VALVE UNIT

Abstract: A valve unit, in particular for use at the suction inlet of a pumping apparatus, comprises a ball valve having a ball (2) and a seat (1) for controlling the flow of liquid between an inlet (18) and an outlet (19). A spring (3) forces the ball (2) into the seat (1). An actuating pin (8) projects through the valve seat (1) and contacts the ball (2). The actuating pin (8) is designed such that liquid can pass alongside the pin through a passageway (22). The armature (14) of a solenoid (12) can be pushed against a membrane (10) which is in contact with the actuating pin (8) so that the ball (2) is lifted from the seat (1) and the valve is opened. Since the opening and the closing of the valve unit can thus be actively controlled, the switching operation of the valve unit is very well reproducable resulting in a more uniform flow of the liquid delivered by a pumping apparatus connected to the outlet (19) ofthe valve unit.

Description: The invention relates to a valve unit, in particular for a pumping apparatus, according to the preamble of claim 1. Such a valve unit is used, for example, as an inlet valve arranged at the suction inlet of a pumping apparatus, e.g., a piston pump having one or several pis­ tons. Such an inlet valve enables flow of liquid from a liquid supply into the pump during the intake stroke of the pump and it prevents that liquid delivered during the expulsion stroke is fed back to the supply. A field of application for such an inlet valve are pumps used in liquid chromatography for compressing and delivering liquid to the chromatographic column. A valve unit according to the preamble of claim 1 is known from US-A-3,810,716. In this publication, a pumping apparatus for a liquid chromatograph is disclosed which comprises a pump head with a bore and a piston for reciprocation within the bore. Check valves are provided at the suction inlet and at the outlet of the pump. The check valves comprise a ball and a corresponding seat and a surrounding housing which confines the movement of the ball. When liquid pushes against the ball from the side opposite the bearing face of the ball, the ball rises from the seat so that the valve is opened. When liquid pushes against the ball from the opposite side, the valve is closed. In the pumping apparatus, the check valves are arranged such they prevent backflow of the sucked in and of the delivered liquid , forcing the liquid being pumped to flow in the desired direction. Thus, during the suction stroke of the pump, the inlet check valve is open and the out­ let check valve is closed; during the delivery stroke of the pump it is just the other way round. The opening and closing behaviour of a check valve, e.g., the distance by which the ball is raised from the seat or the switching time of the valve, depends on the flow rate and pressure of the liquid through the valve, the viscosity of the liquid and other parameters. Thus, the operation of such a valve is not completely reproducible which can result in variations of the flow of the liquid delivered by the pump­ ing apparatus. Such variations are particularly disturbing in high performance liquid chromatography, because they would impair the ac­ curacy of the chromatographic measuring results. Relative to the above described prior art, it is an object of the in­ vention to provide a valve unit, in particular for a pumping appara­ tus, according to the preamble of claim 1 which has a higher reproducibility of its opening and closing operation ensuring a more uniform flow of the liquid delivered by the pumping apparatus. According to the invention, this object is solved by the characteriz­ ing features of claim 1. According to a preferred embodiment of the invention, the check valve at the suction inlet of the pumping apparatus is replaced by an active inlet valve which can be opened or closed via controllable actuating means and is therefore not dependent on the imponderabilities in the operation of a check valve . The valve unit according to a preferred embodiment of the invention comprises a ball valve with a valve ball and a corresponding valve seat and an actuating element for lifting the ball off the seat in order to open the valve. Thus, the opening and closing of the valve can be precisely controlled and the switching times of the valve are kept small, resulting in a more constant flow rate of the liquid delivered by the pumping apparatus. Typical switch­ ing times are in the range of less than 3 milliseconds. If the inven­ tion is used in a liquid chromatograph , the constant flow rate achieved ensures high reproducibility of the chromatographic measuring process. According to a further advantage of the invention which is particularly essential for liquid chromatography, the valve unit has a low dead volume and it can easily be flushed since it is substantially free of zones where residues of liquid or gas bubbles might accumu­ late. Due to the low dead volume and the easy flushing of the valve, the invention permits faster changes of solvent gradients and there­ fore a higher efficiency of the chromatographic process when low pres­ sure gradient operation is performed wherein several types of liquids are mixed before reaching the pumping apparatus. It is understood that the invention is not limited to a ball valve but that other types of actively controlled valves are possible, for example a conical plug and a corresponding seat into which the plug can be moved for closing the valve. It is furthermore understood that the valve unit according to the invention can be used not only at the inlet of a pumping apparatus, but also at other locations in a liquid transporting system where reproducibility and reliability of the valve operation is essential. The valve unit could for example be used as a flushing valve in a liquid chromatograph. The invention has the further advantages that the switching times of the valve are independent of the type of liquid flowing through the valve, so that the reproducibility of the valve does not suffer from a change of the type of liquid being pumped. Furthermore, no backflow of liquid is required for the closing of the valve, and the valve opera­ tion is independent of the spatial position of the valve, in contrast to some prior art valves wherein gravity provides the restoring force for the valve. Also, the motion of the valve ball in an embodiment of the invention is independent of the flow rate, solvent viscosity and other parameters of the liquid, thus also contributing to a higher reproducibility. Furthermore, the susceptibility to contamination of the valve is low and it can easily be flushed. If the valve ball is sticking to the seat, which may happen when salt solutions have passed through the valve and the valve has not been flushed by liquid for a long time so that salt crystals have deposited, the invention permits to open the valve again, whereas this would not be possible with check valves. By the controllable actuation of a valve unit of the inven­ tion, it is avoided that the two sealing partners, e.g. the ball and the seat, are sticking when they should be charged electrostatically, a phenomenon which may occur with certain solvents and with certain materials, for example sapphire and ruby. According to an embodiment of the invention, the actuation of the actuating element for pushing out the ball of the seat may occur by applying a resilient force, e.g., by the use of a spring. The magni­ tude of the resilient force, e.g., the spring constant, is selected such that the ball can only be pushed out of the seat when the pres­ sure in the liquid at the other side of the ball is below a certain value. Otherwise, if the valve would open immediately at the beginning of the suction stroke of the pump, the liquid in the pump would sud­ denly expand which may lead to the formation of gas bubbles, particu­ larly with low-boiling liquids. Subsequently, an embodiment of the invention is explained in more detail with reference to the drawings. Figure 1 schematically shows a pumping apparatus for delivering liquid at high pressure incorporating a valve unit according to the invention. Figure 2 is a cross section of a valve unit according to an embodiment of the invention. Figure 3 is a perspective view of the actuating pin of the valve unit according to Figure 2. Figure 1 schematically depicts a pumping apparatus for delivering liquid at high pressure of the type that is used in a liquid chromato­ graph for delivering solvent to the chromatographic column. The pump­ ing apparatus shown is a two-piston pump with two serially connected pump heads , each comprising a piston 32, 34 and having an inlet valve 31 according to the invention at the inlet of the first pump head and a check valve 33 between the first and the second pump head. The inlet valve 31 is an active valve which can be opened or closed under exter­ nal control by means of an actuating mechanism schematically indicated by block 31a. The check valve 33 is a passive valve which permits flow of liquid from the first to the second pump head and which inhibits flow of liquid in the reverse direction. The check valve 33 may com­ prise a spring for urging the valve ball into the valve seat. The sol­ vent to be delivered is contained in a reservoir 30 and is sucked in when the first piston 32 retracts provided the inlet valve 31 is opened. In the example shown, the pistons 32,34 operate with a phase difference of 180 degrees, that means the piston 34 is at its top dead centre when the piston 32 is at its bottom dead centre and vice versa. The stroke volume of the first piston is larger than the stroke volume of the second piston; preferably, the stroke volume of the second pis­ ton is half the stroke volume of the first piston. In that way, a sub­ stantially smooth flow of the liquid delivered by the pumping appara­ tus to the chromatographic system can be achieved. It is understood that the invention is not limited to a pumping ap­ paratus as shown in Figure 1. Instead of the serial connection of the pistons shown, a parallel connection of the pistons could also be used, or several pistons or only one piston. Furthermore, the inven­ tion might also be used with different types of pumps, for example with diaphragm pumps. In the following, the inlet valve 31 schematically depicted in Figure 1 is explained in detail with reference to Figure 2. An inlet port 18 can be connected to the reservoir 30 and an outlet port 19 can be con­ nected to the first pump head of the pumping apparatus as shown in Figure 1. Inside the valve, liquid flows from the inlet port 18 via a passageway 20 to a cavity 21 which is sealed off at one side by a membrane 10. From this cavity, liquid can flow through a passageway 22 and a ball valve (provided it is opened) comprising a ball 2 and a corresponding seat 1 to the outlet port 19. The seat 1 and the ball 2 of the ball valve are preferably made of a hard material, e.g., a sapphire or ceramic seat and a ruby ball. The ball 2 is pressed into the seat 1 by the spring 3 which is preloaded as shown in Figure 2. The seat 1 is jammed between an insert element 4 and a valve body 5. Sealing washers 6, 7 are provided between the in­ sert element 4 and the seat 1 as well as between the valve body 5 and the seat 1 , respectively. The insert element 4, together with the seat 1, the ball 2, the spring 3, and the sealing washer 6 are pressed into place in the valve body 5 such that the seat is in contact with the sealing washer 7. Instead of the press fit just described the in­ sert element 4 and the body 5 could also be welded or glued together. In Figure 2 , the ball valve is shown in its closed state. The valve can be opened by means of an actuating pin 8 which extends from the cavity 21 through the passageway 22 and the seat 1 to the ball 2. The actuating pin 8 comprises a head located in the cavity 21. Details of the pin head are explained in more detail below with reference to Fig­ ure 3. The portion of the pin extending into the passageway 22 has a substantially rectangular cross section with the longer side being somewhat smaller than the cross section of the passageway 22 such that the pin can be moved in the passageway 22. Due to the substantially rectangular cross section of the pin 8 , liquid can flow through the passageway in the gap between the pin and the wall of the passageway. At its lower end, the pin narrows and extends through the seat 1 of the ball valve and touches the ball 2. The diameter of the portion of the pin inside the seat is selected such that liquid can flow between the pin and the inner wall of the seat. The screw 17 and the snap ring 23 serve for adaptation of the valve to a pump head. The gasket 24 provides a frictional resistance connection between the screw 17 and the valve body 5. The cavity 21 wherein the head of the actuating pin is located is sealed off by a membrane 10. The membrane 10 is clamped between the valve body 5 and an adapter plate 9. The valve body 5 comprises a V-­ shaped groove and the adapter plate 9 comprises a corresponding coun­ terpart with the membrane being clamped between these two mating parts such that it is kept stretched. On the other side of the membrane 10 is arranged a rubber plate 11 which serves as a damping element in the actuation of the pin 8 . The pin 8 is actuated by a solenoid 12 which is connected to the adapter plate 9, e.g., by means of screws. The solenoid comprises a housing 13 , an armature 14 , a spring 15 and a stop 16. The electri­ cal coil necessary for the activation of the solenoid is arranged in the housing 13. When the solenoid is activated, the armature 14 is pulled against the resilient force of the spring 15 towards the stop 16. Consequently, no force is exerted by the armature on the actuating pin 8 so that the ball 2 of the ball valve is pressed into the seat 1 by the spring 3, thus keeping the valve closed. When the valve is to be opened, the solenoid is deactivated so that the spring 15 pushes the armature 14 against the rubber plate 11 leading to a displacement of the actuating pin 8 away from the armature. Due to this displace­ ment, the ball 2 is lifted off the seat 1 so that the valve is opened. The maximum distance by which the ball 2 can be lifted off the seat 1 corresponds to the distance between the lower edge of the head of the pin 8 and the wall of the cavity 21 when the armature exerts no force on the rubber plate 11. According to a practical example, this dis­ tance and therefore the travel of the ball 2 is 0,2 mm. The gap between the armature 14 and the rubber plate 11 is adjustable by cor­ responding adjustment of the stop 16. In a practical example of the invention, this gap is 0,1 mm. The solenoid used in the embodiment according to Figure 2 may be a conventional solenoid except that the spring constant of the spring 15 is specially selected as will be ex­ plained below in more detail. In the following, the actuating pin 8 is described in more detail with reference to Figure 3. The actuating pin substantially comprises three regions: a pin head 40, a centre piece 41 , and an end piece 42. When the pin is inserted in the inlet valve, the pin head 40 is arranged in the cavity 21 (Figure 2) such that its top face is in contact with the membrane 10, the centre piece 41 is arranged in the passageway 22, and the end piece 42 in the central bore of the valve seat 1 such that its bottom face is in contact with the ball 2 of the ball valve. The pin head 40 has a recess 43 which ensures that when the pin head has been pushed by the armature 14 against the valve body 5 , liquid can flow from passageway 20 and cavity 21 into passageway 22. The centre piece 41 of the pin has a substantially rectangular cross section such that liquid can flow through passageway 22 when the pin is inserted. The end piece 42 has a circular cross section and is dimensioned such that sufficient space is left for the passage of liquid through the bore. It is understood that various modifications of the design of the actu­ ating pin are possible. In the following, the operation of the valve according to the inven­ tion is explained in connection with a piston pump whereby the valve is arranged at the inlet of the pump (see Figure 1). The control of the valve and the pump are synchronized such that at the beginning of the suction stroke of the pump, the electric current through the coil of the solenoid is interrupted so that the magnetic field of the solenoid collapses and the spring 15 pushes the armature into the di­ rection of the ball valve. As a consequence thereof, the armature strikes the rubber plate 11. The rubber plate 11 dissipates the kinetic energy of the armature and thereby protects the membrane 10. At the beginning of the suction stroke, the pressure in the pump still is at an increased level resulting from the preceding compression/­ expulsion stroke. This increased pressure is also present at the ball valve since there is an hydraulic connection to the pump. The pressure created by a pumping system in high performance liquid chromatography typically ranges to 400 bar (4·10⁷ PA). According to an embodiment of the invention, the spring 15 has such a spring constant that, even if the solenoid has been deactivated so that the armature is free to move, the ball valve is not opened as long as the pressure in the pump is larger than approximately 10 bar (10⁶ Pa). In that case, the force of the spring 15 is not sufficient to lift via the actuating pin 8 the ball 2 from the seat 2 against the counterforce exerted on the ball from the other side by the pressurized liquid. By this dimensioning of the springs, a sudden expansion of the pressurized liquid is avoided. When the piston is further retracting in the course of its intake stroke, the pressure in the pump is further reduced. At about 10 bar, the force exerted on the ball 2 by the liquid is so small that the pin 8 can lift the ball out of its seat. Thereby, the armature 14, the rubber plate 11, the membrane 10 and the pin 8 are moved by a distance corresponding to the valve lift of the ball. Now, the pump can suck in liquid through the valve. When the piston 32 is at its bottom dead centre and starts changing its direction of movement, the solenoid is activated again by supply­ ing it with current so that the armature 14 is pulled back towards the stop 16. Since there is no counterforce, the spring 3 presses ball 2 back into its seat so that the pin 8 , the membrane 10, and the rubber plate 11 are pushed back to their original position. Now, the valve is closed, the pump can build up pressure and then deliver the pres­ surized liquid to the chromatographic system. When the piston is back in its top dead centre, the above-described cycle starts anew.

Claims: 1. A valve unit, in particular for a pumping apparatus, comprising: - an inlet (18) through which liquid can flow into the valve unit , - an outlet (19) through which liquid can flow out of the valve unit, for example to the suction port of a pumping apparatus (32,33,34) , - a valve (1,2) for interrupting flow of liquid between the inlet and the outlet,characterized in that actuating means (8,12,3) coupled to the valve (1,2) are provided for the controllable opening or closing of the valve (1,2). 2. Valve unit according to claim 1characterized in that - the valve comprises a valve ball (2) and a valve seat (1), - the actuating means comprise an actuating element (8) projecting through the valve seat (1) from the side opposite the bearing face for the valve ball (2) such that the ball can be lifted off the seat (1) when the actuating element is moved, and means for providing a resilient force to urge the valve ball (2) into the valve seat (1). 3. Valve unit according to claim 2,characterized in that - the actuating element comprises a pin (8) having a first end for contacting the ball (2) and a second end for directly or indi­ rectly contacting the armature (14) of a solenoid (12), and - the means for providing a resilient force to urge the ball (2) into the valve seat comprise a spring (3). 4. Valve unit according to claim 3,characterized in that a membrane (10) is provided between the actuating element (8) and the armature (14) for sealing off the region of the valve where liquid flows from the armature (14). 5. Valve unit according to claim 4 ,characterized by a kinetic energy absorber (11) arranged on the membrane (10) for damping the impact of the armature (14) on the membrane. 6. Valve unit according to any of the preceding claims ,characterized in that the actuating means (8,12,3) comprise a spring (15) exerting a resilient force on the actuating element (8) when the actuating means are not activated, with the spring constant being selected such that the valve (1,2) is not opened before the pressure on the side of the valve closer to the pumping apparatus has fallen below a predetermined value. 7. Use of a valve unit according to any of the preceding claims at the suction inlet of a pumping apparatus (31,32,33,34) for delivering liquid at high pressure.

Inventors: WEISSGERBER, HANS-GEORG
Applicants: HEWLETT-PACKARD GMBH
Application year: 1988
Main classification: F04B49
IPC codes: F16K31, F04B49, F16K15, G01N30