Detecting and Controlling Water in Oil

Marianne Duncanson, 

Moisture is considered a chemical contaminant when suspended or mixed with lubricating oils. It presents a combination of chemical and physical problems for the lubricant and machinery, respectively. The potential problems, states of existence and methods for measuring moisture are discussed here.

Effects of Water on Equipment and Lubricants

The effects of water are insidious. Failure due to water contamination may be catastrophic, but it may not be immediate. Many failures blamed on lubricants are truly caused by excess water. The following are some of the effects of water on equipment:

• Shorter component life due to rust and corrosion
• Water etching/erosion and vaporous cavitation
• Hydrogen embrittlement
• Oxidation of bearing babbitt
• Wear caused by loss of oil film or hard water deposits

Rust and Corrosion

Water attacks iron and steel surfaces to produce iron oxides. Water teams up with acid in the oil and corrodes ferrous and nonferrous metals. Rust particles are abrasive. Abrasion exposes fresh metal which corrodes more easily in the presence of water and acid.

Water Etching

Water etching can be found on bearing surfaces and raceways. It is primarily caused by generation of hydrogen sulfide and sulfuric acid from water-induced lubricant degradation.

Erosion

Erosion occurs when free water flashes onto hot metal surfaces and causes pitting.

Vaporous Cavitation

If the vapor pressure of water is reached in the low-pressure regions of a machine, such as the suction line of a pump or the pre-load region of a journal bearing, the vapor bubbles expand.

Should the vapor bubble be subsequently exposed to sudden high pressure, such as in a pump or the load zone of a journal bearing, the water vapor bubbles quickly contract (implode) and simultaneously condense back to the liquid phase.

The water droplet impacts a small area of the machine’s surface with great force in the form of a needle-like micro-jet, which causes localized surface fatigue and erosion. Water contamination also increases the oil’s ability to entrain air, thus increasing gaseous cavitation.

Hydrogen Embrittlement

Hydrogen embrittlement occurs when water invades microscopic cracks in metal surfaces. Under extreme pressure, water decomposes into its components and releases hydrogen. This explosive force forces the cracks to become wider and deeper, leading to spalling.

Film Strength Loss

Rolling element bearings and the pitch line of a gear tooth are protected because oil viscosity increases as pressure increases. Water does not possess this property. Its viscosity remains constant (or drops slightly) as pressure increases. As a result, water contamination increases the likelihood of contact fatigue (spalling failure).

The effects on lubricating oil can be equally harmful:

• Water accelerates oxidation of the oil
• Depletes oxidation inhibitors and demulsifiers
• May cause some additives to precipitate
• Causes ZDDP antiwear additive to destabilize over 180°F
• Competes with polar additives for metal surfaces

Maximum Recommended Water Concentrations

Oil, unless it is dried, contains some dissolved water. Figure 1 shows the amount of dissolved water that can be found in ISO 220 paper machine oil and ISO 32 turbine lubricant before it turns cloudy.

Figure 1. Dissolved Water as a Function of Temperature
in Paper Machine Oil and Turbine Oil

Table 1 helps determine the relative life of mechanical equipment versus the amount of water in the lubricant. To use the chart, estimate the current moisture level in the system along the y-axis, move toward the right to the target moisture level. The top of the chart gives the estimate of how much the life of the oil is extended. For example, by reducing moisture from 2,500 ppm to 156 ppm, machine life is extended by a factor of 5.

	Life Extension Factor
Current Moisture Level	ppm	2	3	4	5	6	7	8	9	10
	50,000	12,500	6,500	4,500	3,125	2,500	2,000	1,500	1,000	782
	25,000	6,250	3,250	2,250	1,563	1,250	1,000	750	500	391
	10,000	2,500	1,300	900	625	500	400	300	200	156
	5,000	1,250	650	450	313	250	200	150	100	78
	2,500	625	325	225	156	125	100	75	50	39
	1,000	250	130	90	63	50	40	30	20	16
	500	125	65	45	31	25	20	15	10	8
	250	63	33	23	16	13	10	8	5	4
	100	25	13	9	6	5	4	3	2	2
Table 1. Moisture Life Extension Method

Tests for Water in Oil

The guidelines in Table 1 help only if it is known how much water is in the oil. There are several qualitative and quantitative tests to determine water content. The easiest one to perform is a simple visual test. An ISO 68 turbine lubricant was observed at room temperature with controlled amounts of water. Table 2 shows the results of the test.

Amount of water, ppm	Appearance of oil
0	Bright and clear
100	Trace of translucent haze
200	Slight translucent haze
250	Translucent haze
500	Opaque haze
1000	Opaque haze with slight water drop out
Table 2. Visual Check of Water in Turbine Oil

Bear in mind that several factors can affect the cloudy or hazy appearance of the oil. First, as the oil sits, it will clear up and the oil may become supersaturated. Second, dye and dark-color oil can mask cloudiness.