The main dimensions of cyclone separators for emergency relief systems have been optimized by measuring the gas loading capacity of geometrically similar cyclones having diameters of 290 and 1,000 mm. Experiments have been carried out with mixtures of air and water, aqueous Luviskol® (poly[vinylpyrrolidone]), having viscosities of 180 to 1,100 mPas, as well as two aqueous foams, the surfactant K8 and methylcellulose. Overall, the measurements have shown that the optimized devices are equally suitable for the separation of aqueous, viscous, and foaming media. The fluids vented during emergency reactor discharges can thus be reliably separated. Compared with design recommendations given in the literature, significantly smaller equipment is required for the same separating performance.