The present invention relates to a tapping fitting for application to a pipe in order to direct fluid conducted therethrough to an ancillary device, such as a branch conduit or an outlet tap. The invention is particularly useful in electrofusion fittings for plastic pipes wherein the fitting is bonded to a plastic pipe by electrofusion, and is therefore described below with respect to this application, although it will be appreciated that the invention could also be used in other applications.
Tapping fittings of the foregoing type are known. They generally include a rotatable blade assembly formed with external threads threadedly received within the housing and including a cylindrical cutting blade such that rotation of the blade assembly moves it axially from an initial, retracted position within the housing, to a projected position, to cut a cylindrical slug from the plastic pipe to which the fitting is attached. Such tapping fittings are applied to the plastic pipe while the blade assembly is in its initial, retracted position, i.e., within the housing of the fitting. After the fitting has been so applied, the blade assembly is rotated from externally of the fitting to its projected position to cut a cylindrical slug from the plastic pipe, and then it is rotated in the opposite direction back to its initial, retracted position to remove the slug and retain it within the housing main section of the fitting. The fitting thus establishes communication between the plastic pipe and the ancillary device, and generally remains in place for the life of the installation.
Such tapping fittings have come into extensive use for tapping branch conduits to high-pressure liquid or gas supply pipes of a plastic material. The housing is generally made of polyethylene or of a similar plastic material as the pipe. It has been found, however, that the plastic material used for the housing tends to creep over a long period Of time. Transverse creep (i.e., transversely to the axial movement of the blade assembly) may result in the blade assembly jumping the threads; whereas axial creep may result in the failure of the valve to completely close.