As emissions standards across the globe grow more stringent, original equipment manufacturers are seeking to design new levels of efficiency into all forms of hydraulic equipment. To satisfy some of these new requirements and to help maintain the essential functionality of important hydraulic equipment, high-performance hydraulic fluid formulations are ever more important. 

Consider this: It is estimated that between 60% and 90% of hydraulic fluids are taken out of service due to contamination. This may come as no surprise, as it is not uncommon for hydraulic equipment to operate in dirty, dusty or humid environments—such as construction sites and industrial facilities—where contaminants will inevitably entrain themselves within the hydraulic fluid. Nevertheless, it is an increasingly pressing concern.

How Dirty Fluids Damage Hydraulic Systems

Today’s hydraulic systems have very tight tolerances and fine clearances—measurable in individual microns—implemented by OEMs to boost efficiency within the systems. In such applications, any type of particulate matter has the potential to interfere with the reliable operation of critical componentry, like pumps, valves and more. Contaminating debris can cause significant damage to hydraulic systems, including wearing, scoring and other types of damage. Hydraulic fluids requiring frequent change-out due to contamination can cause more frequent shutdowns for maintenance purposes. Taken cumulatively, these circumstances have the potential to increase maintenance costs and contribute to a higher total cost of ownership for critical machinery. 

For these reasons, high levels of fluid performance depend on a fluid’s ability to remain free of contaminants throughout its life cycle. Its ability to do so hinges upon several factors, including two critical performance characteristics: filterability and hydrolytic stability, which are both becoming increasingly necessary in modern hydraulic fluids.

Filterability

Filterability is the first performance characteristic that contributes to a fluid’s ability to stay clean, and it can be broken down into a few distinct categories:

• New oil filterability. Before they are placed into service, it is important that hydraulic fluids be highly clean, pure and free of any contaminants. A brand-new fluid must be able to pass through pre-filtration devices with minimal resistance while filling the hydraulic system. 
• In-service filterability. Once in use, hydraulic fluids need to be compatible with a hydraulic circuit’s internal filtration devices. These filters are especially important because hydraulic fluids that operate in severe, dirty or wet environments will inevitably become contaminated by either water or some other particulate matter. The fluid must be able to flow through these devices without issue.
• Old fluid compatibility. When fresh hydraulic fluid is introduced to the system, it is important that it is compatible with any remnants of previous hydraulic fluids that are left over within the circuit. Two incompatible fluids may chemically react with each other to create chemical complexes that have the potential to block filtration devices.

Consequences of Poor Filterability

Several negative consequences can occur when a hydraulic fluid exhibits poor filterability during its service life.

Importantly, a fluid with poor new oil filterability may be stripped of critical additive components by the filter as it is pumped into the hydraulic system. Antifoam additives and demulsifying agents are particularly susceptible to stripping if the filterability characteristics of a fluid are less than ideal. 

Both antifoam agents and demulsifiers are especially critical. For example, a hydraulic fluid that has been stripped of its demulsifier additives, which serve the purpose of keeping water from mixing with the fluid, will become more susceptible to hydrolytic instability. As a result, the fluid will likely be more susceptible to sludge formation; excessive sludge can block the filter and interfere with its ability to perform its essential function. 

Furthermore, this is not simply a contamination issue; a blocked filter that restricts proper flow and pressure can create unsafe backpressure conditions within the system, which can in turn harm operators if pressure builds too much. Meanwhile, if water is entrained within the hydraulic fluid, the circuit will essentially be more susceptible to rust and corrosion formation, an undesirable condition that can lead to leaks or other forms of failure.

Poor in-service filterability can lead to other challenges. If a fluid is unable to pass through filtration devices at optimal flow rates, the results can lead to lower pressure, poor circuit efficiency, decreased speed, compromised precision and lower accuracy in hydraulic systems. These conditions can compromise safety and operational efficiency for end users, especially in high-stakes applications.

Fluid filterability can be improved through the incorporation of the right mix of additives. Additives will maintain suitable cleanliness and help resist the formation of sludge and deposits that can interfere with in-service filtration. Of course, filterability must play a role in modern hydraulic fluid formulation, but it is just as important to the development of the right additive solutions to meet the needs of today’s hydraulic equipment. Additives should be resistant to hydrolytic breakdown while offering thermal stability and oxidative stability. These characteristics will help to enable outstanding fluid cleanliness and filterability.

During the formulation stage, testing for initial cleanliness and in-service filterability in fluids is increasingly important. This can be accomplished via the industry standard ISO 4406 test for new fluid cleanliness, which specifies the purity code to be used in defining the maximum quantity of contamination in the fluid used in a given hydraulic fluid power system. Various other industry tests can evaluate filterability, including a speed and volume displacement test that evaluates the rate at which a fluid passes through a membrane filter. The faster the performance, the more likely the fluid will contribute to the desired level of operation. 

Hydrolytic Stability

Hydrolytic stability defines a hydraulic fluid’s capacity to withstand and resist chemical decomposition when the fluid is exposed to water, which is one of the most common contaminants with which a hydraulic fluid will regularly come into contact.

Water intrusion in the hydraulic fluid has the potential to negatively interact with the fluid’s additive system—and in some cases, environmentally acceptable base oils—resulting in the formation of damaging acids. This can lead to a number of issues that may eventually cause hydraulic failure, including greater likelihood of sludge formation and accelerated rust formation and corrosion. These circumstances may produce a range of undesirable outcomes, including costly system leaks, reduced hydraulic pressure, inefficient operation, and loss of machine accuracy and precision.

It is important to note that there are minimal industrywide requirements for hydrolytic stability performance. Indeed, some industry specifications do not require hydrolytic stability testing at all. This means that products may be made available that have not been evaluated for their hydrolytic stability characteristics. These can be more susceptible to water intrusion issues and may contribute to a greater potential for machine downtime. 

However, many leading OEMs have begun to recognize hydrolytic stability as a critical performance characteristic in hydraulic fluids and request that fluids undergo testing via the ASTM D2619 test (also known as the beverage bottle test), which differentiates the relative stability of hydraulic fluids in the presence of water under the conditions of the test. Some OEMs even stipulate more stringent requirements, highlighting the critical importance of hydrolytic stability in modern hydraulic fluid formulations.

All of this makes formulating for hydrolytic stability just as important as doing so for good filterability performance. Hydrolytic stability performance requires advanced additive chemistry in fluid formulations, properly balanced to achieve the specific performance characteristics that end users rely on. Properly formulated hydraulic fluids are designed to be resistant to interactions with water, helping to extend the life of the equipment. For lubricant manufacturers, working with the right formulation partner to help develop optimal fluid solutions can enable them to deliver better value for their customers.

High-Performance Hydraulic Fluids Matter

As new trends in the hydraulic equipment market create new demands on the hydraulic fluids, it is critical that those fluids are up to the challenge. For fluid formulators, constant research is essential to develop enhanced formulations that best protect today’s continually evolving equipment.

For OEMs, the specification of hydraulic fluids that prioritize hydrolytic stability and filterability can help to elongate the lifetime of their equipment. And for end users, selecting high-performance hydraulic fluids can deliver real-world benefits, including reduced maintenance needs, less downtime and a lower total cost of ownership of the machine. 

Hydraulic equipment continues to play a critical role in the modern world, and maximizing its potential with the right fluids stands to benefit everyone. Fortunately, companies like Lubrizol are committed to contributing to the development of optimal hydraulic fluids that can contribute to higher productivity levels, less downtime and lower total cost of ownership.  

Stephen Gotheridge is technology manager, industrial additives with the Lubrizol Corporation.