The objective of this paper is to discuss site tests, where the swirling flow and the resulting vortex rope were registered. The vortex rope is a low-frequency phenomenon occurring in the Francis turbine draft tubes. The draft tube surge (instability in the draft tube containing swirling flow) is flow-induced and results in vibration in hydroelectric machinery, which causes instability, restricts the operation of the turbine in specific modes and as a result does not allow to generate as much power as the user would like. The tests also showed that draft tube surge, if severe, could have effect on the generator. The selected data were obtained on the set of the Francis turbine hydropower units (98 MW rated output, 13.8 kV, 50 Hz hydropower units with umbrella-type hydrogenerators) through many years of measurements in the Latvian hydro-electric power plant. The site tests showed that draft tube surge appeared at low generating power, as expected from the literature review, but this phenomenon was detected also quite close to the best efficiency point. The conditions necessary for the swirling flow to appear is not simply small generating power. It rather depends on the water head and the guide vanes opening position, which is adjustable in Francis turbines. For 98 MW unit the swirling flow would appear at 60 MW power, but it could also appear at 80 MW power at specific conditions presented in the paper.