The appearance and disappearance of dissolved gases, in the form of entrained air, is an elusive phenomenon. Accelerated fluid through an orifice causes a local static pressure drop. If the pressure drops below atmospheric, dissolved gas is drawn out of solution and appears in the form of tiny bubbles (entrained air). If these bubbles do not conglomerate into larger bubbles, and if the velocity of fluid is low, most of the air bubbles are readsorbed down-stream where the static pressure is greater than atmospheric. This phenomenon agrees with Henry’s law.

An exception to this condition occurs when the fluid is accelerated above a velocity of 100 ft./sec. The air bubbles expand to larger size and are reluctant to go back into solution despite subsequent exposure to higher pressures. Erosion of materials has been known to take place in the vicinity of bubble growth. The reluctant of the bubbles to be readsorbed downstream appears strange at first until it is realized that the fluid immediately adjacent to the bubbles is undoubtedly saturated and cannot adsorb additional air.

When a stream containing bubbles is stopped suddenly, 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.

Results of air removal

The hydraulic cylinder, described above, was tested while assembled in a system to evaluate the effects of air removal. A vacuum-type oil deaerator was operated for eight 15-minute cycles. A floating piston covered the reservoir fluid (“closed reservoir”) and the dissolved air content versus running time is shown in Figure 6. The actuator was then recycled at low pressures, as before, and air measurements were taken every six cycles, Figure 7.

System pressure was then raised, as before, to 1000 psig. Ten cycles were made to achieve a specific level of air content diminished (in Figure 7 compared to Figure 4a).


As a general conclusion, it is readily apparent that several areas of performance can be improved as a direct result of deaerating system fluid. Three other specific conclusions are drawn from these tests.

Effects of flushing. The effect of continuous, hard-over cycling upon air content agreed well with what could be expected intuitively. In fact, the final values achieved were far lower than what would be imagined for a cylinder with “built-in air pockets. Assiduous cycling can therefore be expected to effectively purge any given system of air.

Aeration due to dissolved air. The results of the high pressure cycling indicate that systems using air-saturated MIL-H-5606 fluid, or similar hydraulic fluids, at pressures of about 1000 psig or greater, will generate entrained air across orifices in the system.

Low pressure hard-over cycling of actuators can help remove the resultant air if the cylinder-to-valve lines are short.

Cylinders working unloaded and only in the mid-stroke range, with infrequent hard-over to hard-over signals, can expect increasing air contents with time.

Effects of Deaeration. Deaerating the system fluid prevented completely the generation of entrained air across the simulated valve orifice.

Previous tests, conducted with unsaturated fluid, indicated that accelerated purging also takes place because of the adsorption of small bubbles into the fluid. Further work is now underway to define qualitatively the reduction of pump nose and material erosion by use of unsaturated hydraulic fluid.

Caution with bulk modulus figures

A word of caution is necessary. The values shown are not submitted as pragmatic numbers to be used with abandon. If anything, they point out that for any specific system, actual measurements should be made rather than rely on “text book” values.