1. Some valves like Durco BX6 Series Metal seated valves meet ANSI FCI 70-2 Class VI leakage.
2. Normally all PTFE Seated valves are Class VI Tight Shut Off valves.
3. Double Flanged and Lug (Flanged) Valves are different. Double Flange has two flanged on both edges of valve body whereas Lug type is having single flange in the center of valve body.
Normally Valves designed in AWWA standards are Double flanged type. This valves are very large in size (above 900mm to 2000mm) mainly used in cross country water pipelines.
Flanged or Lug type valves are manufactures as per API609 or BS5155 and normally sizes varies from 50mm to 700mm.
I dont know about valves you have mentioned Graphite etc., but Generally Metal Seated Butterfly valve  Dampers are with Inconel or SS Seats and they achieve 100% Sealing by using a Seal Air Fan.
Generally Graphites are used as Seals if the valves are Fire Safe in design.

MORE NEWS