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: