For oil analysis on a vehicle using liquid natural gas, which test is better, TBN or TAN?

Mr. Smiley,

If it is my car, I put some money on it and perform both analysis. BN will give me information regarding my detergent and dispersant additieves and AN will give me information regarding the acid generation due to burn gas as fuel.

Smiley,

Considering only TBN/TAN for Natural gas Engines
is insufficient.Other important tests like Oxidation,Nitration,Wear metals,Viscosity has to be run regualrly.TBN,TAN is of little importance since oil is been topped up regularly
due to intense heat produced by the combustion
of natural gas.Natural gas increases oxidation,
nitration by-products,causes corrosive punch in
engine components.The organic acids,Nitric acids
elevates Bearing corrosion.It is important to
remember that oil change intervals has to be
optimized based on Viscosity,Oxidation,Nitration
and wear analysis.....

Good Luck

Bala.

If TAN is the measurement of total acid present in the oil ,acid is undesirable, what should be the maximum TAN allowed for Hydraulic oil SAE 68. Shell people in Vietnam told me that their new Hydraulic oil Tellus 68 has TAN equal to 1, if acid is undesirable how new oil has so high a TAN, In any oil how TAN and TBN coexist. If oil is alkaline it will neutralize acid whenever it will be formed, still oil will be alkaline unless and untill total alkali is neutralized . So at any time oil is either alkaline or acidic, how it can be both

Prabhakar,

Any Oil can have an acid and base number at the same time because they measure different constituents with two different titration procedures.Infact some additives particularly
in engine oils are amphoteric-they can behave as either an acid or base.Both the values are
valid.Acid and Base numbers are not reciprocal
in nature,although acid numbers do trend up,as
base numbers trend down through an oils life span.

Hydraulic oil(ISO 68) is acidic in nature due to the ZDDP(Zinc Dialkyl Dithiophosphate)chemistry.Over a period of time additive depleates resulting in decreased TAN.Oxidised oil increases organic acids resulting in elevated TAN levels.Needless to say that treating the cause is good maintenance practice.

Bala.

Dear Bala,
I would request you to explain TAN more elaborately.You have given the example of Hydtaulic oil 68 which has a TAN due to ZDDP, Shell claims the TAN of new oil to be 1, if this oil is run its TAN will go on decreasing due to depletion of additives, but at the same time the oxidation products with moisture will generate acid which should increase TAN, how then this combined effect is taken into consideration?

Prabhakar,
-The first thing we need to clarify is the fact that AN and BN can coexist. There is a close relationship between both. However is not an absolute relationship. The presence of one does not mean the absence of the other.
- The AN value gave to you by Shell for their hydraulic fluid is measuring additives. During the operation of the oil the additive package experiment a normal adjustment and the base oil start to deteriorate (oxidation process) these cause a variation of the AN
- The best way to analyze AN is comparing the values with the base line

Velasco,
your reply is slightly not clear to me.Take the example of Tellus 68 of Shell for which they have declared a TAN equal to 1. Now this new oil is put into machine and run regularly, Oxidation products will combine with moisture to form acid which should increase the TAN value of the oil. Isn't it?
But you have said that the initial TAN represents the additive package present in oil, it means that with more and more use the additives will deplete and TAN should decrease.
In one case TAN is increasing , in another it is decreasing, how they will adjust.
Suppose after 4000 hours the TAN of this hydraulic oil comes to 1.5, now in comparison to intial TAN=1 what is the condition of oil? Every other parametres being within allowable limits, at what TAN this oil will have to be rejected. Kindly go through the whole case thoroughly, then reply. i have received many answers which have not satisfied me

Prabhakar,
Suggest to draw the sample at every 500 Hrs.
Apart from TAN,Run Viscosity @ 40'C,Particle
Count,Wear metal Analysis.At 4000 Hrs it's quite
obvious that TAN increases due to age.Particle
Count is essential for Hydraulic System.Sampling
at regualar intervals establishes trend/pattern
of wear elements,contaminants.Oil change periods
can be optimized based on these tests.Post your
analytical results,Hydraulic system details,Topup added oil etc..for interpretation.

Regards

Bala.

Mr Smiley,

The thumb rule is check TBN for engine oils and check TAN for oils other than engine oils. It should be no different for Gas engine oils.

Hussam Adeni

Bala and Adeni,
I don't think that you have read my posting carefully.I am talking about Hydraulic oil only, if all the parameters like viscosity, moisture and NAS value are within allowable limits , should the value of TAN be the deciding factor.What should be the limit of TAN at which the oil should be rejected? If the TAN of new Tellus 68 is 1, will it go down gradually or will increase with more and more use of oil. If TAN will go down, will it go upto zero and then again increase? Or it will go on increasing from the initial value 1. Kindly read my both the posts thoroughly, then if you have a clear concept of the whole thing , then reply.
Prabhakar

Mr Smiley,

I recently read an article by Mike Barnes of Noria, which answers your question. The article is pasted below.

Hussam Adeni

--------------------------------------------------------------------
Copy of articles by Mike Barnes, Noria is pasted below:

Which Acid Number Test is Best?

"In used oil analysis of natural gas engine oils, most original equipment manufacturers (OEM) have total acid number (TAN) as a criteria for an oil change. However, certain gas engine OEMs have strong acid number (SAN) as a criteria for oil change. Why do these OEMs prefer SAN over TAN?"

Strong acid number can be determined using the same methods as total acid number (ASTM D664 or D974). With total acid number, the test titrates to the final inflection end point and determines both strong acids, like sulfuric and nitric acids, and weaker acids like organic acids formed by oil degradation.

Strong acid number, however, titrates to the first inflection point, corresponding to a pH equivalent of around 6, which does not account for weaker acids.

For natural gas engines - particularly those burning sour gas (that is, a high content of sulfurous impurities), or other impure fuel sources like landfill and biogas, SAN can be instructive in determining the degree of contamination due to stronger, more corrosive acids formed by burning lower-quality fuel.
Under these circumstances, it may offer a more precise method of determining a condition-based oil change parameter.

Mark Barnes, Senior Technical Consultant, Noria Corporation

Prabhakar,

I was carried away by the original post, by Mr Smiley, which sought to know whether to "Analyse TBN or TAN for LNG Engines."

Coming to your query on TAN, I thought it apposite to paste two "Lube tips" that appeared in Noria.

Hussam Adeni
------------------------------------------------
Article appearing in Lube Tips.

ACID NUMBER

Oil Analysis Tip

Acid Number monitors the level of organic acids produced from the oxidation of the oil. All systems in which an extended drain interval is contemplated or in which the potential exists for acidic contamination should be monitored for an increase in acidic contaminates. As the oil ages and oxidizes, the acidic by-products cause the acid number to increase. An increase in acid number is an indication of oxidation in the lubricant, which could lead to lubricant degradation.

-------------------------------------------------
Article appearing in Lube Tips

Acid Number Helps Time Oil Changes

"Should I consider adding the acid number test to my predictive maintenance program?"
Acid number (AN) tests are typically performed as part of a proactive oil analysis strategy. The principal reason we would consider acid number is to determine a condition-based oil change interval rather than a time-based oil change interval.

Because acid number changes when organic acids build up in the oil due to oxidation, the test can be used to warn of incipient oxidation so that an oil change can be scheduled. Used correctly, it allows oil analysis users to optimize oil life, without jeopardizing equipment by leaving oil in service for too long.

It is typically used in large (>50-100 gallon) systems, where the cost to perform AN tests can be justified by the extended oil drain intervals and is applicable to most oils, with the exception of diesel engine oils.

Most quality labs are able to accurately determine AN. We do, However, strongly suggest the lab be requested to use ASTM D664 (potentiometric titration), rather than D974 (colorimetric) because this is generally more accurate for used oil analysis - particularly darker oils.

--------------------------------------------------

Adeni,
Do you think that you have answered my query ? I think you have not even read my posting. I would again request you to first read my posting , then answer it point by point. How can New oil have a TAN of 1, have you considered it.
Prabhakar

Prabhakar,
I think what you are looking for is why does the new oil have a TAN of 1. This can best be determined by the chemical make up of the oil. It has a great deal to do with the additives the company is using. In most hydraulic oils you want to keep the oxidation process down. The oxidation process in your hydraulic oils is what will drive the TAN up. I would refer you to George Totten's book titled "Handbook of Hydraulic Fluid Technology." Look in particular at chapter 14 and specifically section 6.1. While it may not give you all of the answers you are looking for, I am sure it will help give you a better understanding of some of the whys. If you do not have the book, it is available at Noria's book store. Good Luck.

Quoted from Prabhakar:

"If TAN is the measurement of total acid present in the oil ,acid is undesirable, what should be the maximum TAN allowed for Hydraulic oil SAE 68. Shell people in Vietnam told me that their new Hydraulic oil Tellus 68 has TAN equal to 1, if acid is undesirable how new oil has so high a TAN, In any oil how TAN and TBN coexist. If oil is alkaline it will neutralize acid whenever it will be formed, still oil will be alkaline unless and untill total alkali is neutralized . So at any time oil is either alkaline or acidic, how it can be both"

Prabhakar, you need to look up the chemical term 'amphoteric'. Some compounds can behave both as acids and bases, as strange as this may seem. As said above, some of the additives used in lubricants are amphoteric, especially the metal detergency additives.

From a web site with a reasonable definition of amphoteric:

Amphoteric
--------------------------------------------------------------------------------

(1) A compound that can behave both as an acid and as a base. For example, with strong acids aluminium hydroxide acts as a base, Al(OH)3, forming aluminium salts. With strong bases it behaves as an acid, H3AlO3, forming salts containing the ion AlO33-. The formation of amphoteric hydroxides is a characteristic of metalloids.

(2) Compounds that contain both basic and acidic functional groups. See, for example, the amino acid glycine.



As you pointed out, this behavior can make tracking oil condition by TAN very difficult. As a result, you cannot rely only on TAN to determine the oil condition. Several previous posters said the same thing when they told you to do other testing as well as tracking TAN. TAN will sometimes initially drop as the additives are consumed, and may then rise later as the oil is oxidized. Unless you are continually tracking the TAN value it is difficult to tell where you are in this process.

Guy
Thanks a lot, you have given a very clear concept. It means TAN is not a very important parameter for deciding the condition of used hydraulic oil.It may be due to additives which is desirable and at the same time it may be due to oxidation products which will be undesirable.
Just one question: I have taken 600 litres of oil from the hydraulic tank of a Melamine products producing machine, it has run for 6000 or even more hours, now this oil is cleaned resulting in Viscosity of 67.6 at 40 degree C , moisture 100ppm, NAS value 5, how to know whether this oil can be continued further in use or not. Will the measurement of TAN help in decision making? If yes, what decision you will take in 6 different cases when TAN is 0.33 or 1.02 or 1.3 or 1.5 or 1.9 or 2.4 Please remember topping up has been done with oil of different companies , but same viscosity 68 (+/- 5%), no info available about the oil charged initially. Should I perform any other test, which one?

Prabhakar,

1# NAS is Outdated.Current Particle count standard is NIST-ISO 11171 calibration method
with three ranges 4/6/14 microns.

2# FTIR Spectrum of used Hydraulic oil can show
Molecular Additive levels(organic anti-wear)and
Oxidation By-products.Comparing the Fresh oil and used oils helps identify the Additive and
Contaminant levels.

3# After cleaning the oil,Recommend to run the
oil property tests like Foaming characteristics,
Wear test by Four Ball method,Rotating Bomb
oxidation stability,Rust preventive test,
Demulsibility test.Compare the analytical results with fresh oils.

4# If the above oil property tests fails,discuss
with lube oil supplier for further action.If
test passes,it can be reused in hydraulic system
with periodic oil condition monitoring.

5# Treating the cause of High oxidation will
Extend both the machine and oil life.

Regards

Bala.

Bala,
Thanks. You have given very useful information , but have been silent on TAN-TBN of hydraulic oil. So far as I understand from your writing , they need not be given so much importance. Am I correct?
Will you please give me a chart of Particle-counts associated with NIST ISO 11171, however I think it does not make much difference as NAS also considers particles of many ranges.
Can you give me address of some labs in India which are considered authority, if they are Govt labs that will be better.
What do you think , for the tests whch you have mentioned except FTIR, how much will it cost to buy those apparatus, what should be the testing charges in some lab?

I have refrained from reacting to the TAN-TBN debate, as I belong to the old school. In my 18 years with Castrol in India, Burmah Castrol Technical experts taught us some thumb rules that I giving below:

1. Seek TBN values for Marine Engine oils on account of the high sulphur content of most of bunker fuels and poor fuel distillates.It is the quality ( poor quality of Fuels) that demand high TBN. For other engine oils (EO's), which largely run on HSD and possibly LDO, tracking of moisture, Flash, KV 40 will reveal good guestimates of the condition of lube.

2. Track TAN for Gas engine oils other than on LPG.

3. Check TAN for lubes (not EO's) that are aged, stored for a long time without use, or have been contaminated with water to check for organic acidity.

Hussam Adeni