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Close-Fit Slip Lining
A new thermoplastic pipe can be installed in a host pipe with the outside diameter (OD) of the new pipe in close fit with the inside diameter (ID) of the host pipe. Close-fit pipe slip lining is an ideal application for the rehabilitation of pressure pipes that are relatively straight or have only modest bends, and that have largely maintained their circular profile.   Close-fit slip-lining is possible because of the "memory" properties of thermoplastic materials.  Thermoplastic materials will change shape when force is applied to the material either through the application of compression or tension but will return to original shape when the external force is removed or internal pressure is applied. This property allows the thermoplastic pipe to be temporarily deformed and pulled into the host pipe. When the new pipe has been pulled to the desired position, tension on the pipe is no longer applied or internal pressure is applied and the pipe will return to its original shape.   The versatility of thermoplastic pipe has spawned the development of a wide range of innovative close-fit pipe lining systems.  These systems can be classified under one of two generic system types, i.e., concentric reduction/expansion liners and folded liners. Concentric Reduction / Expansion Liner Techniques can be subdivided into two categories tension deformed liners, i.e., the roller die and static die systems, and compression deformed liners.  The tension deformation techniques use either a roller reduction die or static die to reduce the diameter of the thermoplastic pipe. Tension reduction techniques are normally applied to relatively thick-walled (i.e., structural) thermoplastic pipes, owing to the relatively high processing forces they impose on the pipe.


In the first case, the thermoplastic pipe is winched through a die reducing the diameter of the pipe as well as slightly reducing the pipe wall thickness. Tension on the deformed pipe is maintained as it is pulled into the pipe to be renovated as shown in the above drawing. During the installation processing the outside diameter of the thermoplastic pipe is reduced to less than the minimum bore diameter of the host pipe. The outside diameter of the reduced thermoplastic liner is maintained through the application of longitudinal tension on the pipe as it is pulled into the host pipe. Once the thermoplastic liner pipe is in the required position, it is reverted to a close fit with the host main by releasing the longitudinal tension reversing the deformation process.  A properly installed close-fit thermoplastic pipe using the roller reduction or the die method needs to be pulled beyond the end of the exit pit or manhole to allow enough exposed pipe to deal with the shortening of the pipe as it returns to its pre-stressed shape. Compression deformed liners is achieved by gripping and pushing the liner through a series of rollers reducing the pipe diameter. The resulting reduction causes an increase in the pipe's wall thickness, which is substantially retained until the pipe is reverted back to its original form through the application of internal water pressure. The reduced diameter achieved by the compression techniques is relatively stable, i.e., it may take several days or weeks to revert to its natural diameter without the application of internal pressure. The compression technique, like the tension deformation technique, is normally applied to relatively high thick-walled thermoplastic pipe.Folded Liner Insertion Techniques - Close fit slip-linings can also be achieved via deformation of the thermoplastic pipe to form a "c" shape.  The liner pipe outside diameter is chosen to be slightly less than the minimum bore diameter of the host pipeline to be lined. This ensures that the liner deploys fully on reversion to avoid the formation of residual longitudinal folds. The folding process may be done either hot during manufacturing or cold during manufacturing or on the job site. The shape of hot-folded liners is maintained without any additional constraint, while the shape of cold-folded liners has to be restrained using temporary strapping or sheathing. 


Hot-folded liners are wound onto a reel for transportation to site. Factory-folded liners typically are used for smaller diameter pipes up to 500mm. The liner is slip-lined into the host pipe and reverted to a round section and close fit with the host pipe using a combination of heat (typically steam at 125oC) and air pressure.   Cold-folded liners are relative thin-walled PE liners, which are typically designed as interactive liners. These liners are favoured for the rehabilitation of large diameter pipe up to 1,500 mm. Cold-folded liners are pulled into the host pipe where they are reverted by pressurization with cold water to a pressure of 1.5 times the working pressure of the pipe. During pressurization the temporary strapping breaks allowing the pipe to return to its fully round shape.

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