Abstract: A safety valve has a body with a passage extending through it that intersected by a chamber and inclined guide surfaces located in the chamber. A single slab gate with an aperture has ramp surfaces on one side. When moving from the open position to the closed position, the ramp surfaces slide onto the guide surfaces and push the gate into sealing engagement with a side of the chamber to block the passage. The safety valve is particularly useful during a casing-while-drilling operation wherein it is mounted to the upper end of the casing when the casing gripper and top drive are disconnected from the casing string.