In This Issue:

• Up Front: Localized Temperatures Cause Varnish

• Book Bits: Tips for Flushing Hydraulics

• Today's Tip: Lube Tag It

• Q & A: Hot Enough for Synthetic Lube?

Readers Challenge

Localized Temperatures Cause Varnish

At our plant we have a turbo expander that is used to change gaseous oxygen to a liquid. The gas is used to turn a turbine connected by a shaft to another turbine that turns in oil, which acts as resistance.

The turbine runs at about 14,000 rpm. As the energy is used from the gas, it turns to a liquid and is then stored for use in the waste treatment plant. We had been using 32 cSt turbine oil for some time. We had a problem with the equipment and had to take it apart for repairs. Upon inspection, we found that all of the parts were heavily coated with varnish even though the temperature of the oil was always about 100 degrees F. After reading some articles on the subject, we determined that the oil must have been getting overheated and stressed at the turbine wheel before it returned to the main oil loop, which was cooler.

After some thought, we decided to change to a 32 cSt synthetic compressor oil. In the last year, we have not seen the varnish build up like before, and the sample reports are much better. (Submitted by Norman Eudaily, Vibration Analyst, Simpson Tacoma Kraft. Thanks Norman!)

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Book Bits

Tips for Flushing Hydraulics

From the "Hydraulic Fluids"

Flushing may often be required when the system is new or during subsequent service, particularly after repairs, if significant contamination has occurred. Large systems assembled on site frequently incorporate components treated with temporary corrosion protectives. Unless such materials are removed prior to filling the system with a clean hydraulic fluid, they will gradually dissolve in the working fluid and may detract from its performance in several respects, for example, by promoting emulsification.

If flushing is required, it is preferable to utilize the grade to be used in service or a lower viscosity grade of similar composition. Critical components (valves, hydraulic pumps and motors) should preferably be isolated or by-passed during the flushing operation to avoid harmful accumulations of particulate contaminants, thus defeating the purpose of the treatment. A minimum charge of flushing oil should suffice, this being circulated through the system and adequately dimensioned supplementary filter(s) by a suitable external pump. Filters should be checked periodically and flushing continued until particle counts on the fluid returning to the filters decrease to a satisfactory level.

The temperature of circulation should preferably be around 40 degrees C; if necessary the flushing medium should be heated, but local overheating must be avoided. If electrical heaters are used they should be of liberal surface area.

More information about the "Hydraulic Fluids"

Today's Tip

Lube Tag It

• Consider using equipment lube tags to avoid adding the wrong oil to a machine. Using lube tags provides the following benefits:

  • Reduces possibility of error by inexperienced lubricator

  • Facilitates training of new lubricators

  • Reduces confusion associated with switching suppliers

Methods for labeling lube tags include:

• Use color and/or shape-coding where possible. Color-code tags indicating lube name and viscosity and affix to each reservoir.

• Tags can be plastic, stainless steel, aluminum or coated paperboard.

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Q & A

Hot Enough for Synthetic Lube?

"My supplier is pushing hard to move his synthetic gear oils into my operation. His position is that the units run too hot for long-term use of mineral oils. At what temperature should I switch to a synthetic lubricant for a noncirculated gearbox?"

First, the term synthetic is very broad, so we assume you are referring to lubricants that are formulated using polyalphaolefin (PAO) synthetic base oil.

There is no single temperature point that dictates a time to move to synthetics. The decision is dependent on equipment continuous loads and shock loads, equipment availability for routine maintenance, criticality of the application, component lifecycles, lubricant lifecycles and failure modes among other things.

Generally the rate of lubricant degradation doubles with every 18 degrees F increase in temperature. Once formulated with antioxidant additives, PAO-based lubricants have a lower baseline rate of oxidative degradation. At low temperature, a PAO's increased oxidative life may not be noticeable - particularly if you have to change the oil at some point for other reasons. At higher temperatures, the synthetic may last noticeably longer.

One typically begins to notice the extra life provided by a PAO above 160 degrees F. Above 180 degrees F, and especially 200 degrees F, the difference in oxidative life becomes quite apparent.

HOWEVER, and we can't stress this enough, the point at which a change to synthetic is justified is dependent perhaps on a handful of additional "program management" parameters such as:

1. Do you intend to run your gear oils, with an appropriate use of filtration and oil analysis to support lifecycle extensions, for multiples of years?

2. Are you currently doing oil analysis and performing condition- based changes?

3. Do you have, and have you communicated to your lab, oxidation limits that flag you to impending oxidation problems?

4. Does the machine's operating temperature vary a great deal (a PAO's high viscosity index enables it to operate across a wider temperature range)?

5. Do you have an effective contamination control program in place that will enable you to fully exploit the PAO's extended life?

With the appropriate management strategy, a change to a high- performance product can actually cost considerably less than the equivalent mineral oil product type. Outside of these considerations, it is my opinion that somewhere around 165 degrees F represents the point at which one should begin to consider the use of synthetics for the sake of lubricant longevity, if not for the sake of reliability.

Mike Johnson, Senior Technical Consultant, Noria Corporation

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