Understanding Acid Number Limits of Turbine Oil

"What are the recommended acid number limits based on ASTM D664/09a for the lube oil of a gas turbine? The lubricant being used is Shell Aeroshell 500."

ASTM D664 (as well as the similar testing methods of D974) can be a very helpful oil analysis testing procedure for used oil to help determine the acidic constituents present within a particular lubricant sample. In order to effectively assess the acid number results obtained from this testing procedure, you must first obtain a baseline from a new oil sample. This is critical, as the acid number for the new oil may vary considerably between all types of oils.

During acid number monitoring, an increase in the value will likely indicate the rise of acidic products present. Even among various types of turbine oils, the baseline acid number will be different. For example, Shell reports that new oil testing of Aeroshell 500 is typically around 0.11 milligrams of potassium hydroxide per gram (mg KOH/g), with a new oil maximum of 1.0 mg KOH/g. While this is a good starting point, the new oil should be tested from the same batch from which the used oil is to be sampled because the actual acid number baseline will vary.

Typical warning limits for the acid number of an in-service turbine oil will be approximately 0.1 to 0.2 mg KOH/g for gas turbines with more than 3,000 hours of oil life. This may be a significant indicator of above normal degradation. An acid number increase of 0.3 to 0.4 mg KOH/g above the initial value will likely indicate that the oil is at or approaching its end of service life. This recommended warning limit falls in line with that of ASTM D4378 as well.

Even though results may vary from sample to sample, any small increase of the acid number, such as a rise from 0.15 to 0.25 mg KOH/g, should cause concern and prompt an investigation as well as more frequent oil sampling. If the warning limit of a 0.3 mg KOH/g increase or greater occurs, an inspection should be conducted to check for signs of increased sediment on filters and centrifuges. If the oil must remain within the system, there should be closer monitoring of all available indicators, including sight glasses, temperature indicators, differential pressure gauges on filters and, of course, the results of more frequent oil sampling.