Choosing the best hydraulic fluid

To ensure the sound design and smooth operation of hydraulic systems, the hydraulic fluid must be carefully selected. Many different types of fluids are available today, each with unique characteristics. Indeed, the ability to transfer power from one place to another—a property exhibited by plain water, the first hydraulic fluid—is only one requirement for a good fluid. Hydraulic applications abound, and fluid innovations have kept pace with industrial needs. As a result, many properties must be taken into consideration when selecting from the wide range of choices. Hydraulic fluids can be petroleum oil-based, synthetic-based or water-based, which usually has additives. The most common base is petroleum oil, which runs the vast majority of hydraulic systems and forms the standard for fluid performance. Most hydraulic components are designed for use with these types of fluids.

Among the factors in hydraulic fluid selection are lubrication and viscosity. Lubrication refers to the fluid's ability to reduce friction and heat between the hydraulic equipment's metal components, which slide against each other. The hydraulic fluid must form a thin film between these components to minimize damage. Film strength is determined mostly by viscosity—or the fluid's resistance to flow. To figure out the best viscosity for a fluid, the hydraulic system's maximum and minimum operating temperatures must be examined. At the highest temperatures, the fluid must be sufficiently thick for lubrication and low internal leakage. At the lowest temperatures, it must be thin enough for easy flow.

Hydraulic fluids must also protect against corrosion, oxidation and the wearing down of components. Water getting into the hydraulic fluid is the primary culprit of corrosion. Even condensation from humid air can corrode the fluid. Corrosion also takes place when the fluid itself, or a product of its decomposition, attacks system components. Making sure that the fluid is compatible with the materials in the components is therefore a crucial step. Additives, such as defoamers and demulsifiers, are used in corrosion prevention. They not only help fluids resist water retention but also air retention, which causes harmful cavitation. Oxidation or fluid aging, on the other hand, affects all hydraulic fluids except for water. During oxidation, fluids can make sludge, varnish and acids, which are all damaging to the system's components. The most effective preventive measure is to keep temperatures below 135° F. An alternative solution is adding special antioxidants, which allow fluids to function for a long time at higher temperatures. The third problem—wear and tear—is common in components such as vane and gear pumps where metal-to-metal contact is a design feature. The fluid must not only be a lubricant, but it must also have anti-wear properties. A solution is special additives, which can be utilized to form a protective barrier for metal parts.

Other considerations include elastomer compatibility and safety. Hydraulic equipment such as seals, hoses and accumulator bladders are made up of elastomer materials, which must be compatible with the fluid through the range of system temperatures and as the fluid ages. To ensure compatibility, manufacturers or general equipment guidelines can be consulted. Safety is another concern because hydraulic systems often operate close to open flames or hot surfaces in applications such as furnaces, forges and steel mills. If leakage occurs and the hydraulic fluid is flammable—like all normal petroleum oil fluids are—then a fire can result. For these applications, hydraulic fluids developed for increased safety must be considered. The International Standards Organization (ISO) currently recognizes four major groups of fire resistant hydraulic fluids: high-water containing fluids (HFA), invert emulsions (HFB), water glycols (HFC), and water-free fluids including synthetics (HFD).

HFA fluids are mostly made of water. While HFA fluids were originally 5% emulsions of oil in water, HFB fluids are emulsions of water in oil. With 60% oil content, the latter can perform almost as well as petroleum oil in some cases. HFA liquids, meanwhile, mirror the strengths and weaknesses of plain water. So while they are extremely fire resistant, they need equipment that has been designed for use with water. Therefore, they cannot replace petroleum oil in typical hydraulic equipment. HFA fluids are commonly used in steel mills and coal mines. HFB fluids, on the other hand, are not often used because of their inherent instability and maintenance needs.

HFC, or water glycols, are the most popular of the fire resistant hydraulic fluids. They consist of 35-45% water with special viscosity-boosting thickeners. These fluids can run most equipment that has been designed for oil, but pump speeds, temperatures and pressures may have to be modified. Their water content, which provides fire resistance, must be monitored and maintained to prevent evaporation. In contrast, the fourth type of fire resistant fluid is considered synthetic because it has no petroleum oil or water. The most widely used HFD fluids are polyol esters, which are based on organic esters. These high-performing fluids are compatible with system materials. Furthermore, they are biodegradable and display low levels of aquatic toxicity. However, polyol esters can be more than twice as expensive as petroleum oil so their use is limited to applications where fire resistance or biodegradability is extremely important.

Environmental issues such as biodegradability and toxicity figure into fluid selection. Petroleum oils are not biodegradable and as a result, fluid leakage often leads to soil contamination and the destruction of marine life. Environmental fluids have been developed as options. Although specifics may vary according to government or local laws, in general, an environmental fluid must be readily biodegradable—meaning that 60% of it must decompose within 28 days of exposure—and non-toxic—passing aquatic toxicity tests on specific species of fish. Vegetable oil—usually rapeseed oil—forms the most common base for these environmental fluids. Vegetable oil-based fluids feature lubrication and anti-wear properties that match that of petroleum oil. However, these products are susceptible to oxidation and perform poorly in cold weather, during which they can congeal and exacerbate cold starting. Synthetic base fluids present another popular environmental option. Examples such as polyol esters and water-free polyalkalene gycols (PAGs) are both environmental and fire resistant. They excel at preventing oxidation and at low-temperature performance. Environmental fluids are continually being improved with vegetable oil suppliers employing genetic engineering to boost fluid stability and ester manufacturers developing molecules that will yield even better performance. Applications for these fluids already include agricultural, forestry, mining and construction machinery, among others.

Today's hydraulic systems have high pressures and tight tolerances. While many hydraulic system users are drawn to petroleum oil's combination of high performance and low cost to fulfill these requirements, other factors must be examined. For example, many applications call for hydraulic fluids that are environmental and fire resistant—two properties that petroleum oil lacks. In these instances, users must find the best fit, considering both hydraulic fluid properties and system needs.

Source: Match Characteristics to Application Needs