Making the Move to Eco-friendly Hydraulic Fluids
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Governments around the world have enacted regulations aimed at protecting the environment, and they usually carry significant fines and remediation costs for violators. That’s become increasingly important to designers and users of hydraulically powered equipment, especially when operating in wetlands and other environmentally sensitive areas.
While hydraulic hose and connectors perform better than ever, they still have the potential to leak or rupture and spill fluid. Fluid suppliers have responded by introducing eco-friendly biodegradable and nontoxic fluids based on a variety of chemistries. Unfortunately, none of these fluids is a direct replacement for petroleum-based hydraulic fluid.
Here’s a look at commonly available bio fluids, and some engineering considerations when making the switch.
Biodegradable fluids
Four basic types of environmentally friendly hydraulic fluids are commonly used. Each is derived from different base stocks and is best applied to a specific range of applications and operating conditions. And because they have different chemistries, interactions with seal materials and other system components vary from fluid to fluid. Engineers must account for these interactions when weighing whether or not one will be a suitable replacement for petroleum-based fluids in a specific hydraulic system.
HETG fluids (hydraulic environmental triglyceride) are water insoluble triglycerides derived from vegetable or animal oils — with soybean, sunflower, and rapeseed (Canola) being the most common sources. They frequently contain soluble thickeners to increase their natural viscosity, which is approximately 35 mm2/sec at 40°C.
Triglycerides are long-chain fatty acids combined with alcohol in the form of glycerin. Natural triglycerides have excellent lubricity but poor thermal and hydrolytic stability. They also oxidize rapidly. Additives, chemical modification, and even genetic modification of the seeds used to produce the base stock can improve hydrolytic stability and oxidation resistance.
HETG fluids offer many advantages. For one, they are highly biodegradable and nontoxic. They offer excellent lubricity and anticorrosion properties. And because they are made from natural, renewable resources, they are readily available. In addition, they have a high viscosity index and high flash point.
But HETG fluids also have drawbacks. High-temperature operation can cause quick aging, rapid oxidation, and extreme thickening and gumming. In addition, they are susceptible to water contamination, which causes hydrolysis and increases total acid number (TAN). They tend to thicken and gel at low temperature, which hurts machine performance. And because they are miscible with mineral oil, this can lower biodegradability in circuits that aren’t properly flushed. Finally, they are double the cost of mineral oils.
HEES fluids (hydraulic environmental ester synthetic) are water-insoluble synthetic esters derived from either petroleum or vegetable (typically rapeseed) oil feedstocks. Petroleum-sourced HEES fluids combine an organic acid and alcohol, whereas vegetable sourced fluids combine a fatty acid and alcohol.
HEES fluids are available as unsaturated, partially saturated, and fully saturated products. Of these, fully saturated versions generally offer the best performance and command the highest price.
HEES fluids offer long service life due to high thermal and oxidative stability and good fluidity at low temperatures. They are also available in a broad viscosity range (ISO VG 32/46/68). However, they have more disadvantages than advantages. For example, they’re expensive and, like HETG fluids, require special system-design requirements. They also hydrolyze in the presence of water. And like HETG, because they are miscible with mineral oil, this can hurt biodegradability.
HEPG fluids (hydraulic environmental poly glycol) are water-soluble polyalkylene glycols (PAG), polymers made from reacting alkylene-oxide monomers such as ethylene oxide, propylene glycol, or propylene oxide with glycol. Those with 50 to 100% ethylene oxide are water soluble, while those with 100% propylene oxide are water insoluble. Both types are inherently fire resistant.
The biodegradability of HEPG fluids depends on the ratio of propylene to ethylene oxides. The higher the molecular weight, the lower the biodegradability of the fluid. HEPG fluids come in a broad viscosity range and have an operating temperature range of –20 to 80°C. In addition, water-soluble polyglycols can be used as anhydrous lubricants.
However, they require special system designs. For instance, they are incompatible with polyurethane seals, and pumps and motors may need to be derated when used with HEPG fluids.
HEPR fluids (hydraulic environmental polyalphaolefin and related) are water-insoluble polyalphaolefins (PAO) and related hydrocarbon-based fluids. These synthetic hydrocarbons are made by polymerizing alpha olefins to produce PAO. Only low viscosity polyalphaolefins are considered environmentally friendly.
A key advantage of HEPR fluids is that they offer excellent oxidation stability and good corrosion protection. They also have good lubricity and aging characteristics, and a long service life. They offer good viscosity performance over a wide temperature range: pour point is –20 to –40°C and operating temperature range is –30 to 100°C. However, like most green fluids, they can be costly and are incompatible with many seal and gasket materials.
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Design considerations
Seal compatibility. Traditional nitrile seals will have shorter lives when used with many environmentally friendly hydraulic fluids. In general, fluorocarbon seals give the best results. HEPG fluids may require special seal materials formulated specifically to work with these more aggressive products. The same is true for back-up rings, gaskets, and other components used in the system.
Defining “biodegradable” and “nontoxic”Biodegradation is the process of chemical breakdown or transformation of a substance by organisms or enzymes. Two common measures of degradation are complete (ultimate degradation), in which the only end products are carbon dioxide, water, and any indigestible inorganic elements; and incomplete degradation (primary degradation) measured by the reduction of hydrocarbons in the original substance. The relevant test standards are OECD 301 or ASTM D5864 for ultimate degradation and CEC-L-33-A-93 for primary degradation. Two other commonly used terms are readily and inherently biodegradable. A readily biodegradable fluid undergoes primary degradation greater than 80% within 21 days or ultimate degradation greater than 60% within 28 days. Inherently biodegradable fluids break down slowly over time, usually measured in years, and cause substantial damage to the environment. Eco-toxicity is a measure of the concentration required to kill various organisms over a short period of time – ranging from 24 to 96 hr. The fluid toxicity is described by a loading rate in parts per million (ppm) of fluid that has a 50% effect, or causes 50% mortality of the organisms after the stated time. The important thing to remember is that both biodegradability and nontoxicity are relative terms. Environmental friendliness does not mean the fluid is totally harmless to the environment and living things, only that it is less harmful than something else. Environmentally friendly or green fluids, like any hydraulic fluid, must be disposed of carefully and responsibly. |
In any case, consult the fluid supplier, equipment manufacturer, and seal supplier before converting any system to an environmentally friendly fluid. Fluid suppliers generally test their fluids with different seal materials per ISO, ASTM, or other specifications.
Hydraulic hose. As with seals, talk to your hose supplier before switching to a biodegradable fluid. Many major suppliers have done extensive testing with these fluids and can recommend a hose compatible with a specific fluid, as some fluids may require specialty hoses.
The same advice holds for other components containing rubber or elastomers, such as bladder accumulators. The material must be tested with the specific biodegradable fluid before using it in a hydraulic system.
Component performance. Biodegradable fluids come in many different formulations, and they differ in many regards from standard mineral-oil-based hydraulic fluids. That, in turn, may affect the performance of components such as pumps. For example, most environmentally friendly fluids have a higher specific gravity than traditional petroleum based fluids. This may require adjusting pump-inlet conditions, say with overhead reservoirs to ensure positive inlet pressure, maintain adequate suction, and avoid cavitation.
Bio fluids also differ from standard petroleum-based hydraulic fluids in terms of properties like oxidation stability, hydrological stability, water compatibility, filterability, and so on, and this can require special design and maintenance considerations.
Experts recommend testing the fluid with say, a specific pump. Use standard rating procedures to establish how that pump and fluid work together. Some fluids permit use at full rating, given proper system design, while others must be derated. But, in general, engineers must take special care when designing a system to run on specialty fluids such as bio fluids.
Water contamination. Some bio fluids, particularly HEES and HETG types, are susceptible to water contamination, which degrades fluid properties. They readily absorb water and, if water remains in the fluid, will hydrolyze the bio fluid. In essence, the fluid will break down and lose lubricity, and acidity will increase. It’s essential to closely monitor water content and acid levels in vegetable based and synthetic bio fluids.
To limit water contamination, use water-absorbent breathers on the reservoir. So when fluid level drops and outside air enters the reservoir, any moisture is captured.
Temperature extremes. Some bio fluids face problems at extremely low and high temperatures. For instance, vegetable-based fluids usually should not be used below rated temperature limits, as they tend to crystallize at low temperatures much faster than other fluids. Likewise, vegetable-based fluids tend to oxidize at high temperatures, which shortens fluid life. This is especially true in mobile equipment, where it is not unusual for hydraulic fluids to reach 180 to 200°F. Synthetic bio fluids, on the other hand, handle a wider temperature range, depending on the degree of saturation.
A rough rule of thumb is to maintain a 160°F upper limit with vegetable-based fluids (good for most industrial hydraulic applications). Synthetics (HEES) are suitable to around 180°F depending on the type of ester and degree of saturation.
Filtration and maintenance. Standard filter elements are compatible with bio fluids. And filtration requirements are generally the same for petroleum based and environmentally friendly fluids in terms of particle sizes. Filter change intervals may be different, however, particularly with HETG fluids that may have shorter service lives and a tendency to produce varnish and other contaminants. Size filters generously.
And keep in mind that all hydraulic fluids, not just biodegradable fluids, must be maintained in good condition. Additives may degrade depending on the fluid condition. Operating a fluid at too high a temperature shortens its life; and the presence of contaminants such as water and particulates can promote oxidation and also shorten the life of the fluid. No fluid is maintenance-free.
While actual life depends on the duty cycle and countless other factors, properly formulated and maintained fluid that’s not exposed to extreme temperatures and contamination will give good life in a wide range of applications. Perhaps the most important system consideration is to maintain the fluid in excellent condition, and monitor it frequently to ensure it performs as intended in the hydraulic circuit.
System flushing. Although traditional petroleum-based fluids will mix readily with environmentally friendly fluids, this should be avoided because petroleum contamination nullifies the biodegradability and toxicity benefits of the green product. Thorough system flushing is necessary, and it is often advisable to perform multiple flushes to ensure the system is perfectly clean before adding environmentally friendly fluid.
Note, especially, that engine oil additives are not compatible with environmentally friendly fluids. Equipment that has operated with engine oil as a hydraulic fluid requires an exceptionally thorough cleaning before environmentally friendly fluid is installed.
Additional recommendations
Engineers and equipment users must keep in mind that environmentally friendly hydraulic fluids are no substitute for regular maintenance and good operating practices. Preventing leaks is much more cost effective than remediation when it comes to the environmental impact of any operation.
Spills of environmentally friendly fluids are still reportable incidents, but many environmental regulatory agencies treat them differently from petroleum-based fluid spills. Because the fluids are less toxic and more biodegradable, cleanup and reclamation costs may be lower. For example, the affected soil often does not have to be dug up and autoclaved, as is the case in most petroleum-based spills. And operations are frequently allowed to continue during the cleanup.
Eco fluids offer a less-toxic alternative to petroleum-based fluids that are more easily assimilated by the environment when properly disposed of, and cause less damage should they spill. The price for these benefits is often measured in higher initial cost, potentially shorter service life, lower efficiency, and additional maintenance requirements.
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