Why do many ball valves show a "seal carrier" with a "carrier o-ring" on only one side of the ball?
If the ball is supported by something other than the seals, e.g. a trunnion, it doesn't necessarily need two seals.
It sounds as though you may be talking about a Double block and bleed valve, the 'o'ring allows the seat to move with the ball slighlty yet still seal against the body of the valve stopping any flow across the back of the seat.
Pressures are lower than normal use because of the 'o' ring limitations.

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