The invention relates to an apparatus for removing impurities from a fluid, in particular to a hydrocyclone apparatus having a lower particle trap, the apparatus comprising a tank (10) with a sidewall (12) of substantially circular cross-section and an inlet (11) for introducing fluid into the tank, the inlet of flow being such that it is caused to circulate around the tank (10). An outlet (13) is arranged for conducting fluid, with impurities removed, out of the tank (10). A wall/screen means (15) is arranged in the tank and extends from a location (16) on the sidewall (12) in the direction of fluid circulation and across the outlet (13). The wall/screen means (15) is arranged at a distance from the sidewall such that fluid leaving the tank (10) will leave in the opposite direction of flow to that of the circulating fluid and will thus be relatively free of impurities.