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Water Activity in Oil

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AGC REFINING & FILTRATION

WATER ACTIVITY IN OIL 2

Contents Water 3

Solubility 3

Relative Saturation 4

Water Activity Related to a Hydroscav Oil Purifier 7

References 9



Water

Oil can contain a certain amount of moisture. Generally, this moisture exists in three forms:

Dissolved water, which is water bonded to the oil molecules

Emulsified water, which is water that exists in a tight combination with the oil

Free water, which is water that separates from the oil The maximum amount of moisture that oil can contain in solution is called its saturation point. Once the oil has reached its saturation point, any additional water that is added will separate out as free water, which then forms a distinct layer underneath the oil.

Figure 1: Comparison of Water in Oil Below and Above the Saturation Point

Courtesy of Vaisala

Where the saturation point of any oil lies depends on the following factors:

The composition of its base stock, (mineral or synthetic)

The type of additives, (oxidation inhibitors, emulsifiers etc.)

The presence of polar contaminants such as oxygen and sulfur

The presence of ionic impurities such as organic salts

The age of the oil

Major factors that influence the saturation point of any oil are temperature, its fluctuations, pressure, as

well as the solubility of water in the oil. Each oil temperature has its corresponding saturation point.

Solubility

Solubility is defined as the amount of water that can be dissolved in the oil at a specific temperature. The

solubility of water in oil varies directly with the oil temperature. As the temperature increases, the amount

of water that can be dissolved in the oil also increases.

Figure 2 shows the solubility of water in three types of oils.



Example 1

In Figure 1, consider the curve for mineral oil (1).At 20° C the amount of water that can be dissolved is

about 70 ppm.At 80° C the amount increases to approximately 300 ppm. Oil at a higher temperature can

hold more water in solution.If the water concentration rises above these values then the oil will become

super saturated and free or emulsified water will form.

Figure 2: The Solubility of Water in Oil at Different Temperature

Relative Saturation

When a measurement of water in oil is taken, the absolute amount of water is obtained in parts per million (ppm) or milligrams per liter (mg/l). This measurement is like a “snap shot” and it does not relate to the saturation point or to the temperature of the oil at the time the sample was taken. Since the saturation point of oil is dependent on the temperature of the oil, in any oil system such as a transformer or lubrication system, the water level will vary with the saturation point. A momentary measurement in ppm will give no indication of how close the moisture level is to the saturation point at that time.

A further complication is the fact that relative saturation also depends on the type of oil, the additive package, the age, the degree of contamination etc.



Some types of oil can be used safely at 750 ppm, while another type may be damaged at 200 ppm at the

same temperature. Figure 3 below shows the variation of solubilities between the various types of oil.

Figure 3: The Variation Between Relative Saturation of Various Oil Types

Courtesy Noria Corp.

When the measured water content approaches the saturation point of the oil, the risk of actually

exceeding the saturation point and forming free water becomes greater. The formation of free water must

be avoided because it is the most destructive contaminant in oil systems.



Example 2

The figure below shows the correlation between absolute water measurements in ppm and percent

relative saturation. The distance between the two measurements indicates the safety margin at the time

the measurement was taken. As can be seen, the saturation point is exceeded at about 82%. At that

moment free water is formed.

Note: In general, the relative saturation should be kept at a maximum of 50% to maintain a safe margin

between the absolute water concentration (ppm) and the point of saturation at which free water will begin

to form.



Figure 4: The Difference Between PPM and Relative Saturation

Courtesy Ron Barrett, Ontario Power

Water Activity Related to a Hydroscav Oil Purifer

Before the Hydroscav Oil Purifier is started, assume the following:

The compressor lube oil system is operating at equilibrium with the oil temperature at 55° C and 250 ppm absolute water concentration.

Assume no oil temperature rise after the bearings.

Assume the oil is ISO 32 with additives (Figure 2, Curve no. 2)

From the solubility graph the solubility of water in the oil at 55° C is about 320 ppm. Thus the Relative

Saturation in the lube oil tank is 250 ÷ 320 •100% = 78%.



Figure 5

The Hydroscav will maintain the lube oil in the tank at 80° C and 50 ppm absolute water concentration.

The solubility at 80° C according to the graph is about 750 ppm. Thus the Relative Saturation is now 50 ÷

750 •100% = 7%. This is well within the safety margin of 50%.

Even if the oil temperature drops to 55°–65° C when it is pumped through the bearings, at that point the

solubility is around 300 ppm and the Relative Saturation RH = 50 ÷ 300 •100% = 17%, which is still within

the safe margin.

Figure 6



References

1. Bauer, C. et al. “Water Contamination in Hydraulic and Lube Systems.” Pall Corp. Practicing Oil Analysis Magazine, September 2007. 2. Bloch, H.P. “On-stream purification of Turbo machinery Lubrication.” (1980). 3. Jiroutek, S. “Understanding Water Activity.” Vaisala Co., www.Vaisala.com. 4. Jiroutek, S. et al. “Dynamic Oil Systems holds quirks.” Vaisala Co., www.Vaisala.com. 5. Lewand, L. “Understanding Water in Transformer Systems.” Doble Engineering, Spring 2002. 6. Mamishev, A.V. et al. “Moisture Solubility for Differently Conditioned Transformer Oils.” IEEE Transac-tion on Dielectrics and Electrical Insulation 5 (October 2001). 7. Rake, B. “Water Contamination of Equipment Lubricating Oil.” Pump & Systems Magazine, January 2001. 8. Simon, R.J. “Establishment of Saudi Aramco-Abqaiq Maintenance Lube Oil Purification Standards.” (1985). 9. Troyer, D. “Establishing Moisture Contamination Targets for Hydraulic Systems.” Machinery Lubrication Magazine, January 2004.

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