BACKGROUND OF THE INVENTION 
     This invention relates to closed water systems and more specifically to an improved diaphragm assembly as used in an expansion tank forming a part of the system. 
     Expansion tanks, as used in domestic water systems, provide an air surge chamber that accounts for variations in pressure within the system. Tanks of this type are divided into chambers by a flexible diaphragm wherein gas under pressure is contained in one chamber while system water is contained in the other. The water chamber is connected to the system and changes in pressure are created by the increase and decrease associated with water volume usage in the normal cyclic operation of the system. Expansion tanks of this general type are described in U.S. Pat. Nos. 2,695,753 and 3,035,614 to C H Kirk, Jr., and 2,815,152 to L E Mills. 
     Recent improvements in tank design include glass or plastic liners that makes assembly of the tanks as taught by the prior art, more difficult, especially as it relates to positioning and sealing of a flexible diaphragm within the tank and closure of the tank without damaging the diaphragm seal and/or liner. 
     Therefore, it is a primary object of this invention to provide a diaphragm assembly that is easily installed, facilitates tank assembly, and provides a positive seal in either lined or unlined expansion tanks. 
     SUMMARY OF THE INVENTION 
     According to this invention, the beforementioned and related objects are accomplished in an assembly comprising a flexible elastomeric diaphragm having a laterally projecting peripheral protrusion, said diaphragm positioned within an expansion tank such that the protrusion deforms in sealing engagement with the tank wall, and a continuous backup ring positioned on the inner surface of the diaphragm in the area of the protrusion having a diameter such that the diaphragm elastomer is in compression when the assembly is installed in the tank. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The features of the invention may best be understood from a consideration of the following description taken in conjunction with the accompanying drawing, in the several figures of which like reference numerals identify like elements, and in which: 
     FIG. 1 is an elevational view, in section, showing the features embodying the invention; 
     FIG. 2 is an enlarged section through the peripheral edge of the diaphragm/ring assembly; 
     FIG. 3 is an enlarged section through the tank and diaphragm/ring assembly showing the sealing relationship therebetween; and 
     FIG. 4 is an enlarged section through the peripheral edge of a second embodiment of the assembly showing its sealing relationship with the tank wall and liner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawing, FIG. 1 illustrates a conventional expansion tank generally indicated by reference numeral 10. Tank 10 includes a substantially cylindrical body 12 and end walls 14 and 16, each of which contain connections 18 and 20 therein for communication to sources of air and water (not shown) respectively. 
     A cylindrical liner 22 is shown installed in the tank, however, its use is incidental and not a requirement in the application of the present invention. This will become clear as the description proceeds. 
     Installed within the tank 10 is an assembly generally indicated by numeral 30 which comprises a substantially flexible elastomeric diaphragm 32 and a rigid backing ring 34. The diaphragm 32 divides the tank 10 into two non-communicating chambers generally indicated by reference numerals 36 and 38, the sizes of which may be determined by the axial dimensions of the diaphragm. Obviously, a shallow diaphragm will restrict the size of chamber 36 while a deeper diaphragm will allow for an enlarged chamber area 36. In either case, the amount of water in the chamber 38 will be a determining factor on the axial excursion of the diaphragm 32. 
     FIGS. 2 and 3 more clearly illustrate the features of one embodiment of the invention as it relates to the diaphragm/backing-ring assembly. The bead or peripheral portion 32a of the diaphragm 32 comprises annular outward projecting wedge-shaped fins 40a and 40b having concave surfaces 42a and 42b and straight-tapered surfaces 44a and 44b. The relaxed diameter of the inside peripheral surface of the diaphragm is slightly less than the outside diameter of the backing ring 34 such that it must be stretched over the ring until it abuts an outwardly protruding flange 34a. The greatest diameter of the diaphragm is reached in its mounted position on the ring 34 and is located at the outward projection of the fins 40a or 40b. This diameter is slightly greater than the inside diameter of the tank walls 12. 
     Mounting of the diaphragm-ring assembly 30 into the tank 12 is accomplished by inserting the closed end of the diaphragm into the tank and applying force to the ring flange 34a at various locations around its periphery and in the direction of arrow A shown in FIG. 3. As the open end of the diaphragm enters the tank, the fins 40a-b are deformed in the direction of their concave surfaces and tend to follow the inside surface contour of the tank walls. In this condition, the fins provide positive sealing engagement with the tank walls and also maintain seating of the diaphragm by virtue of the resistance to movement in a direction opposing arrow A. When a glass or plastic liner 22 is installed in the tank, the fins deform to the inner diameter of both tank wall and liner, thus providing a leakproof seal for both surfaces and installation may be made without thought of breaking or chipping the liner. For extremely heavy gauge liners fin 40b may be of a lesser diameter than fin 40a. In this arrangement the movement of the assembly in the easy direction of arrow A is checked by the extreme compression of fin 40a when moved into the liner area, and in the opposite direction by the resistance of the fins to flip over toward their straight tapered surfaces. Various additional features may be added to the basic concept that provide ease of locating and positive seating of the assembly 30 within the tank. For example, as illustrated in FIG. 2, the backing ring 34 may include one or more outwardly projecting depressions 34b. These may be in the form of annular ribs or else multiple dimples formed in the ring 34. Their purpose is to restrict movement of the diaphragm in the easy direction of arrow A such that separation of the diaphragm and backing ring does not occur. 
     An alternate embodiment may further provide inwardly projecting depressions 12a in the tank walls as illustrated in FIG. 3. The depth of these inward projecting ribs or dimples 12a will be such to preclude further excursion of the diaphragm in the direction of arrow A by the fin 40a. Obviously, such depressions will also increase the difficulty of movement in the opposite direction. In some conventional tanks inwardly projecting ribs are formed by reason of the manner of fabricating the tank. In these, the tank is formed of two or more sections that have overlapping flanges which are welded to form a leakproof seam. The inward flange then acts in the stead of depressions 12a. An advantage of the present diaphragm assembly to this type tank is readily apparent. Because the diaphragm seals at the fins, these may be spaced apart such that the diaphragm bridges the area to be welded and the heat from the welding operation will have less deleterious effect on the elastomer. 
     A further embodiment of the present invention is shown in FIG. 4, wherein the peripheral outward facing protrusion on the diaphragm 32 comprises laterally projecting circular-section rings 46a and 46b. The rings are molded on the outer peripheral edge of the diaphragm 32 in a similar manner as the fins 40a-b and will effect an O-ring type sealing engagement with the tank walls. However, in this embodiment, movement of the diaphragm in either direction may be caused by extreme pressure differentials and restraining ribs 12a in the tank wall and 34b in the backup ring may be required to establish a permanent location within the tank to maintain required chamber volume for air and water. In all other respects, the embodiment of FIG. 4 exhibits the advantages as described for the fins of FIGS. 2 and 3. 
     The invention offers marked advantages over the prior art in fabricating the expansion tank. First, swaging, crimping, cold-forming, or other specialized operations are not necessary to complete installation of the diaphragm. This not only results in a savings in manufacturing costs but also eliminates breakage problems involved with tanks having glass or plastic liners. Secondly, tolerances do not pose problems since the ring/diaphragm dimensions are not critical as long as the peripheral protrusions are sufficiently deformed by the backup ring to effect a sealing relationship with the tank walls. And third, more flexibility is offered in tailoring the diaphragm/ring assembly to the tank rather than visa versa. 
     While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.
An expansion tank is divided into two non-communicating chambers by a flexible elastomeric diaphragm having a peripheral outward projecting, sidewall engaging protrusion that is held in compressed and sealing engagement with the tank walls by a backup ring. The diaphragm-ring assembly facilitates insertion of the diaphragm, provides a positive seal, and prevents the assembly from being forced out of position by extreme pressure differentials.