Patent Application
20120258001
The present invention relates to a progressing cavity pump/motor, and more particularly to a progressing cavity pump/motor suitable for high temperature applications wherein both the interior surface of the stator and the exterior surface of the rotor are formed from a substantially rigid material.
Progressing cavity pumps are used in various applications, including downhole oilfield applications to pump fluids to the surface. Progressing cavity downhole motors are similar tools commonly used to convert hydraulic energy into mechanical energy, e.g., to rotate a drill bit. The interior surface of the stator is typically formed from an elastomeric material which acts as a contact seal with the rotor. The contact areas determine the perimeter of the cavities which contain the working fluid, and these cavities progress from one end of the pump/motor to the other end of the pump/motor during its operation.
In certain applications, the operational temperature range intended for the pump/motor exceeds the practical maximum temperature of elastomeric materials or the corresponding adhesive. Materials, both elastomeric and rigid, have been used for both the surface of the rotor and the stator subjected to these conditions. For rigid material applications, a clearance between the rotor and the stator replaces the contact seal between the two parts formed in other applications when using an elastomeric seal. The clearance between the rotor and stator is either designed, or worn into the rotor and/or stator during operation. Such a design can significantly reduce the efficiency due to the volume of the working fluid passed between the I.D. of the stator and the O.D. of the rotor. Such pump/motor designs are not favored in most applications because of their poor efficiency.
Progressing cavity pump/motors are disclosed in U.S. Pat. Nos. 6,120,267, 6,491,501, 6,695,060, 7,214,042, 7,407,372, 7,553,139, and Publications US 2010/0316518 and US 2010/0322808. Such pumps conventionally contain an elastomeric layer on the inner surface of the stator for deforming during rotation of the rotor to form a contact seal. U.S. Pat. No. 7,837,451 discloses a rotor with lobes and grooves in the casing for use in pulse detonation combustors (PDC's) and engines (PDE's). Grooves are provided on a tip portion of the lobes and presumably increase air pressure to the combustion chamber. Pumps and motors according to the present invention rely upon a working fluid, which conventionally is a mixture of liquids, solids, and often some gas, to generate energy or to transfer fluids.
The disadvantages of the prior art are overcome by the present invention, an improved pump/motor is hereinafter disclosed.
In one embodiment, a progressing cavity pump/motor includes a stator having an interior surface and two or more spiraling internal lobes. The rotor has an exterior surface and one or more spiraling external lobes, with the rotor cooperating with the stator to form progressing cavities between the stator and the rotor during rotation of the rotor. At least one of the stator interior surface and the rotor exterior surface has a plurality of spaced grooves in its surface, such that fluid flowing to a gap between the stator and the rotor is disrupted by the spaced grooves to reduce leakage between the stator and the rotor.
These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.
The substantially rigid stator interior surface and rotor exterior surface in a progressing cavity pump/motor incorporate a clearance or gap between the rotor and the stator surfaces. This clearance allows the rotor to turn inside the stator. The clearance is large enough to allow small solid particles, sometimes carried in the working fluid, to pass through without binding the rotor and the stator. Flow resistance through the non-contact seal formed by the gap between the rotor and the stator is created by choke flow wherein the fluid particles cannot move around each other. The present invention significantly increases the effective flow resistance of this non-contact seal, thereby significantly decreasing fluid loss between the rotor and the stator while there is a nominal gap between the rotor and the stator.
Those skilled in the art appreciate that the seal line between the rotor and the stator is moving in a cyclical manner as the pump/motor operates. The velocity and direction of flow at any point on the rotor or stator surface is therefore constantly changing in a repeating pattern. Near surface flow will be substantially parallel to the stator or rotor surface, but, at locations near the seal line, the flow can impinge at angles approaching perpendicular. This rapid cyclical velocity offers a certain amount of resistance when the rotor and stator surfaces approach each other. This resistance is minimized, however, by the conventional smooth, contoured surface on both the rotor and the stator surfaces.
The present invention incorporates small and preferably parallel grooves on the surface of either the rotor and/or the stator to create turbulent eddies in the near surface flow. This substantially increases turbulence, which results in an increase in flow resistance and thus a significant decrease in fluid leakage. The groove design (shape, depth, spacing between grooves, and orientation) determines the amount of flow resistance achieved. Eddies are created by “tripping” fluid flowing along the contoured surface as it flows across the groove, or by redirecting fluid that impinges the groove from oblique angles. As discussed hereafter, turbulence generation may be increased with the addition of steps, walls between closely spaced grooves, and a combination of walls and steps.
Referring now to
The present invention is substantially different in that the primary sealing mechanism is non-contact with grooves specifically designed to increase motor/pump efficiency.
In other embodiments, each groove set may consist of three or more grooves, as shown in
The grooves on the inner surface of the stator are elongate in that each groove has a length significantly greater than its width. Continuous grooves may be formed along substantially the entire length of the stator housing, or a discrete length axially extending groove may be formed, followed by a short interval of no groove, followed by a continuation discrete length axially extending groove, etc. Each groove may have one outwardly slanted side wall, and a “straight” side wall which is substantially perpendicular to the interior surface of the stator. In another alternative, both the side walls of the groove may be tapered outwardly, but at different angles relative to a centerline of the groove.
Each axially extending groove which matches or follows the contour of a respective lobe on the stator or the rotor will be a substantially uniform spacing from a respective lobe centerline. The spacing between elongate grooves is relatively short so that the lands between the grooves (e.g., the interior surface of stator not having a groove) for a preferred embodiment may occupy from two to ten times the surface area of the grooves.
The benefits of the invention may be realized because downhole application systems use better filtering techniques than in decades past for recovering solid particles before the fluid enters the pump/motor. In some applications, for example, water may be used as a working fluid when drilling out plugs in a well. Also, some applications utilize a higher viscosity working fluid than was used a decade ago, and accordingly this higher viscosity fluid further benefits from the use of grooves as disclosed herein to disrupt fluid flow to the gap between the rotor and the stator.
The pump/motor can become locked by particles wedged between the rotor and the stator if the particles passing through the pump/motor are larger than the gap. Decreasing the gap between the rotor and the stator reduces the amount of “lost” fluid which passes through the gap, but increases the likelihood of solid particles becoming wedged between the rotor and the stator. The addition of fluid flow disrupting grooves reduces this lost fluid while maintaining a sufficient gap between the rotor and the stator of a progressing cavity pump/motor to minimize or eliminated locking. Costs are incurred to provide such grooves in a progressing cavity pump/motor compared, for example, to providing a spaced groove surface and gap between a cylindrical shaft with a uniform diameter and a uniform diameter bore in a housing. Unlike other situations, the spacing between a groove and the ever changing gap location varies as the pump/motor is operated. Moreover, providing grooves along the contoured surface of a lobe is more complicated, particularly since the grooves effectively need to be provided over substantially the entire length of the pump/motor, which may be twenty feet or more in length. Grooves in the rotor and the stator may be formed using machining techniques which are commonly used to form the respective lobes on the rotor and/or on the stator.
The progressing cavity pump/motor of the present invention includes a stator with a substantially rigid interior surface in the form of spiraling internal lobes, and a rotor with a substantially rigid external surface and one or more spiraling external lobes. At the temperatures and pressures in which the pump/motor is operating, these surfaces are substantially rigid. As used herein, “substantially rigid” means a surface of any material with sufficient geometric stability at its operating temperature and pressure such that the flexibility of the surface does not create or contribute to sealing or to reduced loss of the working fluid between the surfaces. This includes pliable or elastic materials which “cure”, “set”, or “age” to meet the definition of “substantially rigid” under operating conditions.
The “lands” or spacing between the grooves allow the interior surface of the stator to act as a guide to facilitate rotation of the rotor within the stator.
Accordingly, these surfaces cyclically engage at several locations along the length of the pump/motor to guide rotation of the rotor within the stator. A contact seal is not achieved apart from these specific locations. Grooves, at intervals along each of the respective lobes, are provided on the interior of the stator or on the exterior of the rotor to minimize fluid losses through the gap by generating turbulence and effectively reducing the gap width. Groove depth may vary from 0.001 inches to 0.100 inches.
A metal stator and rotor may be formed from steel, since it is a suitable substantially rigid material, although composite materials and some thermoset materials also have this substantially rigid feature, and also provide high chemical resistance to various types of downhole fluids. The gap between the rotor and the stator may vary from 0.000 inches to a point of maximum cavity width. Lost fluid passing through this gap is significantly reduced by the use of grooves as disclosed herein.
The addition of grooves in the surface of the stator or rotor reduces the loss of working fluid passing through the pump/motor, thereby increasing efficiency. In other applications, the size of debris in the working fluid mandates a large gap between the rotor and stator, such that the efficiency of the pump/motor becomes unattractive. By providing the grooves as disclosed herein, a sufficiently large gap may be maintained to pass sizable debris while still resulting in a pump of reasonable efficiency.
The pump/motor disclosed herein is particularly well-suited for downhole applications, e.g., in oil and gas drilling and fluid recovery operations. The pump/motor has significant benefits in other applications, particularly in applications involving high temperature environments and/or fluids deleterious to elastomers.
Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.
