Kelly valve also named Kelly cock or drill pipe safety valve. It is a hand control valve in drill string circulating system and it is one of the effective tools to prevent blowout. Kelly valve is divided into Upper Kelly Valve and Lower Kelly Valve. Upper Kelly Valve is connected between swivel sub and Kelly. Lower Kelly valve is connected between lower end of Kelly and top of drill pipe or lower end of Kelly save sub. Rotating the wrench of full opening safety valve 90° to realize on and off. In the drilling operation, in order to avoid malignant accidents, Kelly valves should be connected to both end of Kelly to prevent loss of drilling fluid and blowout.