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Hydraulic Fluid Darkening

On January 22, 2003 we asked ...

"After pulling a fluid sample from a hydraulic system you notice the hydraulic fluid has become darker. Concerned, you rush the sample to a lab for analysis.

The results of the fluid analysis appear normal: acid number, viscosity and FTIR-Ox (oxidation) have all remained stable.

A quick inspection of the hydraulic system reveals that the hydraulic valves have turned goldish in color. What would you do?"

Winning Answer by Dave Lander, PdM Technician, Temple-Inland Forest Products:

The goldish color on the valves is varnish, which you will likely also find on all surfaces of the tank, piping, valves and the sight glasses.

We recently we went through the same problem. Our lab reports were all normal and the only indication of a problem was the oil’s dark color and a change in its smell.

We found that the problem was due to thermal stressing of the oil, as opposed to straight oxidation. After an investigation we discovered that a valve had failed on a subsystem, forcing oil through a small orifice, causing it to over heat. Because it was a small localized hot spot in a large tank and oil cooler, the high temperature was never detected as a rise in bulk oil temperature. Our first indication of a problem was the change in the oil’s color.

My suggestion is confirm this suspicion with further oil analysis and if proven, drain, flush and clean the system with a solvent or an oil that is compatible with the current operating oil. Also you should check with your lube representative or supplier for recommendations and compatibility. There are also companies out there that will come in and drain, flush and refill your system.

What Does an Industry Insider Say?

The fact that the acid number, viscosity or FT-IR-Ox have not changed indicate that oxidation is not taking place in the hydraulic oil. However, the golden residue on the valves is a clear sign that varnish is precipitating out of the fluid. Since varnish is composed of fluid degradation by-products, what could be the cause of the fluid deterioration if it is not oxidation? The answer is thermal degradation which occurs in the absence of oxygen. Left untouched, this varnish will impact the valves capability to function properly.

The cause of thermal degradation is heat. Two of the most common sources of extreme temperatures in a hydraulic system are; a hot spot caused from an external heat source in close proximity to the lube system, or the implosion of entrained bubbles as the bubbles flow from the reservoir into higher pressure zones like the pumps or valves.

I would do three things to address this problem before performance issues started occurring.

First, I would confirm that thermal degradation is indeed the source of the problem. I would ask the laboratory to provide the FT-IR curve, and instead of looking at the wavenumber region where oxidation levels are measured (1714 cm-1), I would look for a peak in the nitration region (1630 cm-1). If so, my theory of thermal degradation would be substantiated.

Second, I would perform a root cause analysis to investigate the source of the heat in the system and determine if it is a system design flaw or if there is a solution that could be engineered to remove this heat source. If the problem appears to be entrained air, there are technologies available to remove bubbles from the hydraulic fluid to solve this problem.

Third, if it is determined that the thermal degradation is a result of system design, I would utilize a separation technology to remove the thermal degradation byproducts from the fluid, providing a permanent band-aid to the varnish formation until system redesign can be accomplished. Greg Livingstone, Team Leader, Clarus Technologies

Editor's Note: Thanks to everyone that responded. You sent a LOT of responses to this Readers Challenge and there were a lot of right answers. We could have picked a lot of winners. Here are just a few of the responses we received:

In regards to the darkening hydraulic fluid it seems like there has been excesive heat build up and the darkening of oil is the result of fine carbon silt in the oil. I would send a sample out for a RBOT test and view the results for oxidation stability and thermal failure. Using the results of the RBOT test would give me the justification to change the hydraulic oil or not.
Butch Shaw, CBM Coordinator, Millar Western Forest Products

There is enough information to start trouble shooting, but I would like to know a lot more. If the viscosity is stable and the test shows no water, or oxidation has not gone up, the reservior temperture is probally still not too hot. I would like to know the size of the reservior, what kind of cooling system does it have, operating pressure, kind of oil, are there any signs of intrained air or foaming, and patrical count. I would begin gathering this information. With out it though, I can only assume that the oil has air and is starting to implode under pressure and is leaving deposits on the valve parts and reservior walls. So my answer to the question it gather the other information needed, analyze the whole picture to determine the root cause. Jeff Tucker, Lubrication Analyst, International Paper

Color alone is not a measure of the condition of hydraulic oil. Since hydraulic oil is hydroscopic and the challenge did not indicate that a moisture test was done, I would check the moisture level of the fluid and change if off specification. Moisture in the fluid is probably the most destructive characteristic, as it will cause an accelerated corrosion of the hydraulic system components. The oil may also have overheated at some point which would account for the color and component discoloration. I would try to determine if the system is functioning properly, or if indeed there was some malfunction or abuse of the system that may have resulted in overheating. Warren Heidt, Manager Technical Services, Coors Brewing Company

I would suspect thermal degradation due to aeration. Adiabatic compression of air bubbles in contact zones can cause extremely high localized temperatures, leading to sludge, varnish and deposits of the sort indicated by the newly formed "goldish color" on the hydraulic valves. The absence of a change in viscosity and the acceptable TAN value supports this theory, since the overall system would not necessarily be oxidized by these localized extreme hot spots. In certain cases, extreme high temperatures can actually lead to a thinning of viscosity due to molecular cracking, or to no change at all in viscosity.

I would verify that the system had been filled to the proper level, since over- or underfilling can cause aeration. I would drain the system and inspect the pump, internal seals, etc., for any sources of leaks that might cause aeration. I would then refill the system, taking care to fill to the correct level. To the extent possible, I would observe temperature readings throughout the system to monitor potential localized hot spots. I would continue to monitor and inspect until the root cause of aeration is identified and corrected.
David Krause, Commercial/Industrial Sales Manager, Parman Lubricants Corporation

I think that the darker color of the hydraulic oil and the goldish color of the valves indicates varnish accumulation originating from thermal oil degradation. Oxidative oil degradation does not appear to have taken place yet due to the satisfactory results of the FTIR-Ox test. All oil should be drained from the system including the reservoir. The oil reservoir should be cleaned by confined-space tank cleaning. Filter the oil back into the reservoir through a high density sieve absorption filter and an electrostatic oil cleaner. Richard Hill, Mechanical Engineering Tech, Norfolk Naval Shipyard

Hydraulic valves turning goldish in color could be a sign of varnishing, despite the oil analysis indicating no problems with the lubricant. Varnishing could be the result of very high operating temperatures/pressures. One might look at hydraulic system pump pressures, cooler functionality if applicable, and check to ensure bypass/pressure reducing valves are working properly. Because varnishing is related to oxidation, it would be prudent to change the oil and add new oxidation inhibitors; especially if it is a small hydraulic system. If the problem persisted and valves did not turn "brownish," a total system flush would be needed to stop the oil darkening. Brian Groff, Maintenance Supervisor, Cargill Salt

There is no problem. The oil will change color with age and the staining is from the oils color at this time. What was the particle count reading? What type of valve and its material structure? How long has the fluid been in the system? Did the fluid color change overnight? These are questions that also need to be answered. Scott Golomb, Maint Tech. Lubes and Vibes, Northampton Generating

Any time oil is used, the color will gradually darken slightly. If the oil properties all appear within the normal ranges, I would smell the oil and check the temperature gauges. If the operating temperatures exceed 140 degrees f. the cooling fan may not be working to its capacity. Larry Young, Maintenance Planner, Florida's Natural Growers

I found a similar situation in a hydraulic power unit used on our main rubber mixer. Closer scrutiny found the filter on the heat exchanger oil loop, was in need of changing. The poor flow caused the oil to overheat. The subsequent discoloration looked very suspicious. Oil analsys revealed no other negative problems. Joe McLinden, Senior Manufacturing Mechanic, Callaway Golf Company

Given the background of the problem with varnishing on the valves but no other oil degradation evident, I would review the infrared spectrum for indication of nitration in the oil. If the spectrum has spikes in the 1600-1700 wavenumbers (cm-1) region then the oil has probably been exposed to either micro-dieseling or an extremely hot spot within the equipment. The root cause of the problem could be pumping aerated oil or the oil being exposed to an external or an internal heating source over 400 degrees F. Brian Knoke, Engineer, BCHydro

Recently we went through the same problem. Lab reports were all normal the only indication of a problem was the dark color and the smell of the oil had changed. The goldish color on you valves is varnish, which you will find on all surfaces of the tank, piping, valves and the sight glasses.

What we found to be the cause of the problem was thermal stressing of the oil. We had a valve fail on a sub system forcing oil through a small orifice and over heating the oil. Because it was a small localized hot spot along with the size of our tank and oil cooler the high temp was never detected. Our first indication of a problem was the change in color of the oil.

My suggestion is to drain, flush and clean the system with a solvent or oil that is compatible with your systems and operating oil. Check with you lube rep. or supplier for recommendations and compatibility. There are also companies out there that will come in and drain, flush and refill you system. Dave Lander, PdM Tech., Temple-Inland Forest Prod.

The collapsing of entrained air bubbles under extreme pressure generates enough heat to cause accelerated oxidation of hydraulic fluid. I would install baffle plates just below the return piping to reduce agitation. An air ejector to remove air and non-condensable gases from the reservoir will also help.

Finally, I would initiate a slow side stream filtration through an Ion-Exchange media while I recirculated the fluid in the reservoir through a vacuum dehydration unit with an attached 2-micron-or-less partical filter. Initially, the Ion Exchange media will create quite a bit of water but don't worry about it, the water will actually act as a solvent and help remove the varnish that has plated out in the system and then be removed by the vacuum dehydration unit or re-absorbed by the Ion-Exchange media.

This process will also increase the resistivity of the fluid and keep the TAN at or below .1. Randy Norton, Plant Maintenance Optimization Coordinator, Tucson Electric Power Company

1. Resample and submit to lab for verification of previous results.

2. Request additional testing to identify abnormal levels of carbon insoluables (fine patch 0.2 micron membrane, pentane insoluables ultracentrifuge, blotter spot test, FTIR for nitration). Check test results indications of additive depletion. Check for increases in specific gravity. Look for grease like suspensions or sludge formation in the oil.

3. Inspect the hydraulic system. Look for low reservoir levels, plunging returns or long vertical drops into the reservoir. Use ultrasound to identify any suction leaks in the system. Use infrared to find operating temperatures of all components of the system.

4. Depending on results from 1,2 & 3, make repairs/corrections to the system, replace oil, contact lubricant and equipment manufacturs to ensure proper product is being utilized. Monitor to verify corrective measures are adequate. David Richardson, Predictive Maintenance Technician, Allied Services

It sounds to me that what is going on is a combination of oxidation and nitration. Since this is the initial form of oxidation it would probably not show up with analysis yet. It appears this oil is on the borderline, and when it fails, like most hydraulic oils, it will fail rapidly. The valves are turning goldish because of deposits being left on them from the oil.

The valves are acting sort of like a filter, and if they're not sticking yet it will only be a matter of time before they start. If I was not having any machine problems from this like sticking valves I would just change the oil and filters. If I was having machine problems like sticking valves I would run a flushing agent through the system for approx. 24 hours to dissolve the deposits.

Next, I would try to determine the cause of the problem. If the oil had been in the system for awhile then it could just be time for an oil change. If the oil had been in for a short time I would start looking for hot spots in the system like snub valves on pressure gages stuck open or oil coolers not functioning / dirty etc. It could also be that the oil type being used is not of a high enough quality to meet the performance requirements of the equipment. Clifford Robinaugh, Engineering Coordinator, Honda of America Mfg.

Even though the oil analysis showed oxidation to be stable, there is something causing a small amount of oxidation that is creating the varnish that has darkened the oil and discolored the valves.

This source of oxidation needs to be corrected or preventative measures must be in place to maintain equipment reliability. A possible source of oxidation could be air getting into the system. As this air passes through the pump and compression cycle it becomes hot, oxidizing the small amount of oil surrounding the air bubble.

Check pump suction lines, turbulence in the tank and other ways that air might become introduced into the system. Regardless of the cause of the varnish, a good preventative measure would be to install an electrostatic separator.

This will remove the varnish particles before they can build up on the valves and create sluggish response and other performance problems. The result will be a reliable system and the oil life will be extended. Robert Niehoff, Supervisor, Plant Services, Iowa State University

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