Abstract:
A valve cap for sealing an aperture in a pump; a plugging assembly for a valve cap to seal at an aperture in a pump; a docking unit for use with a plugging assembly for sealing an aperture in a pump; and a method of sealing an aperture in a pump. A valve cap ( 10 ) comprises a valve plug ( 16 ) including, a first engaging means ( 40 ); a compression unit ( 20 ) including a plurality of springs ( 56 ) to apply a compressive load upon the plug and second engaging means ( 62, 64 ); a docking unit ( 24 ) for landing on the compression unit, including one or more pistons ( 92 ) to apply a compressive load upon said springs and third engaging means ( 74, 76, 78 ) to lock said plug to said compression unit and seal said plug against said aperture.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application is a continuation of U.S. Ser. No. 12/853,000, filed Aug. 9, 2010, which is a continuation of U.S. Ser. No. 11/568,911, filed May 4, 2005, now U.S. Pat. No. 7,770,509, which is a national stage application of PCT/GB05/01673, filed May 4, 2005. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    Not applicable 
       INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0004]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0005]    (1) Field of the Invention 
         [0006]    The present invention relates to valve caps and in particular, though not exclusively, to a valve cap for use on a hole in a mud-pump fluid-end module. 
         [0007]    (2) Description of Related Art including information disclosed under 37 CFR 1.97 and 1.98 
         [0008]    In the oil industry mud pumps are used to pump viscous fluids, such as drilling muds, cement, or other well fluids. Although mud pumps may be either centrifugal or reciprocating type pumps, typically mud pumps are reciprocating pumps using one or more pistons and hydraulic cylinders with liners to generate the high pressures required to pump these viscous fluids in and out of the well. 
         [0009]    Mud pumps include a fluid end and a power end. At the fluid end, low pressure fluid is drawn in and built-up by compression via a pump piston and check valves, until the pressure overcomes well bore pressure so as to pump the mud into the well. The power end contains the gears that reciprocate the pump piston. It will be appreciated that parts within the pump exposed to the fluid and its associated pressure are liable to wear easily. In particular sufficient seals need to be provided at unused inlets/outlets and at the valves. 
         [0010]    These seal covers are typically referred to as valve caps or valve covers. They must provide a seal while closing off the aperture of an end piece at the fluid end of the pump. 
         [0011]      FIG. 1  shows a prior art valve cap A for use with a pump as supplied by Southwest Oilfield Products, Inc, Houston, Tex., USA. A valve plug B is located against a step in the aperture C of an end piece D. A seal E is provided between the parts. The seal is maintained by pressure from a cap body F located against it. The body F is screwed in place through a locking member G attached to the end piece D at an end face H. Once located the locking member G is forced against the end face H by using a stud rods J and retention nuts K, L as is known in the art. This movement is transferred to the body F via the screw threads and effectively locks the body F against the plug B. When the cap A needs to be removed the nuts K, L are released and a steel bar is inserted through a guide hole M in the body F and turned to remove the body F and release the plug B 
         [0012]    A disadvantage of this valve cap is in the use of threaded connections. It is difficult to determine if the threads are correctly tightened. During mud pump operation, the reciprocating nature and peak pump pressures acts on any insufficiently tightened connections, resulting in a tendency for the valve cap to gradually loosen. Alternatively, the threaded connections have been over tightened, making it even more difficult to unthread. Additionally, in using a steel bar it is often necessary to hammer the bar to release the cap. Such activity is obviously dangerous. In some regions of the world local laws prohibit the use of sledge hammers for personnel safety reasons. 
         [0013]    To overcome these problems a spring based retaining valve cap has been developed. This valve cap is illustrated in  FIG. 2  and is supplied by P-Quip Ltd., Linwood, Scotland. Like parts to those of  FIG. 1  have been given the same reference. In this cap, the body F is forcibly pushed against the plug B by a number of piston and spring arrangements located in the locking member G. The member G is initially bolted to the end piece D at the face H by bolts M. Each arrangement comprises a cylinder N adapted to house a slidable piston P and clamping springs Q. The piston P has a threaded rod R extending outwith the cylinder N and through the body F. A retaining nut K is located on the threaded rod R. In use, the cap is assembled as shown in  FIG. 2  with the nuts K on the threaded rods R. Hydraulic fluid is then inserted between the piston P and the cylinder base, such that the piston P is extended to a greater extent outwith the cylinder N and the nut K is tightened further against the body F. The hydraulic pressure is then released and the springs Q apply their force to the plug B through the rods R, the nuts K and the body F. 
         [0014]    A disadvantage of this cap is in the large dimensions of the cap and the respective face on the end piece required. This is because the space must be available both for bolts to connect the locking member to the end piece, and for the cylinders in which the pistons are housed. As a result these caps are generally limited to a maximum of four cylinders which has the disadvantage of causing an uneven pressure to be applied to the body. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    It is an object of the present invention to provide a valve cap which uses a spring clamping force to hold a plug within an aperture of a fluid end of a pump. 
         [0016]    It is a further object of the present invention to provide a valve cup in which a distributed compressive force is applied to the plug. 
         [0017]    According to a first aspect of the present invention there is provided a valve cap for sealing an aperture in a pump, the cap comprising: 
         [0018]    a valve plug for locating against a wall of the aperture, the plug including a first engaging means; 
         [0019]    a compression unit fastened to said pump, the compression unit including a plurality of springs to apply a compressive load upon the plug and second engaging means; 
         [0020]    a docking unit for landing on the compression unit, the docking unit including one or more pistons to apply a compressive load upon said springs and third engaging means; 
         [0021]    wherein said first and third engaging means sequentially interlock with said second engaging means to lock said plug to said compression unit and seal said plug against said aperture. 
         [0022]    By locking the compression unit to the plug, the docking unit can be removed to be used on any number of compression units and plug combinations. Additionally as the pistons are independent of the springs, a large number of springs can be used to distribute load on the plug without the need to find space for the pistons. The large number of springs also allows maximum uplift on the plug (due to mud pressure incl. peak pressures) to be fully restrained. 
         [0023]    Preferably said engaging means comprises one or more keyed profiles. Preferably the first and third engaging means comprise cogs. Advantageously the second engaging means comprises a cylindrical surface on which is arranged internally facing teeth. The teeth may match teeth on the cogs. Preferably also two rows of teeth are provided on the compression unit such that a cog can pass one row of teeth and by rotation be interlocked between the rows of teeth. 
         [0024]    Preferably the plug further comprises upper and lower members. Preferably also the plug includes a first seal arranged around an outer surface of the plug. 
         [0025]    Advantageously the first seal is tapered. Preferably also there is a second seal between the members. Preferably the plug includes an elongate member arranged parallel to a base of the plug. The elongate member may be used to engage a tool for turning the plug within the aperture. 
         [0026]    Preferably the compression unit further comprises an upper plate and a lower plate, the plates sandwiching the plurality of springs. Preferably also fastening means is provided through each plate to attach the plates to the pump. Advantageously the fastening means are stud rods, each passing through a spring and including a retaining nut at one end. Preferably the lower plate comprises the second engaging means. 
         [0027]    Preferably the docking unit further comprises a stem, the stem having a longitudinal bore therethrough for access to the plug, a locating plate including a plurality of recesses for locating on the fastening means and one or more cylinders, the/each cylinder including a piston, the piston extending from the cylinder to impact a tensioning disc located on the stem. Preferably the third engaging means is located at a lower end of the stem. Preferably a locking nut is located on the stem adjacent the tensioning disc. Advantageously there are one or more ports through which hydraulic fluid can enter the one or more cylinders. Preferably an upper end of the stem includes a pair of radially aligned apertures through which a bar may be passed to rotate the stem. 
         [0028]    Preferably the valve cap further comprises a locking tool, the locking tool being used to interlock the first engaging means to the second engaging means. Preferably the locking tool comprises a barrel suitable for locating through the stem and a hook arranged to engage the elongate member. 
         [0029]    According to a second aspect of the present invention there is provided a plugging assembly for use in a valve cap to provide a seal at an aperture in a pump, the assembly comprising: 
         [0030]    a valve plug for locating against a wall of the aperture, the plug including a first engaging means; 
         [0031]    a compression unit fastened to said pump, the compression unit including a plurality of springs to apply a compressive load upon the plug and second engaging means; 
         [0032]    wherein said first and second engaging means interlock when the springs are in full compression and remain locked when the springs are released. 
         [0033]    Preferably said engaging means comprises one or more keyed profiles. Preferably the first engaging means comprise cogs. Advantageously the second engaging means comprises a cylindrical surface on which is arranged internally facing teeth. The teeth may match teeth on the cogs. Preferably also two rows of teeth are provided on the compression unit such that a cog can pass one row of teeth and by rotation be interlocked between the rows of teeth. 
         [0034]    Preferably the plug further comprises upper and lower members. Preferably the members are joined together. Preferably also the plug includes a first seal arranged around an outer surface of the plug. Advantageously the first seal is tapered. Preferably also there is a second seal between the members. Preferably the plug includes an elongate member arranged parallel to a base of the plug. The elongate member may be used to engage a tool for turning the plug within the aperture. 
         [0035]    Preferably the compression unit further comprises an upper plate and a lower plate, the plates sandwiching the plurality of springs and the upper plate including a plurality of surfaces on which a compressive load can be applied. Preferably also fastening means is provided through each plate to attach the plates to the pump. 
         [0036]    Advantageously the fastening means are stud rods, each passing through a spring and including a retaining nut at one end. Preferably the lower plate comprises the second engaging means. 
         [0037]    According to a third aspect of the present invention there is provided a docking unit for use with a plugging assembly for sealing an aperture in a pump, the unit comprising: 
         [0038]    a plurality of surfaces for landing on a compression unit of a plugging assembly; 
         [0039]    one or more pistons to apply a compressive load upon said compression unit and third engaging means; 
         [0040]    wherein said third engaging means interlocks with second engaging means of said compression unit during compression of said unit. 
         [0041]    Preferably said engaging means comprises one or more keyed profiles. Preferably the third engaging means comprise cogs. Advantageously the second engaging means comprises a cylindrical surface on which is arranged internally facing teeth. The teeth may match teeth on the cogs. Preferably also two rows of teeth are provided on the compression unit such that a cog can pass one row of teeth and by rotation be interlocked between the rows of teeth. 
         [0042]    Preferably the docking unit further comprises a stem, the stem having a longitudinal bore therethrough for access to the plug, a locating plate including a plurality of recesses for locating on the fastening means and one or more cylinders, the/each cylinder including a piston, the piston extending from the cylinder to impact a tensioning disc located on the stem. Preferably the third engaging means is located at a lower end of the stem. Preferably a locking nut is located on the stem adjacent the tensioning disc. Advantageously there are one or more ports through which hydraulic fluid can enter the one or more cylinders. Preferably an upper end of the stem includes a pair of radially aligned apertures through which a bar may be passed to rotate the stem. 
         [0043]    Preferably the docking unit further comprises a locking tool, the locking tool being used to interlock a first engaging means of the plugging assembly to the second engaging means. Preferably the locking tool comprises a barrel suitable for locating through the stem and a hook arranged to engage the elongate member. 
         [0044]    According to a fourth aspect of the present invention there is provided a method of sealing an aperture in a pump, the method comprising the steps: 
         [0045]    (a) locating a valve plug against a wall of the aperture; 
         [0046]    (b) fixing a compression unit to an end face of the pump around the aperture; 
         [0047]    (c) landing a docking unit on the compression unit; 
         [0048]    (d) by rotating a portion of the docking unit, locking the docking unit to the compression unit; 
         [0049]    (e) applying a compressive load from the docking unit on the compression unit to compress a plurality of springs within the compression unit; 
         [0050]    (f) tightening a plate over the compressed springs; 
         [0051]    (g) locking the valve plug to the compression unit by rotating the valve plug; and 
         [0052]    (h) removing the docking unit and thereby removing the compressive load. 
         [0053]    Preferably the valve plug, compression unit and docking unit are according to the first aspect. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0054]    An embodiment of the present invention will now be described, by way of example only, with reference to the following Figures of which: 
           [0055]      FIG. 1  is a cross-sectional view through a prior art screw-type valve cap; 
           [0056]      FIG. 2  is a cross-sectional view through a prior art spring-over-piston type valve cap; 
           [0057]      FIG. 3  is a cross-sectional view through a valve cap according to an embodiment of the present invention; 
           [0058]      FIG. 4  is a plan view of a valve plug of the valve cap of  FIG. 3 ; 
           [0059]      FIG. 5  is a plan view of a compression unit of the valve cap of  FIG. 3 ; 
           [0060]      FIG. 6  is a plan view of a docking unit of the valve cap of  FIG. 3 ; 
           [0061]      FIG. 7  is a plan view of a turning tool for the valve cap of  FIG. 3 ; and 
           [0062]      FIG. 8  is an enlarged view of the head of the turning tool of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0063]    Reference is initially made to  FIG. 3  of the drawings which illustrates a valve cap, generally indicated by reference numeral  10 , according to an embodiment of the present invention. Valve cap  10  is used for sealing an aperture  12  at a fluid end  14  of a pump. Cap comprises a valve plug  16  which locates against a wall  18  of the aperture  12 , a compression unit  20  which is fastened to the end  14  via stud rods  22  and a docking unit  24  for landing on the compression unit. 
         [0064]    Reference is now made to  FIG. 4  of the drawings which illustrates the valve plug  16  in greater detail. Plug  16  comprises a two part cylindrical body having upper body  26  and a lower body  28 . The bodies  26 ,  28  are bolted together via bolts  29   a - d  and a seal  30  is provided at the join to prevent the ingress of fluid there between. At the join is also located a plug seal  32  which is arranged longitudinally on an outer surface  34  of the plug  16 . 
         [0065]    An upper end of the outer surface  34  together with a top surface  36  provides a keyed profile, generally indicated by reference numeral  38 . The keyed profile  38  comprises four extensions or lugs  40   a - d  equidistantly spaced around the outer surface  34 . Each extension  40  has a longitudinally arranged portion which meets a step, that is a protrusion radially outwards from the outer surface  34 . Above the step is a planar top surface  36  on which is arranged a raised profile having four teeth extending outwards to the step with each meeting a side of the extension. From an apex of each tooth a longitudinally aligned sweeping surface, perpendicular to the top surface  36 , provides a shelf above each protrusion. Each of the four sweeping surfaces meets the outer surface  34  at an end opposite the apex. The lugs  40  upon the surfaces  34 ,  36  can be considered to comprise a cog. 
         [0066]    On the top surface  36  there is further a central recess  42  into the upper body  26 . At an upper end of the recess  42 , but located totally within the recess  42  is a bar  44 . Bar  44  is cylindrical and located off-centre to the recess  42 . 
         [0067]    Reference is now made to  FIG. 5  of the drawings which illustrates the compression unit  20 . Unit  20  comprises two plates or rings  46 ,  48 . The upper ring or static ring  46  has twelve apertures  50  arranged equidistantly around its surface which provide longitudinal clearance bores through the ring  46 . Although twelve apertures are shown, any number may be selected to suit the dimensions of the ring  46  while providing a sufficient number to effectively spread loading through the unit  20 . Thus there is always likely to be more than four apertures  50 . 
         [0068]    The lower ring or compression ring  48  has matching apertures so that stud rods  22  can be passed from an upper end  52  of the unit to a lower end  54  of the unit. Mounted on each stud bolt  22  is a compression spring  56 . The compression springs  56  are sandwiched between the rings  46 ,  48 . At the upper end  52 , each threaded stud bolt  22  includes a stud nut  58  which can be tightened against the upper end  52  around each aperture  50 . Further, on an inner surface  60  there are arranged two rows of lugs  62 ,  64 . Each row has four equally spaced lugs circumferentially thereon. 
         [0069]    The plug  16  and the compression unit  20  can be considered as a valve plug assembly as together they provide the parts to plug the aperture  12  in the end  14 . The docking unit  24  can be considered as an additional part which activates the plug assembly when in position. 
         [0070]    Reference is now made to  FIGS. 3 and 6  of the drawings to describe a docking unit  24 . The docking unit  24  comprises a number of parts located on a central stem, or active lock stem  66 . The stem  66  is a hollow cylindrical body  68  which provides a bore  70  through the unit  24  and its outer surface has threaded portions against which components of the unit can be threaded. 
         [0071]    At a lower end  72  of the stem  66  there is a flange referred to as an active lock  74 . Active lock  74  is threaded to the stem  66 . The lock  74  provides a funnel  76  which flares outwards to provide a surface on which four outwardly facing lugs  78  are equidistantly arranged. On an upper surface of an end of one lug is a peg, referred to as a lock stop  80 . 
         [0072]    At the upper end of the stem  66  are two oppositely arranged bore holes  67  in the side wall of the body  68 . This is to allow a bar to be inserted through the bore holes  67  to assist in turning the stem  66  in the valve cap  10 . 
         [0073]    Above the active lock  74  is a hydraulic chamber ring  82 . The chamber is a ring or flange which is free floating on the stem  66 . On a lower surface  84 , there is a central recess to provide clearance for the active lock  76  and twelve docking recesses or locating points  86 . The locating points  86  fit over each of the stud bolts  22  when the docking unit  24  is landed on the compression unit  20 . 
         [0074]    On the upper surface  88  of the chamber  82  a cylinder  90  bored into the chamber. Any number of cylinders can be used. Within the cylinder  90  is a hydraulic piston  92  and an access fluid port (not shown) through which hydraulic fluid is fed to the cylinder  90 , to impact on a base of the piston  90 . Arranged across the top of the chamber  82 , over the upper surface  88  is a plate or hydraulic cover  94 , which is bolted down and provides a space through which the piston  90  can travel upwards out of the chamber  82 . Seals are provided around the piston base to prevent hydraulic fluid from escaping. 
         [0075]    The upper end of the piston touches a tensioning disc  96  threaded to the stem  66 . When attached the disc  96  cannot move on the thread. On an outer surface of the disc  96  are arranged three lifting eyebolts  98  which are used to lift the docking unit  24  on and off the compression unit  20 . A lock nut  99  is provided above the disc  96  and can be screwed down onto the disc  96 . Wing bars  100  are provided on the nut  99  to assist in turning it on the stem  66 . The wing bars  100  can accept steel tube extensions to further assist in turning the stem  66 . 
         [0076]    A final piece which is needed to operate the valve cap  10  is a turning tool, generally indicated by reference numeral  102 . Tool  102  is illustrated in  FIGS. 7 and 8 . The tool  102  comprises a rod  104  sized to pass through the stem  66 . The top of the tool  102  includes a cross bar  106  to assist in turning the tool within the valve cap  10 . At the base of the rod  104  is located a puller tip  108 , shown in greater detail in  FIG. 8 . The tip  108  comprises a cylindrical body  110  with an outer diameter sized to fit within the recess  42  of the plug  16 . Further an elongate opening  112  across the base of the body  110  rises through the body and turns to form two hooks  114  in the body  110 . The opening  112  is off-centre and sized so that the bar  44  in the recess  42  will fit within the opening and rest on the hooks  114  when the tool  102  is turned. 
         [0077]    In use, the compression ring  20  is mounted on the fluid end  14  module of a pump. The stud rods  22  are screwed into corresponding fittings on the end  14 . 
         [0078]    The valve plug  16  should first be well lubricated with high temperature grease and is then lowered through the compression ring  20  and into the aperture  12  in the fluid end  14 . Care must be taken to ensure that the lugs  40  of the plug  16  are aligned to travel between the lugs  62 ,  64  of the compression unit  20 . In order to rotate the plug  16  to achieve this the turning tool  102  may be used. Tool  102  operates by hooking the bar  42  of the plug  16  on the tip  108  of the tool  102 . Any rotation of the tool  102  is then mirrored by the plug  16 . The plug  16  is lowered until the lugs  40  abut the wall  18  in the aperture  12 . Leakage is prevented between the plug  16  and the end  14  by the tapered plug seal  32  fitted between the periphery of valve plug upper body  26  and valve plug lower body  28 . The seal  30  is fitted to prevent pressure loss through the plug  16 . 
         [0079]    To energize the plug  16 , the active docking unit  24  is lifted on top of the compression unit  20  by a lifting device attached to eyebolts  98 . Docking unit  24  locating points  86  are securely located over the top of studs  22 . The active docking unit  24  will now rest on top of nuts  58 . At this point, the lifting device holding active docking unit  24  should be lowered slightly until the lifting slings are just slack. 
         [0080]    Stem  66  is now rotated slowly until it is certain that active lock  74  has passed into compression unit  20  with the lugs  80  locating between the lugs  62 ,  64 . Active lock  74  is rotated anti-clockwise until lock stop  80  prevents further movement. The tensioning disc  96  is then tightened against the piston  92  to remove any slack by locking in position via rotation of the lock nut  99 . 
         [0081]    A hydraulic pump is fitted onto a hydraulic connector which feeds the port into the base of the cylinder  90 . Pressure is raised to typically 650 Barg. (9,500 PSI). By movement of the piston  92  upwards against a now static disc  96 , the hydraulic chamber  82  is forced down against the nuts  58  which will fully compress the compression springs  56 . 
         [0082]    With the springs  56  in compression, the turning tool  102  is lowered through the bore  70  of the stem  66  and gently rotated until it drops over bar  42 . The turning tool is then firmly rotated through 45 degrees clockwise. This causes lugs  64  of the compression unit  20  to abut the teeth of the raised profile in the top surface  36  of the plug  16 . 
         [0083]    Hydraulic pressure is now released which allows the full force of compression springs  56  to be exerted through compression ring  54  and so impel the plug into the module valve port i.e. aperture  12  against wall  18 . 
         [0084]    Stem  66  is then rotated 45 degrees anti-clockwise to allow it to be withdrawn from the ring  46 . The active docking unit  24  can now be lifted off the compression unit  20 , if desired. Alternatively, the docking unit  24  can be left on in order to remove the plug when required for maintenance. 
         [0085]    Thus in use, when sealed on the pump, the compression springs  56  are restrained from lifting by the static ring  46  which is restrained by the nuts  58  fitted on the studs  22  which are in turn fitted into the pump module. When pressure is released, the compression springs  56  press very hard down on top  36  of the plug upper body  26 . The compression unit  20  therefore provides a very powerful clamping force to prevent the plug  16  from being forced out of the module by the mud/fluid pressure inside the module. 
         [0086]    Often, the plug  16  can be removed from the module by hand merely by releasing nuts  58  and pulling the plug  16  from the aperture  12 . If, however, the plug proves reluctant to be removed from the module, the active docking unit  24  can be used to remove it 
         [0087]    In this case, the active docking unit  24  is re-attached to the compression unit  20  as described above. The turning tool  102  is then engaged on the bar  42  for the plug  16 . The shut-off valve on the hydraulic pump is opened and the tensioning disc  96  is screwed firmly down as far as possible. The lock nut  100  is then firmly screwed down sufficiently to prevent the stem  66  from being able to turn inside the tensioning disc  96 . A nut  116  on the turning tool  102  is tightened down against the stem  66  to remove any slack. The hydraulic pressure is then pumped up, typically to 400 Barg. (6,000 PSI), which should readily remove the plug  16 . 
         [0088]    While the specification has used the relative terms ‘up’, ‘down’, ‘upper’, ‘lower’ etc., it will be appreciated that with suitable lifting gear, the valve cap may be used in a number of orientations. 
         [0089]    The main advantages of the present invention can be summarised as follows:— 
         [0090]    1. With an increased number of springs, the resulting powerful spring actuation prevents any tendency for a valve cap to gradually loosen as can happen with screw-type valve caps and increases the actuation available as compared to spring-over-piston valve caps; 
         [0091]    2. The active docking unit and its associated hydraulics are only required during maintenance operations when the plug is inserted or removed. At other times, it is stored away from the pump. Only one such unit is thus required, regardless of the number of pumps on an oil rig/platform; 
         [0092]    3. The spring clamping force, as a result of hydraulic pressure and a large number of springs, more than overcomes the maximum uplift force exerted on the valve plug including the peak transient mud pressure produced by a reciprocating-type pump; 
         [0093]    4. The active docking Unit has the ability to remove sticking valve plugs and sticking valve seats hydraulically without introduction of other equipment; 
         [0094]    5. The valve cap allows very fast maintenance of mud-pump valves and valve seats as very little operator judgment is required to set up the valve cap with little manual effort being involved in valve maintenance operations compared with other systems; 
         [0095]    6. When the docking unit is removed there is improved security of closed valve caps; 
         [0096]    7. All the valve cap parts are readily replaceable in-situ on a pump; 
         [0097]    8. In event of a “stuck” plug seal preventing easy removal of plug, the cap screws between the upper and lower plug bodies can be removed to allow the upper body to be removed first, thus permitting quick and easy access to the plug seal. 
         [0098]    It will be appreciated that various modifications may be made to the invention herein described without departing from the scope thereof. For example, the valve cap can be scaled according with the increase or decrease in the number of pistons and the number of springs as appropriate. Other types of springs could also be used.