Flowserve invented the non-lubricated plug valve for the most corrosive and difficult chemical services where drop-tight shutoff is an absolute requirement. All Durco G4 SLEEVLINE valves offer the proven reliability of the plug/PTFE sleeve primary seal with a PFA diaphragm secondary seal. The unique reverse lip diaphragm provides a self-energizing dynamic stem seal where pressure activates the reverse lip to seal against the stem. It also provides a static seal by wedging against the stem with an interference fit.