One trend in industrial manufacturing is for systems, as well as components, to be designed in a more compact fashion.

The benefits of smaller machinery are obvious — economy of materials, less energy consumption, less square-footage required. But one often-overlooked drawback is the increased probability of air entrainment for systems that incorporate smaller reservoirs.

Bubbles in working fluids greatly influence the performance of hydraulic systems and may cause major problems, such as bulk modulus change, cavitation and aeration, degradation of lubrication, noise generation, oil temperature rise, and deterioration of fluid quality. When bubbles in fluid are compressed adiabatically at high pressure, the temperature of the bubbles rises sharply, and the surrounding fluid temperature also rises. Thus, it is important to eliminate the bubbles from the fluid to preserve its quality, system performance, and to avoid possible damage of the components.

Bubbles vs. foam

What is a bubble? It is a general term that can refer to various phenomena that occur at the surface of a gas suspended in a fluid. However, for technical use, the terms bubble and foam must be strictly defined. In this article, bubble means small gas particles entrained and dispersed in the fluid, Figure 1. Conversely, foam is comprised of many gas pockets surrounded by a thin film and is created when fluid is mixed rapidly with air from the free surface. It occurs when bubbles in the fluid become enlarged and rise to the fluid surface, or when fluid is ejected into a reservoir.

Troubles caused by bubbles

Foam causes problems when it overflows the reservoir. In such cases, the problem may be easily solved by adding ester or silicon oil to the fluid as an anti-foaming agent, or by repairing the equipment to eliminate foam generation.

Bubbles can be created in many ways. Following are sources of air ingression, where bubbles may be generated in a hydraulic system:

Release of dissolved air — All hydraulic fluids contain an amount of dissolved air, which can be released when the pressure is decreased rapidly. This can occur at valves and orifices, as well as where the fluid returns to the reservoir.

Mechanical introduction — Entrained air can be introduced at points in the system where there is a vacuum, such as leaks in the pump suction line.

Improper bleeding — When initially filled, the hydraulic system will contain air in all forms (free, entrained, and dissolved). For proper operation, the system should be properly bled to remove air.

Improper addition of make-up fluid — Air may be entrained in the fluid if splashing occurs when fluid is added, or if the added fluid causes increased agitation in the reservoir.

Contamination — One common source of increased air entrainment and foaming is fluid contamination by surface-active compounds. Alternatively, the fluid may be contaminated in such a way that causes precipitation of the antifoam or air release agent, resulting in significant increases in air entrainment.

Improperly designed reservoirs — The overall dimensions should enclose a sufficient volume of oil to permit air bubbles and foam to escape during the resident time of the fluid in the reservoir. The depth must be adequate to ensure that during peak pump demands, the fluid level will not drop below the pump inlet. The pump should be mounted below the reservoir so that a positive pressure is maintained at all times. This is most critical when water-based hydraulic fluids are used, as these fluids have a higher specific gravity as well as a much higher vapor pressure than mineral oil-based fluids.

Baffles should be provided to prevent channeling of the fluid from the return line to the pump inlet. A breather filter cap should be provided to admit clean air and to maintain atmospheric pressure as fluid is pumped into and out of the reservoir. With water-based hydraulic fluids, a pressurized reservoir is recommended to compensate for the pressure drop at he pump inlet due to the vapor pressure of the fluid.

The bubbles themselves often are created at the following locations or by the following conditions within the system:
● where there is a pressure drop,
● at a throttle or orifice,
● at a branch or pipe joint,
● from a valve opening and closing rapidly,
● from shock waves, due to sudden closing of valves or cessation of pump operation, and
● due to pressure drop at the pipe end, due to sudden opening of valve.

Bubbles entrained in the fluid can create many problems in hydraulic systems, such as:
● acceleration of oil degradation by oxidation,
● decreasing lubricity caused by an air emulsion,
● reduction of thermal conductivity,
● cavitation erosion,
● higher noise emission,
● increase in compressibility and decrease in dynamic characteristics, and
● decrease in pump output efficiency.