BACKFLOW PREVEOTER VALVE
This invention relates to a valve assembly. In particular this invention is directed to a valve assembly suitable for use as a reduced pressure zone backflow preventer for water supply mains to prevent backflow and siphonage of water in the event of mains pressure decrease o failure and the invention will be described hereinafter in this context. However it is envisaged that valve assemblies in accordance with the present invention may find use in other applications such as to prevent backflow of any fluid conveyed under pressure in a conduit.
Many municipal authorities now specify that certain businesses such as medical and veterinary services and hairdressers must fit backflow preventers to avoid the possibility of contamination of the mains by backflow in the event of mains pressure failure. The standards applied to such apparatus specify that for moderate hazard applications a backflow preventer having two independent check valves be installed, and that the space between the check valves be drainable. The industry standard backflow preventer achieve this specification by the use of a reduced pressure zone between the check valves which vents to the atmosphere upon mains pressure failure.
One such backflow preventer is that described in Australian Patent Application No. 82433/87. The apparatus disclosed in this application comprises a pair of check valves of the vertical lift type connected in series with th mains supply and defining a reduced pressure zone therebetween. In parallel with the mains is a hydraulic cylinder having a piston mounted therein, one side of the piston being in fluid communication with the mains upstream of the check valves and the other side of the piston being i fluid communication with the reduced pressure zone. The piston is spring biased towards the mains pressure side. In use, the mains pressure drives the piston to close the fluid
communication with the reduced pressure zone. In the event of a mains pressure failure, the spring drives the piston away from the end of the hydraulic cylinder, placing the cylinder in fluid communication with the reduced pressure zone. A drain hole provided in the cylinder and adapted to be closed by the piston under normal mains pressure conditions and opened to the atmosphere when the mains pressure fails causes the pressure in the reduced pressure zone to drop to ambient atmospheric pressure and allows any traces of possibly contaminated water to drain from the zone. Other similar backflow preventers utilizing a diaphragm in place of the piston described above are also in use. Disadvantages of the prior art backflow preventers include the large number of pipe joins required to assemble the apparatus. This increases the likelihood of leaks as well as increasing the amount of plumbing and hence the installed cost of the apparatus. In addition, the prior art apparatus includes check valves of a type not inherently adapted to promote efficient flow characteristics of the apparatus. The usual type of check valve is of the spring biased poppet type wherein water at pressure is applied to the lower face of the valve poppet, causing the valve to lift and water to pass. This type of valve necessarily involves a bend in the water flow path which causes drag and reduces efficiency. s
A third disadvantage lies in the bulk of the apparatus. The use of external check valves and a separate operating mechanism for pressure control in the reduced pressure zone results in the application of the size constraints of conventional plumbing fittings used to assemble the apparatus.
It is thus an object of the present invention to provide a backflow preventer which substantially overcomes the deficiencies of the prior art. It is a further object of the present invention to
provide a unitary reduced pressure zone backflow preventer.
Accordingly, in one aspect, the present invention resides in a valve assembly including:- a valve body having a chamber; respective inlet and outlet fluid connections in said valve body and communicating with said chamber whereby said valve body may be installed in-line in a fluid supply conduit; a vent from said chamber; a displaceable pressure sensitive closure assembly supported in said chamber and having a non-return valve therein through which fluid may flow from said inlet through said chamber to said outlet, said closure assembly being biassed to an open position with respect to said vent whereby fluid may pass from the portion of said chamber downstream of said closure assembly to said vent, and said closure assembly being displaced to a closed position from said open position to close said vent upon application of a predetermined fluid pressure at said inlet. Preferably, the valve chamber is a substantially cylindrical chamber and the closure assembly is axially moveable in the chamber between vent-open and vent-closed positions. The respective inlet and outlet may take the form of end caps provided on the cylindrical chamber and provided with threading or other means of attachment to the fluid conduit.
The pressure sensitive closure assembly may take any form consistent with the function of opening and closing the vent. Preferably, at least part of the cylindrical chamber is configured as a hydraulic cylinder and the closure assembly is a piston assembly axially slidable in the chamber between vent-open and vent-closed positions. Alternatively, the closure assembly may take the form of a diaphragm disposed in the chamber. The preferred piston is preferably spring biassed to the
vent-open position, the spring bias being such that the piston moves to close the vent at a lower pressure than that required to open the non-return valve. This ensures that the vent is fully closed prior to fluid flowing through the non- return valve and thereby avoiding unnecessary loss of fluid. Preferably, the piston assembly includes a piston having an apertured piston head and said non-return valve includes a valve member spring biassed into sealing engagement with said apertured piston head. Preferably, the non-return valve is disposed substantially centrally in closure assembly in order to maximise the flow efficiency of the assembly. The non-return valve is preferably of the self centering type and adapted to operate axially of the preferred cylindrical chamber. One suitable non-return valve comprises a spring biassed, substantially hemispherical valve member adapted to sealably engage the piston at an annular seal provided in the piston. The non-return valve member may be spring biassed within the piston itself. However, it is preferred to spring bias the valve member by interposing a biassing spring between the valve member and a spring support fixed in relation to the cylindrical chamber.
Preferably, the vent comprises an annular space about the portion of the chamber downstream of the closure member and in fluid communication with the atmosphere. The closure member is preferably provided with an annular valve seat adapted to cooperate, when closure assembly is disposed in the closed position, with a complementary annular valve seat in the downstream chamber portion to define the abovementioned annular space and confine fluid flow to the transfer passage so defined from the non-return valve to the outlet. When the closure member is in its open position, the preferred annular space opens to the transfer passage and places the same in fluid communication with the atmosphere. Preferably, the vent is configured such that any liquid in
the annular space may freely drain fro the valve assembly. In one preferred embodiment the valve seat is spaced about the non-return valve.
Preferably, the complementary valve seat is disposed about the transfer passage and is disposed within a carrier slidably supported within said cylindrical chamber.
Preferably, there is provided in the transfer passage a second non-return valve (hereinafter a "check valve") for preventing return flow of fluid through outlet to downstream portion of said chamber between the non-return valves.
Preferably, the check valve includes a valve member spring biassed into sealing engagement with the transfer passage. The check valve may be of the same or of different type to the first non-return valve. The space between the check valve and the non-return valve represents a contained zone at a reduced pressure compared to the pressure applied by the fluid entering the inlet.
Preferably, there are provided additional fluid passages communicating with any of said inlet, said outlet, and said chamber, such as may be useful for the attachment of pressure gauges or the like, or for sample tapping or bleeding the valve assembly.
The invention will be further described with reference to the accompanying drawings illustrating a preferred embodiment thereof.
The invention will now be described with reference to preferred embodiments that are illustrated in the accompanying drawing in which:
FIG. 1 is a sectional view of a valve assembly in accordance with the present invention, and
FIG. 2 is a sectional view of an alternate valve assembly in accordance with the present invention.
In the figures, there is provided a valve assembly having a substantially cylindrical housing 10. The housing 10 is provided with an inlet end cap 1, sealed to the housing
by an O-ring 11 and provided with an attachment port 8 for connecting a fluid supply line (not shown) .
Slidably mounted within a cylindrical portion of the housing 10 is a piston 2, which is sealed to the bore of the housing 10 by the action of ϋ-profile seal 12. A piston ring 22 of low friction polymeric material may be provided to reduce piston wear. Centrally located in the piston 2 and sealing a passage therethrough is a non-return valve cone 7 adapted to seal to the piston 2 via U-section seal 14. The cone 7 is urged into sealing engagement with the seal 14 by the action of the spring 4, which has its free end supported on a spider 23. The spider 23 is itself supported in the cylindrical bore of an insert piece 6, the full function of which will hereinafter become apparent. The piston 2 in provided with an annular seal 13 and seal retainer 20 on the downstream face of the piston 2. The insert piece 6 is mounted in the bore of the body 10 and is sealed to the body 10 by O-ring 11. The insert piece 6 is located against a step 27 in the bore of the housing 10. The insert piece 6 has an annular sleeve integral therewith which extends upstream and is adapted to seal against the annular seal 13 of the piston 2 when the piston 2 is advanced towards the insert 6. The insert piece 6 is retained against the step 27 by an outlet end cap 9 sealed to the housing 10 via an O-ring. A seal is made between the end cap 9 and the insert 6 by the action of an O-ring 15. The annular sleeve of the insert piece 6 defines an inner chamber and an outer annular chamber 19. The outer annular chamber is vented to the atmosphere at 24 and contains spring 5 adapted to urge the piston 2 away from the insert 6.
The downstream end of the insert 6 is provided with a central bore fitted with a check valve cone 3 retained against a seal 16 by a spring 17 supported on a spider 23. The spider 23 is mounted in the outlet end cap 9. The seal 16 is retained in the insert 6 by seal retainer 21. The
outlet end cap is also provided with a threaded outlet for attachment to a delivery line to be supplied with fluid.
The volume between the valve cones 3 and 7 is provided with fluid access to annular measurement space 18 formed between the housing wall 10 and an extension sleeve of the outlet end cap 9. This annular measurement space 18 may be tapped to provide a convenient attachment point for a pressure gauge for monitoring the reduced pressure zone of the valve assembly. Similarly, tappings at 25 and 26 are convenient pressure measurement points for measuring the pressure at the inlet and the outlet of the valve assembly respectively. Each of these ancillary outlets may be plugged if not in use or alternatively the outlets may be provided with valves such as ball valves. In use and while the fluid pressure is at its normal level the piston 2 is forced along the bore of the housing 10 until the seal 13 contacts the annular edge of the insert 6. The valve cone 7 will, if pressure is sufficient, permit the flow of fluid past, which fluid then passes the valve cone 3 and hence to the outlet. The action of the valve cone 7 causes the space between the cones 7 and 3 to always be at less than the supply pressure under normal operating circumstances.
In the event of a failure of the supply pressure where backflow or siphonage may occur, the valve cones 3 and 7 prevent any such backflow. A small amount of contaminant may get past the cone 3. However, as the pressure is removed from the upstream face of the piston 2 the spring 5 urges the piston 2 away from the insert 6, opening the transfer passage to the annular space 19 and permitting the drainage of any fluid between the cones 3 and 7 to drain via the discharge opening 24.
The embodiments illustrated have the advantage over the prior art of being a unitary valve assembly with only two joins to the water mains. The present assemblies also
exhibit lower losses compared with comparable prior art apparatus. For example, a prior art preventer was measured to have a pressure drop across the valve of 110 kPa at a flow rate of 40 1/min. whereas the present valve assembly tests at a drop of 50 kPa at 40 1/min.
Whilst the above has been described with reference to a preferred embodiment it will be clear that the many modifications and variations as would occur to a man skilled in the art are within the broad scope and ambit of the invention as defined in the appended claims.