Problems caused by bubbles


Problems caused by bubbles 

Problems due to air bubbles such as product defects and measurement problems vary widely depending on the type of liquid, but let's look at a hydraulic system as a typical example.


Air Trap 

Dispersed bubbles dissolve in oil when pressurized by a pump, but they will precipitate again as bubbles when the pressure drops. From the above, when the hydraulic system is stopped and left unattended, the deposited bubbles will rise and collect at the highest part of the liquid. This can cause unexpected troubles such as cylinder malfunction, breathing, and pressure fluctuations. Known examples of troubles include sudden operation of hydraulic cylinders and combustion deterioration of cylinder packings.


Oil Temperature Rise 

If the bubbles are momentarily pressurized by a pump, the temperature will rise rapidly. The approximate value can be easily calculated assuming that the gas does not dissolve in oil and is adiabatically compressed. For example, when a bubble at 35℃ is pressurized to 3.5MPa, it reaches 580℃ in calculation. When the bubbles become hot, the oil around the bubbles burns, causing the oil temperature to rise. Since air does not easily transfer heat, inclusion of air bubbles in oil lowers the heat transfer coefficient and reduces cooling performance. The combination of these factors can cause air bubbles to affect the cooler capacity.


Oxidative Deterioration of Oil 

A rise in temperature is a factor that promotes oxidative deterioration. Due to the principle of oxidation reaction, the oxidative deterioration rate of hydraulic oil doubles each time the temperature rises by 10°C above 60°C. The life of hydraulic oil will be shortened. A large amount of tank oil is required to make the oil change intervals the same.


Reduced lubricity due to Aeroemulsion 

Air bubbles reduce the oil film strength, cause metal contact inside the equipment, and accelerate wear. It shortens the life of hydraulic equipment.


Other problems

(5) Increase in noise

Cavitation erosion

Decrease in dynamic characteristics due to increase in compressibility

Decrease in pump discharge efficiency


And so on.


The approximate rising speed of the bubbles can be calculated, but it takes a long time to float the dispersed bubbles in the oil tank. It is difficult to float fine bubbles created by a pump or cavitation bubbles ejected from a relief valve, even if they are retained in a tank for a long time. This is where the idea of ​​forced bubble removal comes into play.