The University of Drilling is a practical learning site, not an accredited school.
A kelly saver sub is connected between the kelly and the drill pipe of the drill string to protect the kelly from wear during make-up and break-out of the threaded connections. A rubber protector also forms part of the kelly saver sub to cushion any adverse rotation of the kelly during rotation.
A kelly cock is a valve immediately above the kelly that can be closed to confine pressures
inside the drill string while a lower kelly valve is essentially a full-opening valve installed immediately below the kelly, with outside diameter equal to the tool joint outside diameter. Valve can be closed to
remove the kelly under pressure and can be stripped in the hole for snubbing operations. (See API RP 53)