DESCRIPTION

The present invention relates to a volumetric pump, in particular to a volumetric pump with improved characteristics in terms of flow rate. More in particular, the present invention relates to a volumetric pump that is characterized by the capacity to provide a practically constant flow rate, or at least with fluctuations reduced to a minimum.

Volumetric pumps are well known machines for compressible fluids whose main characteristic is that of providing the liquid with a volume with variable geometry that is alternatively placed in communication with the suction side during filling and with the delivery side during emptying. In the case of liquids, due to their low compressibility, the pump will simply "displace" the fluid from an environment at lower pressure to an environment at higher pressure. The average speed of the fluid inside the pump is generally very low so that the action of the machine is of static type and manifests itself as variation in the pressure of the fluid, unlike constant flow machines; in fact, in these machines the energy exchange is of dynamic type with combined variations of pressure, of kinetic energy and of momentum of the fluid.

According to the motion of the moving element, volumetric machines are classified as: reciprocating or plunger volumetric machines, when the moving element, plunger or piston, is provided with reciprocating motion, and rotary volumetric machines when the moving element is provided with rotary motion.

The attached Figs. 1 and 2 schematically represent two single-acting reciprocating volumetric pumps 1 and 2 arranged with horizontal axis and with disc plunger. Both the pumps 1 and 2 have an inlet section 20 (suction) and an outlet section 30 (delivery). The pumps 1 and 2 also have a volume 12 with variable geometry that is connected through a suction valve 11 to the inlet section 20 and through a delivery valve 10 to the outlet section 30. The volume of the chamber 12 is varied 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, while in the pump 2 of Fig. 2 the disc 13 is driven by a brushless servo motor 15 whose rotary motion is converted into linear motion through a planetary roller screw system.

During the travel of the plunger 13 from the bottom dead center of the cylinder (i.e., on the left in the representation of Figs. 1 and 2) to the top dead center of the cylinder (i.e., on the right in the representation of Figs. 1 and 2) the delivery valve 10 is held closed as a result of the vacuum pressure created inside the cylinder and the fluid is drawn inside the chamber 12 through the suction valve 11. When the plunger 13 - having reach its end of stroke - reverses its motion (therefore moves from right to left in the representation of Figs. 1 and 2), this causes an overpressure in the chamber 12 that causes automatic opening of the delivery valve 10 and closing of the suction valve 11.

In order for the pump to operate correctly, seal must be guaranteed between the disc of the plunger 13 and the walls of the cylinder, produced by means of elastic seals positioned on the surface of the plunger; the degree of finish of the inner surface of the cylinder must therefore be precise, so as to allow correct operation and duration of the seal. These moving seals cannot be adjusted to take up the slack caused by wear and their replacement requires machine downtime and disassembly of some of its parts. The disc plunger pump is therefore used with liquids without abrasive solid particles and for operating conditions that are relatively light (pmax<80-100 bar).

For higher operating pressures or for turbid liquids, plunger pumps (an example of which is provided by the pump 3 of Fig. 3) are used, wherein the plunger 16 is totally submerged in the liquid and the seals are external and produced on the fixed part, making them easy to adjust or replace even with the machine running.

Given the high pressures that can be reached with volumetric pumps, it is generally necessary to install safety valves on the delivery side of the pump, to protect the machine or components of the system from malfunction of shut-off or regulating members.

Opening and closing of the suction and delivery valves - generally automatic - can also be controlled by means of external servo-mechanisms; in this case, variation of the pressure in the pipes will depend on the valve opening law. Nonetheless, it must be noted that due to the low compressibility of liquids, opening of the delivery valve must take place more or less instantly upon reversal of the plunger motion.

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 ν _ζ · of the plunger 13 is given by the relation:

Vj = ω · r ^■ sin((o · t) = ω · c/2 ^■ sin(a)

where "r" indicates the crank radius, "cu" and "a" are the angular speed and the crank angle, and "c" is the plunger stroke. For finite length connecting rod, the instantaneous speed trend will no longer be represented by a harmonic function, but by a generically periodic function linear combination of several harmonics of different frequencies.

With reference to Fig. 4, in particular Fig. 4a), the speed of the plunger 13 has a pulsating trend and with the same law it will also vary the flow rate flowing into the suction and delivery pipes. Therefore, the motion of the fluid is not constant but pulsating with a trend similar to the one represented in Fig. 4a) and with intervals of time in which the flow rate delivered by the pump is zero.

While in some cases a pulsating motion may not cause any problems, in the majority of industrial systems it produces undesirable effects, such as:

- excessive increase of pressure drops with consequent overloading of the motor that drives the pump and with the danger of triggering suction cavitation;

- vibrations associated with variations of flow rate and of pressure that can give rise to resonance phenomena with consequent possible mechanical damage to the equipment or instruments;

- decrease of the output of any equipment supplied by the volumetric pump when the output thereof depends on the flow speed (e.g., heat exchange efficacy for a heat exchanger).

The amplitude of the oscillations can be reduced by increasing the number of effects, i.e., the number of working strokes per crank revolution.

Fig. 4b) shows the speed trend of the plunger 13 as a function of the crank angle for a double acting pump: it can be seen that, although the trend is still pulsating, there are no longer intervals of time in which the flow rate delivered by the pump is zero and that vmax is v = π 2.

In the same way, Fig. 4c) shows the speed trend of the plunger 13 for a triple-acting pump: it can be observed that the motion has reduced pulsations, that the flow rate delivered is never zero and that vmax v = π 3.

It is therefore clear that a decrease of the pulsed effect and a relatively constant flow rate can only be obtained by complicating the pump considerably from the point of view of construction.

The same considerations can be applied for the pump 2 of Fig. 2. In this case, the instantaneous speed ν _ζ · of the plunger 13 is linked in a directly proportional manner to the speed of the motor, according to the relation:

Vi = (Oi p

where: = angular speed of the motor, p = screw thread pitch.

In this case, we can observe that it is necessary to reverse the speed of the motor each time a reversal of the movement of the thrust cylinder is to be carried out. The use of a brushless motor 15, as servo motor for driving the plunger 13, enables management of the problems that this causes, as it is capable of guaranteeing very precise and flexible control of piston accelerations and speed. However, the sequence of steps: deceleration - stop - stroke reverse - acceleration in any case causes a discontinuous flow rate, even if to a lesser extent than in the single-acting connecting rod-crank systems previously described.

Therefore, it would be desirable to have a volumetric pump capable of overcoming the problems related to the single-acting volumetric pumps of known type.

An object of the present invention is therefore to provide a volumetric pump able to provide a more or less constant flow rate of the fluid.

A further object of the present invention is to provide a single-acting volumetric pump that is able to provide a more or less constant flow rate of the fluid.

Yet another object of the present invention is to provide a volumetric pump that is simple to manufacture at competitive costs.

The aforesaid and other objects and advantages of the invention, which shall be apparent in the description below, are achieved by means of a volumetric pump comprising an inlet section and an outlet section, and which is characterized by comprising a first volume with variable geometry connected through a first suction valve to said inlet section and through a first delivery valve to said outlet section, a second volume with variable geometry connected through a second suction valve to said inlet section and through a second delivery valve to said outlet section, first means for varying the volume of said first volume with variable geometry, second means for varying the volume of said second volume with variable geometry, actuator means of said first means for varying the volume of said first volume with variable geometry and of said second means for varying the volume of said second volume with variable geometry, said actuator means comprising a servo motor.

In this way, a volumetric pump that satisfies the objects set forth above is obtained.

In particular, the system with two independently controlled volumes with variable geometry makes it possible to guarantee a constant flow rate of the fluid as better described below, while the use of the servo motor, in particular a brushless servo motor, has considerable advantages from the point of view of performances compared to conventional drives.

It is in fact known that in brushless motors, given the constant air gap flux, driving torque is immediately available. Magnetic materials with high flux density, such as iron-neodymium- boron alloys or rare earths, allow the construction, with the same torque available at the shaft, of light, compact motors with low rotor inertia moment. There is also the absence of Joule effect drops in the excitation circuit and sliding contacts, which in conventional synchronous machines are necessary to supply the excitation circuit. Moreover, the immediate availability of torque and reduced rotor inertia allow high dynamic performances to be achieved. In particular, brushless motors have the following advantages compared to direct current motors, essentially due to the absence of brushes and commutator:

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

In general, it can be stated that the outputs of brushless motors are on average higher than that of asynchronous or DC motors of the same size: for high powers (tens and hundreds of kW) outputs of 98% can be reached.

Further characteristics and advantages of the present invention will be more apparent from the description of the preferred embodiment, illustrated by way of non-limiting example in the accompanying figures, wherein:

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

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

Fig. 3 is a schematic view of a third embodiment of a volumetric pump of known type; Fig. 4 represents the speed trend of the plunger of the pump of Fig. 1 and of double and triple acting pumps;

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

Fig. 6 represents the thrust phase trend of two single acting pumps controlled by brushless motors;

Fig. 7 represents the flow rate trend obtainable with the system of Fig. 6;

Fig. 8 represents the speed trend of the plungers of the pump of Fig. 5;

Fig. 9 represents an axonometric (sectional) view of a possible embodiment of a volumetric pump according to the present invention;

Fig. 10 represents a top view of the embodiment of a volumetric pump of Fig. 9.

With reference to the accompanying Fig. 5, a volumetric pump according to the present invention, designated with the reference number 5, comprises - in its most general embodiment - an inlet section 53 and an outlet section 54.

From the inlet section 53, the body of the pump then splits into two branches 61 and 62. In a first of these branches, for example the branch 61, there is positioned a first volume with variable geometry 511 connected through a first suction valve 611 to said inlet section 53 and through a first delivery valve 612 to said outlet section 54.

In the second branch, for example the branch 62, there is positioned a second volume with variable geometry 521 connected through a second suction valve 621 to said inlet section 53 and through a second delivery valve 622 to said outlet section 54.

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

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

There are also provided actuator means 510, 520 of said first means 51 for varying the volume of said first volume with variable geometry 511 and of said second means 52 for varying the volume of said second volume with variable geometry 521, said actuator means 510, 520 comprising a servo motor that is advantageously a brushless servo motor.

Although a single servo motor could be used to control the first 51 and second 52 means for varying the volumes with variable geometry 511 and 521 - for example through appropriate mechanisms that allow synchronized drive thereof according to the chosen law of motion - it is preferable for said actuator means to comprise a first brushless servo motor 510 for operation of said first means 51 for varying the volume of said first volume with variable geometry 511 and a second brushless servo motor 520 for driving said second means 52 for varying the volume of said second volume with variable geometry 521.

In fact, considering that the flow rate Q is a function of the speed ν _ζ · according to the law:

Q = (π · r ) ^■ Vi

it is in fact possible to control, through the two brushless servo motors 510 and 520, the speed of the first 51 and second 52 means for varying the volumes, overlapping the thrust phases as schematized in Fig. 6 (in which the part of the graph with the continuous line relates to the thrust phase of one of the means for varying the volume, while the part of the graph with the dashed line relates to the thrust phase of the other means for varying the volume).

In particular, considering only the delivery phases (Fig. 6), a reduction of the speed of the plunger acting on the first volume with variable geometry 511 corresponds to an increase of the speed of the plunger acting on said second volume with variable geometry 521, and an increase of the speed of the plunger acting on the first volume with variable geometry 511 corresponds to a reduction of the speed of the plunger acting on said second volume with variable geometry 521.

In practice, in operating conditions a reduction of the volume of said first volume with variable geometry 511 corresponds to an increase of the volume of said second volume with variable geometry 521, and an increase of the volume of said first volume with variable geometry 511 corresponds to a reduction of the volume of said second volume with variable geometry 521.

In other words, with reference to Figs. 6, 7 and 8, by making the deceleration phase of one of the brushless motors coincide with the start of the acceleration phase of the second, the sum of the speeds and the fluid flow rate displaced will remain constant (Fig. 7).

It can be said that, in operating conditions (Fig. 8), said first 51 and second 52 means for varying the volume of said first 511 and second 521 volume with variable geometry substantially operate in phase opposition.

As they are not connected to the connecting rod-crank mechanism, it is simple to set the suction phase with independent speed and accelerations to those of the delivery phase by decelerating or accelerating the plunger at will to obtain the synchronism described above. Moreover, as the system can be monitored instantaneously, it is possible to offset any delays due to pressure drops during motor operation, restoring the perfect synchronism of the two plungers required to obtain linearity of the flow rate.

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

Moreover, there are advantageously provided a first piston 51 functionally inserted in said first cylinder and adapted to move with reciprocating motion so as to define a first volume with variable geometry 511, and a second piston 52 functionally inserted in said second cylinder so as to define a second volume with variable geometry 521, a first actuator unit 510 of said first piston 51 adapted to selectively move said first piston along said first cylinder to modify said first volume with variable geometry 511 and a second actuator unit 520 of said second piston 52 adapted to selectively move said second piston along said second cylinder to modify said second volume with variable geometry 521.

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

- a servo motor; - a shaft, having a rotation axis connected to said servo motor, wherein said servo motor transmits to said shaft a rotary motion about said rotation axis;

- a slider mounted sliding on said shaft, wherein said slider comprises means for converting said rotary motion into a reciprocating translational motion according to 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 said first direction, corresponds to a second direction of translation of said slider along said shaft.

Advantageously, said slider is connected in one piece with said or first and second cylinder so that the translation of said slider according to said first and second direction of translation causes the variation of said first and second volume with variable geometry.

Preferably, there is also provided a control unit adapted to control said first and second actuator unit, wherein said control unit is adapted to:

- selectively reverse said direction of rotation of said first and second actuator unit to implement said reciprocating translational motion of said first and second piston in phase opposition to each other;

- vary according to an acceleration/deceleration curve said reciprocating translational motion of each of said first and second piston so as to obtain a substantially constant flow rate.

Figs. 9 and 10 schematically represent sectional views of a volumetric pump 5 with two pumping units of volumetric type 151 and 152, with single-acting reciprocating drive, arranged with horizontal axis with piston in the form of a cylinder. The two pumps have an inlet section (suction - represented by the arrows 81 and 82) and an outlet section (delivery - represented by the arrows 83 and 84), where the flow of liquid is appropriately guided by the specific valves.

A brushless servo motor 510 provides the 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 linear motion through a system of planetary rollers 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 generates the stroke of the piston (not visible as inserted in the corresponding cylinder 74) to which it is connected. Rotation of the screw 71 generates, according to the direction, a translation of the piston in one direction or the other.

Displacement of the piston, obtained as described, produces a reciprocating rectilinear motion and consequently the pumping action. The first important characteristic consists in obtaining the reciprocating rectilinear motion, operating only with reversal of motion, produced by the brushless motor that is particularly suitable to produce said movement.

The second great advantage consists in the fact that the brushless motor is capable of modifying its rotation speed proportionally, following precise instructions provided by the electronic drive.

Precise and prompt variation translates into an analogous behavior of the flow rate of the pump; it is therefore possible to manage two pumps so as to obtain the sum of the two flow rates with a constant value varying number of revolutions and direction of motion of the two single units.

It should be noted that it is possible to produce a constant flow rate with only two pumping units, a situation that cannot be obtained by the pumps with more than one pumping unit currently available on the market.

From the point of view of construction, as is known, in connecting rod-crank mechanisms the stroke C and bore D are linked to each other by a characteristic parameter of each pump, which is the C/D ratio. The stroke/bore ratio is generally between 1.2 for short stroke pumps and 2 for long stroke pumps. In the system with planetary rollers that can be used in the pumps of the present invention, it is possible also to use ratios greater than 2 and this means an increase of the duration of the delivery phase and consequently higher outputs obtainable with the pumps according to the present model.

A further parameter to be considered to reduce pressure drops in the pipes and in the valves is the average speed of the plunger "Vm". In fact, based on speed, pumps are classified as:

- slow pumps: Vm = 0.3 ÷ 0.8 [m/s]

- normal pumps: Vm = 0.8 ÷ 1.2 [m/s]

- fast pumps: Vm = 1.2 ÷ 2.4 [m/s].

Brushless motors are able to provide angular accelerations such as to allow, ideally, the desired speed Vm to be reached almost instantaneously. For correct sizing of the pump it must nonetheless be considered that these accelerations would produce high pressure drops in the system (quadratic proportionality) and high stresses on the mechanical members.

In practice, it has been seen how the volumetric pump according to the present invention allows the set objects to be achieved. With the volumetric pump according to the present invention it is in fact possible to have a substantially constant fluid flow rate; moreover, the use of a brushless servo motor allows continuous and precise control of plunger movement, guaranteeing constant flow rate in any condition.

On the basis of the description provided, other characteristics, modifications or improvements are possible and evident to a person skilled in the art. These characteristics, modifications and improvements should therefore be considered a part of the present utility model. In practice, the materials used, the dimensions and contingent shapes can be any according to requirements and to the state of the art.