The invention relates to a device for preventing slag from flowing along when tapping a molten metal out of a metal-lurgical vessel, whereby the lap opening of the vessel is formed out of interchangeable pipes, which are located one above the other, made of wear-resistant refractory material, and enclosed by tap framing blocks, whereby the lower end of the lap interchangeable system, which is constructed as described, is formed by a cup block against which a slide that closes the opening rests. According to the invention, axially extending channels that are open at both ends of the pipe are provided in the pipe wall of at least the interchangeable pipe that leads into the interior of the vessel. Said channels are connected to a gas supply at the pipe end facing away from the interior of the vessel.