Air bubbles rising up from the oil in the container may form a surface foam which can have a disruptive effect within the hydraulic system. Since surface foam causes cavitation in pumps, it must not be drawn in. This can be ensured by means of various design features in the container:
- keeping the return and suction line as far apart as possible,
- using barriers and
- angled walls (covers) to prevent any direct flow from the return line to the suction line.
More foam tends to form if the surface tension, oil contamination and the like are high. This can be counteracted by using an anti-foaming agent, although these do tend to restrict the air release characteristics.