ULTRAPURE EQUIPMENT AND SERVICE
The selection of piping materials is critical to the performance of high-purity water systems. Piping can contribute contamination. Corrosion products from the piping material, biofilms and other organics are common problems associated with high-purity water storage and distribution systems. Corrosion of steel pipe is likely in aerated high-purity water. However, in deaerated high-purity water, steel can be used and, in fact, is the most common material in boiler feedwater applications. Stainless steel is used for high-purity water service. Type 304 and Type 316L are the most common types used. In the last two decades plastic pipe has become a common replacement for metal pipe. Because of its inert properties, plastic pipe offers certain advantages over metal pipe in high-purity applications. However, plastic pipe can cause contamination problems. Degradation of the plastic pipe occurs when an active substance from the environment is absorbed into the plastic pipe and reacts with the polymer chains. Oxidation of the plastic can occur by contact with oxidizing substances such as ozone. Semiconductor plants routinely use plastic piping because integrated circuits are very sensitive to metallic and ionic contamination. PVC pipe is the most common plastic pipe used for DI water service. Polyvinylidenedifluoride (PDF) is commonly used in 18 plus megohm applications. Polyvinylidenedifluoride offers a lower leaching rate and particulate level than PVC. It is considered more inert than PVC when compared against the pickup of organic ions, TOC and particulates. PDF piping is seven times more costly than PVC and three times more expensive than stainless steel. PDF piping can run $75 to $85 per installed foot for pipe 2" in diameter.