US20190242373A1

US20190242373A1 - Fluid End for a Pump

Info

Publication number: US20190242373A1
Application number: US16/329,852
Authority: US
Inventors: Marcus D. Wernig; Pawan Kumar Nerusu; Ronald Mark Sherwood
Original assignee: C&J Energy Services Inc
Current assignee: C&J Energy Services Inc
Priority date: 2016-10-27
Filing date: 2016-10-27
Publication date: 2019-08-08
Also published as: WO2018080496A1

Abstract

A fluid end for a pump is described herein. The fluid end includes a suction end, a discharge end, and a plunger end. The fluid end defines a main chamber which when viewed in cross section has a substantially elliptical outer boundary. The fluid end can further include an access end.

Description

TECHNICAL FIELD

The present disclosure generally relates to pumps. In particular, embodiments described in this disclosure relate to fluid ends for positive displacement pumps such as those used in hydraulic fracturing operations.

BACKGROUND

Many fluid ends for positive displacement pumps, particularly pumps used in hydraulic fracturing operations, often are subjected to high hydrostatic pressures. Exposure to such pressures can cause a fluid end to be subjected to elevated stress conditions. Pumping operations can also subject a fluid end to relatively high frequency pressure oscillations, which can lead to the repeated applications of cyclic stress to the fluid end.
Although the fluid end might be able to withstand a single instance of increased pressure application and the resulting stress and strain, repeated increased pressure applications can make the fluid end prone to failure (e.g., fatigue cracks, and so forth). For example, typical life cycles of fluid ends constructed from carbon steel are often in a range of about 300 to about 500 hours, and in a range of about 1000 to 1500 hours for fluid ends constructed from stainless steel. While improvements to the materials of construction for the fluid end can be made, such improvements typically are not expected to cost effectively prevent failures, extend typical life cycles, and so forth.
A fluid end is depicted in FIGS. 1A-1C, which illustrates a right angle fluid end for use in a conventional pump. FIG. 1B illustrates a cross-sectional view of the right angle fluid end. The section is taken along view line 1B-1B of FIG. 1A. The main chamber 150 of the fluid end is commonly subjected to high stresses and pressures. The fluid end 100 shown in FIGS. 1A-1C has a plunger end 130, discharge end 120, and suction end 110. Each respective end 110, 120, 130 of the fluid end 100 defines a respective bore 111, 121, 131. The fluid end 100 also has an access end 140 which also defines a respective bore 141. Transition portions 112, 122, 132, 142 open into the main chamber 150. Each of the transition portions 112, 122, 132, 142 is delimited by transition portion surfaces 114, 124, 134, 144. As seen in FIG. 1B, the transition portion surfaces 114, 124, 134, 144 have intersecting edges and form non-uniform curves. The fluid end shown in FIGS. 1A-1C can have increased stress and more variation in stress at its corners which are typically located at the intersection of adjacent bores, which can result in decreased life cycle of the fluid end.
U.S. Pat. No. 9,383,015 to Cary discloses a fluid end having a spherical main chamber for a high-pressure pump. A spherical geometry is created in the cross-bore intersection. A spherical surface forms an outer boundary of the main chamber. However, similar to the fluid end shown in FIGS. 1A-1C, a fluid end with a main chamber with a spherical geometry as disclosed by Cary can have more variation in stress at corners located at the intersection of adjacent bores, which can result decreased life cycle of the fluid end.
Thus, there remains a need in the art for a fluid end that is capable of addressing the foregoing, and/or other needs in the art.

NON-LIMITING BRIEF SUMMARY

The present disclosure is directed to a fluid end for a positive displacement pump. The fluid end generally includes a suction end, a discharge end, and a plunger end. The fluid end defines a main chamber which when viewed in cross section has a substantially elliptical outer boundary. The suction end defines a suction bore extending therethrough, and extends along a suction end centerline and further defines a suction end transition portion. The discharge end defines a discharge bore extending therethrough, and extends along a discharge end centerline and further defines a discharge end transition portion. The plunger end defines a plunger bore extending therethrough, and extends along a plunger end centerline and further defines a plunger end transition portion. Each transition portion places its respective bore in fluid communication with the main chamber.
Among the many different possibilities contemplated, in an embodiment, the main chamber of the fluid end comprises a center point, which is distal from the one or more centerlines. The main chamber of the fluid end has a major axis, a minor axis, and a focal distance. The center point of the main chamber is preferably positioned from the one or more centerlines in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis. It is further contemplated that the main chamber can have an outer boundary which forms a substantially ellipsoid shape.
In one or more embodiments, all of the centerlines of the fluid end can share a common intersection that is disposed in the main chamber. Furthermore, the main chamber of the fluid end comprises a center point, which is distal from the one or more centerlines. Still further, the main chamber of the fluid end has a major axis, a minor axis, and a focal distance. The center point of the main chamber is preferably positioned from the one or more centerlines in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis. It is further contemplated that the main chamber can have an outer boundary which forms a substantially ellipsoid shape.
In one or more embodiments, the internal surface of the fluid end preferably has a maximum von Mises stress of less than about 75,000 psi (5.17×10⁸Pa) when subjected to a hydrostatic pressure of about 15,000 psi (1.03×10⁸Pa).
In one or more embodiments, a planar cross section passing through the center point of the main chamber of the fluid end forms an ellipse on a surface of the planar cross section.
In one or more embodiments, the suction end centerline is substantially coaxial with the discharge end centerline. Furthermore, the plunger end centerline can be substantially perpendicular to the suction end centerline and the discharge end centerline.
In one or more embodiments, when the plunger end centerline intersects with the suction end centerline, an angle is formed between the two centerlines that is greater than about 90 degrees.
In one or more embodiments, when the plunger end centerline intersects with the discharge end centerline, an angle is formed between the two centerlines that is greater than about 90 degrees.
In one or more embodiments, the fluid end includes an access end defining an access bore extending therethrough, and extends along an access end centerline and further defines an access end transition portion.
In one or more embodiments, the main chamber of the fluid end can be formed by various techniques including, but not limited to, multi-axis mill machining, electrical discharge machining, and so forth.
In one or more embodiments, the main chamber of the fluid end includes various materials including, but not limited to, an investment casted material, a strength treated material, a surface treated material, and so forth.
The above brief summary of the invention presents a simplified summary of the claimed subject matter in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview of the claimed subject matter. It is intended to neither identify key or critical elements of the claimed subject matter nor delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts of the claimed subject matter in a simplified form as a prelude to the more detailed description that is presented below.
While multiple embodiments are disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description. As will be apparent, certain embodiments, as disclosed herein, are capable of modifications in various obvious aspects, all without departing from the spirit and scope of the claims as presented herein. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings illustrate specific embodiments. However, it is to be understood that these embodiments are not intended to be exhaustive, nor limiting of the disclosure. These specific embodiments are but examples of some of the forms in which the disclosure may be practiced. Like reference numbers or symbols employed across the several figures are employed to refer to like parts or components illustrated therein.

FIG. 1A illustrates a plan view of a conventional prior art fluid end.

FIG. 1B illustrates a sectional view of the conventional fluid end shown in FIG. 1A taken along view line 1B-1B of FIG. 1A.

FIG. 2A illustrates a plan view of a fluid end in accordance with this disclosure.

FIG. 2B illustrates a sectional view of the fluid end shown in FIG. 2A taken along view line 2B-2B of FIG. 2A.

FIG. 2C illustrates a rotated isometric view of the fluid end shown in FIG. 2A.

FIG. 2D illustrates a sectional view of the fluid end shown in FIG. 2A.

FIG. 2E illustrates a rotated isometric view of the fluid end shown in FIG. 2A.

FIG. 2F illustrates another view of the fluid end where the sectional view of fluid end shown in FIG. 2B is rotated 90 degrees.

DETAILED DESCRIPTION

Without any intent to limit the scope of this disclosure, reference is made to the figures in describing various aspects of a fluid end 200 for a positive displacement pump. FIG. 2A generally illustrates the fluid end 200, and FIGS. 2B-2E illustrate various views of the fluid end 200 depicted in FIG. 2A.
The fluid end 200 comprises a suction end 210 for receiving fluid into the fluid end 200, a discharge end 220 for discharging fluid from the fluid end 200, and a plunger end 230 adapted to receive a plunger. Although not required, the fluid end 200 can further include an access end 240 for providing access to one or more internal components of the fluid end 200. Each respective end 210, 220, 230, 240 of the fluid end 200 defines a respective bore 211, 221, 231, 241 extending therethrough and further defines a respective transition portion 212, 222, 232, 242 that places the respective bore 211, 221, 231, 241 in fluid communication with a main chamber 250. Each end 210, 220, 230, 240 of the fluid end 200 also extends along a respective centerline 213, 223, 233, 243.
In one or more embodiments, the suction end centerline 213 is substantially coaxial with the discharge centerline 223. The plunger end centerline 233 is also preferably substantially perpendicular to the suction end centerline 213 and the discharge end centerline 223. If the plunger end centerline 233 intersects with the suction end centerline 213 at a bore intersection point, an angle is formed between the two centerlines 213, 233 that is greater than about 90 degrees. And, if the plunger end centerline 233 intersects with the discharge end centerline 223 at a bore intersection point, an angle is formed between the two centerlines 223, 233 that is greater than about 90 degrees.
Although in some embodiments all of the centerlines 213, 223, 233, 243 share a common intersection 260 that is disposed in the main chamber 250, it should be appreciated that in other embodiments the centerlines 213, 223, 233, 243 do not have to share a common intersection. Centerlines sharing a common intersection can allow for manufacturing processes to be performed from only one or two ends of the fluid end instead of from all ends of the fluid end. Centerlines that do not share a common intersection can be of some advantage for the locating of assembly mating parts, which can also permit non-conventional fluid end designs. Thus, for example, the suction end centerline 213 can be positioned such that it does not share a common intersection with the discharge end centerline 223. Similarly, in embodiments where the fluid end 200 comprises an access end, the plunger end centerline 233 can be positioned such that it does not share a common intersection with the access end centerline 243.
The fluid end 200 further defines the main chamber 250. The fluid end 200 comprises an internal surface 252, which delineates an outer boundary 254 of the main chamber 250. Within the outer boundary of the main chamber 250 is open space. When the main chamber 250 is viewed in cross section, its outer boundary 254 has a substantially elliptical shape. By providing the main chamber 250 the elliptical shaped outer boundary, the fluid end 200 can more smoothly transfer loads in many directions thereby reducing the stresses applied to the fluid end 200. In an embodiment, the main chamber 250 of the fluid end 200 has a substantially ellipsoid shape. The phrase “substantially ellipsoid shape” describes that the internal surface 252 of the fluid end 200 conforms to an outline forming an ellipsoid. It should be appreciated that the substantially ellipsoid shape of the main chamber 250 of the fluid end 200 can be created in a variety of ways, for example, including without limitation revolving an elliptical area about its major axis, revolving an elliptical area about its minor axis, and so forth.
The only discontinuities in the internal surface 252 of the fluid end 200 occur at the transition portions 212, 222, 232, 242 of the fluid end 200. Any edges of intersection between the main chamber 250 and the transition portions 212, 222, 232, 242 are preferably removed or rounded to create smooth fillet radii, e.g., edges of intersection located at corner(s) of the intersection of adjacent bores. In this manner, the corners 253 a, 253 b, 253 c, 253 d, located at the intersection of adjacent bores 111, 121, 131, 141, can be subjected to substantially the same or similar elastic stress as any other corner at the intersection of adjacent bores, which can also result in stress reductions and increase life cycle. However, it should be appreciated that in some embodiments it is not necessary to remove or round any edges of intersection between the main chamber 250 and the transition portions 212, 222, 232, 242.
The main chamber 250 of the fluid end comprises a center point 251, which is preferably distal from the one or more centerlines 213, 223, 233, 243. For example, as shown in FIG. 2D the centerpoint 251 of the main chamber 250 is located at the intersection of the X_OFFSETand Y_OFFSETlines. In one or more embodiments, the center point of the main chamber 251 is positioned so that the stresses on the internal surface of the fluid end 200 are minimized even though there may be a disparity in stress magnitudes among the respective corners. In addition, the main chamber is preferably sized and positioned so that the internal surface of the fluid end has a maximum von Mises stress in a range of less than about 75,000 psi (5.17×10⁸Pa) when the fluid end is subjected to about 15,000 psi (1.03×10⁸Pa) hydrostatic pressure. These pressure conditions are measured in absolute units. The von Mises stress for the internal surface of the fluid end can be modeled using any commercially available finite element analysis (FEA) software package, for example, including without limitation ANSYS® FEA software (e.g., version 15, and version 17) available from ANSYS, Inc., which has a business location at 2600 ANSYS Drive, Canonsburg, Pa. 15317, United States of America, and SolidWorks Simulation®, available from Dassault Systemes SolidWorks Corporation, which has a business location at 175 Wyman Street, Waltham, Mass. 02451, United States of America.
When a planar cross section passes through the center point of the main chamber 251, which has a substantially ellipsoid shape, an ellipse is formed on a surface of the planar cross section. The main chamber of the fluid end has a major axis, a semi-major axis, a minor axis, a semi-minor axis, and a focal distance. For example, FIG. 2D depicts a cross sectional view of the fluid end 200 illustrating the ellipse formed on the surface of the planar cross section. The ellipse is formed by dashed lines in FIG. 2D. The ellipse has a major axis having a length (L_major), a minor axis having a length (L_minor) and two foci located on the major axis at positions that are symmetric about the minor axis. The focal distance (i.e., the distance the two foci are located away from the minor axis) is equal to one half of the square root of the difference between the square of the major axis length and the square of the minor axis length.
The distance between the center point of the main chamber 251 and the one or more centerlines 213, 223, 233, 243 can be used determine the eccentricity of the main chamber 250, and thus can be utilized to approximate the maximum von Mises stress of the internal surface of the fluid end 200. If the center point of the main chamber 250 is positioned from the one or more centerlines 213, 223, 233, 243 in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis, it has been found that the von Mises stress of the internal surface of the fluid end 200 tends to increase. Thus, the center point of the main chamber 251 is preferably positioned from the one or more centerlines 213, 223, 233, 243 in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis.
It should be noted that although the position of the center point of the main chamber relative to the one or more centerlines is one variable to consider for approximating the maximum von Mises stress of the internal surface of the fluid end 200, as one of ordinary skill in the art appreciates, a fluid end with a main chamber having a substantially ellipsoid shape can introduce additional variables, including without limitation the lengths of the ellipsoid axes and corresponding eccentricity, the sizes of the blending and/or fillet radii at the transitions between the main chamber and adjacent features, and the effective wall thickness of a fluid end, that can be controlled during manufacturing to approximate and thus minimize the von Mises stresses of the internal surface of the fluid end. Thus, in some cases, in addition to the position of the center point of the main chamber relative to the one or more centerlines, approximating and thus minimizing the von Mises stresses for the internal surface of the fluid end requires analyses using multivariate polynomial and/or spline mathematics, as understood by those of ordinary skill in the art.
It should be appreciated that the center point 251 need not be distal from each of the centerlines 213, 223, 233, 243. For example, in one or more embodiments, the center point of the main chamber 250 can be distal from only one of the centerlines 213, 223, 233, 243. Additionally, in one or more embodiments in which all of the centerlines 213, 223, 233, 243 share a common intersection that is disposed in the main chamber 250, the center point of the main chamber 251 is preferably positioned from the common intersection in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis.
The fluid end 200 can be constructed from any conventional material of construction, including, but not limited to, at least one material selected from the group consisting of investment casted material, strength treated material, and surface treated material (e.g., material subjected to a surface treatment such as shot-peening, plastic burnishing and/or autofrettage for the Bauschinger effect). Of course, it should be appreciated that the material of construction is preferably selected based on the intended application of the fluid end 200, for example, fluid type, anticipated operating conditions (temperature, pressure, etc.), and so forth.
The elliptical or ellipsoid geometry of the main chamber 250 and rounded edges of intersection between the main chamber 250 and the transition portions 212, 222, 232, 242 of the present disclosure can be formed by a variety of methods including, but not limited to, multi-axis mill machining, electrical discharge machining, investment casting, and so forth. It should also be appreciated that the fluid end of the present disclosure can be configured in a conventional angular or Y-Block configuration.
In operation, as the plunger (not shown) reciprocates in plunger bore 231 in a first direction away from the access bore 241, fluid enters the suction bore 211 through a fluid inlet. A suction valve (not shown), normally in the closed position, opens to allow fluid into the main chamber 250 of the fluid end 200. The plunger then reciprocates in the opposite second direction along the long axis of the plunger bore 231. The reciprocation causes the fluid to exit the fluid end 200 from the discharge bore 221. The fluid exits the discharge bore 221 by first passing through a discharge valve, normally in the closed position, but which opens as the pressure within the main chamber increases to a predetermined value. For example, in a typical configuration, the discharge valve is a spring-loaded, one-way valve. The fluid passes through the discharge valve when the chamber pressure increases sufficiently to displace the discharge valve piston against its spring. This displacement opens the sealing surfaces of the discharge valve and allows fluid to escape until the main chamber pressure reduces to the point where the spring force exceeds the pressure force. At this point the sealing surfaces of the discharge valve are forced back together and the discharge valve returns to the closed position.
Although a fluid end 200 used with a plunger is described, the above description also applies to a fluid end 200 used with a piston. The piston operates with the plunger end but does not extend into the plunger end bore. The piston is in fluid communication with the plunger end bore via a cylinder which extends from the entry end of the plunger end.

Finite Element Analysis (FEA) Computer Simulations

The present disclosure can be better understood by reference to the following results, which were obtained from FEA computer simulations. These results are presented for purposes of illustration and are not intended to limit the scope of the invention.
FEA computer simulations were conducted in an effort to obtain comparative data between alternative fluid end designs and a fluid end in accordance with the present disclosure. Simulations were conducted for the following design cases: (1) a fluid end having an ellipsoid main chamber in accordance with the present disclosure; (2) a fluid end having a spherical main chamber that is the same or similar to the fluid end described in U.S. Pat. No. 9,383,015 to Cary; and (3) a right angle fluid end that is the same or similar to the one depicted in FIGS. 1A-1C.

Simulations Using SolidWorks® Software

A first set of models for Cases (1), (2) and (3) were created using SolidWorks® 2012 x64 Edition. Each model was then evaluated using SolidWorks® Simulation, which calculates estimates of the von Mises stresses that result when a 3-D model with defined material properties is subjected to displacement and load boundary conditions, as understood by one of ordinary skill in the art. The model under simulation is preferably manufactured from a quasi-homogeneous ductile material that adheres to the Distortion Energy failure theory for the von Mises stresses to be the appropriate metric of evaluation. The intensities of the von Mises stresses can be evaluated by the superposition of orthogonal stress states that result from the finite-element, continuum mechanics technique. For the above-mentioned models, the bolt holes of the mounting flange were held as fixed boundary conditions. Also, in these simulations the internal surface(s) within the seal points for Cases (1), (2) and (3) were subjected to normal/orthogonal hydrostatic pressure boundary conditions.
As shown in Table 1 below, for a 15,000-psi hydrostatic pressure loading condition at the internal surface(s) of Case (3), the maximum von Mises stress is estimated to be about 187.9 ksi. Although the maximum von Mises stress as determined by the simulation(s) for Case (3) is about 187.9 ksi, it should be appreciated that in actual practice, the fluid end is expected to undergo non-linear, plastic deformation at or above a maximum von Mises stress of about 140 ksi and would never reach this stress condition. The value of 187.9 ksi is a linear estimation of the maximum stress state in the fluid end body and is included as a comparative reference with all three Cases having the same simulation boundary condition(s). For a 15,000-psi hydrostatic pressure loading condition at the internal surface(s) of Case (2), the maximum von Mises stress is estimated to be about 83.2 ksi, which is a reduction of about 55.7% with respect to Case (3). For a 15,000-psi hydrostatic pressure loading condition at the internal surface(s) of Case (1), the maximum von Mises stress is estimated to be about 74.2 ksi, which is a reduction of about 60.5% with respect to Case (3) and a reduction of about 11% with respect to Case (2).
TABLE 1

Max. von Mises Stress

Design Case (ksi)

Case (1) 74.2

Case (2) 83.2

Case (3) 187.9

Simulations using ANSYS® FEA Software
A second set of simulations was conducted using ANSYS® FEA software (version 15 and/or version 17). The details for these computer simulations (e.g., boundary conditions, software version, etc.) and results of these computer simulations are set forth in Tables 2 and 3, respectively. Simulations were performed for both pre and post autofrettage conditions. Autofrettage is a metal fabrication technique in which a pressure vessel, such as the main chamber of a fluid end, is subjected to enormous pressure, causing internal portions of the part to yield plastically, resulting in internal compressive residual stresses once the pressure is released. In this manner, autofrettage can increase the durability of the pressure vessel. In Table 3 below, corners 1, 2, 3, and 4 for Cases (1), (2), and (3) correspond to locations of the corners 253 a, 253 b, 253 c, and 253 d respectively, as shown in FIG. 2B.

TABLE 2

Boundary
Condition	Case (1)	Case (2)	Case (3)

Material of	Stainless Steel	Stainless Steel	Stainless Steel
Construction
Hydrostatic Pressure	15	15	15
Before and After
Autofrettage
(ksi)
Autofrettage Pressure	45	45	45
Used
(ksi)
Assumed Fatigue	62.5	62.5	62.5
Limit
(ksi)
Meshing Condition	Tetrahedral	Tetrahedral	Tetrahedral
ANSYS ® FEA	Ver. 17	Ver. 17	Ver. 15
Software Version

TABLE 3

	Peak von		Peak von	Peak von
	Mises	Peak von	Mises	Mises
	stress @	Mises stress	stress @	stress @
	Corner 1	@ Corner 2	Corner 3	Corner 4
Design	(ksi)	(ksi)	(ksi)	(ksi)

Pre-	Case (1)	65.6	62.1	63.1	64.1
Autofrettage	Case (2)	65.0	60.6	65.0	63.2
	Case (3)	104.02	93.2	78.0	95.7
Post-	Case (1)	48.6	47.1	51.1	50.5
Autofrettage	Case (2)	50.7	47.4	49.2	51.2

As shown in Table 3, the results illustrate that Case (1), which corresponds to a fluid end in accordance with the present disclosure, exhibits superior performance, with respect to peak von Mises stress at the corner regions of the fluid end, than the corner regions of the fluid end of Case (2) and Case (3). For example, the peak von Mises stresses for Case (3) are in excess of the assumed fatigue limit of 62.5 ksi, and much greater than those for Case (1), both pre- and post-autofrettage. The peak von Mises stresses for Case (1) are more evenly balanced between the corners than those of both Cases (2) and (3). By having more evenly balanced von Mises stresses at the corners of the fluid end, the life cycle of the fluid end can be increased.
Except as may be expressly otherwise indicated, the article “a” or “an” if and as used herein is not intended to limit, and should not be construed as limiting, the description or a claim to a single element to which the article refers. Rather, the article “a” or “an” if and as used herein is intended to cover one or more such elements, unless the text expressly indicates otherwise. Furthermore, aspects of the invention may comprise, consist essentially of, or consist of the indicated elements or method steps.
Any reference to patents, documents and other writings contained herein shall not be construed as an admission as to their status with respect to being or not being prior art. Unless the meaning is clearly to the contrary, all ranges set forth herein are deemed to be inclusive of the endpoints. Although the present invention and its advantages have been described in detail, it is understood that the array of features and embodiments taught herein may be combined and rearranged in a large number of additional combinations not directly disclosed, as will be apparent to one having ordinary skill in the art. The invention disclosed herein may be practiced in the absence of any element which is not specifically disclosed herein. It should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the following claims. There are, of course, other embodiments, which are alternatives to the foregoing descriptions of the invention, which are intended to be included within the scope of the invention, as defined by the following claims.

Claims

What is claimed is:

1. A fluid end for a positive displacement pump, the fluid end comprising:

a) a suction end defining a suction bore extending therethrough, the suction end extending along a suction end centerline and further defining a suction end transition portion;

b) a discharge end defining a discharge bore extending therethrough, the discharge end extending along a discharge end centerline and further defining a discharge end transition portion; and

c) a plunger end defining a plunger bore extending therethrough, the plunger end extending along a plunger end centerline and further defining a plunger end transition portion;

wherein the fluid end defines a main chamber which when viewed in cross section has a substantially elliptical outer boundary; and wherein each transition portion places its respective bore in fluid communication with the main chamber.

2. The fluid end of claim 1 wherein the main chamber comprises a center point, the center point being distal from the one or more centerlines.

3. The fluid end of claim 2, wherein the main chamber has a semi-major axis, a semi-minor axis, and a focal distance, wherein the center point of the main chamber is positioned from the one or more centerlines in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis.

4. The fluid end of claim 1 wherein all of the centerlines share a common intersection that is disposed in the main chamber.

5. The fluid end of claim 4 wherein the main chamber comprises a center point, the center point being distal from the common intersection.

6. The fluid end of claim 5, wherein the main chamber has a semi-major axis, a semi-minor axis, and a focal distance, wherein the center point of the main chamber is positioned from the common intersection in a range that is the greater of (a) about 0% to about 50% of the focal distance, and (b) about 0% to 25% of the length of the semi-major axis.

7. The fluid end of claim 2 wherein the main chamber comprises an outer boundary which forms a substantially ellipsoid shape.

8. The fluid end of claim 5 wherein the main chamber comprises an outer boundary which forms a substantially ellipsoid shape.

9. The fluid end of claim 1 wherein the maximum von Mises stress of an internal surface of the fluid end is in a range of less than about 75,000 psi when the internal surface of the fluid end is subjected to about 15,000 psi hydrostatic pressure.

10. The fluid end of claim 2 wherein a planar cross section passing through the center point of the main chamber forms an ellipse on a surface of the planar cross section.

11. The fluid end of claim 1 wherein the suction end centerline is substantially coaxial with the discharge centerline.

12. The fluid end of claim 11 wherein the plunger end centerline is substantially perpendicular to the suction end centerline and the discharge end centerline.

13. The fluid end of claim 1 wherein when the plunger end centerline intersects with the suction end centerline at a bore intersection point, an angle is formed between the two centerlines that is greater than about 90 degrees.

14. The fluid end of claim 1 wherein when the plunger end centerline intersects with the discharge end centerline, an angle is formed between the two centerlines that is greater than about 90 degrees.

15. The fluid end of claim 1 further comprising an access end defining an access bore extending therethrough, the access end extending along an access end centerline and further defining an access end transition portion.

16. The fluid end of claim 1 wherein the main chamber is formed by multi-axis mill machining.

17. The fluid end of claim 1 wherein the main chamber is formed by electrical discharge machining.

18. The fluid end of claim 1 wherein the main chamber comprises at least one material selected from the group consisting of an investment casted material, a strength treated material, and a surface treated material.

19. The fluid end of claim 7 wherein the outer boundary of the main chamber is defined by an internal surface of the main chamber.

20. The fluid end of claim 8 wherein the outer boundary of the main chamber is defined by an internal surface of the main chamber.