TAN and TBN: are all acids really bad?

Are All Acids Really Bad?

In lubricated machines, there is a test which we can perform to determine if the acid level is approaching its tolerance limit. This is called the TAN (Total Acid Number) ASTM D974. The TAN test indicates the volume of acid present in an oil. However, it is usually performed on non-engine oils. This doesn’t mean that engine oils have no acid present, on the contrary engine oils may have the highest volume of acid present.

Understanding the TAN & TBN results

During the operation of most engines, acids are produced as by-products and these can enter the oil. Hence, for engine oils, one will typically notice that TBN (Total Base Number) values are given on the data sheets. The TBN gives an indication of whether acids have been neutralized by alkalis contained in the additives of oils. For instance, regular diesel engine oils usually have a TBN of 10-15. When an oil analysis is performed, one can determine whether the TBN value has decreased or not.

If the TBN value shows a decrease, this can indicate that acids were neutralized during the lifetime of the oil within the machine. However, we need to understand the limits which should be observed before the neutralization stops. The alkali (base) product in the oil is finite and at some point, it will be reduced to a level where it can no longer protect the oil. At this point, the acids will continue to build and can start negatively impacting the machines causing wear.

One of the most important aspects of oil analysis is trending. When we trend the results of tests (viscosity, metals, TAN, TBN) we can gain a clearer picture of what’s occurring on the inside of the machine. This can in turn help us to identify steps which can be taken to protect the machine and avoid the dreaded downtime!

Typically, OEMs advise on the warning limits for TAN / TBN depending on the type of equipment. One basic rule of thumb regarding TBN is to observe its rate of decrease. If the TBN is 50% lower than its original value then we’ve got some acid in our machine and have some time in which to rectify the situation. Similarly, the TAN test gives an indication of the actual volume of acid present. When the TAN value shows an increase of 0.3mgKOH/g those warning flags start to raise! However, it is always advised that you consult your OEM on these values as they may differ depending on application and environment.

All lubricants degrade during service, this is part of its sacrificial nature. However, the challenge occurs when the lubricant is no longer able to properly execute its required function due to degradation. This is where the warning limits and tolerances can provide critical information to users. These assist users in determining whether the environment or operation of the machine is severely impacting on the lubricant.

When should TAN be used?

For each of the six degradation mechanisms, there are tests which can be performed to help diagnose the type of degradation which has occurred. This in turn, helps to implement measures in rectifying the challenge. The beauty of these tests is that they all tell a story but they cannot be standalone tests, otherwise part of the story would not be told.

Within the industry, some experts have argued that the TAN test (as a standalone test) is not entirely indicative of the presence of degradation. This holds some truth depending on the type of degradation mechanism. In oxidation, acids are not produced until oxidation has reached the termination phase. At this time, varnish molecules are already in production as well as the acids which are conveyed in the TAN test.

The TAN test is relatively inexpensive and is usually packaged with the other basic tests, such as Viscosity, wear metals or contaminants. Therefore, it is wise to include the TAN (or TBN accordingly) as part of your monthly tests on equipment. This can assist in trending any increases in acidity of the oil and possibly prevent damage to the equipment.

Finding out the entire story

As mentioned earlier, the TAN / TBN results should not be used as standalone tests. They form a component of the story which is being told by the equipment. Let’s look at a few examples to help us understand how these values add to the plot of the story being told by the equipment.

Case study 1: Hydraulic Component

The used oil analysis for a particular hydraulic component showed an increase in the TAN value by 0.2mgKOH/g but no increases in wear metals, slight decreases in additive components and no major change to the viscosity. What should the customer do?

Any increase in acidity should be addressed. The increase in acidity, simply indicates that the volume of acid is building compared to the initial acid value in the oil. If we are not seeing any changes to viscosity, or an increase in wear metals it can indicate that the acid has not reached to a level where it has affected the oil. This is good news, as we now have time to determine the cause of the rise in acid before any damage occurs. Additionally, this can help us project the estimated life of the oil.

In this situation, the customer can resample the oil at a shorter interval and trend the increase in acidity over time. Any changes to the environment or operations should also be noted as these may impact on the oil. Some customers may opt to change the oil to avoid these increases in acidity however, if the root cause for the increase in acidity is not addressed then the customer can face shorter oil drain intervals. This in turn, can impact on the health of the asset and lead to unforetold expenditure associated with these shorter oil changes.

Case study 2: Marine Diesel Engine Component

A marine operator noticed the TBN levels decreasing quickly (at a rate of 30% within one month) for one of the diesel engines on his vessel in the fleet. Additionally, the viscosity had decreased by 10% and the wear metals were approaching their warning limits. He also noticed that there was some fuel dilution present (3%) but this had not reached the OEM’s warning limit of 6%. How should we advise this marine operator?

For rapid declines in TBN and a decrease in viscosity, any operator should be concerned. Once the viscosity decrease (or increases) out of its range, we can suspect that something is occurring to cause this deviation. When there are decreases in viscosity, the associated degradation mechanism is thermal degradation. However, in this case, there is also the presence of fuel dilution which typically decreases the viscosity of the oil.

Fuel dilution should be investigated and addressed as one of the possible causes for the degradation of the oil. Additionally, the decrease in viscosity can lead to the increase of the presence of wear metals as seen in the report. The operator can perform some inspections on the rings, fuel injectors and fuel lines to determine if there are any leaks into the system.

Any changes to the operations or environment of the engines should also be noted. The storage and handling of the lubricants for these vessels should also be investigated to rule out fuel dilution via contamination. Sometimes, when engine oil is being transferred, the equipment may have been used to transfer other fluids and the engine oil can become contaminated. Additionally, increases in temperature should also be noted to rule out the possibility of thermal degradation.

About The Author

TAN vs TBN Sanya Mathura,
Sanya Mathura MLE,
Managing Director and Founder of Strategic Reliability Solutions Ltd

Sanya is an entrepreneur based in Trinidad & Tobago with a passion for adding value to the industry through strategic reliability solutions (it is also the name of her company). She is a strong advocate for women in STEM and a published author (with a couple more books on the way). While she has her bachelor’s degree in Electrical & Computer Engineering, after getting into the lubrication industry, she fell in love with reliability and pursued her Masters in Engineering Asset Management. Later on, she attained her ICML MLE certification and became the first person (and only female) in the Caribbean to do so.

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