Piping of Caustic

Question:

Recently, we had a half-inch SCH 80 CPVC or PVC nipple give way. This system contains caustic (sodium hydroxide) at 25% solution and 50% in the past. After each transfer of caustic, the pump is blocked in at the inlet and outlet as well as at the inlet from the storage tank and at the end of the transfer system by a block valve.

I have two questions. Will the caustic solution have any effect on the pipe as far as making it brittle with summer heat stress and the fact that it was also used for ferric chloride transfer at one time in the past before the caustic? Is there a better pipe grade other than CPVC or PVC for caustic? Thank You. T.V.

Answer:

You ask a very good question. As far as making the PVC or CPVC pipe brittle, my experience has been that caustic and summer heat does not make the pipe brittle, but if one of our readers has a different experience, let me know at pollutioncontrol@pfonline.com.What could make the pipe brittle is if it is located outside and exposed to the sun; unless protected by a coating or shade, the sun’s ultraviolet light will attack both PVC and CPVC pipe over time, causing failure.

Another possibility for your nipple breaking is that there may be significant stress on it when the chemical delivery hose is connected; after repeated loading and unloading, stress fractures could develop, causing failure. Evaluate your piping system and see if either the piping can be reinforced and/or supported better or if the chemical delivery hose can be better supported so as to eliminate any stress to the nipple.

To answer your second question, yes that are better types of piping, but they are extremely expensive. Upon reviewing several corrosion resistance charts that we have, I observed all rated CPVC very high for both 25% and 50% caustic while there are some inconsistencies regarding PVC; while most rate PVC fairly high for caustic, one did not rate PVC well. I have had good experiences with both, but to be safe, you may want to make sure you install CPVC piping. The greatest challenge seems to be in the pipe connections, which may be the source of your problem.

As I have previously written in the September 2001 issue, “Preventing Leaks in PVC Piping” (www.pfonline.com/articles/clinics/0901cl_env3.html), the PVC/CPVC glues that are commonly used by mechanical/plumbing contractors contain fumed silica, very small glass-like particles used to thicken the glue. Caustic solutions dissolve the silica, resulting in leaks and failures. Evidence of this is the crystallized white caustic that “balls” around leaky joints. In addition to properly preparing the pipe for joining, there are PVC/CPVC glues available that do not contain silica and are specifically designed for caustic service. For threaded joints, the recommended tape meets military specification, T-27730A, and the joints’ threads must be clean and sharp. Regarding your valves, check the seal materials. With caustic service, we have had better experiences with EPDM seals (of course, teflon is the best if you can afford it) as compared to Viton A; Viton A works best in acid service.

One final note: I strongly recommend that you stay with Schedule 80 piping due to its thicker wall and because it's structurally more stable.