We use a bleach solution to treat cyanide and a caustic solution to raise pH in our wastewater pretreatment system. Ever since this system was installed many years ago, we have been plagued with leaks of the bleach and caustic PVC piping. We have replaced the piping several times and, at best, did not have leakage for 1 yr. Not only is this situation expensive due to lost bleach and caustic and repair costs, we are also very concerned about the safety of our workers. Do you have any suggestions on how we can solve this problem? In a related matter, our PVC ball valves for our caustic feed system also chronically leak. A.H.
Bleach or sodium hypochlorite solutions contain some caustic to help stabilize the chlorine. What is happening is that over time the bleach and caustic solutions are dissolving the PVC glue. Many PVC glues contain fumed silica, very small glasslike particles used to thicken the glue. The very high pH of the bleach and caustic solutions dissolve the silica, resulting in leaks.
To ensure leakproof joints, have your piping installers trained and certified in accordance to the American Society for Testing and Materials (ASTM) D2855-96, "Standard practices for making solvent-cemented joints with polyvinyl chloride (PVC) pipe and fittings." A representative from your PVC pipe supplier or manufacturer can provide training. In addition to properly preparing the pipe for joining, there are PVC glues available that do not contain silica and are specifically designed for bleach and caustic service.
For threaded joints, the recommended tape meets military specification, T-27730A. Be sure the joints' threads are clean and sharp.
Regarding your PVC ball valves, check the seal materials. With caustic solutions, we have had better experience with EPDM seals as compared to Viton A. I hope this helps.