Care for water-glycol hydraulic fluids.
Water-glvcol hydraulic fluids are the closest thing to a permanent hydraulic fluid being offered in the market today. Unlike petroleum oil or anhydrous synthetic fluids such as phosphate esters or polyol esters, water-glycol hydraulic fluids are not degraded by heat or metals in the system that act as catalysts.
Water glycol fluid is composed of polyglycol thickener, glycol (ethylene, diethylene, and propylene), water, and additives. Additives usually account for between 5% and 10% of the total fluid composition. These additives include corrosion inhibitors in the liquid phase or vapor phase, lubrication, and metal passivators for copper, brass, and bronze. Additives may also include a dye.
The water-glycol composition will have major effects on a system's design criteria. Water will strongly affect design. It has higher vapor pressure than oil, so it's prone to cavitation. Higher vapor pressure requires that the pump inlet be free of restrictions.
The pump should not be set too far above the fluid. The positive-head pump mounts below fluid level, allowing fluid weight to supercharge the pump's inlet to offset the effects of water's high vapor pressure. Inlet strainers should be no finer than 100, with 60 mesh as the optimum. If return-line filtration is in place, and a filter breather is being used, no strainer is required in the setup - Filter elements should be compatible with alkaline water solutions. Elements can be as fine as three microns absolute, which is the normal recommendation for servo applications. Filtration can be either pressure, return-line, or side-stream loop built onto the reservoir.
Longer fluid life
The two keys to extending life and maintaining performance are viscosity and pH. Viscosity measurements are an indicator of water content. For water-glycol fluid, pH is a measure of the corrosion-inhibiting properties of the fluid.
High-viscosity is an indication that water is being lost, probably through evaporation. This is a natural occurrence; When viscosity increases to a level approximately 50 SUS at 100 F above the viscosity of new fluid, water adjustments should be made. It is crucial that water be distilled, soft deionized, or boiler condensate. Hardness must not exceed 5 ppm.
Low viscosity in a water-glycol fluid is usually a result of too much water in the fluid. In case of an over-adjustment to correct a problem, the addition of too much water is an attempt to bring high-viscosity fluids back into specificaton. If soft distilled or deionized water is used for this adjustment, one half of the low-viscosity fluid should be removed from the system.
In the case of a heat-exchanger leak, it is important to realize that the water used for cooling in heat exchangers is typically city water or cooling-tower water. Its calcium and magnesium ions will react with the lubricant additive in water-glycol fluids. The reaction product is a white, soapy solid that can be filtered out of the fluid.
Hard water permanently damages water-glycol fluids, and those that have been contaminated should be removed from hydraulic equipment and disposed of properly. Viscosity measurements can be done on site using a viscosity gage. To determine the corrosion-inhibiting properties, pH should be examined. To inhibit rust in a hydraulic system, the pH of water-based fluids must be maintained at 8.0 or above. This is an alkaline or basic pH, with a neutral pH at 7.0 on a scale from 1 to 14. water losses, the pH of a fluid will most likely drift downward. As water adjustments are made to reduce viscosity, amines, the basic materials that contribute to pH, are not being replenished. Fluids with a pH of less than 8.0 should be removed from the hydraulic system and disposed of properly.
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|Author:||Skoog, Peter N.|
|Publication:||Tooling & Production|
|Date:||Apr 1, 1991|
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