It would be a bit of a stretch to say that hydraulic fluids imitate life. But they continue to undergo evolutionary changes that have adapted them to increasingly diverse applications and tougher environments. Consequently, some older formulations may be perceived as being obsolete, and not even considered for new applications.
The two main driving forces for this evolution are fire resistance and environmental compatibility. For example, phosphate esther- and polyol ester-based fluids have been used in applications where fire resistance is required. Now, however, polyol ester-based fluids are being touted as not only fire-resistant, but environmentally friendly as well. Both of these fluids cost considerably more than conventional hydraulic oil, so polyol esters are targeted for applications where fire resistance and environmental compatibility are required.
Weighing the Alternatives
Designers cannot be expected to anticipate every possible circumstance under which their equipment will be operated. Therefore, every hydraulic system can benefit from using a fire-resistant fluid. Compared to conventional hydraulic oil, fire-resistant fluids have a different chemical makeup, performance characteristics, and carry a higher price tag. Therefore, fire-resistant fluids are specified only when designers anticipate that the hydraulic system might be operated in an environment where the potential for fire exists. After all, specifying a fire-resistant fluid for every hydraulic system would be impractical, so they are specified only when a real potential danger is perceived. By the same token, every hydraulic system can benefit from using an environmentally compatible hydraulic fluid. But, again, because environmental fluids cost more, they are only specified when warranted by the application.
Moreover, the mere existence of fire-resistant and environmentally compatible fluids adds importance to the decision of what type of hydraulic fluid to specify for any particular system. After all, if a fire is caused or accelerated by hydraulic fluid flowing from a system, imagine the liability a company would face if designers could have specified a fire-resistant hydraulic fluid, but didn't. A jury would most likely find the company negligent by sacrificing safety in order make a higher profit. Therefore, litigation prevention may serve as another driving force.
Newer formulations of polyol ester-based fluids meet specific testing standards for fire resistance. However, long-established fluids should not be overlooked just because newer alternatives are now available.
The ultimate in fire resistance is offered by water hydraulic systems. Hydraulic systems using water without additives can serve double duty by also acting as a fire suppression system. Most water hydraulic systems, though, operate with additives so that the fluid circulating through the system is 95% water and 5% additives. Even with the additives, water hydraulic systems generally go far beyond fire resistance and are considered fire-proof.
Water hydraulic systems, however, must be engineered and constructed differently from oil hydraulic systems. Consequently, the cost of a water hydraulic system and its components is considerably higher than that associated with oil hydraulic systems. Even so, safety and environmental concerns - and the liability associated with them - have raised the level of interest in water hydraulics in recent years.
Phosphate ester-based fluids offer the next level of performance in fire resistance. Although phosphate esters cannot be considered fire proof, they resist ignition under a wide range of conditions. This would make them the fluid of choice when the potential for fire poses a real threat but where a water-based system would be impractical.
On the other hand, phosphate esters cost considerably more and are more toxic than conventional hydraulic oils. Furthermore, the chemical composition of phosphate ester-based fluids makes them incompatible with common elastomers used in standard hydraulic components. To alleviate this problem, premium materials, such as Viton, must be specified for all sealing components that will come in contact with phosphate ester-based fluids.
Polyol ester-based fluids also have been approved as fire resistant. Depending on the combination of additives incorporated with the base fluid, polyol esters can be environmentally compatible as well. However, because they cost much more than conventional hydraulic oil, polyol ester-based fluids are used primarily in applications where fire resistance, environmental compatibility, or both justify the higher expense.
Hydraulic fluid using vegetable oil as the base stock entered the market in response to a need for a product that would not contaminate the environment in the event of a spill. This type of fluid can dramatically reduce the cleanup costs associated with spills. These spills often are related to poor maintenance practices. For example, a hose could rupture if equipment is operated or maintained properly, or fluid could be spilled when changing a component or adding fluid to the equipment's reservoir. Although the equipment manufacturer normally would not be responsible for these costs, a machine using environmentally compatible fluid would serve as a selling point to a customer who realizes that accidents such as these occur.
The oil used most successfully in these vegetable-based fluids is derived from the rapeseed. A familiar name often associated with this base fluid is Canola oil, which is derived from genetically engineered rapeseeds. Although this oil offers good performance, it tends to have lower oxidation stability than mineral oil, especially at high temperatures. Oxidation resistance is important to prevent the formation of sludge and varnish within hydraulic components. Additives usually are incorporated with the base oil to increase its oxidation stability.
Hydraulic fluid with a soybean oil base is also available. As with rapeseed oil, soybean oil has a relatively low oxidation stability - but oxidation occurs at even lower temperatures with soybean oil. To help alleviate this potential problem, soybean oil can be partially hydrogenated. Just as partial hydrogenation thickens corn oil into margarine, it also tends to thicken the soybean oil. This thickening reduces the oil's fluidity, which can be critical for hydraulic systems operated at low temperatures.
Another area of concern is that vegetable-based oils can tend to absorb water. This can be a problem if components in the hydraulic systems use conventional urethane seals. Water is a byproduct of chemical reactions that occur in the production of urethane. Consequently, exposing it to water at a high enough temperature can cause conventional urethane to deteriorate through a process called hydrolysis.
To prevent this problem from occurring, additives can be incorporated into the vegetable oil to reduce its water retention capacity. However, the more additives incorporated into the base oil, the greater the likelihood that its environmental compatibility could be compromised. An alternative solution using hydrolysis-resistant urethane is presented in the box above.
Matching the Fluid to the Application
Gone are the days when selecting a hydraulic fluid meant simply finding one with performance characteristics that matched the application. Now you have to determine whether fire resistance and environmental compatibility must be thrown into the mix. And if so, how much weight will they carry relative to performance and price. Fortunately, manufacturers have responded by developing a wide variety of products to serve an even wider diversity of applications. The challenge now becomes choosing which particular fluid is best suited to a particular application.
For information on fire resistance test results of hydraulic fluids, contact FM Global (formerly Factory Mutual Research Corp.), Johnston, R.I., 781/255-4681, fax 781/255-4359, e-mail them at [email protected], or visit their website at www.fmglobal.com. To order a copy of their 7-page Property Loss Prevention Datasheet for hydraulic fluids, published in May 1998, call FM Global toll-free at (877) 364-6726. The datasheet costs $30 for those insured by FM Global and $80 for those who are not. Both prices include shipping and handling charges.
Fire-resistant Vegetable Oil
Vegetable-based oils have become an alternative hydraulic fluid because they hold much less potential to damage the environment in case of a spill. However, a new formulation of vegetable oil-based hydraulic fluid - Cosmolubric B-230, from Houghton International, Valley Forge, Pa. - also boasts fire-resistant properties. Fire resistance combined with environmental compatibility are the same characteristics that have made polyol ester fluids popular.
Polyol ester-based fluids, however, cost several times that of conventional hydraulic oil. Cosmolubric B-230, then, is designed to provide environmental compatibility fire resistance of polyol ester-based fluids, but at a lower price. For more information, please refer to the on-line article, "Canola Oil-based Fluid is Gentle on the Environment."
Urethane Seals Resist Hydrolysis
Instead of incorporating additives into a vegetable oil-based hydraulic fluid to reduce water retention (and, therefore, the potential for hydrolysis), a new premium grade urethane has been developed that resists the detrimental effects of hydrolysis. The new urethane is dubbed P4301A90, and was developed by Parker Hannifin's Seals Packing Division in Salt Lake City.
For more information on hydrolysis of urethane seals and the new P4301A90 compound, please refer to the on-line article, "Keeping Environmentally Safe Hydraulic Fluids in their Place."
We thank the following companies for contributing information for this article: Quaker Chemical Co., Conshohocken, Pa.; Houghton International, Valley Forge, Pa.; Akzo Nobel Chemicals Inc., Dobbs Ferry, N.Y.; Mobil Oil Corp., Fairfax, Va.; Royal Lubricants, Inc., East Hannover, N.J.; CITGO Petroleum Corp., Tulsa, Okla.; Union Carbide Corp., Danbury, Conn.; Amsoil, Inc., Superior, Wis.; and Amoco Petroleum Products, Oak Brook, Ill.