This week, we are going to answer another question that we received for this Tech Corner feature. It is a question we have been asked many times, phrased different ways. A customer asked one of our MaxLife Consultants about his test results. “I have a high acid number for one of my units, and your recap lists a recommendation that I have the transformer hot oil cleaned. Can I just change the oil to lower the acid number to an acceptable level?”
Changing the oil will not provide a permanent reduction in oxidation byproducts in the solid insulation, so it is not an appropriate solution to the maintenance needs indicated by a high acid number. (For that matter, this would not correct a lower than acceptable interfacial tension (IFT), either.) As the oil in a transformer ages, it oxidizes as 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 paper that makes up the solid insulation, shortening the life of the equipment. Damage done by breakdown of paper by oil oxidation decay products 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 deposit within and to coat the paper that makes up the solid insulation in the transformer. Results from research documents that this is happening whenever the acid number increases above 0.05 mg KOH/g sample, or the IFT declines below 32 mN/m (dynes/cm). 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 remove them from the solid insulation.
Changing the oil will not remove any appreciable amounts of oxidation decay products from the solid insulation. The decay products will mostly stay in the paper, and they will continue to break down the solid insulation. If you change oil in a transformer with a relatively high acid number, the acid number will be reduced to a low level immediately after the oil change. Over the next few weeks, though, the acid number will start to increase again as some of the oxidation byproducts in the solid insulation are dissolved back into the new, clean oil. In a relatively short period of time, since the products of oil oxidation are concentrated hundreds of times more in the paper compared to the oil, the acid number of the oil will increase back to essentially the value it was before the oil change was done.
To provide a permanent reduction of acid number, and to stop the on-going destruction of the solid insulation, you need to perform the procedure that we call “hot oil cleaning”. In hot oil cleaning, as in the graphic below, “dirty” oil is removed from the bottom of a transformer that has aged oil with oxidation products in the solid insulation. The “dirty” oil is sent to reclaiming rig where the oil passes through a column (or series of columns) filled with an adsorbent such as fullers earth. The adsorbent chemically removes the oxidation decay products from the incoming oil.
After the oil has been cleaned up, it is heated in the rig to its “aniline point” and is returned to the transformer. (The aniline point is 84 oC maximum; in practice, the oil is generally heated to 65 to 70 oC.) The clean, hot oil will dissolve and remove the oxidation decay products in the solid insulation. By recirculating the oil from the transformer through the processing rig several times, the paper can be thoroughly cleaned and the oxidation products can be permanent removed.
The key consideration is to perform the proper number of recirculations or “passes” during the hot oil cleaning. A minimum of six passes through the adsorbent bed of a properly sized reclaiming rig 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 the proper planning of the project, if the maintenance needs for the transformer are addressed in a timely fashion, hot oil cleaning provides a permanent solution to needs for oil maintenance and life extension.
Thank you for following this discussion of hot oil cleaning. We will have another topic starting next week.
