According to this invention there is provided a proportioning valve providing a first flow passage extending from an inlet to an outlet, a first valve element resiliently loaded towards a position in which it closes off flow through the first flow passage against the pressure of fluid flowing along the passage, a second flow passage opening to the first flow passage downstream of the first valve element, a second valve member coupled to the first valve member to move therewith and controlling the area of communication between the second and first flow passages which area is zero when the first valve element is in its closed position and increases progressively with progressive opening movement of the first valve member, wherein the second flow passage terminates in a cylinder the wall of which has therein at least one slot extending lengthwise of the cylinder and opening to the first flow passage downstream of the first valve element, the second valve member being slidably mounted in said cylinder, and the arrangement being such that axial movement of the second valve member varies the effective length of the at least one slot and the area of communication between the second and first flow passages.

The proportioning valve is intended to be used in combination with means for supplying respective fluids to the first and second flow passages at the same pressure. In a preferred arrangement, the cylinder

is disposed coaxially within the first flow passage

Means may be provided for varying the effective width of the slots along its length. Such means may comprise an adjusting member, e.g. a sleeve encircling the cylinder, which is movable circumferentially of the cylinder to blank off part of the width of the slots.

One embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows in axial section a proportioning valve according to the invention, and

Figure 2 shows diagrammatically a known device for equalizing the supply pressures of two fluids.

Referring to Figure 1 , the proportioning valve 10 comprises a casting which provides a first or mam through-passage 11. The casting includes a hollow portion 12 which extends radially of the passage 11, and the inner end part 13 of which extends along the axis of the passage 11 towards the downstream end of the passage. The annular downstream end piece 13a of the part 13 is formed separately from the casting and has its upstream end screw-threaded into the part 13. End piece 13a has an axial slot 14 cut m its peripheral wall and extending upstream from the downstream end of the end-piece to a predetermined position. A valve member 15 in the form of a piston is slidably mounted in the bore of the end-piece 13a and is mounted on one end of a rod 16 extending along the inner end part 13 and through a boss 17 at the upstream end of the part 13. A primary valve member 18 in the form of a disc is secured on the upstream end of the rod 16. Valve member 18 is a sliding sealing fit in a primary valve aperture 22 formed in the casting and is biased into a position closing the primary valve

aperture by a compression spring 19 encircling the boss 17, one end of the spring being seated adjacent the part 13 and the other end abutting the valve member. When the valve member 18 is in its position in which the primary flow in the direction of the arrow 20 is just cut off, the secondary valve piston is in a position abutting an annular shoulder 21 in the end- piece and just obstructing the upstream end of the slot 14. in operation of the valve, primary and secondary liquids are fed at the same pressure into the upstream end of the through-passage 11 and into the outer end of the hollow portion 12, 13a respectively. The pressure of the primary fluid, e.g. water, on the primary member 18 causes the rod to slide, overcoming the force of the spring 19 and causing the piston of the secondary valve to open the upstream end of the slot and permit secondary fluid, e.g. a foaming agent, to flow into the passage 11 to mix with the water for delivery to e.g. a fire-fighting point. The width of the slot 14 is determined according to the viscosity of the secondary fluid and may vary along the length of the slot to match the minimum cross-section of the flow through the primary valve at each valve opening. Also, it is envisaged that more than one slot may be provided in the end piece 13a.

Where different secondary fluids are to be used or when it is required to alter the concentration of the secondary fluid, different end-pieces 13a may conveniently be employed having different slot widths . Alternatively the end piece 13a may have an encircling sleeve with an axial slot which is wider than slot 14, so that by adjusting the rotational position of the sleeve about the end-piece, part of the width of the slot 14 in the end-piece can be blanked off.

One known apparatus for ensuring that the pressures of the primary and secondary liquids are equal is shown in Figure 2 to which reference is now made. The apparatus comprises a rigid outer container 25 which is connected by a pipe 26 to the water conduit 27 at a point upstream of the proportioning valve 10. Within the container is a flexible bag 28 having open ends at the top and bottom which are sealingly secured to closure members of the container. A perforated tube 29 is disposed within the bag and its lower end extends through the bottom container closure and communicates through a line 30 with the inlet for the foam concentrate on the proportioning valve. Thus all fluctuations in the water pressure flowing to the inlet end of the hollow portion 12 of the proportioning valve are transmitted to the container 25 and to the bag 28, causing corresponding fluctuations in the pressure of the foam concentrate supplied to the inlet of portion 12.