Will Changing Oil Solve an Acid Problem?Technical Briefs
Will Changing Oil Solve an Acid Problem?
We are sometimes asked by our customers about changing the oil when test results indicate high acid levels. "I have a high acid number for one of my units, and your Recap Report recommends hot oil cleaning for my transformer. Instead of hot oil cleaning, can I just change the oil to lower the acid number to an acceptable level?" Unfortunately, no.
Changing the oil will not permanently reduce the oxidation byproducts (ie. decay products) in the solid insulation. It is not an appropriate maintenance solution for lowering a high acid number. For that matter, this would not permanently correct a lower-than-acceptable interfacial tension (IFT), either. As the oil in a transformer ages, it oxidizes when the hydrocarbons in the oil react with oxygen dissolved in the oil. This "aging by oxidation" causes oxidation byproducts to be formed. These include acids, aldehydes, ketones, alcohols, soaps, peroxides, and epoxides. All of these oxidation decay products are aggressive to the solid insulation. When they form, they cause premature breakdown and aging of the solid insulation, therefore shortening the life of the equipment. The damage caused by the premature breakdown of the paper and the oxidation byproducts is permanent.
The oxidation decay products are polar compounds. They tend to have an affinity for the solid insulation since paper is also a polar compound. (Clean transformer oil is not polar.) When you start to detect oxidation decay products in the oil, they have already started to form and absorb into the paper. Results from research have documented that this is happening whenever the acid number increases above 0.05 mg KOH/g sample. To properly maintain the transformer and stop the breakdown of paper by oil oxidation products, you have to not only remove the oxidation decay products from the oil, but also from the solid insulation.
Changing the oil will not remove any appreciable amounts of oxidation decay products from the solid insulation. Most of the decay products will remain in the paper and will continue to break down the solid insulation. If you change oil that has a relatively high acid number, the acid number will only be reduced to a low level immediately following the oil change. Over the next few weeks, however, the acid number will start to increase again, because some of the oxidation byproducts in the solid insulation are dissolved back into the clean oil. Since byproducts are concentrated hundreds of times more in the paper than in the oil, within a relatively short period of time, the acid level will increase back to the value it was before the oil was changed.
In order to permanently reduce the acid, and to stop the ongoing destruction of the solid insulation, "hot oil cleaning" should be performed. During the hot oil cleaning process, (see graphic below) "dirty" oil is removed from the bottom of the transformer, using a processing rig, and passed through a column (or series of columns) filled with an adsorbent material such as Fuller’s Earth. The adsorbent material chemically removes the oxidation decay products from the dirty oil.
During the cleaning process, the oil is heated to its "aniline point", and then returned to the transformer. The clean, hot oil will dissolve and remove the oxidation decay products that are in the solid insulation. By recirculating the oil from the transformer through the processing rig several times, the paper is thoroughly cleaned and the oxidation products (acid) are removed.
The key factor is to perform the proper number of recirculations or "passes" during the hot oil cleaning. A minimum number of passes is required if the acid number and/or IFT results are outside the acceptable range. If results are classified as unacceptable, or if the unit has been in need of maintenance for an extended period of time, additional passes through the earth tanks will be required. However, with proper planning, and if the maintenance needs of the transformer are addressed in a timely fashion, hot oil cleaning will provide a long-term solution for life extension.