Nitration is a form of oil degradation similar to oil oxidation but occurs in a reduced oxygen environment when oil reacting with nitrogen oxide compounds form during combustion and high temperatures. Nitration formation occurs with either of two types of nitrogen compounds; organic nitrates and nitro compounds.
Organic nitrates are most commonly present in natural gas applications. Natural gas engines and compressor are susceptible to oil degradation due to high amounts of organic nitrates in natural gases which promote elevated nitration of the in-service lubricant. Organic nitrate reaction during combustion combined with the oil contributes to the formation of sludge and varnish deposits on piston skirts, valve assembly, and the rocker arm. These deposits also cause oil rings to stick. Though natural gas applications are susceptible to nitration as a result of organic nitrates, the rate of this occurrence is dramatically increased due to improper air/fuel mixtures, inadequate combustion processes, mechanical problems, and severe service duty. Lacquering of oil lubricated system components, varnish formation, sludge formation, and increased acidity can be significant problems with natural gas applications due to high nitration. If abnormally elevated nitration is occurring in a two-stroke engine design it could be caused by poor scavenging of the crankcase oil in the combustion cylinder.
Nitro compounds form in the presence of exhaust gases and contribute to oil nitration when these exhaust gases mix with the engine lubricating oil. Excessive piston blow-by of exhaust gas containing nitrous oxide compounds (NOx) enters the oil and causes nitration. Overly extended oil drains can also lead to a buildup of NOx compounds, thus resulting in elevated nitration of the oil. Elevated nitration as a result of exhaust gas blow-by will increase viscosity due to oil thickening and acidity.
Nitration is much less common in over the road diesel engines, so when elevated nitration occurs this indicates something is not typical in the diesel engine operating performance and the oil environment. Typically the cause of elevated nitration in a diesel engine is exhaust gas blow-by and/or a lean air/fuel mixture. Nitrous oxide compounds (NOx) introduced in the lubricating oil through recirculated exhaust gas by way of EGR systems can also contribute to elevated nitration in diesel engines. Where there has been less concern about the creation of sulfuric acid with in-service diesel engine oil in past years, the creation of nitric acid when these nitration compounds react with water has become a greater concern.
An excessive level of nitration in a natural gas engine or compressor application is typically accompanied by an increase in acid number and viscosity. In diesel engine applications, where base number is more commonly tested for, base numbers will experience a premature decrease, which may or may not accompany a significant increase in viscosity. As always, the oil condition should be evaluated using a matrix of test parameters. Lubricants using saturated base oil formulations are better at resisting nitration during use. Proper equipment performance, quality lubricants, and monitoring through testing of the in-service lubricant can keep nitration degradation under control.
Nitration determination of in-service lubricating oils is one of many routine tests ALS Tribology laboratories perform globally. ALS has extensive experience in testing and evaluating both natural gas applications and diesel engine samples for asset reliability and lubricant integrity.
David Doyle, CLS, OMA I, OMA II
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