PESTAN does not recommend installations of the copper and polypropylene mixed systems, where highly aggressive water attacks copper and releases free ions that negatively affect water and any material in the system, even inert material such as polypropylene. Release of free ions is accelerated with higher velocity flows, elevated temperatures and with use of strong oxidants such as traditional chlorine and especially using increasingly becoming popular chlorine dioxide.
To avoid the erosion-corrosion of the copper piping system, please follow recommended design instructions from from The Copper Tube Handbook by Copper Development Association, page 11
Water Velocity Limitations— To avoid excessive system noise and the possibility of erosion-corrosion, the designer should not exceed flow velocities of 8 feet per second for cold water and 5 feet per second in hot water up to approximately 140°F. In systems where water temperatures routinely exceed 140°F, lower flow velocities such as 2 to 3 feet per second should not be exceeded. In addition, where 1/2-inch and smaller tube sizes are used, to guard against localized high velocity turbulence due to possibly faulty workmanship (e.g. burrs at tube ends which were not properly reamed/deburred) or unusually numerous, abrupt changes in flow direction, lower velocities should be considered. Locally aggressive water conditions can combine with these two considerations to cause erosion-corrosion if system velocities are too high. Due to constant circulation and elevated water temperatures, particular attention should be paid to water velocities in circulating hot water systems. Both the supply and return piping should be sized so that the maximum velocity does not exceed the above recommendations. Care should be taken to ensure that the circulating pump is not oversized, and that the return piping is not undersized; both are common occurrences in installed piping systems.