Abstract:
A pump comprises a housing ( 141 ) having a cavity ( 142 ) with an inlet valve ( 143 ), an apparatus ( 144 ) for raising fluid pressure in the cavity ( 142 ) and a discharge valve ( 145 ), the discharge valve ( 145 ) comprises a valve head ( 160 ) movable between an open and closed position and a valve seat ( 167 ), wherein the pump further comprises a booster piston ( 182 ) arranged in a booster cylinder ( 183 ), the booster piston ( 182 ) has a first face ( 191 ) exposed to a low pressure zone ( 189 ) and a second face ( 194 ) exposed to fluid pressure in the cavity ( 142 ), and at least one of the booster piston ( 182 ) and the booster cylinder ( 183 ) is movable relative to the other to add an opening force to the discharge valve. The pump is preferably a mud pump for circulating drilling mud through a drill string in a wellbore or a cement pump which may be of the type for pumping cement into a wellbore. Also disclosed is a method for facilitating opening a valve comprising a valve head ( 160 ) and a valve seat ( 167 ), the valve head ( 160 ) is movable a predetermined distance from a closed position on the valve seat ( 167 ) to a fully open position distant on the valve seat, and the method comprises the step of adding a force to the valve head ( 160 ) along a small portion of the predetermined distance.

Description:
BACKGROUND 
       [0001]    The present invention relates to a pump, a valve assembly for the pump and a method for facilitating opening a valve in a pump. The present invention particularly, but not exclusively relates to a mud pump, cement pump and a method for facilitating opening a discharge valve in a mud pump or cement pump. 
         [0002]    In the drilling of a borehole in the construction of an oil or gas well, a drill bit is arranged on the end of a drill string, which is rotated to bore the borehole through a formation. A drilling fluid known as “drilling mud” is pumped through the drill string to the drill bit to lubricate the drill bit. The drilling mud is also used to carry the cuttings produced by the drill bit and other solids to the surface through an annulus formed between the drill string and the borehole. The density of the drilling mud is closely controlled to inhibit the borehole from collapse and to ensure that drilling is carried out optimally. The density of the drilling mud affects the rate of penetration of the drill bit. By adjusting the density of the drilling mud, the rate of penetration changes at the possible detriment of collapsing the borehole. The drilling mud may also carry lost circulation materials for sealing porous sections of the borehole. The acidity of the drilling mud may also be adjusted according to the type of formation strata being drilled through. The drilling mud contains inter alia expensive synthetic water or oil-based lubricants and it is normal therefore to recover and re-use the used drilling mud, but this requires inter alia the solids to be removed from the drilling mud. This is achieved by processing the returned drilling mud. The returned drilling mud flows from a wellhead located at a top of the well through a flow line. The first part of the process is to separate the solids from the solids laden drilling mud. This is at least partly achieved by flowing the returned drilling mud on to a screen of a vibratory separator to screen the returned drilling mud of large solids. The screened drilling mud flows into a series of partitioned sections in an active mud tank. Further processing equipment such as centrifuges and hydrocyclones may be used to further clean the mud of solids. Each piece of further processing equipment is located on top of the respective portioned section of the active mud tank, drawing pre-processed mud from the previous portioned section and outputting processed drilling mud into the partitioned section therebelow. The solids are: disposed of; cleaned and used as aggregate or the like; or certain solids, such as Lost Circulation Material is returned to clean drilling mud. It is not uncommon to have 30 to 100 m 3  of drilling fluid in circulation in a borehole. 
         [0003]    The clean drilling mud is pumped into an additions unit of the active mud tank. Additives, such as weighting agents, viscosity control agents and lost circulation material are added to the clean drilling mud in the additions unit. The prepared drilling mud is now pumped into further testing unit of the active mud system and tested before being pumped into the suction tank of the active mud tank. 
         [0004]    A mud pump is then used to pump the prepared and tested drilling mud from the suction tank into a top of the drill string to circulate drilling mud through the drill string to the drill bit and back to the surface through the annulus. The mud pump comprises a pumping section and a power section. The power section may comprise an electric motor, hydraulic motor or the like driving a reciprocating piston in the pumping section. The prepared and tested drilling mud flows from the suction tank through a supply hose or pipe, through an inlet valve in the pumping section of the mud pump into a cavity and is pushed out of the cavity through a discharge valve by action of the reciprocating piston into a further pipe or hose which leads to a goose neck on top of a top drive or swivel and into the top of a string of drill pipe extending into the borehole. 
         [0005]    The borehole can be several kilometres long and several kilometres deep, so the mud pump is typically able to discharge drilling mud at between 200 bar and 1000 bar at between 300 and 4800 litres per minute. 
         [0006]    The mud pump may comprise dual acting reciprocating pistons or single action. The mud pump may comprise a duplex reciprocating piston, triplex piston, or any other number of pistons. It is advantageous to maintain a constant flow of drilling mud through the drill string at a constant pressure. Surges in the flow of drilling mud and surges in pressure may cause problems downhole. Such problems may include: the constant return of drilled solids; build up of solids in the annulus; maintaining pressure in the well to inhibit collapse of the borehole; and maintaining a constant weight of mud in the well. 
         [0007]    The flow of drilling mud through the drill string traditionally stops and starts when a section of drill pipe is added or removed from the string of drill pipe during tripping and drilling. However, continuous circulation systems while tripping and continuous circulation systems while drilling are becoming more popular. With these systems, flow of drilling mud downhole is more continuous and pressure can be maintained at a constant level more easily. 
         [0008]    Mud pumps are used on land rigs and offshore rigs. Mud pumps are generally located on or beside a land rig or on an offshore drilling platform. However, the mud pump may be located on the sea bed or between the seabed and a surface of the sea. 
         [0009]    In deep wells or wells in formations which may easily collapse, it is common to line the borehole with casing. The casing is hung from a wellhead. Similarly, liner is used to case a borehole, except that the liner is hung from the bottom of an existing casing. The casing or liner is cemented in place by pumping cement down through the casing and up through an annulus between the casing and the borehole. To and bottom plugs are used to facilitate the cementing operation. A cement pump may be used to flow cement into the well and then flow of drilling mud or water behind a bottom plug may be used to push the bottom plug down and force the cement up through the annulus. The mud pump may be used to pump the drilling mud or water behind the bottom plug. 
         [0010]    A cement pump may also be used in the general construction industry in pumping cement for: below ground structures, such as foundations; above ground structures such as car parks, housing, commercial buildings and sky scrappers; and in the marine industry in construction above and below water structures, such as quays and oil platforms. 
       SUMMARY AND STATEMENTS OF INVENTION 
       [0011]    The inventors have observed that a build up in pressure in the cavity is required to crack the outlet valve open. This build-up of pressure then normalises to a stable pressure which the pump induces for the stroke of the reciprocating piston in the piston pumping section of the mud pump. The build-up of pressure and then sudden return in pressure to the normalised level creates a shock, which may induce a vibration in the mud pump. The vibration may cause an unwanted noise; may loosen fastenings in the mud pump; and may induce fatigue failure in parts of the mud pump or surrounding components. Furthermore, the inventors have observed that a more consistent pressure in the outlet flow of drilling mud is beneficial for controlling the combined flow once the outlet flows from several pumping sections are combined in a manifold to produce a consistent pressure in the outlet flow of drilling mud from the manifold. 
         [0012]    The inventors have observed that a pump requiring having a smoother flow and more consistent pressure in the outlet would also be useful in a cement pump used in the general construction industry. 
         [0013]    In accordance with the present invention, there is provided a pump comprising a housing having a cavity with an inlet valve, an apparatus for raising pressure in a fluid in the cavity and a discharge valve, the discharge valve comprising a valve head movable between an open and closed position and a valve seat characterised in that the pump further comprises a booster piston arranged in a booster cylinder, the booster piston having a first face exposed to a low pressure zone and a second face exposed to fluid pressure in the cavity, at least one of the booster piston and booster cylinder movable relative to the other to add an opening force to the outlet valve. Typically, the pressure in the cavity is in the order of 500 bar and the pressure in the low pressure zone is three bar. 
         [0014]    Preferably, the pump further comprising a transfer member arranged between the valve head and the booster piston. Preferably, the member is a rod, which may be solid or hollow and of any suitable cross-section, such as circular, square or hexagonal. 
         [0015]    Advantageously, the transfer member has a first end and a second end, the first end fixed to the valve head and the second end free. Alternatively, the transfer member has a first and a second end, the first end fixed to the booster and the second end free to selectively abut the valve head. Alternatively, the member has a first and a second end, the first end fixed to the cylinder and the second end free to selectively abut the valve head. 
         [0016]    Preferably, the transfer member passes through the cylinder into the low pressure zone and the second end arranged in the path of movement of the booster in the cylinder. 
         [0017]    Advantageously, the booster is free floating in the cylinder. Preferably, the booster is free floating between end stops to limit the range of movement of the booster, advantageously between a lip on the end of a cylinder and advantageously, with a hollowed out booster abutting a cylinder head. Advantageously, the range of movement is limited to between 2 mm and 30 mm and preferably, 5 mm to 10 mm. 
         [0018]    Preferably, the pump further comprises an inlet for allowing fluid to flow to the inlet valve, wherein the booster cylinder further comprises an opening therein fluidly connected to the inlet to allow fluid in the inlet to flow into and from the low pressure zone in the booster cylinder. It is preferred to use fluid from the inlet to fill the low pressure cavity in the booster piston assembly, so that if there is any leak of fluid from the booster cylinder into the cavity, there will be no contamination. Furthermore, the pressure of the fluid in the inlet is at a suitable pressure, typically three bar. Alternatively, or additionally, the booster cylinder further comprises an opening therein connected to a low pressure source. The low pressure source may be a fluid reservoir open to atmospheric pressure or a bladder open to atmospheric pressure or an accumulator holding the fluid under a low pressure, such as 3 bar. The fluid is preferably water or may be a hydraulic fluid, such as oil. 
         [0019]    Preferably, the pump is a mud pump for circulating drilling mud through a drill string in a wellbore and advantageously, the low pressure zone is filled with drilling mud. 
         [0020]    Advantageously, the pump is a cement pump for pumping cement or the like. Preferably, for pumping cement into a wellbore in the construction of an oil or gas well, such as for cementing casing in a wellbore. 
         [0021]    Preferably, the apparatus for raising pressure in a fluid in the cavity comprises a reciprocating piston. Advantageously, the apparatus for raising pressure in a fluid in the cavity comprises a motor. Preferably, the motor is an electric motor, which may be an AC motor or a DC motor. Advantageously, the motor is a hydraulic motor. Preferably, the motor is a diesel engine. Preferably, the motor drives a crank shaft to reciprocate a piston rod, with a piston head attached thereto. 
         [0022]    Advantageously, the apparatus for raising pressure in a fluid in the cavity comprises a hydraulic pump. Preferably, the hydraulic pump comprises a piston and cylinder for reciprocating the reciprocating piston. 
         [0023]    Preferably, the valve head of the outlet valve further comprises a spider. Preferably, the spider is arranged on the cavity side of the valve head. Advantageously, the spider comprises a plurality of legs, which may be two, three, four, five or more. Advantageously, the valve seat comprises a portion which extends into the cavity, the spider arranged in the portion of the valve seat. Advantageously, the portion is cylindrical, preferably of circular cross-section, but may be oval, square, pentagonal, hexagonal or have more sides. Preferably, the spider improves flow of fluid through the outlet valve, preferably normalizing the flow and distributing the flow through the valve. Advantageously, the spider guides the outlet valve, holding the valve head and transfer member in axial alignment. 
         [0024]    Advantageously, the outlet valve further comprises a spring for biasing the valve head against the valve seat. Preferably, the spring is one of a: coiled spring; Belleville washer; leaf spring; or any other suitable resilient member. Advantageously, a valve stem extending from the valve head. Preferably, the valve stem is arranged in a valve guide. 
         [0025]    Preferably, the inlet valve further comprises a valve head and a valve seat and advantageously, a spring for biasing the valve head against the valve seat. Preferably, the spring is one of a: coiled spring; Belleville washer; leaf spring; or any other suitable resilient member. Advantageously, a valve stem extending from the valve head. Preferably, the valve stem is arranged in a valve guide. 
         [0026]    The present invention also provides a valve assembly for use in the pump of the invention, the valve assembly comprising a valve head movable between an open and closed position and a valve seat characterised in that the valve assembly further comprises a booster piston arranged in a booster cylinder, at least one of said booster piston and said booster cylinder movable relative to the other to add an opening force to the valve head. 
         [0027]    The present invention also provides a method for facilitating opening a valve comprising a valve head and a valve seat, the valve head movable a predetermined distance from a closed position on the valve seat to a fully open position distant to the valve seat, the method comprising the step of adding a force to the valve head along a small portion of the predetermined distance. Preferably, for opening a valve in a pump and most preferably for opening a discharge valve in a pump for pumping drilling mud through a drill string in a wellbore and a cement pump. 
         [0028]    Preferably, the small portion is between 1% and 95% of the predetermined distance. Advantageously, the small portion is between 5% and 33% of the predetermined distance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For a better understanding of the present invention reference will now be made, by way of example only, to the accompanying drawings, in which: 
           [0030]      FIG. 1  is a schematic view of a drill rig comprising a mud pump of the invention; 
           [0031]      FIG. 2  is a schematic view of a prior art pumping section of a pumping end of a mud pump comprising an inlet valve, a cavity and an outlet valve; 
           [0032]      FIG. 3  is a graph showing pressure in the cavity during opening of the outlet valve I&#39;m the pumping section shown in a  FIG. 2 ; 
           [0033]      FIG. 4  is a schematic view of a pumping section of a pumping end of a mud pump in accordance with the present invention comprising an inlet valve, a cavity, an outlet valve and booster piston assembly in accordance with the present invention; 
           [0034]      FIG. 5  is a graph showing pressure in the cavity during opening of the outlet valve in the pumping section shown in a  FIG. 4 ; 
           [0035]      FIG. 6  is a perspective view of part of a booster piston assembly in accordance with the present invention; 
           [0036]      FIG. 7  is a side view in cross-section of a part of the booster piston assembly shown in  FIG. 6 ; 
           [0037]      FIG. 8  is a top plan view of part of the booster piston assembly shown in  FIG. 6  in a cavity of a pumping section; 
           [0038]      FIG. 9  is a side view of a part of a pumping section in accordance with the present invention in a first stage of use, the pumping section incorporating the booster piston assembly shown in  FIG. 6 ; 
           [0039]      FIG. 10  is a side view of the part of the pumping section shown in  FIG. 9  in a second stage of use; 
           [0040]      FIG. 11  is a side view of the part of the pumping section shown in  FIG. 9  in a third stage of use; 
           [0041]      FIG. 12  is a side view of the part of the pumping section shown in  FIG. 9  in a fourth stage of use; and 
           [0042]      FIG. 13  is a schematic view of a view of part of a pumping section in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    Referring to  FIG. 1  there is shown a drilling rig generally identified by reference numeral  1 . The drilling rig  1  has a derrick  2  arranged on a drill floor  3  supported on legs  4 . The legs  4  are seated on a substructure  5  on ground  6 . 
         [0044]    A swivel  7  and raised and lowered on a travelling block  8  on wireline  9 , which passes over a crown block  10  located at a top of the derrick  2  and down to a drawworks  11  on the rig floor  3 . A section of drill pipe  12  depends from the swivel  7 , passes through a rotary table  13  and is connected at a lower end to a drill string  14 . The drill string  14  passes through a wellhead  15  into a bore hole  16  in formation  17 . A bottom hole assembly  18  is arranged on a lower end of the drill string  14 , which has a drill bit  19  on the lower end thereof. An annulus  21  is defined between the borehole  16  and the drill string  14 . 
         [0045]    A flow line  20  is fluidly connected at one end to the annulus  21  at the wellhead  15  and the other end to an active mud system  22 . Returned drilling mud M flows from the annulus  21 , through wellhead  15 , into flow line  20  and to the active mud system  22 . The active mud system  22  comprises an active mud tank  23  and a series pieces of mud processing equipment, such as: a shale shaker  24 , a degassers  25 , a mud conditioner  26 , a centrifuge  27 . Further mud processing equipment may be used, such as gumbo separators and hydro cyclones. The flow line  20  flows returned drilling mud M directly on to screens of the shale shaker  24 . Screened drilling mud S flows from the shale shaker  24  into partitioned section  27 . Each piece of further mud processing equipment is located above a partitioned section  27 ,  28  and  29  of the active mud tank  23 , being fed from the previous partitioned section using a small pump and flowing processed drilling mud into the partitioned section  27 ,  28 ,  29  therebelow. 
         [0046]    The processed drilling mud then passes into additions section  30 , into which additives are added, such as bentonite, barite and lost circulation material. The processed and tested drilling mud P is tested and when acceptable, is pumped or allowed to flow into suction tank  31 . A mud pump  32  is located downstream of the suction tank  31 . The processed and tested drilling mud P flows from the suction tank  31  through a supply hose  33  to the mud pump  32 . The processed and tested drilling mud P flows into the mud pump  32  may flow directly from the suction tank  31  down to the mud pump under pressure provided by the head of drilling mud in the suction tank or may be pumped with a small pump to maintain a relatively constant supply pressure which pressure is typically is between 1 to 5 bar and preferably 3 bar. The processed and tested drilling mud P is then pumped by the mud pump  32  through a hose  34  to the swivel  7  where the hose is fluidly connected to a top of the drill pipe  12  and flows through the drill string  14  therefrom. 
         [0047]    The mud pump  32  comprises: a pumping end  35  comprising an inlet manifold  38  and an outlet manifold  39  connected to a number of pumping sections, such as the pumping section shown in  FIG. 4 ; and a power end  36  comprising at least one motor  37  driving piston rods, such as piston rod  158  of the pumping section  140  shown in  FIG. 4 . 
         [0048]    In a top drive rig (not shown) the rotary table is replaced or supplemented by a top drive. In a top drive rig, the hose  34  is connected to a goose neck and rotary connection which is fluidly connected with a quill in the top drive which is in turn selectively connected to the drill pipe  12  and drill string  14 . 
         [0049]      FIG. 2  shows a prior art pumping section of a pumping end of a mud pump, generally identified by reference numeral  40 . The pumping section  40  has a valve block  41  with an inlet valve  43 , a reciprocating piston  44  and an outlet valve  45  defining a cavity  42 . A supply pipe  46  allows drilling mud P to flow from an inlet manifold  38 ′ to an inlet connection  47  in the valve block  41 . The inlet manifold  38 ′ receives drilling mud P from a suction tank, like the suction tank  31  shown in  FIG. 1 . A flow channel  48  in valve block  41  leads to an inlet face  49  of a generally circular valve head  50  of inlet valve  43 . The valve head  50  has a valve stem  51  fixed on a rear face  52  of inlet valve  43 . The valve stem  51  is slideably arranged in a stem guide  53 . The valve head  50  has a frusto-conical outer edge  54 . A spring  55  biases the valve head away from a ledge  56 , urging frusto-conical outer edge  54  against frusto-conical valve seat  56  of the valve block  41  to form a substantially fluid tight seal therebetween. 
         [0050]    The piston  44  is slideably arranged in a cylinder sleeve  57 . A piston rod  58  is connected at one end to the piston  44  and at the other to a power end (not shown) of the mud pump. The power end of the mud pump may comprise any suitable means for reciprocating the piston  44 , such as a hydraulically driven ram or a motor rotating a crank shaft. Any suitable motor may be used, such as a diesel engine, an electric motor or a hydraulic motor. 
         [0051]    An outlet valve  45  comprises a valve head  60  of generally circular shape in plan view, the valve head  60  having an outlet face  61  exposed to the cavity  42 . The valve head  60  has rear face  63  to which a valve stem  62  is fixed. The valve stem  62  is slideably arranged in a stem guide  64 . The valve head  60  has a frusto-conical outer edge  65 . A spring  66  arranged in an outlet passage  68  biases the valve head  60  away from a top portion of the valve block  41 , urging frusto-conical outer edge  65  against frusto-conical valve seat  67  of the valve block  41 . A discharge passage  68  is provided for conveying flow of drilling mud into a discharge manifold  39 ′. 
         [0052]    Any number of pumping sections  40  are coupled together with the inlet manifold and outlet manifolds (not shown), so that large quantities of drilling mud can be pumped. Any suitable number of pumping sections may be coupled, typically: two, such as with duplex mud pumps; three, such as with triplex mud pumps; or six, as with hex mud pumps; but may be four, five or more. The position of the pistons  44  along each cylinder sleeve  57  may be staggered to obtain a smooth flow of drilling mud through the outlet manifold and into the drill string. 
         [0053]    In use, the pumping cycle starts with outlet valve  45  closed and drilling mud P flowing through supply pipe  46  under low pressure (such as 3 bar) being drawn into the cavity  42  by action of the piston  44  retracting, drawing drilling mud P through inlet valve  43 . Thus very low pressure in the cavity  42  and the head of drilling mud provided by the level of mud in the suction tank provides sufficient force to open inlet valve  43 . Once the reciprocating piston  44  reaches a fully retracted position, the drilling mud P in the cavity  42  is no longer under a very low pressure. The spring force provided by spring  55  overcomes the force on the inlet face  49  of the valve head  50  provided by pressure in the drilling mud in inlet passage  48  moving the valve head to abut valve seat  56  to close the inlet valve  43 . 
         [0054]    The reciprocating piston  44  is pushed forwards through the cylinder sleeve  57  by the power end moving the piston rod  58 , reducing the volume of cavity  42  and pressurising the drilling mud therein, indicated by the steep slope  71  of trace line  70  in the graph in  FIG. 3 . When the force on outlet face  61  overcomes the force on the rear face  63 , the outlet valve  45  should open. The force on the outlet face  61  is provided by the pressure of the drilling fluid over the area of the outlet face  61 . The force on the rear face  63  is provided by pressure in drilling mud in the discharge passage  68  over the area of the rear face  63 , from spring force supplied by spring  66  and a minimal effect of the weight of the valve head  60  may be taken into consideration. However, the inventors have observed that a large pressure differential across the outlet valve  45  is needed to crack the outlet valve  45  open, as shown in portion  72  of trace line  70  in the graph of  FIG. 3 . A pressure P-Crack is noted before the outlet valve  45  opens, typically 675 bar. Once the outlet valve  45  is open, the pressure in the drilling mud normalises, indicated by the horizontal portion  73  of the trace line  70  at a pressure P 1  in the graph. P 1  is the drilling mud pressure required in the outlet flow, which is typically 500 bar, (although may be anywhere between 100 and 2000 bar). Thus P-Crack is typically 20 to 30% higher than the pressure P 1 . When the force on the rear face  63  of valve head  60  overcomes the force on the outlet face  61  and frusto-conical edge  65  provided by the pressure in the drilling mud in the cavity  42 , the outlet valve  45  will close. This will typically occur when the piston  44  reaches the end of its stroke and starts to retract, as the inlet valve  43  begins to open, repeating the pumping cycle. 
         [0055]    The inventors noted that the pressure spike P-Crack is seen in both the cavity  42  and the outlet flow, which can induce unwanted vibrations in the mud pump and associated components. The inventors have noted it is beneficial to obtain a constant pressure P 1  in the cavity  42  and the outlet passage  68 . 
         [0056]    Referring to  FIG. 4 , there is shown a pumping section of the present invention, with similar parts referred to with reference numerals in the one hundred series. 
         [0057]    The pumping section  140  has a valve block  141  with an inlet valve  143 , a reciprocating piston  144  and an outlet valve  145  defining a cavity  142 . A supply pipe  146  allows drilling mud P to flow from the manifold  38  to an inlet connection  147  in the valve block  141 . A flow channel  148  leads to an inlet face  149  of a generally circular valve head  150  of inlet valve  43 . The valve head  150  has a valve stem  151  fixed on a rear face  152 . The valve stem  151  is slideably arranged in a stem guide  153 . The valve head  150  has a frusto-conical outer edge  154 . A spring  155  biases the valve head  150  away from a ledge  156 , urging frusto-conical outer edge  154  against frusto-conical valve seat  156  of the valve block  141 . 
         [0058]    The piston  144  is slideably arranged in a cylinder sleeve  157 . A piston rod  158  is connected at one end to the piston  144  and at the other to a power end (not shown) of the mud pump. The power end may be any suitable power end, such as any discussed herein. 
         [0059]    An outlet valve  145  comprises a generally circular valve head  160  having outlet face  161  exposed to the cavity  42 . The valve head  160  has a valve stem  162  fixed on a rear face  163  exposed to the outlet flow. The valve stem  162  is slideably arranged in a stein guide  164 . The valve head  160  has a frusto-conical outer edge  165 . A spring  166  arranged in an outlet passage  168  biases the valve head  160  away from a top portion of the valve block  141 , urging frusto-conical outer edge  165  against frusto-conical valve seat  167  of the valve block  141 . The valve head  160  moves approximately 30 mm between a closed position and a fully open position. A transfer rod  169  is located substantially concentrically with the valve head  160  fixed and projecting from the outlet face  161 . The transfer rod  169  is preferably 30 mm in diameter, although may typically be 10 mm to 75 mm in diameter and extends preferably 75 mm from the outlet face  161  into the cavity  142 . The transfer rod  169  is rigid and able to withstand compressive forces so that a force can be applied thereto to push on the valve head  160  to facilitate opening. 
         [0060]    The outlet valve  145  is provided with a booster piston assembly generally identified by reference numeral  81  for facilitating the application of an additional force to the valve head  160  to facilitate opening and then preferably, removing the additional force. 
         [0061]    The booster piston assembly  181  comprises a booster piston  182  slideably arranged in a cylinder  183 . The cylinder  183  is fixed in the cavity  142  in a retainer  184  beneath and concentrically in line with outlet valve head  145 . The retainer  184  comprises three arms (only one shown  185 ), spaced at 120 degrees from each other about the cylinder  183  fixed at one end to the cylinder  183  and at the other to cavity wall  186  of the valve block  141 . Three spaces between the three arms  185  allow free flow of drilling mud P through the cavity  142 . 
         [0062]    The transfer rod  169  passes through a seal  187  in an opening in a cylinder head  188  of the cylinder  183 . The transfer rod  169  extends into the cylinder approximately 75 mm. A low pressure zone  189  is defined by the cylinder head  188 , cylinder wall  190 , an upper face  191  of the piston  182 . The low pressure zone  189  is in fluid communication with drilling mud inlet flow channel  148  via a fluid line  192 , so that drilling mud P can flow freely therebetween. The fluid line  192  is of a small diameter, such as 5 mm, but may be any suitable size such as 3 mm to 20 mm. The fluid line  192  may comprise a channel bored through the valve block  141  or any suitable pipe or hose. 
         [0063]    The piston  182  has a seal  193  arranged between the upper face  191  and lower face  194 , which isolates the cavity  142  from the low pressure zone  189  and allows the piston to slide along the cylinder  183  smoothly. The piston  182  is retained in cylinder  183  by a lip  195 . The piston  182  has a range of movement of approximately 2 to 10 mm and may be less than the opening distance of the outlet valve  145 . Preferably, no more than 30 percent of the opening distance of the outlet valve  145 . Advantageously, the range of movement is between 2 to 5 mm. 
         [0064]    An opening  196  in the cylinder is arranged out side of the range of movement of the piston  182 , which is fluidly connected to the fluid line  192 . 
         [0065]    In use, the pumping cycle starts with outlet valve  145  closed and drilling mud P flowing through supply pipe  146  under low pressure (such as 3 bar) being drawn into the cavity  142  by action of the piston  144  retracting, drawing drilling mud P through inlet valve  143 . Thus very low pressure in the cavity  142  and the head of drilling mud provided by the level of mud in the suction tank  31  provides sufficient force to open inlet valve 1   143 . Once the reciprocating piston  144  reaches a fully retracted position, the drilling mud P in the cavity  142  is no longer under a very low pressure. The spring force provided by spring  155  overcomes the force on the inlet face  149  of the valve head  150  provided by pressure in the drilling mud in inlet passage  148  moving the valve head  150  to abut valve seat  156  to close the inlet valve  143 . 
         [0066]    The reciprocating piston  144  is pushed forwards through the cylinder sleeve  157  by the power end moving the piston rod  158 , reducing the volume of cavity  142  and pressurising the drilling mud therein, indicated by the steep slope  171  of trace line  170  in the graph in  FIG. 5 . When the force on outlet face  161  overcomes the force on the rear face  163 , the outlet valve  145  should open. The force on the outlet face  61  is provided by the pressure of the drilling fluid over the area of the outlet face  161  and an additional force is applied to the outlet face  161  by movement of the transfer rod  169 . The large pressure, such as 500 bar (but may be 100 to 1000 bar) in the drilling mud in cavity  142  acts on second face  194  of the booster piston  182 , which is greater than the force provided by the lower pressure zone (typically 3 bar) acting across the first face  191  of the piston  182 . The resultant additive force moves the piston  182  towards an end of the rod  169 , expelling low pressure fluid from the low pressure zone  189  through fluid line  192 . The piston  182  meets the transfer rod and pushes on the transfer rod  169 , applying this additional force to the outlet face  161 . The piston  182  moves forward approximately 4 to 10 mm, pushing the valve head  160  open approximately 2 to 8 mm, whereupon the transfer rod  169  leaves the piston  182  and opens to a fully open position after approximately 30 mm of travel. Thus no additional force is applied to the valve head  160  during the final part of the valve head&#39;s travel. The valve head  160  begins to unseat itself when the total force on the outlet face is greater than the force on the rear face  163 . The force on the rear face  163  is provided by pressure in drilling mud in discharge passage  168  over the area of the rear face  163  and from spring force supplied by spring  166 . The additional force applied by the booster piston assembly  181  provides the additional force required to crack the outlet valve open without a large spike in pressure in the drilling mud, as shown by the smooth transition  172  from pressure build up in cavity  142  to the discharge pressure P 1  shown by horizontal line  173  in the graph in  FIG. 5 . Thus the pressure in the cavity  142  is substantially the same as the pressure in the drilling mud in then discharge passage  168  during opening of the discharge valve  145 . 
         [0067]    Once the outlet valve  145  is open, the pressure in the drilling mud normalises, indicated by the horizontal portion  173  of the trace line  170  at a pressure P 1  in the graph. P 1  is the drilling mud pressure required in the outlet flow, which is typically 500 bar, (although may be anywhere between 100 and 2000 bar). When the force on the rear face  163  of valve head  160  overcomes the force on the outlet face  161  and frusto-conical edge  165  provided by the pressure in the drilling mud in the cavity  142 , the outlet valve  145  will close. This will typically occur when the piston  144  reaches the end of its stroke and starts to retract, as the inlet valve  143  begins to open. The booster piston  182  will fall back on to lip  195 , sucking drilling mud in from the inlet  148  into the low pressure zone  189 . The pumping cycle then repeats. 
         [0068]    A part of a booster piston assembly in accordance with the invention is shown in  FIGS. 6, 7 and 8 , which is generally similar to the pumping section of  FIG. 4 , with similar parts referred to with reference numerals in the two hundred series. 
         [0069]    A free floating booster piston  282  is slideably arranged in a booster cylinder  283 . The booster piston  282  has an upper face  291  exposed to a low pressure zone  289  and a lower face  294  exposed to drill mud pressure in cavity  242 . The booster cylinder  283  comprises a lip  295  at a lower end to retain the booster piston  282  in the booster cylinder  283 . The booster cylinder  282  comprises a main body  283 ′ with three arms  285 ,  285 ′ and  285 ″ extending therefrom to a valve body  241  to centralize the main body  283 ′ in the cavity  242 . The arms  285 ,  285 ′ and  285 ″ are spaced about the body at approximately 120 degrees and define flow paths  242 ′  242 ″ and  242 ′″ through which drill mud can flow. Three raised lugs  297 ,  297 ′ and  297 ″ are spaced about the main body  283 ′ to rigidly fix the booster piston assembly to the valve block  241 . 
         [0070]    Referring to  FIG. 9 , an outlet valve  245  comprises a generally circular valve head  260  having outlet face  261  exposed to the cavity  242 . The valve head  260  has a valve stem  262  fixed on a rear face  263  exposed to the outlet flow. The rear face  263  comprises a planar ring portion  263 ′ and a concave portion  263 ″. The valve stem  262  is slideably arranged in a stem guide (not shown). The valve head  260  has a frusto-conical outer edge  265 , with a frusto-conical seal member  265 ′, which may be an elastomeric, machined metal or nylon material. A spring  266  arranged in an outlet passage  268  biases the valve head  260  away from a top portion of the valve block (not shown), urging frusto-conical seal member  265 ′ against frusto-conical valve seat  267  of the valve insert  241 ′ which forms part of or fitted in valve block  241 . The valve head  260  moves approximately 30 mm between a closed position and a fully open position. A transfer rod  269  is located substantially concentrically with the valve head  160  fixed and projecting from the outlet face  261 . The transfer rod  269  is preferably 15 mm in diameter, although may typically be 10 mm to 75 mm in diameter and extends preferably 75 mm from the outlet face  261  into the cavity  242 . The transfer rod  269  is rigid and able to withstand compressive forces so that a force can be applied thereto to push on the valve head  260  to facilitate opening. The transfer rod  269  passes through a seal nipple  287  on cylinder head  288 . The seal nipple comprises a cap  287 ′ surrounding the transfer rod  269 . The seal nipple  287  also comprises a number of seal elements  287 ″ and guide rings to provide a drilling fluid tight seal about the transfer rod  269 , between the cavity  242  and the low pressure zone  289  during a pressure differential of typically 500 bar and may be between 100 bar and 1000 bar. 
         [0071]    A spider  298  is rigidly fixed or integral with the outlet valve head  260  and depends therefrom into cavity  242 . The spider  298  is slideable with the outlet valve head  260  in a cylinder  241 ″ formed in a lower part of valve insert  241 ′. The spider comprises four legs  298 ′,  298 ″ and  298 ′″ spaced at 90 degrees about the transfer rod  269 . The spider  298  helps hold the outlet valve  245  and the transfer rod  269  centralized during opening and closing. Furthermore, the spider facilitates obtaining a better of flow of drilling mud through the outlet valve  245 . 
         [0072]    The low pressure zone  289  is defined by the cylinder head  288 , cylinder wall  290 , upper face  291  of the piston  282 . The low pressure zone  289  is in fluid communication with drilling mud inlet flow channel (not shown, but generally similar to flow channel  148 ) via a fluid line  292 , so that drilling mud P can flow freely therebetween at a pressure of approximately three bar. 
         [0073]    The piston  282  has a seal  293  arranged between the upper face  291  and lower face  294 , which isolates the cavity  242  from the low pressure zone  289  and allows the piston to slide along the cylinder  283  smoothly. The piston  282  has a range of movement of approximately 2 to 10 mm and may be less than the opening distance of the outlet valve  245 . Preferably, no more than 30 percent of the opening distance of the outlet valve  245 . 
         [0074]    In use, the free-floating booster piston  282  initially sits on lip  295  of the cylinder  283 , as shown in  FIG. 10 . As pressure in drilling mud in cavity  242  builds up, the pressure differential across the booster piston  282  moves the booster piston  282  upwards to contact a foot of the transfer rod  269 , as shown in  FIG. 10 . As the pressure in the drilling fluid in cavity  242  reaches the pressure in the drilling fluid in outlet passage  268 , an additional force provided by the booster piston  282  on transfer rod  269  facilitates cracking the outlet valve head  260  from the valve seat  267 , as shown in  FIG. 11 . The booster piston  282  may only push the transfer rod  269  2 mm in order to accomplish this. The pressure differential on the outlet valve head  260  and the flow of drilling mud therethrough pushes the outlet valve head  260  open further, whereupon the foot of the transfer rod  269  parts from the upper face  291  of the booster piston  282 . The outlet valve head  260  fully opens, which is usually in the order of 30 mm, as shown in  FIG. 12 . 
         [0075]    The opening  296  allowing fluid communication between the low pressure zone  289  and the flow channel (such as  148 ) through fluid line  292 , is covered by the piston  282 , although the seals  293  are located in a lower portion of the booster piston  282 , so that the seals  293  are prevented from being damaged by the opening  296 . 
         [0076]    During the part of the mud pump&#39;s cycle in which drilling fluid is sucked into the cavity  242 , the pressure in the cavity  242  reduces to below the pressure of the fluid pressure in the low pressure zone  289 . The free-floating booster piston  282  seats itself on the lip  295 . The outlet valve head  260  returns to a closed position with seal  265 ′ seated on valve seat  267 . 
         [0077]    Referring to  FIG. 13 , there is shown a part of a pumping section of the present invention, which is generally similar to the pumping section of  FIG. 4 , with similar parts referred to with reference numerals in the one hundred series. 
         [0078]    There is shown an outlet valve  345  and booster piston assembly  380 . The booster piston assembly  380  is generally similar to the booster piston assembly  180 , save for the booster piston  382 , which has a transfer rod  369  fixed thereto which has a free upper foot  399  which selectively engages lower face  361  of outlet valve head  360 . A fluid line  392  is provided with water therein in fluid communication with low pressure zone  389 . The water is supplied from a constant pressure source  399   a , which may be an accumulator primed to a pressure of preferably 3 bars. 
         [0079]    Other alterations and modifications are envisaged, such as using a stationary piston and a moving cylinder for pushing on the valve head.