This design approach has distinct advantages over BBV in main steam,feedwater,turbine bypass,and other plant systems requiring valves that can handle the challenges inherent in high pressure and temperature applications.However,due to its reliance on system pressure to aid in sealing,pressure seal valves are best applied in system where the minimum,consistent operating pressure is in excess of 500psi.
Further view :
The difference of Bolted Bonnet Vs.Pressure Seals