Even if all the air bubbles and pockets were removed from a hydraulic system and the system totally enclosed, after a short run-in period, air bubbles would begin to reappear. The source of these bubbles is the working fluid itself because all fluids (except fully dearaeted ones) contain dissolved air. It is this dissolved air that, under certain conditions, comes out of solution and plagues the system.

Adsorption rate

Adsorption, rather than absorption, better describes the process by which bubbles under pressure in a hydraulic fluid are dissolved into the fluid. Adsorb means adhesion of extremely thin films of gases to surfaces with which the gases are in contact. Absorb means to soak up.

The adsorption rate of bubbles (entrained air) in a system is determined by their size. It is true that a fluid holds more dissolved air as pressure increases (Henry’s law). However, the size of the bubble determines at what pressure it will dissolve. Bubbles, with diameters of 0.020 to 0.030 inch, will dissolve at approximately 100 psi. Larger bubbles, however, will not dissolve until the pressure is proportionately greater. This behavior is true even when the amount of previously dissolved air in the fluid is almost nil.  We may say, therefore, that the rate of adsorption is a function of the pressure and an inverse function of the bubble diameter. Note that the bubble will reappear when the pressure is lowered.

Origin of dissolved air

Dissolved air will come out of solution when the fluid is exposed to a vacuum. A vacuum can occur in a hydraulic system across orifices; inside un-supercharged pumps; and inside double-acting actuators when driven by an external load, at a rate speed greater than the rate at which fluid can fill the opposite end.

Dissolved air, once out of solution, can be partly readsorbed in a moving stream when the local static pressure again exceeds 14.7 psia. When a stream containing bubbles is suddenly stopped, however, the bubbles will begin to migrate upwards and coalesce in the nearest high point. Repressurization may or may not drive the new, larger bubble back into solution.

Techniques of air removal and measurement

There is free air in a system before it is filled. Conventional fill and flush techniques will remove the major portion of it. However, some pockets will remain, depending n component and circuit design. Without a very low-vacuum fill procedure, it is next to impossible to remove them. When filling under vacuum, the fill fluid should be deaerated; otherwise it will release air when it comes in contact with the vacuum. Deareted fluid could adsorb some of the air in these pockets.

Where you deal with entrained air in an open-reservoir-system, a low bubble-point filter or mesh will help screen out the entrained air bubbles. To remove them entirely, the fluid should be processed through a deaerator as described above. Also, reservoir return lines should be placed well below the fluid level to minimize circulation and vortices.

Dissolved air is the hardest of the three types of air to remove y conventional means. The bulk of it can only be removed in the presence of a vacuum.

Creating a vacuum in a reservoir to remove dissolved air only damages the pump. (One of the primary purposes of a pressurized reservoir is to prevent pump cavitation, which occurs when a vacuum is created at the pump inlet.) Since the amount of dissolved air removed is proportional to the degree of vacuum, the best technique is again to use a deaerator, which will remove all forms of air in a circulating system.

Note that even when a deaerator of large capacity is used continuously in a system, the deaerator can bring an open system’s dissolved air content down only so far before the deaeration rate equals the rate of readsorption of air taking place in the open reservoir.

This article originally appeared in the October 1967 issue of Hydraulics & Pnematics. It is republished here for its continuing technical value.