It's not uncommon to see metallic butterfly valves installed with plastic piping. These metal butterflies tend to corrode and not function properly in a relatively short period of time. At next scheduled replacement, consider the VISPAR lug style butterfly valve. It has the same face-to-face dimension as an existing corroded butterfly valve, but offers many years of maintenance-free service going forward. Only the plastic disc and elastomeric liner are in contact with the water.