Abstract:
A valve device includes a body portion having a plurality of openings, a valve seat located within said body portion, an end stop, corresponding to an opening of said body portion, said end stop comprising a port portion through which fluid may flow, and a nozzle, in fluid communication with said port portion, said nozzle configured for sealing against said valve seat and thereby preventing flow through the valve device, the valve device being openable by relative motion of said nozzle with respect to said valve seat, and adjustment means configured to allow independent adjustment of the location of said nozzle with respect to said valve seat.

Description:
CROSS REFERENCE TO OTHER APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/127,050, filed Jul. 12, 2011, which is a national phase application of PCT patent application PCT/GB2009/002584, filed Oct. 30, 2009, which claims priority from UK Patent Application GB0819940.8, filed Oct. 30, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the field of valve devices, in particular valve devices for delivering precise doses of fluids (gas or liquid). The invention also relates to valve devices that operate proportionally and to valve devices that operate as pressure-reducing devices, for example in gas supply systems, and as a method of gas or liquid dosing. 
       BACKGROUND OF THE INVENTION 
       [0003]    Systems that require a supply of gas or liquid often require a well-controlled dose, pressure or flow rate to be delivered. For example, in the field of gas chromatography the provision of a well-controlled flow rate or pressure is usually necessary so that meaningful results may be obtained. 
         [0004]    Systems that require a supply of fluid often have this fluid stored at a high pressure, and often it is necessary to deliver the fluid at a reduced pressure. For example gas used in gas chromatography systems is generally supplied from a high-pressure cylinder, which may contain gas at a pressure of around 200 Bar. In contrast, the pressure required within a gas chromatograph will generally be far lower, and is generally required at a low flow rate. The accurate control of gas in such circumstances is difficult. 
         [0005]    Commonly the reduction in pressure from the high storage pressure to the lower delivery pressure is achieved through use of a pressure stepdown regulator. For example in gas chromatography a regulator may be used to lower the gas pressure from 200 Bar to a much lower pressure in the region of 5 Bar, and subsequently low pressure valves may be used to control the flow of gas. A valve that can deliver a precisely controlled low pressure outlet directly from a high pressure supply may remove the need for additional stepdown pressure regulators. 
         [0006]    A desirable feature of valves is high impermeability to the fluid when in the closed state. This may be important for conservation of the stored fluid or correct operation of the system. A common gas used in gas chromatography systems is Helium. Helium is a relatively difficult gas to supply in accurate doses since it will tend to leak through valves that do not have a suitably high degree of impermeability when closed. 
         [0007]    There is therefore a need for a valve that can both deal with high input pressures, low output pressures and low flow rates. Such valves may also require a high degree of impermeability in their closed state to prevent leakage of the fluid. High impermeability may be achieved through the application of high forces at sealing interfaces of appropriate design. Furthermore, it is beneficial in situations where high purity gases are required that the actuator is outside of the fluid flow path to minimize contamination of the fluid. Furthermore it may be beneficial for the actuator to be outside the fluid flow path for reasons of materials compatibility between the actuator and the fluid. The fluid flow path is sometimes referred to as the wetted area. 
         [0008]    Generally speaking, precise fluid metering from a high pressure source is beneficial in a wide range of systems in which it is desirable to maximise the resource by maximising the pressure of the fluid stored, whilst providing a precisely controlled flow of the gas or liquid. 
         [0009]    Further examples of such systems include Hydrogen fuel cells and spacecraft propulsion systems. Spacecraft propulsion systems, for example, may be used to manoeuvre micro satellites by releasing precisely controlled doses of gas for use in either electric propulsion systems or in cold gas thruster systems. 
         [0010]    EP-A-1204906 discloses a thermally controlled gas valve that is configured to permit the flow of gas when the valve is heated. Upon heating, the effect of thermal expansion causes nozzles within the valve to move away from a valve seat, and this opens the valve. During assembly of the valve, laser welds are used to provide gas-tight seals between the various component parts of the valve and to fix the relative positions of these components. 
         [0011]    During this welding process, the components are also heated and will consequently thermally expand. The thermal expansion allows a closing force to be built into the valve since, after the welding is complete and the components cool, the valve will contract. 
         [0012]    However, aside from varying the temperature of the valve, no adjustment of the valve&#39;s parameters, such as the closing force at a given temperature, is possible post manufacture. The closing force will affect the temperature at which the valve opens, and also the flow rate at a particular temperature. 
         [0013]    As a result of manufacturing tolerances, including uncertainties in the amount of heat applied during welding and therefore the closing force of the cooled valve, there will be some degree of variability in the above parameters of the finished valves. Thus, the provision of some adjustment in these parameters is desirable, to enable a reduction in the variability of performance between valves. Further if, due to manufacturing variability, the closing force for a particular valve is especially high, then the valve can be prone to cold seizure, whereby the valve seat and nozzle become stuck together, thus preventing the valve from opening. Conversely, if the closing force is lower than required, then the valve may not close adequately when cold. 
         [0014]    The valve described in EP-A-1204906 comprises fourteen components and it is desirable to reduce the number of components used in a valve to simplify assembly and reduce the cost. Additionally, the valve of EP-A-1204906 uses laser welding to join the components. Since laser welding is a complicated and expensive process, it may be desirable to avoid using this process in some circumstances. 
       SUMMARY 
       [0015]    It is an object of the present invention to provide a valve having a post-manufacture means of adjustment. 
         [0016]    The invention provides a valve device comprising; a body portion having a plurality of openings, a valve seat located within said body portion, an end stop, corresponding to an opening of said body portion, said end stop comprising a port portion through which fluid may flow and a nozzle, in fluid communication with said port portion, said nozzle configured for sealing against said valve seat and thereby preventing flow through the valve device, the valve device being openable by relative motion of said nozzle with respect to said valve seat, and adjustment means configured to allow independent adjustment of the location of said nozzle with respect to said valve seat. 
         [0017]    By providing an independent adjustment means, the parameters of the valve can be adjusted post manufacture, and may be configured to obtain a desired performance. 
         [0018]    The device may comprise more than one end stop, and each of said end stops may comprise a nozzle. 
         [0019]    The actuation means may comprise forming said body portion from materials that have different thermal expansion properties to the materials from which at least one of said end stop or stops is formed. 
         [0020]    The valve may be configured such that a variation in the temperature of the valve, or a part thereof, causes a change to the ratio of the lengths of said body portion to those of said end stop or stops. 
         [0021]    By configuring the valve in this manner, the valve can be made to open and close in response to such a variation in temperature. 
         [0022]    The valve may further comprise means to apply heat to or extract heat from the body portion. Such means may comprise an electrical resistance element, or other electrical heating means. 
         [0023]    The length of the body portion may alternatively be adjusted by other means, such as by the application of mechanical force and/or use of a piezoelectric element. Use of a mechanical force, for example as provided by a piezoelectric element, has the advantage that the valve is immune to ambient temperature fluctuation. In addition, mechanical adjustment of the body portion length benefits from a fast actuation time and does not suffer from delays which can occur with thermal expansion. Not using a heating wire or coil dispenses with any potentially undesirable effects of electromagnetic radiation which would be produced by the current flowing in the wire or coil. 
         [0024]    The adjustment means may comprise a threaded nut that mates with a thread on the external surface of the tubular body portion, and engages with an end stop such that a variable force may be applied to the end stop in a direction parallel to the axis of the tubular body portion. 
         [0025]    The end stop or stops may be sealed in a flexible manner such that the depth of the end stop in the body portion may be varied, whilst maintaining an effective seal between the end stop and body portion. 
         [0026]    The flexible sealing means may comprise a flexible gasket material. The flexible gasket material may comprise gold, or another malleable metal or alloy. 
         [0027]    The valve seat may be manufactured from a material having a high degree of hardness, such as sapphire or other types of crystal, ceramic, metal or glass. 
         [0028]    The end stop or stops may be manufactured from a material having a lower coefficient of thermal expansion than that of the body portion. 
         [0029]    For example, the end stop or stops may be manufactured from ‘Invar®’ and the body portion may be manufactured from stainless steel. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]    The invention will now be described in further detail with reference to the figures in which: 
           [0031]      FIG. 1  shows a cross-section of a valve according to an embodiment of the invention. 
           [0032]      FIG. 2  is a cross-sectional view showing detail of a valve seat. 
           [0033]      FIG. 3  is a cross-sectional view showing detail of an end stop. 
           [0034]      FIG. 4  is a cross-sectional view showing detail of the tubular body. 
           [0035]      FIG. 5  is a cross-sectional view showing detail of the nut adjuster. 
           [0036]      FIG. 6  shows a valve in accordance with an embodiment of the invention with a heating element attached. 
           [0037]      FIG. 7  shows a further embodiment of the invention, in which a single end stop is used. 
           [0038]      FIG. 8  shows a yet further embodiment of the invention, in which a mechanical means is used to actuate the valve. 
           [0039]      FIG. 9  shows a further embodiment of the invention, in which two valves are formed in combination to produce a three-port valve. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 1  shows a valve for controlling the flow of a gas according to an embodiment of the present invention. The valve can also be used for controlling the flow of a liquid. The valve  1  has a main body  2  in the form of a rigid tubular structure, which may be formed from, for example, a metal such as stainless steel. The main body  2  approximates a cylinder that is open at either end. A valve seat  6  is located inside the main body  2 . The valve seat  6  will be described in greater detail with reference to  FIG. 2 . 
         [0041]    Also located within the main body  2  are first and second end stops  4 ,  8 , each end stop  4 ,  8  having a nozzle  33 ,  35  which is in fluid communication via a central bore  34  to a port  38 ,  39 . The nozzle  33 ,  35  of each end stop  4 ,  8  is adjacent the valve seat  6  within the main body  2 . 
         [0042]    When the valve  1  is in use, and the nozzles  33 ,  35  of both end stops  4 ,  8  is spaced from the valve seat  6 , the valve  1  is open and gas or liquid may pass through the valve  1 . Conversely, when one or both of the nozzles  33 ,  35  abuts the valve seat  6 , then that nozzle  35  or nozzles  33 ,  35  will be blocked, the flow of gas or liquid through the valve  1  will be prevented, and the valve  1  is therefore closed. Similarly, by varying the force which the nozzles  33 ,  35  exert on the valve seat  6 , the rate of flow of fluid through the valve is controlled, which is known as proportional control. 
         [0043]    The ends  38 ,  39  of each end stop  4 ,  8  that are distal to the valve seat  6  protrude from the main body  2  of the valve  1 , and form the valve ports  38 ,  39 . The valve ports  38 ,  39  are configured for connection to a gas handling system by, for example, use of compression fittings so that a gas tight seal between the end stops  4 ,  8  and a gas inlet and outlet (not shown) can be formed. 
         [0044]    In the embodiment illustrated, the second end stop  8  is rigidly attached/fixed to the main body  2  by, for example, electron beam welding. The electron beam weld  10  also forms a gas tight seal between the end stop  8  and the main body  2 . 
         [0045]    The first end stop  4  is not rigidly fixed to the main body  2  and is able to slide relative to and within the main body  2 . A flexible gasket  12  is situated between the end stop  4  and the end face of the main body  2  to provide a gas tight seal between this end stop  4  and the main body  2 . 
         [0046]    An adjustment nut  14  is provided around the non-rigidly fixed end stop  4  and the adjustment nut  14  engages with a thread  15  formed on the outer surface of the main body  2  such that rotation of the adjustment nut  14  provides a variable force to move the end stop  4  within the main body  2  in a direction parallel to the major axis of the cylinder of the main body  2 , to provide proportional control of the flow of fluid through the valve  1 . 
         [0047]    In an alternative embodiment, both end stops  4 ,  8  may be connected to the main body  2  by flexible sealing means, and each of the end stops  4 ,  8  is provided with an adjustment nut  4  and gasket  12  as described above. 
         [0048]    The main body  2  is constructed from a material that has a higher coefficient of thermal expansion than that of the material that forms the end stops  4 ,  8  and the valve seat  6 . Thus, if the valve  1  is heated by some external heat source, the main body  2  will increase in length by a greater amount than the sum of the increases of the end stops  4 ,  8  and valve seat  6 . The net effect of the expansion of the end stops  4 ,  8  and the main body  2  will be motion of the nozzles  33 ,  35  of the end stops  4 ,  8  away from the valve seat  6 . 
         [0049]    Since the valve  1  will be opened by motion of nozzles  33 ,  35  away from the valve seat  6  such that there is a gap between the nozzle  33 ,  35  of each end stop  4 , 8  and the valve seat  6 , the application of heat to the valve  1  can be used to open the valve  1 . Heat may be applied by, for example, an electrical resistance heater  48  (shown only in  FIG. 6 ). 
         [0050]    Similarly, if the valve  1  is subsequently allowed to cool, then the components will all contract. The net effect of the contraction will be to close the valve  1 . 
         [0051]    An example of a suitable material for the main body  2  is stainless steel, while suitable materials for the end stops  4 ,  8  will have a lower coefficient of thermal expansion than stainless steel, and thus differential thermal expansions of the components described above will occur. An example of a suitable material for the end stops  4 ,  8  is ‘Invar®’. 
         [0052]    In an alternative embodiment, the main body  2  is made from a low expansion material, while the end stops  4 ,  8  are made from a higher expansion material. In such an embodiment, the valve  1  will be opened by a reduction in its temperature. 
         [0053]      FIG. 2  shows further details of the valve seat  6 . The valve seat  6  has a section  16  that is approximately cylindrical and of a diameter that permits the valve seat  6  to fit within the main body  2  of the valve. The valve seat  6  is sized relative to the internal diameter of the main body  2  such that, at ambient temperature, the valve seat  6  is held in a fixed position within the main body  2  by friction against the internal surface of the main body  2 . At elevated temperatures, such as during thermal operation of the valve  1 , the valve seat  6  is free to slide axially within the main body  2  as a result of the thermal expansion of the main body  2 . An elevated temperature is also used during assembly of the valve  1  to permit the valve seat  6  to be positioned within the main body  2 . Both ends  18 ,  28  of the valve seat  6  are formed with a convex curvature. The curvature may be spherical or non-spherical in profile. 
         [0054]    The centres of curvature of the curved ends  18 ,  28  of the valve seat  6  are aligned with the axis of the valve seat  6 , which is in turn aligned with the axis of the main body  2 . Thus, the largest dimension of the valve seat  6  is coincident with the central axis of the valve  1 . The cylindrical section  16  of the valve seat  6  is formed with one or more channels  22  that permit the flow of fluid past the valve seat  6  when it is located in the main body  2  of the valve  1 . The channels  22  may be formed by removal of material from the surface of the valve seat  6  that is in contact with the internal surface of the main body  2 , for example by flattening a section of this surface. 
         [0055]    In an alternative embodiment, the channels  22  may be formed by one or more holes that pass through the valve seat  6  in a direction parallel to the axis of the main body  2 . 
         [0056]    The channels  22  are formed such that they do not extend to the portions of the convex surfaces  18 ,  28  that are in contact with the nozzles  33 ,  35  of the end stops  4 ,  8  when the valve  1  is in its closed position. Otherwise, the valve  1  will be permeable to the flow of gas even when in its closed state. 
         [0057]    The valve seat  6  is formed from a rigid material that has a high degree of hardness, such as sapphire. The valve seat may have a Mohs hardness of 8 or greater. 
         [0058]      FIG. 3  shows details of the main body  2 . The main body  2  is formed from a tube in the form of a straight cylinder of material, and has a uniform circular bore  41  that runs along its length. The external surface of one end of the tube has a raised collar portion  40  that is machined with a thread  15 . The axis of the thread is parallel with that of the tube. 
         [0059]      FIG. 4  shows further details of an end stop  4 ,  8 . Both end stops  4 ,  8  may be of substantially the same construction. The end stop  4 ,  8  comprises a tube of material that has an external diameter suitable to permit a sliding fit within the main body  2  of the valve  1 . The end stop  4 ,  8  has a central bore  34  that runs the complete length of the end stop  4 ,  8  such that fluid may flow through the centre of the end stop  4 ,  8 . At the nozzle  33 ,  35  end of the end stop  4 ,  8 , the central bore  34  has a section  32  that is narrower than the rest of the bore  34 . The narrow section  32  facilitates sealing of the nozzle  33 ,  35  to the valve seat  6  when the valve is closed. 
         [0060]    A collar  36  is provided on the external surface of the end stop  4 ,  8  at a position part way along the length of the end stop  4 ,  8 . The diameter of the collar  36  is greater than the internal diameter the main body  2 , thus the collar  36  provides means to locate the end of the end stop at a predefined position within the tube forming the main body  2  by locating the collar  36  against an end of the main body  2 . 
         [0061]    Upon insertion into the main body  2  the collar  36  of one of the end stops  4 ,  8  may be welded in place at the junction of the collar  36  and the end of the main body  2 . The welding may be accomplished by, for example, electron beam welding or various other welding techniques. The weld forms a gas tight seal to prevent leakage of gas from the valve  1 . 
         [0062]    The port portion  38  of the end stop  4 ,  8  protrudes from the main body portion  2 , and is configured to accept standard fittings for gas handling equipment, such as compression fittings. 
         [0063]    The end face of the nozzle  33 ,  35  of the end stop  4 ,  8  is formed into a plane surface that is oriented at right angles to the axis of the end stop  4 ,  8 . The reduced diameter central bore  32  thus forms a circular hole in the end face of the nozzle  33 ,  35 . Thus, when the end face of the nozzle  33 ,  35  is brought into contact with the valve seat  6 , the curved surface of the valve seat  28 ,  18  forms a seal over the end of the central bore  32  of the end stop  4 ,  8 . 
         [0064]    Thus, when the valve seat  6  is in contact with the end stop  4 ,  8  no gas may pass through the central bore  34  of the end stop  4 ,  8 . 
         [0065]    In an alternative embodiment, the end face of the nozzle  33 ,  35  of each end stop  4 ,  8  is coated with a layer of gold (not shown), or other malleable material, such as a metal (e.g., copper) or alloy. By using a malleable material on the nozzle  33 ,  35 , the surface can deform slightly when it is pressed against the valve seat  6 . This deformation will tend to improve the impermeability of the seal formed between the nozzle  33 ,  35  and the valve seat  6 , and thereby improve the impermeability of the valve. 
         [0066]      FIG. 5  shows further detail of the adjustment nut  14 . The adjustment nut  14  has a cylindrical inner surface, which is machined with a thread. The internal diameter and pitch of the thread are suitable to allow the thread to mate with the thread  15  on the external surface  40  of the main body  2 . A portion  44  of the external surface of the nut  14  is machined such that it is suitable to accept a spanner or other device for applying rotational torque to the adjustment nut  14 . One end of the nut  14  has a collar portion  46  having a smaller internal diameter than the rest of the nut. The diameter of the collar portion  46  is sized such that the port  38  of the end stops  4 ,  8  may pass through it, but that the collar  36  of the end stop  4  cannot pass through. 
         [0067]    Thus, with reference to  FIG. 1 , when the valve  1  is assembled with the nozzle  33 ,  35  of the end stops  4 ,  8  inside the main body  2  and the nut threaded on to the thread  15  of the main body  2 , the collar  46  of the adjustment nut  14  can apply a force to the collar  36  of the end stop  4 . This force may be varied by rotation of the nut  14  to provide proportional control of the flow of fluid through the valve  1 . 
         [0068]    Also with reference to  FIG. 1 , a flexible gasket  12 , in the form of a ring of material, is located between the collar  36  of the end stop  4  and the end portion  40  of the main body  2 . The gasket  12  both forms a gas tight seal between the collar  36  of the end stop  4  and the end  40  of the main tube  2 , and also simultaneously permits some flexibility in the location of the end stop  4  within the main tube  2 . 
         [0069]    Thus, by turning the adjustment nut  14  the location of the end of the nozzle  33 ,  35  of the end stop  4  in relation to the valve seat  6  can be adjusted. This adjustment permits variation of the opening temperature of the valve  1  as described above. 
         [0070]      FIG. 6  shows a valve according to an embodiment of the present invention with a heating device attached. The heating device shown is a coil of resistance wire  48  that is wound around the central portion of the main body  2  of the valve  1 . The resistance wire  48  is electrically insulated from the main body  2 , but is mounted on the main body  2  so that the thermal resistance between the wire  48  and main body  2  is low. When a current is applied to the wire, shown in the Figure by means of a battery  50 , the wire  48  heats up, and transfers heat to the main body  2 . 
         [0071]    In further embodiments, the heat may be applied to the main body  2  by use of other sources of heat, such as power resistors, peltier devices or light sources. The light source could be infrared. The entire valve may be housed in a temperature-controlled environment, and the temperature of this environment adjusted to open or close the valve. 
         [0072]    The opening properties of the valve  1  may be set in two ways. Firstly, the valve may be maintained at a predetermined temperature, and the adjustment nut  14  rotated until the flow of gas is just prevented. This then defines a temperature at which the valve will start to open, and start to permit the flow of gas. Raising the temperature of the valve above this point will permit higher flow rates, the flow rate increasing with increasing temperature. 
         [0073]    Secondly, the valve can be maintained at a predetermined temperature, and the flow rate adjusted by rotating the adjustment nut  14  until a desired fluid flow rate is achieved. Thus, if an application requires a particular flow rate to be used, this can be achieved by setting the temperature of the valve to the predetermined temperature. 
         [0074]    A calibration of flow rate versus temperature can be determined in either situation by using a suitable flow gauge to determine the proportional relationship between temperature and flow rate at given inlet and outlet pressures. The valve may be operated in a closed feedback loop by measuring the outlet pressure or flow rate and comparing this to the desired outlet pressure or flow rate and adjusting the valve temperature accordingly to achieve a desired outlet pressure or flow rate. 
         [0075]      FIG. 7  shows a further embodiment of the invention, in which only a single end stop  4  is used. The end stop  4  is located within a central bore  56  of a main body  52 . The main body  52  is modified from that described above by being configured to accept an end stop  4  at only one end, while at the other end the main body  52  is configured to directly receive standard fittings for gas handling equipment, such as compression fittings. 
         [0076]    The end stop  4  is sealed and fastened into the main body  52  by means of a gasket  12  and nut  14  as described above in relation to  FIG. 1 , and a thread  15  provided on a section of the outer surface the main body  52  to provide fixing for the nut  14 . 
         [0077]    A valve seat  6 , the same as described above in relation to  FIG. 2 , is located within the central bore  56  of the main body  52 . The central bore  56  of the main body  52  has a narrowed/tapered section  54  to provide means to locate the valve seat  6 . The valve seat  6  is prevented from moving past the tapered section  54  since the diameter of the tapered section  54  is less than that of the valve seat  6 . 
         [0078]    Once assembled, the end stop  4  abuts the valve seat  6  such that the valve seat  6  is trapped between the end stop  4  and the tapered section  54  of the central bore  56  in the main body  52 . In use, the end stop  4  seals onto the valve seat  6  to close the valve. 
         [0079]    The end stop  4  may be manufactured from a material having a lower coefficient of thermal expansion to the material used to form the main body  53 . Thus, the device shown in  FIG. 7  can function in the same manner as described above in relation to  FIGS. 1 to 5 , and may therefore also be actuated by varying the temperature of the device. 
         [0080]      FIG. 8  shows an alternative embodiment of the valve device, which provides for mechanical, rather than thermal, actuation of the valve. The force required to seal the valve device derives from the mechanical properties of the material used in construction. To actuate the device a mechanical force is applied to deform the material longitudinally, along its major axis. Actuation of a valve by mechanical rather than thermal means may, for example, be particularly useful when applying a valve for use in electric propulsion systems or in a cold gas thruster as used in a spacecraft. Similarly, mechanical actuation may be preferable in other applications where speed of operation, power requirement and reducing electromagnetic emissions are to be considered. 
         [0081]    When using a mechanical force to effect the expansion of the main body portion, the skilled person will understand that the body portion can only be expanded within its elastic limit. In other words, the material from which the main body is made must have sufficient yield strength so that when it is subjected to a mechanical force, which provides a tensile force or stress, permanent deformation does not occur. 
         [0082]    As with the embodiment described in relation to  FIG. 7 , the main body  58  is configured to accept an end stop  4  at only one end. A gasket  12  and nut  14  are again used to seal and fasten this end stop  4  into the main body  58 , and a thread  15  is provided on a section of the outer surface of the main body  58  to provide fixing for the nut  14 . 
         [0083]    The other end of the main body  58  is again configured to accept standard fittings for gas handling equipment, such as compression fittings, and is also provided with a threaded section  68  on its outer surface. 
         [0084]    A valve seat  6 , as described above in relation to  FIG. 2 , is located within the central bore  56  of the main body  52 . The main body  52  again has a narrowed/tapered section of its central bore  56 , such that the valve seat  6  is prevented from moving past the tapered section  54 . In use, the end stop  4  seals onto the valve seat  6  to close the valve. 
         [0085]    The main body  58  has a protruding collar  62  formed on its external surface close to the end configured to accept the end stop  4 . A hollow piezoelectric actuator  60  is located around the main body  58  of the valve device such that an end of the piezoelectric actuator  60  abuts the protruding collar  62 . 
         [0086]    The other end of the piezoelectric actuator  60  is attached to the end of the main body  58  by a washer  64  and nut  70  that mates with threaded section  68  of the main body  58 . Both ends of the piezoelectric actuator  60  are therefore held in fixed relation to the main body  58 . 
         [0087]    The piezoelectric actuator  60  is configured to change in length on application of an electric field. Thus, by applying a suitable drive voltage to the piezoelectric actuator, a tensile force can be applied to the main body  58 . The tensile force will tend to stretch the main body  58  and, when the force is sufficiently large, the end stop  4  will move away from the valve seat  6  thereby opening the valve. Actuation of the valve may therefore be controlled via the drive voltage. Increasing the drive voltage will result in proportionally increased longitudinal expansion and thus an increase in the flow rate. A calibration of flow rate versus voltage can be determined by using a suitable flow gauge to create a feed-back system which will allow proportional control. 
         [0088]    When the drive voltage is lowered, the tensile force produced by the piezoelectric actuation decreases and the original length of the main body is restored, so that the valve closes. 
         [0089]    In a further embodiment, a combination of mechanical and thermal actuation can be used to control actuation of the valve. This could be achieved for example through the use of a mechanical actuator such as a PZT device to apply a longitudinal tensile force to the body of the valve. 
         [0090]      FIG. 9  shows a further embodiment of the invention in which two valves are combined to form a three-port valve. Three-port valves may be formed either by using two single port valves in combination, or by formation of a single three-port valve. 
         [0091]    In  FIG. 9 , two valves  1  according to any of the above described embodiments are combined by connection of one port of each device to a common port  72  by use of a T-connector  74 . Each valve  1  is independently controllable and adjustable as described above. 
         [0092]    In use, fluid (liquid or gas) is supplied to the common port  72  and actuation of the valves  1  is used to switch the flow to either or both of the outlet ports  76 ,  78 . 
         [0093]    It will be apparent to those skilled in the art that various modifications and variations can be made to the valve without departing from the scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims