Patent Application
20180306179
The present invention is directed to a pump producing a constant outflow and avoiding pulses in the output flow.
Conventional positive displacement pumps use pistons or plungers to displace fluid. However, the reciprocating motion produces pulses or surges in the output flow. It can be appreciated that single cylinder pumps generally produce the greatest pulsation as the one cylinder alternates from suction to discharge. To overcome this problem, more cylinders may be added that utilize an overlapping output that is timed to greatly decrease the pulsations. However, even with multiple cylinders, such as Quintuplex pumps, there are still small pulses.
Attempts have been made to reduce the pulsations by modifying the stroke of the plungers to reduce the pulsation. Such a pump is shown for example in U.S. Pat. No. 5,145,339 to Lehrke, which is directed to a two cylinder pump using cams to produce a pulseless output. Although the pump design in the Lehrke patent does reduce the pulsation, Lehrke's use of cams with complex cam shapes and related mechanisms are relatively complex and expensive. The design also has a fixed stroke profile that can't be varied with pumping conditions that affect the output per stroke, such as the compressibility of fluid. A design utilizing cams does not provide flexibility to alter the stroke profile while the pump is running. For high pressure metering applications, the displacement is small compared to the system bulk modulus and a portion of the stroke is needed to build pressure in the pumping chamber before any fluid leaves the discharge valve. With such a design, the cam profile for such an application would only be effective at one pressure. Different cams with different profiles would be required to be effective at different pressures.
It can therefore be seen that a simple and inexpensive positive displacement pump is needed that substantially eliminates pulses in the output flow. Such a pump should be simple and operate with a minimum number of pumping chambers and plungers to minimize cost. Moreover, such a pump should be effective even with two pumping chambers. It should also be possible to change the stroke profile and to change the profile while the pump is running. Such a pump should also be able to adapt to different operating conditions including different pressures. The present invention addresses these problems as well as others associated with eliminating pulses from positive displacement pumps.
The present invention is directed to a pulseless positive displacement reciprocating pump, such as a piston type pump or a diaphragm pump. In place of a conventional cylinder and piston arrangement, the pump preferably has two or more guide bores with associated guide rods reciprocating back and forth within the bore. The guide rod threadably connects to a lead screw, such as a recirculating ball type lead screw. An associated variable speed motor, such as a stepper motor or a servo motor, actuates each lead screw in both directions. The lead screw includes threads that mate with complementary threads of the guide rod so that as the lead screw rotates, the guide rod is moved axially along the lead screw to extend and retract the plunger into and out of the pumping chamber. The stepper motors can be precisely rotated in miniscule discrete stepped movements and are able to precisely control rotation of the lead screw and therefore the axial position of the guide rod to closely control the position of the plunger and its movements into and out of the pumping chamber.
In operation, to start the pump, with the plunger at bottom dead center, the controller would actuate a first one of the stepper motors to rotate a respective leadscrew in a first direction, which causes the guide rod to travel axially. The controller would communicate to the first stepper motor to increase the rotation speed up to the maximum that produces the full output of the guide rod and guide bore. For most of the stroke, the stepper motor rotates at a constant speed. However, as the plunger nears top dead center, the stepper motor is slowed by the controller until the plunger stops at top dead center. The stepper motor would then start turning in the opposite direction and increasing to a maximum speed and then decreasing again near bottom dead center until coming to a stop at bottom dead center.
In a two pumping chamber pump, duration of the pressure stroke is slightly longer than the suction stroke so that the ramp up portion and ramp down portion of the two plunger strokes overlap. This overlap produces the continuous combined flow at the pump discharge. As the plunger proceeds forward from bottom dead center, there is a portion of the stroke that is compressing the fluid and expanding the pressure containing components. During this period of the stroke, no flow exits the pumping chamber while the other guide rod and plunger assembly is still producing full flow. The controller can determine the number of steps that the stepper motor must move before beginning to slow the other guide rod and associated plunger. It can be appreciated that operation of a diaphragm pump would be substantially the same as for a piston type pump, but acting through a diaphragm.
For a pump with three pumping chambers, the pump stroke cycle can be timed with the overlap similar to a shaft driven pump where the strokes have a phase difference 120 degrees apart or ⅓ of a pump cycle apart. Therefore, there is overlap timing on both the pressure and suction strokes of each plunger. The speeds during the overlaps are calculated in the same manner for both the pressure and suction strokes and the displacement of and duration are precisely controlled by the controller and the stepper motors.
These features of novelty and various other advantages that characterize the invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings that form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.
Referring now to the drawings, wherein like reference letters and numerals indicate corresponding structure throughout the several views:
Referring now to the drawings, wherein like reference letters and numerals indicate corresponding structure throughout the several views:
Referring now to the drawings and in particular to
The guide rod (108) threadably connects to a lead screw, such as a recirculating ball type lead screw (122). A variable speed motor, such as a servo motor or a stepper motor (120), depending on the size an application of the pump, actuates the lead screw (122) in both directions. As shown in
Referring now to
In operation, to start the pump (100), with the plunger (130) at bottom dead center, the controller (150) would drive a first stepper motor (120) to rotate the leadscrew (122) in a first direction, which causes the guide rod (108) to travel axially forward and therefore to the right towards the manifold as depicted in
In a two pumping chamber pump with two plungers, duration of the pressure stroke is slightly longer than the suction stroke so that the ramp up portion and ramp down portion of the two plunger strokes overlap. This overlap produces continuous combined flow at the pump discharge. As the first plunger (130) proceeds forward from bottom dead center, there is a portion of the stroke that is compressing the fluid and expanding the pressure containing components. During this period of the stroke, no flow exits the pumping chamber (110) of the first plunger (130) while the other plunger is still producing full flow. The controller (150) can determine the number of steps that the stepper motor (120) must move before beginning to slow the other guide rod/plunger. It can be appreciated that operation of a diaphragm pump (200) would be substantially the same as for a piston type pump (100).
The controller utilizes a formula of
Dv=P*V/K
where:
Example using values for a small metering pump operating at 3000 psi.
dP divided by the plunger diameter will give the stroke travel to build up to system pressure from 0.
The controller (150) calculates the number of steps that the stepper motor (120) moves to advance the plunger (130) from bottom dead center before the fluid starts leaving the pumping chamber (110) and the other plunger (130) can start slowing down. Stepper motors have a fixed number of steps per revolution. A typical motor uses 200 steps per revolution. However, drivers can increase this number to a much higher number of steps per revolution using microsteps for each full step. Stepper motors may commonly have as many as 3200 steps per revolution for certain applications. As each step is a fixed amount of rotation and the lead screw (122) moves the guide rod (108) a constant linear travel distance per revolution, each step will correspond to a fixed axial displacement and therefore a fixed volume of displaced fluid. Therefore, the time for each step determines the output flow rate of the corresponding plunger and pumping chamber. With these parameters being known, the constant output per step makes an algorithm for flow rate and combining flows from multiple pumping chambers and associate guide rods and plungers can be calculated as follows:
The flow rate for one plunger is governed by the formula:
Q=Vs/T
The microcontroller 150 controls the stepper motor speed by varying the duration T. The controller varies the duration T continuously during the ramp pressure or combined flow periods of the stroke.
When multiple plungers are producing flow the total flow Qt is calculated as follows:
T=T1+T2 for a two plunger pump
Where:
Therefore, the volume of fluid can be shown as:
Qt=Vs/(T1+T2)
It can therefore be appreciated that when only one guide rod/plunger is operating, T1=T, and T2=0. When a first plunger starts to slow as it reaches the end of the stroke, the second plunger starts moving at a speed determined by the step time calculation:
T2=T−T1
Therefore, these durations T are active only when the motors (120) are moving in the same direction, since the pump's check valves (112 and 114) combine flows when the plungers (130) move in the same direction. It can be appreciated that the controller (150) stores pump specific constants that may include a system stiffness factor. The controller (150) also has pump specific characteristics including displacement per step and the number of steps per stroke as well as acceleration rates for the rate of change of step durations. The controller (150) also includes a desired output flow rate that may be constant or may be programmable and an output pressure that may be constant or programmable.
Examples of typical components for an exemplary metering pump include: a Hetai Stepper Motor model #57BYGH603; a McMaster-Carr Ball Screw model #; and a McMaster-Carr model #5966K16 Ball Nut.
Referring now to
Zone 1 is the beginning of Plunger 1 pressure stroke. During Zone 1 the plunger of Plunger 1 is moving forward and building pressure while the fluids compress and the chamber expands. During this period, there is no flow exiting the chamber, so the Plunger 2 continues at the maximum output. As soon as Plunger 1 pressure reaches the output pressure at the end of Zone 1, Plunger 2 slows its flowrate rapidly to offset the flow starting from Plunger 1. In Zone 2 Plunger 2 continues to lower its flow to zero while Plunger 1 increases at rates that result in a constant combined flow. The plunger velocities at the start of Zone 2 can be adjusted to match the change in flow when check valve the pumping chamber of Plunger 1 opens. When Plunger 2 stops at top dead center Plunger 1 is producing full flow. Plunger 1 continues output at a constant rate for the duration of Zone 3. During Zone 3 Plunger 2 makes its suction stroke, traveling to bottom dead center and then starts moving forward to build pressure by the end of Zone 3.
Referring now to
The present invention has been described with stepper motors (120, 220), which deliver a fixed displacement per step and are very simple to control. However, stepper motors are relatively inefficient for certain applications. In small metering pumps this is not a big factor, but in larger pumps energy losses could make stepper motors impractical. In such applications a servo motor system could be used. A system using servo motors would include variable speed control of the motor and position encoders to communicate to the microcontroller how fast to run the motor in the various ramp zones.
It can be appreciated that a minimum of two pumping chambers and associated plungers are required to take advantage of the overlap and modified control to produce a steady output flow. This is also the simplest configuration and typically the least expensive. However, in order to have the pressure strokes overlap, the intake strokes of a two plunger pump would not overlap, which may result in a moment of zero flow. For most metering applications, this is negligible and not important. However, for some applications, this may be important and require a different approach. Applications that have a sensitive or viscous fluid may require a smooth inlet flow. For such applications, a three pumping chamber pump may be used in which three of the plungers such as shown in
As shown in
Referring now to
In operation, the controller would include various constants and parameters related to the particular pump being controlled and the desired output flow and operating flow and pressure. The controller would calculate:
Once the operating parameters have been calculated, the controller is able to operate the pump for a desired flow rate and pressure. Referring to
When plunger 1 reaches the end of Zone 2, the controller instructs the stepper motor to continue for N13 steps at Ts step duration to the end of Zone 3. Then, when plunger 2 reaches the end of Zone 2, plunger 2 is at top dead center. As the controller detects plunger 2 reaching top dead center, the controller instructs the stepper motor to reverse its direction to arrive at bottom dead center at a time to start its pressure stroke before plunger 1 finishes its pressure stroke. These strokes are repeated alternating plunger 1 and plunger 2. The resulting flow is substantially pulse free and achieves the desired flow rate and output pressure.
It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
| Number | Date | Country | |
|---|---|---|---|
| 62489244 | Apr 2017 | US |
