Operating conditions and environmental requirements have led to a growing need for fluids non-oil.
Each hydraulic system is characterized by a fluid movement between the various components (pumps, valves and actuators) that transmits and controls power from one place to another. The density at high power requires fluid not only to transmit energy, but lubrication, heat transfer and be compatible with various materials. As the universal ideal hydraulic fluid does not exist, its selection for a given application is often the result of a compromise.
The economic and environmental problems have led to new charges for hydraulic fluids, which is claimed to help reduce losses, to be filtered for a longer life and reduce replacement costs and, in case of losses, not to be harmful to the environment. Industry observers believe that the hydraulic fluid in a proportion which reaches 85% at the end leave the system in which they operate through small but steady losses, catastrophic breaks of circuits or malfunctions of connections and seals. It is estimated that about 7 million barrels of hydraulic fluids are lost each year to one of the reasons mentioned above.
The experts say that of all the problems with hydraulic systems, a percentage between 70% and 85% is directly related to the choice of the wrong fluid, or to improper treatment or its contamination. As the number of fluids available has expanded considerably, moving in just a few years by oil-based to synthetic water-based or vegetable oils and even plain water, it is important that the properties of the fluid used to satisfy the system needs and those of environmental protection.
The hydraulic fluids are divided into two basic categories: inhibitors of rust and oxidation (R&0) and anti-wear fluid (Aw). Fluids “R&0” are sometimes described as being unable to avoid the wear and tear, but this is misleading because they may provide protection against wear while often used the same temperature and pressure of fluids “Aw”. Fluids “R&0” have anti-wear additives very low or zero.
They contain a number of rust, oxidation and corrosion inhibitors, anti-foaming and reduction of the melting point. These fluids are generally suitable for most systems up to 100 bar. Their use is mainly based on the needs of the pumps and engines that are part of the system. The hydraulic fluids “Aw” contain a much higher level of anti-wear additives of liquids “R&O”. As these additives using a corrosive principle to protect from wear, it is necessary to balance the anti-wear additives with additional features such as neutralization of metal and/or detergent dispersing
Fluid oil-based
The widespread use of the term “hydraulic oil” reflects the preponderance of hydraulic fluids based on oil. This is due not only to the fact that they are used for a long time, but also to the possibility to modify them to obtain some of the typical properties of alternative fluids, such as a good resistance fireproof and good control of viscosity, as well as a satisfactory biodegradability.
Oil is a highly functional industrial fluids once refined and formulated with various additives to prevent rust, oxidation, foaming and wear, as long as heat, cold and the risk of non-flammability represent the critical factors in the application.
Some physical and chemical properties of hydraulic fluids oil-based on are due to raw material base, while others to additives. Some of these properties provide useful, others enhance those already present and others reduce the intensity of extreme events. Today virtually all hydraulic fluids contain at least one type of additive, and many have several. The amount used varies from a few fractions to percentages over 30%. Generally, it is not necessary or beneficial to the user add new additives.
The most important properties of hydraulic fluid is probably the viscosity, that varies with temperature: it grows when the temperature decreases and vice-versa. Hydraulic oils have several different rates of change of viscosity under the influence of temperature.
The viscosity index (Vi) is a system to assign a numerical value to these rates of change. The use of additives that improve the viscosity index allows the formulation of oils which ensure a good level of lubrication in the temperature range much wider than that guaranteed by the simple mineral oil.
The anti-wear additives are used to prevent contact in hydraulic components, especially in pumps. The friction is reduced, and the surfaces in contact may move much more freely. The wear is reduced under conditions of slight slip; in the case of strong flow, the additive can be degraded to the point of completely losing its anti-wear ability.
If the operating temperature exceeds 90°C the effects of catalytic metals such as iron and copper are becoming an important factor in the oil oxidation. In this condition inhibitors are used to form a protective coating. However, anti-oxidation does not completely stop the oxidation of oil in particularly severe operating conditions. Thermal stability is the ability of an oil to resist decay under conditions of prolonged exposure to high temperatures, when oxygen is minimal. The decay may manifest with thickening of the oil, increase of acidity, increase in the formation of residue, or a combination of the three symptoms.
The development of organic acids and the entry of contaminants in oil are the two main factors that determine the corrosion. Detergents, dispersants and the corrosion inhibitors help to neutralize acids and keep the system clean. The rust inhibitors can be used in most of the oils, but the choice must be performed carefully to prevent corrosion of non-ferrous metals or the formation of emulsions with water. Because of rust inhibitors are absorbed by the metal surface, with time they tend to deteriorate, especially in hot systems. Detergents chemically neutralize deposits that are formed under conditions of high temperature.
The dispersants are chemical compounds that disperse or suspend the particles which may be capable to create a muddy residue in the oil. There is no clear dividing line between the detergents and dispersants. Both help to prevent the filing of “paint” due to oxidation of the oil.
The silicone polymer (used in a few parts per million) and polymethyl (used 50 times that level) inhibit the formation of foam. Both materials are mixed with oil, but it is important to choose correctly the size of polymers in order to avoid the formation of deposits in case of long-term storage.
Furthermore, an excessive proportion of anti-foaming agents can increase the drag of air in oil. The importance of the melting point depends almost exclusively on the type of use of the fluid. The melting point of a hydraulic fluid suitable for winter must be low enough to enable it to move towards the suction pump to the lower temperatures expected. There is no need for a low melting point in a heated facility or in conditions of service continue.
Synthetic-based fluids
The synthetic-based fluids are used both for their durability and for the benefits they provide in applications with high or low temperatures, and can offer a reduction in energy up to 7%. Their basic classifications R&O and Aw are identical to those of oil-based fluid. The composition of the additives is similar, with the exception of some minor changes aimed at improving the solubility of the additive or its compatibility with the elastomers.
There is a wide variety of synthetic-based fluids, some of which are compatible with oil-based fluid and their constituents while others, such as esters and polyglycols, require special attention. Formulations based on phosphate ester or ester have good flame retardant properties, with an high flash point temperature and viscosity characteristics comparable to those of fluid-based oil.
The index of viscosity is generally very low. The performance of the pumps should generally be reduced when using these fluids, and it is essential to use special paints and special seals. If these fluids are used in a hydraulic system not designed for them, may eventually become necessary special engineering considerations
Fluid water-based
Because of the problem posed by the fires on board during World War II, the navy of the United States began a program to develop hydraulic fluids fire resistant. The first fluid-based water and glycol were developed and marketed in 1947.
Since then, other emulsions and new synthetic products have been added to fluids for hydraulic systems. It should be clear that under certain conditions these fluids are flammable and therefore are not fire-proof in the strict sense. The content of water, equivalent to 40-50% provides the fire-retardant properties to fluids for hydraulic systems based glycol and water emulsions and oil.
The viscosity is comparable to those of oil-based fluid. The temperatures must be maintained below 60°C to prevent excessive evaporation of water and to control the viscosity of the fluid. To obtain a reasonable service life may become necessary required changes to seals, paint and, in addition, the reduction of the performance of pumps. The additives provide the desired properties in hydraulic fluids.
Vegetable oil-based hydraulic fluids
For several years now there is a growing interest in the hydraulic fluids not harmful to the environment in case of losses. Most of the synthetic-based and oil-based fluids don’t degrade and persist in the environment.
To get a better biodegradability and low toxicity, hydraulic fluids based on vegetable oils has been developed in recent years. Their performances are similar to those of oil-based fluids and their cost, while two, three times higher than that of oil-based fluids, is about half that of synthetic-based fluids.
Fluids based on vegetable oils have a very high viscosity index, which is a positive factor in applications with variable or high temperatures. Also they possess a good thermal stability, compatibility with additives, low volatility and high flash point.
On the contrary, in comparison to mineral oil for hydraulic systems they have a lower hydrolytic stability, poor characteristics at low temperature, poor response to reducing the melting point and low anti-oxidant stability. However, despite these flaws, vegetable oils can provide excellent performance, and not forget the environmental benefit consists of the low toxicity in the event of losses and benefit the broad availability and renewable.
Water
Of all the fluids for hydraulic systems, water is undoubtedly the cheapest and most abundant. In special designed applications, it is a possible alternative to conventional hydraulic fluids because it is fireproof, environmentally safe, cheap and less compressible of other fluids.
Sometimes it is treated with chemicals before use. The problems of corrosion, erosion, and lubrication of machines and systems have been reduced through the use of protective coatings, hardened or lubricated materials and gaps reduced. In hydraulic systems pressures greater than 200 bar are possible; moreover, the accessories are many and include pumps, piston engines, stop and control valves, and cylinders. Furthermore, the water does not require a regular change, as long as a functional filtration system is used.
by Stefano Serri
