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Western Dam Engineering Technical Note May 2016 11 Pipes deteriorated to the point of impending collapse may not be a candidate for CIPP, as proper installation is likely infeasible. Careful attention must be paid to design parameters when determining if CIPP is the best option for an outlet rehabilitation. One of the critical design parameters for CIPP is the liner thickness and resin material required to withstand external and internal pressures. Internal pressures can be induced by static head pressures from the reservoir as well as positive and negative pressures under dynamic flow conditions. Outlet control configurations on gravity piping and pressurized flow within outlets connected to supply, treatment or distribution systems are cases that can induce significant internal pressures. Proper venting of the pipe is also required to prevent collapse of the CIPP lining due to internal vacuum pressures that can develop during operation. See the previous Western Dam Engineering article on pipe venting: Design Considerations for Outlet Works Air Vents Figure 1 – CIPP Installed inside Existing Pipeline [8] The minimum required initial modulus of elasticity of the cured liner, per the ASTM Standards, is 250,000 lb/in2. Typically steam and water cure methods provide an initial modulus of elasticity ranging between 250,000 to 450,000 lb/in2 and ultraviolet cure methods provide an initial modulus of elasticity ranging between 1,015,000 to 2,600,000 lb/in2 (see Liner Materials and Curing Methods section for curing procedures). However, the CIPP liner needs to be designed for the long term modulus of elasticity. Typically the long term modulus of elasticity used for a 50-year design life is 50% of the initial modulus of elasticity. Resins used for CIPP are the most important component to the performance of the CIPP including parameters such as strength, chemical resistance, and creep. Thermoset resins used for CIPP generally fall within one of three categories: polyester, vinyl ester and epoxy resins. Polyester or vinyl ester resins are also formulated for cure by UV. In general epoxy and vinyl ester resins are higher performance products compared to polyester resins. They have higher strength, elongation, elevated thermal and chemical resistance compared to polyesters. Mineral fillers, such as aluminum trihydrate or calcium carbonate, can be added to resins to significantly increase the modulus (i.e. stiffness) of the CIPP without decreasing the strength or the chemical resistance. [7] As may be expected, the higher performance products come at a higher cost, and some projects may not warrant higher performance parameters. The fabric tube material will also influence the mechanic properties of the combined resin/tube material. Depending on the type of tube fabric, it can either enhance or reduce the mechanical (e.g. strength) properties of the raw resin. Beside resin type and liner thickness for loading considerations, other key design parameters include entrance and terminal structures, selection of diameter considering both heat-induced shrinkage as well as potential circumferential stretching, and selection of appropriate curing system considering environmental, access, and climate considerations. Installation For proper installation of CIPP, both upstream and downstream access to the pipeline is required. The two installation methods are the inversion method (ASTM F1216), in which the liner is installed by progressively turning it inside-out from its initial as-shipped configuration, and the pulled-in-place method (ASTM F1743 and F2019). The installation method is chosen based on site conditions and design of the CIPP resin and curing system. During installation, the CIPP lining is inflated and pushed tight against the host pipe, compressing the fabric and resin against the interior of the host pipe. Once cured, the CIPP resin bonds to the host pipe to prevent sliding of the CIPP lining.