In this study, the effect of circulation on the critical submergence of an intake pipe is presented. Experiments were conducted on a vertically flowing downward intake pipe in a circulation imposed still-water reservoir. The circulation imposed on the flow causes an outward centrifugal force component in radial direction which increases the critical submergence considerably. For a given flow and geometry, the vane setting angle has a certain value which causes the same critical submergence as in the case of no-vanes (unsteady vortex in no-circulation imposed flow). As the vane setting angle gets larger than its certain value, the critical submergence becomes larger than that in the no-circulation imposed flow (no-vanes). The smaller the vane setting angle, the smaller is the critical submergence. The critical submergence attains its minimum value when the vane setting angle is zero (vanes are set radially). Flow visualization for an intake pipe in a still-water reservoir has indicated that a spherical volume of fluid bounded by a stream surface of a sphere (SSS) develops. This spherical volume, hence SSS, shrinks radially and rotates about its vertical axis. The air-entraining vortex occurs as the SSS collapses. The SSS is a useful concept for the physical explanation of occurrence of the air-entraining vortex.