Oil degassing

Oil degassing is the process of removing undesirable dissolved gases from the oil (separately or all together).^[1] Gas content is usually measured and monitored by volume, expressed as percentage (%).^[2]

The influence of gas on industrial oils

The content of gas in oil at atmospheric pressure is 10%, but can increase with the temperature of the oil. While ratio of nitrogen to oxygen in the atmosphere is 78% to 21%, the ratio in oil, such as transformer oil, is 69.8% to 30.2% ^[3] Oxygen is the cause of oil oxidation and reduction of its dielectric properties. This is also related to the fact the gas inclusions often become the centers of electric discharge development.

Operational factors which contribute to growth of gas content in oil

The following factors increase the concentration of gas solved in oil:

• increased loads;
• mixing of new oil with used gas saturated oil;
• adding used oil with high solved gas content;
• oil pump damage;
• overheating due to cooling system failure etc.

Industrial oil degassing

The most efficient method of industrial oil degassing is vacuum processing, which removes air and water solved in the oil.^[4] This can be achieved by:

• spraying of oil in large vacuum chambers;
• distributing the oil into a thin layer over special surfaces (spiral rings, Raschig rings etc) in vacuum chambers.

Under vacuum, an equilibrium between the content of moisture and air (solved gases) in the liquid and gaseous phase is achieved. The equilibrium depends on the temperature and the residual pressure. The lower that pressure, the faster and more efficiently are water and gas are removed.

Gas content control methods

Gas content control is essential for equipment which can be negatively influenced by gas content in the oil (such as power transformers).^[5] Gas content can be measured by devices which measure the changing residual pressure in an oil sample container. Such tests offer quantitative indications of gas content, but no qualitative analysis. There is a method which allows diagnosing the equipment the oil is used in by analysis of its gas content. This is referred to as dissolved gas analysis (DGA), and is based on gas chromatography.^[6] DGA can help detect a developing problem or determine the cause of a malfunction. Prevalence of a specific gas in the oil corresponds to certain defects, i.e. carbon monoxide and dioxide indicate aging of oil and high water content, ethylene indicates that the oil reaches temperatures in excess of 600°С, methane indicates oil temperatures between 400-600°С etc.

References

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