In Gate valve OS&Y stands for outside stem and yoke, this is most commonly used valve.
A Yoke connects the valve body or bonnet with the actuating mechanism.
A Yoke nut is an internally threaded nut and is placed in the top of a Yoke through which the stem passes.
Fail safe is a position which valves attain in case of a failure of Actuating force.
Fail-close: In-case failure of actuating force, valve gets automatically closed; this is the most common and widely required position.
Fail–open: In-case failure of actuating force, valve gets automatically open, for ex. Coolant which ensures temperature of media does not cross certain prescribed levels.
Cv is a flow coefficient of a valve, it gives us an idea of discharge capacity of a valve. Higher the Cv higher is the discharge capacity.