In this Lab Minute, our expert Vincent Bouillon, Salesforce Manager of BfB Oil Research, will share comments about sources and risks of air introduction in a hydraulic circuit. He will also introduce the  dedicated test run in our laboratories.

Air sources and risks : What are the air introuction sources in a hydraulic system and the associated risks ?

Hydraulic systems are affected by the technical performance of the fluid as well as its physical and chemical properties.

Lubricating oil is not completely free of air.

The amount of dissolved air depends on the nature of the chemicals in the lubricant, but also on the temperature and pressure at which it is subjected.

One of these parameters’ change can transform dissolved air into free air, generating air bubbles in the oil.

The main air introduction ways are related to flow rate, oil quantity, leaks and circuit design.

The air can then cause damage to the system.

Analysis : de-aeration test

The laboratory test that evaluates the ability of air to escape from the oil is called the “Air release test”.

Many similar methods exist (ISO 9120 – ASTM D 3427,…).

Air is blown into the oil (the following datas are applied if its viscosity is between 9 and 90 mm²/s):

• during 420 seconds
• at a temperature of 50°C,
• under a pressure of 20 kPa.

When air blowing is stopped, air bubbles escape from the fluid.

The air release changes the density of the fluid.

We measure this variation to determine the air release value :

Air release value indicates the amount of time it takes to reduce the amount of dispersed air in the oil to 0,2% (V/V).

Conclusion : air release value, a critical parameter 

The air release value is a critical parameter which is hardly improved by the presence of additives.

This property is altered by the presence of polar compounds, such as oxidation by-products, impurities or some additives.

An expert’s advice :

It is important to follow this property on new oil but also to include this check in a full oil condition monitoring program.

Note: This test is also essential for monitoring compressor and turbine oils.

► Click here to download a PDF of “Risks associated with the presence of air in a hydraulic circuit”

This post is also available in: French