Abstract: When running in a hydrocarbon well with a large diameter pipe, such as a liner or wash pipe, on the bottom of smaller diameter drill pipe, a safety sub is screwed into the box of a drill pipe stand by the derrickman. Any low pressure mud flowing upwardly in the drill pipe is diverted by the sub to a standpipe and then to the mud system of the drilling rig. In the event high pressure mud starts flowing in the drill pipe, the safety sub is tightened with a wrench and a valve is manipulated to stop upward flow in the drill pipe. The safety sub is partially dismantled, leaving the valve closed and suitable pressure lines are connected to the sub to kill the well. The valve includes a ball check which is normally held open and a mechanism manipulated to allow the ball check to close against a seat. After closing the valve, the ball check may be pumped downwardly into the well to allow a wire line tool to be run into the hole.