QSB was leaking while it was closed(see attached picture)! So, it was disassembled completely and the following o-rings were replaced: 3 o-rings(Viton) for the shaft one O-ring (Viton) for the butterfly plate two o-rings(Buna) for QSB flanges
Valve was cleaned and put in the place. Leak check was ok and back groung 1e-9torr lit/sec.