Hydraulic oils for high temperature applications

Hydraulic systems are among the most reliable of modern industries. From die-casting and steel mills and offshore drill rigs, as well as glass manufacturing facilities Hydraulic drive processes that require durability in the harshest environments on earth. Of all the elements that can affect a hydraulic system, excessive heat is the most damaging.

If temperatures rise above the limit of what a typical hydraulic oil is able to handle The consequences can be swift and expensive: fluid breakdown and seal degradation, increasing wear and loss of viscosity, and, in extreme instances, the system can fail catastrophically. The choice of the best hydraulic oil for applications that require high temperatures isn't just a secondary concern. It is an important engineering choice.

This article explains the factors that make hydraulic oils suitable for high temperatures and the differences between them and standard formulations and the factors to be aware of when choosing the right one for your industrial environment.

Understanding the challenge of temperature

Mineral-based hydraulic oils are typically designed to be reliable for a range of 60 to 70 degrees Celsius (140-158°F) during continuous operation. Above this temperature range, oxidation speeds up and viscosity decreases dramatically, and additive packages begin to reduce as well as the oil begins to degrade chemically.

In high-temperature processes, however, bulk fluid temperatures ranging from 80 to 120 degrees Celsius (176-248°F) are not uncommon, and the localized temperature at the pump outlet, valves for control, and actuators may rise more. Foundries for steel, injection mold presses, forge workshops, and certain marine applications regularly increase the temperature of hydraulic fluids beyond their limits.

This results in a shrinking gap between acceptable performance and failure. People who fail to recognize this generally face the following:

• Viscosity decreases: The oil is not able to keep the film of lubrication that protects motors and pumps, leading to wear that is more rapid.
• The process of oxidative degradation: heat accelerates this reaction of oxygen with base oil, resulting in varnish, sludge, and acidic compounds that can corrode system components.
• Damage to hoses and seals The degraded oil damages sealing elastomerics and hydraulic hoses, causing leaks and inadvertent downtime.
• Additional burnout: the antiwear antioxidant and anti-foam ingredients that keep a hydraulic oil running are consumed much faster when temperatures are elevated.

High-temperature hydraulic oils tackle all of these failure mechanisms by ensuring the right base stocks and additive chemicals.

The key properties that define high-temperature performance

Viscosity index (VI)

Viscosity index is a measure of the extent to which a fluid's viscosity changes with temperature. A higher value indicates that the oil has a higher level of stability across an extensive temperature range. The standard hydraulic oils usually contain a VI range of 95 to 100. High-temperature oils typically have a VI of at least 150 or more, which is achieved by the utilization of base stocks with high VI or viscosity index enhancers.

The ability to maintain viscosity at the operating temperature is crucial. A thin oil when heated to high temperatures is unable to form an effective protective layer between metal surfaces. Likewise, one that is too thick consumes energy and reduces the system's response.

Stability of the oxidative process

High-temperature oils are made up of either extremely processed Group II and Group III mineral base oils or synthetic base stocks like polyalphaolefins (PAO) or synthetic esters. These base oils possess superior oxidation resistance when compared with traditional mineral oils of Group I.

The stability of oxidation directly affects the lifespan for the liquid. Fluids with a low resistance to oxidation may require replacement every 1,000-2,000 hours of hot service, whereas the most well-formulated synthetic fluid will be serviceable for 6,000 to 10,000 hours or more under similar conditions.

Stability of the thermal environment

In contrast to oxidative stability, which refers to the resistance to oxygen-driven degradation, thermal stability refers to the resistance of the fluid to decomposition by heat alone even without oxygen. This is particularly relevant in situations where the fluids are exposed to high-temperature surfaces, for example, close to hydraulic actuators that are in proximity to casting equipment or furnaces.

PAOs and synthetic esters generally provide excellent thermal stability, with some formulations being rated for continuous use up at 150°C (302°F) or more.

Flash point and resistance to fire

High flash points are crucial in areas where ignition sources are in place. Mineral hydraulic oils are typically used for their flash points that range in the region of 180 to 200 degrees Celsius (356-392°F). High-quality, high-temperature grade oils, including the synthetic esters, may provide flash points that exceed 250°C (482°F).

In the fields of manufacturing steel as well as die casting and glass production, fire-resistant hydraulic fluids (FRHFs) like phosphate and water glycol ester and polyol esters could be mandatory, not just for reasons of performance; however, they are also required for safety reasons.

Types of hydraulic oil employed in high-temperature processes

high-VI mineral oils

They're the most basic alternative for high-temperature service. Made from highly processed Group II or Group III base oils, they can provide enhanced resistance to oxidation as well as more viscosity indices that are higher than standard grades, yet remain cost-effective. They're suitable for use in areas in which temperatures are below 90 to 95 degrees Celsius (194-203°F).

Polyalphaolefin (PAO) synthetics

PAOs are a type of hydrocarbon that has been synthesized and have an extremely homogeneous molecular structure. This gives excellent thermal and stable oxidative properties, a naturally extremely high index of viscosity (typically 130-150), and outstanding low-temperature performance. They're compatible with all seal materials that are used in mineral oils, which makes retrofitting relatively simple. The PAO-based hydraulic fluids are extensively employed in steel mills, die-casting equipment, and presses with high speeds.

Fluids based on synthetic ester

Esters—such as diester, polyol ester, and phosphate ester varieties—are able to provide the best temperature performance that is available. Particularly, polyol esters are highly regarded for their biodegradability, the high flash points, and outstanding fluidity. They are fireproof and are commonly employed in aviation hydraulics as well as generators of power.

The downside of esters is their susceptibility to water contamination as well as, in certain formulations, incompatibility with certain seals. Cleanliness of the system and seal selection is essential.

Water-based fire-resistant fluids

Water glycol and oil-in-water emulsion fluids are primarily used in areas where there is a fire risk that is the main problem—around open flames, extremely high-temperature areas, and in tight areas. Although these fluids are less effective in their lubricating properties when compared with synthetics, formulations have made huge advances in extending the lifespan of equipment. They require strict monitoring of levels of water, pH, and the concentration of additives.

The right oil to choose: an approach to evaluating the right oil

The selection of the appropriate hydraulic oil for a high-temperature application is more than just reading the datasheet. The following aspects should guide the process of selecting:

1. Define the operating temperature band. Measure both the temperature of the bulk fluid as well as the localized hotspot temperature. Many operators underestimate temperatures at peak by relying on the readings of reservoirs only.

2. Determine the viscosity level required at the operating temperatures. Most hydraulic system manufacturers stipulate the minimum and maximum operating viscosity (commonly expressed in the cSt range at 40 degrees Celsius). Work backward from actual operating temperature and the fluid's viscosity-temperature relationship to confirm the correct ISO grade.

3. Assess the risk of fire. If the system is located near sources of ignition such as open flames or over the autoignition temperature of normal mineral oil (~300°C/572°F), the use of a fire-resistant fluid is required regardless of price.

4. Make sure you are compatible. Changing base oil type—especially changing between mineral oils and water-based or synthetic fluids—can have an impact on seals, hoses, or paint coatings, as well as filter media. Check with the original equipment manufacturer (OEM) guidelines before making any modifications.

5. Think about all the costs of the ownership. Premium synthetic fluids have a higher initial expense but provide more drain time, less time to repair, less wear, and a longer life for components. In highly useful industrial environments, most synthetics will provide the highest ROI for three or five years.

6. Create a program to monitor fluids. High-temperature operation can accelerate the degradation of fluids, making regular analysis of oil essential. Monitoring parameters like acid number, viscosity particle count, water content, and metal contamination allows operators to detect problems early and improve drain intervals.

Maintenance practices that help support high-temperature performance

Even the most powerful hydraulic oil could fail in the early stages in the event that the system itself gets too hot. Alongside the fluid selection, important maintenance procedures are as follows:

• Maintenance of heat exchangers: Keep coolants clean and functioning. Fouled coolers are the primary reason for elevated temperatures of hydraulic fluid.
• System sizing: Reservoirs that are too small or pumps operating at high pressure or in poorly designed circuits produce excessive heat. A review of the thermal performance of the system might uncover a fixable issue.
• Filter maintenance: Filters that have become blocked result in pressure drops and create heat. Maintain the recommended service intervals of OEM without exclusion.
• Repair and detection of leaks: The leakage inside damaged valves and actuators can be an important source of heat, since the energy generated by high-pressure fluid is converted into heat as it passes through damaged clearances.

The high-temperature hydraulics applications require more of both the fluid and the person who selects the fluid. The standard mineral oils, while generally regarded as good in their normal use, aren't made to withstand the heat tension of foundry floors and steel mill presses or die-casting machines with high-duty cycles.

The use of a defined high-temperature hydraulic fluid—whether it's the mineral grade high-VI or a PAO synthetic or even a synthetic ester—and a disciplined maintenance of the system and a well-structured oil analysis program is the best way to guarantee reliability and long-term performance in harsh conditions. The cost of doing this right is minimal. The price of doing it wrong is measured by urgent repairs, lost production, and reduced equipment life.

In industrial hydraulics. The failure isn't.