Abstract: A conduit has an upper and a lower open end. A sealing ring surrounds the conduit exteriorly adjacent the lower open end, and a cover is mounted on the conduit to be located beneath the lower open end. The normally upwardly facing side of the cover is provided with a depression surrounded by an upstanding rim which is normally in engagement with the sealing ring. The cover is mounted for swinging movement, about a mounting shaft defining a swinging axis, from this normally closed position downwards to an open position. A counterweight normally holds the cover in the closed position, but when the liquid in the depression reaches a predetermined amount, the weight of the liquid triggers a quick opening of the cover, discharge of accumulated liquid, followed by quick closing of the cover. The cover is so mounted that the end thereof near the mounting shaft can move vertically relative to the mounting shaft.