1 AN IMPROVED DUAL PLATE CHECK VALVE. FIELD OF THE INVENTION. [0001] This invention relates to check valves which are used to allow the flow of a fluid in a pipeline in one direction but prevents the flow of fluid in the reverse direction. In particular this invention relates to those check valves which are known as dual plate check valves. BACKGROUND ART. [0002] Dual plate check valves employ in their construction a pair of flat platelike valve members each configured in the form of the capital letter D which are arranged back to back within a relatively short tubular housing adapted to be mounted within a pipeline and having a fluid inlet means and a fluid outlet means. The valve members being supported within the housing by hinged connection to a rod mounted transversely across the housing and such that the valve members may hinge about the rod to cover or uncover corresponding D shaped fluid inlet openings formed within an end wall of the tubular housing in response to varying fluid flow through the valve. Said rod also supports a coiled metal spring which is employed to ensure that the valve member pair are caused to be hingeingly biased from an open position to a closed position and cover said fluid inlet openings the moment that fluid flow through the valve ceases thereby allowing fluid flow in one direction but preventing fluid flow in the opposite direction. SUMMARY. [0003] Dual plate check valves are in common use but suffer from several disadvantages which can compromise valve performance and cause valve failure. [0004] A common problem with these constructions is that they are totally reliant on the action of the spring which due to the process of coiling begins service in a stressed condition and is prone to failure with the result that the valve fails to close. [0005] A further problem with these constructions is that the valve is frequently required to remain in the open position for extensive periods with the result that 2 over time fine solid particles and salts present in the fluid can build up in the hinge clearances of the valve plates thereby compromising their free hinging action and preventing them from hinging to the closed position when fluid flow through the valve ceases resulting in valve failure. [0006] A further problem with these constructions is that the hinge point of the valve plates is by necessity of construction located some distance downstream from the fluid inlet openings causing the heel of the valve plates to protrude out into the flow stream when biased to the open position. This generates a catchment point for contaminants such as fibrous matter, plastic bags etc present in some fluids which can become caught and prevent the valve from closing. [0007] Accordingly it is the object of the present invention to overcome the aforementioned problems. BREIF DESCRIPTION OF THE DRAWINGS. [0008] The invention will be more fully understood by reference to the following description when read in conjunction with the following drawings. [0009] Figure 1 is an elevation of the valve viewed from upstream when in the fully open position; [0010] Figure 2 is an elevation of the valve viewed from downstream when in the fully closed position; [0011] Figure 3 is a sectional view through A-A of figure 1 when in the fully closed position; [0012] Figure 4 is an enlarged part sectional view through B-B of figure 2 showing the valve member biased to the partly open position; [0013] Figure 5 is an enlarged part sectional view through B-B of figure 2 showing the valve member biased to the fully open position; 3 DESCRIPTION OF AN EMBODIMENT OF THE INVENTION. [0014] Accordingly the invention resides in a dual plate type check valve comprising a relatively short tubular housing 1 adapted to be mounted in a pipeline and provided with two fluid inlets 2 each configured in the form of the capital letter D (as shown in Figure 1 ) formed in an upstream end wall of the housing 1, a fluid outlet 3 at the downstream end of the housing 1, a landing or valve seat 4 formed on the downstream face of the end wall of the housing 1, a valve member 5 mounted in sealing engagement with the landing or seat 4 and retained in place by a retaining member 6 and screws 7, affixed to a bridge 8 formed between the two fluid inlets 2. [0015] Said valve member 5 being made from an elastomeric material such as rubber is provided with a co-axial groove 9 within which the retaining member 6 is mounted. Two metal reinforcing plates 10 each configured in the form of the capital letter D are encapsulated within the valve member 5 to form two reinforced sections 15 to the valve member 5 and arranged such that the reinforced sections 15 of the valve member 5 sealingly overlay the corresponding D shaped fluid inlet openings 2 formed in the upstream end wall of the housing 1 and such that the reinforced sections 15 of valve member 5 may flex about the retaining member 6 similar to the wing action of a butterfly, to uncover and cover the fluid inlets 2 when influenced by increasing or decreasing fluid flow through the valve and by the reaction of the distorted elastomeric section within the flexural point of the valve member 5 adjacent to the retaining member 6. [0016] The shape and dimensions of the groove 9 and retaining member 6 are proportioned such that progressive inward flexing ( or biasing) of the reinforced sections 15 of the valve member 5 causes the opposing inner side faces 11 of the groove 9 to abut the retaining member 6 (as shown in Figure 4) which then forms a fulcrum point 12 about which the reinforced sections 15 of the valve member 5 may pivot causing the elastomeric material between the fulcrum point 12 and the retaining member 6 to stretch as flexing (or biasing) progresses thereby increasing the elastomeric tension over and above that which would have been caused by flexure alone.
4 [0017] The effect of the above described pivoting action is to increase the return biasing force to the reinforced sections 15 of valve member 5 via elastomeric tension and thereby avoids the use of metal springs with their associated problems. [0018] The groove 9, the retaining member 6 and the bridge 8 are crafted such that when the valve is in a fully open position (as shown in Figure 5) the reinforced sections 15 of the valve member 5 lay substantially parallel to the flow stream and when viewed from upstream (as shown in Figure 1) are confined within the outline of the bridge 8 thereby avoiding any protrusion of the reinforced sections 15 of valve member 5 into the flowstream thereby preventing the formation of a contaminates catchment point. [0019] accordingly the herein disclosed construction provides a dual plate type check valve which effectively avoids the herein before described problems associated with prior art constructions.