Differences in ball valve design, especially with respect to Ball/Seat combinations in metal sealing applications.
From what I have seen to get bubble tight sealing, balls are match lapped to their seats, but is it usual to start with a seat which has an initial spherical profile similar to the ball, or with a conical seat profileYou need a minimum contact stress between the ball and seat in order to seal.  So the higher the contact stress, the tighter the seal.  Howver, the ball and seat must slide against each other for the valve to operate.  High force that can be used to increase contact stress will require more torque to turn the ball.  So you want to limit the potential force.  and you need to control the contact area in order to get the desired contact stress.  A third factor to consider is that the ball, seat, or both are usually coated with somthing that allows them to slide against each other without much wear or galling.  Too much contact stress can cause coatings to peel off.  Lots of things to consider.  One more thing.  The harder the coating, it is more difficult and it takes more time to lap the mating surfaces.

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