A valve for installation on a flow line while the flow line is under pressure. The valve includes a two-part valve body and a valve cartridge. Each body portion has a semi-cylindrical channel sized to mate with the flow line. The body portions are secured together with a portion of the flow line enclosed and sealed in the channels of the body portions. A throughbore extends through the two body portions and fully intersects the channels of the two body portions. One end of the throughbore is adapted for attachment of a cutting tool, which is used to cut out the portion of the flow line disposed in the throughbore. After the flow line is cut, the valve cartridge is directed into operating position. A valve stem nut, a valve cap, a valve plug, gaskets, seals and connectors are provided to close and seal both ends of the throughbore.