US20090092511A1

US20090092511A1 - Heart-shaped cam constant flow pump

Info

Publication number: US20090092511A1
Application number: US11/868,024
Authority: US
Inventors: Fangfang Jiang
Original assignee: Schlumberger Technology Corp
Current assignee: Schlumberger Technology Corp
Priority date: 2007-10-05
Filing date: 2007-10-05
Publication date: 2009-04-09

Abstract

A constant flow pump and method of providing a constant fluid flow using such a pump, and more particularly a pump and method that maintain a constant flow by using a heart-shaped cam to directly reciprocate the pistons of the pump at a constant velocity are described. The pump operates by using heart-shaped cam to translate the rotational motion of a power source such as an engine or motor can into a constant speed linear motion.

Description

FIELD OF THE INVENTION

The current invention is generally directed to a constant flow pump, and more particularly to a pump that maintains a constant flow by using a heart-shaped cam to reciprocate the pistons of the pump at a constant velocity.

BACKGROUND OF THE INVENTION

Pumps were described as early as the 7^thcentury BC; however, most modern pumps, including reciprocating pumps, double acting pumps with suction pipes, water pumps, and pistons were first described in the 13^thcentury AD. Modern pumps can be broadly separated into two major groups based on the method used by the pump to move fluids. In a first group there are rotodynamic pumps, these pumps are based on bladed impellers which rotate within the fluid to impart a tangential acceleration to the fluid and a consequent increase in the energy of the fluid. The basic purpose of these rotodynamic pumps is to convert the energy of the impellers into fluidic pressure energy that can be used in the associated piping system.
In the second group are positive displacement pumps. Positive displacement pumps cause a liquid to move by trapping a fixed amount of fluid and then forcing (displacing) that trapped volume into the discharge pipe. Positive displacement pumps can be further classified as either rotary-type reciprocating pumps or the helical twisted Roots pump. Reciprocating-type pumps use a piston and cylinder arrangement with suction and discharge valves integrated into the pump. In simplest terms, these reciprocating positive displacement pumps change the rotational motion of a power source, such as, for example air, steam, engine or motor into a linear motion of a plunger to deliver high-pressure fluid. Usually the constant speed rotation is transferred into linear motion with variable speed. For example, the pump with a crankshaft changes the constant speed rotation into a linear motion with trigonometric function speed. These reciprocating pumps are typically used for pumping highly viscous fluids including concrete and heavy oils.
Pump arrangements are from “simplex” one cylinder to in some cases four (quadriplex), but typically these pumps are either “duplex” two or “triplex” three cylinder. Furthermore, these pumps can either be “single acting” where the suction and discharge strokes are independent, or “double acting” where the suction and discharge occur in both directions. However, all of these conventional reciprocating pumps reduce their deliveries towards the ends of their strokes and accelerate their deliveries at the commencement of their strokes. These effects result in pumping pulses in the delivery of the fluid that can cause premature wear on downstream components.
As a result, these simple reciprocating pumps have been replaced by more complex gear or non-simple reciprocating pumps. More specifically, triplex or quintiplex pumps, which sum three to five trigonometric functions, are usually used to smooth out these pulses. Typically, a triplex or quintiplex pump has pump pistons which are equally phased apart and driven by a common drive shaft so that the velocity, i.e., speed with direction of all pistons in the particular pump is equal to zero at all times. However, these devices have several major disadvantages. First, these pumps can only approximate a constant flow by summing the output of many pistons. As a result, absent an infinite number of pistons there will inherently always be some pulsation in the output of these devices. In addition, as the number of pistons and chambers increase so does the cost and complexity both of manufacturing and maintaining these pumps.
Despite these drawbacks, in many applications, including, for example, cementing, fracturing etc., a constant output flow is desirable to avoid serious pressure spikes within the pump and downstream equipment. As a result, operators are forced to purchase and use these less than ideal pumps. In view of the state of the art, a device that can change the constant speed rotation of an engine or motor into a constant speed linear motion directly would be extremely desirable.

SUMMARY OF THE INVENTION

The current invention is generally directed to a constant flow pump; and more particularly to a pump that maintains a constant flow by using a cam having an approximately heart-shaped outer contour defined by two diametrically opposed 180° arches of an Archimedean spiral to directly reciprocate the pistons of the pump at a constant velocity. Hereinafter this cam will be described as a “heart-shaped cam”.
In one embodiment, the heart-shaped cam of the current invention has a line of reflection defined through its center and each of the halves of the cam defined by said line of reflection can be described by the equation r=kΘ, or in other terms as a 180° arc of an Archimedean spiral.
In another embodiment, the pump of the current invention used a single heart-shaped cam with two pistons slidingly engaged thereto. In such an embodiment the two pistons are rotated 180° relative to each other.
In still another embodiment, the pump of the current invention uses two heart-shaped cams mounted 180° out of phase on the same shaft to provide a constant flow rate, or alternatively two heart-shaped cams mounted in parallel and two pistons engaged to said cams where the pistons are mounted 180° out of phase.
In yet another embodiment, the heart-shaped cams are asymmetric to eliminate flow rate variations when the plunger ends pass the transition between one side of the heart and the other. In such an embodiment the arcs of the cam are asymmetric. The asymmetry of the arcs can be understood with reference to an extending portion of the cam that serves to extend the piston and a retracting portion of the cam that serves to retract the piston. In the case of an asymmetric cam the extending portion spans a larger angle arc than the retracting portion such that both pistons deliver fluid at the same time during the transition period. Alternatively, this asymmetry can be described in reference to the Archimedean spiral constant. In an asymmetric embodiment, the Archimedean spiral constant of the retracting portion is larger than that of the extending portion. Regardless of the definition used, the extending and retracting portions are designed such that the sum of the velocities of the accelerating and decelerating cams is constant.
In still yet another embodiment, the pump is a positive displacement pump directed for use in oil field applications, such as, for example, in cementing and fracturing jobs.
In still yet another embodiment, the invention is directed to a heart-shaped cam for use in pump applications. In one such embodiment, the cams are asymmetric to eliminate flow rate variations when the plunger ends pass the transition between one side of the heart and the other.
In still yet another embodiment, the invention is directed to a method of providing a constant fluid flow using the heart-shaped cams of the current invention. Again, in one such embodiment, the cams are asymmetric to eliminate flow rate variations when the plunger ends pass the transition between one side of the heart and the other.
The above-mentioned and other features of this invention and the manner of obtaining and using them will become more apparent, and will be best understood, by reference to the following description, taken in conjunction with the accompanying drawings. The drawings depict only typical embodiments of the invention and do not therefore limit its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a diagram of a conventional displacement pump fluid chamber;

FIG. 2 provides a plot, in polar coordinates, of the contour of one embodiment of a symmetric heart-shaped cam in accordance with the current invention;

FIG. 3 a shows a schematic diagram of an exemplary embodiment of the current invention comprising a single cam and two pistons;

FIG. 3 b shows a schematic diagram of an exemplary embodiment of the current invention comprising two cams and two pistons;

FIG. 4 provides a plot, in polar coordinates, of the contour of a second embodiment of an asymmetric heart-shaped cam in accordance with the current invention in polar coordinates; and

FIG. 5 provides a plot of plunger velocity and cam rotating angle in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is generally directed to a constant flow pump, and more particularly to a pump that maintains a constant flow by using the unique geometric properties of an Archimedean spiral to directly reciprocate the pistons of the pump at a constant velocity. Although embodiments may be described in reference to pumps for use in operations such as fracturing, cementing, coil tubing and water jet cutting, it should be understood that the current cam design may be used with any positive displacement pump for which constant flow is a functional requirement.
As discussed in the background, positive displacement pumps cause a liquid to move by trapping a fixed amount of fluid and then forcing or displacing that trapped volume into a discharge pipe. A schematic diagram of the intake/discharge chamber of a typical positive displacement pump (10) is provided in FIG. 1. As shown, these pumps use a piston or plunger (12) and cylinder (14) arrangement with suction and discharge valves (16 & 18) integrated into the pump. In simplest terms, these reciprocating positive displacement pumps change the rotational motion of a power source, such as, for example air, steam, engine or motor into a linear motion of a plunger to deliver high-pressure fluid. Usually the constant speed rotation is transferred into linear motion with variable speed. For example, a pump with a crankshaft changes the constant speed rotation into a linear motion with trigonometric function speed. In the prior art the variable pump forces produced by these variable speeds have either been tolerated, or multiple out-of-phase cams are used to smooth the variability of the fluid force generated by the pump.
In more detail, a displacement pump (10) shown in FIG. 1 includes a plunger (12) for reciprocating within a plunger housing (20) toward and away from a chamber (22). In this manner, the plunger (12) affects high and low pressures on the chamber (22). For example, as the plunger (12) is thrust toward the chamber (22), the pressure within the chamber (22) is increased. At some point, the pressure increase will be enough to affect an opening of a discharge valve (18) to allow the release of fluid and pressure within the chamber (22). Thus, this movement of the plunger (12) is often referred to as its discharge stroke. Further, the point at which the plunger (12) is at its most advanced proximity to the chamber (22) is referred to as the discharge position. The amount of pressure required to open the discharge valve (18) as described may be determined by a discharge mechanism (24) such as spring, which keeps the discharge valve (18) in a closed position until the requisite pressure is achieved in the chamber (22). In an embodiment where the pump (10) is to be employed in a fracturing operation pressures may be achieved in the manner described that exceed 2,000 PSI, and more preferably, that exceed 10,000 PSI or more.
As described above, the plunger (12) also affects a low pressure on the chamber (22). That is, as the plunger (12) retreats away from its advanced discharge position near the chamber (22), the pressure therein will decrease. As the pressure within the chamber (22) decreases, the discharge valve (18) will close returning the chamber (22) to a sealed state. As the plunger (12) continues to move away from the chamber (22) the pressure therein will continue to drop, and eventually a low or negative pressure will be achieved within the chamber (22). Similar to the action of the discharge valve (18) described above, the pressure decrease will eventually be enough to affect an opening of an intake valve (16). Thus, this movement of the plunger (12) is often referred to as the intake stroke. The opening of the intake valve (16) allows the uptake of fluid into the chamber (22) from a fluid channel (26) adjacent thereto. The point at which the plunger (12) is at its most retreated position relative to the chamber (22) is referred to herein as the intake position. The amount of pressure required to open the intake valve (16) as described may be determined by an intake mechanism (28) such as spring, which keeps the intake valve (16) in a closed position until the requisite low pressure is achieved in the chamber (22).
As described above, a reciprocating or cycling motion of the plunger (12) toward and away from the chamber (22) within the pump (10) controls pressure therein. The valves (16 & 18) respond accordingly in order to dispense fluid from the chamber (22) and through a dispensing channel (30) at high pressure. That fluid is then replaced with fluid from within a fluid channel (26).
However, as discussed using a traditional circular cam design the velocity of the cam, and therefore the pressure of the fluid being release into the chamber, is constantly in flux, in turn causing pressure spikes and drops. To limit these pressure fluctuations, conventional “constant flow pumps” use three or more cams and pistons that are phase offset such that at any one point the velocity of the sum of the pistons approaches a constant in a trigonometric function. Typical embodiments of such pumps are disclosed in U.S. Pat. Nos. 4,556,371 and 4,687,426, the disclosures of which are incorporated herein by reference. As discussed, one drawback to these pumps is the large number of moving parts and the overall complexity and cost involved in manufacturing and maintaining them in working order.
It has been surprisingly discovered that a constant fluid flow can be generated in a pump using the geometric properties of a cam comprising one 180° arc of an Archimedean spiral together with its reflection (described herein as the “heart-shaped” cam). Specifically, the instant pump utilizes a cam in which the distance from any point on the cam contour to the origin is proportional to the angle from the x-axis. In polar coordinates (r, Θ), the shape of the cam of the current invention can be described by the equation:
r=kΘ, (Eq. 1)
where k is the spiral constant. This is the mathematical definition of an Archimedean spiral. A plot of the contour of one embodiment of a symmetric heart-shaped cam can be shown in FIG. 2, where the left side of the cam contour (solid line) is an Archimedean spiral from Θ=90° to Θ=270°, and the right side of the cam contour is the mirror image of the spiral (dashed line). The line defining the left and ride sides in this plot is called the line of reflection, which and will be used hereinafter to describe the two “halves” of the cam of the current invention.
Utilizing such a heart-shaped cam, the rotational motion of a power source such as an engine or motor can be transformed into a constant speed linear motion. Specifically, rotating the heart-shaped cam with uniform angular velocity about its center will result in uniform linear motion of a plunger or piston whose axis passes through the heart origin in FIG. 2, as such the cam can be used to convert uniform circular motion into uniform linear motion. For a complete discussion of the mathematical principals behind this see, for example, Steinhaus, H. Mathematical Snapshots, 3rd ed. New York: Dover, pp. 136-137, 1999, the disclosure of which is incorporated herein by reference.
Despite the fact that it has been known for some time that a cam comprising a reflected Archimedean spiral, or heart-shape, is able to impart a constant linear motion to a properly situated piston, the ramifications of these properties to the operation of pumps has never been explored. Applicants have surprisingly discovered that utilizing a heart-shaped cam to drive a positive displacement pump provides a direct mechanism for generating a constant fluid flow.
Unlike the conventional triplex pump, the torque at this heart cam shaft is a constant if the shaft rotate at a constant speed and the load at the connected plunger is also constant. Since the plunger moves at a constant speed, the power (where power equals load times velocity) required is constant. Accordingly, when the heart-shaped cam of the current invention rotates at a constant speed, the torque at the cam origin is also a constant (because power equals torque times rotational speed). Using Eq. 1, above, a simple derivation leads to equation 2, below:
T=kF, (Eq. 2)
where T is the torque on the cam, k is the Archimedean spiral constant and F is the load at the plunger. Accordingly, it can be seen from Eq. 2 that the torque is proportional to the spiral constant for a constant load.
As shown in FIG. 3, in one embodiment when the heart-shaped cam of the current invention is incorporated in a pump the cam (34) is disposed on a cam-shaft (36) through which the rotational motion of the power source (not shown) is applied. One end of the plunger (12) touches the outer surface or contour of the cam (34), while the other end is disposed in a cylinder chamber (not shown) like the one described in reference to FIG. 1 (element 22), above. The axis of the plungers passes through the axis of the cam-shaft.
The operation of the device will be described with reference to FIGS. 2 and 3. As shown, when the cam positioned at the “top” edge of the cam contour is rotated, in this case in a counter clockwise direction about the origin, the end of the plunger (12) follows the trace of the solid line of FIG. 2. During this motion the plunger (12) will extend at constant speed. When the end of the plunger reaches the right-hand side of the contour (dashed line), it will retract at constant speed. In one embodiment of the invention, as shown in FIG. 3 a, a single heart-shaped cam (34) can be used upon which two pistons (12 & 12′) are disposed. In such an embodiment, the two pistons are rotatably disposed 180° relative to each other along the single cam. In an alternative embodiment, as shown in FIG. 3 b, two cams (34 & 34′) are arranged 180° out of phase with two adjacently disposed pistons/plungers (12 & 12′) slidingly engaged to each of the cams such that the sum flow at the downstream of the two plungers is constant. Although in this embodiment the cams have been rotated 180° relative to each other, it should be understood that the invention also contemplates a pump where the cams are aligned in phase and the pistons are rotated 180° relative to each other.
Finally, although only single and double cam pumps are shown and described above, it should be understood that any combination of heart-shaped cams and pistons can be used such that the velocities of the cams and therefore the flow rate of the sum of the pistons remains constant. Again, it is preferred that in arranging the cams and pistons, they must be positioned such that the extending and retracting pistons are in balance.
Although the symmetric heart-shaped cam of FIG. 2 operates in principal, it should be noted that there are mathematical discontinuities on the curve at 90° and 270° that are required for proper operation. During the manufacturing process it is possible that these discontinuities might be smoothed. Smoothing these discontinuities would cause a finite deceleration or acceleration in the linear speed of the plungers at these transition points. Accordingly, with a “smoothed” symmetric heart shape and two cams arranged 180° apart as shown in FIG. 3, a flow rate dip would be generated when the plunger ends pass these transition zones.
To remedy this potential problem an asymmetric heart-shaped cam can be used. One exemplary embodiment of such an asymmetric cam is shown in FIG. 4, where the extending contour of the spiral cam spans a larger angle than the retracting contour of the spiral cam. Using such a design would cause the Archimedean spiral constant for the retracting curve to be larger than the extending, which in turn would mean that the retracting speed of the piston is higher than the extending speed allowing for the cancellation of the pressure dip.
For example, when two symmetric heart-shaped cams are arranged 180° out of phase on the camshaft there is an instantaneous transition from extending to retracting, so that while one of them is extending the other is retracting. With two asymmetric cams arranged 180° out of phase, as shown in FIG. 4, both plungers deliver fluid at the same time during the transition period (because of the longer extending curve). As a result, the flow rate dips are canceled by the acceleration of one plunger and deceleration of the other. This cancellation is shown graphically in FIG. 5, which provides a map of piston movement and fluid flow as the heart-shaped cam is rotated.
In such an embodiment, the portions of each of the cams incorporating these extended transition curve shapes should be designed so that the sum of the velocities of the accelerating and decelerating cams is constant. For example, in any particular curve the angular velocity can be described according to Equation 3, below.
r=k(Θ−Θ₀)² +b ₀, (Eq. 3)
Using the above equation it can be seen that a constant angular velocity can be transferred to a linear velocity with a constant acceleration (positive k) or deceleration rate (negative k). Any of the constants k, Θ₀and b₀can be chosen to make a smooth transition from the Archimedean curve to the acceleration or deceleration curve. Although specific embodiments of these transition curves are described above, it should be understood that other suitable curves can also be used in the acceleration and deceleration portions of the cam as long as the sum of the resultant velocities is constant.
As discussed, the cams of the current invention are contemplated for use in any pump application in which a constant linear velocity is required in the movement of the piston coupled thereto. Accordingly, the cam of the current invention can be made of any material suitable for such a pumping application, such as, for example, metal, plastic/polymer, ceramic, composite, or wood. In addition, the cams of the current invention can be incorporated with any piston that can be designed to slidingly engage the cam such that it follows the unique contour of the cam. Likewise, the cams can be incorporated with and into any suitable fluid handling system in any configuration that allows for the normal operation of the engaged piston.
Although the above discussion has focused on embodiment directed to the pump and cam, it should be understood that the current invention is also directed to a method of providing a constant fluid flow using a pump as described above. In such an embodiment, the invention would comprise the following general steps:

- providing at least one heart-shaped cam rotatably engaged to a power source through a drive shaft;
- placing at least two pistons into slidingly engagement with the outer contour of said at least one cam, the pistons being engaged to points along the contour of said at least one cam that are rotated 180° relative to each other;
- connecting the pistons to a fluid delivery system; and
- rotating the cam at a constant speed such that the pistons have a constant linear motion.

Finally, it should be understood that while preferred embodiments of the foregoing invention have been set forth for purposes of illustration, the foregoing description should not be deemed a limitation of the invention herein. Accordingly, various modifications, adaptations and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.

Claims

1. A constant flow positive displacement pump comprising:

a power source;

at least one cam rotatably engaged to said power source through a drive shaft, said at least one cam having an approximately heart-shaped outer contour; and

at least two pistons slidingly engaged with the outer contour of said at least one cam, said pistons being engaged to points along the contour of said at least one cam that are rotated 180° relative to each other.

2. The pump of claim 1, wherein a line of reflection can be defined through the center of said cam, and where each of the halves of the cam defined by said line of reflection is described by the equation r=kΘ.

3. The pump of claim 1, wherein a line of reflection can be defined through the center of said cam, and where each of the halves of the cam defined by said line of reflection is an approximate 180° arc of an Archimedean spiral.

4. The pump of claim 3, wherein the arcs of the cam are asymmetric.

5. The pump of claim 3, wherein one half of the cam is an extending portion that serves to extend the piston and one half of the cam is a retracting portion that serves to retract the piston, and wherein the extending portion spans a larger angle arc than the retracting portion such that both pistons deliver fluid at the same time during the transition period.

6. The pump of claim 3, wherein one half of the cam is an extending portion that serves to extend the piston and one half of the cam is a retracting portion that serves to retract the piston, and wherein the Archimedean spiral constant for the retracting portion is larger than the extending portion.

7. The pump of claim 5, wherein the extending and retracting portions are designed such that the sum of the velocities of the accelerating and decelerating cams is constant.

8. The pump of claim 1, further comprising at least two heart-shaped cams, wherein at least one of said at least two pistons is disposed on each of the at least two cams.

9. The pump of claim 8, wherein the pistons are disposed in parallel relative to said cams and wherein said at least two cams are rotated 180° relative to each other on said shaft.

10. The pump of claim 8, wherein the at least two cams are disposed adjacent and in parallel on said shaft, and wherein said pistons are rotated 180° relative to each other in relation to said shaft.

11. The pump of claim 1, wherein the pump is a positive displacement pump.

12. The pump of claim 1, wherein the pump is used for one of either concrete or fracture applications.

13. A heart shaped cam for use in a constant flow pump, said heart-shaped cam having a line of reflection defined through the center of said cam, where the outer contour of each of the halves of the cam defined by said line of reflection is an approximate 180° arc of an Archimedean spiral.

14. The cam of claim 13, wherein each of the halves of the cam defined by said line of reflection can be described by the equation r=kΘ.

15. The pump of claim 13, wherein the arcs of the cam are asymmetric.

16. The pump of claim 13, wherein one half of the cam is an extending portion that serves to extend a piston and one half of the cam is a retracting portion that serves to retract a piston, and wherein the extending portion spans a larger angle arc than the retracting portion.

17. The pump of claim 13, wherein one half of the cam is an extending portion that serves to extend a piston and one half of the cam is a retracting portion that serves to retract a piston, and wherein the Archimedean spiral constant for the retracting portion is larger than the extending portion.

18. The pump of claim 16, wherein the extending and retracting portions are designed such that the sum of the velocities of the accelerating and decelerating cams is constant.

19. A method of providing a constant fluid flow comprising:

providing at least one cam rotatably engaged to a power source through a drive shaft, said at least one cam having an approximately heart-shaped outer contour;

placing at least two pistons into slidingly engagement with the outer contour of said at least one cam, said pistons being engaged to points along the contour of said at least one cam that are rotated 180° relative to each other;

connecting said at least two pistons to a fluid delivery system; and

rotating said cam at a constant speed such that the pistons have a constant linear motion.

20. The method of claim 19, wherein a line of reflection can be defined through the center of said cam, and where each of the halves of the cam defined by said line of reflection is described by the equation r=kΘ.

21. The method of claim 19, wherein a line of reflection can be defined through the center of said cam, and where each of the halves of the cam defined by said line of reflection is an approximate 180° arc of an Archimedean spiral.

22. The method of claim 21, wherein one half of the cam is an extending portion that serves to extend the piston and one half of the cam is a retracting portion that serves to retract the piston, and wherein the extending portion spans a larger angle arc than the retracting portion such that both pistons deliver fluid at the same time during the transition period.

23. The method of claim 21, wherein one half of the cam is an extending portion that serves to extend the piston and one half of the cam is a retracting portion that serves to retract the piston, and wherein the Archimedean spiral constant for the retracting portion is larger than the extending portion.

24. The method of claim 22, wherein the extending and retracting portions are designed such that the sum of the velocities of the accelerating and decelerating cams is constant.

25. A method of performing an oilwell operation comprising:

providing a constant flow positive displacement pump according to claim 1 at the oilwell; and

operating the pump to inject a fluid at a constant flow into the oilwell.