In the continuous casting of steel, the control of the flow to the mould is a critical issue as a constant mould level is essential for a good quality of the cast product. A stopper rod is a commonly used device to control the flow rate. Agglomeration of solid material near the stopper rod can lead to a reduced cross section and thus to a decreased casting speed or even total blockage (clogging). The mechanisms involved in clogging are still not fully understood. Single phase considerations of the flow in the region of the stopper rod result in a low or even negative pressure at the smallest cross-section. This can cause degassing of dissolved gases from the melt, evaporation of alloys and entrainment of air through the refractory material. In this paper the criteria for degassing of liquid steel at low pressures are analyzed and compared with water. A mathematical model for a two phase flow situation (liquid steel/gas) in the SEN that links the pressure distribution and the throughput with the geometrical parameters, the amount of gas in the SEN and the pressure loss coefficient at the stopper rod is presented. An explanation theory for the diminishing influence of the gas injection on clogging is outlined.