A separator (1) of a boiler drum for separating steam and water, which separator (1) has a separating chamber (5) having a fluid inlet (2) at the bottom of the separating chamber (5) and a steam outlet (3) at the top of the separating chamber (5) and spiral vanes (9) are attached to the wall (6) of the separating chamber (5) between the fluid inlet (2) and the steam outlet (3) for accomplishing circular motion of the incoming fluids and wherein a circular water outlet (4) surrounds the steam outlet (3). Top end (8) of the wall (6) of the separating chamber (5) within the water outlet (4) has a rounded top contour for ensuring smooth flow and keeping a water trap (13) filled up at the top art of the water outlet (4) and is inclined inwards. The inclination at the top end (8) of the wall (6) is concave.