How to Select Machine-Specific Oil Analysis Tests

An important aspect of oil analysis is being able to produce machine-by-machine test slates which serve the performance requirements of the assets under the user’s control. There is no single oil analysis test slate which can serve all the needs of all the different types of oil-lubricated assets on a plant. A knowledge of what oil analysis can provide is invaluable in determining test slates for various different types of equipment. It is also essential to determine the type of maintenance profile suitable for the machine. Armed with this information, one is in a position to determine a test slate which is effective both in cost and performance.

There are a few different perceptions of oil analysis. One view is that oil analysis is there to provide a warning of an imminent breakdown, or the predictive maintenance approach. Another view is that oil analysis is a metric of the overall health of a machine, or, the proactive approach. Oil analysis can fulfill both of these roles admirably, if used correctly. By ‘used correctly,’ means that the correct tests are chosen for the machine to serve its reliability profile.

As an example: an iron mine, as much as they claimed differently, and as much as they wanted to believe differently, were practicing rudimentary preventive maintenance. This company was owned by a large international mining house which demanded their subsidiaries carry out particle counts on all their oil-lubricated assets. It was asserted, and rightly so, that particle counts are an important part of any proactive maintenance program. Because workers at this mine weren’t practicing any type of contamination control, the laboratory they were using was seldom able to produce a meaningful particle count due to the severe dirt and wear metal contamination in the samples. The results: oil analysis data which was meaningless for the mine’s operation, loss of faith in the tool, undetected failures and an overall disdain of lubrication.

Had this mine been able to acknowledge their maintenance abilities and been able to choose tests more suited to their existing maintenance procedures, they would have been able to detect more impending failures and act on them, rebuff the oil analysis skeptics, and use the proven savings to justify improvements to their lubrication program.

Trying to determine the correct test slate for an oil-lubricated piece of machinery can be daunting. There are so many tests available, some appropriate for the application, others not. Having an idea about what the various tests are, what they can accomplish, and taking into account the maintenance philosophy being practiced, test slates can easily be drawn up to accomplish the desired results.

Table 1. Common oil analysis tests.

The Tests

Eleven commonly performed tests are listed in Table 1. These tests are not all the tests that can be performed, but do include all the most common ones. The table also indicates whether the test can be reasonably done as at an onsite laboratory or whether they are more likely to be performed in a commercial oil analysis laboratory. Onsite laboratories range from very simple to very complex, and the third column in Table 1 represents an industry average of what might be found in an onsite laboratory.

It is worth noting that test packages can be purchased from most laboratories at a price substantially cheaper than the sum of the individual tests purchased separately. Where possible these test packages should be used and complimented with extra tests if desired.

Oil analysis can be broadly divided into three different categories: fluid condition, contamination and wear.

Particle Counting

The particle counter produces a count, in different size ranges, of particles per 1 m? of oil. It is concerned primarily with contamination, but as some of this contamination may be internally generated, the wear aspect of oil analysis is also addressed. With most particle counters differentiating between internal and external wear is impossible, but there are new technologies available which are addressing this.

A particle counter produces a number for each of the different size ranges, as shown in Table 2. Various different types of machines are used to generate these counts and different reporting structures are used, but Table 2 probably represents the most common one.

Each column, except the last, reports the number of particles bigger than a certain size in microns per 1 m? of fluid. Trying to absorb all the information presented at once is not that easy, but a summary of the particle count is presented in the last column. The summary reports an index related to the number of particles in each of the following different size ranges: larger than 4 microns; larger than 6 microns, and larger than 14 microns. Increasing numbers as evaluated on a trend basis indicate the fluid is getting dirtier and decreasing numbers indicate the fluid is becoming cleaner.

It is worth mentioning that there are interferences that can cause anomalies in the results. The interferences depend on the technology being used, but can include water droplets, air bubbles and heavily discolored oil. If significant differences in particle counts are noticed, the first course of action should be to ensure, as much as possible, that interferences have been dealt with in the testing process and that other significant test results have not changed, such as water contamination.

Particle counters are not inexpensive, but the results they provide are generally seen as being important enough to warrant their inclusion in many onsite oil analysis laboratories.

Water (crackle test)

The crackle test is one of the simplest tests that can be performed on an oil sample and is an absolute must for any onsite laboratory. The test addresses the contamination aspect of oil analysis and involves heating the oil up to between the boiling point of oil and water. At this temperature, water in the oil will boil and produce noticeable bubbles. In practice, it involves putting a drop of oil onto a hotplate and watching for water bubbles in the drop. It is accurate to approximately 500 parts per million (ppm), or 0.05%, water content.

Interferences are few, but probably the most significant is the presence of contaminants, such as refrigerant gas. The crackle test will suffice for most moisture content determination needs. If it passes the crackle test, moisture levels are acceptable.

A failed crackle test should in most cases be followed up by a test to determine the exact water content. Various options are available, the most common being the Karl Fischer test.

Water (Karl Fischer)

The Karl Fischer method is used to determine the exact water content of an oil sample. It reports results as ppm water. It is most commonly used as an exception test generated by the crackle test, but should absolutely be run as a routine test in situations where water content below 1,000 ppm is important, such as electrical transformers. In most industrial applications Karl Fischer as an exception test from the crackle test should suffice.

Viscosity

Viscosity is a fluid’s resistance to flow. It is an important indicator of the condition of the oil and can also be negatively affected by contamination. There are various means of carrying out the viscosity test and it can be reported at temperatures of 40?C or 100?C. For most industrial applications a viscosity measurement at 40?C is required.

Many onsite laboratory instruments do not carry out the test at 40?C, but rather perform the test at room temperature and then estimate a 40?C measurement. The method used is of secondary importance to consistency in method in performing the test from sample to sample, and it is the trend that is ultimately the most important.

Ferrous Density

Ferrous density is a determination of the content of magnetic debris in the oil. As most wear metal is iron-based, this test is, in most cases, a good indicator of the amount of wear debris in the oil. It does not have a particle size bias, as does the elemental analysis test, but generally does not have good sensitivity at very low levels of wear metal contamination. As such, the nature of the test puts it squarely into the realm of predictive maintenance rather than being a proactive maintenance tool.

There are several different instruments for performing the test, ranging from portable to desktop units, and each instrument reports its results differently. Once again, the particular instrument used to perform the test is of secondary importance compared to consistency of method from sample to sample.

Table 2. A typical particle count.

Analytical Ferrography

Analytical ferrography is the visual analysis of solid contaminants removed from the oil sample. As the name suggests, it is biased toward contaminants of a ferrous nature, i.e., wear metal, but some non-magnetic debris gets trapped as well. The test uses magnetic fields to separate the ferrous debris into different size ranges on a microscope slide, then examined under a compound microscope. It is an expensive test to perform and the results are subjective, so this test is usually only performed as an exception test.

On filtered systems, the results of the test may be misleading due to the possibility of abnormal wear particles being filtered out. It can be used on filtered systems and on systems which are filtered by portable filtration units, but preferably a filter analysis should be carried out in such systems.

The high cost of equipment and the complexity of interpretation make it unlikely that analytical ferrography will be found in most onsite laboratories.

Filter Analysis

Filter analysis is a visual analysis of solid contaminants removed from the filter. It involves washing out a piece of the filter membrane and depositing the contents onto a filter patch for microscopic analysis. The debris can be separated into magnetic and non-magnetic components if desired, but unlike analytical ferrography, the particles are not separated according to size. Like ferrography, the test is time consuming, expensive and subjective. It provides better resolution of non-magnetic debris than analytical ferrography. This test should be carried out on filtered systems as an exception test, possibly generated by an out-of-specification elemental analysis, ferrous density or particle count.

Filter analysis can be successfully performed in an onsite laboratory, however, more advanced diagnoses will probably be available from a commercial laboratory.

Acid Number

The acid number (AN) test measures the acid content of a sample. The AN is an indication of how much the fluid has oxidized or degraded. AN also is used to determine the rate of depletion of the anti-oxidant additive. It is primarily focused on the condition of the oil, although some contaminants can also affect the AN. Units are mg KOH/gram oil. An increasing AN indicates increasing oxidation of the oil. Unlike some conditions, like contamination, which can be reversed, a high AN cannot be.

Acid number can be easily and inexpensively carried out in an onsite laboratory.

FTIR

Fourier-Transform Infra-Red (FTIR) spectroscopy uses infra-red light of varying frequencies to search for the presence of absence of certain compounds in the oil. The scope of the test can range from very simple, or inexpensive, to very complex and expensive, depending on the desired results. FTIR examines both the condition and contamination of the sample.

For most industrial applications the simple tests can give sufficient information. The primary property sought here is oxidation.

FTIR is seldom found in onsite laboratories due to its high costs and moderate complexity of operation. It is worth noting that prices on the spectrometers are decreasing and the feasibility of including one of the devices in an onsite laboratory is increasing.

Patch Test

Like the crackle test, the patch test is one of the easiest and most inexpensive tests to perform and is a must for an onsite laboratory. The test involves filtering oil through a filter patch and then examining the filtergram through a microscope. This test focuses on the contamination and wear aspects of oil analysis. If desired, the contents of the oil sample can be separated into magnetic and non-magnetic components and each part examined individually. It is worth attaching a camera to the microscope to record the resulting images on a computer for comparison purposes.

Elemental Analysis

Arguably the most important test in the oil analysis arsenal is elemental analysis and it provides information on all three aspects of oil analysis: the condition of the fluid (levels of some additives), contamination and machine wear. The commonly used method is inductively-coupled plasma (ICP) spectroscopy, which utilizes light in the visible and ultra-violet ranges. It reports results in ppm of various elements.

The major drawback of this test is the size of the particles it can detect. Particles larger than 5 to 8 microns in size are not detected by this test. However, in most wear situations, there will be an increase in wear particle sizes across the range, so elemental analysis can still provide excellent results. A knowledge of the metallurgy of the machine is vital in the interpretation of the results. It is also important to employ exception tests when anomalies in the elemental analysis are detected.

Due to the high capital costs and complexity of operation, ICP spectrometers are found in only the most sophisticated of onsite laboratories.

Table 3. Gearbox test profiles. R = Routine. E = Exception (triggering tests in parentheses).

 

Table 4. Clean-oil systems.

The Profiles

The most commonly encountered industrial test classes are going to be examined-gearboxes and clean-oil systems.

The test profiles have been divided up into three categories: screening, predictive and proactive.

A screening test can be run in a few different applications. It might be used on small, non-critical pieces of equipment where regular full-slate oil analyses aren’t cost-effective. Screening tests also can be run on any piece of equipment where a problem is suspected. The benefits of a screening test should be its low cost and high speed turnaround. Because of those reasons, a screening test would be performed at an onsite laboratory. A screening test should be seen as an enhancement to a routine oil analysis test slate rather than a replacement.

The routine oil analyses have been divided into two categories: predictive and proactive. Ideally, one wants to be performing proactive maintenance as much as possible, but there are times when performing predictive maintenance is the correct course of action. Such occasions might include low replacement costs or low criticality of particular pieces of equipment, when more complex maintenance strategies are unwarranted. Proactive maintenance strategies would typically be performed on newer, more expensive equipment and where criticality of operation makes a high reliability desirable.

In Table 3 and Table 4, the predictive and proactive test profiles have two types of tests indicated, routine and exception. Routine tests are performed each and every time the sample is tested. The screening profile has no exception tests indicated - the exception test for a failed screening test is a full routine analysis.

The profiles presented are designed to serve as guidelines only to help with creating test slates for the most common industrial applications. There are instances when criticality of operation, cost, safety factors, environmental factors or fluid selection, make changes to the suggested test slates desirable. In all instances the previously mentioned factors need to be taken into account in deciding the final test slate. A knowledge of what oil analysis can provide in conjunction with the reliability goals are essential when making the final choice.

Gearboxes

Some suggested test profiles for gearboxes are presented in Table 3.

There is more emphasis on abnormal wear in the predictive test slate, and more emphasis on contamination and oil condition in the proactive one. As an example, while ferrous density and patch tests are suggested as routine tests in the former, they are only exceptions in the latter. It can also be seen that AN is included in the proactive test slate more to monitor abnormal additive depletion rather than oxidation.

Clean-oil Systems

Clean-oil systems include machines such as compressors, hydraulics and circulating systems, such as turbines. As can be seen in Table 4, there is more emphasis placed on contamination in clean-oil systems than with gearboxes. Due to the nature of these machines they are generally less tolerant of contamination than gearboxes.

Oil analysis can serve many goals, including failure prediction and overall health monitoring. But it is only able to do its job when the correct tests are chosen to serve the goals in mind. Before one can choose a test slate for a machine, one needs to take cognizance of both the reliability profile of the machine and have a thorough understanding of the basic tests. Once this has been done the user can confidently select an oil analysis test slate to accomplish the task at hand.

Machinery Lubrication (3/2011)