A triple offset valve is a rotary, butterfly-style valve.  A standard butterfly has no offset: THe shaft and vane are coincident with the centerline of the pipe.  A double-offset butterfly has the shaft offset back from the centerline of the seating surface, and offset to one side of the centerline.  This helops the disc "cam" into the seat but the seat must still be resilient to accept the edge of the vane in the last degrees of seating.
A triple-offset valve is like a double-offset, with the addition that the edge of the seating surface is generated as the surface of a cone, which itself is offset so that one of its sides is parallel to the wall of the pipe.  This makes the seating surface slightly elliptical, but it meets the seat without needing to be distorted.  Metal seats are possible, eliminating the limitations of elastomer seats.  A slightly flexible laminated stainless/graphite seat seal is used to conform to the small irregularities in the seating surfaces, but the seating torque is necessarily high.Triple offset valves can survive higher temperatures and higher pressures than other types.

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