Air in a hydraulic system has many detrimental effects on both component and system performance. The more obvious symptoms of an aerated system are system oscillation, power loss, cavitation, increased fluid heat, response lag, foaming, and “spongy” controls. Loss of power and a “soft” system can be directly attributed to a decrease in the bulk modulus (greater compressibility) of the fluid because of an increasing air content

Aspirator vacuums air from fluid

Unfortunately, bubbles get smaller with increasing pressure. The lowest pressure in the F-104 is about 30 psig. This pressure was far too high because even at this pressure the free air bubbles would be compressed and driven through the filter screen. Total bubble removal was not possible. The next logical step was to introduce an aspirator, which would reduce the pressure in the chamber so the bubbles would expand and not penetrate the filter screen.

It was at this stage that the scheme to use a jet pump (aspirator) to lower the pressure in the separating chamber was tried. By lowering the pressure acting on the fluid, large quantities of dissolved and free air were released and the filter screen barrier was able to separate the bubbles from the fluid.

Air reabsorbed quickly

During the development testing it was noted that fluid release of its air by the separator would quickly reabsorb air when left in an exposed container. The deaerated fluid acted somewhat like a sponge. This phenomenon has proved very beneficial in bleeding complex systems such as that of the F-104. The most remote reaches of the system are bled effectively by circulating treated (deaerated) fluid throughout the system, reabsorbing pockets of air. Nooks and crannies that would normally never get bled using conventional bleeding methods are easily cleared of air.

On F-104 aircraft, two separators–one for each hydraulic system–are in operation whenever the aircraft hydraulic systems are used, either in flight or on ground test stand power. Extracted air is easily bled-off during ground servicing with a  conveniently located pushbutton-operated bleed valve in each system that sequentially blocks jet pump flow and opens the air storage chamber to the atmosphere.

Sizing separator to system

An air-oil separator can be designed to fit most hydraulic system requirements by varying the volume of pressurized flow, separating chamber and filter size, and servicing frequency. The volume of separating flow and separating chamber vacuum pressure can be balanced by adjusting the size of the inlet restrictor. The jet pump should be sized according to the amount of pressurized flow that can be spared from the system and the amount of heat that the system can tolerate. (Pressure drop across the pump nozzle adds heat to the fluid.)

If an air-oil separator is considered during the early stages of system design, the separating filter can be integrated with system filtration requirements since the separator is ideally situated in a bypass loop of the system with relatively low flow rates and low return pressure.