Unsteady flows in steam turbine control valves are becoming more and more a subject of interest for many reasons. In general, steam conditions have increased for both industrial and power plant turbines. Flexible operation conditions of industrial turbines are a field of research ever since, but became more and more interesting for power plants due to the significant rise of renewable energies and their natural inconstant supply. In this paper the results of experiments in an original sized industrial steam turbine control valve are presented. The experiments with air as fluid focus on the flow in the diffuser and are recorded using PIV techniques. With the results typical flow patterns are derived and allocated to a number of operating points with variation in pressure and opening ratio and therefore also in mass flow.