Online Sensor Measures Dissolved Water in Lubricating Oils and Hydraulic Fluids

It is almost certain that some amount of water is present in hydraulic and lubrication systems. Because of its destructive potential, hydraulic and lubrication systems are best operated with no free or emulsified water present.

Water normally enters a system through external influences; the most common sources are moisture from the surrounding air entering through reservoir openings and breathers, condensation, “splash” from process water, system wash down water and contaminated new oil.

Water contamination accelerates the aging process resulting in oxidation, hydrolysis, additive depletion, reduced lubricant film strength, corrosion and damaged components. Most of these expensive problems can be avoided by monitoring and controlling the operating fluid’s water content.

Figure 1. Example of a Hydraulic Oil Saturation Curve

Monitoring Percent Water Saturation Levels

A practical method for monitoring water in oil is to report water concentration as a percent of the saturation level. Water above the saturation point is defined as free or emulsified water and is more harmful than dissolved water (levels below the saturation point).

Different oils are capable of dissolving varying amounts of water depending on their basestock and additive composition and, therefore, have varying water saturation curves. The curve in Figure 1 shows the relationship of water saturation level versus fluid temperature in a typical mineral-based hydraulic or lubricating oil. The curve shows that most lubricants are capable of holding more water, or have higher saturation levels, as the temperature increases.

A New Way to Monitor Saturation

The new HYDAC AS 2000 series Aqua Sensor is a stationary, microprocessor-based measurement unit designed to continuously monitor water saturation and temperature in hydraulic and lubrication systems. The saturation level indicates the percentage of water dissolved in the oil relative to the maximum possible amount of water that can be dissolved in the oil at a specific temperature. A reading of zero percent would indicate oil is free of water, while a reading of 100 percent would indicate oil that is completely saturated.

Hydac AS2000 Aqua Sensor

Consider the example of a plant having an oil with a saturation curve similar to the graph in Figure 1, where the ambient temperature is 68°F and the oil operating temperature is 104°F. The saturation level curve shows the saturation level to be 600 parts per million (ppm) at 104°F but only 200 ppm at 68°F.

It is important for the user to know when his operating fluid reaches its ambient temperature saturation limit so he can avoid the introduction of free water into his system during shutdown hours; therefore, the user at the plant would want to know when his oil is 33 percent saturated at 104°F.

If the saturation characteristics of the oil are known, ppm can be calculated by multiplying the percent saturation reading at a given temperature by the known ppm saturation limit at that temperature. The HYDAC AS 2000 reports saturation level and temperature continuously to the integrated display and built-in alarm relays. This information can be captured via PLC or PC.

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