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Water-glycol Hydraulic Fluid:
Water-glycol fluids contain from 35 to 60 percent water to
provide the fire resistance, plus a glycol antifreeze such
as ethylene, diethylene, or propylene which is nontoxic and
biodegradable, and a thickener such as polyglycol to provide
the required viscosity. These fluids also provide all the
important additives such as antiwear, foam, rust, and
corrosion inhibitors. Operating temperatures for
water-glycol fluids should be maintained below 49°C
(120°F) to prevent evaporation and deterioration of the
fluid. To prevent separation of fluid phases or adverse
effects on the fluid additives, the minimum temperature
should not drop below 0° C (32° F).
Viscosity, pH, and water hardness monitoring
are very important in water-glycol systems. If water is lost
to evaporation, the fluid viscosity, friction, and operating
temperature of the fluid will increase. The end result is
sluggish operation of the hydraulic system and increased
power consumption. If fluid viscosity is permitted to drop
due to excessive water, internal leakage at actuators will
increase and cause sluggish operation. A thin fluid is also
more prone to turbulent flow which will increase the
potential for erosion of system components.
Under normal use, the fluid pH can be
expected to drop due to water evaporation, heat, and loss of
corrosion inhibitors. The fluid pH should be slightly
alkaline (i.e., above pH8) to prevent rust. However, because
of their volatility and toxicity, handling of the amine
additives that stabilize the pH is not recommended.
Therefore, these essential additives are not usually
replenished. Fluids with pH levels that drop below 8 should
be removed and properly discarded.
Make-up water added to the system must be
distilled or soft deionized. The calcium and magnesium
present in potable water will react with lubricant additives
causing them to floc or come out of solution and compromise
the fluid’s performance. When this condition occurs the
fluid is permanently damaged and should be replaced. To
prolong the fluid and component life, water added to the
system should have a maximum hardness of 5 parts per million
(ppm).
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