Three offsets, one purpose
The first offset puts the shaft behind the plane of the sealing face, keeping the seal circle uninterrupted. The second moves the shaft off the pipe centreline, creating the cam action that swings the disc clear of the seat on opening. The third is different in kind: the axis of the conical seat surface is inclined relative to the pipe axis, so the seat is a slice of an oblique cone rather than a symmetric ring. The geometric consequence is that no point of the seal ring is in contact with the seat at any disc position except fully closed. There is no rubbing, no interference and no elastomer; the ring lands on the cone evenly around its whole circumference at the final degree of travel and is then loaded into it by torque, the way a tapered plug seats in a tapered bore.
