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Does your fluid provide enough protection?

Dec. 19, 2007
Contamination doesn’t just reduce system performance and reliability — it can also degrade the hydraulic fluid itself.

You’ve spent a lot of time and effort designing and building your hydraulic system; now you’re ready for production. After the equipment with its brand new hydraulic system has been shipped, your colleagues in accounting can calculate your profit.

But what will happen when (not if) contaminants enter the hydraulic system? Will contamination degrade fluid performance? Will it cause equipment malfunctions and downtime? If so, you can bet it will eventually jeopardize future orders from customers. Of course, adequate filtration will eliminate or reduce many contamination related failures, but your choice of hydraulic fluid help can help mitigate the problems associated with contamination.

Figure 1. Mining equipment is susceptible to contamination from dirt and dust and from water condensation that results from equipment repeatedly warming up during operation and cooling down when shut off.

Hydraulic systems have changed dramatically in the last 20 years. Generally, they are smaller, contain less hydraulic oil, and require the same or more output as the systems of yesteryear. The smaller size places a greater burden on the hydraulic oil to help protect the equipment and components.

One thing that hasn’t changed over time, however, is the fact that hydraulic oil can become contaminated in a variety of ways and from a variety of sources. Contaminants in a hydraulic system can be:

  • present when the system is new,
  • generated by the system during operation, or
  • be introduced to the system during operation or even during idle periods.

Examples of contaminants that could be present in a new system include metal chips or dust left from manufacturing processes. Contaminants generated by the system include wear debris and thermal degradation by-products from heat, such as sludge and varnish. Contaminants, including dirt, water, and even other fluids, can enter the system through reservoir breathers, rod and shaft seals, and whenever a hydraulic component is replaced.

You can also classify contaminants by type, such as dirt and wear debris, water, air, by-products of heat, and other fluids such as gear oils, cleaners, and motor oils. Systems are designed to have filters in place to remove many of these contaminants. But what role does the hydraulic fluid itself play with respect to these contaminants?

Figure 2. Excessive heat can cause oxidation of hydraulic oil, so measures should be taken to keep hydraulic lines and components away from heat sources, such as in steel mills.

Dirt and wear debris
Eaton Vickers has stated that 90% of failures due to contamination come from abrasive wear, which is caused by metal-to-metal contact between components. The result is that small particles of the components break off and infiltrate the hydraulic fluid. In turn, they can cause wear of other components, clog filters, and chemically react with the fluid. Vane and gear pumps especially need protection against abrasive wear. Hydraulic fluids help prevent abrasive wear by using an effective anti-wear additive. Zinc dialkyl dithiophosphate (ZDDP) is typically the anti-wear additive used. ZDDP is both relatively inexpensive and very effective at reducing mild wear by preventing metal surfaces from contacting each other. The result is reduced abrasive wear. Over time, anti-wear additives can become less effective, therefore it is important to use a fluid that exhibits wear protection over an extended time period. These fluids are said to have good durability, meaning that their performance does not diminish over extended periods of time. The timing depends on system operating conditions such as temperatures, pressures, and the operating environment.

Water as a contaminant
It is not uncommon for water to enter a hydraulic system through access plates in reservoirs or simply condensed from ambient air drawn into the reservoir. Because water can cause poor lubrication, hinder filtration, and potentially form rust, water content should be monitored closely by analyzing the oil. Depending on the application, water content of more than 1000 ppm (0.1%) generally is considered excessive.

Hydraulic fluids are designed to separate from water (demulsify) quickly so it can be drained from the bottom of a reservoir. Some hydraulic fluid additives containing some of the less stable ZDDPs also can react with water to form acids that can corrode yellow metals, such as copper and brass.

When a fluid does not react with water, it is hydrolytically stable. It is important to use a fluid with high demulsibility and hydrolytic stability, referred to on fluid suppliers’ technical data sheets as hydrolytic stability — ASTM D 2619 and water separation (or demulsibility) ASTM D 1401.

Rust in a hydraulic system often can be tracked back to water ingression. The interaction of iron, water, and oxygen forms rust. Rust can be prevented by using a fluid containing rust inhibitors built into the additive system.

Water also can cause problems with filtration. Today, many systems are designed to use very fine filters, 3 μm or less, to help remove wear debris and other contaminants. In the development of hydraulic fluids, filtration is measured to make sure that the fluid can be filtered, both in its original condition and as contaminated with water. Generally, fluids will filter much more slowly when contaminated with water. Some OEMs have set specifications to ensure that fluids filter well both in their original condition and when contaminated with water.

Air as a contaminant

Air contains oxygen, and when it is mixed with oil at high temperatures, the oxygen converts oil molecules into acids — a process known as oxidation. These acids can thicken the oil, which will decrease pump efficiency. The thickened oil may also result in cavitation, which can cause catastrophic failure. In some cases, acid buildup forms deposits that can block filters and strainers. In general, an oil’s oxidationrate doubles with every 10° C increase in temperature.

To combat oxidation, good quality hydraulic oils are formulated with antioxidants, which interfere with the oxidation process (air reacting with the oil molecules). As a result, the fluid lasts longer and is less likely to increase in viscosity.

Air in hydraulic oil also can lead to foam-related problems. Foam can cause pump cavitation and decrease lubricity, which shortens component life. Foam carried by the fluid will deteriorate system performance and usually can be prevented by eliminating air leaks. Surface foam can be eliminated with proper reservoir design or by using a defoaming additive.

Figure 3. Overhead hydraulic cranes used for ship loading are prone to water contamination that may cause corrosion.

Another general problem related to air contamination is entrained air. Entrained air is made up of bubbles suspended in the fluid. It can be introduced during release of dissolved air within the fluid when pressure is decreased, from leaks on the suction side of the pump, from splashing in the reservoir, or from contamination Entrained air can result in spongy controls, cavitation, noise, and loss of horsepower. The best prevention for entrained air is a fluid formulation designed with proper additive and base oil selection.

Hydraulic systems often operate at elevated temperatures. For mineral oil-based hydraulic systems, temperatures from 140° to 180° F are considered normal. If the oil temperature goes higher, oxidation rate increases and oil life decreases. The problem is exacerbated by the use of more powerful systems with smaller reservoirs, which have become a trend in the design of many of today’s hydraulic systems. Over time, the additives and oil can degrade to form sludge and varnish.

Sludge and varnish are by-products of oxidation and thermal degradation of the fluids that are meant to protect expensive hydraulic systems. Varnish is a dark, sticky deposit that adheres to metal parts. It can increase friction of moving parts to cause erratic operation, especially in valves, or prevent valves from shifting altogether. Sludge is a thick deposit that can accumulate through a hydraulic system. It can also cause erratic operation and often clogs filters, necessitating premature element replacement. In extreme cases, sludge and varnish can collect in orifices and narrow passageways and block them.

These problems prove costly not only in the expenses associated with labor and component replacement, but with productivity lost due to down time. Furthermore, the cost of replacing the degraded fluid with fresh fluid, disposal costs, and possibly having to flush the system can also be substantial.

So it is best to keep varnish from forming in the first place. To do that, a combination of carefully selected additives can provide all the traditional benefits expected in a properly formulated hydraulic oil. They help prevent varnish from forming on surfaces, thus keeping the system clean.

Other contaminants
Other contaminants, such as gear oils, motor oils, and even cleaners, can contaminate hydraulic oils. This often happens if poor transfer methods are used to fill systems or if fluids or equipment are not labeled correctly. The result can vary depending on the contaminant.

Often, viscosity is an issue if a different viscosity grade of another fluid is added. Viscosity increase can result in sluggish pump performance. The viscosity can be too low if a low viscosity contaminant is added by mistake. If contaminated with motor oil, water separation properties can be compromised, leading to potential wear and/or rust issues.

Hydraulic oil contamination is a hot topic with hydraulic systems. The impact on the system varies depending on the type of contaminant. Effective filtration systems help remove particulates in order to minimize performance- related issues. However, hydraulic oil additive chemistry can also be effective in reducing the effect of potential contaminants. Overall, this helps to extend the life of the fluid and boost reliability of the equipment.

For more information, contact the authors at [email protected] or [email protected] or visit www.lubrizol.com.

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