FIELD OF INVENTION 
     The present invention relates to water hammer arresters. Water hammer arresters are devices for preventing the development of water hammer in conduits conveying water under pressure and under conditions of interrupted flow. 
     BACKGROUND OF THE INVENTION 
     The phenomenon of water hammer is caused by the development of hydraulic shock waves generated by the sudden stopping of fluid flow within the confines of a conduit system carrying water under pressure. This condition results from the rapid closing of positive valves incorporated within the system. In addition to the production of unpleasant noise effects, fluid hammer, if allowed to persist for any length of time, will result in broken conduits and damage to other components of the conduit system. 
     Various types of water hammer arresters are known in the prior art. Some devices include expandable bellows or diaphragms that absorb the shock. Another type of water hammer arrester involves the application of a piston working against the pressure developed by a compressed gas chamber. When the water hammer arrester is properly installed with its inlet orifice facing the pressurized water supply line, it must reduce spike pressures as high as 1,000 p.s.i. or more (in large pipe diameters) to 150 p.s.i. or less. Nevertheless, an echo effect still results from the dampening effect of the initial spike shock. 
     Conventional piston-type water hammer arresters are typically manufactured from copper tubing with solder attached brass/copper fittings that provide either pipe thread or sweat solder fittings in order to install into the supply lines. They utilize either brass or plastic pistons with a number ( 2 - 3 ) of rubber o-ring seals. Some problems exist with this particular design. In order to reduce the echo effect it is necessary to utilize acoustical/fluid directional methods to break up shock wave patterns. For example, there have been some experiments with conical interiors of the body and limiting orifices to improve and reduce echo. 
     Long term life conditions of this design of water hammer arrester are also a concern. Hard deposits of iron, calcium, and or sand, which exist in all municipal water, infiltrate the rubber o-rings due to a clearance gap much larger than these contaminant particle sizes. When this infiltration occurs, the hard deposits are imbedded into the o-rings, especially due to a silicone grease requirement used to lubricate the o-rings. This imbedding of deposits causes scoring of the interior wall of the copper tubing and degradation of the o-rings. In time, the pressurized chamber (normally 60 p.s.i.) behind the piston leaks and is absorbed into the water supply. The resulting loss of pressure renders the water hammer arrester ineffective. 
     Another problem with conventional water hammer arresters is denting of the thin-walled copper body. During installation, when a plumbing contractor may have personnel installing hundreds of water hammer arresters, fatigue occurs and heavy wrenches required for the installation work are dropped and may strike the thin-walled copper body causing a dent that may prevent piston movement. 
     There have been water hammer arresters formed entirely out of plastic. However, the pipe thread connections may easily cross thread or fracture during installation. 
     What is needed is a water hammer arrester that will reduce echo, provide superior dent resistance, improve long term life and provide a high strength non-corrosive metallic connection to the supply lines. 
     SUMMARY OF THE INVENTION 
     The present invention meets the above-described need by providing a water hammer arrestor having an insert molded plastic body with a metal attachment at one end to install to the supply line. A piston disposed inside the bore of the plastic body includes a flexible skirt that faces the water supply. The skirt includes a sharp edge that provides a scraping/cleaning action. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which: 
     FIG. 1 is a fragmentary view in side elevation of a piping system including a water hammer arrester; 
     FIG. 2 is an exploded perspective view of the water hammer arrester of the present invention; 
     FIG. 3 is a cutaway front elevational view of the body of the water hammer arrester; 
     FIG. 4 is a top plan view of the end cap; 
     FIG. 5 is a cross sectional view taken along lines  5 — 5  of FIG. 4; 
     FIG. 6 is a partial detailed view of the end cap; 
     FIG. 7 is a perspective view of the piston; 
     FIG. 8 is a top plan view of the piston; and, 
     FIG. 9 is a cross-sectional view taken along lines  9 — 9  of FIG.  8 . 
    
    
     DETAILED DESCRIPTION 
     The presently described water hammer arrester is indicated generally at  10 , FIG.  1 . It is adaptable for installation in fluid conveying systems wherever there is a likelihood of the development of fluid hammer. Such a situation is present in the illustrated example where there is fluid flow in the direction of arrow  11  through a conduit system  12  including a valve  14 . As is well known, closing the valve  14  suddenly and thus arresting the flow of the fluid through the conduit system  12  may result in setting up water hammer conditions. 
     In general, the water hammer arrester  10  should be placed as near the source of shock as possible. It also should be installed in such a manner that there is an unobstructed shock path (indicated by arrow  15 ) to the arrester  10 . Accordingly, in FIG. 1 the arrester may be included in the conduit system  12  simply by inserting a tee  16  to which the arrester  10  is coupled. 
     Turning to FIG. 2, the water hammer arrester  10  is formed from a hollow cylindrical body  18  having a round opening  20  at a first end  22 . The outside surface  24  of the body  18  has a set of external threads  26  disposed thereon. The threads  26  engage with a set of internal threads  28  disposed on end cap  30 . 
     At the opposite end  32  of the body  18 , a metal fitting  33  having a set of external threads  34  is attached to and extends from the body  18 . A set of wrench flats  36  are disposed adjacent to the fitting  33 . The fitting  33  may be formed out of any suitable metal including brass. The wrench flats  36  provide for installing the arrester  10  into a conduit system  12  by means of engagement of the threads  34  on the fitting  33  with a cooperating set of internal threads in the tee  16  or other member of the conduit system  12 . 
     The body  18  is preferably formed out of a suitable rigid plastic material. The metal fitting  33  may be attached to the body  18  during the molding process. As known to those of ordinary skill in the art of plastic molding, the metal fitting  33  may be placed inside a mold and the plastic may be molded onto the fitting  33  in interlocking fashion. The attachment of the fitting  33  to the body  18  is described in greater detail below. 
     A piston  40  is sized to fit inside the hollow body  18  and to slide relative to the body  18  in response to pressure changes. The piston  40  is provided with at least one o-ring  42  to provide a seal between the piston and the inner walls of the body  18 . A first side  44  of the piston  40  faces the flow of the water inside the conduit system  12 . The surface of the piston  40  facing the water supply can be textured, ribbed, and or spiked to fragmentize the fluid spike in order to reduce echo. A second side  46  of the piston  40  is exposed to pressure resulting from a compressed gas such as air disposed inside the pressurized chamber created inside the body  18 . The pressurized chamber will be described in greater detail hereinafter but the pressure is typically around 60 p.s.i. 
     A gasket  48  seals the connection between the body  18  and the end cap  30 . 
     Turning to FIG. 3, a cutaway view of the body  18  and the fitting  33  illustrates the interlocking connection. As shown, the metal fitting  33  has a set of recesses  50  disposed around the circumference of the fitting  33 . The recesses  50  receive plastic material from the body  18  during the molding process such that an interlock is formed. Finger-like projections  52  from the molded body  18  extend into the recesses  50  around the entire circumference of the fitting  33 . As a result of the molding, a mechanical interlock between the body  18  and the fitting  33  is formed. Depending on the materials and the molding process it may be possible to also form a chemical bond between the materials. 
     As shown, the body  18  and the fitting  33  are symmetrical about a longitudinal axis  53 . The body  18  has a centrally located aperture  54 . Inwardly extending cylindrical wall  56  surrounds aperture  54 . The cylindrical wall  56  is supported by a plurality of gussets  58  disposed around the periphery of the wall  56 . 
     The fitting  33  also has a centrally located aperture  60 . Aperture  60  is smaller than the aperture  54  in the body  18 . 
     Referring to FIGS. 4-6, the end cap  30  is shown in greater detail. The end cap  30  has a cylindrical side wall  70  that terminates in an opening  72  at a first end  74 . At the opposite end  76 , the side wall  70  meets a bottom wall  78 . As shown in FIG. 4, the end cap  30  has a centrally disposed cylindrical member  80  with a central aperture  82 . The cylindrical member  80  is supported around its perimeter by a plurality of gussets  86 . The cylindrical member  80  provides high strength and a stop for the piston  40 . This member  80  also provides a place for drilling and pipe tapping in order to install an external check valve which can be used in high performance applications requiring field pressurizing of air or nitrogen gas. 
     As shown in FIG. 5, the cylindrical member  80  has solid walls supported by the gussets  86  which are triangular in shape. The internal threads  28  that engage with the external threads  26  on the outside of body  18  are also shown. 
     In FIG. 6, the seat  90  for gasket  48  is shown. At the bottom of the threads  28 , the seat  90  is formed around the entire perimeter of the end cap  30 . The gasket  48  is positioned in the seat  90  when the end cap  30  is attached to the body  18 . When the cap  30  is secured onto the end of the body  18  by means of the cooperating threads  26  and  28 , the bottom edge of the body  18  engages with the gasket  48  to form a seal. This arrangement allows for installation of the piston and o-rings from the open end of the plastic body. While assembling the threaded end cap  30 , pressurized air or nitrogen gas can backcharge the gas chamber behind the piston  40  to various pressure levels as dictated by particular applications. 
     Turning to FIG. 7, the piston  40  is preferably formed out of a plastic material. At least one O-ring  42  is positioned in a groove  100  formed in the outside wall  102  of the piston  40 . As shown, there are two O-rings  42  disposed in a pair of grooves  100  (FIG.  9 ). The piston  40  includes a side wall  104  that terminates at an opening  106  at a first end  108 . The side wall  104  is angled outwardly such that the diameter at the first end  44  is larger than the diameter at the second end  46 . At the first end  44 , the sidewall  104  has a flexible skirt  105  with a sharp edge. This design recirculates deposits and hard particles back into the water line. At the opposite end  110 , there is a second opening  112 . A wall  114  is disposed across a midportion of the piston  40 . The wall divides the piston into a first section and a second section. The first section faces the water side of the arrester  10  when the arrester  10  is installed in a conduit system  12 . The wall  114  defines a first water chamber when the arrester  10  is absorbing a shock wave resulting from water hammer created in the conduit system  12 . The second section faces the end cap  30  when the arrester  10  is installed. The second section created by the wall  114  defines a second chamber for holding compressed gas such as air. 
     As shown in FIG. 7, a projecting member  116  extends from the center of the wall  114  on the water side of the arrester  10 . The projecting member  116  fits into the aperture  60  in the fitting  33 . A web structure  115  adds strength and rigidity to the wall  114 . On the opposite side of the wall  114 , there is additional support in the form of triangular gussets  120  (FIG. 9) that are disposed about the perimeter of the piston  40  and extend to support the wall  114 . 
     In operation, the arrester  10  is provided with internal pressure through the end cap  30 . The pressure on the back side of the piston  40  is normally set at a level greater than the line pressure inside the water line. When the pressure inside the line spikes due to a water hammer triggering event such as the sudden interruption of flow caused by closing the valve, the water hammer arrester  10  provides a path for the water under pressure. Once inside the arrester  10 , the pressurized water works against the pressure of the piston to dissipate the shock from the interruption in flow. Once the line pressure returns to normal, the piston returns to its normal position resulting from the pressurized chamber within the piston. 
     While the invention has been described in connection with certain embodiments, it is not intended to limit the scope of the invention to the particular forms set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.
A water hammer arrestor having an insert molded plastic body with a metal attachment at one end to install to the supply line. A piston disposed inside the bore of the plastic body includes a flexible skirt that faces the water supply. The skirt includes a sharp edge that provides a scraping/cleaning action.