Slug flow occurs when the speed of the vapor phase pushes the waves from the wavy flow regime onto each other. The waves grow until the liquid waves touch the top of the pipe and form frothy slugs. The velocity of frothy slugs and the "slugs" of vapor is faster than the liquid velocity. Slug flow is found in lower velocities in piping running uphill than in horizontal piping. When running downhill, it takes higher vapor rates to establish slug flow than in horizontal pipe.
Because the slugs are moving faster than the average liquid velocity, care should be taken to avoid slug flow around fittings. Severe water hammer may occur when changing flow direction when slug flow is occurring.