HK40003786A - Volumetric pump - Google Patents

Description

Displacement pump

Technical Field

The present invention relates to a displacement pump, in particular a displacement pump with improved characteristics in terms of flow rate. More particularly, the invention relates to a volumetric pump characterized by being able to provide an almost constant flow rate or at least with fluctuations reduced to a minimum.

Background

A volumetric pump is a known machine for compressible fluids, which is mainly characterized by the provision of a volume of liquid with variable geometry, which is alternately placed in communication with the suction side during filling and with the delivery side during emptying. In the case of a liquid, the pump will only "move" fluid from an environment at a lower pressure to an environment at a higher pressure due to the low compressibility of the liquid. The average velocity of the fluid inside the pump is generally very low, so that, unlike constant flow machines, the action of the machine is of the static type and manifests itself as a variation in the fluid pressure; in fact, in these machines, the energy exchange is of the dynamic type, with combined variations of pressure, kinetic energy and fluid momentum.

Volumetric machines are classified according to the movement of the moving element as: reciprocating or plunger-type volumetric machines when reciprocating motion is provided to the moving element, plunger or piston, and rotary volumetric machines when rotary motion is provided to the moving element.

Figures 1 and 2 show schematically two single-acting reciprocating positive displacement pumps 1 and 2 provided with a horizontal axis and a disc plunger. Both pumps 1 and 2 have an inlet portion 20 (suction) and an outlet portion 30 (delivery). The pumps 1 and 2 also have a variable geometry volume 12 connected to the inlet portion 20 through a suction valve 11 and to the outlet portion 30 through a delivery valve 10. The volume of the chamber 12 is changed by the stroke of the disc plunger 13. In the pump 1 of fig. 1, the disc 13 is driven by a connecting rod-crank system 14, whereas in the pump 2 of fig. 2, the disc 13 is driven by a brushless servo motor 15, the rotary motion of which is converted into a linear motion by a planetary roller-screw system.

During the travel of plunger 13 from the bottom dead center of the cylinder (i.e. left side in the illustrations of fig. 1 and 2) to the top dead center of the cylinder (i.e. right side in the illustrations of fig. 1 and 2), delivery valve 10 remains closed due to the vacuum pressure created within the cylinder and fluid is drawn into chamber 12 through suction valve 11. When the plunger 13, which has reached the end of its stroke, reverses its movement (thus moving from right to left in the illustrations of fig. 1 and 2), this causes an overpressure in the chamber 12, so that the delivery valve 10 is automatically opened and the suction valve 11 is closed.

For the pump to operate correctly, it is necessary to ensure a seal between the disc of the plunger 13 and the cylinder wall, the seal being formed by means of an elastic seal on the plunger surface; thus, the degree of polishing of the inner surface of the cylinder must be precise to allow correct operation and duration of the seal. These mobile seals cannot be adjusted to take up slack due to wear, and replacement of the seal requires machine downtime and disassembly of some of its components. The disc piston pump is thus used for fluids without abrasive solid particles and for operating conditions at relatively low pressures (pmax <80-100 bar).

For higher operating pressures or for turbid liquids, a plunger pump (an example of which is provided by the pump 3 of fig. 3) is used, in which the plunger 16 is completely immersed in the liquid and the seal is external and formed on the fixed part, making it easy to adjust or replace the seal even when the machine is running.

Given the high pressures that a positive displacement pump can reach, it is often necessary to install a safety valve on the delivery side of the pump to prevent shut down failures or regulatory components of the machine or system components.

The opening and closing of the suction and delivery valves (which are typically automatic) can also be controlled by means of external servos; in this case, the pressure change in the conduit will depend on the valve opening law. It should be noted, however, that due to the low compressibility of the liquid, the delivery valve must be opened more or less immediately upon reversal of the plunger movement.

With reference to the pump 1 of fig. 1, it can be observed that, in the case of an infinitely long connecting rod (ideal crank mechanism), the instantaneous speed v of the plunger 13_iGiven by the following relationship:

v_i＝ω·r·sin(ω·t)＝ω·c/2·sin(α)

for a finite length connecting rod, the instantaneous velocity trend is no longer represented by a harmonic function, but rather by a linear combination of fundamental periodic functions of multiple harmonics (harmonic) of different frequencies.

With reference to fig. 4, and in particular fig. 4a), the speed of the plunger 13 has a tendency to pulsate and, according to the same law, it will also vary the flow rates into the suction tube and the delivery tube. Thus, the movement of the fluid is not constant, but is pulsed with a trend similar to that shown in fig. 4a and with time intervals in which the flow rate delivered by the pump is zero.

Although the pulsating motion may not cause any problems in some cases, in most industrial systems it produces undesirable effects, such as:

an excessive increase in the pressure drop with consequent overloading of the motor driving the pump and the risk of causing suction cavitation;

vibrations associated with the variations of flow rate and pressure, which cause resonance phenomena with the consequent possibility of mechanical damage to the equipment or apparatus;

-reducing the output of any equipment fed by the positive displacement pump when the output of the positive displacement pump is dependent on the flow rate (e.g. the heat exchange efficiency of the heat exchanger).

By increasing the number of actions, i.e. the number of working strokes per crank rotation, the amplitude of the vibrations can be reduced.

Fig. 4b) shows the trend of the speed of the plunger 13 as a function of crank angle for a double-acting pump: it can be seen that although the trend is still pulsatile, there is no longer a time interval in which the pump delivers a flow rate of zero and vmax is pi 2.

Likewise, fig. 4c) shows the speed trend of the plunger 13 for a triple action pump: it can be observed that the motion has reduced pulsation and the delivered flow rate is never zero and vmax v pi 3.

It is thus clear that a reduction of the pulsation effect and a relatively constant flow rate can only be achieved by structurally making the pump rather complicated.

The same considerations may apply to the pump 2 of fig. 2. In this case, the instantaneous velocity v of the plunger 13_iIs directly proportional to the speed of the motor according to the following relationship:

v_i＝ω_ip

wherein: omega_iThe angular speed of the motor, and p the thread pitch.

In this case, it can be observed that the speed of the motor must be reversed each time the reverse movement of the thrust cylinder is to be carried out. As a servo motor for driving the plunger 13, the use of the brushless motor 15 can deal with the problems caused by this, because the brushless motor can ensure very precise and flexible control of the piston acceleration and velocity. However, the sequence of steps, in any case deceleration-stop-stroke reversal-acceleration, causes a discontinuous flow rate, even to a lesser extent than the single-acting connecting-rod-crank system described previously.

Disclosure of Invention

It is therefore desirable to enable a displacement pump to overcome the problems associated with single-action displacement pumps of known type.

It is therefore an object of the present invention to provide a volumetric pump capable of providing a more or less constant fluid flow rate.

Another object of the invention is to provide a single-action volumetric pump capable of providing a more or less constant fluid flow rate.

It is a further object of the present invention to provide a volumetric pump which is easy to manufacture at competitive costs.

The foregoing and other objects and advantages of the invention, as will become apparent in the following description, are achieved by a positive displacement pump comprising an inlet portion and an outlet portion, and characterized by comprising: a variable geometry first volume connected to the inlet portion by a first suction valve and to the outlet portion by a first delivery valve, a variable geometry second volume connected to the inlet portion by a second suction valve and to the outlet portion by a second delivery valve, first means for varying the volume of the variable geometry first volume, second means for varying the volume of the variable geometry second volume, the first means for varying the volume of the variable geometry first volume and actuator means for varying the volume of the variable geometry second volume, the actuator means comprising a servo motor.

In this way, a volumetric pump is obtained that meets the objectives described above.

In particular, a system with two variable-geometry and independently controlled volumes enables a constant fluid flow rate to be ensured, as better described hereinafter, while the use of servomotors, in particular brushless servomotors, has considerable advantages from a performance point of view compared to conventional drives.

It is in fact known that in brushless motors, given a constant air gap flux, the driving torque is immediately available. Magnetic materials with high flux density (e.g. iron neodymium boron alloy or rare earth) allow to build lightweight compact motors with low rotor inertia moment with the same available torque at the shaft. There is also no drop in joule effect in the excitation circuit and the sliding contact, which would have to be supplied in a conventional synchronous machine. Moreover, the immediate availability of torque and the reduced rotor inertia allow high dynamic performances to be obtained. In particular, brushless motors have the following advantages compared to direct current motors, essentially due to the absence of brushes and rectifiers:

less maintenance; the reliability is stronger; the speed variation range is wider; the output is higher; heat removal is easier because the windings are arranged on the stator and the heat they generate encounters lower thermal resistance; the power size ratio is higher to assist heat removal; due to the presence of permanent magnets on the rotor, inertia is limited and dynamic performance is higher; the noise is less.

In general, it should be noted that the output of a brushless motor is on average higher than the output of an asynchronous or direct current motor of the same size: for high powers (tens or hundreds of kW), an output of 98% can be obtained.

Drawings

Further characteristics and advantages of the invention will become better apparent from the description of a preferred embodiment, illustrated by way of non-limiting example in the accompanying drawings, in which:

figure 1 is a schematic view of a first embodiment of a volumetric pump of known type;

figure 2 is a schematic view of a second embodiment of a volumetric pump of known type;

figure 3 is a schematic view of a third embodiment of a volumetric pump of known type;

FIG. 4 shows the speed trend of the pumps of FIG. 1 and the plungers of the double-acting and triple-acting pumps;

FIG. 5 is a schematic view of a possible embodiment of a volumetric pump according to the invention;

FIG. 6 shows thrust phase trends for two single-acting pumps controlled by brushless motors;

FIG. 7 illustrates the flow rate trends that can be obtained using the system of FIG. 6;

FIG. 8 illustrates a velocity trend of the plunger of the pump of FIG. 5;

figure 9 shows an axonometric (cross-sectional) view of a possible embodiment of the volumetric pump according to the invention;

fig. 10 shows a top view of an embodiment of the volumetric pump of fig. 9.

Detailed Description

With reference to fig. 5, a volumetric pump according to the invention, designated by the reference numeral 5, comprises, in the most basic embodiment, an inlet portion 53 and an outlet portion 54.

From the inlet portion 53, the pump body is then divided into two branches 61 and 62. In a first of these branches, for example branch 61, a first variable-geometry volume 511 is located, which is connected to the inlet portion 53 by a first suction valve 611 and to the outlet portion 54 by a first delivery valve 612.

In a second branch, for example branch 62, a second variable-geometry volume 521 is positioned, connected to the inlet portion 53 by a second suction valve 621 and to the outlet portion 54 by a second delivery valve 622.

The pump 5 according to the invention also comprises first means 51 for varying the volume of said first variable-geometry volume 511 and second means 52 for varying the volume of said second variable-geometry volume 521.

For example, the first means 51 for varying the volume of said first variable-geometry volume 511 comprise a first disc plunger, and the second means 52 for varying the volume of said second variable-geometry volume 521 comprise a second disc plunger. However, other plunger or piston devices or equivalent systems may also be used.

Actuator means 510 of said first means 51 for varying the volume of said first variable-geometry volume 511 and actuator means 520 of said second means 52 for varying the volume of said second variable-geometry volume 521 are also provided, said actuator means 510, 520 comprising servomotors, advantageously brushless servomotors.

Although it is possible to use a single servomotor to control the first means 51 and the second means 52 for varying the volume of the first and second variable-geometry volumes 511, 521 (for example by means of a suitable mechanism allowing their synchronous driving according to a chosen law of motion), it is preferable for said actuator means to comprise a first brushless servomotor 510 for operating said first means 51 for varying the volume of said first variable-geometry volume 511 and a second brushless servomotor 520 for operating said second means 52 for varying the volume of said second variable-geometry volume 521.

In fact, considering that the flow rate Q is the velocity v, according to the following law_iFunction of (c):

Q＝(π·r²)·v_i

in fact, by means of the two brushless servo motors 510 and 520, it is possible to control the speed of the first means 51 and the second means 52 for varying the volume so that the thrust phases as illustrated in fig. 6 overlap (where the portion of the graph with continuous lines refers to the thrust phase of one means for varying the volume and the portion of the graph with broken lines refers to the thrust phase of the other means for varying the volume).

In particular, considering only the delivery phase (fig. 6), a decrease in the speed of the plunger acting on the first variable-geometry volume 511 corresponds to an increase in the speed of the plunger acting on said second variable-geometry volume 522, whereas an increase in the speed of the plunger acting on the first variable-geometry volume 511 corresponds to a decrease in the speed of the plunger acting on said second variable-geometry volume 522.

In fact, in operating conditions, a decrease in the volume of said first variable-geometry volume 511 corresponds to an increase in the volume of said second variable-geometry volume 521, while an increase in the volume of said first variable-geometry volume 511 corresponds to a decrease in the volume of said second variable-geometry volume 521.

In other words, referring to fig. 6, 7 and 8, by coinciding the deceleration phase of one brushless motor with the beginning of the acceleration phase of the second brushless motor, the sum of the moved fluid flow rate and speed will remain constant (fig. 7).

It can be said that, in the operating condition (fig. 8), said first and second means 51 and 52 for varying the volume of said first and second volumes 511 and 521 of variable geometry operate substantially in opposite phases.

Since they are not connected to a connecting rod-crank mechanism, it is easy to set the suction phase at a speed and acceleration independent of the delivery phase, by arbitrarily decelerating or accelerating the plunger, to obtain the above synchronization. Moreover, since the system can be monitored instantaneously, any delay due to the pressure drop during the operation of the motor can be compensated, restoring the perfect synchronization of the two plungers required to obtain the linearity of the flow rate.

In fact, the volumetric pump 5 advantageously comprises an inlet portion 53 and an outlet portion 54. The volumetric pump 5 further comprises a first cylinder 511 connected to said inlet portion 53 through a first suction valve 611 and to said outlet portion 54 through a first delivery valve 612, and a second cylinder 521 connected to said inlet portion 53 through a second suction valve 621 and to said outlet portion 54 through a second delivery valve 622.

Furthermore, it is advantageous to provide: a first piston 51 functionally inserted within said first cylinder and adapted to move with a reciprocating motion to define a variable geometry first volume 511, and a second piston 52 functionally inserted within said second cylinder to define a variable geometry second volume 521, a first actuator unit 510 of said first piston 51 adapted to selectively move said first piston along said first cylinder to change said variable geometry first volume 511, and a second actuator unit 520 of said second piston 52 adapted to selectively move said second piston along said second cylinder to change said variable geometry second volume 521.

Advantageously, said first actuator unit 510 and second actuator unit 520 each comprise:

-a servo motor;

-a shaft having a rotation axis and connected to the servo motor, wherein the servo motor will impart a rotational motion to the shaft about the rotation axis;

-a slider slidingly mounted on said shaft, wherein said slider comprises means for converting said rotary motion into a reciprocating translational motion along said longitudinal axis, so that a first direction of rotation of said shaft corresponds to a first direction of translation of said slider along said shaft, and a second direction of rotation of said shaft, opposite to said first direction of rotation, corresponds to a second direction of translation of said slider along said shaft.

Advantageously, the slider is integrally connected to the first and second cylinders, so that a translation of the slider along the first and second translation directions causes a change of the first and second volumes of variable geometry.

Preferably, a control unit adapted to control the first and second actuator units is further provided, wherein the control unit is adapted to:

-selectively reversing the direction of rotation of the first and second actuator units to effect the reciprocating translational movements of the first and second pistons in opposite phases to each other;

-varying the reciprocating translational motion of each of the first and second pistons according to an acceleration/deceleration profile to obtain a substantially constant flow rate.

Fig. 9 and 10 schematically show a cross-section of a volumetric pump 5 with two volumetric pumping units 151 and 152 with a single-acting reciprocating drive, provided with a horizontal axis, and pistons in the form of cylinders. Both pumps have an inlet part (suction-represented by arrows 81 and 82) and an outlet part (delivery-represented by arrows 83 and 84), wherein the flow of fluid is appropriately directed by specific valves.

The brushless servo motor 510 provides a rotary motion to the corresponding screw 71 (the screw operated by the servo motor 520 and the corresponding screw thread not visible) and is converted into a linear motion by a planetary roller system belonging to the screw thread 73 (the screw connected to the piston 75 operated by the servo motor 520 and the corresponding screw thread not visible). The screw thread 73 translates and produces a stroke of the piston (not visible due to insertion into the corresponding cylinder 74) to which it is connected. Rotation of the screw 71, depending on its direction, causes the piston to translate in one direction or the other.

The movement of the piston obtained as described above generates a reciprocating rectilinear motion and therefore a pumping action. A first important feature is the achievement of a reciprocating rectilinear motion generated by a brushless motor (which is particularly suitable for generating said motion), operating only by the reversal of the motion.

A second great advantage lies in the fact that the brushless motor is able to vary its rotation speed proportionally to the precise instructions provided by the electronic driver.

The precise and rapid change translates into a similar behavior of the pump flow rate; it is thus possible to manage two pumps to obtain a sum of two flow rates with constant values by varying the number of rotations and the direction of movement of the two single units.

It should be noted that a constant flow rate can be created with only two pumping units, which is not possible with pumps currently available on the market having more than one pumping unit.

As is known, from a structural point of view, in a connecting rod-crank mechanism, the stroke C and the orifice D are correlated with each other by a characteristic parameter of each pump (which is the C/D ratio). The stroke/orifice ratio is typically between 1.2 for short stroke pumps and 2 for long stroke pumps. In a system with planetary rollers that can be used in the pump of the invention, it is also possible to use a C/D ratio of more than 2, and this means that the duration of the delivery phase is increased and thus a higher output can be obtained with the pump according to the invention.

Another parameter that will be considered to reduce the pressure drop in the tube and valve is the average velocity "Vm" of the plunger. In fact, pumps are classified, according to speed, as:

-a slow pump: vm is 0.3-0.8[ m/s ]

-a conventional pump: vm is 0.8-1.2[ m/s ]

-a rapid pump: vm is 1.2-2.4 [ m/s ]

Brushless motors are capable of providing angular acceleration, ideally allowing the desired speed Vm to be reached almost instantaneously. In order to correctly select the pump dimensions, it must still be taken into account that these accelerations will generate high pressure drops (quadratic ratio) in the system and high stresses on the mechanical components.

In practice it has been seen how the volumetric pump according to the invention allows to achieve the set aims. With the volumetric pump according to the invention, it is in fact possible to have a substantially constant fluid flow rate; moreover, the use of brushless servo motors allows continuous and precise control of the plunger movement, ensuring in any case a constant flow rate.

Other features, modifications, or improvements will be possible and apparent to those skilled in the art based on the description provided. Accordingly, such features, modifications and improvements are intended to be part of this disclosure. In practice, the materials used, the dimensions and the contingent shapes may be any according to requirements and to the state of the art.

Claims (9)

1. A volumetric pump (5) comprising an inlet portion (53) and an outlet portion (54), characterized in that it comprises: a variable-geometry first volume (511) connected to the inlet portion (53) by a first suction valve (611) and to the outlet portion (54) by a first delivery valve (612), a variable-geometry second volume (521) connected to the inlet portion (53) by a second suction valve (612) and to the outlet portion (54) by a second delivery valve (622), first means (51) for varying the volume of the variable-geometry first volume (511), second means (52) for varying the volume of the variable-geometry second volume (512), actuator means (510) of the first means (51) for varying the volume of the variable-geometry first volume (511) and second means (52) for varying the volume of the variable-geometry second volume (512), 520) Said actuator means (510, 520) comprises a servo motor.

2. A volumetric pump (5) according to claim 1, characterized in that the actuator means (510, 520) comprise a brushless servo motor.

3. A volumetric pump (5) according to claim 1 or 2, characterized in that the actuator means comprise a first brushless servo motor (510) for operating the first means (51) for varying the volume of the first geometrically variable volume (511) and a second brushless servo motor (520) for operating the second means (52) for varying the volume of the second geometrically variable volume (512).

4. A volumetric pump (5) according to one or more of the preceding claims, characterized in that said first means (51) for varying the volume of said first variable-geometry volume (511) comprise a first disc plunger and said second means (52) for varying the volume of said second variable-geometry volume (521) comprise a second disc plunger.

5. A volumetric pump (5) according to one or more of the preceding claims, characterized in that in an operating condition a decrease in the volume of said first variable-geometry volume (511) corresponds to an increase in the volume of said second variable-geometry volume (521), while an increase in the volume of said first variable-geometry volume (511) corresponds to a decrease in the volume of said second variable-geometry volume (521).

6. A volumetric pump (5) according to one or more of the preceding claims, characterized in that in an operating condition said first means (51) for varying the volume of said first geometrically variable volume (511) and said second means (52) for varying the volume of said second geometrically variable volume (521) operate in opposite phases.

7. A volumetric pump (5) according to one or more of the preceding claims, characterized in that, in operating conditions, the fluid flow rate into/out of the inlet portion (53) respectively the outlet portion (54) is substantially constant.

8. A volumetric pump (5) according to one or more of the preceding claims, characterized in that said first (510) and second (520) actuator units each comprise:

-a servo motor;

-a shaft having a rotation axis and connected to the servo motor, wherein the servo motor transmits a rotational movement about the rotation axis to the shaft;

-a slider slidably mounted on said shaft, wherein said slider comprises means for converting said rotary motion into a reciprocating translational motion along said longitudinal axis, such that a first direction of rotation of said shaft corresponds to a first direction of translation of said slider along said shaft, and a second direction of rotation of said shaft, opposite to said first direction of rotation, corresponds to a second direction of translation of said slider along said shaft.

9. A volumetric pump (5) according to claim 8, characterized in that it comprises a control unit capable of controlling the first (510) and second (520) actuator units, the control unit being capable of:

-selectively reversing the direction of rotation of the first and second actuator units to effect the reciprocating translational movements of the first and second pistons in opposite phases to each other;

-varying the reciprocating translational motion of each of the first and second pistons according to an acceleration/deceleration profile to obtain a substantially constant flow rate.