex plates shown within circle  2  of  FIG. 1 . 
         FIG. 3  is a cut away top plan view of the tank of  FIG. 1 . 
         FIG. 4  is an enlarged isometric view of the sand dam overflow and associated water outlet nozzle. 
         FIG. 5  is a diagram showing the automation system associated with the tank of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings and initially to  FIGS. 1 and 3 , there is illustrated a highly retentive automatically skimmable tank  10  that is constructed in accordance with a preferred embodiment of the present invention. The tank  10  is provided with a water inlet nozzle  12  that enters the tank  10  adjacent the bottom  14  of the tank  10 . The water inlet nozzle  12  is provided with an extension in the form of a 90 degree long radius elbow  16  that points upward within the tank  10  at approximately a 15 degree angle. 
     The 90 degree long radius elbow  16  is turned to the right so the flow is counterclockwise in applications above the earth&#39;s equator, and turned to the left so the flow is clockwise in applications below the earth&#39;s equator. 
     The 90 degree long radius elbow  16  is positioned so the inlet flow is directed at an angle slightly less than parallel to the inside diameter (ID) or inside surface  18  of the tank wall  17  of the tank  10  so the inlet fluid is forced to contact and impinge upon the ID  18  of the tank  10 , thereby using the inside tank surface  18  as an impingement baffle. This impingement augments oil-water separation as droplets of oil collected on the inside wall surface  18  of the tank  10  tend to stay attached to the inside surface  18 , wet its surface area, and eventually wick upward to disengage into and become a part of the oil layer that is located above the water layer within the tank  10 . 
     As previously stated, the 90 degree long radius elbow  16  is affixed to the water inlet nozzle  12  so that it is turned upwards at approximately a 15 degree angle. This directs the inlet flow stream slightly upward in an upwardly rising helical flow pattern around the inside diameter  18  of the tank  10 . This helical flow pattern prevents the inlet stream from taking the path of least resistance directly from the water inlet nozzle  12  to the water outlet nozzle  20  of the tank  10 , and instead causes the inlet flow stream to circulate in an upward rising, spiraling, helix flow path directed to the top of the fluid level in the tank  10 , thereby increasing the retention time of the inlet flow stream as the velocity of the stream is slowed by the distribution of the fluid across the outer 50% of the tank&#39;s horizontal cross section. 
     As the fluid flow rate slows smaller and smaller droplets of oil naturally separate consistent with Stokes&#39; Law of gravity separation. At the top of the fluid level oil-water separation has reached its maximum. 
     Then, as the helical flow path nears the top of the liquid level in the tank  10  the momentum of the upward flowing fluid redirects the flow path of the fluid toward the center or middle 50% of the tank  10 . As the flow concentrates toward the middle of the tank  10  near the top of the fluid level the radius of rotating helix begins to collapse on itself in a cyclonic, downward flowing decreasing radius flow path through the smaller retentive area of the tank  10  in the center of the tank  10 . As the radius of the now downward flowing stream diminishes, the flow stream begins to accelerate and a reducing radius helical flow path downward toward the middle of the tank  10  is established. 
     At this point any separation that was going to occur by virtue of impingement on the tank walls and by virtue of Stokes&#39; Law ceases, and the accelerating water stream moves down to near the bottom  14  of the tank  10 , across the tank bottom  14  under the inlet flow path toward the water outlet nozzle  20 , over the associated sand dam  22 , into the water outlet nozzle  20  and leaves the tank  10  via the water outlet nozzle  20 . The sand dam  22  is best shown in  FIG. 4 . The sand dam  22  is open on top and is secured to the inside diameter  18  and the bottom  14  of the tank  10 . The sand dam  22  helps to prevent sand and other sediment from entering the water outlet nozzle  20 . The water outlet nozzle  20  is preferably located on an opposite side of the tank  10  from the water inlet nozzle  12 , as illustrated in  FIG. 3 . 
     The result is that an oil layer accumulates on top of the water layer, regardless of the tank&#39;s fluid level. As more water flows through the tank  10  more oil accumulates, building an ever-thicker or deeper layer of oil above the water. 
     Since the throughput rate of the inlet water stream in most process plants is rarely 100% consistent, the overall level in tanks used for this purpose is ever changing. The challenge therefore is to allow the liquid levels to rise and fall consistent with the incoming fluid loading, while removing the more valuable oil layer as frequently as circumstances allow. 
     To automatically skim and remove the separated oil from the tank  10 , a dedicated oil withdrawal system is installed within the tank  10 , as illustrated in  FIG. 2 . This is a piping system designed to collect oil and prevent vortexing. It consists of an oil outlet nozzle  24 , oil outlet riser  26  and anti-vortex plates  28 U and  28 L. 
     The oil outlet nozzle  24  extends through the tank wall  14  and is turned upward to form the oil outlet nozzle riser  26  within the tank  10 . The oil outlet nozzle  24  and riser  26  are normally one or two pipe diameters oversized to minimize pressure drop and assure the low velocity extraction of a horizontal layer of oil above the water to prevent vortexing of either gas above the oil or water below the oil layer. 
     In order to augment the nozzle velocity consideration and upper horizontal anti-vortex plate  28 U and a lower horizontal anti-vortex plate  28 L are added to the top of the oil outlet nozzle riser  26 . These horizontal plates  28 U and  28 L are each approximately 24 inches in diameter and are separated by 4 inches using four pieces of thin strap material  30  aimed parallel to the flow path between the plates  28 U and  28 L. The bottom or lower plate  28 L is perforated in its center to allow the outlet riser nozzle to penetrate it though a similarly sized hole  32 , as illustrated in  FIG. 2 . Therefore, oil flowing out of the tank  10  must flow horizontally between the two anti-vortex plates  28 U and  28 L, into the hole  32  in the bottom plate  28 L, down through the oil outlet riser  26 , and then turn horizontally within the riser  26  to exit the tank  10  via the oil outlet nozzle  24 . 
     Automating the skimming function is the key to maximizing oil recovery and preventing tank overflow and resulting oil losses. Automating this skimming function also takes the human element out of the skimming process, freeing up operation personnel to perform other tasks which may require less precision. Referring to  FIG. 5 , the automation system associated with the tank  10  for controlling its operation is illustrated. The automation system necessary to accomplish automatic skimming employs a sophisticated level transmitter  34 , an electrically actuated oil valve  36 , a low shear oil transfer pump  38 , and a dedicated software algorithm contained within a programmable logic controller (PLC)  40 . 
     The level transmitter  34  used here is a device that electronically detects both the oil-gas interface, i.e. the tank liquid level, and the oil-water interface located below. This can be a dual channel guided wave radar transmitter in applications where the oil layer has little or no entrained emulsion, or a combination transmitter with guided wave radar to detect the oil-gas interface and a capacitance feature to detect the emulsion-water interface. In either case the detection of these two interfaces is the key. 
     Once the level transmitter  34  is calibrated to detect the two key interfaces, it is then tasked to do so, interfacing with the PLC  40  programmed to trigger the automatic electrically actuated oil valve  36  and its associated low shear transfer pump  38 . The dedicated automation software algorithm installed in the PLC  40  which allows the automatic valve  36  to open and the pump  38  to start only when two specific conditions are met simultaneously. 
     These conditions are: 1) the oil-gas interface is above the elevation of the upper anti-vortex plate  28 U on the oil outlet riser  26 , and 2) the oil-water interface or alternately the emulsion-water interface is below the lower anti-vortex plate  28 L on the outlet oil nozzle riser  26 . When both of these two conditions are met, the automatic valve  36  is opened and the low shear pump  38  is started and oil is pumped out of the skimmer tank  10  and directly into oil storage tanks  42 . When either or both of these conditions are not met, the PLC  40  prevents the valve  36  from opening if the valve  36  is closed and prevents the low shear pump  38  from starting, or alternately, closes the valve  36  if the valve  36  is open and stops the low shear pump  38 . 
     The low shear pump  38  by definition prevents the shearing of oil and water droplets by its design, thus minimizing the re-emulsification of oil and water. This allows all freely separable water to separate from the oil according to Stokes&#39; Law when it reaches the oil storage tanks  42 , rendering the oil emulsion-free. 
     While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for the purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.