Field of the Invention
The disclosed and claimed concept relates to a stator assembly for a progressing cavity pump and, more specifically, to a stator assembly wherein the helical passage is a flexible helical passage.
Background Information
Progressing cavity pumps are often referred to as “Moineau” pumps, in recognition of their inventor, Rene Moineau, who obtained U.S. Pat. No. 1,892,217. Progressing cavity pumps are used in various industries to pump materials such as, but not limited to, viscous fluids, semi-solids, fluids with solids in suspension, and solids. Exemplary materials transported by a progressing cavity pump include, but are not limited to, oil, sewage, fracking fluids or the like. Generally, a progressing cavity pump (also known as a helical gear pump) includes an elongated rotor having one or more externally threaded helical lobes, or “splines,” rotatably disposed in a stator assembly or stator body defining a helical passage. In one embodiment, the helical passage includes one more lobes than the helical rotor. The elongated helical passage includes a plurality of helical grooves that form a plurality of cavities with the stator. As the rotor turns within the stator, the cavities progress from a suction end of the pump to a discharge end. In other embodiments, there are an equal number of rotor splines and stator lobes, but the rotor splines are sized and shaped so as to define cavities within the stator lobes. In an exemplary embodiment, each lobe of the rotor is, in theory, constantly in general contact with the stator at any transverse cross section; this has the effect of creating a plurality of empty spaces between the stator and the rotor. It is noted that the clearance, or interference, at a location wherein a rotor spline is not fully seated in a stator lobe, may be variable, i.e., less than substantial engagement. That is, for example, in an embodiment wherein a stator passage has an arcuate end surface and a linear lateral surface, it is desirable to ensure the rotor seals against the arcuate end surface of the stator; this ensures the cavity, and therefore the fluid therein, moves forward. It is desirable, but less important, that the rotor seals against the linear lateral surface of the stator.
As the rotor rotates, the empty spaces advance from the suction end of the helical passage to the discharge end of the helical passage. Further, the empty spaces are isolated from each other by the points of contact between the rotor and the stator, which are often referred to as “seal lines.” As the rotor rotates within the stator, the empty spaces “move” or progress with a helical motion along the length of the helical passage. In operation of a progressing cavity pump, the empty spaces are filled with a material that is to be moved. Thus, as the empty spaces progress, the material is moved from one end of the stator to the other end of the stator as the rotor rotates relative to the stator. Due to the shape and geometry of the stator and the rotor, the rotor will move laterally or precess relative to the stator as the rotor rotates within the stator. In other words, the rotor moves eccentrically relative to the stator in addition to rotating within the stator.
In an exemplary embodiment, shown in FIG. 1, a progressing cavity pump 1, includes an elongated helical rotor 2, and a stator assembly 3 defining an elongated helical passage 4. In the exemplary embodiment shown, the rotor has a single lobe and, therefore, has a generally circular cross-sectional shape. The helical passage (shown in cross-section) has an obround shape. As used herein, an “obround” shape includes opposed generally arcuate surfaces and opposed generally parallel, generally linear surfaces; what may be colloquially identified as a “pill” shape. In operation, the rotor 2 reciprocates between the two ends of the helical passage.
To ensure that the rotor is “constantly in substantial contact with the stator at any transverse cross section” the stator helical passage is typically lined with a resilient material, such as but not limited to an elastomeric material. That is, in an exemplary embodiment, the stator assembly includes a rigid support assembly defining the helical passage and the liner is disposed thereon. As the rotor rotates and reciprocates between the two ends of the helical passage, in the exemplary embodiment shown in FIG. 1, the resilient material is compressed between the rotor and the support structure. Further, if the material being moved is a fluid with suspended solids, the solids may pass between the resilient material and the rotor.
This configuration has several disadvantages including the degradable nature of the resilient material liner. That is, the compression of the resilient material liner causes rapid wear and tear on the liner leading to the need for replacement. As used herein, “rapid” degradation is a relative term; a resilient material degrades more rapidly than a durable material. Further, solids passing between the resilient material and the rotor also damage the resilient material liner. Also, the resilient material liner may react with, or be degraded by, the material being moved. Another disadvantage is that rigid stator assemblies are difficult and/or expensive to construct. That is, such stator assemblies are typically created by hydroforming, rolling a metal tube, cold drawing a metal tube, hot extrusion of a metal tube, boring a metal tube using a method such as, but not limited to, electrical discharge machining, and electroforming with metal deposition.
In another embodiment, not shown, the stator assembly is made substantially of a resilient material. While the resilient material may have a rigid outer housing, the helical structure and support is formed by the resilient material. This embodiment also allows for substantial constant contact between the rotor and the stator assembly, and, allows for solids to pass between the rotor and stator. This embodiment is, however, also subject to rapid degradation. Further, as the stator helical passage is generally resilient, the progressing cavity pump of this embodiment is limited to lower pressures and lower transfer speeds. That is, at a higher pressure, the stator will distort allowing back-flow of the material over the rotor.
In another embodiment, not shown, the stator assembly is made of a rigid material with no liner. Typically, both the rotor and the stator are made from a durable material, i.e., a non-resilient material. While a durable material is less subject to wear-and tear, the friction between the two durable material elements will cause wear-and-tear to both the rotor and the stator. Further, with rigid materials forming both the rotor and the stator, particles cannot pass therebetween. That is, a solid trapped between the rigid rotor and stator will be crushed causing additional wear and tear to the components. Alternatively, with a larger or more durable particle, the rotor will flex, possibly bending the rotor permanently. As such, and as used herein, a progressing cavity pump wherein a durable rotor engages, or moves over, a durable stator is a “self-damaging” progressing cavity pump. One solution to the issue with particles in a self-damaging progressing cavity pump is to allow for a small gap between the rotor and the stator; that is, the rotor and stator are not “constantly in contact.” This configuration, however, allows for back-flow of the material between adjacent cavities. That is, this configuration is less efficient. Further, in this embodiment, the stator is typically made by one of the expensive methods noted above.
Further, as noted in U.S. Pat. No. 8,905,733 there is an advantage to having turbulent flow of a fluid adjacent the stator surface within a progressing cavity pump. In that patent, the turbulent flow is created or enhanced by grooves in, for example, the surface of the stator helical passage. These grooves, however, must be machined into the stator helical passage surface either during the formation of helical passage or sometime thereafter. As such, the grooves are expensive to incorporate into the stator.
It is understood that a progressing cavity pump includes a drive assembly with a drive shaft that causes the rotor to rotate within the stator thereby creating the pump action. That is, a rotary motion is converted to a fluid action, i.e., pumping. As is known, however, the rotor/stator assembly with minor geometric differences may have a fluid pumped therethrough thereby causing the rotor to rotate. That action is then transferred to the drive shaft and drive assembly. That is, a fluid motion is converted into a mechanical motion. Thus, it is understood that while the following discussion addresses a rotor/stator assembly as a pump, the same rotor/stator assembly may be used to create a rotational motion, i.e., may be used as a drive device, e.g., for a drill.
There is, therefore, the need for an improved progressing cavity pump wherein the components are not subject to rapid degradation, are not self-damaging, and do not allow for back flow of the material being transported.