Water in Oil White Paper

1. Water Activity in Oil
WHITE PAPER
© 2017 AGC Refining & Filtration LLC

2. 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

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WATER ACTIVITY IN OIL 3
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.

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WATER ACTIVITY IN OIL 4
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.

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WATER ACTIVITY IN OIL 5
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.

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WATER ACTIVITY IN OIL 6
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.

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WATER ACTIVITY IN OIL 7
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%.

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WATER ACTIVITY IN OIL 8
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

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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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