We also supply our own Wegman position indicator for dual expanding plug valves, not only for General Valve Twin Seal but also suitable for other brands.
Dual expanding plug valve
80 year ago this type of valve was designed by General valve as an answer to have 1 valve that could offer double block and bleed service to replace a 2 valve configuration with spool piece and bleed valve.
The upstream and downstream seals (slips) provide the same function as the 2 block valves. The body replaces the spool piece and the bleed verifies the integrity.
With most other valves the closure member rubs / grinds against the seat and closure member and or seats are in the flow which causes wear or damage of some sort. The slips in the dual expanding plug valve are completely retracted out of the flow and before doing this are moved away from the seat. So no wear, damage or erosion.
The sealing on most valves is done (supported) by the medium pressure, dual expanding plug valves seal mechanically, i.e. independent of medium pressure. This makes them one of the safest valves.
These valves are typically used in meter stations, multiproduct manifolds, tank storage isolation, hydrant isolation, fuel loading and unloading on tank terminals, airports, refineries, etc. handling gasoline, diesel, kerosene, jet fuel, heating oil, LPG, crude oil & natural gas.
The easiest valve to repair if the soft sealing slips need to be replaced, they can be removed through the bottom plate while the valve is in the line.