Contamination control falls under the category of proactive maintenance, whereby the strategy is to minimize the ingression of particulate liquid and other undesirable contaminants from the lubricants, thus maximizing both lubricant and machinery life. The most commonly addressed contaminants in power plant lubricants are particulate contamination from dust, dirt, wear, etc., and moisture. It is important for the plant to recognize the impact that contaminants can have on machinery life, and some level-of-awareness training is typically required to communicate this knowledge throughout the plant. The control and prevention of lubricant contamination is a process that touches nearly every workgroup in the plant. From the handling of new lubricants by storeroom personnel to the maintenance practices during overhauls, the opportunities to
introduce contaminants are numerous. Figure 5-1 shows a typical machine and the various checkpoints that are evaluated for potential contaminant ingression. Contamination control is a three-step process of identification, elimination, and exclusion. By adopting a program that addresses the most important contamination scenarios threatening a plant, effective protection from contaminants can become a basic part of the work practices of the utility.
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Contaminant Identification
The identification process involves establishing the proper sampling and analysis practices that ensure routine evaluation of lubricants for the presence and quantity of important contaminants. The tests that are chosen and their frequency of performance are functions of the types of predominant contaminants and the degree of opportunity for their insertion into the lubrication process. Typical and opportune contaminants for each system must be defined in order to establish the required testing that would be needed to detect their presence. For example, in the coal handling sections of a coal-fired plant, the presence of abrasive coal particles is dominant and universal. The reliable operation of machinery in these areas is almost a direct correlation to the ability to provide adequate protection from coal dust ingression. In these areas, it would be desired to establish oil analysis test slates and sampling frequencies that provide maximum sensitivity to the presence of these particles. In pulverizer ball mills, some plants do not perform particulate count, because of the typically high levels of coal dust present. In fact, this is just a case of hiding from the problem. Just because testing is not done for something does not mean that it is going to go away. Instead, focus is needed on the analysis to highlight the presence and trends in the quantity of destructive contaminants present. Only by doing so can corrective and protective strategies to minimize the negative effects of contaminants be adopted.
Table 5-1 shows typical lubricant tests and their applicability in detecting lubricant contamination.
Table 5-1
Lubricant Test/Contaminations
Once the appropriate testing regimen has been established, acceptance criteria must then be set (both alarm and alert levels), which will indicate when action must be taken to prevent damage, either to the lubricant itself or to the machine that it lubricates. These limits should have a clearly prescribed course of action that seeks to either minimize the ingression of the contaminant or
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actively works to remove its presence in the reservoir. In addition to condemning limits and warning levels, targets can also be set to try to achieve, proactively, an even lower level of contamination, which would further result in the extended life of the lubricant and the machine. Ultimately, when the source is discovered it must be possible to evaluate the opportunity to correct it, or to possibly recommend or employ counter measures in the short term that will allow the machinery to achieve its needed function until the work evolution required to correct the problem can be scheduled. If it is found that the designed defensive measures against
contamination are functioning as they should, and yet are inadequate in reducing the sources of contamination to acceptable levels, it may require investigating design changes to improve the ability of the machine to keep out these contaminants. Examples of this include the use of desiccated and high-efficiency filter breathers on many types of older oil reservoirs as shown in Figure 5-2.
These help to minimize ingression through natural breathing of the reservoir through an open or coarse-meshed screen. The high efficiency filtration of the breather minimizes the ability of particulates to enter the reservoir through the headspace. The use of desiccants in areas prone to moisture allows the breathing process to continue without the ingression and condensation of an unacceptable level of moisture.
Figure 5-2
High-Efficiency Filter Breather
When there are indications of the excessive presence of contaminants, a first look should be directed to the machine itself and any design features that are typically employed to exclude
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filtration systems should be checked to see if they are operating properly, or if hatch or penetration seals have failed. These conditions can be indicated through monitoring analyzed particulate concentrations in the oil, visual inspections of hatches and openings, and the use of pressure differential gages on filters.
Following are some of the key areas of contamination concern in a power plant, and a discussion of appropriate monitoring strategies.
Particulate Contamination
Particulate is the enemy of bearings. SKF, a manufacturer of rolling element bearings, states that, “bearings can have an infinite life when particles larger than the lubricant film are removed.” Of course, there are many factors that bring bearings to premature demise, but contamination of the lubricant can account for as much as 70% of all bearing failures. Once the impact that these unwanted particles can have on the reliability of the machinery is recognized, every effort must be made to minimize their presence in the lubrication oil. Some of these strategies have been discussed in the specific sections of this document where they apply, such as Section 3, “Storage and Handling,” and Section 8, “Lubrication/Relubrication Practices.”
The most common unit of reporting fluid cleanliness is the International Organization for Standardization (ISO) Code System. This convention is covered under ISO Standard 4406:99 (Figure 5-3). In this standard, the number of particles in three different size categories, >4 µm, >6 µm and >14 µm, are determined in a one-milliliter sample. However, the ISO standard is not the only method by which the cleanliness of an oil sample can be reported. Other standards include NAS 1638 and MIL-STD 1246C, as well as outdated standards such as the Society of Automotive Engineers (SAE) fluid cleanliness rating system. Whichever method of reporting is selected, the first step is to count the number of particles in a volume of fluid.
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Figure 5-3 ISO Standard
As outlined below, there are three basic methods that can be used to determine the absolute number of particles in any given sample:
Optical Microscopy (ISO 4407)
The original method for determining fluid cleanliness levels was to take a representative portion of the sample and examine it under an optical microscope. In this procedure, the particles are manually counted, which can then be used to determine the fluid cleanliness of the bulk sample. Although this method may seem outdated, slow, and cumbersome, it is still in use today and considered by many to be the most reliable and accurate method of particle counting, because it is unaffected by some of the limitations of the more modern, automated methods.
Automatic Optical Particle Counting (ISO 11500)
Perhaps the most widely employed method today for determining fluid cleanliness is to use an automatic optical particle counter. There are a variety of instruments commercially available to optically count particles, from portable units for on-site use that cost as little as $15,000, to large, sophisticated lab-based instruments that may cost in excess of $40,000. There is even a low-cost, on-line optical particle counter available for under $1,000. However, all instruments, whether they are a hand-held unit or a full lab instrument, use one of two methods, either a white light source or, more commonly today, a laser.
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In a laser-based instrument, due to the near-parallel nature of the laser beam, light scattering from the unimpeded laser beam is minimal because it is focused into a beam stop until a particle passes through the instrument. As the laser strikes the particle, light scatters and hits the
photocell. Just like a white light instrument, the change in voltage across the photocell is directly related to the size of the particle. In the laser-based instrument, one looks at an increased signal against what should be a zero background (in theory). Laser optical particle counters are generally considered to be slightly more accurate and sensitive than white light instruments. There are several subtleties that must be considered with the automatic optical particle counters. First, for the most part, particles from used oil samples are not perfectly spherical. This can create problems for optical counters because of the one-dimensional ISO 4406:99 coding
scheme, which classifies particles that are 5 microns across the minor axis, but 40 microns across the major axis. To resolve this issue, developers of automatic optical particle counters have devised a compromise known as the equivalent spherical diameter. With the equivalent spherical diameter method, a particle is counted in the size-range under which the shadow, or scattering effect observed, would have appeared if the particle had been a perfect sphere. This allows the average fluid cleanliness to be estimated, permitting the ISO code to be trended over subsequent samples.
Another concern is the effect of false positives. For example, both air bubbles and free and emulsified water appear as if they are a particle using the optical particle counting method. Although the effect of air and water can be negated by using an ultrasonic bath and vacuum de- gassing to remove air, and solvent extraction to dissolve free and emulsified water, other false positives are possible with multiple particle coincidences and additive floc. For this reason, care and attention to procedural details must be exercised when performing optical particle counts.
Pore Blockage Particle Counting (BS3406)
The pore blockage method is a widely used method of obtaining an automatic particle count. In this method, a volume of fluid is passed through a mesh screen with a clearly defined pore size, commonly 10 microns. There are two instrument types that use this method. One instrument measures the flow decay across the membrane as it becomes plugged while pressure is held constant, first with particles greater than 10 microns and, later, by smaller particles as the larger particles plug the screen. The second measures the rise in differential pressure across the screen while the flow rate is held constant. Both instruments are tied to a software algorithm, which turns the time-dependent flow decay or pressure rise into an ISO cleanliness rating according to ISO 4406:99.
Although pore block particle counters do not suffer the same problems as optical particle
counters with respect to false positives caused by air, water, dark fluid, etc., they do not have the same dynamic range as an optical particle counter. Because the particle size distribution is roughly estimated, they are dependent on the accuracy of the algorithm to accurately report ISO fluid cleanliness codes according to ISO 4406:99. Nevertheless, they accurately report the aggregate concentration of particulates in the oil and, in certain situations (particularly for dark fluids such as diesel engine oils and other heavily contaminated oils), pore block particle counting does offer advantages.
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There can be little doubt that the ability to quantify an oil’s fluid cleanliness using particle counting is the most valuable tool in any proactive maintenance program. Therefore, this makes consistent and accurate particle counting one of the most important tasks in oil analysis. By adopting a valid particle counting method, and obtaining representative samples, the proper actions for addressing oil cleanliness issues can be directed by the oil analysis results.
Moisture
Water is perhaps the most harmful of all contaminants, with the exception of solid particles. Although the presence of water is often overlooked as the primary root cause of machine problems, excess moisture contamination can lead to premature oil degradation, increased corrosion, and increased wear. Proper sampling to determine the quantities of water present requires evaluation of the lubrication system for each machine. To determine the moisture levels that can potentially affect the formation of the lubricating film and the corrosion of bearing surfaces, the oil in the live-zone should be observed. In circulating systems, samples taken from the supply lines indicate the quality of the oil supplied for the lubricated components. Samples taken on the return lines include the same information, plus any additional water being
introduced to the system at the lubricated components. The primary sampling location for a return line sample, taken to show the maximum amount of moisture present in the live-zone path, would typically be a position after any in-line cooler. This is different, however, than for the maximum concentration of moisture in the system. This would typically be found in the bottom drain of the reservoir, where moisture can accumulate by design. That sample is also important, however, to indicate excessive accumulation of moisture in the sump, the need to investigate sources, and possibly to clean the tank.
When unacceptable levels of moisture are indicated from the primary sampling point, additional follow-up samples are typically taken to help pinpoint the source. All too often, plant staff will dismiss a low, but higher than normal, moisture level in a sample as condensation. Although condensation of ambient moisture is a feasible route of ingression, normal system design should prevent it from adding significantly to the moisture levels in the oil. Even low levels of moisture in a large reservoir, when showing a clear increasing trend, can be valuable indicators of
emerging moisture problems such as leaky coolers, leaky pump seals, or other continuous ingression. When discovered and rectified in the early stages, such problems can be headed off before leading to reliability threatening conditions. The following tests can be used to determine the presence of water.
Visual Crackle Test
The simplest way to determine the presence of water in oil is to use the Visual Crackle test. Although this is an effective test for identifying free and emulsified water, down to
approximately 500 ppm, its biggest limitation is that the test is non-quantitative and fairly subjective. False positives are possible with entrained volatile solvents and gases. Nevertheless, as a screening tool in the lab and the field, the Crackle test will always have a role to play where
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Fourier Transform Infrared Spectroscopy (FTIR) Analysis
FTIR can be an effective method for screening samples containing in excess of 1,000 ppm of water, provided a correct new oil baseline is available for spectral subtraction. However, due to its limited precision and comparatively high detection limits, FTIR is not adequate in many situations where precise water concentrations below 1,000 ppm, or 0.1%, are required.
Dean and Stark Method
The classic method for determining the presence of water in oil is the Dean and Stark distillation method (ASTM D95). This test method is fairly cumbersome and requires a comparatively large sample to ensure accuracy, which is why it is rarely used in production-style oil analysis labs today. The method involves the direct co-distillation of the oil sample. As the oil is heated, any water present vaporizes. The water vapors are then condensed and collected in a graduated collection tube, such that the volume of water produced by distillation can be measured as a function of the total volume of oil used.
Karl Fischer Moisture Test
The Karl Fischer Moisture test is the method of choice when accuracy and precision are required in determining the amount of free, dissolved, and emulsified water in an oil sample.
All Karl Fischer procedures work in essentially the same way. The oil sample is titrated with a standard Karl Fischer reagent until an end-point is reached. The difference in test methods is based on the amount of sample used for the test and the method used to determine the titration end-point. The results can be reported as parts per million, or as a percent of water in the sample.
Calcium Hydride Test Kits
One of the simplest and most convenient ways to determine water concentrations in the field is by using a Calcium Hydride test kit. This method employs the known reaction of water with solid calcium hydride to produce hydrogen gas. The amount of hydrogen gas liberated is directly proportional to the amount of water present in the sample. Therefore, the water content of the sample can be determined by measuring the rise in pressure in a sealed container due to the liberation of hydrogen gas as any water in the sample reacts with the calcium hydride. Used correctly, these test kits are reported to be accurate down to 50 ppm free or emulsified water.
Saturation Meters
When the amount of water present in an oil sample is below the saturation point, saturation (dew-point) meters can be used to indirectly quantify water content. The saturation point of an oil is simply the point at which the oil contains as much water in the dissolved state as possible, at a given temperature. At this point, the oil is saturated or has a relative humidity of 100%. Most saturation meters use a thin film capacitive device, whose capacitance changes depending on the relative humidity of the fluid in which it is submerged. Saturation meters have proven to be accurate and reliable at determining the percent saturation of used oils. The biggest drawback with saturation meters is the fact that the saturation point is strongly dependent on temperature,
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as well as on the presence (or absence) of polar species (including additives, contaminants, and wear particles). In addition, with water levels in excess of the saturation point, typically 200 to 600 ppm for most industrial oils, saturation meters are unable to quantify water content
accurately. Despite these limitations, saturation meters can be useful trending tools to determine moisture on-site, provided that they are used frequently and routinely.
Monitoring and controlling water levels in any lubricating system is important. Whether it is a large diesel engine, a steam turbine, a hydraulic system, or an electrical transformer, water can have a significant impact on equipment reliability and longevity. Regular water monitoring, whether it be a simple on-site Crackle test or a lab-based Karl Fischer moisture test, should become a standard condition-monitoring tool.
When moisture is the culprit contaminant, look for indications of a possible ingression source,