The S. D. Myers Inc. laboratory reports the ratio of interfacial tension (IFT) and acid number as an indicator of the extent of oxidation of insulating oil. This ratio is the inspiration for TOA4’s Fluid Quality Index, which is calculated with an extra factor of 1000 to ensure that typical values of the index are not tiny decimal fractions. Here is the formula:
fqindex = 1000*acidnum/ift
Here, acidnum is the acid number (mg KOH/g), and ift is the interfacial tension (mN/m). The fqindex is calculated so that low values are good and high values are bad. Clean new oil has an fqindex below 1.0, and when fqindex reaches 10, the oil is in poor condition.
While fqindex is good for trending the approximate oil quality and for detecting sudden changes which may need investigation, the acidity and IFT values provide a direct basis for triggering maintenance activity such as oil treatment.
For in-service mineral transformer oil, the acid number starts at about 0.01 mg KOH/g and is considered poor when it has increased to 0.10 or 0.20; at 0.40 it is completely unacceptable because sludge formation begins at about that level of acidity. The IFT starts at about 50 mN/m and declines as oil oxidation produces polar byproducts. At an IFT of about 25 or 30 mN/m, the oil should be reclaimed, and at about 22 mN/m sludge formation begins. How can we use this information to derive norms for fqindex?
Observe that if acid number is less than 0.10 and IFT is greater than 30, then fqindex is less than 3.33; that is, if fqindex exceeds 3.33, then either acid number or IFT must be outside the acceptable range. Using the more relaxed limits of 0.20 for acid number and 25 for IFT, the corresponding limit for fqindex is 8.00. On the other hand, if acid number is greater than 0.40 and IFT is less than 22, then fqindex is greater than 18 and sludge formation has probably started.
The arithmetic exercise above suggests the following set of critical values for fqindex:
|Warning level||FQ index||Meaning|
|2 (alert)||3.3||Consider oil reclamation|
|3 (warning)||8.0||Oil reclamation needed|
|4 (alarm)||18.0||Sludge formation assured|