Q. Recently, I have been promoted and now have responsibility for both our plating operations and its wastewater pretreatment system. When I reviewed our operations, I was struck by how many of our PVC and CPVC piping joints that carried either caustic for wastewater pretreatment or alkaline cleaner exhibited a crusty scale, apparently dried caustic residue. Also, many joints also exhibited weeping of the caustic or alkaline cleaner. I was told that this has “always been like this.”
I find this unacceptable, mainly due to safety concerns. Do you have any suggestions on how to remedy these caustic leaks? D.E.
A. It seems like every several years I get a question similar to the one you posed. I have been involved in the metal finishing industry for over 30 years, and for most of the time, I, too, wondered why PVC caustic piping always leaked.
As I have written in previous articles, the PVC/CPVC glues commonly used by mechanical/plumbing contractors as well as your own maintenance personnel 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 the leaky joints as you have observed. In addition to properly preparing the pipe for joining, there are PVC/CPVC glues available that do not contain fumed silica and are specifically designed for caustic and/or sodium hypochlorite (bleach that contains some free caustic as stabilizer) service; these are available, but you must specifically request them.
Also, we strongly recommend that you use glued joints for piping and valves, but if you must use a threaded joint, make sure the joints are clean and sharp and use sealing tape that meets the military specification T-27730A.
The following are other recommended actions to minimize leaky joints and improve piping reliability:
- Review the supports of the piping and consider additional supports, especially at elbows and valves, to prevent sagging and movement during pumping,
- Consider using schedule 80 piping because of its thicker walls and more stable structure,
- An alternative to PVC/CPVC is polypropylene piping with joints connected by fusion welding; obviously this option is much more expensive,
- Because of surfactant packages in alkaline cleaners that can cause brittleness in PVC/CPVC piping and fittings, some facilities have found success using black iron piping,
- For valve and union seals, we have had better experiences with EPDM (of course, Teflon is the best if you can afford it) as compared to Viton A; Viton A works best in acid service, and
- Have your pipe installers trained in accordance to ASTM D 2855-96, “Standard Practices for Making Solvent-Cemented Joint with Polyvinyl Chloride (PVC) Pipe and Fittings;” a rep from your PVC/CPVC pipe supplier can usually provide training or refer you to someone who does.
Using the right methods and material will give you the best shot at achieving your goals.