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CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to fluid valve arrangements that permit flow under pump pressure and automatically close against flow when the pump is turned off. In one preferred aspect, the invention relates to mud saver valves of the type used in oil drilling operations. In other aspects, the invention relates to knockout caps useful for such mud saver valves. 
     2. Background of the Invention 
     It is standard practice in drilling operations to insert a mud saver valve between the kelly and the drill pipe in order to help prevent loss of drilling mud when the connection between the kelly and the drill pipe is broken. The recognized advantages of such valves include the saved cost of lost drilling mud, less pollution and greater safety for drilling rig personnel since less lost mud results in fewer slippery floors and surfaces in the rig. 
     Conventional mud saver valves incorporate a spring-biased check-valve or poppet-type valve that opens to permit mud flow downwardly into the drill pipe. When the mud flow is turned off, the spring biases the poppet valve closed so that mud cannot pass through the valve. 
     Unfortunately, conventional poppet-type mud saver valves usually need to be machined to close tolerances and may be susceptible to wear from the abrasive muds that are passed through them, particularly around the area of the valve seat. Over time, this wear can deteriorate the ability of the valve to seal. Also, if the seals of the poppet valve have a slight leak, the valve will likely not seal properly, and under pump pressure, the valve may begin throttling in an undesirable manner. The valve seat may also be vulnerable to impact damage. 
     In addition, under normal operating conditions when such a valve is open, turbulent flow develops through the valve body which leads to washing out or eroding of portions of the valve body. This turbulence results at least partially because fluid passing through these types of valves is directed radially outwardly through the space between the valve body and the valve seat, thus changing the direction of flow. Further, the flow is often directed toward and into the walls of the flowbore, creating further turbulence in the flow. 
     Vent caps are known for use in mud saver valves. These caps permit venting of excessive downhole pressure through the kelly valve. Some vent caps are designed to be broken away in the event that it is desired to pass tools downward through the mud saver valve. One such cap is disclosed in U.S. Pat. No. 3,965,980 issued to Williamson. In order to replace this type of cap, however, stop pins must be removed from the guide and cap. The cap then is removed. Afterward, the cap must be replaced and the stop pins replaced. 
     Other vent caps are known that are removable from the kelly valve in the event that tools must be passed downward through the kelly valve. A vent cap of this type is described in U.S. Pat. No. 4,364,407. Unfortunately, a wireline tool is required in order to remove the cap from the valve and then to replace it later. 
     A need exists for improved mud saver valves that can more effectively resist wear from abrasive drilling muds. A need also exists for an improved knockout cap that can be easily replaced and does not require stop pins or other connectors to hold it in place during operation. 
     SUMMARY OF THE INVENTION 
     The present invention provides a mud saver valve that features an outer housing or sub that retains upper and lower valve pistons. The pistons are reciprocably disposed within the housing and coordinate to provide a check valve though which fluid, such as drilling mud, is permitted to flow in one direction under pump pressure. Both the upper and lower valve pistons are provided with apertured plates that can be aligned in order to selectively open or close fluid passages defined by the apertures. 
     The valve configuration generates largely laminar flow through the valve. Turbulence is minimized because the direction of flow is not changed by the valve components. 
     In the preferred embodiment described here, the upper piston is disposed within the housing so that axial movement of the upper valve piston within the housing will also rotate the upper valve piston within the housing. In the described embodiment, a camming action is provided to rotate the upper piston within the housing and close the ports. The plates are secured within the piston sleeves using a keying arrangement. The plates are readily replaceable. 
     In operation, the spring causes axial movement of the piston sleeves within the housing and, thus, angular rotation of the plates with respect to one another, thereby opening a plurality of fluid flow ports to permit flow therethrough. 
     The invention also describes a frangible knockout vent cap that is readily replaceable and self-securing. The cap permits venting of excessive downhole pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For an introduction to the detailed description of the preferred embodiments of the invention, reference is made to the following accompanying drawings wherein: 
     FIG. 1 is a side cross-section depicting an exemplary mud saver valve constructed in accordance with the present invention. The valve is shown in a closed position. 
     FIG. 2 is a cutaway view of the valve taken along the line  2 — 2  in FIG.  1 . 
     FIG. 3 is a cutaway view of the valve taken along the line  3 — 3  in FIG.  1 . 
     FIG. 4 is a side cross-section of the valve shown in FIG. 1 with the valve in an open position. 
     FIG. 5 is a cutaway view of the valve taken along the line  5 — 5  in FIG.  4 . 
     FIG. 6 is a cutaway view of the valve taken along the line  6 — 6  in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1-6, an exemplary mud saver valve is depicted which is constructed in accordance with the present invention. A tubular body  10  is shown having a threaded box connector  12  at its upper end  14  and a threaded box connector  16  at its lower end  18 . 
     An interior flow bore  20  is defined along the length of the body  10  made up of an upper, enlarged-diameter polished bore section  22 , and a reduced diameter lower section  24 . An upwardly-facing annular shoulder  26  is located between the upper and lower bore sections  22 ,  24 . 
     An upper piston  28  is reciprocably retained within the flow bore  20 . The upper piston  28  generally includes a tubular sleeve  30  and a flat circular plate  32 . The tubular sleeve  30  includes an upper, enlarged portion  34  which is adapted to fit within the upper bore section  22 . A plurality of annular seals  36  are secured around the circumference of the enlarged portion  34  to assist in creating a fluid seal between the enlarged portion  34  and the upper bore section  22 . 
     As FIGS. 1 and 2 illustrate, the plate  32  contains a central opening  38 . A plurality of surrounding apertures  40  are also provided in the plate  32 . In this case, there are eight apertures  40 . Plate portions  41  are located between each pair of apertures  40 . It should be understood that there could be more such apertures or fewer, although eight apertures are currently preferred. 
     The circular plate  32  is secured to the sleeve  30  within a complimentary recess  42 . A keying arrangement is used to secure the plate  32  within the recess  42 . In the described embodiment, the keying arrangement employs pin passages  44 ,  46  disposed in the plate  32  and sleeve  30 , respectively. The pin passages  44 ,  46  are coaxially aligned, as shown in FIG. 2 so that a pin  48  can be inserted into the two passages, thus securing the plate  32  and sleeve  30 . As shown in FIG. 2, there are two sets of pin passages  44 ,  46  and two pins  48 . 
     The outer housing  10  includes three upper apertures  50  spaced at approximately 120° from one another around the periphery of the housing  10 . Camming pins  52  are disposed through the apertures  50  and reside within angled slots  54  in the outer surface of the sleeve  30  of upper piston  28 . The camming pins  52  cause rotation of the upper piston  28  within the housing  10  when the upper piston  28  is moved axially within the housing  10 . 
     A lower piston  60  is disposed below the upper piston  28  within the valve housing  10 . The lower piston  60  is formed from a generally tubular piston sleeve body  62  and a flat circular plate  64 . The sleeve body  62  includes an axial fluid flowbore  66  disposed therethrough. Preferably, the inner surface of the flowbore  66  is coated with chrome or another finish to prevent frictional resistance to fluid flow along the flowbore  66 . 
     The circular plate  64  is nearly identical to the circular plate  32  described above. The plate  64  also contains a central opening  68  and a plurality of radially disposed apertures  70 . Eight such apertures  70  are shown in FIG.  3 . It is pointed out that the number of apertures  70  should equal the number of apertures  40  in the circular plate  32 . 
     Just as with the upper piston  28 , a keying arrangement is used to secure the circular plate  64  within the sleeve body  62  of the lower piston  60 . Pin passages  72 ,  74  are disposed in the plate  64  and sleeve body  62 , respectively. The pin passages  72 ,  74  are coaxially aligned, as shown in FIG. 3 so that a pin  76  can be inserted into the two passages, thus securing the plate  64  and sleeve body  62 . As shown in FIG. 3, there are two sets of pin passages  72 ,  74  and two pins  76 . 
     Three lower apertures  78  are included through the outer housing  10 . Like the upper apertures  50 , the lower apertures  78  are spaced at approximately 120° from one another around the periphery of the housing  10 . Alignment pins  80  are disposed through the apertures  78  and reside within vertically-oriented slots  82  in the outer surface of the sleeve body  62  of the lower piston  60 . The alignment pins  80  function to prevent rotation of the lower piston  60  with respect to the housing  10 . It is also noted that the slots  82  might be angled in a direction opposite that of angled slots  54 . 
     An annular spring chamber  84  is defined between the sleeve body  62  of the lower piston  60  and the outer housing  10 . A compressible spring  86  is disposed within the chamber  84  and biases the upper and lower pistons  28 ,  60  upwardly. The spring  86  should provide adequate closing force to ensure closure of the valve against the force provided by a static load from the kelly hose (not shown) above the valve being filled with mud. The spring chamber is filled with air at atmospheric pressure. The spring  86  should compress as the lower piston  60  is moved downwardly within the housing  10  to allow the valve to open when mud is pumped down through the valve under pressure. 
     The circular plates  32 ,  64  are urged against one another by the spring  86 . The sleeve bodies  30 ,  62  of the two pistons  28 ,  60  do not contact one another. As a result, the entire spring force is transferred directly through the plates  32 ,  64 , thereby assuring a better fluid seal. 
     FIGS. 1-3 depict the valve assembly in a closed configuration wherein fluid flow across the valve is blocked. The valve will be in this configuration absent downward fluid flow through the bore  22  such that fluid pressure above the valve exceeds the pressure provided by the static mud load on the valve with the mud pumps turned off. The spring  86  biases the upper and lower pistons  28 ,  60  upward thereby camming the upper piston  28  angularly so that the upper piston  28  is rotated within the housing  10 . When this occurs, the plate portions  41  are aligned with the apertures  70  of the lower plate  64 . The apertures  40  of the upper plate  32  are also positively closed against fluid flow therethrough by complimentary plate portions of the lower plate  64 . Wear around the periphery of the apertures  40 ,  70  is unlikely to result in deterioration of the valve&#39;s ability to seal since there is no peripheral seal to be worn away. 
     FIGS. 4-6 depict the valve assembly in an open position such that fluid is capable of flowing through the aligned apertures  40 ,  70  of the plates  32 ,  64 . As shown clearly in FIG. 4, fluid passages are defined by the aligned apertures  40 ,  70  in the plates  32 ,  64 . Drilling mud can be pumped downwardly through these fluid passages. 
     The valve is easily moved from the closed position shown in FIGS. 1-3 to the open position depicted in FIGS. 4-6 by increasing fluid pressure above the valve. An increase in fluid pressure is normally accomplished by turning on the mud pumps used to pump drilling mud downward through the flowbore  22 . As fluid pressure is increased, the upper and lower pistons  28 ,  60  are urged downwardly within the housing  10 . The spring  86  is compressed within the spring chamber  84 . As the upper piston  28  is moved downwardly within the housing  10 , the camming pin  52  moves within the slot  54  to the position shown in FIG. 4 thereby causing the upper piston  28  to rotate with respect to the housing  10 . Rotation of the upper piston  28  causes the apertures  40  in the upper plate  32  to become aligned with the apertures  70  in the lower plate  64  thereby forming fluid passages which permit the communication of fluid through the upper and lower plates  32 ,  64 . It is noted that fluid flow through the aligned apertures  40 ,  70  will be substantially laminar rather than turbulent. 
     Upon a reduction of fluid pressure above the valve, the spring  86  will urge the upper and lower pistons  28 ,  60  upwardly within the housing  10 . The camming pin  52  will move within the slot  54  to the position shown in FIG.  1 . Again, the upper piston  28  will be rotated with respect to the housing  10 . The apertures  70  of the lower plate  64  will be covered by the plate portions  41  of the upper plate  32 , closing them against fluid flow. 
     The lower piston  60  can be thought of as a translational member in that it translates axially within the housing  10  without rotating with respect to the housing  10 . The upper piston  28  can be thought of as a rotational member because it will be rotated with respect to the housing  10  when it is moved axially within the housing  10 . 
     A frangible vent cap  100  is disposed within the openings  38 ,  68  of the two circular plates  32 ,  64 . The cap  100  includes a generally cylindrical elongated body  102  with a dome-shaped top  104 . A plurality of slots  106  are disposed within the body  102 . A plurality of perpendicularly-extending axial collet fingers  108  are defined by the slots  106 . The collet fingers  108  each include an outward radial protrusion  110  that has an upwardly facing stop face  112  that is oriented perpendicularly with respect to the axis of the cap  100 . The protrusion  110  also presents a downwardly-facing cam face  114  that is oriented at an angle to the longitudinal axis of the cap  100 . The cylindrical body  102  also includes a plurality of lateral fluid ports  116 . 
     The cap  100  is normally seated in a “lower” position, as shown particularly in FIGS. 1 and 4, such that the dome-shaped top  104  is resting upon the upper plate  32 . In this position, the lateral ports  116  are covered by edges of openings and the slots  106  are disposed below the plates  32 ,  64 . In this lower position, fluid is not communicated across the valve through either the ports  116  or the slots  106 . 
     It should be understood that excessive fluid pressure below the cap  100  will cause the cap  100  to move upwardly within the openings  38 ,  68  until the stop faces  112  on the protrusions  110  of the collet fingers  108  engage the lower plate  64 . In this upper position, the lateral ports  116  are raised above the plates  32 ,  64  and are uncovered so that fluid may be communicated through them. In addition, portions of the slots  106  become disposed above the plates  32 ,  64  so that fluid can be communicated through them as well. 
     In operation, the cap  100  permits venting of excessive wellbore pressures below the valve when the mud pumps are shut off. When these pumps are shut off, the pressure below the valve may exceed the pressure provided by standing mud above the valve  100 . This higher pressure will cause the vent cap  100  to move upwardly so that the excess pressure will escape through the slots  106  within the body  102  and lateral ports  116  and be transmitted through the kelly to a pressure gauge (not shown). The vent cap  100  thus also allows standpipe pressure to be read when the mud pumps are turned off. The dome shape of the top  104  assists in directing downwardly-pumped fluids toward the fluid passages formed by apertures  40 ,  70 . 
     The vent cap  100  is easily inserted into the valve but cannot be easily removed. Insertion of the cap  100  into the valve is accomplished by aligning the cap  100  with the openings  38 ,  68  in the two circular plates  32 ,  64  and pushing the cap  100  downwardly. The edge of the upper opening  38  will engage the cam faces  114  of the collet fingers  108  urging them radially inward and permitting the protrusion  110  to pass through both openings  38 ,  68 . 
     The presence of the stop face  112  on each of the collet fingers  108  will prevent withdrawal of the cap  100  from the openings  38 ,  68 . If the cap l 00  is lifted upwardly, the stop faces  112  will engage the lower side of the plate  64  in a mating relation. 
     If desired to destroy the vent cap  100 , a sinker bar can be dropped into the well to break the cap  100 . The cap  100  will be destroyed, permitting a wireline tool to be passed through the openings  38 ,  68  of the plates  32 ,  64 . The cap  100  can be easily replaced by inserting a new cap into the openings  38 ,  68  in the manner described. 
     While various preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are only exemplary and are not limiting. Many variations in modifications of the invention and apparatus disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by this description set out above, but is only limited by the claims which follow, that scope, including all the equivalence of the subject matter of the claims.

Summary:
A mud saver valve is described that features an outer housing that retains upper and lower valve pistons therewithin. The pistons coordinate to provide a check valve so that fluid, such as drilling mud, is permitted to flow in one direction while under pump pressure and works as a relief valve in the event of excessive wellbore pressure when the pump is turned off. Both pistons are provided with apertured plates that selectively define fluid passages through the valve. In the described embodiment, the valve also includes a frangible vent cap that is self-securing and easily replaceable. The cap permits venting of excessive downhole pressures.