I once bought one of the controllable irrigation valves from Rona or HD because I just had to find out the answer. I found out basically what Steve just said (only this was 4 or 5 years ago so nobody knew the answer). There's a metal screw and a spring that I thought would not hold up over time. I'm sure the parts would be replaceable with ones that would hold up with a little ingenuity, but I decided against it.
There are motor controllable valves used in industrial applications that are rated for semi-corrosive environments (which is what our SW tanks would be considered), but the problem is it's difficult to buy these in small quantities. I bought a 3 way ball valve from Chemline Plastics, I wanted to get the valve for a proof of concept so I ordered the valve without the motor (as it was a $100 difference). I had to go through the whole sales process with the guys, a sales guy from the Edmonton office came and paid me a visit at my office when he was down to Calgary for the day anyhow, and I bought the valve for $80. Kind of a ridiculous process and I never really got back to seeing about getting the motor mount, but I guess it was an educational experience. Now I have all kinds of sales material/documentation on things like flow meters, foot valves, needle valves, gate valves, flanges blah blah blah blah etc. etc. the list goes on and on so I guess I ever wanted to make skimmers for a living or something like that, I'd be set. Too bad that I don't (want to make skimmers for a living..).
