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Hydraulics: The Unvarnished Truth


Although it is accepted practice for end users to routinely change or clean servo valves in hydraulic equipment, is it really necessary? What are the cost implications? Are there alternatives?

Quite often, a valve may be changed because it is stuck in one position or not responding as it should to commands. Because the system no longer operates effectively, the only alternative is to stop the machine and replace the valve. Unfortunately, for some operations changing valves is the norm, and the expense is accepted as the cost of operation.

Some commonly accepted alternative solutions, including electrostatic filtration systems, can remove contaminants. But these systems dont deal with varnish formation causes and tend to be expensive. In addition, some are susceptible to water contamination.

However, use of new additive technology in hydraulic oil can lessen the need to frequently clean or change servo valves. It does this by suspending in the oil any varnish that does form, thus preventing it from depositing on critical work surfaces. The result is longer component life and a hydraulic system that stays clean, which improves overall operational efficiency.

The Inside Story

A major use for industrial lubricants is hydraulic equipment, and a major issue in many of these applications is varnish formation.

What is varnish? As oil ages, fluid degrades by oxidation and thermal decomposition in addition to the natural process of additive consumption. Remember that additives are performance-enhancing chemicals that are consumed during the life of a fluid. The decomposition byproducts increase as the oil ages, eventually forming varnish.

Since varnish is polar, it is attracted to metal surfaces – including servo valves. Although it starts as a sticky, soft residue (sludge), it attracts wear debris, forming a sandpaper-like surface. In time, it ends up as a tenacious, hard lacquer.

Why is varnish so bad? Oil that has been oxidized generally doesnt lubricate very well. It can result in reduced oil flow, plugged filters, stuck valves (especially proportional and servo types), higher friction, poor heat transfer and elevated operating temperature. (Varnish acts as an insulator so cooling capacity can be diminished). In addition, it shortens the lives of components such as valves, filters, pumps, bearings and seals. The bottom line generally is that hydraulic system performance suffers.

Bad for Pumps, Too

Mr. Philippe Parreau, laboratory manager of Parker Denison Hydraulics in Vierzon, France, is well aware of varnish issues in todays modern hydraulic systems. Mr. Parreau, who leads the companys fluid approval program, made this observation: Varnish is bad for pumps, valves and filters. For example, in high-performance vane pumps found in the Parker Denison T6H20C and the new Parker Denison T7 series, varnish adhering to the vanes can cause the vane to stick in the rotor slot.

There are five main consequences of this, he added:

1. Increased noise.

2. Decreased volumetric and mechanical efficiency, with an equivalent increase in energy consumption.

3. Side plate scuffing.

4. Rotary seal damage.

5. Potential bearing damage.

Parker Denison has found that the phenomenon is most obvious at low pressures, when there is little centrifugal force and low fluid pressure.

In the case of piston pumps, varnish can increase piston land friction against the wear plate, leading to leakage and possible seizure. It is also well known that sticking valves can cause unscheduled stoppages during equipment use, e.g., regulation valves found on piston pumps. Finally, the varnish that forms can also cause filter blockage, leading to high use of filter cartridges and increased maintenance costs, Mr. Parreau explained.

Costly Fixes

What is the cost of having varnish? Varnish can result in a need to change or clean servo valves in the hydraulic system or risk system failure. A single servo valve can cost $3,000 new and in the neighborhood of $2,000 to clean and refurbish. And the cost to clean or change the valve doesnt end there. Dont forget the associated labor and shutdown costs, both of which impact the bottom line. And if the system fails, the operator loses income.

Lets put that into real-world terms. A large plastic injection molding company produces between 20 million and 30 million parts per month with over 200 machines that range in size from 33 to 770 tons. The hydraulic fluid reservoir size in these machines ranges from 80 to 250 gallons. The equipment operates 24 hours a day, five days a week. The company estimates that its cost per year due to varnish, not including production loss, is approximately $135,000.

In addition to potentially triggering premature replacement of control valves, if left uncorrected, varnish can reduce the filter load-carrying capacity and plug supplemental cooling system orifices – among other things.

What is the solution for varnish? In the past, end users have simply replaced servo valves or cleaned them as needed to keep their systems operating. Electrostatic filters and precipitators have been used successfully but have some shortfalls, including their cost and the loss of productive floor space in the plant.

Fixing the Fluid

The use of a hydraulic fluid that does not deposit varnish on surfaces is an ideal solution. While minimizing varnish deposits, the hydraulic fluid also must maintain all other important properties, such as antiwear performance and the ability to separate water rapidly and prevent corrosion. Because hydraulic formulations are carefully balanced to meet OEM requirements, adding a new varnish-mitigating feature to the fluids performance profile requires a unique solution. Fluids are now available that incorporate additive chemistry that reacts with the precursors to varnish, minimizing the formation of resinous films on system hardware. This technology has recently gained Denison HF-0 approval.

Some laboratory testing nicely demonstrates the clean feature offered by these new fluids. Many technologies show varnish formation within 500 hours of operation in industry-accepted pump tests. Even after 1,000 hours of use, there is no evidence of varnish formation with this new solution to an age-old problem.

Industrial hydraulic systems typically operate at approximately 140 degrees F, although temperature spikes up to 180 degrees F are not uncommon. High-temperature applications that place thermal stresses on the oil, such as plastic injection molding machines, glass transfer systems, heavy presses and mobile equipment, are ideal candidates for this new technology. It also is suitable if in general the user wants to improve equipment productivity and extend the life of oil, equipment and components such as valves, filters and pumps.

Hydraulic systems are improving by using tighter filtration limits (less than 3.0 microns is not uncommon) and high-efficiency filters to keep the oil clean. At the same time, hydraulic fluids are being subjected to increasingly tough operating conditions. Demands to raise production at the same time that oil volume is decreasing have emphasized the need to use high-quality hydraulic fluids. Increased operating temperatures have resulted in todays hydraulic systems developing varnish deposits over time that can lead to problems. The availability of new additive chemistries goes hand in hand with todays harsher operating conditions.

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