Methods and Apparatus for Enhancing Elastomeric Stator Insert Material Properties with Radiation

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national stage application of PCT/US2011/061782 filed Nov. 22, 2011, which claims the benefit of U.S. Provisional Application No. 61/416,589 filed Nov. 23, 2010, both of which are hereby incorporated herein by reference in their entireties for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

1. Field of the Invention

The invention relates generally to progressive cavity pumps and motors. Still more particularly, the present invention relates to the treatment of elastomeric stator inserts with ionizing radiation to enhance the material properties of the elastomeric material.

2. Background of the Technology

A progressive cavity pump (PC pump) transfers fluid by means of a sequence of discrete cavities that move through the pump as a rotor is turned within a stator. The transfer of fluid in this manner results in a volumetric flow rate proportional to the rotational speed of the rotor within the stator, as well as relatively low levels of shearing applied to the fluid. Consequently, progressive cavity pumps are typically used in fluid metering and pumping of viscous or shear sensitive fluids, particularly in downhole operations for the ultimate recovery of oil and gas. Progressive cavity pumps may also be referred to as PC pumps, progressing cavity pumps, “Moineau” pumps, eccentric screw pumps, or cavity pumps.

A PC pump may be used in reverse as a positive displacement motor (PD motor) by passing fluid through the cavities between the rotor and stator to power the rotation of the rotor relative to the stator, thereby converting the hydraulic energy of a high pressure fluid into mechanical energy in the form of speed and torque output, which may be harnessed for a variety of applications, including downhole drilling. Progressive cavity motors may also be referred to as progressing cavity motors (PC motors), positive displacement motors (PD motors), eccentric screw motors, or cavity motors.

Progressive cavity devices (e.g., progressive cavity pumps and motors) include a stator having a helical internal bore and a helical rotor rotatably disposed within the stator bore. An interference fit between the helical outer surface of the rotor and the helical inner surface of the stator results in a plurality of circumferentially spaced hollow cavities in which fluid can travel. During rotation of the rotor, these hollow cavities advance from one end of the stator towards the other end of the stator. Each of these hollow cavities is isolated and sealed from the other cavities.

Since a PC motors have few components, they can be made to have a relative small outer diameter while being able to generate considerable torque. This design can be applied to subsurface boring motors (i.e. mud motors) for the drilling of wellbores. In such applications, the drilling mud that is used to cool and lubricate the drill bit and to bring cuttings to the surface up the annulus area between the drill string and the wellbore is typically used as the drive fluid for the downhole PC motor. The drilling fluid or mud may contain a certain amount of solid particles without risking damage to the motor, which is another advantage of utilizing eccentric screw motors in the drilling of wellbores.

Conventional stators often comprise a radially outer tubular housing and a radially inner component disposed within the housing. The inner component has a cylindrical outer surface that is bonded to the cylindrical inner surface of the housing and a helical inner surface that defines the helical bore of the stator. Alternatively, the housing may have a helical bore and the inner component may comprise a relatively thin, uniform thickness coating on the helical inner surface of the housing. In either case, the inner component is typically made of an elastomeric material and is disposed within the stator housing, and thus, may also be referred to as an elastomeric stator liner or insert. The elastomeric stator insert provides a surface having some resilience to facilitate the interference fit between the stator and the rotor.

Typically, stator manufacturers use an injection molding process to form the elastomeric stator insert. The injection molding process requires a relatively low viscosity elastomeric material, which often limit the ultimate stiffness and resilience of the material. During operation, the rotor and stator insert are in constant frictional engagement along a plurality of sealing lines defining the fluid filled cavities. Materials with low stiffness, strength, and/or resilience may wear quickly, thereby reducing the efficiency, power, and useful life of the PC device. Thermally curing injection molded elastomers is known to enhance certain elastomeric properties, but may also detrimentally affect other elastomeric properties.

Accordingly, there remains a need in the art for progressive cavity devices exhibiting reduced friction between the rotor and the stator insert to enhance efficiency, power, and durability. It would be desirable to increase the efficiency, power, and durability of the progressive cavity device by enhancing the resilience, strength, and resistance to stress cracking of device components such as the elastomeric stator insert. Such improvements in the stator insert would be particularly well-received if they could be achieved in addition to or separate from the thermal curing process.

BRIEF SUMMARY OF THE DISCLOSURE

These and other needs in the art are addressed in one embodiment by a method for manufacturing a stator for a progressive cavity motor or pump. In an embodiment, the method comprises (a) forming an elastomeric stator insert. In addition, the method comprises (b) exposing the elastomeric stator insert to ionizing radiation. Further, the method comprises (c) positioning the elastomeric stator insert in a stator housing to form a stator.

These and other needs in the art are addressed in another embodiment by a method for manufacturing a stator for a progressive cavity motor or pump. In an embodiment, the method comprises (a) generating a beam of electrons. In addition, the method comprises (b) positioning a target between the beam of electrons and an elastomeric stator insert. Further, the method comprises (c) emitting ionizing X-ray radiation from the target after (b). Still further, the method comprises (d) exposing the elastomeric stator insert to at least 100 KiloGrays of the ionizing X-ray radiation. Moreover, the method comprises (e) forming a plurality of polymer cross-links in the elastomeric stator insert with the ionizing X-ray radiation during (d).

These and other needs in the art are addressed in another embodiment by a progressive cavity pump or motor. In an embodiment, the progressive cavity pump or motor comprises a stator having a central axis and including a stator housing and a stator insert disposed within the stator housing, wherein the stator includes a helical bore defined by the elastomeric stator insert. In addition, the progressive cavity pump or motor comprises a rotor rotatably disposed within the helical bore of the stator. The rotor has a radially outer helical surface. The stator insert comprises an elastomeric material including a plurality of polymer chains connected by a plurality of cross-links induced by ionizing radiation.

Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

FIG. 1 is a perspective, partial cut-away view of an embodiment of a progressive cavity device in accordance with the principles described herein;

FIG. 2 is an end view of the progressive cavity device of FIG. 1;

FIG. 3 is a schematic view of a system for treating the elastomeric stator insert of FIGS. 1 and 2 with ionizing radiation; and

FIGS. 4 and 5 are graphical illustration of results from the test described in Example 1.

DETAILED DESCRIPTION OF SOME OF THE PREFERRED EMBODIMENTS

The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, 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 intimate 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. 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 should 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, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis.

Referring now to FIGS. 1 and 2, an embodiment of a progressive cavity (PC) device 10 is shown. In general, PC device 10 may be employed as a progressive cavity pump or a progressive cavity motor. PC device 10 comprises a rotor 30 rotatably disposed within a stator 20. Rotor 30 has a central or longitudinal axis 38 and helical-shaped radially outer surface 33 defining a plurality of circumferentially spaced rotor lobes 37. Rotor 30 is preferably made of steel and may be chrome-plated or coated for wear and corrosion resistance.

Stator 20 has a central or longitudinal axis 28 and comprises a housing 25 and an elastomeric stator insert 21 coaxially disposed within housing 25. In this embodiment, housing 25 is a tubular (e.g., heat-treated steel tube) having a radially inner cylindrical surface 26, and insert 21 has a radially outer cylindrical surface 22 engaging surface 26. Surfaces 22, 26 are fixed and secured to each other such that insert 21 does not move rotationally or translationally relative to housing 25. For example, surfaces 22, 26 may be bonded together and/or surfaces 22, 26 may include interlocking mechanical features (e.g., surface 22 may include a plurality of radial extensions that positively engage mating recesses in surface 26). Insert 21 includes a helical through bore 24 defining a radially inner helical surface 23 that faces rotor 30. Although housing 25 and insert 21 have mating inner and outer cylindrical surfaces 26, 22, respectively, in this embodiment, in other embodiments, the stator housing (e.g., housing 25) may have a helical-shaped radially inner surface defined by a helical bore extending axially through the housing, and the elastomeric insert may be a thin, uniform radial thickness elastomeric layer or coating disposed on the helical inner surface of the housing.

Referring still to FIGS. 1 and 2, rotor lobes 37 intermesh with a set of circumferentially spaced stator lobes 27 defined by helical bore 24 in insert 21. As best shown in FIG. 2, the number of lobes 37 formed on rotor 30 is one fewer than the number of lobes 27 on stator 20. When rotor 30 and the stator 20 are assembled, a series of cavities 40 are formed between the helical-shaped outer surface 33 of rotor 30 and the helical-shaped inner surface 23 of stator 20. Each cavity 40 is sealed from adjacent cavities 40 by seals formed along the contact lines between rotor 30 and stator 20. The central axis 38 of rotor 30 is parallel to and radially offset from the central axis 28 of stator 20 by a fixed value known as the “eccentricity” of PC device 10.

The manner and method in which a PC device 10 operates is well known in the art. In general, the intermeshing stator insert 21 and rotor 30 generate a plurality of cavities 40 separated in the circumferential and longitudinal directions. When PC device 10 is operated as a pump, the rotation of rotor 30 relative to stator 20 drives the axial movement of cavities 40 through device 10 in the direction towards the end with the higher fluid pressure, and when PC device 10 is operated as a motor, the flow of fluid through cavities 40 from the end with a high fluid pressure to the end with the lower fluid pressure drives the rotation of rotor 30 relative to stator 20.

In general, elastomeric stator insert 21 may be constructed from any suitable elastomer or mixture of elastomers. In embodiments described herein, the elastomeric stator insert (e.g., stator insert 21 or uniform radial thickness stator insert disposed on the inner surface of the stator housing) is preferably made from nitrile rubber, hydrogenated nitrile (HNBR), ethylene propylene diene monomer rubber (EPDM rubber), Chloroprene (neoprene), fluoroelastomers (FKM), epichlorohydrin rubber (ECO), natural rubber (NR), or combinations thereof. In general, elastomeric stator insert 21 may be formed by any suitable means known in the art including, without limitation, injection molding, transfer molding, extrusion, compression molding, or any other molding method.

An elastomer is a polymer with the property of viscoelasticity (i.e., elasticity), generally having notably low Young's modulus and high yield strain compared with other materials. In particular, an elastomer is composed of a plurality of hydrocarbon polymer chains, which may have the same general orientation (e.g., substantially parallel). Cross-links may be formed between polymer chains when one polymer chain bonds with an adjacent polymer chain. Such cross-links may occur naturally or may be initiated by a variety of means including, without limitation, exposing the elastomer to heat, pressure, radiation, or changes in pH; curing the elastomer; reacting the elastomer with catalysts; or combinations thereof. Without being limited by this or any particular theory, the density of the cross-links in an elastomeric material impacts the physical properties of the elastomeric material. For example, increasing the number of cross-links between polymer chains may increase the tensile strength, Young's modulus, and resilience of the elastomeric material. Increasing the number of cross-links may also increase the resistance to stress cracks, deformation, and abrasion.

In embodiments described herein, the elastomeric stator insert (e.g., stator insert 21 or uniform radial thickness stator insert disposed on the inner surface of the stator housing) is exposed to ionizing radiation. In particular, the elastomeric stator insert is preferably exposed to at least 100 KiloGrays of ionizing radiation, at least 500 KiloGrays of ionizing radiation, at least 1000 KiloGrays of ionizing radiation, at least 2500 KiloGrays of ionizing radiation, at least 5000 KiloGrays of ionizing radiation, at least 7500 KiloGrays of ionizing radiation, or at least 10,000 KiloGrays of ionizing radiation. As is known in the art, a “gray” is a unit of absorbed radiation dose of ionizing radiation, and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter (e.g., elastomeric material).

Without being limited to this or any particular theory, exposing the elastomer stator insert to ionizing radiation increases the polymer chain cross-linking in the elastomeric material by breaking some polymer chains and forming cross-links with other polymer chains within the elastomeric material, thereby offering the potential to increase one or more of the following properties of the elastomeric material—the tensile strength, the Young's modulus, the resilience, the stiffness, the resistance to stress cracks, the resistance to deformation, and the resistance to abrasion. For example, ionizing radiation may increase the tensile strength of the elastomeric material to 20 MPa (or by about 50% as compared to the same elastomeric material prior to treatment with the ionizing radiation), increase the modulus to 10 MPa (or by about 100% as compared to the same elastomeric material prior to treatment with the ionizing radiation), increase the hardness to 90 Shore A (as compared to the same elastomeric material prior to treatment with the ionizing radiation), or combinations thereof. Depending on the composition of the elastomeric material of the stator insert, the formation of cross-links in response to a given level of radiation exposure (e.g., 1000 KiloGrays) may occur at different rates, and the formation of cross-links in response to radiation exposure may occur at different levels of radiation exposure (e.g., 500 KiloGrays vs. 7500 KiloGrays).

In generally, any type of ionizing radiation may be applied to the elastomeric stator inserts described herein including, without limitation, alpha rays, beta rays, gamma rays, neutron rays, proton rays, UV rays, X-rays, and combinations thereof. Moreover, the ionizing radiation may be generated by any suitable means. For example, a relatively high-flux neutron source may serve as a neutron generator. As another example, the ionizing radiation source may be a DC accelerator such as a Dynamitron that directs an electron beam at a target to produce high-energy X-rays.

Referring now to FIG. 3, an embodiment of a system 100 for treating elastomeric stator insert 21 (or any other elastomeric stator insert) with ionizing radiation is shown. In this embodiment, system 100 includes a DC accelerator 110, an electron beam acceleration tube 118, an electron scan magnet 120, and a target 130. DC accelerator 110 generates a stream or beam of electrons 115 via thermionic emission from a heated filament or cathode 111 in an electron gun 112. Within gun 112, electrodes generate an electric field that focuses the stream of electrons 115, and one or more anode electrodes accelerate and further focus the stream of electrons 115. A relatively large voltage differential is applied to accelerates the electron 115 from gun 112 through beam tube 118 and scan magnet 120. Scan magnet 120 provides an oscillating magnetic field that sweeps electrons 115 back and forth across a scan window 121. The beam of electrons 115 is directed toward target 130, which is positioned between scan magnet 120 and elastomeric stator insert 21. Target 130 is made of an element with a Z-number sufficient to produce high-energy X-rays 122 capable of forming polymer cross-linking within elastomeric stator insert 21. In this embodiment, target 130 is a water-cooled tantalum plate. Thus, electrons 115 impact target 130, and in response, target 130 emits X-rays 122 to which elastomeric stator insert 21 is exposed.

In some embodiments, the elastomeric material of the stator insert (e.g., stator insert 21) and/or the stator housing (e.g., housing 25) may incorporate one or more energy activated elements that influence how the ionizing radiation affects the elastomeric material of the stator insert. The energy activated elements may enhance the ionizing radiation effects, increasing the formation rate of polymer cross-linking, increase the strength of the polymer cross-links, or combinations thereof. In an embodiment, the energy activated elements comprise a material capable of emitting secondary ionizing or non-ionizing radiation, upon exposure to the initial ionizing radiation. In another embodiment, the energy activated element material is a material that increases the capture efficiency of the ionizing radiation within the stator. The energy activated element(s) may be positioned in any suitable location(s) including, without limitation, incorporated within the elastomeric material, incorporated within the stator housing, incorporate in any other stator component, formed as an insert, lining, coating, or film on the stator insert, formed as an insert lining, coating, or film on the stator housing, or combinations thereof. Materials that may be employed as energy activated elements include, without limitation, peroxides, coagents, vinyl containing acrylates and methacrylates, modified bismaleimides, and combinations thereof.

In general, the elastomeric stator insert (e.g., stator insert 21) may be exposed to the ionizing radiation before or after being positioned (e.g., injected or installed) in the stator housing (e.g., housing 25), and before or after complete assembly of the PC device (e.g., PC device 10). Since the density of polymer cross-linking in an elastomeric material is generally directly proportional to elastomer viscosity, it may be preferable to maintain the cross-link density, and hence elastomer viscosity, relatively low prior to injection molding, transfer molding, extrusion, or compression molding of the stator insert. However, once the elastomeric stator insert is formed, the cross-link density, viscosity, and other properties may be enhanced by exposure to ionizing radiation.

Embodiments of elastomeric stator inserts described herein (e.g., stator insert 21) may be subject to additional processes to increase the polymer cross-linking density including, without limitation, thermal cure, pressure cure, or pH cure. In particular, embodiments of elastomeric stator inserts described herein are preferably peroxide cured to further enhance polymer cross-linking. These additional processes may be applied to the elastomeric stator insert before or after exposure to ionizing radiation. Thus, exposing the elastomeric stator insert to ionizing radiation may be the only process applied to increase polymer cross-linking density within the elastomeric stator insert, or may be one of many process applied to the elastomeric stator insert to increase polymer cross-linking density.

It should be appreciated that the ionizing radiation employed in embodiments described herein may damage molecules in addition to causing cross-linking. Such damage may increase elastomeric rigidity by breaking of polymer chains. However, such destruction of polymer chains and chemical retarders may increase the mechanical strength of the elastomeric stator insert.

Increased polymer cross-linking density induced by the ionizing radiation offers the potential to increase the strength, resilience, and stiffness of elastomeric stator inserts (e.g., elastomeric insert 21), as well as increase resistance to stress cracking and abrasion. In addition, added cross-linking induced by ionizing radiation offers the potential to decrease wear of the stator insert due to frictional engagement with the rotor, thereby increasing the efficiency of the PC device (e.g., PC device 10) and durability of the stator insert.

EXAMPLE

To assess and quantify the effect of ionizing radiation exposure to an elastomeric stator insert, two B3000 stators with standard nitrile elastomeric insert were sent to the IBA Industrial Inc.'s facility in Edgewood, N.Y. to be exposed to ionizing radiation. At the facility, a 3 MeV Dynamitron directed a high energy electron beam at each stator. A water cooled tantalum plate was interposed between the electron beam and each B3000 stator to expose each B3000 stator to ionizing X-ray radiation. A first of the two B3000 stators was exposed to 180 KiloGray of ionizing radiation and a second of the two B3000 stators was exposed to 250 KiloGray of ionizing radiation.

Each irradiated B3000 stator was then employed in a PC motor, which was subjected to testing by way of a dynamometer and compared to a PC motor including a standard non-irradiated B3000 stator having a nitrile elastomeric insert. The three PC motors tested (i.e., the PC motor including the B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation, the PC motor including the B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation, and the PC motor including the non-irradiated B3000 stator) were identical with the exception of the ionizing radiation treatment.

FIG. 4 graphically displays test results showing the power produced by each PC motor tested (i.e., the PC motor including the non-irratiated B3000 stator, the PC motor including the B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation, and the PC motor including the B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation) at various differential operating pressures between 0 psi and about 500 psi. The power output of the PC motor including the non-irradiated B3000 stator is labeled “PC Motor w/ Non-Irradiated Stator” in the legend of FIG. 4; the power output of the PC motor including the irradiated B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/ Irradiated Stator (180 KiloGray)” in FIG. 4; and the power output of the PC motor including the irradiated B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/Irradiated Stator (250 KiloGray)” in FIG. 4. As shown in FIG. 4, for differential operating pressures greater than about 115 psi, the power output of the PC motor including the non-irradiated B3000 stator was lower than the power output of each PC motor including an irradiated B3000 stator. Thus, both PC motors including X-ray treated B3000 stators exhibited a higher power output than the PC motor including the non-irradiated B3000 stator for differential operating pressures greater than about 115 psi.

FIG. 5 graphically displays test results showing the rotational speed of the rotor of each PC motor tested and the torque output of each PC motor tested (i.e., the PC motor including the non-irratiated B3000 stator, the PC motor including the B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation, and the PC motor including the B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation) at various differential operating pressures between 0 psi and about 500 psi. The torque output of PC motor including the non-irradiated B3000 stator is labeled “PC Motor w/ Non-Irradiated Stator—Torque” in the legend of FIG. 5; the torque output of the PC motor including the irradiated B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/ Irradiated Stator (180 KiloGray)—Torque” in the legend of FIG. 5; and the torque output of the PC motor including the irradiated B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/ Irradiated Stator (250 KiloGray)-Torque” in the legend of FIG. 5. The rotational speed of the rotor of the PC motor including the non-irradiated B3000 stator is labeled “PC Motor w/ Non-Irradiated Stator—Speed” in the legend of FIG. 5; the rotational speed of the rotor of the PC motor including the irradiated B3000 stator exposed to 180 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/Irradiated Stator (180 KiloGray)—Speed” in the legend of FIG. 5; and the rotational speed of the rotor of the PC motor including the irradiated B3000 stator exposed to 250 KiloGray of ionizing X-ray radiation is labeled “PC Motor w/ Irradiated Stator (250 KiloGray)—Speed” in the legend of FIG. 5. As shown in FIG. 5, for differential operating pressure greater than about 115 psi, the rotational speed of the rotor of the PC motor including the non-irradiated B3000 stator was lower than the rotational speed of the rotor of each PC motor including an irradiated B3000 stator. Thus, the rotors of both PC motors including X-ray treated B3000 stators exhibited higher rotational speeds than the rotor of the PC motor including the non-irradiated B3000 stator for differential operating pressures greater than about 115 psi. In addition, for differential operating pressure greater than about 115 psi, the torque output of the PC motor including the non-irradiated B3000 stator was lower than the torque output of each PC motor including an irradiated B3000 stator. Thus, both PC motors including X-ray treated B3000 stators exhibited a higher torque output than the PC motor including the non-irradiated B3000 stator for differential operating pressures greater than about 115 psi.

The trend lines on FIGS. 4 and 5 connecting the actual data points are for visualization of the data trends and are not meant to mean that actual data points were taken at each location on the trend lines. The actual testing measurements are shown in Table 1 below.

TABLE 1

Differential

Rotational

Operating

Speed

Torque Output

Power Output

Temp

Pressure (psi)

(rpm)

(ft-lbs)

(bhp)

(° C.)

Non-Irradiated B3000 Stator

374

123

1020

23.9

70.3

268

138

860

22.61

66.5

166

157

600

17.94

70.6

118

161

450

13.8

70.9

76

164

300

9.37

70.4

33

165

150

4.71

69.4

0

168

0

0

69.3

Irradiated B3000 Stator (180 KiloGray)

464

113

1250

26.9

71.2

331

133

1030

26.09

67.5

168

159

620

18.78

72.2

113

161

440

13.49

72.2

76

165

300

9.43

71.8

32

165

150

4.71

70.6

0

169

0

0

70

Irradiated B3000 Stator (250 KiloGray)

425

118

1160

26.07

76.7

342

135

1030

26.49

72.2

165

159

610

18.47

75.6

117

161

460

14.11

75.4

58

165

250

7.86

73.8

31

165

150

4.71

73.5

0

169

0

0

72.8

While preferred 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 systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments 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. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simply subsequent reference to such steps.