Dear electricpete,
Thanks for the enticing and memorable "conversation". I too go by your decision to include IFT based on #1 and #2 of your last reply.
Something equally interesting follows - about your question
"why IFT is important enough for transformers but not for rotating machines?" The thermosyphon mechanism that is largely responsible for making the oil go around in the ON cooling phase of a oil filled/cooled transformer apparently has less scouring action/dislodging force compared to the assisted flow that is encountered in rotating equipment application is my wild guess
Oil Deterioration in Transformers: In transformers, sludge sticks to the surfaces through which heat is dissipated; the sludge forms a barrier to the flow of heat from the oil to the coolant and from the core and coils to the oil. If allowed to continue, the sludge may block off the flow of oil through the cooling ducts. As a result, the transformer insulating oil becomes hotter and the transformer could be damaged, particularly between the coil turns of the windings. Deterioration of the coil-turn insulation may eventually lead to short circuits between coil turns and the breakdown of the transformer. When oxidation progresses to the point where sludge is being precipitated, the first step should be to remove the sludge from the transformer by a high-pressure stream of oil and to either replace the sludged oil or treat it with activated clay to remove the acid and sludge precursors. Complete treatment of the oil is normally less costly than replacing it with new oil.
Acidity Test: If properly refined, new transformer oils contain practically no acids. The acidity test
measures the content of acids formed by oxidation. The oxidation products polymerize to form sludge which then precipitates out. Acids react with metals on the surfaces inside the tank and form metallic soaps, another form of sludge. Sludging has been found to begin when the acid number reaches or exceeds 0.4 mg of KOH/g (0.4 is considered to be the normal service limit). New oil has an acid number of less than 0.05 mg of KOH/g. The acid number (also referred to as the neutralization number) equals the milligrams of potassium hydroxide (KOH) required to neutralize the acid contained in 1 g of oil. It is questionable whether an oil that has deteriorated to the point where its acid number exceeds 0.6 mg of KOH/g can be restored to good condition by a single renovation. It is almost certain that two or more renovations, spaced 6 months to 1 year apart, would be necessary. It is recommended that an upper limit of 0.2 mg of KOH/g be used to determine when oil should be renovated, as at that point a single renovation would probably restore the oil to very good condition. Oil showing an acid number of 0.15 mg of KOH/g or larger can be expected to show accelerated acid formation. Tests have been made which indicate that acidity is proportional to the amount of oxygen absorbed by the oil. It is estimated that 0.0015 m3/L (0.2 ft3/gal) of oxygen absorbed in oil will cause an acidity of about 0.4 mg of KOH, which is the approximate acidity number at which sludging is assumed to start.
Interfacial Tension (IFT) Test: The acidity test determines conditions under which sludge may form, but does not necessarily indicate that actual sludging conditions exist. The IFT test is employed as an indication of the sludging characteristics of power transformer insulating oil. It is a test of interfacial tension of water against oil, which is different from the surface tension of oil against air. The attraction between the water molecules at the interface is influenced by the presence of polar molecules in the oil in such a way that the presence of more polar compounds causes lower interfacial tension. The test measures the concentration of polar molecules in suspension and in solution in the oil and thus gives an accurate measurement of dissolved sludge precursors in the oil long before any sludge is precipitated. It has been established that an interfacial tension of less the 0.015 N/m (15 dyn/cm) almost invariably shows sludging. An IFT of 0.015 to 0.022 N/m (15 to 22 dyn/cm) shows an uncertain condition, and an IFT value of more than 0.022 N/m (22 dyn/cm) is generally indicative of no sludging. Transformer oils whose IFT is in the range of 0.015 to 0.022 N/m (15 to 22 dyn/cm) should be scheduled for reclaiming, regardless of acidity values.
Interpretation of Test Results: When the results from all tests are integrated, a sound conclusion can be drawn as to the condition of the oil. Qualitative relationships exist between acidity and IFT which are useful in interpreting test results. Note that the rate of change of IFT is a more sensitive indication of early stages of deterioration. The IFT measurements are particularly useful in judging the effectiveness of oil renovation processes; identifying at an early stage new oil with poor life expectancy, thus, permitting treatment while it is still practical; and indicating when oil should be discarded and replaced. The rate of change of acidity is often a more sensitive indication of deterioration near the sludge point. However, this is only true if the oil does not
contain alkali impurities. Such impurities neutralize the acids as they are formed, resulting in a low-acidity value.