Patent Publication Number: US-8534185-B2

Title: Reciprocating pump having a pressure compensated piston

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND 
     1. Field of Art 
     The disclosure relates generally to pumps, particularly reciprocating pumps, such as mud pumps used in the recovery of oil and gas. More particularly, the disclosure relates to piston components for creating and maintaining a seal between the piston and a surrounding pump cylinder. 
     2. Background of Related Art 
     Mud pumps are commonly used for conveying drilling mud during well drilling operations, such as for the recovery of oil and gas. Because of the need to pump the drilling mud through several thousand feet of drill pipe, such pumps typically operate at high pressures. Also, it is necessary for the drilling mud to emerge from the drill bit at a high flow rate in order to provide lubrication and cooling to the bit and to provide a vehicle for removal of drill cuttings from the earth formation being drilled. Further, the pressure generated by the mud pump contributes to the total downhole pressure, which is important and provided to prevent well blowouts. 
     Conventional mud pumps generally require interference between the sealing element of the piston and the surrounding cylinder to assure a seal between the two components and to provide sufficient material to maintain the seal while allowing for wear over the effective service life of the piston. This interference, however, results in a frictional load on the piston, which reduces pump efficiency. Moreover, the combined effect of the frictional forces resulting from the reciprocating contact between the piston seal and the cylinder, and the abrasive nature of drilling mud passing through the pump at high pressure is especially harmful to the sealing element. As the piston moves, edges of the sealing element experience wear and may become damaged. In some instances, the frictional force may be sufficient to cause the sealing element to detach from the piston. 
     Accordingly, means for maintaining a seal between a pump piston and a surrounding cylinder that also minimize wear to the piston components and frictional loads between the piston and cylinder are desirable. 
     SUMMARY OF SOME OF THE DISCLOSED EMBODIMENTS 
     A reciprocating pump having a pressure compensated piston is disclosed. In some embodiments, the piston includes an annular body having a radially-facing outer surface and an annular sealing element disposed radially outward of the annular body. The sealing element has an inner surface adjacent the radially-facing outer surface of the body. The annular body further includes an axially-facing surface with an inlet port and a flowpath extending between the inlet port and the radially-facing outer surface. 
     In some embodiments, the pump includes a rod having a cavity and a piston disposed about the rod. The piston includes an annular body having a radially-facing outer surface and an annular sealing element disposed about the annular body. The sealing element has an inner surface adjacent the radially-facing outer surface. A first fluid path extends between an end of the rod and the cavity, while a second fluid path extends between the cavity and the radially-facing outer surface. A resilient pressure transfer element disposed in the cavity and separating the first and the second flow paths. 
     Thus, embodiments described herein comprise a combination of features intended to enable enhancement of certain prior pumps and pump components. The various features and characteristics described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed description of the disclosed embodiments, reference will now be made to the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a piston rod assembly including a pressure compensated piston made in accordance with the principles described herein; 
         FIG. 2  is a cross-sectional view of another pressure compensated piston rod assembly made in accordance with the principles described herein; and 
         FIG. 3  is a cross-sectional view of a reciprocating pump comprising a pressure compensated piston rod assembly made in accordance with the principles described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS 
     The following discussion is directed to various exemplary embodiments of the invention. The embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment. 
     Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function or structure. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in interest of clarity and conciseness. 
     In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus are to be interpreted to mean “including, but not limited to.” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. Further, the terms “axial” and “axially” generally mean along or parallel to a central or longitudinal axis, while the terms “radial” and “radially” generally mean perpendicular to a central longitudinal axis. 
     Referring now to  FIG. 1 , piston rod assembly  100  includes a rod  105  disposed within a pressure compensated piston  110 . A retainer  115  couples piston  110  to rod  105 . In this embodiment, retainer  115  includes a washer  120  and a nut  125  threaded onto an end  130  of rod  105 . Pressure compensated piston  110  is annular, and includes an axial throughbore  135  and a circular recess  132 , both configured to receive rod  105 , as shown. Rod  105  includes an elongate extension  140  connected to a base portion  145 . Extension  140  of rod  105  is inserted through axial throughbore  135  of piston  110 . Base portion  145  of rod  105  has a diameter greater than that of extension  140  and thereby forms a shoulder  150 . Circular recess  132  of piston  110  receives shoulder  150  of rod  105  such that piston  110  is seated against rod  105 . 
     Piston  110  further includes a front cover  155 , a rear cover  160  and a body  165  disposed therebetween, all of which are annular. Body  165  includes a circular recess  163  that receives a circular projection  167  of rear cover  160 . When rear cover  160 , front cover  155  and body  165  are assembled together or constructed as shown, these components form axial throughbore  135 , which receives rod  105 . Front cover  155  and rear cover  160  each include one or more throughbores  170 ,  175 , respectively. Piston body  165  includes one or more screwholes  180  which align with throughbores  170 ,  175  when body  165  is disposed between front and rear covers  155 ,  160 , as shown. To secure these components together, and thus form piston  110 , a screw  185  is inserted through each throughbore  170 ,  175  of front and rear covers  155 ,  160 , respectively, and threaded into an aligned screwbore  180  in body  165 . Front cover  155 , rear cover  160  and body  165  are preferably made of metal, and in some embodiments, are made of stainless steel. 
     Piston  110  further includes an annular sealing member  190  disposed radially outward and adjacent to body  165 . Sealing member  190  includes a generally cylindrical inner surface  215 , a generally cylindrical outer surface  217 , and two irregular end surfaces  195 ,  200 . Outer surface  217  includes one or more seal grooves  192 . Irregular end surface  195  includes an annular recess and an annular extending lip. Front cover  155  includes an inner mating surface  205  that includes an annular extending lip and an annular recess that are shaped to receive the annular recess and the annular extending lip, respectively, thus allowing irregular end surface  195  of sealing element  190  to interlock with inner mating surface  205  of front cover  155 . Similarly, irregular end surface  200  includes an annular recess and an annular extending lip. Rear cover  160  includes an inner mating surface  210  that includes an annular extending lip and an annular recess that are shaped to receive the annular recess and the annular extending lip, respectively, thus allowing irregular end surface  200  of sealing element  190  to interlock with inner mating surface  210  of rear cover  160 . 
     Once sealing element  190  is sandwiched between front and rear covers  155 ,  160  about body  165 , as shown, the shape of irregular surfaces  195 ,  200  and their respective mating surfaces  205 ,  210  on front and rear covers  155 ,  160  hold sealing element  190  in position and prevent translational movement of sealing member  190  relative to the other components of piston  110 . Sealing member  190  is not, however, connected in any other way to the adjacent components of piston  110 . As will be described, sealing member  190  is instead free to expand in the radially outward direction when a pressure load is applied along its inner surface  215 , and to subsequently contract or relax when the pressure load is removed. To enable such expansion and contraction, sealing member  190  is preferably made of a resilient material, such as an elastomer, and in some embodiments, is made of polyurethane. 
     Front cover  155  further includes a series of axial flowbores  220 , each spaced circumferentially about piston rod assembly  100 . Body  165  further includes a series of radial flowbores  227 , each flowbore  227  coupled to an axial flowbore  225 , also spaced circumferentially about piston rod assembly  100 . When piston  110  is assembled as shown, an axial flowbore  220  of front cover  155  aligns with an axial flowbore  225  of body  165  to form an “L-shaped” flowpath  230  extending from an inlet  235  in front cover  155  to inner surface  215  of sealing element  190 . 
     Body  165  further includes an annular groove  240  along its inner surface adjacent rod  105 . Similarly, rear cover  160  further includes an annular groove  245  along its inner surface adjacent rod  105 . Grooves  240 ,  245  are configured to receive annular sealing elements  250 ,  255 , respectively. In some embodiments, including those illustrated by  FIG. 1 , sealing elements  250 ,  255  are O-rings. Sealing elements  250 ,  255  prevent loss of fluid from flowbores  220 ,  225  other than through inlet  235 . 
     As will be described, piston rod assembly  100  may be installed within a reciprocating pump and used to pressurize fluid, such as drilling mud. During operation of the pump, the fluid, referred to henceforth as drilling fluid, enters inlet  235  of front cover  155  and flows along flowpath  230  through flowbore  220  of front cover  155  and flowbores  225 ,  227  of body  230  until reaching sealing element  190 , where the drilling fluid applies a pressure load to inner surface  215  of sealing element  190 . In response to the applied pressure load, sealing element  190  expands in the radially outward direction. Moreover, the higher the pressure of the drilling fluid, the greater the pressure load to sealing element  190  and the more sealing element  190  expands radially outward. 
     Conversely, when the drilling fluid exits flowbores  220 ,  225 ,  227  through inlet  235  or when the pressure of the drilling fluid contained within flowbores  220 ,  225 ,  227  decreases, the pressure load applied to inner surface  215  of sealing element  190  also decreases. In response to the pressure load reduction, sealing element  190  relaxes or contracts. In the absence of drilling fluid pressure, sealing element  190  relaxes to its unexpanded configuration, as shown in  FIG. 1 . 
     Some drilling fluids, such as mud, which may be pressurized by a pump including a pressure compensated piston, contain abrasive particles that may damage the sealing element of the piston, or be otherwise incompatible with the piston, and cause excessive wear and ultimately loss of the seal between the piston and surrounding cylinder. In such circumstances, it may be desirable to include a barrier within the piston rod assembly to prevent exposure of the sealing element to the abrasive drilling fluid.  FIG. 2  depicts a pressure compensated piston rod assembly that includes such a barrier. 
     Referring now to  FIG. 2 , piston rod assembly  300  includes a rod  305  disposed within a pressure compensated piston  310 . A retainer  315  couples piston  310  to rod  305 . In this embodiment, retainer  315  includes a washer  320  and a nut  325  threaded onto an end  330  of rod  305 . Pressure compensated piston  310  is annular, and includes an axial throughbore  335  and a circular recess  332 , both configured to receive rod  305 , as shown. Rod  305  includes a first base portion  348  coupled to a second base portion  345  having an elongate extension  340 . Extension  340  of rod  305  is inserted through axial throughbore  335  of piston  310 . Second base portion  345  of rod  305  has a diameter greater than that of extension  340  and thereby forms a shoulder  350 . Circular recess  332  of piston  310  receives shoulder  350  of rod  305  such that piston  310  is seated against rod  305 . 
     Piston  310  further includes a front cover  355 , a rear cover  360  and a body  365  disposed therebetween, all of which are annular. Body  365  includes circular recesses  363  that receive circular projections  367 ,  369  of rear cover  360  and front cover  355 , respectively. When front cover  355 , rear cover  360  and body  365  are assembled or constructed as shown, these components form axial therethrough  335 , which receives extension  340  of rod  305 . Front cover  355 , rear cover  360  and body  365  are preferably made of metal, and in some embodiments, are made of stainless steel. 
     Piston  310  further includes an annular sealing member  390  disposed radially outward and adjacent to body  365 . Sealing member  390  includes a generally cylindrical inner surface  415 , a generally cylindrical outer surface  417 , and two irregular end surfaces  395 ,  400 . Outer surface  417  includes one or more seal grooves  392 . Irregular end surface  395  includes an annular recess and an annular extending lip. Front cover  355  includes an inner mating surface  405  that includes an annular extending lip and an annular recess that are shaped to receive the annular recess and the annular extending lip, respectively, thus allowing irregular end surface  395  of sealing element  390  to interlock with inner mating surface  405  of front cover  355 . Similarly, irregular end surface  400  includes an annular recess and an annular extending lip. Rear cover  360  includes an inner mating surface  410  that includes an annular extending lip and an annular recess that are shaped to receive the annular recess and the annular extending lip, respectively, thus allowing irregular end surface  400  of sealing element  390  to interlock with inner mating surface  410  of rear cover  360 . 
     Once sealing element  390  is sandwiched between front and rear covers  355 ,  360  about body  365 , as shown, the shape of irregular surfaces  395 ,  400  and their respective mating surfaces  405 ,  410  on front and rear covers  355 ,  360  hold sealing element  390  in position and prevent translational movement of sealing member  390  relative to the other components of piston  310 . Sealing member  390  is not, however, connected in any other way to the adjacent components of piston  310 . As will be described, sealing member  390  is instead free to expand in the radially outward direction when a pressure load is applied along its inner surface  415 , and to subsequently contract or relax when the pressure load is removed. To enable such expansion and contraction, sealing member  390  is preferably made of a resilient material, such as an elastomer, and in some embodiments, is made of polyurethane. 
     Rod  305  further includes a cavity  455  therein and an axial flowbore  460  coupled thereto. Axial flowbore  460  extends from cavity  455  through end  330  of rod  305 , terminating at an inlet  435 . As will be described, drilling fluid enters rod  305  through inlet  435  and flows through flowbore  460  into cavity  455 . 
     A hydraulic system  490  is also coupled to cavity  455  and extends initially from cavity  455  in the opposite direction as that of axial flowbore  460 . Hydraulic system  490  includes an axial flowbore  465 , which extends from cavity  455  through first base portion  348  of rod  305 , terminating at two radial flowbores  505 ,  510 . Radial flowbore  505  extends between axial flowbore  465  and an outer surface  472  of first base portion  348  of rod  30 , where radial flowbore  505  terminates at a vent port  470 . Hydraulic system  490  further includes a series of flowbores  515 ,  520 ,  525   530 ,  535  extending from radial flowbore  510  through first base portion  348  and second base portion  345  of rod  305 , rear cover  360  of piston  310  and body  365  of piston  310  to an outer surface  540  of body  365 . Hydraulic fluid is delivered between cavity  455  and outer surface  540  of body  365  adjacent inner surface  415  of sealing element  390  via flowbores  510 ,  515 ,  520 ,  525 ,  530 ,  535 . Hydraulic system  490  further includes an inlet port  485  along an outer surface  480  of second base portion  345  of rod  305  and a series of flowbores  545 ,  550 ,  555 ,  560  extending from inlet port  485  through second base portion  345  of rod  305 , rear cover  360  of piston  310  and body  365  of piston  310  to outer surface  540  of body  365 . Hydraulic fluid is delivered from input port  485  to outer surface  540  of body  365  adjacent inner surface  415  of sealing element  390  via flowbores  545 ,  550 ,  555 ,  560 . 
     To prevent loss of hydraulic fluid between cavity  455  and outer surface  540  of body  365 , sealing members  518  are disposed between first base portion  348  and second base portion  345  around flowbore  520 , between second base portion  345  and rear cover  360  around flowbore  520 , and between rear cover  360  and body  365  around flowbore  530 . Similarly, to prevent loss of hydraulic fluid between input port  485  and outer surface  540  of body  365 , sealing members  518  are disposed between second base portion  345  and rear cover  360  around flowbore  545  and between rear cover  360  and body  365  around flowbore  555 . To prevent loss of hydraulic fluid applied to inner surface  415  of sealing element  390 , and instead allow that fluid to return to hydraulic system  490  via radial flowbore  535 , sealing members  518  are also disposed between body  365  and rear cover  360 , between body  365  and front cover  355 , and between rear cover  365  and shoulder  350  of rod  305 . In some embodiments, sealing members  518  are O-rings seated in annular grooves formed in second base portion  345  of rod  305 , rear cover  360  of piston  310  and body  365  of piston  310 . 
     An incompressible fluid, such as oil, is contained within hydraulic system  490 . During assembly of piston rod assembly  300 , hydraulic fluid is injected into hydraulic system  490  at input port  485 . Any air that may be trapped in the hydraulic fluid is then bled off through vent port  470 . If necessary, additional hydraulic fluid is injected into hydraulic system  490 , and again, any air trapped in the hydraulic fluid is bled off. This process is repeated until hydraulic system  490  is completely fill and contains a solid column of hydraulic fluid. Input port  485  and vent port  470  are then closed. When necessary or desired, the hydraulic fluid may be drained from hydraulic system  490  through vent port  470 . 
     A pressure transfer element  525  is disposed within cavity  455  of rod  305 . Pressure transfer element  525  is a barrier between drilling fluid that enters axial flowbore  460  through inlet  435  and the incompressible fluid contained within hydraulic system  490 . As such, the drilling fluid, which may contain abrasive particles or be otherwise incompatible with sealing element  390 , is prevented by pressure transfer element  525  from mixing with or contaminating the fluid contained within hydraulic system  490 . Thus, pressure transfer element  525  prevents exposure of sealing element  390  to the potentially abrasive or incompatible drilling fluid, such as mud. 
     Pressure transfer element  525  also transfers the pressure of drilling fluid contained in rod  305  to the fluid contained within hydraulic system  490 , and vice versa, such that the fluid pressure on both sides of pressure transfer element  525  is substantially balanced. During operation of a pump including piston rod assembly  300 , high pressure drilling fluid enters axial flowbore  460  of rod  305  through inlet  435  and exerts pressure on pressure transfer element  525 , which, in turn, pressurizes fluid contained within hydraulic system  490 . As the hydraulic fluid pressure increases, the hydraulic fluid pushes against inner surface  415  of sealing member  390  with increasing force. In response, sealing member  390  increasingly expands in the radially outward direction. Conversely, as the drilling fluid pressure decreases, the pressure exerted by this fluid on pressure transfer element  525 , and in turn, on the hydraulic fluid also decreases. In response, the force exerted by the hydraulic fluid on inner surface  415  of sealing element  390  is reduced, allowing sealing element  390  to contract or relax. 
     In the embodiment shown in  FIG. 2 , pressure transfer element  525  is a diaphragm. Diaphragm  525  is a hollow, bell-shaped cup made of neoprene, or other suitable material, that collapses under pressure and expands again when the applied pressure is reduced or removed. Diaphragm  525  includes a generally cylindrical thin wall with an open end to receive hydraulic fluid in hydraulic system  490  and a closed end proximate axial flowbore  460 . At the open end, the cylindrical wall is flanged. This flanged end is compressed between first base portion  348  and second base portion  345  of rod  305  to hold diaphragm  525  in place within cavity  455 . The dimensions, e.g., length and/or internal volume, of diaphragm  525 , measured at its natural state in the absence of any pressure exerted upon it by the hydraulic fluid or drilling fluid, are chosen such that when diaphragm  525  is fully collapsed, the pressure exerted on sealing member  390  by the hydraulic fluid is sufficient to maintain a seal between piston  310  and a surrounding pump cylinder under the full range of expected drilling fluid pressures. 
     During operation of piston rod assembly  300 , drilling fluid enters the flowbore  460  of rod  305  and exerts pressure on diaphragm  525 . Upon application of pressure from the drilling fluid, diaphragm  525  collapses, expelling hydraulic fluid contained within its cup-like shape, thereby pressurizing the fluid contained within hydraulic system  490 . The hydraulic fluid then exerts pressure on inner surface  415  of sealing element  390 , forcing sealing element  390  to displace in the radially outward direction. Conversely, as the drilling fluid pressure decreases, diaphragm  525  expands and again receives hydraulic fluid within its cup-like shape. In response, the pressure of fluid within hydraulic system  490  decreases, and sealing element  390  subsequently contracts or relaxes. 
     One of ordinary skill in the art will readily appreciate that the components of piston  310  may take other forms while still performing the same functions. For example, the position of cavity  455  may vary along the length of rod  305 . Moreover, a rod extension may be coupled to end  550  of rod  305 , and cavity  455  disposed within the rod extension. The locations and dimensions of the components forming hydraulic system  490  could then be modified to accommodate the new position of cavity  455 . However, their function and the principles of operation of pressure compensated piston  310  would be as described above. Further, the general layout of hydraulic system  490  may be modified from that shown in  FIG. 2 , while still providing transfer of fluid pressure from the drilling fluid through pressure transfer element  525  and the hydraulic fluid to sealing element  390 , and vice versa. 
     Embodiments of a pressure compensated piston rod assembly, including assembly  100  of  FIG. 1  and assembly  300  of  FIG. 2 , find application in pumps, and in particular, reciprocating mud pumps used in connection with well drilling operations. Turning to  FIG. 3 , reciprocating mud pump  10  includes a fluid end  20  and a power end  30 . Fluid end  20  includes a piston, which in this example is piston  110 , shown in and described with reference to  FIG. 1 . Fluid end  20  further includes cylinder liner  24 , module  26 , intake valve  27  and outlet valve  28 . Power end  30  includes a crankshaft  32 , connecting rod  34  and crosshead  36 . Fluid end  20  is coupled to power end  30  by an extension rod  42 , rod sub  46  and rod  105 , also shown in and described with reference to  FIG. 1 . Extension rod  42  connects to crosshead  36  and is coupled by clamp  50  to rod sub  46  and rod  105 , which connects to piston  110 . Although extension rod  42  is coupled to rod sub  46  by clamp  50  in this embodiment, these components may be coupled by other equivalent means, such as but not limited to a threaded connection. As previously described, rod  105  with piston  110  coupled thereto forms piston rod assembly  100 . It will be understood that, instead of piston rod assembly  100 , pump  10  may instead include piston rod assembly  300 , shown in and described with reference to  FIG. 2 . 
     The design of piston  110 , and in particular sealing element  190 , is such that, after installation of piston rod assembly  100  within pump  10 , there is radial interference between sealing element  190  ( FIG. 1 ) of piston  110  and surrounding cylinder  24 . In some embodiments, the diametrical interference of sealing member  190  with cylinder  24  after installation is 0.060 inches. This interference compresses sealing element  190 , causing sealing element  190  to exert force against cylinder  24 . The force exerted by sealing element  190  against cylinder  24  creates an initial seal between piston  110  and cylinder  24 . 
     During operation, pump  10  draws drilling mud through intake valve  27  into module  26  where the drilling mud is pressurized by piston  110 . Drilling fluid is then expelled at high pressure from pump  10  through outlet valve  28 . During this pressurization process, piston rod assembly  100  is exposed to the pressurized drilling mud. Although the interference between sealing element  190  ( FIG. 1 ) of piston  110  and cylinder  24  is sufficient to maintain the seal between these components when piston rod assembly  100  is exposed to low pressure drilling fluid, higher contact force between sealing element  190  and cylinder  24  is desired to maintain the seal when piston  110  is exposed to higher drilling fluid pressures. 
     To accommodate this goal, pressure compensated piston rod assembly  100  is configured to exert increasing force on cylinder  24  with increasing drilling fluid pressure. Thus, piston rod assembly  100  can maintain the seal between piston  110  and cylinder  24  as drilling fluid pressure increases. As previously described, during operation of pump  10 , drilling fluid enters piston  110  through inlet  235  ( FIG. 1 ) and flows through flowbores  220 ,  225 ,  227  until reaching inner surface  215  of sealing element  190 , where the drilling fluid applies a pressure load to sealing element  190 . In response to the applied pressure load, sealing element  190  expands in the radially outward direction and applies increased force to cylinder  24 . The increased force exerted by piston  110  on cylinder  24  enables the seal between these components to be maintained. Moreover, as the drilling fluid pressure continues to increase, the pressure load exerted by the drilling fluid on sealing element  190 , and, in turn, by sealing element  190  on cylinder  24  also continues to increase. In this manner, piston rod assembly  100  compensates for increasing drilling fluid pressure so as to maintain the seal between piston  110  and cylinder  24 . 
     Conversely, when drilling fluid pressure decreases, the pressure load exerted by the drilling fluid on sealing element  190  decreases. In response, sealing element  190  contracts or relaxes, and the force exerted by sealing element  190  on cylinder  24  is reduced while still maintaining the seal between these components. Further, the friction load to piston  110  due to contact between sealing element  190  and cylinder  24  also decreases. 
     In this manner, pressure compensating piston rod assembly  100  applies to sealing element  190  only the minimum pressure needed to maintain the seal between piston  110  and cylinder  24 , where the minimum pressure needed to maintain the seal depends on the drilling fluid pressure. Moreover, by adjusting the force exerted by piston  110  on cylinder  24  to only that required to maintain the seal, the frictional load created by contact between sealing element  190  and cylinder  24  is minimized. This results in increased pump efficiency and less wear to sealing element  190 , thereby increasing the service life of piston  110 . 
     By contrast, the frictional load between a sealing element and a cylinder in many conventional mud pumps is constant. That is the case because the force exerted by the sealing element against the surrounding cylinder does not vary, whether the pump experiences a minimum or maximum drilling fluid pressure. For this reason, the pump is designed to provide interference between the piston and surrounding cylinder such that the seal between the piston and the cylinder is maintained under the entire range of expected drilling fluid pressures. In other words, the interference is chosen based on worst case conditions. This means that when the pump is not operating under such worst case conditions, the interference is more than needed to maintain the seal. This creates an excessive frictional load between the sealing element and cylinder, causing unnecessary wear to the sealing element and reductions in pump efficiency. 
     While various embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible. For example, the relative dimensions of various parts and the materials from which the various parts are made can be varied. Accordingly, the scope of protection is not limited to the embodiments specifically described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.