Evaluation of Industrial Lubricants: From Selection to Periodic Lubricant Analysis – Part 2

In the first part of this article, we briefly discussed the criteria for the initial evaluation of a suitable lubricant, as well as the importance of conducting periodic analyses during its service life. In this section, we will provide a more detailed discussion on how to choose the appropriate tests, select a laboratory, and employ the proper sampling method, as well as the most important tests that should be performed as part of routine periodic lubricant analysis.

Selection of Tests, Laboratory, and Sampling Method

Tests should be selected to provide sufficient information about the condition of the oil, the equipment, and any contaminants that may be present. The importance of the equipment is also a key factor in this selection. For critical or high-value equipment, it may be necessary to perform a wider range of tests compared to standard machinery.

For periodic lubricant analysis results to be effectively used in preventive maintenance planning, the laboratory must be equipped to perform all necessary tests and should be ISO 17025 certified. A reputable laboratory should also have the ability to issue warnings when warranted based on test results. When selecting a laboratory, the quality of testing is far more important than the cost, and the higher expenses of a qualified laboratory should not be compared with the lower prices offered by less capable ones. To ensure accurate interpretation of periodic lubricant analysis results, laboratories should be provided with the oil type, its initial viscosity, the number of operating hours or kilometers, and the manufacturer and model of the equipment or system from which the sample was taken.

Oil sampling should be carried out regularly and according to a planned schedule, but only after the system has operated long enough for the oil temperature to reach its normal operating level. This ensures that the sampled oil has fully circulated and is truly representative of the entire system. Sampling should be performed using special containers, which must be tightly sealed immediately after sampling and sent to the laboratory as quickly as possible.

Routine tests in Periodic Lubricant Analysis

1- Analysis of Wear Metals

The measurement of metals resulting from wear is usually performed in parts per million (ppm) using spectroscopic methods. This test can reveal the presence of wear in various machine components. A significant increase in the amount of one or more metals in the sample indicates wear and the potential for issues in certain parts. The most commonly measured elements include iron, copper, aluminum, chromium, and lead. These elements reflect the metallurgy of the materials used in most system components.

2- Silicon in Oil

High levels of silicon often indicate contamination. Any dust, dirt, or debris present in the oil that causes wear in the system is usually introduced into the oil through the breather system. Other potential sources of silicon contamination include silicone leaks from silicone-based sealants and contamination from coolant that contains silicone inhibitors. The amount of these contaminants is measured and reported in parts per million (ppm). However, the presence of silicone in the oil can also be attributed to the addition of antifoam agents. This must be taken into account when evaluating the results of an oil analysis to avoid misinterpretation.

3- Total base number (TBN)

This number indicates the residual alkalinity of diesel and gasoline engine oils. This property is directly related to the detergent and dispersant additives and serves to neutralize acids formed as a result of oil oxidation. The measurement of Total Base Number (TBN) is conducted based on the ASTM D-2896 method. The longer the oil is in use, the lower its TBN will become. When the TBN of used oil reaches half the TBN of unused oil, the oil should be replaced.

4- Oil Dilution by Fuel

This test measures the amount of unburned fuel that has entered the oil. Oil dilution by fuel occurs when fuel mixes with engine oil, typically due to incomplete combustion or other issues, leading to a decrease in the oil’s viscosity and lubricating properties. This can cause increased wear, reduced engine performance, and potential engine damage. The presence of fuel in the oil indicates injector or fuel line leaks or improper calibration of the injector pump. All of these can be identified by periodic lubricant analysis.

5- Suspended solids in oil

Suspended solids in oil refer to the total amount of carbon particles and other contaminants resulting from combustion. High levels of carbon contamination in oil cause wear on some engine parts that must be controlled. Unusually high levels of suspended solids in oil can indicate excessive fuel intake, oxidation or nitration of the oil, a saturated oil filter that is no longer able to separate contaminants, a blocked air passage, and poor-quality fuel (high sulfur content).

6- Contamination by Water

Depending on the type of equipment, the amount of water contamination in the oil must be precisely measured in periodic lubricant analysis. Small amounts of water in the oil should not always be considered as a serious contamination.

Typically, small quantities of water range from 1000 to 2000 ppm. However, in some systems, even this amount can be excessive and should not be ignored. In some systems, even levels below 100 ppm can cause serious problems. For example, in systems where a large volume of oil circulates and the flow is turbulent, water levels below 100 ppm can lead to foaming issues. In systems with bronze components and lacking temperature control, elevated water content can lead to serious corrosion issues. In biodegradable oils, controlling water content has a major impact on the service life of the oil. To precisely determine water content in ppm or as a percentage, the Karl Fischer method should be employed.

7- Antifreeze

Even slight amounts of antifreeze in oil can cause serious damage to engine components. This should be considered during periodic lubricant analysis. Detection of glycol-based antifreezes in oil is best achieved using the chemical colorimetric method. Sodium- and boron-based compounds are among the most widely used inhibitors in coolant formulations. If small amounts of these elements are present in the oil, it indicates that coolant has likely entered the engine oil. However, sodium is also present in some oil additives. Therefore, this issue should be taken into account when analyzing oil test results.

8- Oil Viscosity Grade

The oil viscosity grade is a system used to classify oils based on their thickness or thinness at various temperatures, which is essential for effective engine lubrication and protection. An industrial oil may be classified by ISO viscosity grades such as ISO 32 or ISO 46. Engine oils are also graded by the SAE system, such as SAE 10W30.

9- Total Acid Number (TAN)

This number indicates the increase in the acid content of an oil. The increase in acid content during the oil’s service life serves as a guide for determining when to replace it. When the acid number of the used oil reaches twice that of the unused oil, the oil should be replaced.

10- Oil Viscosity

The viscosity of oil indicates its flow rate at a particular temperature over time. Viscosity is usually measured at two temperatures: 40 °C and 100 °C. If the used oil shows high resistance to flow, it indicates that the oil has become thicker. This can be due to excessive contamination, oxidation of oil, or degradation of the oil additives. Any increase or decrease in viscosity greater than ten percent should be investigated immediately, and the cause should be identified.

11- Sediment

Sediment is the suspended contaminants in an oil, reported as milligrams per liter. If there is an accepted normal limit for sediment, sediment levels exceeding this limit indicate the entry of external contaminants into the oil, ineffective oil filtration, improper maintenance, poor filtration system design, or a change in the machine’s operating environment.

12- Oil Degradation

Determining the degree of oil degradation is also one of the important objectives of periodic lubricant analysis, which is carried out by infrared spectroscopy (IR) testing. This test measures the amount of carbon particles, nitration, oxidation, and other conditions that may adversely affect oil performance. Oxidation resistance can also be tested using the Rotating Bomb Oxidation Test (RBOT), which measures the remaining service life of industrial oils in operation.

13- Oxidation and Nitration

Extreme operating temperatures, whether high or low, can significantly harm oil and trigger chemical alterations. Early detection of these changes is one of the important goals of periodic lubricant analysis. Chemical changes in the oil can be determined by measuring the base number in diesel and gasoline oils and the acid number in natural gas engine oils, gear oils, hydraulic oils, compressor oils, and turbine oils. As the temperature increases, the rate of oil oxidation increases. On the other hand, lowering the oil temperature increases the rate of nitration. This is why the air-fuel ratio in natural gas engines is very sensitive.

In summary, the reliability of industrial equipment greatly depends on selecting suitable lubricant tests for periodic lubricant analysis, working with accredited laboratories, and following correct oil sampling procedures. Regular periodic analysis of indicators such as wear metals, contaminants, viscosity, and chemical stability helps identify issues early and increases machinery longevity.