CM oil analysis of turbine oil

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Oil Analysis and Ferrography

• Material compiled and presented by
• Mr. V.K.Gupta,
• Faculty, Mechanical Maintenance, NPTI

2
Oil Function

• Very important for any rotating equipment
• Performs two well-known, primary functions
• Lubricates surfaces - reduces friction
• Separates surfaces - oil film prevents
  metal-metal contact
• Typical oil film thickness is 10 microns - 400
  microns
• At actual load/contact point may be less than 1
  micron
• Oil also performs several other, related
  functions
• Protects surfaces from corrosion
• Can remove contaminants and particulates in
  circulating systems
• Assists with temperature control by absorbing /
  transferring heat
• Transmits power in hydraulic applications

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Oil Types

• Oil comes in two basic forms
• Natural (or Mineral)
• Petroleum based
• From oil deposits in the ground
• More commonly used type
• Synthetic
• Man-made lubricant
• Based on manufactured polymers
• There are many sub-categories of each form

• Grease
• Suspension of lubricant (both natural and
  synthetic) in a chemical solution.
• Lubricant is temperature released (temperature
  rises, lubricant gets released until the
  temperature drops again and so on)

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Why Analyze Oil Condition?

• Oil analysis is the leading indicator of any
  problems developing on a large number of
  different mechanical systems
• Oil analysis will arguably catch more potential
  problems across the broad range of mechanical
  systems than any other predictive technology
  (including vibration analysis).
• That is not to say that other technologies are
  not necessary or that oil analysis does not have
  its weaknesses (rolling element bearings being
  one notable example). It simply illustrates the
  value of a well-run oil analysis program.

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Where is Oil Analysis used?

• Oil analysis works particularly well on
• Oil lubricated rolling element bearings 
• Oil lubricated sleeve bearings
• Gearboxes
• Engines
• Turbines
• Hydraulic Systems
• Compressors
• Chillers
• Any system that uses lubrication and lends itself
  to drawing and testing samples of that
  lubrication.

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What does Oil Analysis Reveal ?

• Oil analysis can be broken into three main areas
  of testing 
• Chemical composition of the oil (including the
  additives).
• Presence of contaminants (e.g. water).
• Particle analysis (wear particles in the oil -
  primarily metals - a.k.a. ferrography).
• It is obvious that tests should be performed on
  oil systems on machines that are operational. But
  what about incoming and stored oil - are you
  getting what you should be getting (what you
  ordered) ?
• Has the oil been stored properly ?
• Has moisture or other contaminants gotten in ? Is
  recycled or reclaimed oil up to the standards it
  should be ?

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Where to take sample
A representative sample should be taken at the
same location using the same method each time, as
the following table indicates
Equipment Sampling Points
Engine Crankcase Through dip stick holder Sample valve on crankcase wall Return line before filter
Hydraulic System Sampling Valve on return line before filter Reservoir about mid-point, away from reservoir walls
Compressor Crankcase about mid-point away from crankcase walls Return line after oil separator
Gearboxes Sump mid-point Avoid sump floor sludge and side walls deposits
Turbine system Reservoir mid point keep away from sidewalls and baffles. Main bearings return lines Secondary points - after bypass filtration system

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Commonly Measured Parameters

• Viscosity
• An oil characteristic related to it's resistance
  to flow.
• Moisture
• A contaminant that causes rust, corrosion and
  oxidation.
• Acid Number
• A measure of acidity related both to additive
  presence and oxidation likelihood.
• Base Number
• A measure of alkalinity related both to additive
  presence and oxidation likelihood.

• Particle Count
• Related to component wear, contaminant
  ingression, corrosion and others.
• Presence of Various Additives
• Depletion of these can lead to serious problems.
• Dielectric Properties
• Related to the likelihood of impending oxidation.
  
• There are numerous other tests that can be
  performed and specific tests within these general
  categories.

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Oxidation

• Caused by heat, air bubbles, water and metal
  particles
• Chemical process that changes oil molecular
  structure
• Very common problem monitored by oil sampling and
  testing
• Results in an increase in oxides, acids, polymers
  and sludge
• Oil properties change in that the viscosity
  increases (it gets thicker), it darkens and
  changes its odor.
• Can be monitored by (amongst others)
• Viscosity Measurement
• Fourier Transform Infrared Spectroscopy (FTIR)

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Viscosity Testing

• Kinematic Viscosity
• Measures oil's resistance to flow due to gravity.
• Units are "centistokes" (cSt)
• Capillary viscometer
• This U-shaped tube is filled with oil.
• Suction is applied that lifts the oil up one side
  of the tube to a "start" line.
• The tube is then submerged in a temperature
  controlled water bath (40C or 100C)
• Oil allowed to flow from the start line to the
  stop line under gravity.
• The time it takes represents the viscosity value

• Absolute Viscosity
• Measure's an oil's resistance to flow due to
  internal friction.
• Units are "centipoise" (cPs)
• Brookfield (rotary) viscometer
• Glass tube in a temperature controlled block is
  filled with oil
• Rotating spindle is submerged in the oil
• The amount of torque necessary to turn the
  spindle at a particular rate determines the
  viscosity value

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Effects of Incorrect Viscosity

• Too high (thick oil)
• Excessive resistance to flow causes
• Additional heat to be generated - one of main
  causes of oxidation and sludge
• Oil may not flow to or through areas that it is
  supposed to flow (e.g. bearings, return or drain
  lines). Problem compounded when oil (system) is
  cold
• Cavitation
• Increased energy consumption

• Too low (thin oil)
• Insufficient resistance to flow causes
• Loss of proper oil film thickness - leads,to
  increased friction, heat buildup and effects such
  as oxidation
• System is more susceptible to loss of oil film in
  high load and slow speed areas
• System susceptible to problems generated by
  smaller particles than would be the case with a
  normal (thicker) oil film
• Increased likelihood of thermal breakdown of the
  oil

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Chemical Makeup FTIR

• Fourier Transform Infrared Spectroscopy
• Common method to assess the oxidation level (or
  potential) in oil
• Especially useful because it allows the analysis
  of the additives and the presence of a variety of
  contaminants.
• Some of the molecules that can be tested with
  this method include water, oxidation by-products,
  nitration, sulphation, glycol, anti-oxidants,
  anti-wear, soot and many more.

• Method
• Infrared light passed through a fixed film of oil
  (100 um thick)
• Absorbance of IR is examined at range of
  wavelengths
• Compared to the same test on "base", or new oil. 
• Many of the molecules being tested for (additive,
  contaminant molecules plus the oil molecules)
  absorb the IR light only at very specific
  wavelengths. By comparing the used oil to the new
  oil, an accurate assessment of the quantity and
  presence of these molecules can be made.

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Transmittance Spectra Engine Oil
NEW
USED
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Chemical Contaminants

• Some generated by processes taking place in the
  oil (e.g. oxidation)
• Others are result of outside chemical
  contaminants getting into the oil and include
• Water
• Glycol
• Fuel
• Air

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Chemical Contaminants Water

• Water is possibly the single most destructive
  contaminant that commonly gets into oil. It can
  get into the oil in any number of ways
• Oil drum stored improperly, water standing on top
  slowly leaks in.
• Reservoir that gets water condensation on the
  lid, eventually drips back into the oil.
• Leaking or no seals
• Often people make the mistake of thinking that
  since oil and water separate (oil on top, water
  on the bottom), you can see water contamination.

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Effects of Water Contamination

• Causes oxidation -significantly worse when water
  is present. The chemical process causes acid
  formation, sludge and varnish are formed and the
  oil is thickened.
• Viscosity changes - Contrary to what many
  suspect, water will cause the viscosity to
  increase (oil thickens) especially when oil
  emulsions are created.
• Dielectric changes - Water, since it conducts
  electricity, reduces the insulating properties of
  oil.
• Aeration - Water can accelerate aeration problems
  such as the formation of tiny air bubbles and the
  generation of foam.
• Attacks additives - Water chemically reacts with
  additives to cause effects such as sludge, acids,
  sediment and many more.
• Reduces oil film strength - Water will cause film
  failure and other side effects.
• Bacteria - Bacteria can actually can form in the
  water.
• Water also affects machine components
• Corrosion - Water causes components to rust (one
  of the solid contaminants mentioned previously).
• Acids - The acids formed will also cause
  corrosion. Embrittlement - Loss of oil film
  strength and instantaneous water vaporization can
  cause hydrogen embrittlement of the metallic
  components.

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Testing for Water Contamination

• Testing for water in oil can be done through a
  variety of methods with the most common and
  simple being the "crackle" test.
• A crackle test is a test where a couple of drops
  of oil is put on a hot plate  and heated to about
  300 F (150 C).
• If water is present, audible crackling will be
  heard as the water heats up, bubbles form and
  grow and finally pop.
• Another method is the FTIR (discussed earlier)
  where the presence of water will be indicated in
  a particular wavelength.
• In order to quantify the water in the oil, the
  Karl Fischer test is often used. This test will
  provide a ppm or percentage water value. Measures
  all water - free, emulsified and dissolved.

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Solid Contaminants

• Solid (particle) contaminants can be quite
  destructive
• Destructiveness
• Quantity
• Size
• Hardness
• Sharpness of edges
• Weight

• Examples include
• Wear metals (depends on system components)
• Soot (combustion by-product)
• Rust
• Dirt / Dust
• Fibers
• Silt (class of very small particles ( 1 micron)
  and can be composed of many different materials

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Introduction To Ferrography

• Also known as Wear Debris Analysis
• A powerful tool in analyzing the health of
  machinery.
• Wear particles suspended in the oil are separated
  by magnetic or filtration methods
• Examine this debris under optical microscope
  (100x is standard)
• Analyst can gain much information on the health
  of the machinery from which the sample was taken
• Relatively simple technique
• periodically quantify amount of wear taking place
  over time
• identify location and mechanism of such wear

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Example
Large Ferrous Abrasive Wear 100 Magnification
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Wear Debris Particle Recognition

• Appearance of wear debris is related to
  conditions under which they were formed
• This facilitates the identification of wear
  mechanisms.
• Only a limited number of ways in which surface
  wear can occur.
• Each mechanism generates particles of a specific
  appearance.

• Damage due to wear can occur by any of the
  following specific wear mechanisms
• Abrasion
• Gouging
• Adhesion
• Cavitation
• Erosion
• Micro fatigue
• Fretting
• Corrosion

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Particle Characteristics

• Most important parameters of the wear debris are
  
• Extent of wear
• Quantity
• Texture / Hardness
• Color
• How wear is occurring
• Size
• Shape
• Composition

• The shape of wear particles can be classified
  into any of the following categories
• Platelets (P)
• Ribbons (R)
• Chunks (C)
• Spheres (S)
• Heat Related Fused Particles (T)
• Abrasive Debris (A)
• Fretting Wear (F)
• Needles (N)
• Corrosion (oxidant product) (O)

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Particle Characteristics

• Color
• Severity of the wear (and hence temperature
  involved) is indicated by particle color.
• Colors range from light straw ? brown ? purple ?
  blue as the temperature progressively rises (from
  230 C to 300 C).
• Brass or bronze (copper based alloys) show a deep
  red or green discoloration from tempering.
• Dark discoloration on the particle surface may
  indicate surface oxidation (corrosion).

• Size
• Four descriptions for categories of size
  classification may be used
• Fine lt10 microns
• Small 10-25 microns
• Medium 25-60 microns
• Large gt60 microns

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A Lubricating oil sample analysis report showing
trending.
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References

• Schalcosky, D.C. and Byington, C.S. "Advances in
  Real Time Oil Analysis". Practicing Oil Analysis
  Magazine. November 2000.
• Barnes, M. "Fourier Transform Infrared
  Spectroscopy". Practicing Oil Analysis Magazine.
  March 2002