The invention relates to a proportional pinch valve for controlling the pressure in a continuous flow system, especially for use in a cross-flow filtration system. The invention further relates to a method of controlling the pressure of a fluid in a continuous flow system using a proportional pinch valve.

BACKGROUND OF THE INVENTION

According to its basic operation principle, a pinch valve usually employs a member that directly acts on a length of elastomeric process tubing. Forcing the tubing together will create a seal that is equivalent to the tubing's permeability. Pinch valves are typically used to control medium pressure in applications where the medium needs to be completely isolated from any internal valve parts or entrapments. Pinch valves are thus preferably employed in sterile single use systems. Whereas standard proportional valves for controlling pressure are typically of a design that does not provide a simple disposable flow path, pinch valves enable the use of a clean and very low cost disposable flow path, namely the process tubing itself. The process tubing can be made from an elastomer material that is suitable for gamma sterilisation, for example.

However, pinch valves are generally used for simple on/off flow control only. In theory, small solenoid driven pinch valves for small tubing might be designed to optimise proportional control, but in practice pinch valves are typically designed as mere shut-off valves. It has to be further considered that larger tubing and/or higher pressures require high pinch forces, and solenoid driven pinch valves become accordingly large and costly. Therefore, larger pinch valves often use a motor driven leadscrew or ballscrew to move an anvil to directly pinch the tubing.

A significant problem in using pinch valves for proportional control is the fact that small changes in the tubing internal “pinched” size creates large changes in pressure or flow. This means that small movements of a pinching anvil cause large changes to the pressure or flow. During operation this results in coarse pressure or flow control and/or frequent control point changes. For good control, very small anvil movements in the order of 1 micrometre are required but still do not necessarily lead to stable control. Very small movements can be achieved by using a fine resolution leadscrew and/or other precision mechanisms. However, a fine resolution leadscrew will typically only move at a slow linear speed, and this can be a problem when the valve is required to open or close quickly.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a low cost pinch valve allowing for a stable proportional control of pressure over a large operating range.

The above problem is solved by a proportional pinch valve according to claim1. Advantageous and expedient embodiments of the invention are apparent from the dependent claims.

The invention provides a proportional pinch valve for controlling the pressure in a continuous flow system. The proportional pinch valve comprises an anvil for pinching a length of a tubing in the continuous flow system, and a drive mechanism including a displacement element for moving the anvil towards the tubing. According to the invention, the anvil is indirectly coupled to the displacement element via an elastic spring element. The elastic spring element provides a defined play (i.e. elasticity) for the anvil at least as long as the tubing is not fully pinched so that displacement of the anvil is force-controlled by the elastic spring element. This means that the anvil is not simply pressed into the tubing by the moving displacement element. According to the invention, pinching of the tubing is rather controlled by the force exerted to the anvil via the spring element interposed between the displacement element and the anvil. The invention makes use of the finding that changing from a displacement-controlled anvil, as used in common designs, to a force-controlled anvil, as realised in the present invention, provides significant advantages with respect to pressure control. In common designs, pressure control is very sensitive to the displacement caused by the drive mechanism as the anvil is directly coupled to the displacement element and directly acts on the tubing. This means that pinching of the tubing is directly proportional to the movement of the displacement element. However, these common designs do not take into account the influence of the back pressure caused by the medium flowing through the tubing and possible variations thereof during operation. If, for example, the back pressure increases, there is no possibility to increase the flow through the pinched tubing in order to offset the pressure increase. By contrast, in the design according to the invention, the anvil has a defined “play” (i.e. elasticity) when the tubing is pinched due to its coupling to the elastic spring element. Accordingly, if the back pressure increases at a given pinch force, the increased pressure of the fluid is able to force the pinched surfaces of the tubing apart, thus resulting in an increased flow. This provides greater stability of the pressure setpoint and reduces the control sensitivity. In this way, the proportional pinch valve according to the invention also advantageously provides the basic functionality of a pressure release/relief valve which will be discussed further below in connection with a preferred embodiment.

The intended application of the invention is to control pressure in ultrafiltration/diafiltration (UF/DF) systems, but it is not limited thereto. As already indicated, the basic benefit of the invention is stable pressure control over a large pressure range with a low cost disposable flow path.

According to a preferred embodiment of the proportional pinch valve, the elastic spring element is a coil spring. A coil spring can easily be integrated in the drive mechanism. Depending on the actual application and the desired force/displacement behaviour, a coil spring with an appropriate spring rate is usually easily available. However, it is also possible to use a specially configured gas cylinder or an appropriate elastomer member instead of a coil spring.

The displacement element used in the drive mechanism preferably is a leadscrew nut arranged on a leadscrew. A leadscrew can easily be turned by a stepper motor or by a handwheel, while the leadscrew nut translates this turning motion into the required linear motion. Leadscrews and leadscrew nuts are proven linkage components and can be chosen according to the given requirements.

In a combination of a leadscrew and a leadscrew nut used in the drive mechanism together with a coil spring used as the spring element, one end of the coil spring can be supported on the leadscrew nut and the other end of the coil spring can be supported on the anvil to provide the desired elastic coupling.

In an automated version of the proportional pinch valve, the drive mechanism includes an automated drive, preferably a stepper motor, for displacing the displacement element. A stepper motor has the advantage that the motor's position can be commanded to move and hold at one of a number of equal steps without any feedback sensor, as long as the motor is carefully sized to the application in respect to torque and speed.

For holding and fixing the tubing in the proportional pinch valve, a support member having one or more receptacles and a cover plate cooperating with the support member can be used. The cover plate preferably provides a resting surface against which the tubing is pressed when the anvil is moved towards the tubing.

In a conventional pinch valve, the pinched length is rather small, i.e. less than 5 mm, to minimize the force required to close the valve. However, it has been recognized that it is possible and advantageous to lengthen the distance over which the tubing is pinched, as compared to previous designs. Therefore, the length of a front surface of the anvil contacting the tubing is application specific but preferably in the range of 5 to 10 mm. For low pressure applications (less than 1 bar) a longer contact length may be desirable. In addition to increasing control performance, the lengthened pinched tubing also reduces the peak shear stress on liquid passing through the valve as the pressure drop through the valve occurs over a longer distance compared to conventional (pinch) valve designs.

In cases where a high differential pressure is needed across the proportional pinch valve, it is a problem that a significant proportion of the internal surfaces of the pinched tubing are in contact with each other with flow being possible only at either edge of the pinched tubing. Under these circumstances sensitivity to the pressure in the tube is significantly reduced. To overcome this problem, a (further) enhancement of the length of pinched tubing is achieved by providing a ridge projecting from a front face of the anvil facing the tubing. The ridge preferably extends perpendicular to a longitudinal direction of the tubing. The purpose of the ridge is to more effectively use the pressure in the tube to act on the anvil and hence on the spring. In particular, the ridge is used to create most of the pressure drop across the short length of the ridge and therefore allowing the internal tube surfaces elsewhere in the pinched section to not be in contact with each other. In this way, those tube surfaces are subject to the pressure within the tube and therefore can act on the anvil. The ridge can be either upstream, downstream, in the middle or any other position along the length of the pinched tubing, depending on whether it is important to control pressure upstream or downstream. A universal proportional pinch valve would have the ridge in the middle of the pinched tube.

According to an advantageous aspect of the invention, a seal is provided, preferably a rubber diaphragm, for sealing a free space around the pinched tubing from the drive mechanism. The seal prevents process fluids from entering and contaminating the drive mechanism in the interior of the valve in case the pinched tubing is damaged.

According to another advantageous aspect of the invention, a pin coupled to the displacement element cooperates with a slot decoupled from the displacement element in order to prevent the displacement element from unintentional rotation. The slot may be formed in a housing member of the proportional pinch valve in which the displacement element moves.

A position sensor may be provided for detecting at least a home position of the displacement element. For the detection of the home position, it is possible to use the above-mentioned pin as a position indicating element, giving the pin a double function.

Regarding the behaviour of the proportional pinch valve in an end position, in which the tubing is fully pinched by the anvil, the invention offers different concepts as will be explained below.

According to a first design principle, the displacement element is in contact with the anvil when the displacement element is displaced into the end position in which the tubing is fully pinched. Thus, a high force can be applied to the tubing, and it is ensured that the anvil remains in its position.

According to a second design principle, the displacement element is not in contact with the anvil when the displacement element is displaced into the end position in which the tubing is fully pinched by the anvil. This design allows the proportional pinch valve to act as a pressure release/relief valve if a maximum allowed system pressure is exceeded. The maximum pressure can be controlled during operation or by the design by setting the maximum displacement of the displacement element to define the end position and hence the maximum force that the spring element applies to the anvil.

According to a more sophisticated design, a switch mechanism is provided for switching the driving mechanism between a first mode, in which the anvil is directly coupled to the displacement element, and a second mode, in which the anvil is indirectly coupled to the displacement element via the spring element. Depending on the circumstances, the more appropriate control, i.e. force-controlled pinching or displacement-controlled pinching, can be chosen as desired.

The invention also provides a method of controlling the pressure of a fluid in a continuous flow system using a proportional pinch valve as described above. The spring of the proportional pinch valve allows self-correction of the pressure, especially in a case of a back-pressure or other circumstances undesirably deviating the pressure away from a setpoint.

In particular, according to an important aspect of the invention, the elastic spring element provides a defined play (elasticity) for the anvil at least as long as the tubing is not fully pinched so that displacement of the anvil is force-controlled by the elastic spring element.

Accordingly, in response to a back-pressure of the fluid flowing through the tubing at a given pinch force acting on the surrounding wall of the tubing, the pinched surfaces of the tubing can be forced apart against the bias of the elastic spring element, thus resulting in an increased flow. This provides greater stability of the pressure setpoint and reduces the control sensitivity to noise.

The method according to the invention is preferably used in, but not limited to, a cross-flow filtration system.

