Butterfly plate shaft is at an angle to improve the sealing.  It will be great help if any one could explain how it is achieved?
There are three "Major" types of butterfly valves.
1. Rubber lined valves have the shaft through the centerline of the vane.  The vane swings through an interference for woth the rubber liner to seat.  Shutoff rating vary, but 150 psi is typical and 250 is about as high as you will see for a rubber-lined valve."Rubber" van be almost any elastomer, but nitrile, EPDM, and Viton are the most popular. Operating torque is high because the center of pressure is ahead of the vane centerline and the dynamic torque tries to close the valve.  Also, there is always high friction because the vane is always in contact with the rubber liner. 
There is one manufacturer that makes an inclined vane valve.  The centerline of the vane is concentric, but the edge of the vane is tilted so that it has a continuous sealing surface against the liner.
2. Double-offset, or "High-Performance" butterfly valves. The shaft is not at an "Angle", but is offset behind the seating line.  ALso the shaft is offset some to the side of the centerline.  this lets the valve use a resilient, but not elastomeric, seat.  Wven flexible metal seats are available.  As soon as the HP valve begins to open, the vane is clear of the seat.  Friction drops to only the packing friction.  The offset helpo to reduce the dynamic torque as well, because the center of pressure becomes more closely aligned with the center of rotation. 
3.  Triple-Offset butterfly valves.
A triple-offset valve is much like a double-offset valve in appearance. The shaft is offset behind the vane, and the shaft is offset from the pipe centerline just like the double offset. The difference is that the seating surface of the vane and the seat is machined as a section of a cone.  The leading edge of the vane seats on a surface that is parallel to the flow, and the trailing edge of the vane seats on a surface that is about 40 degrees inclined to the flow.  The apex of the (imaginary) cone is alongside the pipe wall, upstream of the valve.  This geometry lets the valve have rigid seats and still give good shutoff, or rubust metal seats are used that have very little flex.  SO the Triple-offset valve is all-metallic and can serve at very high temperatures.  But it's difficult to machine, made of more expensive alloys, and therefore relatively expensive. 

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