In the March 2004 Pollution Control Clinic, “Piping of Caustic,” I answered a reader’s question regarding caustic and PVC pipe (www.pfonline.com/articles/clinics/0304cl_env3.html). I received two excellent responses from other readers.
I really enjoy your column. Beware of using CPVC pipe with caustic. At one time I worked for a large manufacturer of pumps and filtration equipment. We had a rash of failures with caustic soda, alkaline non-cyanide plating solution and black oxide solutions used to promote better inner layer adhesion in printed circuit boards. All of these involved CPVC pumps, filter chambers and interconnecting piping.
The failures came in three distinct types: 1) Thread leaks in threaded pipe; 2) Glue failure in socket glued fittings and 3) Cracking of the pipe and other molded and extruded products. Pipe fittings and other injection molded products were more vulnerable than extruded pipe, itself, but there were still failures in all products.
The resin producer was at a loss to explain the failures, but later amended their chemical charts to warn of trouble with caustic and CPVC. We found that virgin polypropylene worked best, but all joints must be done by fusion welding for best integrity. Threaded joints in polypro pipe will leak with time, especially if temperatures vary 20-30°F. You are absolutely correct about EPDM being the preferred elastomer for caustic. Viton will usually fail rapidly when exposed to caustic. C.R.
I read the question from your reader regarding piping of caustic. We have seen problems with brittleness of the PVC and CPVC SCH 40 or 80 on pretreatment systems in the alkaline cleaner stages. The brittleness that our customers have experienced is not due to the caustic itself, but the surfactant packages in the cleaner that attack the plasticizers of the pipe. The brittleness is most apparent on formed fittings, i.e. elbows, valves, nipples, unions and so on. It has been seen in pipe sections, but only in areas where it is clamped and the process experiences heat up and cool down. In most of the pretreatment stages that use alkaline cleaners and sometimes the first city water rinse after an alkaline cleaner stage, we recommend that black iron piping is used due to this specific concern. D.M.