Float accumulator is suitable for use with a hydraulic system for an oil well blowout protector. It is provided with a vented float movable along a transversely centered, normally vertical guide rod according to the level of the liquid fill within the vessel. The vent outlet for the float and a ballast weight for the float are positioned on opposite sides of the guide rod and the float is mounted for free turning around the guide rod. Whatever the degree of liquid fill of the vessel, sidewise tilting of the vessel will be accompanied by rotation of the vent outlet to the then-high side of the guide rod to maintain the float directly vented to the ullage space of the vessel.