VARIABLE CAPACITY PLUNGER PUMP

[0001] This disclosure relates to hydraulic fracturing in general and, more specifically, to plunger pumps used for pumping pressurized fracturing fluid down wellbores for purposes of reservoir stimulation.

[0002] Subterranean hydraulic fracturing is conducted to increase or stimulate production from a hydrocarbon well. To conduct a fracturing process, high pressures are used to pump special fracturing fluids, including some that contain “proppants”, down the well-bore and into a hydrocarbon formation to split or “fracture” the rock formation along veins or planes extending from the well-bore. Once the desired fracture is formed, the fluid flow is reversed and the liquid portion of the fracturing fluid is removed. The proppants are intentionally left behind to stop the fracture from closing onto itself due to the weight and stresses within the formation.

[0003] The proppants support the fractures in open positions, yet remain highly permeable to hydrocarbon fluid flow since they form a packed bed of particles with interstitial void spaces that provide permeability. Sand is one example of a commonly-used proppant. Fractures that are propped open with proppant clusters can thus serve as new formation drainage areas and new flow conduits from the formation to the well-bore, thereby providing increased flow rates, and hence increased hydrocarbon production.

[0004] Plunger pumps are commonly used in the oil and gas industry for hydraulic fracturing purposes. Plunger pumps have a fluid end and a power end that drives the fluid end. Plunger pumps that are used to pump fracturing fluids down a well-bore generally require power ends rated at 2,000 hp (1492 W) or more. To reach this power level, multiplex plunger pumps (i.e., more than one plunger) may be used as disclosed in US 2012/0063936.

[0005] The fluid ends of plunger pumps frequently have a service life of less than 200 hours as the fracturing fluid proppant slurries are abrasive. Further, other well stimulation fluids may be corrosive. Still further, well stimulation fluids including fracturing fluids are pumped at rates of up to 100 bbl/min (16 kl/min) and pressures of 10,000 psi (5.2×10⁵ mmHg) or more.

[0006] The fluid ends (or outlets) of plunger pumps have many parts that are releasably fastened to one another so that the plungers and valves can be easily repaired or replaced as disclosed in US 2012/0222760. However, the connections between the parts and the supporting features for the valves and the 90° turns that tend to weaken a fluid end, limiting its pressure rating, and making it susceptible to cracking under high, cyclical stresses. Thus, while many improvements for extending the service life and/or facilitating the maintenance of plunger pumps have been developed, short service lives and/or complex maintenance requirements persist.

[0007] In one aspect, a pump is disclosed. The pump may include an inlet for receiving fluid and an outlet for discharging fluid. The pump may also include a housing assembly that defines a housing bore. The pump may also include an axial plunger slidably disposed within the housing bore to form a compression chamber disposed between a distal end of the plunger and the outlet. The compression chamber may be in axial alignment with the outlet. The plunger may be configured to receive fluid from the inlet and pass fluid to the compression chamber. When the plunger is at a first position, the compression chamber may be at a first volume for receiving fluid from the plunger at a first pressure. Further, when the plunger is at a second position, the compression chamber may be at a smaller second volume to pressurize the fluid within the compression chamber to a higher second pressure. The fluid at the second pressure may then be discharged from the compression chamber into the outlet.

[0008] In another aspect, a pump with an inlet for receiving fluid and an outlet for discharging fluid is also disclosed. The pump may also include a housing assembly that defines a housing bore. The pump may also include an axial plunger slidably disposed within the housing bore to form a compression chamber disposed between a distal end of the plunger and the outlet. The compression chamber may be in axial alignment with the outlet. The plunger may have a side wall that defines a plunger bore. The plunger may include at least one opening formed in the side wall of the plunger that is configured to pass fluid from the inlet into the plunger bore. The plunger may then be configured to communicate fluid from the plunger bore to the compression chamber. When the plunger is at a first position, the compression chamber may be at a first volume to receive fluid from the plunger at a first pressure. When the plunger is at a second position, the compression chamber may be at a smaller second volume to pressurize fluid within the compression chamber to a higher second pressure. Fluid at the second pressure may then be discharged from the compression chamber into the outlet.

[0009] A method for pumping hydraulic fracturing fluid is also disclosed. The method may include retracting a plunger to increase a volume of a compression chamber defined within a housing bore of a pump between a distal end of the plunger and an outlet of the pump. The plunger may also provide communication between an inlet of the pump and the compression chamber in order to receive fluid in a plunger bore of the plunger. The compression chamber may be in axial alignment with the outlet of the pump. The plunger may be configured to communicate fluid from the plunger bore to the compression chamber. The method may further include moving the plunger to reduce a volume of the compression chamber to pressurize fluid within the compression chamber and the method may further include dispensing the pressurized fluid from the compression chamber to the outlet.

[0010] In another aspect, a pump module is disclosed. The pumping module may include a plurality of pumps arranged in a side by side configuration. Each pump may include an inlet for receiving fluid, an outlet for discharging fluid and a housing assembly defining a housing bore. Each pump may further include an axial plunger slidably disposed within the housing bore to form a compression chamber disposed between a distal end of the plunger and the outlet. The compression chamber may be in axial alignment with the outlet. The plunger may be configured to receive fluid from the inlet and pass fluid to the compression chamber. When the plunger is at a first position, the compression chamber may be at a first volume to receive fluid from the plunger at a first pressure. When the plunger is at a second position, the compression chamber may be at a smaller second volume to pressurize the fluid within the compression chamber to a higher second pressure, which results in the fluid at the second higher pressure being discharged from the compression chamber to the outlet.

[0011] A discharge valve assembly for a pump is also disclosed. The discharge valve assembly may include a discharge valve sleeve coupled to a bore of a pump. The discharge valve sleeve may include a radial inwardly directed valve seat at a proximal end of the discharge valve sleeve. The discharge valve assembly may also include a retainer that may include a proximal side spaced apart from the valve seat and a distal side that faces an outlet of the pump. The retainer may have a hub that extends proximally from the proximal side of the retainer and the retainer may also include at least one passageway axially extending through the retainer to provide communication between the proximal side of the retainer and the outlet of the pump. The discharge valve assembly may also include a discharge valve head disposed between the retainer and the discharge valve seat. The discharge valve head may have a proximal side to sealably engage the valve seat when the discharge valve assembly is in a closed position. The discharge valve head may also include a shaft that extends from a distal side of the discharge valve head that is slidably received in the hub of the retainer.

[0012] In yet another aspect, a pump is disclosed that includes an inlet for receiving fluid and an outlet for discharging fluid. The pump may also include a housing assembly that defines a housing bore. The pump may further include an axial plunger slidably disposed within the housing bore to form a compression chamber disposed between a distal end of the plunger and the outlet. The compression chamber may be in axial alignment with the outlet. The plunger may be configured to receive fluid from the inlet and pass fluid through the compression chamber. A discharge valve sleeve may be included that is coupled to the housing bore. The discharge valve sleeve may include a valve seat at a proximal end of the discharge valve sleeve. The pump may also include a retainer that includes a proximal side spaced apart from the valve seat and a distal side that faces the outlet of the pump. The retainer may have a hub extending proximally from the proximal side of the retainer. The retainer may include at least one passageway extending through the retainer to provide communication between the proximal side of the retainer and the outlet. The pump may also include a discharge valve head disposed between the retainer and the discharge valve seat. The discharge valve head may have a proximal side that sealably engages the valve seat when the discharge valve assembly is in a closed position and a shaft extending from a distal side of the discharge valve head that is slidably received within the hub of the retainer. The discharge valve head may be biased towards a closed position where the discharge valve head engages the discharge valve seat. When the pressure of the fluid in the compression chamber is beyond a threshold pressure, the discharge valve head may be movable away from the closed position to permit pressurized fluid to move from the compression chamber to the outlet.

[0013] A method for pumping hydraulic fracturing fluid is also disclosed. The method may include biasing a discharge valve head against a discharge valve seat to close an outlet of a pump. The discharge valve head may be connected to a shaft that may be slidably received within a hub that may be connected to a proximal side of a retainer. The method may include providing communication between an inlet of the pump and an axial housing bore defined by a housing assembly of the pump. The method may further include moving a plunger disposed axially within the housing bore towards the discharge valve head thereby pressurizing a portion of the housing bore disposed between the plunger and the discharge valve head to form a compression chamber in axial alignment with the discharge valve head thereby overcoming the bias of the discharge valve head against the valve seat and providing communication between the compression chamber and the outlet of the pump. The method may further include dispensing fluid through the outlet.

[0014] A plunger for a plunger pump is also disclosed. The plunger may include as sidewall extending between a proximal end and an open distal end. The sidewall may have a plunger bore extending axially through the sidewall. The sidewall may also include at least one opening to receive fluid from an inlet of the plunger pump into the plunger bore. The plunger may also include a valve seat that is coupled to the distal end of the plunger. The plunger may also include an outlet valve head for sealable engagement with the valve seat. The plunger may further include a rod that extends axially from the outlet valve head within the plunger bore. The rod may also include a proximal end coupled to a piston that may be slidably received within the plunger bore. The rod may also include a distal end coupled to the outlet valve head. The outlet valve head, the rod and the piston may form a valve assembly movable between open and closed positions. In the closed position, the piston is at a first position and the outlet valve head is configured to sealably engage the valve seat. In the open position, the position is moved distally away from the first position and the outlet valve head is moved away from the valve seat.

[0015] A plunger pump is also disclosed. The plunger pump may include a housing assembly including an inlet, an outlet and an axial housing bore extending therebetween. The housing bore may slidably accommodate a plunger. The plunger may include a sidewall that extends between a proximal end and a distal end of the plunger and the sidewall may define a plunger bore extending axially within the sidewall. The plunger may be configured to receive fluid from an inlet of the plunger pump into the plunger bore. The plunger may also include a valve seat and an outlet head for sealable engagement with the valve seat. The plunger may further include a rod that extends axially from the outlet head within the plunger bore and a piston may be coupled to a proximal end of the rod. A distal end of the rod may be coupled to the outlet valve head. The piston may be slidably received within the plunger bore. The outlet head, rod and piston form a valve assembly that is movable between open and closed positions. In the closed position, the piston may be at a first position and the outlet head may be configured to sealably engage the valve seat. In the open position, the piston may be moved distally away from the first position and the outlet valve head may be moved away from the valve seat.

[0016] In another aspect, a method for pumping hydraulic fracturing fluid is also disclosed. The method may include providing a pump with an inlet, an outlet, a proximal end, a distal end and an axial housing bore which slidably receives a plunger. The method may further include retracting the plunger axially away from a discharge valve assembly connected to the outlet of the pump that may be disposed at a distal end of the pump. The retracting of the plunger may reduce pressure in the axial housing bore of the pump which may open an outlet valve assembly disposed at a distal end of the plunger. The method may further include providing communication between an inlet of the pump and the housing bore of the pump through the open outlet valve assembly of the plunger. The method may further include moving the plunger towards the discharge valve assembly thereby increasing pressure in the housing bore thereby closing the outlet valve assembly. The method may further include pressurizing at least a portion of the housing bore disposed between the outlet valve assembly of the plunger and the discharge valve assembly thereby opening the discharge valve assembly. Finally, the method may further include dispensing fluid through the open discharge valve assembly to the outlet.

[0025] FIG. 1 illustrates a cross-section of a disclosed plunger pump 10 in a static position where the discharge valve assembly 11 shown at the right in FIG. 1 near the outlet 12 and the outlet valve assembly 13 disposed at the end of the plunger 14 are both in a closed position. The pump 10 may include a housing assembly 15 that may be unitary in construction or that may be provided in multiple parts as shown in FIG. 1. Specifically, the housing assembly 15 includes an inlet housing 16 having a proximal end 17 that may be lined with a proximal seal 18. The inlet housing 16 may also include a distal end 21 that may include a flange 22 that may be used to couple the inlet housing 16 to the main housing body 23 of the housing assembly 15 by way of a plurality of an attachment mechanism such as mechanical fasteners 24, welding, or other attachment mechanisms known by one skilled in the art. The inlet housing 16 may also include an inlet 25 having an inlet axis 26.

[0026] The main housing body 23 may also include a proximal end 27 that may include a flange 28 that may be connected to the flange 22 of the inlet housing 16 as shown in FIG. 1. The proximal end 27 of the main housing body 23 may be lined with a middle seal 31. The purposes of the proximal and middle seals 18, 31 will be described below in connection with the plunger 14. Continuing with the description of the main body housing 23, the main housing body 23 may also include a distal end 32 that may be connected to an outlet body 33 by way of one of the aforementioned attachment mechanisms such as mechanical fasteners 34. The outlet body 33 may include a flange 35 for accommodating the fasters 34. Further, the outlet body 33 may be equipped with an annular groove 36 for accommodating a ring seal 37 as shown in FIG. 1.

[0027] The discharge valve assembly 11 may include a discharge valve head 38 that may be connected to a shaft 41. The shaft 41 may be received in a hub 42 that is connected to a retainer 43. The hub 42 may accommodate a spring or biasing member 44 for biasing the discharge valve head 38 against the discharge valve seat 45. The spring 44 may also surround the hub 42. Other biasing schemes will be apparent to those skilled in the art. A discharge valve seat 45 may be part of a discharge valve sleeve 46 which may include a distal flange 47 that is trapped between the outlet body 33 and the main housing body 23 as shown in FIG. 1. The discharge valve sleeve 46 may also include an annular slot 48 for accommodating a ring seal 51. The housing assembly 15 also defines a housing bore 67 which becomes filled with fluid to be discharged through the outlet 12. The filling of the housing bore 67 with fluid will be described in connection with FIG. 4 below.

[0028] Turning to the plunger 14, the plunger 14 may include a proximal end 52 that may be coupled to an actuator 53 as shown in phantom in FIG. 1. The flange 59 may be used for this coupling. The actuator 53 may be a motor, either electric, internal combustion or hydraulic with the requisite linkages needed to move the plunger 14 in a reciprocating fashion as illustrated in FIGS. 1, 2 and 4. The plunger 14 may also include a distal end 54 that may be detachably coupled to a valve seat 55 to facilitate replacement of the valve seat 55 if the valve seat were to wear. In the embodiment shown, the outlet valve seat 55 is threadably connected to the distal end 54 of a sidewall 57 of the plunger 14. Of course, other means for attaching the outlet valve seat 55 to the sidewall 57 will be apparent to those skilled in the art. The outlet valve seat 55 may also be unitary or integral with the sidewall 57. The outlet valve head 56 can sealably engage the valve seat 55 when the outlet valve 13 is in the closed position as shown in FIG. 1.

[0029] The sidewall 57 of the plunger 14 may also include at least one opening 58. When more than one openings 58 are provided, the openings 58 may be disposed circumferentially along the sidewall along a common region or disposed offset from one another. The sidewall 57 may define a plunger bore 61. Fluid may enter the plunger bore 61 through the inlet 25 and at least one opening 58 thereby loading the plunger 14 with fluid. The outlet valve head 56 may be connected to a distal end of a rod 62 which, in turn, is connected to a piston 63 circumferentially disposed about the proximal end of the rod 62. The rod 62 is shown extending axially through the plunger bore 61. The piston 63 may engage the interior 64 of the sidewall 57. However, the piston 63 may also include a metered passageway 65. For example, the metered passageway may include one or more axial bores formed in the piston 63. The purpose of the metered passageway 65 is to permit fluid trapped in the proximal cavity 66 to pass through the metered opening 65 as the outlet valve assembly 13 moves from the open position shown in FIGS. 4-5 to the closed position shown in FIG. 1. In other words, fluid disposed within the plunger 14 and, more particularly, fluid trapped in the proximal cavity 66 in combination with the metered passageway 65 may act as a damper to the closing of the outlet valve head 56 and prevent the outlet valve head 56 from engaging the outlet valve seat 15 with excessive force. This dampening of the closing action of the outlet valve assembly 13 can facilitate the reduction of wear and tear on the outlet valve head 56 and the outlet valve seat 55. Thus, at least one of the outlet valve head 56, the rod 62, the piston 63, and the metered opening 65 can provide a dampening effect when the outlet valve assembly 13 is closed. Further, the outlet valve head 56 may be detached from the rod 62. For example, the outlet valve head 56 may be threadably connected to the rod 62. Other means for attaching the outlet valve head 56 to the rod 62 will be apparent to those skilled in the art and the two components may be unitary in structure as well.

[0030] In FIG. 2, the plunger 14 has been moved to the right to a dispense position from the static position of FIG. 1. When in the dispense position, fluid disposed in the housing bore 67 has been pressurized by the rightward of movement of the plunger 14 to form a compression chamber out of a portion of the housing bore 67 as the outlet valve head 56 moves towards the discharge valve head 38. The reader will note that when the pressure in the housing bore 67 has increased beyond a threshold pressure to permit the discharge valve head 38 to overcome the bias of the spring 44 and become separated from the discharge valve seat 45. Thus, fluid flows from the housing bore 67 past the discharge valve head 38 and through one or more passageways 68 in the retainer 43 before exiting the pump 10 through the outlet 12. Pressure is able to be increased in the housing bore 67 due to the presence of the middle seal 31 which provides a seal between the sidewall 57 of the plunger 14 and the main housing body 23 or, more specifically, the proximal end 27 of the main housing body 23 in combination with the rightward movement of the plunger 14.

[0031] FIG. 3 illustrates the separation between the discharge valve seat 45 and the discharge valve head 38. As shown in FIG. 3, the discharge valve head 38 may include a frusto-conical section 71 that is configured to sealably engage the similarly configured discharge valve seat 45 when the discharge valve assembly 11 is in the closed position as shown in FIG. 1. FIGS. 3 and 4 also illustrate the ease in which the discharge valve assembly 11 can be replaced. The outlet body 33 may be easily removed by removing the fasteners 34. Then, the retainer 43 may be disconnected, or in the example shown, unscrewed from the discharge valve sleeve 46 thereby enabling the retainer 43, spring 44 and discharge valve head 38 to be removed through the distal end 32 of the main housing body 23. Then, the discharge valve sleeve may be disconnected or, in the example shown, unscrewed from the threaded inner surface 69 of the housing bore 67.

[0032] In FIG. 4, the pump 10 is shown in a reload position as the plunger 14 is being moved to the left in the shown orientation away from the dispense position of FIG. 2. The leftward movement of the plunger 14 decreases the pressure in the housing bore 67. A decrease in the pressure in the housing bore 67 below the threshold pressure can permit the discharge valve head 38 to close because of bias of the spring 44 and re-engage the discharge valve seat 45. Further, the lower pressure in the housing bore 67 can permit the outlet valve assembly 13 to open as shown in FIG. 4. Specifically, during the leftward movement of the plunger 14, the pressure in the housing bore 67 can be less than the pressure within the plunger bore 61 of the plunger 14 thereby causing a pressure difference to allow the outlet valve head 56 to become unseated from the outlet valve seat 55 as shown in FIG. 4. The outlet valve head 56 may include a frusto- conical section that is configured to sealably engage a similarly configured face of the outlet valve seat 55 when the discharge valve assembly 11 is in the closed position as shown in FIG. 1. While the outlet valve seat 55 is shown in the form of a replaceable liner connected to the plunger sidewall 57, those skilled in the art will realize that the outlet valve seat 55 may be integrally connected to the sidewall 57 of the plunger 14.

[0033] Referring to FIGS. 1, 2 and 4, in one example, a replaceable cylindrical liner 73 may line the housing bore 67 and, accordingly, reduces the effective outer diameter of the housing bore 67 (or the effective inner diameter of the main housing section 23). Therefore, one way to change the volume of the housing bore 67 is to change the replaceable liner 73 with a liner that is either thicker or thinner than the liner 73. Hence, the capacity of the pump 10 can be altered by simply changing the liner 73. This can be achieved in one of two ways. Perhaps one way would be to remove the fasteners 34 and outlet body 33. Then, the retainer 43 can be removed before the discharge valve sleeve 46 and discharge valve head 38 can be removed together thereby exposing the sleeve 73 which may be secured to the main housing body 23 via a threaded connection or another connection that will be apparent to those skilled in the art. Also, the sleeve 73 may simply be snugly accommodated or friction fitted within the housing bore 67 as it is trapped between the middle seal 31 and the discharge valve sleeve 46 as shown in FIGS. 1, 2 and 4.

[0034] The reader will note that the inlet axis 26 is perpendicular to or at least substantially perpendicular to a pump axis or plunger axis 19. Fluid enters the pump 10 through the inlet 25 and then enters the plunger 14 through the opening 58. After fluid passes through the opening 58 and into the plunger bore 61, the fluid travels linearly through the plunger 14, around the outlet valve head 56 and then linearly into the housing bore 67. During a dispense stroke as illustrated in FIG. 2, high pressure fluid is then ejected axially past the discharge valve assembly 11 and axially out through the outlet 12. Thus, once the fluid has entered the pump 10, it travels in a more or less linear fashion through the pump 10 and out through the outlet 12.

[0035] FIG. 5 illustrates the outlet valve assembly 13 in an open or reload position as the outlet valve head 56 has been lifted off of the seat 55 as the plunger 14 moves to the left. As noted above, the discharge valve seat 55 may be threadably secured to the sidewall 57 of the plunger 14 and, of course, other means for connecting the seat 55 to the sidewall 57 will be apparent to those skilled in the art, in addition to making the sidewall 57 and seat 55 a single integral part.

[0036] Returning briefly to FIG. 4, as the plunger 14 is retracted to the left thereby causing the pressure within the housing bore 67 to be reduced and the outlet valve assembly 13 to be opened, fluid will flow through the open outlet valve assembly 13 to recharge the housing bore 67 with fluid. One or more radial orifices 60 may be formed in the hub 42 to allow fluid to pass therethrough. Without one or more radial openings in the hub 42, a vacuum can be created in the space occupied by the spring 44. Such a vacuum effect could hinder the closure of the discharge valve head 38 against the valve seat 45. Using one or more radial orifices 60 and controlling the size of the one or more radial orifices 60 can be used to advantageously dampen the closing of the discharge valve head 38 against the seat 45. Similarly, as discussed above, as the outlet valve head 56 is closed against the outlet valve seat 55, the dampening feature provided by the rod 62, piston 63 and metered opening 65 provides a dampening effect thereby providing a less violent collision between the outlet valve head 56 and the outlet valve seat 55 which extends the useful life of both components.

[0037] Finally, because the pump 10 delivers fluid to the plunger bore 61 before the fluid is transferred to the housing bore 67 and out through the outlet 12, the pressurized fluid is delivered through the outlet 12 in a linear fashion which may reduce stresses against various parts of the pump thereby reducing the chances of pump failure.

[0038] FIGS. 6 and 7 illustrate the coupling of three and five pumps 10 respectively together for increased capacity. The pumps 10 of FIGS. 6-7 are connected in parallel and therefore, regardless of whether the liners 73 are of the same or different thicknesses, only the capacity will be increased, not the output pressure. As shown in FIG. 7, a single actuator 53 may be used for the entire group of pumps 10 or, in the alternative, individual actuators (not shown) may be utilized. The pumps 10 may be coupled together by various means, which will be apparent to those skilled in the art.

[0039] Turning to FIG. 8, the relationship between the fluid pressure in the housing bore 67 and the position of the plunger 14 is illustrated. Specifically, the x-axis represents time (t) and the y-axis represents fluid pressure (p) in the housing bore 67. The pressure values in the housing bores 67 are represented by the line 75 and the position of the plunger is represented by the line 76. The point 77 along the x-axis represents the plunger 14 in the far left position shown in FIG. 1 where the housing bore 67 is loaded with fluid. The apex 78 of the plunger curve or line 76 represents the plunger in the far right position shown in FIG. 4. Thus, as the plunger moves from the position shown in FIG. 1 as represented by the point 77 in FIG. 8, the pressure increases to a maximum pressure 79 which, within a certain time period, will open the discharge valve assembly 11 thereby causing the pressure to drop as shown in FIG. 8. The point 81 represents the plunger 14 being retracted and moved back to the left as shown in FIG. 1. As the plunger 14 is retracted, the pressure within the housing bore 67 is at a minimum, thereby opening the outlet valve assembly 13 as explained above.

[0040] A modular plunger pump is disclosed that can be used in a side-by-side configuration to build multiple pump units. Each individual pump includes a housing having a housing bore for receiving a slidable plunger. The plunger includes a piston bore and a sidewall opening in communication with the pump inlet. Low pressure fluid passes from the inlet into the piston bore of the plunger during a retracting or reloading stroke of the plunger. The pump inlet may be oriented in a perpendicular fashion to the plunger or main pump axis. Pressure of the fluid within the plunger bore can build beyond a threshold pressure to overcome the biasing of an outlet valve assembly in the closed position in order to urge the outlet valve assembly to move away from its closed position. Pressure from the pump inlet is then communicated out of the plunger through the outlet valve assembly disposed at a distal end of the plunger and into a housing bore where continued movement of the plunger in the compression direction within the housing bore pressurizes the fluid. During such a compression or dispense stroke, the fluid in the housing bore is pressurized to a sufficiently high pressure beyond a threshold pressure to open a discharge valve assembly which is in communication with an outlet.

[0041] The output capacity of the disclosed pumps may be altered by changing the lining of the housing bore with the lining of a different thickness. Thus, the volume of the housing bore can be increased or decreased rather easily as described above. The closure of the discharge valve assembly and the outlet valve assembly may also be dampened. Specifically, with respect to the outlet valve assembly, the outlet valve head may be coupled to a piston with a metered passageway that allows fluid to flow through the piston as the outlet valve head moves towards and engages the outlet valve seat. In contrast, the closure of the discharge valve head against the discharge valve seat may be dampened by fluid entering the housing bore through the outlet valve assembly as the plunger is retracted.

[0042] A method of pumping hydraulic fracturing fluid down a wellbore is also disclosed using one of the disclosed pumps. The method may include retracting the plunger away from the outlet in an expansion direction such that the openings formed in the plunger wall can communicate with the inlet to open the outlet valve assembly and provide communication between the pump inlet and the housing bore through the plunger and the open outlet valve assembly of the plunger. The method may then include moving the plunger towards the discharge valve assembly thereby closing the outlet valve assembly disposed at the distal end of the plunger and causing the fluid disposed in the housing bore to become pressurized thereby eventually opening the discharge valve assembly. The method may also include dispensing fluid through the open discharge valve assembly and the outlet.

[0043] In another aspect, a method of pumping hydraulic fracturing fluid down a well bore may, in a more simplified manner, include biasing the discharge valve head against the discharge valve seat, providing communication between the pump inlet and the housing bore, moving the plunger towards the discharge valve head thereby pressurizing a portion of the housing bore disposed between the plunger and the discharge valve assembly thereby opening the discharge valve assembly and dispensing fluid through the opened discharge valve assembly and the outlet. The method may also include dampening an engagement between the discharge valve head and the discharge valve seat by releasing pressurized fluid disposed in the hub and between the distal end of the shaft and the proximal side of the retainer to provide additional pressure in the housing bore as the discharge valve head closes against the discharge valve seat. A radial orifice in the hub can be utilized for releasing this pressurized fluid.

[0044] Still further, the method may further include replacing the discharge valve seat and/or the discharge valve head by removing the outlet from the housing assembly, removing the retainer from the discharge valve sleeve, removing the discharge valve sleeve and discharge valve head from the housing assembly, providing a new or repaired discharge valve sleeve and/or a new or repaired discharge valve head, coupling the new or repaired discharge valve sleeve to the housing assembly, inserting the new or repaired discharge valve head into the discharge valve sleeve, coupling the retainer to the discharge valve sleeve and placing the hub over the shaft over the discharge valve head and recoupling the outlet to the housing assembly.

[0045] Another method may include retracting the plunger away from the discharge valve assembly to open the outlet valve assembly and provide communication between the inlet and the housing bore through the plunger. The method may further include moving the plunger towards the discharge valve assembly thereby closing the outlet valve assembly, pressurizing the housing bore disposed between the outlet valve assembly and the discharge valve assembly and opening the discharge valve assembly. The method may then further include dispensing fluid through the open discharge valve assembly and the outlet. The method may also include dampening an engagement between the outlet valve head and the outlet valve seat as the outlet valve assembly closes by restricting flow of fluid from a proximal side of the piston, through a metered passageway, through the piston and to the distal side of the piston as the piston moves towards the proximal end of the plunger.

[0046] Methods of replacing the outlet valve head and outlet valve seat are also disclosed that include withdrawing the plunger from the proximal end of the housing, unscrewing the outlet valve head from the distal end of the rod, screwing a new outlet valve head onto the distal end of the rod and inserting the plunger back into the proximal end of the housing. To replace the outlet valve seat, after withdrawing the plunger from the proximal end of the housing, the outlet valve head, rod and piston are pulled through the distal end of the plunger, the outlet valve seat is unscrewed from the distal end of the sidewall of the plunger and replaced with a new outlet valve seat. Then the piston, rod and outlet valve head are reinserted back through the distal end of the plunger until the outlet valve head engages the new outlet valve seat. The plunger is then reinserted through the proximal end of the housing.