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Industrial fluid power applications have increased worldwide over the years. Hydraulic fluid performance demands have increased in operating pressures, safety and reliability. As operating pressures increase, the risk of fire from ruptured lines also increases. It is necessary to balance management’s regulatory and insurance interests with equipment requirements for effective lubrication, wear and corrosion protection.
Fire-resistant fluids include synthetics such as phosphate esters or ester-mineral blends and water-based formulas such as water-oil emulsions or water-glycols. Water glycol fluids have proven to be an excellent fire-resistant hydraulic fluid option.
The fire-resistance of these fluids depends upon the vaporization of the water and the smothering effect of the steam. The other performance characteristics important to these fluids are viscosity, lubrication quality, operating temperature range, corrosion resistance, system compatibility and fluid maintenance. Excellent fire-resistance coupled with good cost and performance makes water glycol fluids the right choice for many industrial applications.
Water glycol fluids consist of a solution of water, ethylene or diethylene glycol, a high molecular weight polyglycol and an additive package. The water-to-glycol mixture typically contains 38 to 45 percent water. These fluids usually contain red or pink dye to aid in their identification.
With water in the fluids’ formulation, evaporation is ongoing and upper operating temperature limits must be considered. Checks must be made periodically of the water content. The fluids’ typical operating temperatures should be kept below 150°F.
The polyglycol is a water-soluble polymer thickener, which can be formulated to cover a wide range of viscosities. The resulting viscosity-temperature properties are Newtonian and give water glycols good low-temperature cold-start pump wear protection as well as minimizing cavitation.
The additive package imparts corrosion resistance, metal passivation, seal and hose compatibility, oxidation resistance, antimicrobial properties and antiwear properties. With a density of about 1.0, mineral oil contaminants may float on the fluid surface and be skimmed off. Finally, water glycol fluids have better thermal transfer properties over other fire-resistant fluids.
Water glycol fluids usually have an operating range up to 2000 psi at less than 150°F. Their lubricating quality is very good where loads are moderate and where only hydrodynamic lubrication is involved. When the application has high bearing loads and extreme boundary lubrication conditions, higher wear rates should be expected. Typical applications include:
There are some application limitations due to compatibility when using water glycol fluids. Regarding metals, the fluid is corrosive to zinc, cadmium and nonanodized aluminum, and the reaction with these metals causes rapid deterioration of the fluid.
Synthetic rubber seal and gasket compatibility is good, however polyurethane, leather or cork materials should be avoided. Typical paints will soften in the presence of water glycols; therefore painted surfaces should be painted with epoxy resin paints.
Testing should be conducted initially to measure the water glycol fluid’s ability to meet performance specifications. When charged to a system and during use, water glycol should be periodically tested as part of a maintenance condition-monitoring program.
Given that fluid performance results may differ significantly, as shown in Table 2, fluid performance based on standardized (ASTM) tests should have a major influence in product selection. A Midwestern steel plant recently requested five major commercially available water glycol fluids be evaluated for lubrication performance. Table 2 shows that significant variation is possible. The fifth fluid is unqualified for use.
Note the apparent lack of correlation in Table 2 between the vane pump test (combination boundary and hydrodynamic lubrication) and the boundary lubrication measurement of the Four-ball Wear Test. Figure 1 shows the sample specimen for ASTM D2882 vane pump testing on fluids A (left) and E (right). Fluid E clearly shows the excessive metal scuffing and wear.
Testing used fluid samples by industry standard practices should be scheduled with a competent testing laboratory. The testing lab should be qualified to provide recommendations from fluid analysis for both fluid and equipment long-term care. The frequency of the sampling will depend on the same factors common to most lubricant condition-monitoring programs:
Severity of operation, environmental cleanliness, maintenance philosophy and testing observations. The initial frequency for testing systems that are already in production should be monthly, or more frequently given operating conditions and mission criticality, and should include the criteria in Table 3.
Low water levels can be corrected by following the OEM’s published table or curve of water amounts to add per testing results.
The most common fluid faults noted in samples tested from water glycol users are particle contamination, contamination with other fluids and water loss or accumulation shifting the viscosity.
|Figure 1. Hydraulic Pump Test Ring for Fluid A and Fluid E|
Particle and dirt contamination is a problem for water glycols more than mineral oils because of the affinity of the polymers to hold the fine particles in suspension. Good maintenance practices and filter management is required.
Contamination from mineral oils is readily observable by visual appearance (pink milky emulsion sample) or FTIR. Figure 2 shows a used normal fluid and a used contaminated fluid. The milky appearance and the layer of mineral oil on the top of the sample fluid suggest an oil/water emulsion condition. Contamination often occurs due to the widespread use of mineral oils near or on the equipment using the water glycols, or as a result of direct contamination resulting from poor reservoir top-up practices.
Figure 2. Water Glycol Fluid. Used Normal Sample (left), Mineral Oil Contaminated (right)
Water loss due to evaporation or accumulation due to the intrusion of free water such as cooling water can be measured accurately by Karl Fischer titration or a refractometer. Make-up water must be distilled or deionized (DI), such as from boiler feed water condensate. Water concentrations should be maintained according to guidelines provided by the OEM. This may mean adding either glycol concentrate or DI water to systems during the lifecycle of the product. This is done to retain proper viscosity and fire-resistance properties.
Water glycols must not be mixed with nonwater-based hydraulic fluids and preferably not with other brands of water glycols. Additive packages in various brands may conflict, resulting in loss of fluid performance. The alkalinity reserve additive does deplete by evaporation. The manufacturer can help users manage fluid alkalinity by supplying supplemental additive.
Water glycol fire-resistant hydraulic fluids are a reliable, cost-effective option for hydraulic power. When maintained properly, they give long, predictable life.