A once-through steam generator including one or more steam-generating circuits extending between inlet and outlet ends thereof and including one or more pipes, the steam-generating circuit having a heating segment at least partially defining a heating portion of the once-through steam generator, and one or more heat sources for generating heat to which the heating segment is subjected. The steam-generating circuit is adapted to receive feedwater at the inlet end, the feedwater being subjected to the heat from the heat source to convert the feedwater into steam and water. The pipe has a bore therein at least partially defined by an inner surface, and at least a portion of the inner surface has ribs at least partially defining a helical flow passage. The helical flow passage guides the water therealong for imparting a swirling motion thereto, to control concentrations of the impurities in the water.