The selection factors for Cocentric vs eccentric butterfly valves.
Centric/Flat disc/rubber-lined butterfly valves were the first developed.  Originally conceived to isolate dry granular solids, they were later found to be useful on liquid streams. 
They are very simple to make and they are the most inexpensive possible valves. 
THe rubber (Elastomer) liner performs multiple duties of stem seal, flange gasket, and process isoplation. 
THe elastomer also limits the temperature and pressure that the valves can tolerate. 
Friction is high because the disc never fully breaks away from the elastomer liner. 
High-Performance, or Double-offset butterfly valves move the sealing line of the vane away from the stem. The sealing point a continuopus line, not interrupted by the stem.  The stem is then sealed by a conventional packing set instead of just interference between the liner and the shaft. 
Friction is much lower because the disc breaks completely away from the sealing line (seat) as soon as the valve moves a couple of degrees.
The seal can be made of higher-tempearture materials, including flexible metallic designs, and published limits of Double-offset valves can exceed 1000F.

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