Diverter systems, used as a means of well control while drilling surface hole, have a history of occasional failure. This paper addresses excessive back pressure, which can result in mechanical failure of surface equipment or foundation collapse. Flows at critical rates are shown both experimentally and theoretically to have a significant effect on back pressure. Critical flow is modeled by quantifying exit pressures and by including fluid acceleration pressure losses in back-pressure calculations, Experiments were then conducted in I-in., 2-in., and 5-in. pipes to verify these mathematical models. A "systems analysis" approach was used for the diverter design, which allowed for consideration of wellbore and reservoir performance effects. This procedure allows the relationship of diverter vent line diameter, conductor depth, and drilling depth to be better identified, yielding improved design criteria.