WO2014049349A1

WO2014049349A1 - Valve

Info

Publication number: WO2014049349A1
Application number: PCT/GB2013/052501
Authority: WO
Inventors: Stuart Malcolm DRYSDALE; Johnnie William DRYSDALE
Original assignee: Drysdale Brothers (Larbert) Limited
Priority date: 2012-09-25
Filing date: 2013-09-25
Publication date: 2014-04-03
Also published as: GB201217096D0; EP2901055A1

Abstract

The invention provides a check valve comprising valve components adapted to be retained in position by a spacer which holds carriers in a spaced position such that they are retained in the correct position within the valve housing. Removal of the spacer allows convenient removal of the valve components from the housing.

Description

Valve

This invention relates to fluid check valves, i.e. non-return valves, and particular, but not exclusively, to dual plate check valves. The invention also relates to methods of producing and using such valves, and kits for installing such valves.

Dual plate check valves (also known as twin-plate check valves) comprise an annular housing with two substantially semi-circular or D-shaped valve members, referred to as plates or flaps (and also sometimes as flappers or wafers), pivotally mounted on a hinge pin. These plates can take up a closed position in which they shut respective semi-circular or D- shaped apertures defined in the housing by a diametral cross piece. The plates are generally urged towards the closed position, e.g. be a spring, so that they close without the need for reverse flow of the fluid within the housing to push them closed. Pressure of a fluid, e.g. being propelled by a pump, on the upstream side of the valve overcomes the spring force on the plates and pushes them towards an open position in which they lie generally parallel to the housing axis. Often a stop pin is provided parallel to and downstream of the hinge pin to prevent either plate from pivoting more than 90° from the valve closed position.

Dual plate check valves thus open in response to pressure on the upstream side and, as pressure decreases, e.g. as the flow rate drops or stops, the plates are urged by the spring and/or reverse pressure towards the closed position.

The hinge and stop pins can be fixed into the housing through holes bored in the housing and then retained therein by means of screwed plugs applied from outside the housing. In such cases it is useful if the screw plug can be readily removed to permit easy disassembly so that valve componentry can replaced as required. However, corrosion, thermal cycling, poor installation and the like can lead to loosening and leakage of the plugs. Clearly this is undesirable. Furthermore, corrosion of the plugs can make them difficult or impossible to remove. Furthermore the presence of bolted flanges of other apparatus to secure the valve in position in the pipeline can make it difficult to access the plugs.

EP 0160576 discloses a dual plate check valve with flaps mounted on one or more hinge pins where the or each hinge pin extends at each end into a bore in an insert, the inserts being received in respective slots in the body. The inserts are held in place within the slots by a retaining system. The preferred retaining system comprises a ring which extends into a groove at the end of the body, from which groove said slots extend, wherein the ring abuts against inserts and is held in position in the valve body by screws.

l In some check valves an arrangement is provided when there is some play between various components of the valve system such that the "D" shaped valve plates lift off the seat before pivoting occurs, so that the plate does not scuff on the cross member of the seat. In US 5819790, which is another document disclosing dual plate check valves mounted in an insert, this is achieved by providing play between the hinge pin and the inserts. Play between hinge pin and inserts will eventually cause wear, but these parts are comparatively cheap and can be replaced. The inserts are held in place by screws, pins, dowels or the like, which are located into bores provided in the valve body.

US 5301709 discloses another system which avoids the need for apertures through the wall of the body. In the system disclosed therein extendible sleeves provided on the hinge pin(s) are adapted to extend into blind bores provided on the inside of the body. This allows the hinge pin(s) to be moved into position within the body with the plates mounted thereon, and then secured by extending the sleeves and securing the sleeves in position using spring clips on the hinge pins.

There remains a need for improved systems for mounting check valve components in a valve body. The systems of the prior art do not provide systems which are easy to manufacture and which are easy to assemble and disassemble. For example, the systems described US 5819790 require a fairly complex system in which various bores in the inserts and the housing must be formed and then screwed together. Such a system is complex to manufacture and install, requires tight tolerances and would also be difficult to remove, particularly if the valve components or fixings have corroded. In EP 0160576 the ring-based system disclosed is likewise relatively complex and would be difficult to remove if corroded. The system of US 5301709 is again complex and requires spring clips which could be fiddly to both install and replace, and the difficulties would be increased if they become corroded. It would be advantageous to have a check valve which is simpler to manufacture, which is simple to install and remove, even where corrosion of the valve components has occurred.

According to the present invention there is provided a dual plate check valve comprising an annular housing and two valve plates adapted to close a passage through the housing, the plates being mounted on one or more hinge pins, the or each hinge pin extending at each end into a carrier, the carriers being engaged in respective retainers in the annular housing and wherein each of said carriers is retained in said retainer by means of a spacer which is located between the carriers and which holds the carriers spaced from one another and engaged with the retainers. It is preferred that the retainers comprise recesses that are adapted to receive the end of the carriers. However, the retainers could alternatively or additionally comprise one or more projections from the wall of the housing. The retainers must be able to retain the carriers when they are in the spaced position, and this can be achieved providing a retainer which prevents the spaced carriers from moving significantly in the axial and circumferential directions within the housing. This can be achieved by surrounding the ends of the carrier with a continuous shoulder or wall, e.g. as would occur in a simple recess, or by providing a plurality of discrete projections surrounding and thereby retaining the end of the carrier. In a preferred embodiment the recess is formed directly into the wall of the housing, e.g. in the general manner shown in the specific example below. In another example, the housing could be formed with a projection, e.g. a boss, typically a shallow projection, into which the recess is formed. In such an embodiment the recess would thus be defined by a ridge which surrounds it, and the base of the recess need not be recessed relative to the remainder of the wall of the housing.

A significant advantage of the present invention is that the valve components can be installed and removed from the valve housing by locating (i.e. inserting) or removing the spacer. No other fixing means or device is required. This is a simple operation which can be carried out simply and rapidly. Significantly, the spacer can be easily removed even when the valve components have become, for example, corroded, covered in deposits, or if access to the valve interior is difficult. The spacer can be removed very simply by cutting, e.g. with a grinder, bolt cutters or a cutting torch, breaking the spacer or pulling the spacer out.

Optionally the spacer can be frangible. This allows the spacer to be even more easily removed. For example, the spacer could be adapted to be strong in compression (i.e. in respect of inward radial forces applied to it by the carriers), but weak when exposed to lateral loads beyond those experienced in normal operation. Thus, an approximately axial force applied to the spacer could be used to break the spacer and allow removal of the carriers and other valve components.

Alternatively or additionally, the spacer could be formed from a material having a low melting point (e.g. lead, tin or a polymer) so that it can be readily removed by melting the spacer.

Preferably the spacer is located generally centrally within the valve housing. By located centrally it is meant that, when the spacer is in position holding the carriers apart, the spacer is located away (i.e. spaced radially) from the walls of the valve body and towards the centre (i.e. axis) of the valve body. One can visualise the major internal dimension of the housing (e.g. the diameter of a cylindrical housing), in which case the spacer is preferably located at or near the midpoint of the this dimension. Suitably at least a portion of the spacer, and more preferably substantially the whole spacer, is located within the central third of the cross sectional area of the housing. When the spacer is located centrally it allows access to the spacer for its removal (e.g. by cutting) without a risk of damaging the walls of the valve housing. Preferably the spacer is accessible from the downstream direction without being obstructed by any valve components. Thus the spacer is preferably located downstream of the plates, one or more hinge pins, springs and other such components. This means that the spacer can be readily accessed to be removed without the need to manoeuvre tools around valve components. For example, the spacer can be easily cut using a grinder or other form of cutter without significant risk of damaging the walls of the valve housing or the valve seat. This makes removal of the spacer very straightforward.

In a preferred embodiment the spacer comprises a plate. The plate is preferably formed from a malleable material such that it can be deformed (e.g. swaged) once located in position such that it cannot accidentally become dislocated. In a preferred embodiment the spacer comprises a plate having an edge which is pre-bent to form a lip. The opposite end from the lip can conveniently comprise on or more prongs which can be bent over to secure the spacer in position. The spacer could, of course, be secured in position by other means, such as adhesive, clip means, screws, or the like.

Preferably the spacer is adapted to be a single-use item, i.e. it is non-reusable. Thus once the spacer has been located and removed once, it cannot be used further.

For example, the spacer can be formed from a material that is suitable to be deformed once, but which tends to fracture when the same portion is repeatedly deformed. Thus the spacer can bent into position once, but once it has been removed (e.g. by straightening the bent portion) if it is bent again the spacer will fracture, or at least be significantly weakened. For example, many aluminium alloys are tolerant of being bent significantly in one direction, but when bent back they tend to fracture. Other suitable materials for forming the spacer will be apparent to the person skilled in the art, and could readily be tested by attempting to bend spacers of the relevant material more than once. The spacer could be made from a brittle material such that it can be inserted into position, but will likely break upon removal.

Exemplary materials for the spacer when it is to be used in a chemically aggressive environment, e.g. sea water, include stainless steel and Alloy 400 (also known as Monel).

Preferably the spacer is provided with a unique identifier. For example, the spacer could be provided with a serial number or the like. An advantage of having a single-use spacer, and especially one that is uniquely identified, is that it allows control over the servicing and maintenance of the valve and for warranty assessment. For example, it allows a supplier of valves to determine if the valve components have been replaced with unauthorised replacements or has been tampered with. Once the spacer has been removed it cannot be re-used, and thus a replacement must be used. If this spacer does not carry the correct unique identifier then it will be evident that something is amiss, e.g. an unofficial repair or modification has been carried out. Of course, it is possible that an identical spacer could be fabricated with the same unique identifier applied, but this would be inconvenient and potentially costly to carry out and thus would not typically be worth the effort.

Preferably the carrier comprises an elongate portion and at least one projection, the projection having a receptacle adapted to receive the at least one hinge pin. The receptacle preferably comprises an aperture (e.g. a bore) adapted to receive a hinge pin. More preferably the carrier comprises two projections, each comprising a receptacle, the receptacles being adapted to receive the same hinge pin at different points along its length.

In a particularly preferred embodiment the carrier comprises an elongate member in the form of a substantially straight bar, the bar having a proximal and a distal end. The proximal end is the end which, in use, is nearest the centre of the housing and the distal end being that nearest the wall. At the proximal end of the elongate member there is a recess, e.g. a slot, adapted to receive a portion of the spacer. At the distal end of the elongate member there is a protrusion which extends substantially normal to the elongate member, and which has an aperture for receiving a hinge pin near its end. At an intermediate point between to two ends a second projection extends substantially normal to the elongate member, the projection ending in an annular collar with an aperture for receiving the same hinge pin - thus the two annular collars lie on the same axis. In preferred embodiments the carrier is essentially an F-shape, with the arms of the F being the projections, the arms being of equal length, the back of the F being the elongate member, and the base of the F being provided with a slot to receive the spacer. The retainers of the housing are preferably diametrically opposed. Preferably the retainers align with a diametral cross-member provided on the housing that defines two symmetrical flow openings (i.e. the passageway through the valve housing).

The retainers suitably have a shape that corresponds to the shape of the distal end of the carrier. Where the retainer comprises a recess, preferably the footprint of the recess (i.e. when viewed from the axis of the housing) and the cross section of the end of the carrier correspond. Furthermore, the profile of the base of the recess may correspond to the profile of the end of the carrier. Suitably the footprint of the recess and the distal end of the carrier are both generally rectangular, but other shapes could of course be used. The recess preferably comprises a projection which extends from the base of the recess, the projection being positioned such that it abuts against the end of the hinge pin which is located in position within the receptacle of the carrier. Suitably the distal end of the carrier has a suitable profile to correspond with the sides of the projection. Suitably the projection is frustoconical, but other shapes could be used, e.g. frustopyrimidal, cylindrical, etc. The projection preferably extends from the base of the recess so that its extremity is at least flush with the inner wall of the housing, and optionally it can extend slightly beyond the inner wall. It is generally preferred that the projection is substantially flush with the surface of the housing. In any event, the distance between the extremities of the projections preferably substantially corresponds to (i.e. is the same or very slightly larger than) the length of the hinge pin. The projections thus serve to hold the pin in position and limit side-to-side (radial) movement thereof.

Preferably the valve comprises a resilient means adapted to urge the plates into a closed position. Suitably the resilient means is a spring. For example, a preferred resilient means is a helical torsion spring mounted on the hinge pin, with one end abutting against one of the plates and the other end abutting against the other plate, such that the plates are urged into the closed position. Other resilient means could of course be used.

Preferably the valve comprises stop means to limit the opening angle of the plates. Suitably the stop means comprises a projection extending from the downstream face of one, other or both of the plates. Preferably a projection extends from both of the plates. A suitable projection is a cylindrical or frustoconical projection. The projections on each plate are positioned such that when the plates are in an open position they abut against each other and thereby limit the opening of the plates. Alternative stop means could comprise a suitable surface provided on the one, other or both of the carriers or a second pin. The plates are typically substantially D-shaped, although any shape could be used provided it can effectively seal against the valve seat on the valve housing, and can pivot in order to open the valve. The valve plates preferably comprise at least two projections which comprise annular collars for mounting on the hinge pin. Typically the plates each comprise two such projections, though they could have more, or even a single elongate collar thus preventing undesirable twisting of the plate relative to the pin. The projections can be located at any suitable point on the plates. In general the projections are offset such that the projections of one plate do not interfere with the projections of the other plate. Suitably the two plates are identical, or they could e mi. The valve may comprise washers, bushes or the like mounted on the shaft located between the protrusions on the plates, carriers and/or the springs.

According to a second aspect, the present invention provides a valve kit comprising at least one hinge pin, a pair of carriers, a pair of plates and a spacer. Various preferred features of these components are described above. Such a valve kit is adapted to be mounted in a valve housing, e.g. at initial installation of the valve or at a subsequent replacement of the valve components. The kit may further comprise the housing. The kit may suitably further comprise one or more of a resilient member, at least one washer or bushing, and a valve housing having a pair of recesses adapted to engage the carriers.

The kit can optionally be pre-assembled as a valve component cassette ready for installation into a suitable valve housing, with only the spacer required to be inserted to secure the carriers, and hence the valve cassette, in position. In a third aspect the present invention provides a method of producing a valve, the method comprising the steps of:

- providing a valve housing comprising a pair of retainers;

- providing a valve component cassette comprising at least one hinge pin, a pair of carriers slidably mounted on the hinge pin, and a pair of plates pivotally mounted on the hinge pin;

- providing a spacer; - locating the cassette within the valve housing,

- sliding the carriers outwards along the hinge pin to engage with the retainers in the valve housing;

- inserting the spacer into the gap between the carriers and securing the spacer in position to fix the carriers in position in the retainers of the valve housing.

The method may suitably comprise the step of mounting the plates and the carriers onto the hinge pin to form the valve component cassette. The method may comprise providing one or more of:

a resilient means adapted to urge the plates into a closed position,

- washers; and

bushings. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawing in which:

- Fig 1 shows an exploded view of a valve according to the present invention;

- Fig 2 and 3 show views of the valve in the closed and open positions respectively, viewed from the downstream axis;

- Fig 4 shows a line drawing of a section of the valve along line A-A in a closed position;

- Fig 5 shows a line drawing of a section of the valve along line B-B in an open position; and

- Fig 6 shows a line drawing of a section of the valve along line C-C in a closed position. A valve 10 according to the present invention comprises an annular valve housing 12, the housing comprising a diametral cross-member 14 extending across the housing and forming two substantially semi-circular flow apertures 16, 18. The upstream and downstream annular end faces of the housing 20,22 are provided with grooves to provide a sealing engagement with a gasket (not shown). The valve 10 is mounted in a pipeline (not shown) and secured in place by any suitable means. In the present case the annular flanges 24,26 at each end of the housing do not have bolt holes and such a valve is typically held in place by compression. In other cases the housing comprises annular flanges with bolt holes to allow the flange to be secured with bolts to a corresponding flange on the adjacent section of pipe. The housing also comprises an annular shoulder 28 which, along with the downstream face of the cross member defines a valve seat 30. Two D-shaped plates 32,34 are provided, which are adapted to pivot about a hinge pin 36, allowing the valve to change from a closed mode, where the plates form a sealing engagement with the valve seat 30, two an open mode where the plates are pivoted away (downstream) from the valve seat, thus opening the valve. The plates each comprise a frustoconical projection 35 on the downstream face of the plate. The projections 35 abut against each other when the plates swing open, thus acting as stop means to limit the opening angle of the plates. A helical torsion spring 38 is provided on the hinge pin 36, the spring having two ends, one of which abuts against the downstream face of one plate 32, and the other of which abuts against the downstream face of the other plate 34. Grooves 40,42 are provided on the faces of plates to positively locate the ends of the springs. The spring urges the plates 32,34 into the closed position.

When the force acting on the upstream face of the plates is greater than the force acting on the downstream face, the plates 32,34 pivot to open the valve. This occurs, for example when a fluid is flowing through the valve from upstream to downstream under pressure, e.g. when the fluid is being pumped. Should the flow of fluid stop or reverse, the plates swing back to the closed position as a result of the torsion spring and/or the pressure applied by the fluid to the downstream face of the plates. It is desirable that the plates should close before significant reverse flow occurs to prevent hammering and potential damage to the valve or upstream pant such a pump. The torsion spring helps achieve this.

The hinge pin is retained in position by a pair of carriers 50,52, which are generally F- shaped. The carriers have a proximal end, i.e. the end which in use is located towards the centre of the housing, and a distal end, i.e. the end which in use is located in recesses provided in the wall of the housing 54,55. The carriers comprise an elongate bar 56 (forming the back of the F) from which two parallel projections 58,60 extend normal to the axis of the bar. The first projection 58 extends at the distal end of the bar, and the second 60 extends at an intermediate point between the proximal and distal ends, in the specific example shown it is about ¼ of the total length of the bar from the proximal end. The projections each comprise a round aperture 62,64 located near their distal ends. The apertures are coaxial and have the same diameter, i.e. corresponding to the diameter of the hinge pin 36. At the proximal end of the bar there is provided a slot 66. The slot is sized and shaped to receive the edges of a spacer plate 68. When the carriers are located for use, the slots are parallel to the plane of the valve seat, i.e. such that the spacer lies parallel to the valve seat when in position.

The plates 32,34 and carriers 50,52 are mounted onto the hinge pin through the relevant apertures and in the appropriate arrangement, as best shown in Figs 1 and 6. The torsion spring 38 is mounted on the hinge pin 36 through the helix of the spring. Washers 70 can be positioned between the various protrusions, as required.

Once assembled, the carriers 50,52 can be pushed together so that they abut against one another. There is thus formed a valve component cassette, ready for insertion into the valve housing 12.

The housing comprises two recesses 54,55 adapted to receive and retain the carriers. The recesses are diametrically opposed, and are arranged such that the carriers and hinge pin will be aligned with the cross-member 14. The recesses are generally rectangular in footprint, and are shaped and sized to match the ends of the carriers. The recesses each have a frustoconical projection 72 which projects from the base of the recess. The frustoconical projection is positioned such that it aligns with the position of the aperture 62 on the first projection of the carrier.

The pre-assembled cassette is moved into position within the valve housing such that the carriers 50,52 are aligned with the recesses 54,55. The carriers are then moved outwards such that they enter into the recesses. Once the carriers have been moved outwards a gap is formed between them. Into this gap the spacer 68 is slid, the spacer sliding into the slots on the proximal end of each of the carriers. The spacer comprises a plate in which one edge has been folded over to form a lip 74. The lip 74 acts as a stop to prevent the spacer from sliding all the way through the slots, i.e. when it abuts against the carriers. The spacer also comprises a pair of prongs 76 extending from the edge opposite the edge with the lip. Once the spacer is in position these prongs can be bent over, thus securing the spacer in position, and holding the carriers in a spaced position. The cassette is thus firmly located in position by the carriers 50,52 which are engaged in the recesses 54,55 in the valve hosing 12.

The hinge pin 36 is mounted between the frustoconical projections 72 which prevent excessive side to side movement of the hinge pin, i.e. movement along the axis of the pin. The hinge pin has a length substantially corresponding to the diameter of the valve housing. If the projections 72 were not present, the hinge pin would be free to move from side to side to a greater extent which would create a wear problem and reduce stability of the valve components. The use of such projections is made possible by the sliding carrier arrangement and represents a simple and effective mechanism for securely locating the hinge pin appropriately. Furthermore, the projections provide a simple but effective system for the securing the hinge pin, without the need for sliding sleeves or the like from the prior art. The projections 72 extend outwards so that they are flush with the internal wall of the valve housing. This means that a hinge pin 36 which has a length equal to the diameter of the housing will fit snugly between the projections. It is possible that the projections could extend further, in which case a shorter hinge pin would be used. However, the carriers 50,52 must be able to clear the protrusions when they abut against each other. If the protrusions were absent, or did not extend to be flush with the wall then either the hinge pin would need to be able to extend to some extent, or there would be free play.

The valve 10, now fully assembled, can be used for its intended purpose, i.e. as a check valve in a pipeline to prevent reverse flow. When the time comes to service or replace the valve, e.g. due to wear or failure of the valve, access to the valve interior can be achieved, e.g. by removing the section of pipeline immediately downstream of the valve. The valve cassette can then be removed quickly and conveniently by removing the spacer 68, e.g. by cutting the spacer using a grinder. Because the spacer is mounted centrally in the valve housing, and because it is downstream of, and spaced from, the components of the valve it is easy to gain access and cut or otherwise remove the spacer without risking damage to the valve housing, or indeed the various components of the valve. It is a significant advantage that the valve components can be removed so simply. There is no need to remove screws, access spring clips, dowels or the like, which is the case with prior art valves. Such components are prone to seizing or otherwise jamming due to wear or corrosion, or can be fouled with deposited materials. Even if they are not corroded, jammed or fouled with deposits, they are typically inconvenient to access and remove. In the valve of the present invention removal is very straightforward - simply remove the spacer and then remove the cassette. There are no fiddly components to be accessed and unfastened. Furthermore, in prior art devices, when the fixing means (e.g. screws or the like) are jammed or corroded, they often need to be cut or drilled out. Because the relevant components are typically mounted in or adjacent to the valve body and other valve components, it is almost impossible to do this without damaging the valve housing and/or the valve components. In the present system the spacer is presented in an ideal position for easy removal. This not only protects the valve housing, but also means that the undamaged valve components can be reconditioned and reused if desired.

The spacer is adapted to be a single use (non-reusable) item. For example it is adapted to break or be severely weakened when it is removed from its location in the carriers. The spacer is typically formed of an aluminium alloy (e.g. a aluminium bronze) which is not tolerant of repeated bending. Other exemplary materials for the spacer include Alloy 400 (Monel Alloy 400 is binary alloy of the same proportions of nickel and copper) and stainless steel. The prongs can be bent once while retaining adequate strength to ensure the spacer is retained in position. However, if the prongs are straightened out and then re-bent, they will either break or be very weak. This prevents the same spacer being re-used. Some materials (e.g. mild steel) are capable of being bent back and forth several times without breaking or becoming severely weakened, and such materials are not well suited where a single-use spacer is desired. In addition, each spacer is marked with a serial number, e.g. by stamping, which uniquely identifies the spacer. This allows the valve supplier to monitor whether a valve has been subject to unauthorised tampering, servicing or replacement, e.g. if a warranty claim is made. If the spacer in the valve is incorrect or it has been removed then the supplier can infer that something is amiss.

Numerous modifications to the specific embodiment can be made without departing from the spirit of the invention. For example, the carriers and spacers need not have the specific form described, and could be shaped in any suitable way provided that they are suitable to engage with a spacer which holds them in a spaced arrangement. The recesses could be replaced with a ridge on the wall of the housing which surrounds and defines a region adapted to receive and retain the end of the carrier. Alternatively, a plurality of projections could be provided which are arranged in a manner to receive and retain the end of the carrier. Thus the retainers could take a variety of suitable forms. The housing need not be circular in cross-section. The components of the valve could be modified to allow a degree of play to allow the valve plates to move slightly away from the valve seat before they pivot, as is known in some prior art devices. The edges of the valve plates could be thinned to an extent that a typical working pressure applied to them causes them to deform slightly and accurately fit the valve seat. Alternatively, the valve seat could be rubberised to facilitate sealing.

The materials from which the various components are manufactured can obviously be selected according to the particular use intended. The valve housing can be made from any suitable metal, for example, gunmetal or other bronze alloys such as aluminium bronze, steel (e.g. stainless steel) or the like. The valve plates can also be made from such metals. The hinge pin is typically made from stainless steel, although other materials could be used. It is also possible that at least some of the components could be made from polymeric materials.

Claims

I . A dual plate check valve comprising an annular housing and two valve plates adapted to close a passage through the housing, the plates being mounted on one or more hinge pins, the or each hinge pin extending at each end into a carrier, the carriers being engaged in respective retainers in the annular housing and wherein each of said carriers is retained in a said retainer by a spacer which is located between the carriers and which holds the carriers spaced from one another and engaged in the retainers. 2. The valve of claim 1 wherein the retainers comprise recesses formed in the wall of the housing.

3. The valve of claim 1 or 2 wherein the spacer is frangible. 4. The valve of any preceding claim wherein the spacer is located centrally within the valve housing.

5. The valve of any preceding claim wherein the spacer is accessible from the downstream direction without being obstructed by any valve components.

6. The valve of any preceding claim wherein the spacer comprises a plate.

7. The valve of any preceding claim wherein the spacer comprises a plate having an edge which is pre-bent to form a lip.

8. The valve of any preceding claim wherein the spacer is a single-use item.

9. The valve of any preceding claim wherein the spacer is provided with a unique identifier. 10. The valve of any preceding claim wherein the carrier comprises an elongate portion and at least one projection, the projection having a receptacle adapted to receive the at least one hinge pin.

I I . The valve of claim 10 wherein the receptacle comprises an aperture adapted to receive a hinge pin.

12. The valve of claim 10 or 1 1 wherein the carrier comprises two projections, each comprising a receptacle, the receptacles being adapted to receive the same hinge pin at different points along its length.

13. The valve of any preceding claim wherein the carrier comprises an elongate member in the form of a substantially straight bar, the bar having a proximal and a distal end.

14. The valve of any preceding claim wherein at the proximal end of the carrier there is a recess adapted to receive a portion of the spacer.

15. The valve of any preceding claim wherein the carrier comprises an elongate portion and at the distal end of the elongate member there is a protrusion which extends substantially normal to the elongate member that has an aperture for receiving a hinge pin near its end, and at an intermediate point between to two ends a second projection extends substantially normal to the elongate member, the projection has an aperture for receiving a hinge pin near its end for receiving the same hinge pin, wherein the apertures lie on the same axis.

16. The valve of any preceding claim wherein the carrier is F-shaped.

17. The valve of any preceding claim wherein the retainers of the housing are diametrically opposed.

18. The valve of claim 17 wherein the retainers align with a diametral cross-member provided on the housing that defines two symmetrical flow openings.

19. The valve of any preceding claim wherein the retainers comprise a recess having a shape that corresponds to the shape of the distal end of the carrier.

20. The valve of any preceding claim wherein the retainer comprises a recess, and wherein the recess comprises a projection which extends from the base of the recess, the projection being positioned such that it abuts against the end of the hinge pin which is located in position within the receptacle of the carrier.

21. The valve of claim 20 wherein the projection extends from the base of the recess so that it extremity is at least flush with the inner wall of the housing.

22. The valve of any preceding claim which comprises a resilient means adapted to urge the plates into a closed position.

23. The valve of any preceding claim which comprises stop means to limit the opening angle of the plates.

24. A valve kit comprising at least one hinge pin, a pair of carriers, a pair of plates and a spacer, wherein the carriers and spacer are adapted to inter-engage to hold the carriers in a spaced position within a valve housing.

25. The valve kit of claim 24 which is be pre-assembled as a valve component cassette ready for installation into a suitable valve housing, with only the spacer required to be inserted to secure the carriers, and hence the valve cassette, in position. 26. A method of producing a valve, the method comprising the steps of:

- providing a valve housing comprising a pair of retainers;

- providing a spacer;

- locating the cassette within the valve housing;

- sliding the carriers outwards along the hinge pin to engage with the retainers of the valve housing; and

- inserting the spacer into the gap between the carriers and securing the spacer in position to fix the carriers in position engaged with the retainers of the valve housing.

27. The method of claim 26 comprising the step of mounting the plates and the carriers onto the hinge pin to form the valve component cassette. 28. The method of claim 26 or 27 comprising providing one or more of:

a resilient means adapted to urge the plates into a closed position,

- washers; and

bushings.