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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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