What is the high performance type for butterfly valve?
A high performance butterfly valve is normally a triple or occasionally double eccentric butterfly valve (could be centric???) but where the valve in all cases is metal to metal seated. The sealing construction might be 'laminated': several layers of metal or carbon (or metal and teflon), but where the metal construction also normally will give a firesafe design. ( If the valves are firesafe tested with valid certification is a separate issue)
The sealing has a mechanical sealing function (not depending on helping pressure or forces from the fluid) where the torque forcing the seat sealing into the seat is giving the sealing effect.
The sealing is either from the formed edge of the metal disc itself, but normally a mounted on disc sealing ring construction where at least the main and backup sealing is metal.Comment to above: offset = double or triple eccentric, and yes, a valve with this description will have an almost plan-parallell movement the first degrees of the disc movement in and out of the seat, contributing to less wear than a valve without offset.
Eccentric: stem is 'off to one side' from the center of the disc.
Double eccentric : stem is in addition slightly off to one side from the center of the valve (pipeline)
Triple eccentric : the seatsealing construction or seat is slightly off from plan parallell to the valve/pipeline. (Giving even better geometrical plan-parallell movement of seat-sealing in and out of the seat)
Tee HP-valve however would always have the metal to metal sealing as the necessary item to be deemed a 'HP' valve.
I have not seen any official standard describing a HP valve, but the name is commonly used, and has probably an origin in a factory trying to describe 'a better quality metal-seated constructed valve' in a shorter and more commercial way, and then being used further by others.

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