Abstract The steam flow in a bell-shaped control valve, located before an intermediate pressure cylinder of an ultra-supercritical steam turbine unit, was numerically investigated. Toward this end, the Reynolds-averaged Navier–Stokes (RANS) simulation approach using a shear stress transport (SST) turbulence model was first validated against wall pressure measurements in tests using a scaled valve model. The dependency of the full-scale valve’s steam flow patterns on its open ratio and pressure ratio was then established. The annular attachment flow and central detachment flow were classified and analyzed in terms of flow quantities including velocity vectors, Mach number contour, pressure distribution and turbulent kinetic energy contour. The numerical results demonstrate that the attachment flow was produced mainly by the annular wall-attached jet and the central reverse flow, whereas the detachment flow was mainly produced by the central detached-jet expansion and the ambient reverse flow. Further analysis of the flow field indicates that the transition from attachment flow to detachment flow could be satisfactorily explained in terms of the Coanda effect and was influenced mainly by the ratio of the valve seat’s radius to the valve’s lift displacement. In addition, when decreasing the valve’s pressure ratio to the critical detachment pressure ratio, two phenomena were observed: discrete elliptical regions with jet’s expansion-and-recompression in the Mach number contour and a spatial variation at the pressure profile along with reducing magnitude, which was induced by the interaction between the expansion wave and the compression wave in a Prandtl–Mayer expansion process. These findings are of great practical significance for the valve’s operation and design optimization.