Oil Analysis

Adam AdgarSchool of Computing and Technology

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

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)

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.

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.

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 ?

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.

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)

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

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

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.

Transmittance Spectra – Engine Oil

NEW

USED

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

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.

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.

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.

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

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

Example

Large Ferrous Abrasive Wear Large Ferrous Abrasive Wear 100100×× Magnification Magnification

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

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)

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: <10 microns Small: 10-25 microns Medium: 25-60 microns Large: >60 microns

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