1. Field of the Invention
The present invention relates to an apparatus and method of forming a fusion welded butt joint between thermosplastic pipe sections.
2. Description of the Prior Art
Pipelines made of medium to high density polyethylene or similar thermoplastic material are commonly used for transporting natural gas or the like. While, upon original installation a joint may be made by applying heat direct to the ends of pipe sections to fuse them directly together to form a butt joint, in certain applications, as for instance in repairing pipe previously buried underground, the pipe is so restricted against movement that a direct butt joint cannot be made. This is because, when a section of replacement pipe is installed in close fitting relationship within a gap formed between the ends of the pipe left when a damaged section of pipe has been removed, there is insufficient space between the confronting pipe ends to receive a heating rod for melting the pipe to its molten, or fusing, temperature thereby achieving direct fusion. In these instances there have been efforts to joint the pipe sections together by a thermoplastic coupling collar which receives the pipe sections within its opposite ends. The collar incorporates a helical heating coil formed by continuous wraps of coil spaced longitudinally along the inner surface thereof. Energization of the coil raises the temperature of the interface between the coupling and the abutting pipe sections to its molten level to cause fusion. This type of fusion welded joint is characterized by a number of significant shortcomings.
More particularly, when the interface between the coupling collar and the pipe sections is heated to a molten state, the wraps or lops of the heating coil tend to float in the molten material frequently resulting in some becoming disposed in more closely spaced relationship than others. In the areas of the more closely spaced wraps there will be a concentration of heat while in the area of the wraps of greater spacing there will be an absence of heat resulting in an uneven temperature profile along the length of the heating coil. Oftentimes the location of the elevated temperature will be at the butt joint interface resulting in the pipe wall becoming more molten than the remainder of the length of collar. This results in the thermoplastic material forming the wall of the pipe adjacent the butt joint losing its structural rigidity causing it to deform or flow. As a consequence, the end portions of the pipe wall adjacent the butt joint will sag radially inwardly, and the tendency of the wraps, or loops, of the helical heating element to migrate toward the sagging area will be enhanced. The resulting concentration of heat at the butt joint compounds the sagging problem, accelerating the flow of molten material and the migration of the heating coil loops toward the butt joint area, all before the interface throughout the remainder of the collar has reached its fusion temperature. Since the heating coil must remain heated until fusion has been achieved between the collar and pipe sections for some distance from the pipe ends, the temperature in the sagging areas will continue to elevate creating "hot spots" where the thermoplastic pipe walls will become burned thus leading to deterioration of the structure of the plastic itself. These shortcomings are compounded by the fact that the pipe ends are frequently of an irregular cut thus leaving gaps of as much as 1/8 inch one one side or the other when the ends are abutted together for fusion. The amount of parent material required from the wall of the pipe to fill such gaps during fusion is limited thus frequently leading to the wall of the joint in that area being relatively thin thereby resulting in the section being heated to an even higher temperature during fusion. The fusion welded joint which develops under this method is imperfectly formed and susceptible to structural failure and gas leakage.
Attempts have been made in the prior art to solve the development of such "hot spot" heating adjacent the butt joint by employing two separate helical coils spaced apart on opposite sides of the butt joint to thus avoid applying heat directly to the interface of the pipe ends. This is a relatively expensive expedient which is not completely effective and leaves the unheated portion at the interface of the pipe ends unfushed and susceptible to subsequent entry of fluids under pressure between the pipe wall and sleeve, a condition which in use may create stress risers thus further deteriorating the joint.
Another prior art attempt to preserve the structural integrity of thermally fused pipe section end portions is disclosed in U.S. Pat. No. 3,788,928, issued Jan. 29, 1974 for a "Method of Forming a Lap Joint Between Tubular Articles of Thermoplastic Material". As its title indicates, the patented method is directed to a lap joint. It utilizes a reinforcing sleeve of a material having a melting temperature higher than that of the thermoplastic pipe. The sleeve is inserted into one pipe section end portion to support the joint area such that when the pipe is heated only to its fusion temperature, the insert will not melt and will afford support against radially inward sagging during the fusion welding step. However, since the method is limited to an insert of dissimilar material to lap joints which requires axial movement of the pipe sections to make the joint, it is not practical for use in pipe repair as described above. Furthermore, the method itself contemplates that the temperature will not reach the fusion temperature of the sleeves, thus having the disadvantage that material of the flange thereof will not be available to fill any spacing that may exist at any location around the circumference of the pipe ends.
Other efforts for a satisfactory joint include the construction of a piston style dynamic coupling as shown in U.S. Pat. No. 4,386,796 assigned to the assignee of this application. That coupling, which satisfactory for its intended purpose, is relatively expensive to manufacture and is not satisfactory for all applications.