Abstract:
A check valve having a profiled entrance that reduces net positive suction head for piston and plunger pumps. The valve inlet surface has a cross-section that may include a curved portion that corresponds to a portion of a cone, circle, ellipse, hyperbola, or parabola.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The invention generally relates to check valves used in pumping operations. More specifically, the invention relates to a check valve with a profiled entrance for reducing net positive suction head for piston and plunger pumps.  
         BACKGROUND OF THE INVENTION  
         [0002]    Check valves are devices that allow fluid to flow through a passageway in one direction but block flow in the reverse direction. The force of gravity and/or the action of a spring aids in closing the valve. FIG. 1 shows an example of a conventional check valve assembly. As shown therein, the major components of a check valve include: a valve body  16 , a spring retainer  17 , a valve  18 , and biasing member  12  in compression between the valve and the spring retainer.  
           [0003]    Check valves are used in a variety of applications, from regulating flow in HPLC machines to downhole drilling operations. Because check valves are used universally, in many types of media, they are prone to damage, including stuck or missing discs, backstop tapping, seat tapping, disc flutter, disc stud pin wear, hinge pin wear, and flow leakage. One of the major problems occurring with check valves without sufficient suction head(pressure), is cavitation.  
           [0004]    Cavitation is the process in which a liquid changes to a vapor due to a reduction in pressure below liquid vapor pressure. Currently, almost all check valves for piston and plunger pumps have sharp corners at valve entrances or have a very small chamfer or radius, just enough to break the sharp corner. The result of this configuration is vena contracta. Vena contracta is defined as the contracted portion of a liquid jet at and near the orifice from which it issues. The fluid stream  50  shown in FIG. 2 contracting through a minimum diameter  51 , is the prime mover for cavitation at check valve inlets. The sharp edges  52  in the entrance  53  cause flow separation, which results in non-recoverable pressure loss. Basically, the sudden increase in the velocity of the pumped liquid as fluid passes from a large flow area to a smaller flow area reduces the inlet pressure, sometimes below the liquid vapor pressure, resulting in the formation of gas and bubbles. The bubbles are caught up and swept upward along the inside cavity. Somewhere along the cavity, the pressure may once again drop below the vapor pressure and cause the bubbles to collapse. Implosions of these vapor pockets can be so rapid that a rumbling/cracking noise is produced. The hydraulic impacts of the shock waves caused by the collapsing bubbles are strong enough to cause minute areas of fatigue on the metal piston or plunger surfaces. Depending on the severity of the cavitation, a decrease in pump performance may also be noted. Cavitation damage to the pump may range from minor pitting to catastrophic failure and depends on the pumped fluid characteristics, energy levels, and duration of cavitation.  
           [0005]    Thus, if the suction head of a given pump, namely the energy per lb. (due to pressure, velocity or elevation) required by a liquid to remain a fluid, cannot be raised above the vaporization line by decreasing the temperature or increasing the pressure, cavitation will occur. Cavitation often occurs on pumps in offshore platforms due to space constraints; there is not room available for equipment to house large flow regions, which would allow for minimal pressure reduction, thereby reducing the risk of cavitation. Instead, the equipment promotes small flow regions with many pressure drops, leading to frequent cavitation and premature damage of fluid end components.  
           [0006]    The first reaction to a cavitation problem is typically to check the net positive suction head available (NPSHa), measured at the suction flange, and compare it to the net positive suction head required (NPSHr). The NPSHa is a characteristic of the system and is defined as the energy which is in a liquid at the suction connection of the pump over and above that energy in the liquid due to its vapor pressure. The NPSHr is a characteristic of the pump design. It is determined by test or computation and is the energy needed to fill a pump on the suction side and overcome the friction and pressure losses from the suction connection to that point in the pump at which more energy is added; the NPSHr is the minimum head required to prevent cavitation with a given liquid at a given flowrate. The ratio of NPSHa/NPSHr must be sufficiently large to prevent formation of cavitation bubbles.  
           [0007]    Normally, the NPSHr plotted on the traditional pump curve is based on a 3% head loss due to cavitation, a convention established many years ago in the Hydraulic Institute of Standards. Permitting a head loss this large means that at some higher flow condition cavitation would already have begun before performance loss was noticed.  
           [0008]    For this reason, it is imperative that a margin be provided between the NPSHr and the NPSHa at the desired operating conditions. Further, the NPSHr will actually tend to increase with a reduction in flow.  
           [0009]    A reasonable margin of 8 ft of water at rated flow rate is commonly accepted by end users for most services. For known problem applications, such as vacuum tower bottoms and some solvents, this margin is often increased to 10 ft.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    The present invention is a check valve that includes a profiled entrance for reducing net positive suction head required. Profiled is defined as being shaped into a particular, predetermined form to streamline flow and minimize vena contracta. The profiled entrance offers an improvement over traditional sharp-cornered entrances by allowing the nozzle to require a lower pressure at the same flow rate. By requiring a lower inlet pressure, the total pressure loss in the pump is reduced, which in turn, reduces the net positive suction head required. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    For a more detailed understanding of the present invention, reference is made to the following Figures, wherein:  
         [0012]    [0012]FIG. 1 is a schematic representation of a typical, check valve with a sharp-cornered entrance (prior art).  
         [0013]    [0013]FIG. 2 is a schematic representation of vena contracta;  
         [0014]    [0014]FIG. 3 is an assembly drawing of a check valve constructed in accordance with a preferred embodiment, having a rounded entrance, single radius;  
         [0015]    [0015]FIG. 4 is an assembly drawing of a first alternative embodiment of the present check valve, having a rounded entrance, double radii;  
         [0016]    [0016]FIG. 5 is an assembly drawing of a second alternative embodiment of the present check valve, having a conical entrance;  
         [0017]    [0017]FIG. 6 is an assembly drawing of a third alternative embodiment of the present check valve, having a taper entrance;  
         [0018]    [0018]FIG. 7 is a representative drawing of an ellipse;  
         [0019]    [0019]FIG. 8 is an assembly drawing of a fourth alternative embodiment of the present check valve, having an elliptical entrance;  
         [0020]    [0020]FIG. 9 is a representative drawing of a hyperbola;  
         [0021]    [0021]FIG. 10 is an assembly drawing of a fifth alternative embodiment of the present check valve, having a hyperbolic entrance;  
         [0022]    [0022]FIG. 11 is a representative drawing of a parabola; and  
         [0023]    [0023]FIG. 12 is an assembly drawing of a sixth alternative embodiment of the present check valve, having a paraboloidal entrance.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 is a cross-section of a check valve assembly  10  containing: a valve body  16  having a bore therethrough, the bore having an inlet  30  and an outlet  11 , a valve  18  engaging body  16  at outlet  11 , a spring retainer  17  engaging body  16  and surrounding valve  18  and outlet  11 , a valve seat  13 ; a first biasing member  12  in compression between body  16  and spring retainer  17 ; and a second biasing member  15  in compression between body  16  and valve  18 . The prior art typically has a small chamfer  31  in the valve body at the inlet  30 . Valve assembly  10  has a longitudinal axis  99 .  
         [0025]    The valve is designed to open and allow fluid passage when the force of the working fluid in the positive flow direction  32  exceeds the compressive load of biasing member  12  (shown as a coil spring), which maintains valve  18  against valve seat  13 . If the flow pressure decreases or reverses in direction, biasing spring  12  will act to close valve  18  against valve seat  13  and prevent reverse fluid flow.  
         [0026]    FIGS.  3 - 6 ,  8 ,  10 , and  12  are alternative embodiments of check valves in accordance with the present invention. The check valves include all the components of FIG. 1, except chamfer  31  in the valve body at inlet  30 . In each embodiment, the valve body at inlet  30  has been modified to minimize vena contracta.  
         [0027]    [0027]FIG. 3 is a cross-section of a check valve assembly with a rounded inlet surface  60  and an inlet diameter  61 . Inlet surface  60  has a single radius of curvature  62 . The radius of curvature  62  is preferably limited by R≧0.05 D, where R is radius  62  and D is diameter  61 .  
         [0028]    [0028]FIG. 4 is a cross-section of a check valve assembly with a curved inlet surface  70  and a diameter  73 . Curved inlet surface  70  is a continuous curve having a radius of curvature that ranges from radius of curvature  71  to radius of curvature  72 . The radii  71  and  72  are preferably each limited by R≧0.05D, where R is radius  71  or  72  and D is diameter  73 .  
         [0029]    [0029]FIG. 5 is a cross-section of a check valve assembly with a frustoconical inlet surface  81  having a height  80  and an inner diameter  82 . An angle a is defined between surface  81  and axis  99  and is preferably between 10° and 75°. The ratio of height  80  to inner diameter  82  is preferably greater than 0.05.  
         [0030]    [0030]FIG. 6 is a cross-section of a check valve assembly with a tapered inlet having a frustoconical inlet surface  90 . Unlike surface  80  in FIG. 5, surface  90  extends inward all the way to valve disk  18 . An angle γ is defined between surface  90  and axis  99  and is preferably between 5° and 75°.  
         [0031]    [0031]FIG. 7 is a cross-section of a representative ellipse  105 . Ellipse  105  is vertical and defined by the equation y 2 /a 2 +x 2 /b 2 =1 wherein a is a value on major axis  106  and b is a value on minor axis  107 . For an ellipse having its center at the origin (0, 0), the foci c are defined by a 2 −b 2 =c 2 . The major axis is on the y-axis and has a length of  2   a . The minor axis is on the x-axis and has a length of  2   b . The foci are located at (0, c) and (0, −c). The vertices are at (0, a) and (0, −a). The convertices are at (b, 0) and (−b, 0).  
         [0032]    [0032]FIG. 8 is a cross-section of a check valve assembly wherein the cross-section of inlet mouth  102  is defined by one quadrant of an ellipse  101 . The inlet mouth has an inner diameter, d,  100  and an outer diameter, D,  130 . Ellipse  101  is defined by the same equation as ellipse  105  of FIG. 7. Thus, ellipse  101  is characterized by major and minor axis  103  and  104 , respectively, of which major axis  103  is parallel to inlet axis  99 . In a preferred embodiment,  103  is ≧0.05D and  104  is ≧0.05(D-d) and 0.05d.  
         [0033]    [0033]FIG. 9 shows a representative hyperbola  115 . Hyperbola  115  is vertical and defined by equation x 2 /a 2 −y 2 /b 2 =1 wherein a is a value on transverse axis  116 , b is a value on conjugate axis  117 . The center is at point (0, 0). The asymptotes are at y=(b/a)x and (−b/a)x. The vertices are at (a, 0) and (−a, 0). The foci are at (c, 0) and (−c, 0) where a, b, and c are related by c 2 =a 2 +b 2 . The transverse axis is on the x-axis and has a length of  2   a . The conjugate axis is on the y-axis and has a length of  2   b.    
         [0034]    [0034]FIG. 10 is a cross-section of a check valve assembly wherein the cross-section of inlet mouth  112  is defined by a portion of hyperbola  111  and the inlet mouth has an inner diameter, D,  110 . Hyperbola  111  is defined by the same equation as hyperbola  115  of FIG. 9 and is positioned such that transverse axis  113  defines an angle φ with respect to the inlet axis  99 . In the embodiment shown φ is 45°. In other preferred embodiments, φ is preferably between 0° and 90° and a and b are ≧0.01D.  
         [0035]    [0035]FIG. 11 shows representative parabola  125 . Parabola  125  is vertical and defined by the equation x=4py wherein p is the focus of the parabola located on the y-axis  126 . The vertex  127  is located at point (0, 0). The focus is at (0, p). The directrix is at y=−p. The quantity 4p is known as the latus rectum 4p.  
         [0036]    [0036]FIG. 12 is a cross-section of a check valve assembly wherein the cross-section of inlet mouth  123  is partially defined by a portion of a parabola  121  and has an inner diameter, D,  120 . In the embodiment shown, the inlet surface defines one-half of parabola  121 . Parabola  121  is defined by the same equation as parabola  125  of FIG. 11 and is characterized by y-axis  124  and x-axis  123 . In a preferred embodiment, y-axis  124  is parallel to inlet axis  99 . In other embodiments, y-axis  124  can be at an angle of from 0° to 90° degrees with respect to inlet axis  99 , and p is ≧0.01D.  
         [0037]    Reducing the pressure loss due to vena contracta is advantageous for a number of reasons. First of all, by profiling the body of the valve at the inlet, the change in velocity of the pumped liquid as fluid passes from a large flow area to a smaller flow is reduced. This is because the liquid undergoes a gradual flow change, which results in a smaller reduction in the inlet pressure. If the change in the inlet pressure is kept to a minimum, the required pump suction head will be met, and cavitation cannot occur.  
         [0038]    In order to prove that pressure loss due to vena contracta can be reduced by simply changing the shape of the valve body at the inlet, the following experiments were conducted using nozzles. A check valve inlet in a pump can be viewed as a nozzle because the valve seat is short and the through bore diameter is smaller than the fluid end chamber diameter.  
       EXAMPLE 1  
       [0039]    Experiment  
         [0040]    Nozzles were made to ⅛ scale of the actual valve size to determine profiled inlet&#39;s effects on pressure and through flow volume. The new profile selected was the rounded inlet with a single radius, shown in FIG. 3.  
         [0041]    Results: Flow Rate  
         [0042]    Some of the test results are shown on Table 1. It is clear that more flow goes through the nozzle with the new profiled inlet than the nozzle with the standard sharp corner inlet at the same pressure. On average, there is a 27.4% increase in fluid flow at an average 25.33 gpm through the new profiled inlet, as compared to the standard inlet.  
         [0043]    Results: Pressure  
         [0044]    Still looking at Table 1, it is clear that lower pressure is required by the profiled inlet nozzle than the standard nozzle at the same flow rate. On average, there is a 34.9% reduction in pressure loss at an average 12.83 gpm through the new profiled inlet, as compared to the standard inlet.  
                                                                 TABLE 1                                   Flow Rate @   Flow Rate @   Flow Rate @   Pressure @   Pressure @           30 psi   26 psi   20 psi   12.22 gpm   13.44 gpm                                    Standard   13.06 gpm   12.07 gpm   10.57 gpm   26 psi   32 psi       New Profile   16.46 gpm   15.32 gpm   13.67 gpm   16 psi   20 psi       Improvement   26.0%   26.9%   29.3%   38.5%   31.3%                  
 
       EXAMPLE 2  
       [0045]    Experiment  
         [0046]    Based on the results from Experiment 1, valves were made to actual size with new profile inlets, and tested in a pump driven by an engine to determine the profiled inlet&#39;s effect on cavitation.  
         [0047]    Results: Cavitation  
         [0048]    Results are shown on Table 2. With a standard valve, the pump starts to cavitate at an engine speed of 1450 rpm, and is severely cavitating at 1500 rpm. However, with a new profiled valve, the pump starts to cavitate at 1550 rpm and only slightly cavitates above 1550 rpm.  
                                             TABLE 2                                   Engine Speed               (rpm)   Observations                                        Standard   1450   Starts to cavitate at 1450 rpm;                   at 1500 rpm, very bad                   cavitation           New Profile   1550   Starts to cavitate at 1550 rpm                      
 
         [0049]    The embodiments described herein are exemplary only, and are not limiting. Many variations and modifications of the invention and the principles discussed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims that follow, that scope including all equivalents of the subject matter of the claims.