30 Analytical Instrumentation - Focus on Lubricant & Condition Monitoring
DEVELOPMENT OF A NEW ASTM METHOD USING MEMBRANE PATCH COLORIMETRY
Varnish is often considered a transparent or translucent coating applied to various surfaces to provide a protective and decorative fi nish. However, in the petroleum industry, varnish has a very diff erent meaning compared to machinery. Varnish, in this context, is defi ned as an insoluble fi lm deposit that forms on surfaces [3]. This insoluble fi lm can often cause detrimental damage to machinery [3].
Varnish is formed through a complex process involving the degradation and oxidation of lubricating oils in machinery and can cause detrimental damage to machinery, such as reduced oil fl ow, sluggish operation, loss of system control, bearing deposits, and pump failure [9]. Lubricating oils exposed to high temperatures, pressure, and other operating conditions undergo chemical changes that can form varnish due to the degradation of its physical and chemical properties. Varnish thereby forms on surfaces inside turbine lubrication systems such as pipes, tanks, bearings, heat exchangers, and servo-valves [4]. With the presence of varnish in machinery, companies may experience increased maintenance costs, unplanned downtime, and reduced equipment lifespan. By implementing varnish condition monitoring programs, such as the use of Standardized Testing and Enhanced MPC Methods, organizations can detect and mitigate varnish-related issues at an early stage, leading to improved equipment reliability, optimized maintenance schedules, and enhanced operational effi ciency [7].
Several factors infl uence the formation of varnish. According to the National Library of Medicine, high temperatures accelerate the oxidation process and expedite the breakdown of the oil, leading to faster varnish formation [1]. Contaminants, such as water, solid particles, or degraded additives, can also contribute to varnish formation by acting as catalysts or providing reactive sites for chemical reactions. Improper synthesis of oils is another source of varnish, as improper oils react with lubricating oils to produce precipitates [1]. Further examples can be seen in Figure 1 on lesser-known factors.
The most common mechanism behind varnish formation is oil degradation caused by oxidation. As the oil degrades, various byproducts are formed, including polar compounds, acids, sludge, and insoluble particles. These byproducts can combine and undergo chemical reactions to form varnish-like substances, which can vary depending on factors such as the type of oil, operating conditions, and the presence of additives or contaminants. As varnish is typically a complex mixture of oxidized oil components, resins, polymers, and other insoluble materials, varnish can range in color depending on the severity of the degradation process. Varnish formation is a gradual process that occurs over time as the oil undergoes continuous degradation in service. If not addressed, this degradation can lead to reduced lubrication effi ciency, increased friction, restricted fl ow, and potential equipment failure. To mitigate varnish formation and its detrimental effects, various strategies are employed. These include the use of high-quality lubricating oils with improved oxidation stability, proper fi ltration and maintenance practices, and the implementation of varnish condition monitoring techniques such as colorimetric patch analyzer method and membrane patch colorimetry test. By monitoring varnish levels and taking proactive measures, such as oil fi ltration, fl uid replacement, or equipment cleaning, operators can minimize varnish formation and maintain the performance and longevity of machinery.
Even with the previously stated mitigation techniques being used, Exxon Mobil has reported that approximately 40% of the 192 power plants were reported to experience varnish-related problems [2]. Furthermore, according to Chevron Lubricants downtime and repair costs caused by varnish can run from $100,000 to the millions [6]. As a result, researchers and industry professionals have dedicated efforts to develop further and refi ne varnish condition monitoring techniques to safeguard the performance and longevity of equipment. Cases of this can be seen in ASTM D7843, which provides a standardized approach for quantifying insoluble color bodies in turbine oils. This quantity can serve as an indication of varnish formation.
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ASTM D7843 involves fi ltering a representative oil sample through a membrane patch and evaluating the color intensity of the retained insoluble particles [5]. The determined color intensity is then compared to a standard color chart to evaluate the varnish level. This test method enables operators to monitor varnish formation in turbine oils and assess the potential for detrimental effects on equipment by employing membrane patch colorimetry. It provides a quantitative measurement that can be used for trend analysis and comparison against established thresholds or industry guidelines. Developing and utilizing such standardized test methods demonstrate the industry’s commitment to monitor and manage varnish-related issues effectively. Thus, by implementing these techniques, operators can identify varnish formation early on and take appropriate actions, such as fl uid fi ltration or replacement, to prevent further accumulation and mitigate potential damage to machinery.
Figure 1: Causes of Varnish
inaccurate and ineffective maintenance decisions. Thus, ASTM D7843 was modifi ed to work with soluble MPC along with the insoluble outlined in the original ASTM D7843.
While ASTM D7843 specifi cally focuses on Insoluble Varnish (I-MPC), many researchers are investigating soluble varnish (S-MPC). Insoluble varnish is typically the primary concern in machinery, however, soluble varnish can also have detrimental effects on equipment performance and reliability. Soluble varnish refers to a type of varnish that remains suspended in the oil and does not form deposits within the system. Insoluble varnish occurs when the oil becomes overly saturated with soluble varnish. This causes it to begin to plate out and adheres to the surfaces of the system, which leads to hot spots, micro- dieseling, and wear. [7]. Researchers actively study soluble varnish and its implications in industrial settings. This research involves understanding soluble varnish’s formation, composition, behavior, and potential applications. Researchers investigate soluble varnish to uncover new insights and strategies to manage varnish-related issues in machinery effectively. It is important to note that soluble varnish is not applicable for formulations that are dyed, and colloidal carbon can also infl uence test results. Soluble varnish should be monitored along with Insoluble varnish considering L*a*b* values and their correlations.
In a study conducted by Wasan Chokelarb and his team, Chokelarb investigated the signifi cance of both soluble and insoluble varnish test methods in monitoring varnish buildup in mineral turbine oil. Chokelarb believed that the use of only insoluble varnish data would be insuffi cient and may cause
The study used the in-service turbine oil samples of three different power plants to perform the previously mentioned ASTM D7843 as well as the modifi ed method. Condense The fi rst oil sample was analyzed every two months within a period of two years. As seen in Table 1, the sample results caused the team to conclude that ASTM D4373 is not applicable to all oil types and equipment combinations, for Sample One shows that the turbines from which the oil was taken were already experiencing an accumulation problem. Therefore, even if the amount of MPC did not reach the limit, the turbine is currently experiencing issues with accumulation. This was unaccounted for in the ASTM method. Thus, the consensus made from the results is that the ASTM limits should be used only as a general guideline, and that in the future the use of oil analysis results along with physical examination of the equipment should be used to set a limit specifi ed for each oil and equipment combination. The results of the sample turbine can be seen in Table 1.
Sample Two of this study was used to monitor quality and varnish build-up. This sample used a VRT, or depth media fi lter, to clean the oil system. This method of monitoring and controlling varnish buildup resulted in the cleansing of large amounts of insoluble varnish from the system, allowing it to go below the threshold of ∆E 30. While the insoluble varnish levels were reduced, the levels of soluble varnish were not addressed and increased over the course of the study. Chokelarb and his team suggested that the use of electrostatic fi lters could be a possible
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