Acid Number is but one part of the overall testing requirements for evaluating industrial lubricant serviceability. But in many cases, where initial testing is limited, Acid Number is the main factor in evaluating serviceability.
In most applications, when the Acid Number is outside allowable limits, re-sampling for confirmation and/or a simple oil change is the usual recommendation. But in large capacity industrial applications with oil reservoirs that contain thousands of gallons of oil, additional testing is highly recommended before condemning that much lubricant based on one test result. When an oil change is not possible, a “feed and bleed” approach may be recommended to keep the lubricant in service until the necessary maintenance can be performed.
The interpretation of Acid Number results can vary greatly depending on lubricant type, component application and the test method used. In some instances, there are very strict allowable limits applied and in other circumstances trend analysis is more useful.
There are four ASTM procedures for determining Acid Number – ASTM D-664, ASTM D-974, ASTM D-1534 and ASTM D-3339. All use the same diluent 0.1N KOH (Potassium Hydroxide) in alcohol solution. ASTM D-664 is the only procedure in the group with a potentiometric process for determining the end point. The remaining three use a colorimetric procedure to determine the end point using p-naphtholbenzne as a color indicator in a diluent mixture of toluene, isopropanol and water as described in the procedure. All four procedures are reported in mg KOH/g.
ASTM PROCEDURES FOR DETERMINING ACID NUMBER
ASTM D-664 is generally used for compounded lubricants – engine, gear and AW hydraulic oils. This method may be used to detect both weak organic acids as well as strong inorganic acids (SAN).
ASTM D-974 is best used for light-colored lubricants. Titration is quick and easy on samples expected to have low Acid Number results – turbine (R&O) oils, refrigeration oils, phosphate esters, polyolefins, polyolesters and polyglycols.
ASTM D-1534 is the same methodology as ASTM D-974 but is designed for low viscosity oils used in transformers. This procedure’s upper limit for viscosity is 24 cSt at 40 c.
ASTM D-3339is also the same methodology as ASTM D-974 with the exception of using a limited sample volume of 2 grams. With the exception of ASTM D-3339, the procedures call for a sample volume to be use based on the expected number result. The expected acid number is usually known based on the oil type. If the starting point of the acid number is not known, review the lubricant supplier’s specification for the information. A high sample volume may be use, the titration process will take longer to determine the end-point. Each procedure has a calculation to determine the acid number based on the sample weight used for the titration.
ACID TYPES DETECTED:
The types of acids all four of these ASTM procedures detect may include but are not limited to the following:
Organic acids represent oil oxidation in all general lubricants.
Sulfuric acids are generally a result of sulfur in diesel fuel, natural gas engines and compressors and AW hydraulic and EP gear oils.
Nitric acids are produced by blow-by in diesel, gasoline and natural gas engines and compressors.
Hydrochloric and hydrofluoric acids result from refrigerant break down in refrigeration compressors.
REPEATABILITY AND REPRODUCIBILITY
Repeatability and reproducibility vary between procedures and various factors can affect the results, including outside contaminants. The limits for each are listed within each procedure.
APPLICATION FOR TESTING ACID NUMBER -
Most engine oils have a starting point of 1.5 to 2.5 mg KOH/g when new depending on the additives type and volume used to meet the associated API requirements. The Acid Number is stable until the Base Number is reduced to a sufficient level that will allow the acid number to increase. The Acid Number is usually deemed out of tolerance once it reaches 3.5 to 4.0 mg KOH/g with the exception of no zinc EMD/GE engine oils or marine applications using heavy fuel oils. Fuel type, as well as lubricant oxidation, may affect Acid Number results.
Power Train Oils
These fluids also fall within the general numbers used for the engine oils above. The Acid Number will start in the area of 2.5 and is considered out of tolerance at 3.5 to 4.0 gm KOH/g.
Automatic Transmission Oils
The starting point for these oils is relative to the additives used. The full product name and starting point is necessary to determine a condemning limit. The acid number may start from 0.5 to 3.0 gm KOH/g.
Gear oils with a general EP sulfur phosphorus additive package start around 1.5 to 2.0. Water content may affect the formation of acidic products even in new or low-time-in-service products. The condemning limit may start at 3.5 to 4.0 mg KOH/g.
Hydraulic AW Oils
Depending on additive levels, Hydraulic anti-wear oils usually start at 1.0 to 1.5 mg KOH/g. The allowable limits may range from 2.5 to 3.5 depending on oil type an application. Biodegradable hydraulic oils must be closely monitored for degradation/oxidation.
Refrigeration & Air Compressor Lubricants
These oils are separated into different groups.
These oils usually have a starting point of 0.01 to 0.05 mg KOH/g. The maximum allowable limit is 0.1 mg KOH/g requiring an oil change. Polyolefins may start at 0.1 to 0.15 with an allowable limit generally at mg KOH/g, depending on the application.
These may start at 0.1 to 0.15 with an allowable limit generally at 0.35 mg KOH/g.
Polyglycols are being used more often in a variety of applications. The starting point is around 0.2 and should be closely monitored after 1.2 mg KOH/g is reached.
Alkylbenzenes and other compressor lubricants require starting acid number be determined and judged on application and component manufacture.
Phosphate Esters and other synthetic products also have varying starting points and allowable limits for Acid Number. Other factors (such as?) are generally used to determine overall serviceability of these fluids.
Interpretation of Acid Number results will vary based on customer needs and requests. Trend analysis is a valuable approach for accurately determining the remaining useful life and serviceability of industrial oils. Environmental contamination is also a factor to consider as it can adversely affect Acid Number results.