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FIELD OF THE INVENTION 
   This invention relates to apparatuses for preventing the loss of drilling mud or other fluids when a top drive unit or kelly are disconnected from a tubular string in order to add additional tubulars to the tubular string or to perform other tasks. 
   BRIEF BACKGROUND 
   When tubulars and/or tubular strings are lowered into or raised out of a wellbore, including, but not limited to, drilling the wellbore, it is common practice, particularly in the oil and gas field, for the tubulars and/or tubular strings to be filled with a fluid or mud. The fluid is typically pumped into the top of the tubular after it has been connected to the tubular string below it and/or as it is being lowered into a wellbore. As the next tubular joint is added to the tubular string, the fluid connection is typically disconnected from the tubular string to allow the next tubular or tubular joint to be connected to the tubular string. When the fluid connection is disconnected, there should preferably to be a valve in place to retain this fluid and prevent it from flowing out onto the work area and environment. The advantages of using such a valve are well known and include saved mud cost, decreased chances of pollution, and increased safety to rig personnel. 
   In the drilling operation, these valves are typically inserted between the kelly and the tubular string. Typical valves of the mud retaining type are illustrated in the following patents: 
   
     
       
             
             
             
           
         
             
                 
                 
             
             
                 
               Patentee 
               U.S. Pat. No. 
             
             
                 
                 
             
           
           
             
                 
               Taylor 
               3,331,385 
             
             
                 
               Garrett 
               3,698,411 
             
             
                 
               Litchfield, et al 
               3,738,436 
             
             
                 
               Williamson 
               3,965,980 
             
             
                 
               Liljestrand 
               3,967,679 
             
             
                 
                 
             
           
        
       
     
   
   All of the above listed patents include a downwardly opening spring loaded poppet type valve enclosed in a body having at least two parts. These two extra pieces in the drill string replace a conventional single piece kelly saver sub, which functions to reduce wear on the kelly pin. The two-part body is generally longer than a standard kelly saver sub and consequently increases the length of the string which must be handled at the rig. In most oil and gas drilling and/or production operations, it is mandatory that a lower manually operated kelly inside blowout prevention (“IBOP”) safety valve be included in the string at all times, which is another addition to the length of the string which must be handled. Thus, on most oil and gas drilling and/or production rigs, where the height of the derrick or mast is usually limited, it may be impossible to include a mud retaining type valve with a two-part body. 
   An additional disadvantage inherent in mud retaining valves with two-part bodies is that the pin of the lower body member replaces the pin of the kelly saver sub and is therefore subject to tremendous wear. This wear limits the longevity of the pin and therefore the longevity of the valve. A solution to this problem has been to insert an additional short sub below the lower body member. However, this solution is not entirely satisfactory because it adds still more length to the string. 
   A further disadvantage of heretofore existing mud retaining valves is in the fact that none of them include means for adjusting the force with which their respective closure members are driven upwardly. The force may be insufficient to close the valve when heavy muds are used. 
   U.S. Pat. No. 4,128,108 to Bill Parker, et. al. is yet another example of a mud saver valve, and shows in its FIGS. 2 and 3 a mud saver valve which, when the mud pumps are on, mud can flow through the interior of the valve, but which closes when the mud pumps are turned off based upon a spring-loaded closure mechanism which does not have the spring strength to close the valve until the mud pumps are turned off. As with this mud saver valve and with the other ones above referenced, once the mud pumps are turned off, the valve closes and the mud saver valve provides its desired purpose, that of preventing the mud from being spilled out onto the rig floor when the tubular string is being broken down. 
   The valves disclosed above are unusable in top drive units. In a top drive unit, space below the top drive and above the tubular string is at a premium and must be kept to a minimum. Typically, the conventional top drive comprises two IBOP valve subs. The upper IBOP sub typically contains a remote controlled shut-off valve and the lower IBOP sub typically contains a manual shut-off valve. These valves are typically utilized to prevent damage from wellbore kicks or pressure surges. However, neither of these IBOP sub valves are automatic. Thus, these valves cannot automatically allow fluid or mud flow into the tubulars and/or tubular string, when the mud pumps are running, or prevent flow through the top drive, when the mud pumps are de-energized or shut down. Further, these IBOP sub valves do not provide a simple monitoring of the pressure in the tubular string connected to the top drive. Further, constant use of the IBOP valves as mudsaver valves may cause premature wear requiring costly repair or replacement; in a worst case, the IBOP valves may not be operable when needed to control the wellbore pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a cutaway view of an improved mud saver valve sub. 
       FIG. 1A  is an illustration of a conventional upper IBOP in a cutaway view. 
       FIG. 1B  is an illustration of a conventional lower IBOP in a cutaway view. 
       FIG. 1C  is an illustration of a conventional upper IBOP in a cutaway view further illustrating a mud saver valve therein. 
       FIG. 1D  is an illustration of a tool for removing/installing a mud saver valve. 
       FIG. 1E  is an illustration of an adapter for the tool illustrated in  FIG. 1D . 
       FIG. 2  is an illustration of a cutaway view of the improved mud saver valve sub, illustrated in  FIG. 1 , further illustrating a mud saver valve therein. 
       FIG. 3  is an illustration of a cutaway view of a mud saver valve retaining ring. 
       FIG. 3A  is an illustration of a special spanner type tool which may be used for the installation and removal of the retaining ring illustrated in  FIG. 3 . 
       FIG. 4  is an illustration of a cutaway view of the mud saver valve body. 
       FIG. 5  is an illustration of a top view of the mud saver valve check valve retainer nut. 
       FIG. 6  is an illustration of a bottom view of the mud saver valve check valve retainer nut illustrated in  FIG. 5 . 
       FIG. 7  is an illustration of a cutaway view of the mud saver valve check valve retainer nut illustrated in  FIG. 5 . 
       FIG. 8  is an illustration of a top view of the mud saver valve. 
       FIG. 9  is an illustration of a side view of the mud saver valve piston. 
       FIG. 10  is an illustration of a cut away view of the mud saver valve piston. 
       FIG. 11  is an illustration of a spring for the mud saver valve and piston. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Referring now to  FIG. 1  there is illustrated a valve sub  10  having a through bore  16  and an upper end with a box connection  12  which is preferably threaded. Further illustrated is the valve pocket  14  wherein preferably is inserted the mud saver valve  21  ( FIG. 2 ). The valve  21  is preferably retained by a conventional snap ring (not shown) which preferably fits in the retaining groove  18 . It should be appreciated that the retainer is not limited to a conventional snap ring but can be a variety of retaining devices including, but not limited to, springs, retaining pins, retaining rings, shear pin, shear screws, screws, rivets, bolts, and the like. Preferably, the valve  21  is retained in a manner to secure the valve body against wellbore pressure kicks while still allowing the removal of the valve  21  without destroying the valve sub  10 . The lower end of the valve sub  10 , preferably, comprises a threaded pin end connection  11 . It should be appreciated that the upper  12  and lower  11  valve sub connections are not limited to a box and pin connection respectively. The connections can be reversed or can comprise other connection methods as necessitated by the tubulars, tools or other equipment that may be attached either above or below the valve sub  10 .  FIG. 2  illustrates the valve  21  positioned within the valve sub  10 . It should be appreciated, by those skilled in the art, that the valve sub  10  is preferably attached at the threaded pin end connection  11  to a tubular and/or tubular string. It should be further appreciated that a tubular is preferably a drill pipe but can also include, but not be limited to, pipe, casing, tubing, other oilfield tools, equipment and tubulars, and the like and thus a tubular string is preferably such multiple tubulars connected together. 
     FIG. 1A  illustrates a conventional upper IBOP  29 U. It should be appreciated that  FIGS. 1A–1C  are for clarification purposes only and are not intended to be a detailed depiction of IBOP&#39;s but more for aiding in the description of the interaction of the present apparatus with such conventional IBOP&#39;s.  FIG. 1B  illustrates a conventional lower IBOP  29 L. For simplicity, both IBOP&#39;s are shown with a conventional internal block valve  29   a  and corresponding stem  29   b . The upper IBOP  29 U is typically configured for remote operation and is illustrated herein with a conventional remote controlled operator  70 . The upper IBOP  29 U is typically attached to a conventional top drive  71 . Typically, both the upper  29 U and the lower  29 L IBOP each have a box end  34  and pin end  35 . It should be appreciated that the end connections of the IBOP&#39;s can vary and should not be viewed as a limitation of the present apparatus. 
     FIG. 1C  illustrates the placement of the valve  21  within an upper IBOP  29 U. Preferably, the valve  21  is inserted through the bottom or pin end  35  of the upper IBOP  29 U. However, with different configurations, of an upper IBOP, the valve  21  may also be inserted in the top of an IBOP. Preferably, the valve  21  is retained within the upper IBOP  29 U by a suitable retainer in retaining groove  18   a . As with the retainer for the valve  21  within the valve sub  10 , it should be appreciated that the method of retention can include, but is not limited to, snap rings, springs, retaining pins, retaining rings, shear pin, shear screws, screws, rivets, bolts, and the like. It should be further appreciated, by those in the art, that it would preferably be more convenient to retain the bottom of the valve  21  as opposed to the top as in the valve sub  10 . However, either place of retention could satisfy the need for detachably retaining the valve  21  within the upper IBOP  29 U. 
   It should be appreciated that the valve  21  is not limited to only placement within valve sub  10  or within the upper IBOP  29 U. Valve sub  10  can be installed below a conventional upper IBOP  29 U and above a conventional lower IBOP  29 L. In such an embodiment, upper connection  12  will preferably be threadably connected to the lower end of the upper IBOP  29 U and lower connection  11  would preferably be threadably connected to the lower IBOP  29 L. It should further be appreciated that in order to save available vertical length space, the valve sub  10  and the valve  21  may replace the upper IBOP  29 U. Thus, only the lower IBOP  29 L would be utilized. Still further, the valve  21  may be placed directly into the top or bottom of an upper IBOP  29 U which has been modified to enclose the valve  21  ( FIG. 1C ). 
   The upper  29 U and lower  29 L IBOP&#39;s allow the insertion of certain tools or wireline equipment into the tubular string. Should a need arise, for such insertion, the valve  21  will preferably be removed. If the valve  21  is positioned within the valve sub  10 , then preferably the valve sub will be removed. If the valve  21  is carried within the upper IBOP  29 U, the valve  21  is preferably removed using a special tool  40  ( FIG. 1D ). 
   The special valve removal tool  40  ( FIG. 1D ) preferably comprises an all thread shaft  41  having a “T” handle  42 , at one end and a reduced threaded portion  43  on the end opposite of the “T” handle  42  which is preferably made up into the top connection  26  ( FIGS. 4 and 8 ) or into a removal tool adapter  47 . It should be appreciated that the “T” handle  42  is preferably not for turning the tool, other than for threadedly engaging the valve removal tool  40 . The preferred function, of the “T” handle  42  is to aid in maintaining physical control of the valve  21  during installation or removal particularly when installing or removing the valve  21  from a suspended sub or IBOP. 
   When used, the removal tool adapter  47  is preferably threadedly attached to the internal threads  37  of the retaining ring  30  ( FIG. 3 ).  FIG. 1E  illustrates the removal tool adapter  47 . Preferably, the removal tool adapter  47  will comprise an external threaded area  48  which is adapted to threadably connect to the internal threads  37  of the retaining ring  30 . Further, the removal tool adapter includes an internally threaded connection  49  which can threadedly mate with the reduced threaded portion  43  of the special valve removal tool  40 . It should be appreciated that the adapter  47  is utilized when removing the valve  21  from the bottom end of a sub or IBOP. 
   Referring again to  FIG. 1D , it is illustrated that preferably the tool  40  further comprises a pull plate  45  and a hex bushing  46 , which when preferably turned clockwise, applies pressure against the pull plate  45  causing the valve  21  to be removed from the valve sub  10  or the upper IBOP  29 U. It should be appreciated that, although the preferred design of the tool  40  is illustrated, other variations of the tool  40  could be envisioned with the goal of removing the valve  21  and are within the scope of this invention. An example, which is not intended as limiting, the pull plate  45  may comprise threads on the external circumference (not shown) which may engage the box threads of a sub or IBOP to aid in the valve  21  installation or removal. It should further be appreciated that the use of the tool  40  is not intended to only be limited for use in conjunction with the upper IBOP  29 U or the valve sub  10  but can be utilized, as is or somewhat modified, to remove the valve  21  from substantially all installations of the valve  21 . For example, but not in a limiting sense, a nut  44  may be fixedly attached at some predetermined distance from the reduced threaded portion  43 . The nut  44  may allow for the tool  40  to be threadedly locked onto the valve  21  at connection  26  ( FIGS. 4 and 8 ) or to the removal tool adapter  47  at connection  49 . Still further, it should be appreciated that the tool  40  can be used to remove the valve  21  from either the top or the bottom of a valve sub or an IBOP. Preferably, the removal tool adapter  47  is used when removing the valve  21  from the bottom of a valve sub or IBOP. It should be noted that although the preferable use of the tool  40 , with or without the adapter  47 , is to remove the entire valve  21 , it is envisioned that an adaptation of the tool to only remove parts of the valve  21  is within the scope of this invention. 
   It is also envisioned that the valve sub  10 , with the valve  21 , may be used between a conventional kelly and the tubular string being lowered into the wellbore. The preferable advantage, is that the valve sub  10  will provide a much more compact design primarily by conserving the valuable vertical space on the rig described herein above. 
     FIG. 3  illustrates the retaining ring  30  which preferably fits in the bottom of the valve body  20  ( FIG. 4 ) in the bottom threaded area  22 . Preferably, a set screw  90  may be used to prevent the retaining ring  30  from rotating after installation. It should be appreciated that although a set screw is preferred, a variety of fasteners including, but not limited to, rivets, shear pins, shear screws, bolts, and the like may be used. The upper surface  31  may preferably contact spring  32  ( FIG. 11 ). It should be appreciated that when desired upper surface  31  may not directly contact the spring  32  and may instead include a type of spacer, washer, gasket, or similar element between the upper surface  31  and the spring  32 . Further, the upper surface  31 , may be coated or treated so as to have a harder surface for contact with the spring  32  in order to avoid undesired wear between the upper surface  31  and the spring  32 . Preferably, after installation or assembly, the bottom surface  33 , of the retaining ring  30 , will not protrude out beyond the bottom surface  23  of the valve body  20 . The retaining ring will preferably perform at least two functions including, but not limited to, retaining the spring  32  within the valve body  20  and also compressing the spring  32  such that the force of the spring  32  against the piston  50  ( FIG. 9 ) urges the piston  50  in an axial direction toward the top of the valve body  20 . (See  FIG. 2  illustrating a general arrangement of the valve  21 ). The above will be more fully described herein below. 
   Still referring primarily to  FIG. 3  and secondarily to  FIG. 3A , the retaining ring  30  can be threaded in and/or removed, from the valve body  20  utilizing a special spanner type tool or wrench  62 . Such special spanner type tool  62  may be substantially tubular or another shape such that the pins  65  would preferably be inserted into holes  36  located on the bottom surface  33 . Preferably, the special spanner type tool  62  is turned clockwise while pressure is applied to compress the spring  32  ( FIG. 11 ) to aid in installing the retaining ring  30 . Preferably, the retaining ring  30  is inserted, into the valve body  20 , a certain pre-determined distance to create a pre-calculated compression on the spring  32 ; thus preferably providing the required spring force against the piston  50  ( FIG. 9 ). To remove the retaining ring  30 , the special spanner tool  62  is preferably rotated in a counter clockwise direction while the tool  62  preferably aids in controlling the de-compression of the spring  32 . It should be appreciated, by those in the art, that the dimensions and number of the removal holes  36  are dependant on the dimensions of the retaining ring  30  and can be increased if so desired and/or required. It should be understood that directions of rotation, disclosed herein, are the preferred directions of rotation and should not be viewed as a limitation on the operations of the present apparatus nor the scope of the invention. It should be noted that the pressure, to be applied to maintain the spring pressure during the installation or removal of the retaining ring  30  may be preferably attained through the use of a conventional hydraulic cylinder (not shown). In such an embodiment, the conventional hydraulic cylinder would preferably exert pressure on the special spanner tool  62 , towards the spring. The hydraulic cylinder would preferably be in communication with shaft  64 . This pressure, would preferably counter the spring force exerted on the retaining ring  30  and thus allow the retaining ring to be rotated. The rotation is preferably supplied through the special spanner tool  40  by inserting a pin, rod, stem, or the like, into the rotation holes  63  of the special spanner tool  62 , or by the use of a wrench across the flats  66 . It should be appreciated that the configuration of the special wrench  62  can vary and that it is deemed within the spirit of this invention. Examples of such may include, but are not necessarily limited to, the location, shape, and number of pins  65 , the length and shape of shaft  64 , the location and type of wrench connection  66 , and the location, size and shape of the rotational holes  63 . 
   Referring now to  FIG. 4 , a cutaway view of the valve body  20  is illustrated. Preferably, grooves  24  are provided for conventional seals, such as, but not limited to, o-rings (not shown). The seals prevent fluid or pressure leakage around the valve body  20  when it is installed in a sub, such as valves sub  10  or in the upper IBOP  29 U. Preferably, the upper surface  81  comprises ball check valves  80  and flow slots  82  which will be described in more detail herein below. The upper surface  81  will preferably further comprise a threaded female connection  26 . Connection  26  is preferably used to raise and lower the valve  21 . It should be understood that lifting and lowering of the valve includes the installation and removal of the valve. It should further be understood that although the preferred configuration of connection  26  is having female threads, any variety of connections could be used to such as but not limited to eye bolts, hooks, internal lift slots, internal lugs, and the like and that such modifications would also include modifications of any installation and removal tools such as the removal tool  40  ( FIG. 1D ). 
     FIG. 4  further illustrates a cutaway view of check valve  80 , whose function will be described more fully herein below. The check valve  80  preferably includes, but is not limited to, a ball  83 , a ball or valve seat  86 , and a ball retainer  84 . Preferably, the check valve  80  has a throughbore  87  which passes through the ball retainer  84  ( FIG. 7 ) and the upper surface  81  of the valve body  20 . The check valve  80  preferably seals against flow moving from the upper surface  81  toward the lower surface  23 . 
     FIGS. 5 ,  6 , and  7  illustrate several views of the ball retainer  84  of the check valve  80 . The check valve seat  86  is preferably integral to the valve body upper end  81 . The ball  83  is retained in the check valve  80  by the ball retainer  84 .  FIG. 5  illustrates a top view of the ball retainer  84  and further illustrates the preferred internal hex drive for removal and installation.  FIG. 6  illustrates a bottom view of the ball retainer  84  and further illustrates flow channels  85 . The flow channels  85  preferably allow flow in the upward direction to pass through the ball retainer throughbore  87 . It should be appreciated that the flow channels  85  are provided so as to prevent the ball  83  from plugging the throughbore  87  as it passes through the ball retainer  84 .  FIG. 7  illustrates a side view of the ball retainer  84 . 
   It should be understood that the preferred purpose of the check valve  80  is to prevent flow in one direction. Therefore, many varieties of such a check valve can be envisioned within the spirit of this invention. Such variations may include, but are not limited to, a drive configuration, for the ball retainer  84 , that is not hexagonal, a shaped plug as opposed to as ball, a one piece pocket type valve that can be inserted in the valve body upper end  81 , a multi-piece check valve, an external check valve, or a bypass which might eliminate the need for the check valve. 
     FIG. 8  illustrates a top view of the upper end  81  of the valve body  20 . This view illustrates further detail of the connection  26 , the check valves  80  and flow slots  82 . As can be seen in  FIG. 8 , the preferred number of check valves  80  is three. It should be appreciated that this arrangement provides advantages such as a redundancy feature wherein a check valve  80  can become plugged from the fluid passing therethrough or the ball  83  may deform in such a matter as to plug the throughbore  87  passing through the ball retainer  84 . A further advantage may be to allow required flow rates or flow volumes to pass through the multiple check valves, particularly if one of the check valves is blocked or otherwise prevents flow. It should be understood that the preferred purpose for the through bore  87  is to allow fluids to pass from portions of the wellbore below the mudsaver valve  21  up through the piston bore  52  and out through the throughbore  87  up to the earth&#39;s surface to allow the wellbore pressure to be measured. Therefore, it should be appreciated that the availability of three such passages preferably insures that such fluids will be accessible, from the wellbore, even give a situation wherein one passage becomes blocked. 
     FIG. 9  is a side view illustration of the valve piston  50 .  FIG. 10  illustrates a cutaway view of the piston  50 . Referring to  FIGS. 9 and 10 , the piston has a through bore  52  which preferably allows the flow of fluid or mud into and out of the wellbore. The piston  50  further comprises at least one seal groove  54 . The seal groove  54 , is preferably fitted with seal, such as, but not limited to, an o-ring (not shown). The o-ring preferably seals against the valve body internal cavity  28  ( FIG. 4 ). It should be appreciated that the specific type of seal is dependant on the fluid environment and thus should not be seen as being limited only to an o-ring, an elastomeric seal, or a single seal. The piston  50 , preferably, still further comprises an upper surface  56 , a lower surface  57 , and a spring contact surface  58 . 
   Referring still to  FIG. 10 , the piston  50  may preferably comprise an erosion and/or corrosion resistant insert or inserts. In one embodiment, a first insert  59  may form a part of the upper surface  56  and an upper portion of the piston  50  bore wall. A second insert  60  may be fixedly attached, forming a portion of the piston bore  52 , below the first insert  59 . It should be appreciated that these inserts are preferably of a hard metallic material such as, but not limited to, tungsten carbide. Further, the first  59  and second  60  inserts may be combined as a single insert. Still further, the insert or inserts may be attached by welding, may be threaded into the upper piston bore, or attached by another industry acceptable method. It should also be further appreciated that the piston  50  could be entirely made or cast of an erosion resistant material or that the hard metal surface could be deposited onto the piston by welding, spraying, coating, or the like; thereby eliminating the need for the inserts  59 ,  60 . It should be understood, by those in the art, that the lower surface  89 , of the upper end  81  of the valve body  20  ( FIG. 4 ), may also be coated with a hard surface, such as, but not limited to, tungsten carbide. The preferred purpose of harder metallic surface is to prevent pre-mature failure of the piston and/or the corresponding valve body contact surface  89  due to excessive erosion from the fluid flow. 
   It should be appreciated that in order to allow the necessary flow, through the valve  21 , the piston bore  52  must be sufficiently large to allow the necessary flow rate. Preferably, the piston bore  52  will have some pre-determined flow area or cross-sectional area. This cross-sectional area or flow area is preferably sized so that a pre-determined flow is allowed through the piston bore. This flow rate is, in turn, preferably based upon the necessary flow of fluid as required to be introduced into the tubular string attached downstream of the valve  21 . To that end, the flow slots  82 , of the upper surface  81 , are preferably sized such that the total flow area or total cross-sectional area, of all of the flow slots  82 , is at least equal to or greater than the flow area or cross-sectional area of the piston bore  52 . It should be noted that the cross-sectional area, or flow area, of each slot  82  is preferably pre-determined before the flow slots  82  are manufactured. This will preferably insure that the sum of the flow slot  82  cross-sectional areas is greater than or substantially equal to the cross-sectional area of the piston bore  52 . Thus, preferably there will be no substantial flow restriction or reduction, due to flow area reduction, caused by the installation of the valve  21 . 
   It should be appreciated that the materials of construction, of the valve sub  10 , the valve  21 , and all of its parts are known in the industry and are preferably metallic with the possible exception of the seals. However, as described herein above, some of the metals are harder or are coated with a harder substance to resist erosion. The specific choice of materials is preferably dependant on the environment to resist erosive and corrosive attack and to resist deformation from pressure or contact, as well as for compatibility with parts that are in contact with each other. 
   For operation, the valve is assembled, in no particular order, but as described herein below. The check valve balls  83  are inserted into the ball cavity  88  ( FIG. 4 ). The ball retainers  84  are preferably threaded into the ball cavity  88  above the check valve balls  83 . The piston is fitted with the selected seal in the seal groove  54 . The piston  50  is fitted inside the valve body cavity  28 . The spring  32  ( FIG. 6 ) is slidably mounted over the piston and inside the valve body cavity  28 . Preferably, the spring  32  contacts the spring contact surface  58  of the piston  50  ( FIG. 10 ). The retaining ring  30  is preferably threadedly engaged into the bottom threaded area  22  of the valve body  20 . When fully threaded, the retaining ring will preferably compress the spring  32  some pre-determined amount. The compressed spring  32  will preferably exert a pre-calculated force on the piston  50 . This pre-calculated force is preferably sufficient for the piston to block the flow of fluid or mud through the flow slots  82  when the mud pumps are de-energized but to allow the piston to be pushed down to allow flow through the flow slots  82  when the mud pumps are energized (i.e. the force of the mud pumps will preferably overcome the spring force exerted upwardly on the piston  50 ). Further, the fluid or mud, will maintain the balls  83  in contact with the check valve seat  86 , whether the mud pumps are energized or de-energized, thus preventing any flow downward into the valve  21 . It should be appreciated that the retaining ring  30  ( FIG. 3 ) will preferably allow some adjustment of the spring  32 . Thus, the retaining ring  30  can increase or decrease the spring force, exerted against the piston surface  58  ( FIGS. 9 and 10 ) by being further engaged into the bottom threaded area  22  ( FIG. 4 ) or disengaged out of the bottom threaded area  22 . 
   The valve  21  is fitted with the selected seals in the seal grooves  24  ( FIG. 4 ). If the valve  21  is inserted into the valve sub  10 , then a suitable retainer is placed in retaining groove  18  ( FIG. 1 ) to preferably retain the valve  21  within the valve sub. If the valve  21  is inserted into the upper IBOP  29 U ( FIG. 1C ), the valve  21  is preferably inserted through the top or bottom end of the upper IBOP  29 U. A suitable retainer is placed in a retaining groove  18   a  located in the bottom of the IBOP  29 U to preferably retain the valve  21  within the upper IBOP  29 U. It should be appreciated that when installed into the top of a sub or IBOP, the retainer may be above, below, or on both ends of the valve  21 . 
   When the mud pumps or other fluid pumps are energized and/or operating, the mud or fluid will preferably flow through the flow slots  82 . The pressure of the pumped fluid or mud will preferably overcome the spring force of spring  32  and urge the piston  50  in a downward direction. As the piston  50  moves away from contact with the piston sealing surface  89  ( FIG. 4 ) the fluid or mud will flow through the piston bore  52  and into the tubular and into the wellbore. It should be understood that preferably there is no flow through the check valves  80  as the balls  83  are seated in the check valve seats  86 . 
   Whenever the mud pumps are shut down or de-energized the spring pressure, exerted by spring  32 , will preferably urge the piston  50  up against the piston contact surface  89  of the valve body  20 . Thus, when the tubular joints are broken out, the mud is prevented from passing through the valve  21  preferably because of the seal formed between the piston contact surface  89 , of the valve body  20 , and the upper surface  59 ,  56  of the piston  50 . 
   Although the mud saver valve  21 , according to the present invention, is substantially shut in when the mud pumps are turned off or de-energized, the downhole pressure of the fluids can be measured by the fact that the balls  83 , of the check valves  80 , are moved off of their engagement with the seats  86  because there is no longer any pressure or flow, from the mud or fluid pumps, being exerted on the balls  83  in a downward direction. It should be appreciated that there is some pressure existing above the balls  83 . However, this is typically only a static or head pressure that is a factor of line size and length directly above the check valves  80  on which gravity would act. Therefore, any significant pressure, in the wellbore, would over come this static pressure and move the balls  83  off of the seats  86 . Thus, the fluid or mud can flow through the check valve flow bores  87  and the pressure and other parameters related to the downhole fluids can be measured. It should be appreciated that this action only allows flow in one direction. Therefore, if the wellbore pressure falls below the mud or fluid pressure above the check valves  80 , the balls  83  will preferably return to the seats  86  and block any flow into the tubular below the valve  21 . 
   From the foregoing, it can be seen that the present invention is one well adapted to seal against mud loss particularly in top drive assemblies and in conjunction with a conventional kelly while reducing axial length, allowing full fluid flow, and allowing measurements of desired parameters of the fluid or mud. It should be appreciated that certain embodiments of the present invention are not limited to specifically interact with top drive assemblies, they can likewise be adapted to kelly subs or set between the kelly sub and the tubular string as required or desired. It should be further appreciated that other advantages which are obvious and which are inherent to the present invention should not be limited by the examples presented in the foregoing descriptions. It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of the claims. 
   As many possible embodiments may be made of this invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Summary:
A mud saver valve being operable in conjunction with a top drive unit to retain mud in the top drive unit when a tubular is disconnected therefrom. The valve utilizes a spring-loaded piston to control the flow of mud or other fluid onto the work area and environment while the top drive unit is being connected to the new tubular and re-connected to the original tubular string. The valve further comprises multiple check valves for evaluating as well as monitoring wellbore pressure. The valves provides for full bore flow passages for the mud or fluid being pumped into a tubular fluidly connected to the top drive.