This invention relates to valves, particularly, although not exclusively, to flood-closable valves such as tidal outlet valves. BACKGROUND OF THE ART

An outfal ^' l discharging into a body of water such as a river or the sea is conventionally provided wifh a flap valve, to prevent back-flow through the outfall when the water level of the river or sea is higher than the outlet of the outfall. However, such flap valves frequently become fouled, with the result that they fail to close properly. DISCLOSURE OF THE INVENTION

According to one aspect of the present invention there is provided a valve comprising a chamber having an inlet and an outlet, one wall of the chamber comprising a diaphragm which is movable into contact with a seat provided in the chamber to prevent commτιnication between the inlet and the outlet, characterized in that the diaphragm is exposed to the ambient medium whereby the condition of the valve is dependent on the ambient pressure

A preferred embodiment in accordance with the present invention provides a lood-closable valve comprising a chamber having an inlet and an outlet, one wall of the chamber comprising a diaphragm which is movable into contact with a seat provided in the

chamber , to prevent communication between the inlet and the outlet, characterized in that the diaphragm, in use of the valve,, is disposed horizontally with its upper surface exposed to the ambient medium, and in that movement of the diaphragm into contact with the seat takes place in the downwards direction.

When, for example, the lood-closable valve is used as a tidal outlet valve, it may be open at low tide, when the level of the sea is below the diaphragm, but closed at high tide, when the level of the sea is above the diaphragm and the hydrostatic pressure of the sea water presses the diaphragm into contact with the seat.

The valve may be provided with .means for biasing the diaphragm towards its closed position. Such means may, for example, comprise a spring or, particularly where the valve is a flood-closable valve, a weight attached to the diaphragm.

'The chamber may be cylindrical, the diaphragm then being circular and affording one end wall of the cylinder. With such a construction, the outlet may be circular, and may be disposed coaxially with the chamber in the end wall of the chamber opposite the diaphragm. The inlet may comprise a duct which extends radially of the chamber. Alternatively, it may be desirable for the duct to extend tangentially of the chamber so as to promote swirl within the chamber.

The seat may comprise an annular rib surrounding the outlet. The diaphragm may be provided with an annular projection which, when the diaphragm is closed, engages the seat to provide a reliable seal. BRIEF DESCRIPTION ■ ^• OF DRAWINGS

Figure 1 is a sectional view through a tidal outlet valve;

Figure 2 is a plan view of the valve of Figure 1;

Figure 3 is a sectional view showing an alternative

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construction for a tidal outlet valve; and

Figures 4A, 4B, 5A and 5B show diagrammatically two further constructions of tidal outlet valves. MODES FOR CARRYING OUT THE INVENTION The valve of Figures 1 and 2 comprises a housing

2 defining a chamber 4 haying an inlet 6 and an outlet 8. A diaphragm 10 is clamped to the housing 2 by a ring 12. The diaphragm is made, for example, of rubber or neoprene. Tihe housing is in the form of a flat cylinder, having a cylindrical wall 14 and a bottom wall 16, to which the outlet 8 is secured. An annular rib 9 surrounds the outlet 8 and serves as a seat for the diaphragm 10. The diaphragm 10 is circular, and the outlet 8 is coaxial with the housing 2. The inlet 6 is connected to the cylindrical wall 14 and extends tangentially of the housing 2.

A circular stainless steel weight 18, which is slightly larger in diameter than the outlet 8,_is fixed to the diaphragm 10.

In use of the valve on an outfall discharging into tidal waters the inlet 6 is connected to the outfall pipe, and the valve is disposed with the diaphragm 10 horizontal, at a level between the high and low water levels. When the tide is out, the valve is above water level. . '.The water in the outfall exerts a pressure head at the inlet 6 of, for example, 3 metres, and the weight 18 is selected such that this pressure head is sufficient to lift the diaphragm 10 off the seat 9, to permit flow from the inlet 6 through the outlet 8 to the sea.

Should the pressure head at the inlet 6 drop below, for example 1 metre, the weight 18 will be sufficient to overcome this pressure to bring the diaphragm 10 into contact with the seat 9, so closing the valve until the pressure head at the inlet 6 has built up again to a sufficient value to cause the. valve to open. As the tide ccracs in, , - the level of the sea. will rise

above the valve. Assuming the valve is open, it will stay open until the total downward force on the diaphragm 10, resulting from the hydrostatic pressure of the rising water and the weight 18, exceeds the force applied by 5 the pressure head at the inlet 6. Assuming that the weight 18 is equivalent to a pressure head of approximately 1 metre and that the pressure head at the ^" inlet 6 is approximately 3 metres, the valve-will- close when the water level is approximately 2 metres above the 1° diaphragm 10. Once the yalve is closed, the area of the diaphragm 10 over which the pressure head at the inlet 6 is effective is reduced by the area of the outlet, thus reducing the tendency of the valve to re-open. The valve will thus remain ^' firmly closed

^■■ without fluctuating between the open and closed positions unless the effect of the pressure head ar τne inlet 6 acting on the restricted area of the diaphragm 10 exceeds th.e combined effect: of the hydrostatic pressure of the sea water and the weight. - Thus the valve.. ill open

20 only when the pressure head at- the inlet' is sufficiently high to overcome the pressure'head at the -outlet 8, so preventing backflow into the outfall pipe. When the tide falls again, the valve will remain closed until the combined effect of the hydrostatic pressure' of the sea

25 water and the weight falls below-the effect of the pressure head s c the inlet 6 acting on the reduced area of the diaphragm.

The characteristics of the valve can be modified to suit the desired operation, for example by suitable.

30 selection of the weight 18 and of the ratio of the diameters of the outlet 8 and the housing 2.

Figure 3 shows a modified construction for the valve, in which the parts are designated by the same reference numerals as the corresponding parts in the

35 embodiment of Figures 1 and 2. The housing 2 has a smooth, continuously curved contour, which avoids excessive stresses in the diaphragm 10. The diaphragm

10 is provided with a lip 20 which engages over the periphery of the housing 2 to secure the diaphragm to the housing 2. In the embodiment of Figure 3, the inlet 6 extends radially of the housing 2 and the outlet 8. The valve of Figure 3 operates in the same way as that of Figures 1 and 2.

The valve of Figures 4A and 4B has an inlet 6 which extends radially of the housing 2. The lower wall of the inlet 6 is at the same level as the lower wall of the housing 2, thus avoiding accumulation of

" de _b ris, which could occur, in the constructions of Figures 1 to 3, where the inlets 6 extend below the bottom of the housing 2.

In the valve of Figures 5A and 5B, the inlet 6 is wholly above the bottom of the housing 2.

Furt _h ermore, the housing 2 is dished. As a result, the diaphragm 10 can seal, separately, the ends of both the inlet 6 and the outlet 8. INDUSTRIAL APPLICABILITY As described, a principal use of valves in accordance with the present .invention is as tidal outlet valves controlling discharge into tidal waters such as the sea. However, the valves could also be used in outfalls which discharge, for example, into non-tidal ^" .rivers, where the purpose would be to prevent backflow through the outfall when the river was in flood. It will be appreciated that there are numerous other uses for the valve of the present invention. Fo ^' r example, the valve could be used as an outlet valve for an aeration system in a slurry tank, serving to prevent backflow of slurry into the aeration system when the air supply was discontinued. The valve could also be used to maintain a constant level of liquid in a bulk storage tank. As liquid is drawn from the tank, and the level drops, the valve would open ^" to admit further liquid in response to the - drop in hydrostatic pressure at the diaphragm 10.