Patent Application
20210222686
The disclosure relates to a pumping system. More particularly, it relates to a pumping system to pump fluid at a nearly constant flow rate.
The practice of fracking has greatly increased the amount of oil and natural gas produced within the United States. Fracking involves pumping large quantities of fluid into wells. Conventionally, this is accomplished by reciprocating pumps driven by diesel engines. Due to the availability of natural gas on site, it would be preferable to use electric power from natural gas turbine driven generators.
Conventional fracking pumps utilize a crankshaft and connecting rod mechanism to convert rotational motion into axial reciprocating motion of a piston. Each cycle of the piston produces a pulse of flow, with the flow rate during each pulse being a function of the crankshaft and connecting rod geometry. Use of a large number of pistons with offset pulses allows the total flow rate to be smoothed out, but never completely constant. The variations in flow rate are called flow ripple. Flow ripple causes pressure pulses that increase failure rates of various components in the system. Also, for a given system size, such a pump has a very limited stroke distance. Therefore, many strokes per unit time are required to achieve a desired flow rate. This increases wear on valves which must open and close once per stroke.
A pumping unit includes a housing, a hollow threaded shaft, a linear actuator, a first cylinder, a first hollow piston, and first inlet and outlet valves. The hollow threaded shaft is supported for translation along an axis with respect to the housing and restrained against rotation about the axis with respect to the housing. The linear actuator is configured to rotate a nut about the axis, in response to power provided to at least one motor and to translate the hollow threaded shaft along the axis, in response to rotation of the nut. The at least one motor may include a plurality of motors arranged circumferentially around the axis. The linear actuator may include the nut, a ring gear fixed to the nut, and a plurality of pinion gears, each pinion gear meshing with the ring gear and fixed to a rotor of a respective one of the plurality of motors. The first cylinder is fixed to an interior of the hollow threaded shaft to translate with the hollow threaded shaft along the axis. The first hollow piston is fixed to the housing and extends into the first cylinder such that the first cylinder, first piston, and housing define a first chamber having a first volume which varies in response to translation of the hollow threaded shaft. The first inlet valve is configured to alternately fluidly connect the first chamber to a fluid source and separate the first chamber from the fluid source. The first outlet valve is configured to alternately fluidly connect the first chamber to a fluid sink and separate the first chamber from the fluid sink. The pumping unit may also include a second cylinder, a second hollow piston, and second inlet and outlet valves. The second cylinder may be fixed to the interior of the hollow threaded shaft to translate with the hollow threaded shaft along the axis. The second hollow piston may fixed to the housing and may extend into the second cylinder such that the second cylinder, second piston, and housing define a second chamber having a second volume which varies in response to translation of the hollow threaded shaft. The second inlet valve may be configured to alternately fluidly connect the second chamber to the fluid source and separate the second chamber from the fluid source. The second outlet valve may be configured to alternately fluidly connect the second chamber to the fluid sink and separate the second chamber from the fluid sink. A sum of the first volume and the second volume may be independent of the axial position of the hollow threaded shaft. The first cylinder may be mounted within the hollow threaded shaft such that a radial gap exists between an outer wall of the cylinder and an interior wall of the hollow threaded shaft.
A pumping system includes a plurality of pumping units as described above and a controller. The controller is configured to vary a flow rate of each of the pumping units such that a total flow rate is substantially constant. The flow rate of each pumping unit may include a series of alternating increasing phases and decreasing phases. Each increasing phase may be coordinated with a decreasing phase of another of the pumping units. Similarly, each decreasing phase may be coordinated with an increasing phase of another of the pumping units. Each decreasing phase may be separated from the previous increasing phase by a constant flow phase.
Embodiments of the present disclosure are described herein. It should be appreciated that like drawing numbers appearing in different drawing views identify identical, or functionally similar, structural elements. Also, it is to be understood that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations.
The terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the following example methods, devices, and materials are now described.
Each electric linear actuator 20 includes at least one electric motor 32 having a fixed stator and a rotatable rotor. The motors may be, for example, alternating current motors such as a permanent magnet synchronous motors. With a synchronous alternating current motor, the rotational speed of the rotor is adjusted by adjusting the frequency of the electric current using an inverter. With other types of motors, a speed or position feedback signal may be required. The motor 32 drives a nut of a planetary screw drive mechanism 34 as described, for example, in U.S. Pat . No. 9,267,588. Rotation of the nut in response to rotation of rotor 32 causes shaft 22 to displace along its axis. The nut may be fixed to a ring gear 36 which meshes with a pinion gear 38 fixed to the rotor of the electric motor.
A control unit continually monitors a control signal or multiple control signals from a sitewide controller which controls multiple pumping systems. These signals indicate a desired flow rate and pressure from the pumping system. The controller calculates a trapezoidal motion profile for each actuator unit in the local pump system, the sum of which meets the demand. The controller utilizes various types of feedback signals which may include: back-emf voltage from the motors, current supplied to the motors, linear position sensors attached to the reciprocating portion of the pumps, rotary position sensors on the integrated nuts, pressure sensors in the fluid chambers of the pumps, strain sensors on the load-bearing elements of the pumps, and condition monitoring sensors in the bearings. The controller adjusts the motion of each actuator's motors to achieve: close adherence to the commanded motion profile, even sharing of torque load on each motor within an actuator unit, and protection from damaging conditions such as cavitation, low pressure, and incomplete fillage. The controller adjusts the motion profiles of each actuator unit in the local group to achieve: even wear and maximum life of each unit, real-time compensation for flow ripple (as discussed below), and special operating conditions as instructed by sitewide controller such as: pulsation or shockwave generation, ramp up/down, and/or idle. The controller relays real-time operating parameters (position, velocity, status) to the sitewide controller.
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With three pumping units, these phases are staggered to maintain constant total flow. At any given time, one pumping unit is operating in either phase 60 or 66, another pumping unit is operating in either phase 62 or 68, and a third pumping unit is operating in either phase 64 or 70. With three total pumping units, the length of phase 50 and 54 should be half as long as the length of phases 52 and 56. With different numbers of pumping units, the relative durations of the phases may be adjusted such that one unit is always in a declining flow phase and one unit is always in an increasing flow phase.
In addition to establishing a constant flow rate, the pumping system described above offers several advantages. Each of the pumping units has a relatively long stroke relative to its overall size. As a result, the valves do not need to open and close as often as they would for a shorter stroke pump at the same average flow rate. This improves the durability of the valves. Furthermore, the pumping system can continue to operate with one of the pumping units offline which simplifies maintenance.
A radial gap separates the outside wall of the cylinder 80 from the interior wall of the hollow threaded shaft 22. Therefore, slight expansion of the cylinder due to pressure of the fluid therein does not alter the dimensions of the threaded shaft.
While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the disclosure that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. As such, to the extent any embodiments are described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics, these embodiments are not outside the scope of the disclosure and can be desirable for particular applications.
This application claims priority to U.S. Provisional Application 62/963,584 filed Jan. 21, 2020, the entire disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62963584 | Jan 2020 | US |
