Does butterfly valves with EPDM or BUNA-N seats require a gasket?

Wafer and lug style butterfly valves don't require a separate gasket. The same gasket acts as a seat and a seal against the flanges that hold the valve in place. An AWWA butterfly is flanged and would require flange gaskets to seal against the flanges on the pipe. The seat on an AWWA butterfly valves is to provide a seal to stop the flow of water only.