Oil analysis and preventive maintenance
Imagine the implications of neglecting to check the lubrication systems of plant equipment. Failure here would be both catastrophic and expensive. Not only because of the cost of repairing the physical damage, but also the time it takes to fix the problem and the revenue lost from the equipment not being in use.
Oil Analysis Programs
The smooth running and long-term performance of plant machinery has proved to be so critical, that some equipment manufacturers offer their customers a preventative maintenance oil analysis program. They will take samples of oil on a regular basis and by a combination of physical and chemical tests, gauge how well a gearbox or hydraulic system is performing. They monitor its physical properties, contamination levels, and wear debris fingerprint over time to get a better understanding of the lifetime of the moving parts.
This type of program has traditionally been offered by some of the larger plant equipment manufacturers, but forward-thinking oil companies are also beginning to realize its importance. By offering a comprehensive preventative maintenance oil analysis program they are getting an early warning of their customers’ problems before they get too serious.
What will oil analysis tell us? Let’s use the example of a hydraulic system. Make the basic assumption that it consists of two essential components, moving parts and the lubricant. Anything else has to be considered foreign material. As the equipment begins to wear, small microscopic particles and products of heat and the oxidation process will be suspended in the oil. These particles can provide critical information as to what moving parts are wearing or whether there is contamination from external sources. Even though there are many different test methods and approaches to this problem, it is generally accepted that to be effective, an oil analysis program must carry out a number of physical and chemical measurements. Some of the most important tests include:
Viscosity: As oil is used, hydrocarbons break down with heat and the viscosity of the oil increases. Monitoring the viscosity of oil using a viscometer is a good indicator and can help determine whether the oil is in need of a change.
Water: In small amounts, this contaminant will not cause major damage. But in larger amounts, it may produce emulsions that can plug filters, contribute to the formation of metal-corroding acids, and accelerate oil oxidation. Moisture analysis can either be carried out by infrared spectroscopy or the Karl Fischer method.
Total Acid Number (TAN) and Total Base Number (TBN): These monitor organic acids and bases respectively that are produced from a combination of heat-generated oxidation products and the breakdown of additives in the used oil.
Wear Metals: Metallic particles in the oil, generated by wear, can reveal the condition of individual components within a machine. Trends in the relative concentrations of major, and minor elements in the oil can help to identify their likely source (Table 1).
Wear metals analysis is typically carried out by optical emission techniques. A typical wear metal analysis by emission spectroscopy (Fig 1), shows a trend in the lead and copper content over time of an oil sample used in a bronze bearing. It can be clearly seen that after approximately 6000%%MDASSML%%7000 hours of use, the lead and copper levels in the oil are beginning to increase, indicating a potential problem with the bearing.
Monitoring signature additive metals for a given product formulation can also indicate whether an additive package is being depleted or whether contamination by another oil has occurred. This is an important part of any used oil analysis, especially where multiple products of varying product compositions are used at a single location.
Ferrous Metal Content: A sensitive magnetometer is used to measure the mass of ferrous material in a sample of oil and displays it as a particle quantifier (PQ) index. In wear metal content tests for all metals, the PQ index is only an indicator of ferrous-based wear.
Particle Counting: This approach uses the principle of light scattering to measure the size of particulate matter suspended in the oil. Used oil passes a laser beam, where any particles will block light from reaching a detector. For used oil work, the particle counter is typically set up to read three different sizes, 4-, 6- and 14-micron particles, which can indicate the type and severity of the wear taking place.
High Throughput Testing
For a preventative oil analysis maintenance program to be successful, it is absolutely essential that the oil be analyzed on a regular basis. Analyzing oil when a piece of equipment is new provides a baseline. Then, over the lifetime of the equipment, the used oil is sampled and analyzed on a frequent basis. A change in these chemical and physical properties over time produces a trend analysis of the oil, which can indicate a number of potential problems including:
Thermal or oxidation breakdown of the oil
Machine component wear
Contamination from external sources
The key to avoiding major damage is to analyze oil samples with high frequency, fast turnaround times, and good accuracy. Unfortunately, high sample throughput can sometimes have a negative impact on the quality of results, unless the equipment being used is optimized for the task. For this type of demanding application work, which can generate a few hundred thousand tests per year, there has to be a close working relationship between the manufacturer and the user of the equipment. The instrument manufacturer understands the technology and how it works, whereas the user of the equipment supplies the application knowledge and the expertise to make critical decisions based on data generated. There are two main analytical techniques to carry out the majority of chemical tests used in a plant machinery based oil analysis program.
ICP-OES is a rapid multi-element technique for the analysis of aqueous and organic-based solutions. It uses a high temperature, argon plasma (ICP) to excite ground state atoms in a sample so they emit wavelength-specific photons of light, characteristic of a particular element. The photons are then focused by an optical system onto a detector where they are counted using sensitive measurement circuitry. The emission intensity (total number of photons), corresponding to the individual trace metals in the sample is then quantified by comparing it to known calibration or reference standards.
Infrared Spectroscopy relies on the principle that different molecular structures in a sample have different wavelength-specific absorption characteristics, known as an IR fingerprint. By comparing the intensity and shape of molecular bands produced by this absorption, the component relating to that molecular structure can be identified and quantified. For example, water can easily be identified in the presence of lubricating oils by looking for the H 2 O molecular band.
The benefits of oil analysis programs cannot be disputed. There are countless examples where it has saved companies large amounts of money by minimizing equipment downtime. And by extending this service to an Internet-based system, oil companies can get a comprehensive view of a customers’ entire oil analysis program, zero-in on problem areas, and offer expert help when and where it is needed.
Authors Carl Tolas, David Hilligoss and Robert Thomas would like to thank members of ExxonMobil’s Signum Oil Analysis Laboratory (Kansas City, KS). PerkinElmer’s Chemical and Environmental Group (Shelton, CT), and Scientific Solutions (Gaithersburg, MD) for their contributions to this article.
If you have any questions about oil analysis or preventative maintenance, contact David Hilligoss at 630-368-3405. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8774, JFoszcz@ReedBusiness.com
Table 1: Typical sources of wear metal contamination in oils
Element Found Possible Source Aluminum (Al) Bearings Copper (Cu) Thrust bearings, Roller bearing cage wear Iron (Fe) Roller bearings Lead (Pb) Bearings Vanadium (V) Surface coatings Silicon (Si) Dirt