Kelly valve is a manually operated ball type drill stem valve; it is divided into upper Kelly valve and lower Kelly valve. The upper Kelly valve is connected between lower end of swivel and upper end of Kelly; the lower Kelly valve is connected between upper end of drill pipe and lower end of Kelly or connected between lower end of Kelly saver subs. The Kelly valve can be opened on or closed off by only turning the special operating wrench at 90° according to indicating direction. During drilling operation, the Kelly valve shall be connected at upper end and lower end of Kelly.

The sealing principle for Kelly valve is that the ball is positioned and provided with proceeding stress by the lower seat supported by spring load and a close seal between ball and ball seat is provided. During normal drilling operation, the hole is kept unblocked by turning the stem to be “on” position; In case of kick or blowout, turn the operating stem to be “off” position to close off the internal bore of drill string, the kick or blowout accident is avoided due to a high pressure sealing situation between ball and ball seat.