One of the questions that had to be considered in the design of the IT3 system was "Did the liner need to be centralized to maintain the effectiveness of the concept?"
Without centralizers, a buoyancy effect, arising from a plastic tube being immersed in a cement slurry (specific gravity 1.8), tends to float the liner upward just after the liner has been grouted in. In regards to rejuvenation applications, however, most of the localized corrosion occurs at the bottom of the pipeline since aqueous media tends to concentrate in this area and also due to the very mechanism of pit propagation. Hence, it very often becomes an advantage to retain a greater thickness of cement grout on the bottom of the pipeline.
Moreover, the cement thickness affects the stress state of the plastic liner very little. It was found that with a uniform cement thickness of .75", a PVC pipe bears the stress of 160 psi, which is well below its plastic limit of 2660 psi. If now the liner is not centralized even in the extreme fashion of the corresponding diagram, the stress in the plastic liner varies insignificantly from 175 psi at the top to 150 psi at the bottom. In these situations, therefore, it even becomes preferable to avoid the use of centralizers. However, with other more peculiar conditions, centralizing methods are utilized.
Mechanical centralizers are used in the fabrication of new IT3 plant manufactured pipe. Centralizers are primarily used so that double random length joints of pipe can be field cut to length, thus ensuring that all pipe can be matched and lined up properly during the installation process.
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