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On-site oil analysis laboratories are becoming ever more popular as reliability professionals realize the profit-enhancing benefits that a well-designed and well-managed lab can reap.
The Patch Test
An essential test for any on-site laboratory is the patch test, which is used to quantify and qualify either internal or external contaminants. The patch test evaluates many different wear processes as well as different types of contaminants.
Patch makers may be portable or desktop-mounted units. A desktop unit, consisting of a vacuum pump and the appropriate glassware, is desirable for higher sample throughput rates. A portable patch maker is shown in Figure 1.
An essential tool in the patch-analysis process is the microscope. If an advanced particle analysis is required, the use of a compound microscope is necessary. Because these are expensive, advanced particle analysis should typically be left to a commercial laboratory specializing in analytical ferrography or advanced patch microscopy.
Essential features of a typical on-site lab microscope include:
The ability to magnify up to at least 200x, although 500x is preferable
The ability to store images electronically
Today, many microscopes are able to connect to a camera, or may even contain a built-in camera. A digital image capture facility should be considered an essential trending tool.
Two types of viscometry should be considered. The first is a viscosity comparator. Viscosity comparators evaluate the flow characteristics of a known reference oil against the flow characteristics of the test oil. They have a high rate of accuracy (approximately 95 percent) as compared to laboratory-quality viscometers, but can be negatively influenced by degraded oil and oils made up of different viscosity indices from the reference oil. These devices should always be used at the same temperature for trending purposes, hence the desirability of having a climate-controlled environment for the lab. It is important to use these devices as the first line of defense, and if inconsistencies are noted or suspected, the comparator should be backed up by a proper viscosity reading.
The second type of viscometry uses a viscometer to determine viscosity. Viscometers are more expensive than a comparator, but are also more accurate. Consider a viscometer as part of a future expansion plan, or if a generous budget is available. There are many manufacturers of these products.
Depending upon the principle of operation, some on-site viscometers should always be used at the same temperature to ensure consistency for trending.
Two water tests should be performed: water detection and water quantification.
At an on-site lab, water detection will be performed using the crackle test. This test involves placing a couple of drops of oil onto a hotplate, which is maintained between the boiling points of water and oil, at approximately 320°F (160°C). If water is present in the oil, it will reveal itself with bubbles in the oil drops. The crackle test has a lower detection limit of 500 to 1,000 ppm, depending on the additive package and the state of oil degradation (Figure 2).
If lower than 1,000 ppm of water limits is required, the crackle test should not be assumed accurate enough for this purpose.
Once the presence of water has been detected, it should be quantified. This determines whether the quantity of water present is actionable, and if so, determines the correct course of water removal.
Perhaps the most cost-effective method for an on-site laboratory to quantify water is to use calcium hydride-based test kits. These kits measure the pressure of the hydrogen gas released when calcium hydride reacts with water.
Acid Number/Base Number
The acid number (AN) or base number (BN) are essential indicators of the oil's health for industrial oils and engine oils. Quantification of these parameters is necessary to determine a lubricant's remaining life.
Test kits are available to make these determinations and should be considered an essential feature in an on-site lab. These tests take approximately five minutes to complete. All reagents are sealed, which reduces hazard factors.
Blotter Spot Test
The blotter spot test is generally classified as a rough-and-ready field test, something that can be carried out on a business card. Its simplicity does not diminish its importance as a useful on-site lab test, particularly where engines are concerned. This test has been performed since the 1950s2, long before Fourier transform infrared (FTIR) was introduced into used-oil analysis.
The basic procedure is to place a couple of drops of oil onto a piece of blotting paper and watch what happens.
Interpretation requires a little experience, but as a general rule, if significant structure is involved in the resulting spot, further investigation is needed.2
Demulsibility is typically not a routine test; however, it is simple to perform and is worth investing ($50) in the capability, particularly if a plant has large circulating systems. Apart from assessing the demulsibility of the oil, if this test fails it could mean that other parts of the additive package have also failed, and further investigation is needed. A variation of this test (without the water) can be used to evaluate foaming tendencies of the oil.
The test requires two components: a graduated measuring cylinder and a home blender. Equal quantities of water and oil are blended together at high speeds, the mixture is poured into the graduated measuring cylinder and the separation of the oil and water can be observed over a period of time (Figure 4). Results take a little bit of experience to interpret, but it's a skill worth developing.
Voltammetry is a technique used to measure various aspects of the oil's condition, such as AN or BN and the remaining antioxidant. It measures the electro-oxidizable content of the oil and provides an estimate of the remaining useful life. It works by passing a varying voltage over two electrodes and measuring the current flow (linear sweep voltammetry). The current passing between the electrodes is a function of the additive content.4
Voltammetry is suited to many different situations, however it is typically used in on-site laboratories that monitor sumps of larger volumes.
Bench-top minilabs combine various tests into one single unit. They are often modular, therefore a basic unit may be purchased and later expanded as needs demand and budgets permit. Suited to high sample throughput, the minilabs are usually accompanied by software to integrate tests and handle sample reporting.
Two options should be considered when setting up a laboratory: the basic and the advanced.
The following should be considered as a basic minimum for an industrial environment:
Patch test kit, including a simple microscope
AN/BN test kits
Calcium-hydride water test kits
Blotter spot test (if engines are being analyzed)
Demulsibility (applies particularly to circulating systems and hydraulics)
An on-site lab will cost approximately $3,000 (Table 1). Note that this outlay is for test equipment only, and variable costs per sample must still be taken into account.
For later expansion, or larger budgets the following should be considered: (instrument selection should depend upon the type of equipment from which samples are being analyzed and the goals of the lube analysis program).
Particle counter (optical or pore-blockage, depending on needs), where fluid cleanliness is important. This should be purchased with a paint shaker.
A ferrous density tester, especially if a high proportion of gearbox samples are analyzed.
Bench-top minilab, particularly if multiple samples need to be tested and software integration is important.
Table 2 presents some options for expanding the lab's capabilities for more advanced testing. Costs indicated are on the lower end of the scale and may vary.
Table 1. Basic Capital Lay-out for On-site Laboratory Setup
Integrating On-site and Commercial Labs
No on-site oil analysis program will reach its full potential without employing the services of a commercial laboratory. It is unreasonable to expect a low-cost, on-site operation to produce the same results as an operation capitalized with several million dollars.
Here are some reasons for integrating on-site and commercial lab analysis:
Benchmark progress. Comparing the results from the on-site lab and commercial labs will indicate if the lab is producing consistent numbers. Do not expect the numbers to be the same because the tests are performed using different methods and equipment. However, the numbers should be expected to differ with a fair degree of consistency.
More advanced testing. Some tests, like elemental analysis, will be found only in the most advanced on-site labs. Elemental analysis is one of the most important single tests that could be performed on an oil sample. The commercial lab will also have the ability to carry out more nonroutine testing, such as analytical ferrography and RPVOT. It employs staff who are familiar with the more unusual tests and are able to aid in the selection of appropriate tests.
Table 2. Advanced Testing Options
Send in routine samples on a regular basis for baselining purposes, even if they are expected to be normal. Do not send in abnormal samples only. The heart of oil analysis diagnostics is trend analysis, and not always the absolute numbers. If there is not a baseline of normal operation, trying to determine the abnormal becomes difficult and less accurate. Decide on a percentage of routine samples that will be sent to the lab and stick to it.
An on-site oil analysis laboratory is a useful addition to the condition monitoring arsenal. However, users must be aware of the failure potential, and the root causes that may be attributed to this - including incorrect instrumentation selection, incorrect test selection, poor personnel and information-flow management, and incorrect or no assimilation with a commercial laboratory. Understanding the pitfalls of embarking on such a project is vital to ensuring its success.