ASTM D6224 Standard Practice for In-Service Monitoring of Lubricating Oil
ASTM D6224 Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment
8. Significance of Tests
8.1 In determining the condition of the oil and equipment for continued service, important properties of used oils include:
8.2 Viscosity - Most commercial oils are sold under ISO (International Standards Organization) viscosity classification system. Industrial fluids fall into ISO VG-32, VG-46, VG-68 and higher viscosity grades corresponding to 32, 46, 68 cSt at 40°C (Classification D2422). Diesel engine oils are tested at 100°C with cSt units and use the SAE classification. Gear oils are tested at 40°C with cSt units and use the AGMA or SAE classification. The viscosity (for example, for multi-grade oils) can be measured at 40° and 100°C in order to calculate the viscosity index and determine that the correct oil has been used. The main purpose for checking the viscosity of used oil is to determine if the correct oil is being used and to detect contamination. In extreme cases, used oils will experience a significant increase in viscosity due to thermal or oxidative degradation. Contamination can cause the viscosity to either increase or decrease, depending on the contaminant. Emulsified water and diesel fuel soot will increase the viscosity, while diesel fuel, Freon, or solvents will decrease the viscosity. Dissolved water in phosphate ester fluids can reduce the fluid viscosity slightly. Contamination from a different lubricant can change the viscosity of the oil in either direction. The method normally used for viscosity determinations is Test Method D445.

8.3 Acid Number - The test most used to indicate the extent of oxidation is the acid number (Test Method D664 or D974). With phosphate esters, acidity is most frequently an indication of hydrolysis. Many rust inhibitors used in lubricating oils are acidic and contribute to the acid number of the new oil. An increase in acid number above the value for new oil indicates the presence of acidic oxidation products or, less likely, contamination with acidic substances. The acid numbers determined by these two test methods are not identical and only loosely correlate; a single method should be used consistently. The use of Test Method D974 on aged phosphate ester fluids which have significantly darkened in color, and especially those which have been dyed prior to use, is not recommended.

8.4 Water Content - If a mineral oil is clear and bright, the amount of dissolved water present is of little significance. Most mineral oils will remain clear with up to 75 ppm water at room temperature. Phosphate ester fluids can hold more than 1000 ppm water at ambient temperature and still be clear and bright. The presence of water determined by screening methods (such as the hot plate splatter test for mineral oils) may be confirmed using a standard test method. Adequate lubrication cannot be maintained by an oil which contains a significant quantity of water. The analytical range for Test Method D95 is 0.05 % to 25 % and the range for Test Method D1744 is 50 to 1000 µg/g. Other methods (such as Test Methods D96, D1533 and E 1064) are available for measuring the water content in oils.

8.5 Oxidation Inhibitor - A common method for measuring the concentration of phenolic (or amine) antioxidants is infrared spectrometry. Each antioxidant is a specific chemical substance and will absorb infrared light at a particular wavelength and with its own absorptivity; some antioxidants may not be detectable by infrared spectroscopy. (Test Method D2668 may be used for antioxidants if the wavelength and absorptivity are known.)

8.6 Oxidation Stability (RBOT) - One of the most important properties of new oil containing oxidation inhibitor is its ability to resist oxidation while in service. This is accomplished by the addition of antioxidants that are formulated to protect the oil itself from degradation. Measuring the concentration of the antioxidant additive or the ability of the additive to resist oxidation (as compared to the new oil) is an important factor in monitoring the degradation of industrial lubricants. The result of this measurement will offer the user a rating of oxidation stability reserve. When the operating conditions of a machine are known (that is, oil temperature, oil service time), the oxidation stability reserve serves as an indication of the optimum oil change interval. Methods of measuring the resistance to oxidation are generally thermal, catalytic degradation techniques such as Test Method D943 or D2272. Test Method D943 is generally reserved for new lubricant comparison and is too lengthy to be used for in-service monitoring. Test Method D2272, however, is widely used for in-service monitoring of oxidation stability reserve when compared to the new oil.

8.7 Color - New lubricating oils have colors ranging from light to moderately-dark. Darkening will occur in service but the change is usually slow over a period of time. Frequent checks for color are therefore useful as a quick on-the-spot test. A significant color change would be indicative that something has changed. A more detailed examination would be necessary to find the cause. Test Method D1500 is the standard method for defining the color of lubricants. Color darkening alone is not itself a cause for alarm (unless supported by additional tests).

8.8 Gravity or Density - This test only has significance with respect to contamination. As an example, phosphate ester EHC fluid cannot tolerate contamination with mineral oil. This test is of little value to determine the degree of degradation. The method normally used is Practice D1298 (hydrometer).

8.9 Flash Point - Lubricants must have flash points well above the minimum applicable safety requirements. Flash point is useful for detecting contamination with diesel fuel oil or low-boiling solvents. However, flash point is of little significance for determining the degree of degradation of used oils, since normal degradation has little effect on the flash point. The method in common use is Test Method D92 (Cleveland Open Cup).

8.10 Insolubles - Pentane insolubles (Test Method D893) may include oil-insoluble materials and some oil-insoluble resinous matter originating from oil or additive degradation, or both. Toluene insoluble materials may come from (1) external contamination, (2) fuel carbon and highly carbonized materials from degradation of fuel oil, and additives, and (3) engine wear and corrosion materials. Industrial oils may also be tested for insolubles using Test Method D2276. A significant increase in insolubles indicates a potential lubrication problem. The measurement of insolubles can also assist in evaluating the performance characteristics of a new oil or in determining the cause of equipment failure.

8.11 Water Separability - Water can get into lubrication systems due to oil cooler leaks, normal system breathing, and other means. Water adversely affects oils by acting with metals to catalyze oxidation. It also depletes water sensitive oil additives such as some types of rust inhibitors, and can cause rusting and corrosion. For some oils, water will settle to the bottom of the storage tank where it should be drained off as a routine operating procedure. Purification systems will also assist in removing the water. Unfortunately, if the oil has developed poor water separability properties (poor demulsibility), significant amounts of water may stay in the system and create problems. In addition to chemical effects on the oil and additives, the lubricating properties of the oil can be adversely affected. The water separability characteristics of an oil are adequately measured using Test Method D1401. This test determines the time required for significant amounts of water to separate from an oil-water emulsion. By design, water will not separate from some lubricating oils such as engine oils. The dispersant additive present in these oils disperses water as well as combustion residues, dirt, and oxidized compounds.

8.12 Rust Evaluation - Anti-rust protection provided by the lubricant is important for certain systems. Protection is required in areas of fluid flow, for surfaces covered by static drops of water, and for areas which are only occasionally splashed by the lubricant. New oil containing an anti-rust inhibitor additive must meet test requirements such as Test Method D665. In service, this additive can become depleted by (1) performing its proper function, (2) by removal with water, (3) by adsorption on wear particles and other solid debris, or (4) by chemical reaction with contaminants. In exceptional circumstances where alkaline or polluted water enters the system, additive removal will be much more rapid. Test Method D665 Procedure A (distilled water rust test) is usually adequate for determining a satisfactory level of anti-rust inhibitor for inland equipment. For marine usage, Test Method D665 Procedure B (seawater rust test) is recommended.

8.13 Foaming Characteristics - Foaming characteristics are measured by Test Method D892 which indicates both the tendency of the oil to foam and the stability of the foam after it is generated. This test may be useful in troubleshooting oil foaming problems in equipment. System foaming problems have three possible origins.

8.13.1 Mechanical - The system design can have a major impact on the foaming tendency of a lubricant.

8.13.2 Antifoam Additive Depletion - Anti-foam agents may be removed mechanically (due to fine filtration, centrifuging, mechanical shearing, or adsorption) because they are dispersed and not dissolved in the oil.

8.13.3 Contamination - An attempt should be made to identify and remove the contaminants. In cases where this cannot be done adequately, this may be corrected with an addition of defoamant.

8.14 Air Release Properties - Agitation of lubricating oil with air in equipment such as bearings, couplings, gears, pumps, and oil return lines may produce a dispersion of finely divided air bubbles in the oil. If the residence time in the reservoir is too short to allow the air bubbles to rise to the oil surface, a mixture of air and oil will circulate through the lubricating oil system. This may result in an inability to maintain oil pressure (particularly with centrifugal pumps), incomplete oil films in bearings and gears, and poor hydraulic system performance or failure. The time required for air entrained in the oil (in Test Method D3427) to reduce in volume to 0.2 % is recorded as the air bubble separation time.

8.15 Base Number - New and used petroleum products (especially diesel engine lubricants) can contain basic additives. The relative amount of these materials can be determined by titrating with acids. This base number is a measure of the amount of basic substances in the oil and, hence, is a measure of additive depletion. Condemning limits must be empirically established. The base number methods do not provide equivalent test results; one method should be used consistently.

8.16 Chlorine Content - The chlorine content in phosphate ester EHC fluids can be determined via microcoulometry. Excessive chloride ion can cause electrochemical corrosion since the fluid flows through small openings at a rapid velocity.

8.17 Resistivity - Phosphate ester EHC fluids with low electrical resistivity can cause servo valve erosion.

8.18 Mineral Oil Content - The mineral oil content in phosphate ester EHC fluids must be minimized to preserve the fire-resistant properties.

8.19 Glycol Content - Leakage of glycol-based antifreeze into the crankcase is serious because the coolant tends to interfere with the lubricant and its ability to lubricate; it also promotes sludging. Ethylene glycol present in the coolant can increase varnish deposit formation in the crankcase due to glycol oxidation and the interaction between glycol and lubricant. Lubricant displacement, sludging, and deposit formation all lead to engine malfunction and possible seizure. It is important to detect glycol-base coolant contamination at low levels because early detection enables corrective measures to be taken to prevent leaking coolant from accumulating and seriously damaging the diesel engine.

8.20 Fuel Dilution - Some fuel dilution of the diesel engine oil may take place during normal operation. However, excessive fuel dilution is of concern in terms of possible performance problems.

8.21 Particle Counts - The most deleterious solid contaminants found in oil systems are those left behind when the system is constructed and installed or when it is opened for maintenance and repair. The need for proper cleaning and flushing of new or repaired oil systems is emphasized. In operation, there are few opportunities for solids to enter the lube oil system, although in very dusty areas where units may be outdoors, some solids can enter through improperly installed or operating vents as well as seals and air intakes. During operation, the equipment may begin to accumulate significant particulates. Some may enter the system through the make-up oil when it is added. Fly ash may be drawn in with the air at bearing shaft seals. Other contaminants may be abrasive degradation (that is, wear particles) and corrosion products developed in the system. Whatever the source, the presence of abrasive solids in the oil cannot be tolerated since they will promote scoring and damage to bearings and journals as well as causing malfunction and sticking of control mechanisms. These must be removed by the use of filters or centrifuge, or both. Cleanliness of the system oil can be determined by particle counting (by means of electronic particle counters). Cleanliness levels can be represented by classification systems such as ISO 4406. ISO 4406 uses a numeric code to reference the number of particles larger than 2 µm (proposed), 5 µm, and 15 µm/mL of oil. ISO 4406 assigns integer values to denote a range of particles whose upper limit doubles with each successive number. Desired cleanliness levels are sometimes designated by the equipment manufacturer or user.

8.22 Wear Particle Concentration (WPC) - It has been shown that the separation and measurement of large and small wear particles is beneficial in the detection and diagnosis of related machine wear regimes. Techniques used to measure WPC should not be considered to be particle counts, rather a measurement of specific wear metal concentrations. Caution should be used when evaluating results since separation and measurement techniques may be alloy-specific. The wear particle concentration for a particular piece of equipment is monitored over time in order to detect a sudden increase in the wear trend and reduce the likelihood of catastrophic failure.

8.23 Wear Debris Analysis (WDA) - A microscopic analysis of wear debris can be used to describe a wear condition as normal rubbing wear, severe sliding wear, cutting wear, gear wear, or bearing wear. In addition, this debris analysis can reveal fibers, sand/dirt, lubricant degradation residue, red oxides, black oxides, and ferrous spheres. A rating can be given to each type of wear condition which is a combination of the size of the wear debris and the concentration of particles of the same type.

8.24 Elemental Analysis - Emission spectroscopy can be used to analyze for elements found in used lubricating oil. This analysis is generally limited to dissolved materials or particles smaller than about 8 µ. The elements found derive from wear debris, additives, and contamination. (See 9.5.)

8.24.1 Some wear on metal parts can be considered to be normal (although not desirable). Large amounts of metal contaminants usually indicate a serious machine problem. Since different machine parts are made from different metals, the presence of particular metals indicate which components are wearing. When a machine has been sampled several times (or when multiple machines of the same type have been tested), an evaluation is made to determine whether the metal concentrations are outside of the normal range. Samples from a piece of equipment that have metal concentrations increasing at an unusually high rate or outside of the normal range indicate that the equipment may have a problem. When no historical data is available, there is a chance that not enough significance will be given to a particular wear metal and its concentration.

8.24.2 Certain elements which are found as lubricant oil additives can also be analyzed to ensure that the appropriate additives are present and that there are no other inorganic additives which indicate that cross-contamination has occurred. An analysis should be performed on unused oil in order to establish a baseline for future comparisons.

8.24.3 Contaminants (such as dirt and water) in the oil can be carried throughout the machine and cause severe wear or corrosion. The presence of dirt can be detected by the presence of silicon (or aluminum). Inorganic constituents of fresh and treated water may be detected using elemental analysis. Examples of these may be: calcium from untreated water, sodium and magnesium from sea water, and potassium, sodium or boron from cooling water. In EHC fluids, the presence of sodium, aluminum, calcium, and magnesium is most commonly found as a result of either release of Fuller's earth or activated alumina into the system or the production of soluble/insoluble metal soaps in the fluid (which can have an adverse effect on foaming and air release properties).