Patent Publication Number: US-6213928-B1

Title: Method and apparatus for measuring the thickness of sludge deposited on the sidewall of a centrifuge

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to centrifuges, and more particularly, to an improved method and apparatus for measuring the thickness of sludge being deposited against the sidewall of a rotating turbine can in a centrifuge, by measuring the electrical admittance between the sidewall and the base, shaft or body of the centrifuge which are usually at ground potential. 
     Centrifuges are widely used in many fields of science and industry. Centrifuges use centrifugal force for various applications such as separating solids from liquids and separating heavier liquids from lighter liquids, etc. One application of a centrifuge is to separate metal and other particulate matter from lubricating oils. In this application, the centrifuge acts as a device for filtering dirty oil. Dirty oil under high pressure is inserted into the rotating turbine can of a centrifuge. The turbine can is rotating at a very high speed. Due to centrifugal force, heavier particles are repelled outwards, removed from the oil, and deposited on the sidewall of the rotating can. This results in the formation of a layer of sludge or particulate matter on the sidewall of the can which keeps increasing as the cleaning process continues. From an application point of view, it is very useful to be able to monitor the thickness of the sludge layer continuously as it is increasing during operation of the centrifuge. 
     The prior art discloses methods and apparatus for measuring the thickness of sludge being deposited on the sidewall of a rotating centrifugal basket or can while the centrifuge is operating. For example, U.S. Pat. No. 4,229,298 to Bange discloses a method and apparatus for determining the thickness of a charge wall being formed on the sidewall of a centrifugal basket. An electrically conductive plate is disposed in the open basket inwardly of the sidewall to provide a source of voltage for establishing an electric field between the plate and the sidewall. A capacitance sensor is used to sense a change in the capacitance resulting from the formation of a charge wall on the sidewall of the rotating basket. 
     The addition of the conductive plate requires a centrifuge with an open rotating basket or can. The electrode cannot be inserted into a centrifuge having a closed basket for filtering liquids under high pressure. Thus, the method and apparatus disclosed in the Bange patent is limited to centrifuges having an open rotating basket. 
     In the present invention, no additional conductive plate, electrode, or rod is introduced. Instead, the present invention measures the admittance (capacitance or conductance) between the turbine can sidewall and the base, shaft, or body of the centrifuge. In a typical centrifuge, the turbine can is usually electrically shorted or connected to the base, shaft and body of the centrifuge. Therefore it is not apparent that one can measure the admittance between them. This is the reason the Bange patent introduces an extra conductive electrode. The present invention modifies the centrifuge by electrically isolating the turbine can from the rest of the centrifuge structure (i.e. the combined base, shaft and body). The modification to the centrifuge is relatively simple and does not affect its operation. 
     Frequently, it happens that the thickness of sludge on the sidewall of the basket or can is not uniform. The method disclosed by the prior art carries out a local measurement on a small portion of the sidewall. Thus, the method measures the sludge thickness in an area opposite the fixed electrode. The method used by the present invention has an integral character and provides the average value of sludge thickness deposited on the sidewall of the centrifuge. This is parameter that is more accurate and useful than the local thickness at a certain limited region of the centrifuge. 
     The advantages of the present invention over the prior art are that the present invention is cheaper and simpler to implement, the invention works with minimal modifications to the centrifuge, and the prior art can only be used in centrifuges having an open rotating basket or can. The prior art method and apparatus cannot be used in cases where the rotating turbine can is closed and/or includes high pressure liquids inside it. It should also be noted that it is very difficult to place an additional fixed electrode or rod inside a rotating can. The addition of an extra conductive electrode may disturb the symmetry of the centrifuge and the thickness of the sludge near the electrode may become non-uniform. In this case, the data obtained from the device will be erroneous. In the present invention, the symmetry of the centrifuge is not disturbed. 
     SUMMARY OF THE INVENTION 
     The method of the present invention is based on measuring the electrical admittance between two suitable points in the centrifuge. One point is located on the base shaft body, while the other point is located on the turbine can. The base shaft body is usually connected to ground. The electrical connection made to the turbine can is shown in FIG.  1 . Between the electrical connection and ground, the centrifuge looks essentially like a coaxial capacitor. The dielectric of the capacitor is a mixture of oil and sludge. 
     The centrifuge includes an outer body connected to a base. Extending upwardly from the center of the base is a center shaft or spindle which allows rotation of a rotor turbine rotatably mounted on the spindle. Connected to the rotor turbine is a baffle screen assembly with a plurality of openings extending therethrough. Also attached to rotor turbine is a rotatable turbine can having a sidewall with an inner surface and an outer surface. The centrifuge further includes an o-ring for sealing the connection between the rotor turbine, the baffle screen assembly, and the turbine can near the bottom of the rotating parts of the centrifuge. The rotatable turbine can is closed at both ends for cleaning liquids under high pressure. The rotor turbine, the baffle screen assembly, and the turbine can all rotate around the center shaft. 
     The centrifuge operates by accepting a pressurized liquid such as dirty lubricating oil into an input in the center of the base of the centrifuge. The liquid moves up through a channel in the center shaft, exits the channel through openings in the side of the shaft, and moves up through a space between the center shaft and the rotor turbine. The liquid passes through openings in the rotor turbine, and moves up through a space between the rotor turbine and the baffle screen assembly. The liquid then enters the turbine can of the centrifuge which is rotating at a high rate of speed to cause particulate matter to be separated from the liquid and deposited as sludge on the inner surface of the rotating turbine can sidewall. The filtered liquid passes through a plurality of openings in the baffle screen assembly, moves down through a space between the baffle screen assembly and the rotor turbine, and comes out through openings at the bottom of the rotor turbine. The filtered liquid finally flows out of the centrifuge through openings in the base of the centrifuge for collection. The thickness of sludge deposited on the inner surface of the turbine can sidewall keeps increasing during operation of the centrifuge. The present invention discloses a novel method and apparatus for measuring the average thickness of sludge being deposited on the rotating turbine can entire sidewall during operation of the centrifuge. 
     The method of the present invention is based on measuring the electrical admittance between two points on the centrifuge in a temperature-independent maner. In order to perform this measurement, the rotating turbine can must be electrically isolated from the rest of the centrifuge structure. The electrical isolation is achieved by adding two insulating rings to the centrifuge structure between the rotor turbine and the turbine can, and between the turbine can and structure holding the internal components of the centrifuge together. 
     To perform the measurement, an electric circuit is coupled to the centrifuge at a point on the sidewall of the turbine can and a point on the base, shaft or body of the centrifuge which have been electrically isolated from the turbine can. The circuit generates a signal at a given frequency which is coupled to the input on the turbine can. This signal is continuously monitored during operation of the centrifuge to determine changes in voltage, admittance, capacitance or conductance. These measured values all vary as a function of average sludge thickness, thereby making it possible to measure small changes in these values and determining an accurate value of average sludge thickness on the entire sidewall of the rotating turbine can. 
    
    
     Various other features, objects, and advantages of the invention will be made apparent from the following detailed description and the drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawings illustrate the best mode presently contemplated of carrying out the invention. In the drawings: 
     FIG. 1 is a cross sectional view of a centrifuge used in accordance with the present invention; 
     FIG. 2A is a graph which plots capacitance as a function of frequency for three different thicknesses of sludge deposited on the sidewall of the centrifuge; 
     FIG. 2B is a graph similar to the graph of FIG. 2A which plots conductance as a function of frequency for three different thicknesses of sludge deposited on the sidewall of the centrifuge; 
     FIG. 3 is a schematic diagram of an electrical circuit which is used to measure the electrical admittance in the centrifuge; and 
     FIG. 4 is a graph which plots the output voltage of the electrical circuit in FIG. 3 as a function of sludge thickness. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a cross sectional view of a centrifuge  13  used to remove particulate matter from liquids, especially lubricating oils. The centrifuge  13  includes an outer body  7  connected to a base  16 . Extending upwardly from the center of the base  16  is a center shaft or spindle  1  which allows rotation of a rotor turbine  2  rotatably mounted on the spindle  1 . the top of spindle  1  is provided with a fixed nut  11 . Connected to the rotor turbine  2  is a baffle screen assembly  3  with a plurality of openings  21  extending therethrough. Also attached to rotor turbine  2  is a rotatable turbine can  5  having a sidewall  20  with an inner surface  25  and an outer surface  26 . The centrifuge  13  further includes an o-ring  6  for sealing the connection between the rotor turbine  2 , the baffle screen assembly  3 , and the turbine can  5  near the bottom of the rotating parts of the centrifuge  13 . The rotatable turbine can  5  is closed at both ends for cleaning liquids under high pressure. The rotor turbine  2 , the baffle screen assembly  3 , and the turbine can  5  all rotate around the center shaft or spindle  1  during operation of the centrifuge  13 . The can  5  is free of internal structure (e.g., rods) as shown in FIG.  1 . 
     As shown in FIG. 1, a pressurized liquid such as dirty lubricating oil enters through an input  14  in the center of base  16  of the centrifuge  13 , moves up through a channel  15  in the center shaft  1 , exits channel  15  through openings  17  in the side of the shaft  1 , and moves up through a space  18  between the center shaft  1  and the rotor turbine  2 . The liquid passes through openings  19  in the rotor turbine  2 , and moves up through a space  27  between the rotor turbine  2  and the baffle screen assembly  3 . The liquid then enters the turbine can  5  of the centrifuge  13 . Rotation of the turbine can  5  causes the particulate matter to be separated from the liquid and deposited as sludge  4  on the inner surface  25  of the can sidewall  20 . The filtered liquid passes through a plurality of openings  21  in the baffle screen assembly  3 , moves down through a space  22  between the baffle screen assembly  3  and the rotor turbine  2 , and comes out through openings  23  at the bottom of the rotor turbine  2 . The filtered liquid finally flows out of the centrifuge  13  through openings  24  in the base  16  of the centrifuge  13 . The thickness of sludge  4  deposited on the entire inner surface  25  of the turbine can sidewall  20  keeps increasing during operation of the centrifuge  13 . The path of a liquid flowing through the centrifuge  13  is shown by the plurality of arrows in FIG.  1 . 
     The method of the present invention is based on measuring the electrical admittance (capacitance or conductance) between two suitable points on the centrifuge  13  in a temperature-independent manner. One point is located on the base  16 , shaft  1 , or body  7  of the centrifuge  13 , while the other point is located on the turbine can  5 . The structure of the centrifuge  13  is usually totally connected together, so that the turbine can  5  must be electrically isolated from the rest of the centrifuge structure in order to attach a circuit to the centrifuge and perform the admittance measurement of the present invention. The electrical connection of the circuit to the turbine can  5  is shown at  12  in FIG.  1 . Between the sidewall  20  of the rotating can  5  and the shaft  1 , the centrifuge  13  looks essentially like a coaxial capacitor, with the dielectric being the mixture of liquid and sludge. 
     Electrical isolation of the rotating turbine can  5  from the rest of the centrifuge structure (i.e. the combined base  16 , shaft  1  and body  7 ) is achieved by adding two insulating rings  8  and  10  to the centrifuge  13 . A first dielectric ring  10  is placed between the rotor turbine  2  and the turbine can  5  to isolate the turbine can  5  from the rotor turbine  2 . A second dielectric ring  8  is placed between the turbine can  5  and a lock nut  9  located at the top of the centrifuge to isolate the turbine can  5  from the lock nut  9 . Now the turbine can  5  is electrically isolated from the outer body  7 , the base  16 , the center shaft  1 , the rotor turbine  2 , and the baffle screen assembly  3  which are all at ground potential. This modification of the centrifuge  13  does not affect its operation, since the centrifuge  13  is still balanced. 
     Theoretical analysis shows that the admittance (capacitance or conductance) of this lossy capacitor formed of the centrifuge  13  will vary with respect to average sludge thickness deposited on the sidewall  20  of the rotating turbine can  5  during operation of the centrifuge  13 . This hypothesis has been verified by carrying out measurements of capacitance and conductance for different values of sludge thickness. 
     FIG. 2A shows a graph of the capacitance  31  as a function of frequency  32  for three different thicknesses of sludge deposited on the sidewall of the rotating turbine can  5 . The first set of points  28  indicates no sludge present on the sidewall  20  of the turbine can  5 . The second set of points  29  indicates a sludge thickness of approximately 0.3 inches. The third set of points  30  indicates a sludge thickness of approximately 0.4 inches. As can be seen from FIG. 2A, at a given frequency, the capacitance of the turbine can  5  in the centrifuge  13  varies as a function of sludge thickness. 
     FIG. 2B shows a graph similar to FIG. 2A with plots of the conductance  33  as a function of frequency  34  for three different thicknesses of sludge. The first set of points  35  indicates no sludge deposited on the sidewall  20  of the turbine can  5 . The second set of points  36  indicates a sludge thickness of approximately 0.3 inches. The third set of points  37  indicates a sludge thickness of approximately 0.4 inches. It can be seen from FIG. 2B that at a given frequency, the conductance of the turbine can  5  in the centrifuge  13  also varies as a function of sludge thickness. Thus, a measurement of capacitance and/or conductance can be used to determine the thickness of a sludge layer being deposited on the sidewall of a rotating turbine can in a centrifuge. This measurement can be performed continuously while the centrifuge is operating. 
     FIG. 3 illustrates an electrical circuit  38  which is coupled to the centrifuge  13  for measuring the admittance (capacitance or conductance) between point  12  on the sidewall  20  of the turbine can  5  and the base  16 , shaft  1 , or body  7  of the centrifuge  13  which are all at ground potential as shown in FIG.  1 . As mentioned previously, the sludge thickness can be determined by measuring the admittance (capacitance or conductance) between point  12  on the centrifuge  13  and ground. The circuit  38  includes a free running oscillator  39  which generates a square wave signal at a frequency of approximately 1 MHz. The free running oscillator  39  is comprised of two inverters UA 1  and UA 2 , resistors R 1 , R 2 , R 3  and a capacitor C 1 . Point A of the circuit  38  is electrically coupled to the sidewall  20  of the turbine can  5  at point  12 . The circuit  38  measures the admittance (capacitance or conductance) between the sidewall  20  of the rotating turbine can  5  and the base  16 , shaft  1 , or body  7  of the centrifuge (ground) in the form of a voltage. The voltage varies as a function of sludge thickness as shown in FIG.  4 . In addition, as the sludge thickness increases, the admittance between point A (point  12 ) and ground increases as well. This results in a decrease in the voltage between point A (point  12 ) and ground. The second part of circuit  38  is an amplifier circuit  40  for amplifying the voltage between point A and ground. The amplifier circuit  40  includes operational amplifiers U 3  and U 4 , resistors R 4 , R 5 , R 6 , R 7 , R 8  and capacitors C 2  and C 3 . 
     A graph of the output voltage  41  as a function of sludge thickness  42  for a typical centrifuge is shown in FIG.  4 . Note that the output voltage  41  decreases as the sludge thickness  42  increases. Thus, a very small change in sludge thickness can be accurately measured by measuring the output voltage of the circuit  38  coupled to a centrifuge  13 . 
     It is recognized that other equivalents, alternatives, and modifications aside from those expressly stated, are possible and within the scope of the appended claims.