Abstract:
A fluid degasification system for a hydraulic circuit includes a gas/fluid separation tank, a fluid entry passage directing fluid into a small foam generating cup containing foam and a small amount of additional fluid, thereby stimulating foam formation. A separation screen is positioned below the foam generating cup to receive bubbles formed in the cup and to allow liquid to pass through the screen to a degasified-fluid collecting chamber below the screen as the bubbles resting on the separation screen decompose.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application 61/470,300, “On-Board Hydraulic Fluid Degasification System for a Hydraulic Hybrid Vehicle,” filed on Mar. 31, 2011. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The invention is directed to removal of dissolved gases from hydraulic fluid in fluid power systems, in particular for hydraulic hybrid vehicle applications. 
         [0004]    2. Description of the Related Art 
         [0005]    Some fluid power systems use hydraulic accumulators. Hydraulic accumulators store hydraulic fluid that is pressurized against a gas cushion. The gas is physically separated from the fluid by a structure, such as a flexible bladder or a sliding piston. The most commonly used gas is nitrogen, due to its low cost and relatively inert properties. Unfortunately, all known bladder materials are somewhat permeable to nitrogen. This causes the fluid in any bladder accumulator to become contaminated with dissolved nitrogen over time, leading to degraded performance of the hydraulic system. Piston accumulators can also be susceptible to nitrogen contamination due to gas permeation across the seal of the piston. Gas permeation tends to increase with higher operating pressures, and is a significant problem at the high pressures found in hydraulic hybrid vehicle systems (e.g., 5,000-7,000 psi). 
         [0006]    It can therefore be important to manage nitrogen contamination in a high pressure hydraulic system that employs a hydraulic accumulator. In order to separate dissolved gas from fluid, previously known degasification devices have generally utilized vacuum chambers, swirling/vortex methods, long-delay settling tanks, or mechanical or other external agitation methods to stimulate degasification. A simple, fast, and inexpensive degasification system tailored for a hydraulic hybrid vehicle is needed in the art. While it is possible to simply replace the fluid at periodic intervals, a more practical and less service-intensive solution is desired. For hydraulic hybrid vehicle applications, it would be particularly preferable to devise a fluid degasification system that could be carried on-board the vehicle by which dissolved nitrogen could be continuously or periodically removed and maintained within acceptable limits. Because hydraulic fluid tends to form foamy bubbles when dissolved gas is rapidly released from it, a fluid degasification device preferably will also provide for efficient separation of the resulting foam into its gas and liquid components. 
       OBJECT OF THE INVENTION 
       [0007]    It is therefore an object of the invention to remove dissolved and entrained gases from hydraulic fluid within a working hydraulic circuit, particularly for hydraulic hybrid vehicles. 
       SUMMARY OF THE INVENTION 
       [0008]    According to the present invention, a degasification system includes a separation tank for separation of gas from hydraulic fluid, with the tank having a bottom portion for collection of degasified fluid, and a top portion for collection of gas that has been separated from the hydraulic fluid. Fluid needing degasification enters the separation tank in the top portion of the separation tank and is directed into a small foam generating cup that contains a small volume of foam and fluid. The air pressure in the separation tank is preferably near ambient atmospheric pressure, but less than the pressure of the entering hydraulic fluid, since the reduction in pressure of the hydraulic fluid that occurs as it enters the separation tank improves bubble formation. The small foam generating cup is affixed inside the separation tank in a position to have the entering untreated fluid cross an air gap and contact the small volume of fluid within the foam generating cup. The contact and resulting turbulence from the impact of the untreated fluid with the small volume of fluid and foam in the cup greatly facilitates and accelerates bubble formation. Resulting foam that overflows the cup&#39;s brim then falls and collects on a separation screen placed somewhere below the cup, which screen separates the top and bottom portions of the separation tank. Holes in the separation screen are designed to prevent bubbles and foamy oil from passing through, but to allow degasified oil to drip through as the foam bubbles coalesce and decompose. The degasified fluid at the bottom of the separation tank may then be pumped out for reuse within the fluid power system. Gas that accumulates at the top of the separation tank is vented to the atmosphere. 
         [0009]    One or more foam level sensors, preferably in the form of a float or similar device, may be used to monitor the height of the foam that has collected on the separation screen. This is because the flow rate through the separation screen depends on the foam height, such that fluid flow through the degasification device may be determined and regulated through foam height, e.g., by targeting a preferred height of foam above the screen. 
         [0010]    Over prior art, this invention has the advantages of (1) greatly increasing the rate of deaerating a fluid into a foam, and (2) greatly increasing the rate of gas and liquid separation from the resultant foam, (3) in an inexpensive assembly easily carried on-board a motor vehicle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a sectional view of a fluid/gas separation tank. 
           [0012]      FIG. 2  is a schematic of a fluid degasification circuit that utilizes the separation tank. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , degasification device  100  is shown, including separation tank  101 , separation screen  106 , foam generating cup  104  and fluid inlet  103 . Separation screen  106  effectively separates the interior volume of tank  100  into an upper section  151 , meant for collection of gas and foam, and a lower section  152 , meant for collection of degasified liquid. Fluid enters the upper section  151  from the low pressure side of a hydraulic fluid power circuit (not shown) by means of tube  103  and fluid inlet port  105  (alternatively, fluid could enter through an orifice in the separation tank  101  above cup  104 , which orifice would also constitute a fluid inlet port, without need for a tube) into cup  104  which is open to upper section  151 . A gap  110  exists between fluid inlet port  105  and fluid surface level  102  in cup  104 . Gap  110  is occupied with air. Cup  104  may be attached to the interior of tank  101 , or tube  103 , or any static structure. The volume within separation tank  101  is held at a low pressure (near ambient) by being open to the atmosphere, for example via a breather cap  108  and optionally a weakly sprung relief valve  109 . Therefore the pressure in tank  101  is lower than the pressure in the low pressure side of the hydraulic circuit (in  FIG. 2 ). For example, the low pressure side of a sealed hydraulic circuit might be perhaps 50 psi, while the degasification tank may be near atmospheric pressure. The volume of tank  101  is preferably in the range of one to five gallons, but may be any volume that is appropriate to the specific application. 
         [0014]    In operation, low pressure fluid from a hydraulic circuit passes through tube  103 , past optional orifice  107 , and into foam generating cup  104  through fluid inlet port  105 . Alternatively, orifice  107  may be replaced by a narrowing of the tip of fluid inlet port  105 . Owing to the brief pressure change that occurs as the fluid passes the narrowed cross sectional area of optional orifice  107  (or the narrowed tip if provided), dissolved gas present in the fluid begins to form entrained bubbles so that fluid exiting at fluid inlet port  105  is actually a mixture of liquid and entrained bubbles. Upon exiting fluid inlet port  105  this mixture is then additionally exposed to the relatively lower pressure within tank  100 , causing the bubbles to expand rapidly and form a foam in cup  104 . The contact and resulting turbulence from the impact of the untreated fluid with the small volume of fluid and foam in the cup greatly facilitates and accelerates bubble formation. The foam may then overflow cup  104  (e.g., at the brim of cup  104  or through one or more alternative openings in the cup  104  to tank section  151 ) to enter and collect within the greater volume of upper section  151 . By the force of gravity, this overflowed foam will then collect upon separation screen  106 , which has a preferred mesh size of about 300 to 400 openings per linear inch. As the foam contacts the separation screen  106 , some of the foam decomposes into deaerated liquid which will fall through the separation screen  106  into lower section  152 , while the gas thus freed rises to collect in head space  153 . 
         [0015]    It should be noted that while cup  104  is generally described herein as a small cup, with a diameter of perhaps just a few inches, an equivalent small vessel of different shape could also perform the same function. In addition, references to a “small” volume of fluid in the cup will mean a volume of 5 ounces or less for purposes of this application. 
         [0016]    As mentioned above, the invention preferably uses one or more layers of very fine screen (e.g. 300 to 400 mesh) to separate foam from the deaerated fluid, while promoting the decomposition of the foam into gas and liquid. In contrast to using a coarse screen, applicants have discovered that such a fine mesh screen not only promotes the breakage of bubbles but also serves to better physically separate the foam phase from the liquid phase, with deaerated fluid able to pass through the small openings in the separation screen while the gas bubbles, including the very fine bubbles characteristic of foam, are blocked and remain above the screen. The bubbles, of course, are lighter than the fluid and either larger than the openings in the screen or are collapsed as they enter the openings. The gas that was within the bubbles rapidly rises above the foam layer on the screen and is removed or vented from the space above the foam. 
         [0017]    Affected by the rate at which foam is being generated at cup  104  relative the rate at which it decomposes on separation screen  106 , a volume of bubbles/foam  112  preferably collects upon screen  106 . Depending on the pressure and temperature within the tank, there exists an optimum foam height (height  113 , which may be, for example, 3-5 inches) that will maximize the rate of foam decomposition. The optimum height will depend on conditions such as the fluid used, the screen mesh size, and other factors, and should be determined experimentally for the expected conditions. Thereafter, it is a concern of the invention to monitor the height of foam above screen  106  in order to maintain it as near as possible to the predetermined optimum height by regulating the flow of fluid into the tank. 
         [0018]    One or more foam level sensors (for example, foam level sensor  111 ) measure the foam level, resulting in an estimation of the foam level which can then be used to estimate a foam height  113  above screen  106 . A value representing this height may then be transmitted to a CPU (not shown) that may use the value to determine a rate of flow to be allowed into tank  101 . Sensor  111  is preferably of the float type that returns a signal representing a height within a range of heights. Alternatively, sensor  111  may simply be mechanically positioned at a maximum desired foam level height. In normal operation the fluid level would remain well above the level at which deaerated fluid is drawn out, by controlling the flow of fluid from the hydraulic system into the degasification system by means of a valve or similar device. 
         [0019]    Liquid that has passed through separation screen  106  will collect in lower section  152  of tank  100 . This liquid (represented here by liquid surface  116 ) is deaerated fluid that is ready to re-enter the low pressure side of the hydraulic circuit from which it came. Fluid return line  117  is provided to allow this fluid to thereby be drawn back into the circuit. Separation screen  106  not only promotes the breakdown of foam into deaerated liquid, but also prevents any foam from being present in lower section  152  so that any fluid drawn back does not contain foam. Additionally, one or more liquid level sensors such as sensor  114  are provided to monitor the level of liquid so that fluid will not be drawn into the circuit if the level is below a certain threshold. Sensor  114  is also preferably of the float type that returns signals representing a height within a range of heights. 
         [0020]    One or more much coarser screens (not shown) may optionally be provided anywhere above the separation screen  106 . An additional screen may, for example, be 10 to 30 mesh, or a finer screen such as 100 to 200 mesh to provide some bubble/gas removal of a cold or viscous foam/fluid mixture while allowing sufficient fluid flow through the screen. 
         [0021]    Referring now to  FIG. 2 , the operation of the degasification device is seen in the context of a hydraulic circuit. Degasification tank  101  is disposed to receive, deaerate, and return low pressure fluid from hydraulic machine case  130 . Case  130  may be the low-pressure, fluid-filled case of any hydraulic device, such as a bent-axis hydraulic pump/motor, hydraulic drive module, or a hydrostatic transmission, having a low pressure fluid port  131  preferably residing at a high point on the device. Alternatively, case  130  may simply be any fluid-filled point on the low pressure side of a hydraulic circuit, such as a low pressure manifold, a low pressure reservoir, or a low pressure fluid line. Fluid may also be taken from the high pressure side of a hydraulic circuit, but using such high pressure fluid is energy inefficient and the available higher pressure drop across an orifice is not needed to stimulate gas/fluid separation in the present invention. 
         [0022]    Degasification tank  101  is here depicted with additional components useful for its effective operation in the context of the hydraulic circuit. For example, control valve  140  regulates flow into tank  101  from case  130 . Pump  123  draws fluid from tank  101  via drain port  117   a  and return line  117 . Rupture disc  121  (alternatively, a relief valve) prevents damage to tank  101  in case of a sudden high pressure release into the tank, such as a pump/motor blow-off event. CPU  122  receives signals from foam height sensor  111  indicating foam height, and from liquid level sensor  114  indicating liquid level. CPU  122  also controls control valve  140  and pump  123 , and may be part of a vehicle controller. Optional switching valve  125  directs flow from pump  123  either back toward case  130  or to a secondary system (not shown), for example, a priming circuit that pre-pressurizes the high pressure side of the hydraulic circuit at system startup. Relief passage  143   a ,  143   b  connects tank  101  via port  143   c  with low pressure port  132  of wet case  130  to prevent overpressurizing of case  130  by relief valve  142 . Relief valve  142  may alternatively be a rupture disk. A relief valve would be preferred over a rupture disk, because although a rupture disk would provide protection against overpressurization, it would also permit fluid to continue leaking into the separation tank until the low pressure accumulator could be shut off. 
         [0023]    The degasification circuit operates as follows. Low pressure fluid having a component of dissolved gas flows from low pressure port  131  to regulating valve  140 . If regulating valve  140  is open, fluid then passes through inlet line  103   a , orifice  107 , inlet line  103   b  and inlet port  103   c . The degasification components in the separation tank  100  function as described for  FIG. 1  above. Liquid having thus collected in lower section  152  is then drawn through drain port  117   a  and return line  117  by pump  123 , and then conducted through line  124 . Optional switching valve  125  may operate to selectively route the fluid either to line  128   a  (toward case  130 ) or to a secondary system (not shown) via auxiliary line  126 . Fluid thus routed to line  128   a  then proceeds through check valve  127  and enters wet case  130  via line  128   b  and low pressure port  133 . 
         [0024]    CPU  122  receives a liquid height signal from liquid level sensor  114 . Based on the indicated liquid level, CPU  122  then may issue a control signal to either or both of control valve  140  and/or pump  123  to modify the flow into and/or out of tank  101 . For example, if the liquid level is at or above a maximum level, the flow through control valve  140  might be reduced in order to prevent overfilling of the lower section  152  of tank  101 . If the liquid level is at or below a minimum level, the flow might be increased in order to prevent lower section  152  from running dry, and/or the flow through pump  123  might be reduced or stopped for the same purpose. 
         [0025]    Further, CPU  122  receives a foam height signal from foam height sensor  111 . Based on the indicated foam height, CPU  122  then may issue a control signal to control valve  140  to modify the flow into tank  101 . For example, if the foam level is at or above a maximum level, the flow through control valve  140  might be reduced in order to reduce the formation of incoming foam and therefore prevent the foam height from exceeding this height. If the foam level detected by foam sensor  111  is below a desired foam level, the flow might be increased in order to allow the height of foam to increase. 
         [0026]    Furthermore, it is known in the art that a hydraulic machine may in some rare circumstances experience what is called a “blow-off” event in which high pressure fluid is rapidly released to the low pressure side because the cylinder barrel has momentarily become unseated, or because of some failure in the high pressure components that results in leakage to the low pressure side. This could cause a momentary surge in the pressure on the low pressure side of the circuit, such as in case  130 , possibly enough to rupture the case or any other devices connected to the low pressure side. To prevent damage to tank  101  in this event, pressure relief valve  121  is provided at port  121   a . Alternatively the function of relief valve  121  could be provided by a rupture disk. 
         [0027]    Because pump  123  acts to return fluid to case  130 , it is conceivable that, in some unexpected failure condition, the flow through this pump into case  130  might exceed the flow able to exit through valve  140 , causing the case to overpressurize. Relief passage  143   a ,  143   b  with relief valve  142  are provided to allow fluid to vent through port  143   c  to tank  101  in this event. Should tank  101  then begin to overpressurize from this incursion of fluid, rupture disk/relief valve  121  will then act to release pressure. Valve  121  may also be connected to a separate vessel to collect any fluid which may be expelled. Additionally, check valve  127  acts to prevent fluid from exiting improperly through return line  128   b.    
         [0028]    Another feature that may be beneficial when employed in a hydraulic hybrid vehicle application is the ability to use the volume in the tank  101  to buffer the fluid volume in the overall hydraulic system. For example, the effects of temperature changes on fluid volumes on cold or hot days can lead to overpressuring or underpressuring the hydraulic system in a closed system. The ability to use the separation tank to dynamically change the working volume of the hydraulic system, and control the effective accumulator precharge to counteract temperature effects, can help avoid overpressuring or underpressuring the hydraulic system.