What is in this article?:
Hydraulic fluid can be the most vital component of a hydraulic system, so you must carefully consider dozens of characteristics before making a final selection.
The readily biodegradable question
One question that often comes up is whether a fluid is readily biodegradable or just biodegradable. Most things are biodegradable, given enough time and proper conditions. Readily biodegradable means that a substance exhibits a result equal to or greater than a pre-set requirement in a standard test.
For example, XYZ Standard requires 80% or higher biodegradation by CEC L-33-A93 in 21 days. If a lubricant meets this requirement, it is considered readily biodegradable by the XYZ Standard. Ideally, any claim that a lubricant is readily biodegradable also also specify the test and standard.
Vegetable oils or synthetic esters?
Being naturally occurring esters, vegetable oils are susceptible to hydrolysis, which leads to fluid decomposition and degradation, especially in the presence of heat. Because of their polarity, vegetable oils tend to cause elastomers to swell, though in most cases the degree of swell is insufficient to cause any serious concern in hydraulic applications.
On the other hand, vegetable oils offer excellent lubricity, intrinsic high viscosity index, and good anti-wear and extreme-pressure properties. Well-formulated, biodegradable hydraulic fluids based on vegetable oils can easily pass the demanding Vickers 35VQ25 or Denison T5D-42 vane-pump wear tests. They also can meet the requirements of major OEMs for premium hydraulic fluids, except hydrolytic, thermal, and oxidation stability. Experience has shown that vegetable oil-based biodegradable hydraulic fluids can perform satisfactory for years under mild climate and operation conditions (temperatures below 160° F, and hydraulic systems kept free of water contamination).
The use of synthetic esters - typically polyol esters - provides better hydrolytic, thermal, and oxidative stability, and excellent low-temperature fluidity, while preserving the high biodegradability and low toxicity of the fluids. For nearly 30 years, polyol esters have been used to formulate aviation gas turbine lubricants, which demand high thermal and oxidation stability at extreme temperatures. While a vegetable oil-based hydraulic fluid can perform between 0° to 180° F, a similar fluid based on synthetic esters can be used between 25° and 200° F. Similar to vegetable oils, synthetic esters have the tendency to swell and soften elastomers, although again, the swell should not be a concern for most hydraulic applications.
Vegetable oil or synthetic ester-based biodegradable hydraulic fluids are fully miscible with each other and with petroleum hydraulic fluids.
However, when a biodegradable hydraulic fluid is mixed with petroleum lubricants, its biodegradability typically decreases, and its toxicity increases. Because of their susceptibility to hydrolysis, vegetable oil- or synthetic ester-based fluids should be kept free of water contamination, both in storage and in everyday use.
There is no regulation permitting shortcuts in the disposal of biodegradable hydraulic fluids. Such disposal should be handled in the same manner as the disposal of petroleum fluids, in accordance with applicable federal, state, and local laws and regulations.
The future of biodegradable fluids
Government regulations and codes, and the environmental awareness of lubricant users are the driving forces for the growing use of biodegradable hydraulic fluids. However, the lack of definition and standards for biodegradable fluids in the United States impedes the market development for these fluids. Development of new standards and guidelines by ASTM and other industrial and governmental organizations will inevitably influence the growth of biodegradable fluids.
Meanwhile, lubricant suppliers continue to develop and evaluate new additive chemistries that provide greater oxidative, thermal, and hydrolytic stability properties for biodegradable fluids. Vegetable oil suppliers are using genetic engineering to produce new vegetable oils with improved stability. Ester manufacturers are considering improving ester performance by incorporating additive-type functional groups into molecular structures. The improvement in the performance quality of biodegradable hydraulic fluids will eventually lead to more applications and increased popularity of these important fluids.
Environmentally safe and fireproof
A drawback of most hydraulic fluids, including some fire-resistant fluids, is their toxicity - either to personnel, the environment, or both. Furthermore, they are only fire resistant, and most will burn under certain conditions. Recently introduced synthetic water additives, on the other hand, mix with water (usually in a concentration of 5%) to become fire proof; the solution actually could extinguish a fire.
These water-based fluids, in general, also offer a cost advantage over most other fluids because one gallon of concentrate produces 20 gallons of hydraulic fluid. When disposal expenses enter calculations, the cost differential becomes even greater - especially with a solution containing non-toxic, readily biodegradable synthetic water additives that require no treatment. The accompanying table summarizes characteristics of common fire-resistant and fire-proof fluids.
There are, however, important performance and operating characteristics of water-based fluids that cannot be ignored. First, water-based fluids in general have much lower viscosity, film strength, and lubricating qualities than oil-based fluids. This means that system components - especially pumps, valves, and actuators - must be designed specifically for operation with water-based fluid. You can't just drain fluid from a system containing oil-based fluid and expect it to run on water-based fluid.
A perception remains today that components for water-based fluid are much more expensive and larger - especially valves - than their conventional counterparts. While this may have been true 20 years ago, the cost premium for valves and other components designed for water-based fluid has narrowed to about 30%. This investment can easily be recovered in the cost of fluid alone, not to mention disposal and treatment costs. Moreover, valve size has been reduced dramatically: many are available with standard NFPA footprints.
Next, any potential for freezing must be considered. Traditionally, ethylene glycol is added to water to lower the solution's freezing point. However, using highly toxic ethylene glycol in a solution containing the synthetic additive would completely negate the purpose of using an environmentally safe additive. Using propylene glycol instead as anti-freeze maintains the environmental integrity of the solution because propylene glycol is so non-toxic that it is approved for use in food by the U. S. Food & Drug Administration.
Finally, because the fluid is non-toxic, it naturally tends to support microbial growth. To minimize or prevent consequences associated with this problem, judicious use of bacteriostatic additives and effective sealing and reservoir design should be practiced.
Glossary of hydraulic fluid terminology
Absolute viscosity - the ratio of shear stress to shear rate. It is a fluid's internal resistance to flow. The common unit of absolute viscosity is the poise. Absolute viscosity divided by fluid density equals kinematic viscosity.
Absorption - the assimilation of one material into another.
Additive - chemical substance added to a fluid to impart or improve certain properties.
Adsorption - adhesion of the molecules of gases, liquids, or dissolved substances to a solid surface, resulting in relatively high concentration of the molecules at the place of contact; e.g. the plating out of an anti-wear additive on metal surfaces.
Anti-foam agent - one of two types of additives used to reduce foaming in petroleum products: silicone oil to break up large surface bubbles, and various kinds of polymers to decrease the amount of small bubbles entrained in the oils.
Asperities - microscopic projections on metal surfaces resulting from normal surface-finishing processes. Interference between opposing asperities in sliding or rolling applications is a source of friction, and can lead to metal welding and scoring. Ideally, the lubricating film between two moving surfaces should be thicker than the combined height of the opposing asperities.
Bactericide - additive included in the formulations of water-mixed fluids to inhibit the growth of bacteria.
Boundary lubrication - form of lubrication between two rubbing surfaces without development of a full-fluid lubricating film. Boundary lubrication can be made more effective by including additives in the lubricating oil that provide a stronger oil film, thus preventing excessive friction and possible scoring.
Bulk modulus - the measure of a fluid's resistance to compressibility; the reciprocal of compressibility.
Cavitation - formation of a vapor pocket (bubble) due to sudden lowering of pressure in a liquid, and often causing metal erosion and eventual pump destruction.
Corrosion inhibitor - additive for protecting wetted metal surfaces from chemical attack by water or other contaminants. Polar compounds wet the metal surface preferentially, protecting it with a film of oil. Other compounds may absorb water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Still others combine chemically with the metal to present a non-reactive surface.
Demulsibility - ability of an oil to separate from water.
Dewaxing - removal of paraffin wax from lubricating oils to improve low temperature properties, especially to lower the cloud point and pour point.
Emulsifier - additive that promotes the formation of a stable mixture, or emulsion, of oil and water. Common emulsifiers are: metallic soaps, animal and vegetable oils, and polar compounds.
Emulsion - intimate mixture of oil and water, generally of a milky or cloudy appearance. Emulsions may be of two types: oil-in water (where water is the continuous phase) or water-in-oil (where water is the discontinuous phase).
EP additive - lubricant additive that prevents sliding metal surfaces from seizing under conditions of extreme pressure (EP). At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that prevents scoring that destroys sliding surfaces under high loads.
Fire-resistant fluid - hydraulic oil used especially in high-temperature or hazardous applications. Three common types of fire-resistant fluids are: water-petroleum oil emulsions, in which the water prevents burning of the petroleum constituent; water-glycol fluids; and non-aqueous fluids of low volatility, such as phosphate esters, silicones, polyolesters, and halogenated hydrocarbon-type fluids.
Full-fluid-film lubrication - presence of a continuous lubricating film sufficient to completely separate two surfaces, as distinct from boundary lubrication. Full-fluid-film lubrication is normally hydrodynamic lubrication, whereby the oil adheres to the moving part and is drawn into the area between the sliding surfaces, where it forms a pressure, or hydrodynamic wedge.
Hydraulic fluid - fluid serving as the power transmission medium in a hydraulic system. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high-pressure, and high-capacity pumps that require extra lubrication protection.
Immiscible - incapable of being mixed without separation of phases. Water and petroleum oil are immiscible under most conditions, although they can be made miscible with the addition of a proper emulsifier.
Inhibitor - additive that improves the performance of a petroleum product through the control of undesirable chemical reactions.
Kinematic viscosity - absolute viscosity of a fluid divided by its density at the same temperature of measurement. It is the measure of a fluid's resistance to flow under gravity.
Lubricity - ability of an oil or grease to lubricate (also called film strength).
Miscible - capable of being mixed in any concentration without separation of phases; e.g., water and ethyl alcohol are miscible.
Newtonian fluid - fluid, such as a straight mineral oil, whose viscosity does not change with rate of flow.
Non-Newtonian fluid - fluid, such as a grease or a polymer containing oil (e.g. multi-grade oil), in which shear stress is not proportional to shear rate.
Oxidation inhibitor - substance added in small quantities to petroleum product to increase its oxidation resistance, thereby lengthening its service or storage life; also called anti-oxidant.
Polar compound - a chemical compound whose molecules exhibit electrically positive characteristics at one extremity and negative characteristics at the other. Polar compounds are used as additives in many petroleum products.
Pour point - lowest temperature at which an oil or distillate fuel will flow, when cooled under conditions prescribed by specific test methods. The pour point is 3° C (5° F) above the temperature at which the oil in a test vessel shows no movement when the container is held horizontally for five seconds.
Shear rate - rate at which adjacent layers of fluid move with respect to each other, usually expressed as reciprocal seconds.
Shear stress - frictional force overcome in sliding one layer of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity.
Surfactant - surface-active agent that reduces interfacial tension of a liquid. A surfactant used in a petroleum oil may increase the oil's affinity for metals and other material.
Vapor pressure - pressure of a confined vapor in equilibrium with its liquid at a specified temperature; thus, a measure of a liquid's volatility.
Viscosity - measurement of a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. In addition to kinematic viscosity, there are other methods for determining viscosity, including, Saybolt Universal viscosity, Saybolt Furol viscosity, Engier viscosity, and Redwood viscosity. Since viscosity varies inversely with temperature, its value is meaningless until the temperature at which it is determined is reported.
Viscosity index (VI) - empirical, unitless number indicating the effect of temperature changes on the kinematic viscosity of an oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the V.I. of an oil, the lower its tendency to change viscosity with temperature.
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