Introduction
Lubricating oil is as important for an engine as blood for a human. Just as a lot of information about human health can be obtained from blood tests, similarly UOA can give deep insight into the machine’s condition.
The practice of Used Oil Analysis (UOA) started more than 75 years ago when an American railroad company started using wear metal detection in used oil to assess the health of locomotive engines. Now UOA is one of the most potent tools of NDT and plays a central role in any planned/condition-based/predictive maintenance program for all kinds of machinery.
Regular testing of machinery lubricants enables us to:
- Assess the condition of the oil – to provide recommendations on its suitability for further use and to optimize oil change intervals.
- Check for contaminants in the oil – ingress of water, dirt, fuel, process fluid, wrong oil grade can significantly reduce oil life and accelerate wear.
- Assess the condition of the machine – to provide early warning of impending problems, and help reduce shutdown frequency, prevent equipment failure, improve production efficiency, and reduce maintenance expenses. All resulting in significant economic benefits.
UOA comprises of a series of Physical & Chemical tests along with Spectrometry, Particle count, Ferrography, IR, PQ, etc.
Common tests/tested parameters for marine lubricants.
Kinematic Viscosity
Viscosity is the most important property of any lubricant. Kinematic Viscosity is a measurement of resistance to flow at a specific temperature in relation to time. The two most common reference temperatures for checking lube oil viscosity are 40 °C and 100 °C. Viscosity is generally measured by a kinematic method and reported in centistokes (cSt). In UOA, the sample’s viscosity is compared to that of the new oil to determine whether the oil has thickened or thinned excessively. KV is tested for all lube samples.
Viscosity Index (VI) of lubricant is calculated based on viscosity values at 40 °C and 100 °C and can identify the lubricant grade as mono/multigrade.
Water Contamination
Presence can lead to rapid oil degradation and loss of equipment life. It is particularly damaging for bearing lubrication. Water content is often measured by Karl Fischer titration test, which can check for all forms of water: dissolved, emulsified, and free. Crackle test & FT-IR may be used as screening tests for water content. Water content is tested for all UOA samples and is usually expressed as a percentage.
Base Number (BN)
Base Number (BN) is a measure of the acidity neutralisation potential of the lubricant. Products of combustion are the main source of acids. It is an important property for engine oils & an indirect indicator of detergency. BN is tested only for over-based engine oils.
Acid Number (AN)
Acid Number (AN) is the quantity of acid or acid-like derivatives in the lubricant. AN is usually measured in non-crankcase oils. The AN of a new oil is not necessarily nil since some oil additives can be acidic in nature. Increase in AN from that of the new lubricant is monitored. Increases in AN usually indicate lubrication oxidation or contamination with an acidic product.
Both BN & AN are titration tests using standard reagents and the results are expressed in mgKOH/g.
Flash Point
Change of lube flash point is mainly influenced by fuel contamination and to a certain extent by high temperature oil degradation. Above 200 °C the test is a FLASH/NO FLASH test. Below 200 °C Flash Point is measured. Usually only engine oil & thermal oil samples are tested for FP.
Insolubles/Soot
Insolubles represent a measurement of all solids in a lubricant. The nature of solids depends on the system. In diesel engine oils, Soot is the main component, and its level is good indicator of combustion efficiency. In other machinery, wear debris, dust and oil oxidation products are the main components. Lubricating oils are typically soluble in pentane. Oxidation products are typically insoluble in pentane, but soluble in toluene. Wear debris, soot, sand and asphaltenes are typically insoluble in both pentane and toluene. This test determines the amount of pentane insolubles and toluene insolubles in lubricating oils. The results are expressed as a percentage.
Fourier Transform Infra-Red (FTIR)
FTIR is a quick method for determining chemical changes in a lubricant. The instrument checks for changes of various characteristics by measuring the shift in infrared absorbance at specific wavelengths of a used oil sample, in comparison with levels in a fresh oil sample. The results are presented as a plot of infrared light absorbed against wavelength and expressed as ‘Absorbance per 0.1mm’ (Figure 1).
It is particularly good for measuring oxidation & nitration; somewhat less reliable but cost effective for measuring contaminants like fuel, water, soot, glycol, etc. Since it relies on comparison with fresh oil results, future tests should preferably be carried out in the same lab.
(More on FTIR in a later article on the subject)
Oxidation
Lubricating oil in engines and other machinery combines with available oxygen under certain conditions to form a wide variety of harmful by-products. High temperatures, excessive aeration, presence of catalyst materials (e.g., metallic wear particles) accelerate the oxidation process. By-products of oxidation form lacquer deposits, organic acids, corrode metal parts and thicken oil (increase viscosity). Most lubricants contain antioxidant additives which retard the rate of oxidation. Tested by FTIR and expressed as Abs/0.1mm
Spectrometry
Metallic elements in the oil sample are measured by various spectroscopic methods. Most commonly ICP Emission Spectrometry (ICP – Inductively Coupled Plasma) is used. Wear, additive & contaminant elements can be measured. The results are usually expressed in ppm. Most spectroscopic methods have a limitation on size of particles that can be measured. ICP-ES does not effectively measure particles larger than 5 – 7 μm as larger particles are not fully vaporised in the plasma due to mass effects and so wear element concentration can be underestimated in cases of particularly high wear.
(More on Spectrometry in a later article on ELEMENTAL ANALYSIS)
Particulate Quantifier (PQ)
As the oil sample is brought into proximity to a controlled magnetic flux field in the PQ instrument, this magnetic flux distorts proportionally to the number of ferromagnetic debris within the sample. The amount of distortion is expressed as the “PQ Index” and is independent of particle size. PQ is a valuable adjunct to spectrometry.
Particle Count
Measuring the number of particles in the lubricant sample helps determine the overall cleanliness of the system being monitored. PC is primarily used for hydraulic systems. Reducing particulates in the lubricant can greatly increase the life of these systems. With the increasing use of marine 2-S main engine system oil for the Servo/Hydraulic systems of the engines, particle counting of Mn Eng. servo/hydraulic systems has assumed great importance.
(More on PC in a later article on MEASURING OIL CLEANLINESS).
Trend Analysis
A single UOA report is like a paragraph. Trend analysis tells the story. Trend Analysis helps establish patterns within data that can be important for identifying problems early, which if left undetected could later lead to catastrophic failures. The historical trended data may also enable prediction of certain outcomes.
Sampling
Successful UOA begins with sampling. How, when and where a sample is taken from the lubrication system is especially important. This is because only a small amount of oil is collected during sampling. Hence it is essential to collect a truly representative sample of the system. Bad data is worse than no data. If samples are collected incorrectly, the oil analysis is a waste of time, effort & money.
(More on this in a later article on COLLECTING UOA SAMPLES)
A version of this article was first published in MER(I) in the JULY 2021, Vol. XV; Issue. VIII.
References:
- Used Engine Oil Analysis-User Interpretation Guide, CIMAC No. 30/2011
- Oil Analysis Explained, Machinery Lubrication 12/2013
- Test Instrument photos from LUKOIL Marine-Tribocare Lab & Spectro Scientific
- Title photo from STLE
About the author:
Sanjiv Wazir is a Technical Adviser with LUKOIL Marine Lubricants. He is a mechanical engineer from IIT-Bombay. He is a marine engineer and a member of the Institute of Marine Engineers. He is a Certified Lubrication Specialist from the Society of Tribologists & Lubrication Engineers (STLE), USA and is a member of the Tribological Society of India. He has contributed to MER on marine lubrication developments in the past, and on oil contamination issues under “Lube Matters”, earlier.
He can be reached at sanjiv@lukoil.com
