Oil analysis is a powerful predictive maintenance tool for machinery, both for fleet management (agricultural, transport, forestry, maritime, etc.) and for industry. Through this practice, it is possible to anticipate critical failures, extend the time between oil inventory changes, identify components with premature wear, among other advantages.
In the case of fleet management, fuel dilution can cause severe engine malfunctions if not detected and remedied in time. The appearance of fuels mixed with lubricating oil is due to internal leaks, which can occur due to faulty seals. This is commonly found, for example, in machines or engines operating at low temperatures, associated with prolonged idleness [1, 2].
In principle, a small amount of fuel contamination may not cause major problems, considering that, during operation, the machine reaches a temperature high enough to cause its evaporation. The bigger problem is when the contamination is severe and the operating temperature is low or has been low, worsening when there is also contamination with water. In these cases, in addition to the risk that oil thinning can bring to the lubrication of moving parts, the fuel can combine with water, causing the precipitation of additives important for the protection of the system in question [1].
How to identify contamination?
There are two quick ways to check for this type of contamination: by analyzing the flash point of the lubricant, or by measuring its density.
O flash point It is described as the lowest temperature at which a flammable mixture is obtained, without, however, the spark being sufficient to sustain a flame.
There are several techniques for its determination, depending on the characteristics of the fluid to be analyzed (viscosity, flash point, etc.). However, in all methods, the analysis consists of gradually heating the sample and exposing it to an ignition source at regular intervals of 1 to 3°C, depending on the method. The flash point can be detected both visually in manual equipment and through an abrupt difference in temperature or pressure in the vapor space between the lid and the sample vessel, in the case of automatic systems.
Due to the large difference between the flash points of diesel (40 to 100°C) and oil (≈ 200°C), fuel contamination is noticeable by the decrease in value over time. It can also, in some cases, be related to the degradation of certain types of oil, generating volatile compounds [1]. It is possible to obtain empirical tables with the percentage of fuel contamination in relation to the flash point. However, this analysis is only qualitative, since the variation between this variable and the volume of contamination does not show a linear relationship.
Another very common analysis for oils used in various applications is... density (specific gravity), which can be obtained by several methods. The main methods applied to lubricants are the glass hydrometer [3] and the automated oscillating tube method [4]. The latter, in addition to being more precise and less susceptible to visual assessments, is more suitable for analysis in darkened oils, where visual assessment is significantly impaired.
Although it is a quick and accurate analysis in the case of the automated method, a new or used lubricating oil will tend to have density values close to 0,8800 g/mL, slightly higher than the maximum limit of the diesel specification [1, 5]. Thus, density analysis is more commonly used to monitor the condition of the lubricant itself, especially in heavy and constant service, unless this contamination is intentional or gross, such as due to a solvent or low-density fuel (gasoline, for example). Furthermore, monitoring the trend of the results – increase or decrease in density over time – can be useful, in conjunction with other analyses, to determine whether or not a lubricant change is necessary.
How can Alutal help in investigating lubricant contamination?
For investigating fuel dilution and lubricant condition, Alutal can offer two solutions in its portfolio.
- Miniflash Vision – the flash point analyzer of Grabner Instruments It is a robust and extremely precise instrument, based on the modified continuously closed cup method (MCCCFP), in accordance with ASTM D 7094 [6]. In addition to using an extremely small amount of sample (only 2 mL), the Miniflash performs the entire analysis in a closed circuit, making it a much safer piece of equipment than traditional flash point analysis methods. Portable, the equipment can be taken to the field for quick assessments, or coupled to an autosampler for up to 12 samples, ideal for service laboratories with high demand for analyses.
- DS 7800 Hydrometer – the digital densitometer of Kruss Optronic It is an excellent cost-effective alternative for automatic density analysis in lubricating oils, in accordance with ASTM D 4052 [4], with an accuracy of 0,0001 g/mL. For samples with low to medium viscosity, the equipment can be fitted with a peristaltic pump and fully automatic sampler, increasing the productivity of lubricant analysis laboratories.
References
[1] Gresham, RM; Totten, GE Lubrication and maintenance of industrial machinery: best practices and reliability. CRC Press. New York, NY. 2009.
[2] Blau, PJ Tribosystem analysis: a practical approach to the diagnosis of wear problems. CRC Press. New York, NY. 2016.
[3] ASTM D1298. Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. ASTM International. West Conshohocken, PA. 2017.
[4] ASTM D4052. Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter. ASTM International. West Conshohocken, PA. 2018.
[5] ANP Resolution No. 50/2013. This establishes the specifications for diesel fuel for road use, as contained in ANP Technical Regulation No. 4/2013, which is an integral part of this Resolution, and the quality control obligations to be met by the various economic agents that market the product throughout the national territory. National Agency of Petroleum, Natural Gas and Biofuels. Rio de Janeiro, RJ. 2013.
[6] ASTM D7094. Standard Test Method for Flash Point by Modified Continuously Closed Cup (MCCCFP) Tester. ASTM International. West Conshohocken, PA. 2017.
