Abstract:
A backflow prevention assembly has a unitary housing with a passage therethrough, two threaded regions formed in an inner surface of the unitary housing, and two valve assemblies positioned in the passage to prevent fluid flow in a direction. Each valve assembly has an annular mount having a threaded outer surface that engages one of the threaded regions on the inner surface of the housing. The threads can be formed by placing a die having a thread on an outer surface thereof in contact with an inner surface of the unitary housing, and pressing on the outer surface of the housing.

Description:
This application is a continuation-in-part of U.S. application Ser. No. 09/454,680, filed Dec. 3, 1999, U.S. Pat. No. 6,192,933, entitled Backflow Prevention Assembly. 
    
    
     BACKGROUND 
     This invention relates to a backflow preventer. 
     Backflow preventers are principally used to prevent contamination of a public water distribution system by reducing or eliminating backflow or back-siphonage of contaminated water into the system. Usually, the backflow prevention assembly is installed in a pipeline between a main supply line and a service line that feeds an installation, e.g., a hotel factory or other institution, or even a multiple or single family residence. A backflow prevention assembly typically includes two check valves that are configured to permit fluid flow only in the direction from the main supply line to the service line. 
     FIG. 1 illustrates a conventional connection between a valve module  40 ′ and a housing  20 ′ of a prior art backflow preventer. As shown, an annular mount  90  having an threaded inner surface  92  and a smooth outer surface  94  is secured to a cylindrical inner surface  96  of housing  20 ′, e.g., by solder  98  or welding  99 . The valve module  40 ′ is then screwed into mount  90  to secure it to the housing. 
     SUMMARY 
     In one aspect the invention is directed to a backflow prevention assembly. The backflow prevention assembly has a unitary housing with a passage therethrough, two threaded regions formed in an inner surface of the unitary housing, and two valve assemblies positioned in the passage to prevent fluid flow in a direction. Each valve assembly has an annular mount having a threaded outer surface that engages one of the threaded regions on the inner surface of the housing. 
     Implementations of the invention may include the following features. The two valve assemblies may be cam-check valves. The threaded regions may be formed by a thread rolling process, e.g., by placing a threaded die against the inner surface of the housing and pressing on an outer surface of the housing. Threaded regions may also be formed in the outer surface of the unitary housing. 
     In another aspect the invention is directed to a method of forming threads in a tubular housing. In the method, a die having a thread on an outer surface thereof is placed in contact with an inner surface of a unitary housing having a passage therethrough. The outside of the housing is pressed to drive the inner surface against the threads on the outer surface of the die to form threads on the inner surface of the housing. 
     Implementations of the invention may include the following features. The thread on the outer surface of the first die may be generally the inverted shape of the threads to be formed on the inner surface of the housing. A second die having a thread formed on an outer surface thereon may be used to press on the outside of the housing. Two valve assemblies may be positioned in the passage to prevent fluid flow in a direction, each valve assembly having an annular mount having a threaded outer surface that engages one of the threaded regions on the inner surface of the housing. A port may be formed in a wall of the housing. 
     Advantages of the invention may include the following. The backflow prevention assembly is easier to assemble, e.g., in the field, and is less expensive to manufacture than currently available backflow prevention assemblies for similar applications. 
     Other features and advantages of the invention will become apparent from the following detailed description including the drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a conventional connection between a check valve and a housing in a prior art backflow preventer; 
     FIG. 2 is a cross-sectional view of a backflow prevention assembly according to an embodiment of the present invention; 
     FIG. 3 is a cross-sectional view of a check valve from the backflow preventer of FIG. 2; 
     FIG. 4 is a cross-sectional side view illustrating the formation of the threads in the housing of the backflow preventer; 
     FIG. 5 is a view along line  5 - 5  of FIG. 4; 
     FIG. 6 is an expanded view of the connection of the check valve to the housing in the backflow preventer of FIG. 2; and 
     FIG. 7 is cross-sectional view of a backflow prevention assembly according to an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 2, a backflow prevention assembly  10  includes a unitary tubular housing  20  having a first end  22  and a second end  24 . A through bore  26  extends between the ends  22 ,  24  of housing  20  for the flow of water therethrough. A pair of valve modules  40 ,  60  are located inside through bore  26  to permit fluid flow from the first end  22  of housing  20  to the second end  24  of housing  20 , while preventing fluid flow in the opposite direction. An inner surface  28  of tubular housing  20  includes two threaded annular regions  30 ,  32  with internal threads  34 ,  36 . 
     Valve module  40  may be a cam-check valve. e.g., Check Assembly Kit No. RK-775CK1 available from Watts Industries. North Andover, Mass., although the invention is applicable to other valves, such as a ball valve. As shown in FIG. 3, cam-check valve  40  includes an annular seat  42 , a clapper  44  pivotally connected to annular seat  42 , and a cam arm  46  to prevent over-extension of clapper  44 . An outer surface  48  of seat  42  includes external threads  54  and an annular recess  50  to secure an O-ring  52  near an end  56  of the seat adjacent clapper  44 . When valve module  40  is inserted into through bore  26 , external threads  54  on seat  42  cooperate with threads  34  in threaded annular region  30  to secure valve module  40  in housing  20 . In addition. O-ring  52  is secured between outer surface  48  of seat  42  and housing  20  to prevent fluid leakage therebetween. A similar cam-check valve, lacking external threads and an annular recess for an O-ring on the outer surface of the seat, is described in U.S. Pat. No. 5,855,224, the entirety of which is incorporated herein by reference. 
     Returning to FIG. 2, valve module  60  can also be a cam-check valve, and can be constructed in a fashion similar to valve module  40  with an annular seat  62 , a clapper pivotally connected to annular seat  62 , and a cam arm to prevent over-extension of clapper. An outer surface of the seat includes external threads  64  and an annular recess  66  to hold an O-ring  68 . External threads  64  cooperate with internal threads  36  to secure valve module  60  in housing  20 . Like valve module  60 . O-ring  68  is secured between the outer surface of seat  62  and housing  20  to prevent fluid leakage therebetween. However, in valve module  60 , annular recess  66  can be located near an end  58  of seat  62  opposite clapper  60 . 
     Housing  20  includes a port  70  which provides access to bore  26  and valve modules  40  and  60 . Port  70  is closed by securing a cover plate  72  to a rim  74  with a groove coupler  76  and a groove coupler gasket  78 . 
     Referring to FIGS. 4 and 5, threads  34 ,  36  are formed in housing  20  by a thread rolling process that uses a threaded internal die  80 . Internal die  80  has threads  82  that are the inverse shape of the threads to be formed on housing  20 . Unlike conventional thread rolling processes, in which a die with non-inverted threads is held against the outside surface of a housing, in this method, internal die  80  is held against inside surface  28  of housing  20 . Specifically, housing  20  begins with an inner diameter D 1  that is slightly larger than the major diameter D 2  of threads  82  on internal die  80 . The threaded internal die  80  is inserted into through bore  26 , and external die  84  are positioned around an outer surface  29  of tubular housing  20 . The external die force a portion  30  of tubular housing  20  inwardly to mold inner surface  28  of housing  20  to internal die  80 , thereby forming threads  34 . Alternatively, tubular housing  20  can be held steady and internal die  80  can be moved in an orbit as it is pressed against the inner surface of the housing. In either case, the threads penetrate the blank inner surface to form the thread roots and displace material radially outward to form the die crests. Then the internal die is unscrewed from threaded portion  30  to remove the internal die from the housing. This thread rolling process is then repeated to form threads  36  in threaded portion  32  of housing  20 . Alternatively, two die could be inserted into the through bore, and threaded portions  30  and  32  could be formed simultaneously. In addition, the external die  84  can have a thread  86  formed on its outer surface to force the material of the housing into the gaps between threads  82  on internal die  80 , thereby also forming threads on outer surface  29 . The port  70  could be formed in housing  20  before or after threaded portions  30  and  32 . 
     Since the inner surface of the housing directly contacts the die, the threads more closely match the shape intended by the die than if a die with non-inverted threads was pressed against the outside surface of the housing. In contrast to other thread forming processes, such as grinding, thread rolling does not remove metal. In addition, the cold forming process can strengthen the threads by work hardening and form reinforcement. 
     As previously discussed, in the conventional backflow preventer shown in FIG. 1, the threads are formed in a mount that is soldered to the housing. In contrast, as shown in FIG. 6, threaded portions  30  and  32  are formed directly on inner surface  28  of unitary housing  20 , and valve module  40  is secured directly to housing  20 , without an intervening mount or soldering, thereby decreasing the time and expense in constructing the backflow prevention assembly  10 . 
     Referring to FIG. 7, in an alternative embodiment, an n-shaped backflow prevention assembly  100  includes a unitary tubular housing  110  having a first end  112  and a second end  114 . Attached to end  112  of housing  110  is a first member  116 , and attached to end  114  of housing  110  is a second member  118 . Members  116 ,  118  have open ends  120 ,  122 , respectively. Member  116 , unitary tubular housing  110 , and member  118  define an n-shaped throughbore  123  for the flow of water therethrough. Tubular housing  110  has an inner surface  124 , including threaded annular regions  126 ,  128  in the vicinity of ends  112 ,  114 , respectively, formed as described above. 
     Member  116  is formed as follows. Pipe  130  has an end  132  which is flared to fit over housing end  112 . Pipe  130  is cut at  134  at 90 degrees to form two lengths of pipe  136 ,  138 . The second length of pipe  138  is welded to first length  136  to form a 90-degree bend. Second length of pipe  138  has an end  140  to which an extension  142  is welded. Extension  142  has an end  144  to which a flange  146  is welded. Member  118  is formed in the same manner, with a flange  148  being welded to an end  122  of a second extension  150 . Members  116 ,  118  are welded to  110  at ends  112 ,  114 . 
     As described above, two valves (not shown) are attached to tubular housing  110  at threaded annular regions  126 ,  128  by passing valves through a port  152 . 
     Other embodiments are within the scope of the claims.