Solubility of Dissolved Gases

Different gases/parameters have different solubility levels in mineral oil [DiGiorgio1]. It is important to correctly identify the levels of gases produced in the associated defective conditions before any dissolved gas analysis is attempted. The following table provides solubility levels of the gases in transformer mineral oil with a static equilibrium at 760mm Hg and 25 °C [DiGiorgio1]:

Table: Typical solubility levels of gases in mineral oil [DiGiorgio1]

Gases	Solubility
H2	7% by volume
N2	8.6%
CO	9%
O2	16%
CH4	30%
CO2	120%
C2H6	280%
C2H4	280%
C2H2	400%

The figure below provides some indication of the relative solubility of the combustible gases as a function of temperature [DiGiorgio1]. Gases such as hydrogen, nitrogen, carbon monoxide and oxygen have an increasing solubility rate with increasing temperature. The hydrocarbon gases methane, acetylene, ethylene and ethane together with carbon dioxide have a decreasing solubility rate with increasing temperature. It is noted that both hydrogen and carbon monoxide are not very soluble at low temperatures but the solubility rate does improve drastically for hydrogen at operating temperatures around 70-80°C. The solubility rate of methane is moderate, decreasing with increasing temperatures. These aspects play an important role and must be taken into consideration when assessing the gas production rates as assessed from oil samples.

Figure : Relative Solubility as a function of Temperature [DiGiorgio1]

References

[Digiorgio1]

DiGiorgio, J. B., “Dissolved Gas Analysis of Mineral Oil Insulating Fluids,” DGA Expert System: A Leader in Quality, Value and Experience 1, Northern Technology and Testing, pages 1-17, http://www.nttworldwide.com/tech2102.htm, 2005