I had copper in a previous shop. Changed to galvanized about 10 yrs ago with a change in shops. Have never seen any flakes in the filters. Had occasion to rearrange some piping about 5 yrs ago. What I took apart was clean on the inside.
I've been in several chemical plants around the world. Many of them used galvanized for their instrument air lines (though most have converted to electronic instruments now).
I saved a heat pump condenser once and started to make an in-line cooler but gave up when it looked like it was more trouble than what it was worth to do all the plumbing needed to make it work well. Ended up selling it for scrap
The pressure ratings on most PVC pipe is based on an ASTM specification that is for liquid service only.
