Translated by Francisco J. Páez Alfonzo

Posted inMachinery Lubrication (6/2013)

Oils by nature are hygroscopic, which means they absorb moisture from the air. The tendency of an oil to undergo this process is known as hygroscopicity. Ester-type fluids, especially polyol esters and phosphate esters, readily absorb moisture from the environment.

Since the lubricant is contaminated with water, the question we ask is how stable the oil is relative to water. The ability of a lubricant and its additives to withstand chemical decomposition in the presence of water is known as its hydrolytic stability. The ASTM test used to determine the hydrolytic stability of oils is D2619-09. The test is known as the Coca-Cola bottle test, as it uses a soda bottle that is capped during test run.

The test is performed by mixing 75 ml of the oil to be tested with 25 ml of water. Then, a strip of copper is placed inside the sample. The bottle is capped, heated to 93 ° C (200 ° F) and rotated for 48 hours. At the end of the test, the copper strip is removed and its weight loss as well as the change in color is determined (reported as in ASTM D130). The acid number of the oil – AN is determined, as well as the acidity level of the water. The results indicate the hydrolytic stability of the oil and how well it behaves against acid formation, which coincides with the hydrolysis.

Some factors influence test results, including water purity, fluid contamination, viscosity, and the additive package. For example, the zinc dialkyldithiophosphate (ZDDP) antiwear additive produces acids when hydrolyzed. When analyzing the test results, the weight loss of the copper strip is determined. The zinc covers the copper strip (as expected), but once the copper strip is washed (usually with heptane or trichloroethane), it is determined exactly how much was the weight loss of the strip.
This table displays the results of an ASTM D2619 test. Note the difference in fluid and acidity results as well as damage to the copper strip.

Over time, even oils with a high hydrolytic stability will begin to hydrolyze. In oils, mineral lubricants and additives degrade. The degradation of these molecules with the addition of water results in a restructuring of the bonds and a modification of the compounds within the fluid. This process is accompanied by a change in pH and can be monitored by analyzing the acid number of the oil. As previously stated, ester-based fluids are highly susceptible to hydrolysis and should be monitored closely to detect any signs of hydrolysis, especially in equipment with a high risk of moisture ingress.

Hydrolysis in lubricants causes a wide variety of problems. Not only does it affect the physical properties of the oil (viscosity, color, etc.), it also affects its chemical properties. One of the most common effects of hydrolysis is the formation of acids, the predominant being carboxylic acids. These acids are weak compared to sulfuric acid, but still cause damage to the machinery. These acids are detected using Fourier Transform Infrared (FTIR) and can be monitored using a routine oil analysis program.

As the hydrolysis process continues, the viscosity of the oil begins to decrease. This decrease in viscosity poses a real threat to the health of the machine. As the viscosity decreases, its load bearing capacity also decreases, resulting in an operation in a limiting film lubrication regime and a more pronounced wear.

Being proactive and preventing the entry of water into the oil, the hydrolysis process can be mitigated. Monitoring water content and acid number along with FTIR are the best weapons to determine if you are presenting hydrolysis in the oil. Keeping the oil in it will prevent the devastating effects of this chemical process.

References

Forest, M. and Araud, C. «A New Approach for Oil Formulations.»

Papay, Andrew G., and Harstick, Christian S. «Petroleum-B

Translated by Roberto Trujillo Corona,

Critical machinery should definitely be inspected more frequently than non-critical machinery. Based on the definition of «critical», we mean machines with the highest level of importance to you, your company and your process. These assets deserve the attention of their limited time, money and effort.

Of course, it is important to know how to define an asset as critical. There are many approaches to determining the criticality of an asset. Some plants choose to employ a simple 5-degree scale and assign numbers subjectively, while others use long, endless processes and even hire consulting firms to help them determine it. However, the decision can be reduced to a few key attributes:

Impact on production
Impact on safety
Impact on the environment
Replacement cost
Availability of spare parts or redundancy
Probability of failure
Historical data about reliability and preventive maintenance
Most plants are very limited in terms of labor force and financial resources. If you have few of those resources in your «bank», you should spend them wisely. In order to make an informed decision on how to spend them, you need to have a good knowledge of the criticality of some assets relative to others.

These criticalities can also be used to help you prioritize work. With limited resources, you always seek to work first on tasks with the highest priority. This classification of work is almost impossible if one does not understand the criticality of the machinery.

Oil sampling is a good example of the use of criticality for decision making. Any asset that has a high impact on production, safety and the environment, which is more costly to replace, has little or no spare or redundancy, together with a high probability of failure and with low reliability and preventive maintenance, Would be an excellent candidate for continuous online monitoring in real time.

As asset attributes become less and less severe, the sampling frequency is relaxed, shifting from continuous monitoring to perhaps hourly, daily, monthly, quarterly, or yearly. You may even get to the point of deciding that the oil analysis simply offers no benefit because the machine has little or no impact on production, safety or the environment. It could be cheaper to replace it, or you could easily dispose of the spare parts.

Criticality is a very important element in making informed decisions about programming, whether it is a simple peephole inspection or as complex as repairing a turbine.