Nitration of Lubricating Oil and Benefits of Oil Analysis

Transcription

1 T E C H N O L O G Y Lubricants Nitration of Lubricating Oil and Benefits of Oil Analysis This article enumerates the benefits of have an oil analysis program in place to monitor lubes and how nitration of lubes can be detected and prevented by studying the causes. L ubricants are the lifeblood of industrial machinery. They protect critical components an excessive level is reached, then they drop out to sludge and varnish. They are soluble in oil until and help promote enhanced operation, just form light amber to maroon deposits around the as blood supports health in the human rocker arm and valve assembly, and on piston skirts. body. And, while a doctor learns valuable information about a patient s health through a blood oil consumption, and shorten filter life. These deposits also cause oil rings to stick, increase test, you can discover valuable insights about Nitro compounds result from several conditions: piston blow-by caused by stuck, worn, or your equipment using routine oil analyses. Nitration is an broken compression rings; scored undesirable condition which indicates that the oil in round liners; or or worn out-of- natural-gas-fueled exhaust gas leaking into the oil engines is becoming saturated with because of high the soluble and/or valve guide wear insoluble nitrogen or poor valve oxide compounds. seating. Other The reaction of causes include: nitrogen with the leaking turbocharger seals; base oil forms two kinds of nitrogen critical engine compounds: organic nitrates and combustion pat- ignition and nitro compounds. They are independent of the terns; or excessive service length of the oil. oxy-products that lead to oil oxidation, which is A higher-than-expected concentration of nitro another form of oil degradation. compounds means that there are unreacted nitrogen oxide gases in the oil. They will thicken the oil Organic nitrates comprise the most significant amount of nitrogen compounds in used gas engine abnormally and cause premature dropout of varnish and sludge, evidenced by reddish piston oils. As oil is thrown onto the cylinder walls and wiped down, these compounds are washed into the skirt varnish and sludge in the lower compression ring grooves and oil crankcase where they play a major role in forming rings. 44 HYDROCARBON ASIA, APR-JUN 2013 Visit our website at:

2 Causes of Nitration There is a correlation between the rate of nitration of a gas engine oil and a combination of operating conditions, such as air/fuel ratio, engine load, and oil temperature. Charts 1, 2, 3, and 4 show the effects of these factors on nitration. tests have shown that nitration increases when ambient air temperatures increase and/or loads are higher. While there are no specific data on the degree to which spark timing influences nitration, there are strong indications that it is one of the more important factors. Of the various mechanical conditions which affect the rate of nitration, three are especially important: rate of oil makeup to the crankcase; poor ring sealing; and crankcase ventilation. Although the rate of oil make-up alone does not affect nitration, the dilution of new oil and removal of nitrated oil through leakage changes the rate at which the bulk crankcase oil combines with nitrogen oxides and deteriorates. The higher the oil makeup rate in a given engine, the slower the rate of oil deterioration. Nitrogen oxides formed during combustion are also influenced by ambient air conditions, spark timing, and final combustion temperature. Field Blow-by of combustion gases into the crankcase adds to a buildup of nitro compounds in the oil. When ring sealing is poor, more highly nitrated oil will migrate back into the crankcase instead of out through the exhaust port. Tests on laboratory engines have shown a correlation between reduced crankcase ventilation and oil deterioration. This suggests that nitro compounds in the oil may be more rapidly removed when crankcase ventilation is improved, thereby reducing deterioration. HYDROCARBON ASIA, APR-JUN

3 Degradation Patterns Four-cycle gas engines: Organic nitrates decompose rapidly at temperatures above 300 F. They are the main cause of oil deterioration in low-speed (below 700 rpm), four-cycle gas engines because cylinder wall temperatures usually are below 320 F, even in ebullient-cooled units. Where cylinder wall temperatures exceed 320 F, the higher temperatures promote oil oxidation which is the main cause of oil deterioration in small, highspeed, four-cycle gas engines. Two-cycle gas engines: Oxidation is the main cause of deterioration in two-cycle engines with separate power cylinder lubrication systems. Nitrated products are scavenged out of the exhaust ports and thereby prevented from contaminating the crankcase charge. The presence of even moderate nitration in these units, however, is a strong indication that excessive oil feed to the cylinders is being scraped down into the crankcase. Detection Visual inspection of the rocker arm and valve assembly area and the piston skirt of an engine will reveal the amber-to-maroon coloured varnish deposits indicative of nitration. Nitration will also cause the oil control rings to stick and will form sludge in the crankcase. Performance indicators, such as excessive oil consumption and shorter filter life can be indications of nitration inside the engine. Infrared absorbance, commonly known as an IR scan, is a rapid, qualitatively accurate method of differential analysis which determines inherent chemical changes in used oil, as well as the amount and nature of the contaminants. In the process, a sample of used lubricant is compared to a reference sample of new oil. Infrared rays are passed through cells of 0.1mm ( in) in thickness, which contain the samples. The net difference in the chemical composition is recorded. The Mobil SignumSM oil analysis laboratory program uses infrared absorbance to determine nitration contamination levels by looking at both trends and sudden changes. Table 1 shows unsatisfactory engine conditions that can be caused by nitration and nitro compounds, as detected by Signum oil analysis. Lubricants 46 HYDROCARBON ASIA, APR-JUN 2013 Visit our website at:

4 Troubleshooting The following is a general troubleshooting guide for various nitration conditions. Nitration: Check trend leading up to the condemning value. If the value is the result of a gradual increase, the cause may be either: a) the combustion mixture, which may be improved by adjusting the air/fuel ratio; b) slightly low bulk oil temperatures; c) minor ignition problems, such as spark plugs, wiring, or timing. Rapid increases in nitration values are caused by the same problems as above, but to a more severe degree. On two-cycle engines, check for excessive power cylinder oil-feed rates. Nitration tendency: Indicates improper ignition and combustion, which may be caused by: o Unfavorable air/fuel ratios o Uneven fuel/air distribution o Poor scavenging o Detonation or preignition o Unbalanced loads and firing pressure o Faulty ignition, spark timing, spark plugs o High blow-by o Leaking fuel valves o High combustion pressure o Engine overloading; improper cooling o Low oil temperatures o Excessive cylinder oil scrape-down (two-cycle) Nitro compounds: Check hours since overhaul; values tend to be high following overhaul. While these values will not condemn oil, they suggest operating problems. Possible causes could be: o Piston blow-by or turbocharger seal leakage oexcessive power cylinder oil-feed rates (two-cycle engines) Correcting the Problem Laboratory tests and field-sample analyses show that gas engine oils become unfit for service when the concentration of organic nitrates approaches five percent. Excessive amounts of organic nitrates act as oxidizing agents that rapidly accelerate oil oxidation. Continued build-up of nitration products will deteriorate the oil. Check air/fuel ratio: Low rates of nitration may be maintained if the oxygen level in the exhaust is outside the range of 0.5 to 4.5 percent, with nitration reaching a peak at 3.3 percent oxygen. Oil temperature: Decreasing the oil temperature from 150 F to 135 F appears to boost nitration appreciably. This may be due to the heating effect on the oil film exposed to nitrogen fixation. Oil temperatures from the engine should not be lower than 150 F, and preferably greater than 160 F, if organic nitration is to remain at a moderate level. Organic nitrates decompose rapidly at temperatures above 300 F; therefore, they are not retained in oil films when the cylinder wall temperature exceeds 320 F. Oil oxidation, however, is related directly to high engine temperatures, as in highspeed, four-cycle gas engines where cylinder wall temperatures exceed 320 F. Adjust load: High loads and load imbalance between cylinders will increase nitration. An increase from 75 to 105 percent of rated load can sharply increase the slope of the nitration curve. Prevention Oil analysis is based on a slate of tests designed to help evaluate the condition of internal hardware as well as the in-service lubricant. It is a quick and non-invasive way to gauge the health of a machine by looking at what s in the oil. Factors such as a lubricant s properties, suspended contaminants, and wear debris are analyzed by specialists in a laboratory. Regular testing allows personnel to monitor contamination levels and guarantee the opti- HYDROCARBON ASIA, APR-JUN

5 mum lifespan of machines and other critical plant equipment. When oil analysis sample results are tracked over a period of time, maintenance trends can be identified, helping machine-owners avoid costly repairs and sudden breakdowns. Today, oil analysis is performed in line with routine maintenance programs to provide precise, significant data on lubricant and machine condition. With regular oil analysis and a good maintenance program, machine downtime can be reduced significantly, hence reducing the potential dangers of a repair process and allowing workers to function with improved safety. In addition, oil analysis can also extend oil life especially when used effectively in pinpointing the proper oil drain and filter change intervals of different lubrication systems, thereby reducing waste and supporting environmental awareness. Oil analysis also supports environmental awareness, as in maintains the cleanliness and efficiency of hydraulic oil systems, combustion engines, and other industrial machinery. The Signum Oil Analysis program has improved the industrial equipment performance of hundreds of companies around the globe. In China, Tibet Dongga Power Plant, the main electricity supplier for Tibet, has benefited from the program. Its Sulzer 16ZAV40S Engine Dynamotors endures severe working conditions. Therefore, periodic monitoring of engines and lubrication status were vital. ExxonMobil offered Signum oil analysis and PES services when the business was secured in Based on the Signum analysis reports, a Mobil Field engineer submitted a Flushing Proposal to clean the company s lubrication system, and suggested re-engineering oil purifier program when detected the filter and lubricants status were subdued. As a result, the Tibet Dongga Power Plant was able to save costs over the span of three years. These savings come from reduced filter consumption as well as reduced labor and lubrication oil analysis costs. Assimilating the Oil Analysis Program into Maintenance Regimes The importance of a routine oil analysis program is clear, but knowing where to start requires some guidance. Here is a simple seven-step process to get an oil analysis routine started: Step 1: Identify Mission Critical Equipment It is not necessary to perform oil analysis on every single lubricated system in a plant. Identify critical applications that would seriously jeopardize production if they were to shut down unexpectedly. Step 2: Register Your Equipment It is important to have equipment properly registered with the laboratory. This supports routine trending and plays a key role in early detection of lubricant or equipment problems. There s no need to decide which tests are appropriate for a particular application because the laboratory has already established test slates for specific applications. Step 3: Establish Best Practices Establish a consistent how-to practice for taking oil samples and train maintenance personnel to use this practice. Correct sampling practices are critical to the value received from the analysis data. This extremely important step rarely gets the attention it needs. Step 4: Retrieve Samples Retrieve samples in accordance with best practices and send to the lab as soon as possible. Samples that are set aside may deteriorate and give non-representative results. Step 5: Analyze A thorough analysis, keyed on trends, helps determine the systems conditions. General laboratories may have a pretty good idea of what is being measured, but often know little about specific formulations and don t always understand whether or not differences are significant. Step 6: Interpret Reviewing the results and determining what, if any, action is required can make or break a successful program. It is important to remember that a sample alert does not necessarily mean imminent failure. Seek consultation on alert samples and re-sample to confirm present Lubricants 48 HYDROCARBON ASIA, APR-JUN 2013 Visit our website at:

6 data before taking massive corrective action. Step 7: Take Corrective Action and Document, Document, Document!!! As always, documentation is the key to knowing what has been done and what will be or needs to be done. Historical oil analysis and corrective action documentation will ensure that safety, environmental care and productivity are maximized. Signum Oil Analysis with Proven Results Choosing a proper oil analysis program is key to optimizing the reliability of industrial machinery. Backed by 40 years of expertise in oil analysis, ExxonMobil s Signum is the global brand of in-service oil analysis that brings the expertise and resources of ExxonMobil to clients/customers in a convenient way. It is supported by the full technical rigor of the ExxonMobil team of formulators, scientists and engineers who not only develop the products, but understand their performance and life cycle in your field. HA Enquiry Number 04/06-03 This publication thanks ExxonMobil for providing this article. Have you read our other magazine? see us on the web at HYDROCARBON ASIA, APR-JUN