The present invention relates to a metering pump device for medical use comprising a pumping chamber, an annular pumping diaphragm the outer edge of which is secured to the said pumping chamber and the inner edge of which is secured to a central drive part that is more rigid than the said diaphragm, able to be displaced parallel to itself between an extreme return position and an extreme displacement position in its expulsion stroke and its intake stroke, respectively, these two positions lying one on each side of the plane containing the outer edge of the said annular diaphragm, the said pumping chamber comprising an inlet valve and an outlet valve which are respectively opened by a reduced pressure and by a raised pressure in the pumping chamber as a result of the movements of the said annular pumping diaphragm, the intake stroke of the said diaphragm resulting from the energy accumulated by the elastic deformation of the diaphragm during its expulsion stroke.
Pumps of this type are already known. Production of such a pump as a metering pump for medical use, particularly as a single-use perfusion pump, presents numerous problems that known diaphragm pumps are unable to overcome.
This pump needs to be accurate, and small while at the same time providing optimum flow rate, and needs to represent good value for money because it is not re-usable. The outlet valve of the pump has to provide safety against a free flow of liquid under a certain pressure, which is typically that of the column of liquid between a pouch of perfusion liquid and the pump, or the pressure resulting from accidental pressure applied to the pouch of liquid. Given this safety, the pumping diaphragm has to be able to withstand the working pressure, which is made up of a pressure to open the outlet valve that has to remain closed up to a pressure that is determined by safety standards, of a pressure drop in the downstream pipe and of a service pressure at the end of the downstream pipe.
The pumping diaphragm has to be able to take in the liquid by moving from its extreme displacement position to its extreme return position creating enough of a reduced pressure in the presence of the pressure drop in the upstream and downstream pipes of the pumping device and of the pressure threshold of the inlet valve.
This intake has to be achieved as quickly as possible in order to provide an optimum flow rate given the volume of the pumping chamber. However, in order to avoid shockwaves in the pipes, vaporization of the pumped liquid through a pressure drop greater than the vapour pressure of the liquid, and the effects of cavitation, the reduced pressure created by the return of the diaphragm must not be too great.
Admittedly, the pumping diaphragm could be connected to the actuator for a two-way drive. However, such a solution would make the single use more complicated and therefore more difficult to manufacture and would make it more difficult to fit into the drive device. This method of driving also allows great precision in the control of the position of the diaphragm, and therefore great precision in the flow rate.
Meeting all of these conditions, some of which oppose others, is therefore not straightforward.
It is an object of the present invention to provide a solution which is able, at least in part, to meet the aforementioned conditions.
To this end, the subject of the present invention is a metering pump device for medical use according to Claim 1.
The various specifics and advantages of the invention will become better apparent from reading the following description of two embodiments of the metering pump device that forms the subject of the invention which are given by way of examples and illustrated schematically in the attached drawings.
FIG. 1 is a schematic general arrangement of the metering pump device;
FIG. 2 is a diagram illustrating a typical operating pumping range using a flat pumping diaphragm;
FIG. 3 is a cross section through a pumping diaphragm according to the invention intended to be connected to a pumping chamber;
FIG. 4 is a pressure-displacement diagram for a pumping device using the diaphragm of FIG. 3;
FIG. 5 is a comparative pressure-displacement diagram of a pumping device using a flat diaphragm with the same dimensional ratios as the diaphragm of FIG. 3;
FIG. 6 is a diagram showing the pressure sensitivity of the diaphragm of FIG. 3;
FIG. 7 is a comparative pressure-sensitivity diagram for the flat diaphragm of FIG. 5;
FIG. 8 is a diagram of the diaphragm of the pumping device that forms the subject of the invention, showing the characteristic dimensions the dimensional ratios of which will be discussed further in the description.
The metering pump device that forms the subject of the invention is illustrated very schematically in FIG. 1, given that it is the elastically deformable pumping diaphragm 1, and the geometry and structure thereof, which constitute the innovative part of this invention.
Aside from the pumping diaphragm 1, this device comprises a pumping chamber 2 into which there open an upstream pipe 3 controlled by an inlet valve 4, a downstream pipe 5 itself controlled by an outlet valve 6. The pumping diaphragm 1 is intended to move between an extreme displacement position that reduces the volume of the pumping chamber 2, leading to a raised pressure able to open the outlet valve 6 and an extreme return position that induces a reduced pressure able to close the outlet valve 6 and to open the inlet valve 4.
It is a more particular object of the invention to determine how to produce a diaphragm that is able to meet a certain number of conditions.
In order to expel the liquid from the pumping chamber 2, a drive mechanism 7, here symbolically depicted by a pushrod, pushes against the pumping diaphragm 1 in the direction of the inside of the pumping chamber. During the intake phase of the pump, it is the elasticity of the diaphragm which produces the return stroke generating, on the one hand, the intake and, on the other hand, returning the drive mechanism 7 to its starting position. As a result, suitable sizing of the diaphragm 1 is of key importance in order:
To have sufficient intake (be capable of creating enough of a reduced pressure) at the time of filling to combat any reduced pressure (pressure drop in a pipe, height of water column, valve with pressure threshold, etc.) and achieve sufficiently rapid filling of the pumping chamber, and to do so over the entire operating range thereof.
However, in order to avoid shockwaves in the lines, effects of vaporization of the liquid contained in the pumping chamber 1 as a result of a pressure drop beyond the vapour pressure of this liquid, or alternatively the effects of cavitation, the intake must not create too great a pressure drop.
Not to be too sensitive to the pressure in the upstream and downstream pipes, so as to maintain the precision of the incremental volumes pumped and therefore the precision of the flow rate, irrespective of the pressures upstream and downstream of the pumping device.
We are now going to look at the sizing of the pumping diaphragm in order to obtain adequate filling.
In what follows of the description, the behaviour of an annular diaphragm of flat profile surrounding a central core the thickness of which is chosen such that it deforms as little as possible, ideally not at all bearing in mind the stresses to which it is exposed, will be compared.
The feature of foremost interest to us is the pressure reduction that the diaphragm is capable of supplying as a function of the displacement of its central core.
Dimensioning the pumping diaphragm 1 first of all goes through the step of defining an operating range. In order to allow the diaphragm to pump right from the beginning of its stroke giving rise to a raised pressure, it is necessary for the diaphragm to be subjected to a preload, as will be seen later on.
In the example adopted here, the preload of the diaphragm corresponds to a displacement by 0.4 mm from its rest position, the operating range extending from 0.4 mm to 1.2 mm. This operating zone is delimited by two vertical dotted lines in the diagram of FIG. 2. The pressure in the pumping chamber must not drop below the vaporization pressure of water, represented by the lower line. Finally, the operating range needs to lie at a pressure below −2×104 Pa, in order to counter the pressure of the liquid on the upstream side of the inlet valve 4 (1×104 Pa), the height of the water column to be taken in (5×103 Pa) and a minimum of 5×103 Pa in order to return the drive member 7 to its starting position. The working zone of the pumping diaphragm 1 is therefore defined by the rectangle bounded by the two vertical dotted lines and the two horizontal lines. Nonetheless, in order to take tolerances into consideration, a margin of 0.1 mm on the displacement of the pumping diaphragm is provided, as illustrated by the two continuous vertical lines. The possible operating zone then ranges between 0.3 and 1.3 mm, as illustrated in FIG. 2.
It is evident from that figure that the annular pumping diaphragm 1 has to have a rigidity such that the absolute pressure in the pumping chamber in the range of operation of the diaphragm situated between the extreme displacement position and the extreme return position lies within the following range:
- Patm=atmospheric pressure
- Pvap=vapour pressure of the pumped fluid
- ΔPvalve—in=pressure difference between the upstream and downstream side of the inlet valve for opening it
- ΔPΔh—in=pressure difference in the upstream pipe between its distal end and the upstream side of the inlet valve as a result of the weight of the column of fluid
- ΔPpd=pressure drop in the upstream pipe for the desired flow rate.
FIG. 2 again depicts the pressure-displacement curve for the aforementioned flat diaphragm. It may be noted that a situation is reached where the reduced pressure generated by the moving diaphragm x>0.7 mm is far too great, which could give rise to shockwaves (pressure waves in the upstream pipe 3), to boiling phenomena resulting from a drop in pressure and/or to cavitation. In other words, the gradient which is about −1×104 Pa/0.1 mm is too steep. Conversely, if attempts are made to reduce the gradient as far as possible, there is a risk of being at a pressure equal to −2×104 Pa at the prestress position of the diaphragm corresponding to a displacement of 0.3 mm and of not being able to complete the intake, or, put another way, of having an intake smaller than the pumping volume of the pump.
In order to address this problem, the challenge is to create a diaphragm which can work at the most constant pressure possible over the operating range. That would make it possible, firstly, to operate in the permissible operating zone (an essential condition) and secondly to soften the shockwaves in the upstream pipe 3 and avoid vaporization of the liquid or cavitation.
In order to achieve a diaphragm something like this a diaphragm geometry that had a frustoconical profile in the state of rest was studied, this therefore resulting in the rigid central core of the flat diaphragm moving in a parallel plane, the annular part of the diaphragm then being conical in the state of rest. The forces resulting from the displacement of the rigid central core parallel to its plane in such a frustoconical diaphragm can be broken down into tension-compression forces and to forces of bending of the flexible annular part.
A distinction is made between three types of behaviour as the central core gradually moves parallel to its plane:
- The diaphragm works chiefly in compression, bending being of secondary importance.
- Compression decreases with the displacement of the core as far as a threshold where the diaphragm buckles and compression forces are relaxed. Bending forces increase.
- The compression forces are completely relaxed and the diaphragm works in tension. The bending forces continue to increase with displacement.
Hence, a diaphragm of frustoconical shape has a pressure-displacement characteristic in the shape of a wave resulting from the buckling phenomenon.
A distinction can be drawn between two different types of behaviour according to the thickness of such a diaphragm:
- When the thickness is very small and the cone angle is pronounced, the diaphragm undergoes a great deal of buckling, and behaves like a bi-stable diaphragm, the pressure-displacement characteristic of which forms a very pronounced S shape. In this case, it is the tension-compression forces which dominate.
- If the thickness is great and the gradient small, the buckling phenomenon appears little if at all; this then is a stable diaphragm, the pressure-displacement characteristic of which does not form an S shape. It is the bending forces that dominate.
A combination of these two extreme solutions, like the one illustrated in FIG. 3, gives an interesting curve. FIG. 4 illustrates the pressure-displacement curve of an annular diaphragm of the kind in FIG. 3 that forms the subject of the invention and the geometry of which is essentially characterized by a convex annular profile domed towards the outside of the pumping chamber. In this embodiment, the outer edge of the diaphragm 1 is secured to an annular connecting element 1b which has an annular groove 1c intended to allow it to be attached to the wall of the pumping chamber 2.
The type of profile of pumping diaphragm 1 of FIG. 3 makes it possible to alter the distance between the plane of the central core 1a of this diaphragm 1 and the plane that passes through the outer edge of the annular diaphragm 1, where it is connected to the pumping chamber by the annular connecting element 1b. The diameter of the central core 1a can also be varied. Thus, by varying various geometric parameters of the pumping diaphragm 1, it is possible to modify the amplitude of the pressure-displacement curve of FIG. 4. The following effects are noted according to the characteristics of the diaphragm 1 of FIG. 3:
- The more acute the cone angle, the more the tension-compression forces dominate and the more the pressure-displacement characteristic of the diaphragm adopts a pronounced S shape.
- The thicker the annular diaphragm 1, the more the bending forces dominate:
- the less the pressure-displacement characteristic adopts a pronounced S shape
- the steeper the gradient of the pressure-displacement characteristic
- the narrower the operating range of the diaphragm
- The more domed the shape of the annular diaphragm 1, the more the bending forces dominate and the lesser the extent to which the pressure-displacement characteristic adopts a pronounced S shape.
One way of reducing pressure sensitivity of the conical annular pumping diaphragm would be to reduce the operating range and to increase the preload, for example by switching from an operating range of 0.3 to 1.3 mm to an operating range of 0.7 to 1.2 mm. In such a case, it would be possible to reduce the rigidity of the diaphragm, the gradient of which would then typically be 1×104 Pa/0.2 mm and which would work over a narrower range, like that of FIG. 5.
Nevertheless, a diaphragm such as this would have the following disadvantages:
- The preload of the diaphragm is increased, which means that the forces within the diaphragm are greater.
- The flow rate of the pumping device is reduced in the same ratio as the operating range.
- A device such as this is far more sensitive to pressure because the thickness of the diaphragm has been reduced in order to reduce the gradient of the pressure-displacement curve which means that the pressure sensitivity of such a diaphragm is greatly increased, this significantly reducing the precision of the flow rate.
In order to illustrate this sensitivity to pressure, FIGS. 6 and 7 indicate the pressure sensitivity of the diaphragm of FIG. 3, as compared with a flat annular diaphragm with the same ratios of inside/outside diameters:
In each case, the diagrams illustrate the central core 1a of the diaphragm in two positions parallel to its plane with respect to its outer edge fixed to the pumping chamber.
The diaphragm is illustrated in a first extreme position under preload of the central core 1a, that corresponds to the extreme return position of the diaphragm, once under zero pressure and once under a pressure of −3×104 Pa.
The diaphragm is illustrated in its extreme displacement position corresponding to the end of the stroke used to expel liquid from the pumping chamber, once at zero pressure and once at 1.2×105 Pa.
It may be seen in FIG. 6 that the diaphragm according to the present invention is not very sensitive to pressure. Specifically, very little difference can be observed between the curve in continuous line and the curve in dotted line in which it is subjected to a reduced pressure of −3×104 Pa or even between the curves in chain line and the curve in dashes and crosses in which it is subjected to a pressure of 1.2×105 Pa. By contrast, in FIG. 7, a large effect that pressure has on the shape of the diaphragm can be seen under the same measurement conditions.
The various preferred dimensional parameters for the diaphragm in order to obtain the desired effects as far as the pressure-displacement characteristic is concerned, and as far as the reduction in pressure sensitivity is concerned are indicated in FIG. 8 which is a cross section through half of the diaphragm 1 from its centre to its periphery.
The dimensional parameters of this diaphragm that contribute to obtaining the aforementioned characteristics are as follows, and need to fall within the following ranges:
for an elastic material the elastic modulus of which ranges between 0.1 MPa≦E≦100 MPa. It is also the parameters b and e that can influence the pressure-displacement curve.
Increasing ΔR/Rext leads to a reduction in the compression/relaxation of compression, a reduction in tension. This increase in ΔR/Rext also has the effect of reducing the pressure amplitude of the pressure-displacement curve (see FIG. 4), making it flatter.
Increasing H/Rext has the effect of increasing the compression, relaxation of compression, of increasing the tension, of increasing the possible stroke of the diaphragm and of increasing the pressure amplitude of the pressure-displacement curve.
Increasing e/ΔR increases the bending effects and the rigidity and reduces the amplitude of the pressure-displacement curve (FIG. 4), flattening it.
The table below gives, by way of example, the dimensional and frequency/flow rate parameters of three pumping devices that form subjects of the present invention.
Advantageously, the diaphragm is made of silicone. It could also be made of polyurethane or of EPDM. The pumping chamber is advantageously made of polycarbonate and the diaphragm of FIG. 3 is welded.