Why Oil Changes Won’t Fix High Acid Numbers in Transformers

This article is on transformer oil maintenance and the limitations of oil replacement as a corrective action for oxidative degradation.

Transformer operators managing elevated acid numbers may wonder whether a straightforward oil change is sufficient to restore acceptable insulation fluid quality. It is not. Understanding why requires a closer look at how oxidation byproducts behave in a transformer system — specifically, their affinity for solid insulation and their persistence long after new oil is introduced.

How Oil Oxidation Degrades Transformer Insulation

As transformer oil ages, dissolved oxygen reacts with hydrocarbons in the oil, producing a range of oxidation byproducts: acids, aldehydes, ketones, alcohols, soaps, peroxides, and epoxides. These compounds are chemically aggressive to solid insulation, accelerating the breakdown of paper and shortening equipment service life. Critically, this degradation is permanent — once the paper has been compromised, it cannot be restored.

Why Oxidation Byproducts Concentrate in the Paper

Oxidation decay products are polar compounds. Because paper (cellulose) is also polar, these byproducts have a strong chemical affinity for solid insulation — clean transformer oil, by contrast, is non-polar and does not share this attraction. Research has established that byproduct absorption into the paper begins as soon as the acid number rises above 0.05 mg KOH/g sample. By the time elevated acid numbers are detectable in oil testing, decay products are already concentrated in the solid insulation at levels hundreds of times higher than those present in the oil itself.

What Actually Happens After an Oil Change

Replacing high-acid oil will briefly reduce the measured acid number — but only temporarily. Within weeks, acid levels begin climbing again as oxidation byproducts migrate out of the paper and dissolve back into the fresh oil. Because the byproduct reservoir in the solid insulation is so much larger than what was present in the oil, the acid number will return to its pre-change level relatively quickly. An oil change addresses the symptom in the fluid, not the source of contamination in the paper.

This same limitation applies to interfacial tension (IFT): oil replacement will not produce a lasting improvement in IFT results when the underlying cause is byproduct saturation of the solid insulation.

Hot Oil Cleaning: The Correct Maintenance Response

How the Hot Oil Cleaning Process Works

Hot oil cleaning uses a processing rig to continuously draw oil from the bottom of the transformer, circulate it through columns packed with an adsorbent material — typically Fuller’s Earth — and return the cleaned oil to the unit. The adsorbent chemically captures and retains oxidation decay products from the oil stream. The oil is heated to its aniline point before re-entry into the transformer, enabling it to dissolve and extract byproducts that have been absorbed into the solid insulation. Through repeated recirculation, the paper is progressively cleaned and acid levels are durably reduced.

Determining the Required Number of Passes

The number of recirculation passes required is not fixed — it depends on the severity of the contamination and how long maintenance has been deferred. A minimum pass count is established for results that fall outside acceptable ranges. Units with acid numbers or IFT values classified as unacceptable, or those with a history of prolonged neglect, will require additional passes through the Fuller’s Earth columns. When maintenance is performed in a timely manner and contamination has not advanced significantly, hot oil cleaning delivers a durable, long-term solution for transformer life extension.

Summary:

• Oil changes do not address solid insulation contamination. Oxidation byproducts are polar and absorb preferentially into the cellulose paper — oil replacement removes only the fraction present in the fluid, leaving the bulk of the contamination behind.
• Byproduct absorption begins early. Research shows migration into paper starts when acid numbers exceed 0.05 mg KOH/g sample — often before operational concerns are apparent.
• Acid number rebound is predictable. After an oil change, byproducts migrate back from the paper into clean oil, restoring elevated acid levels within weeks.
• The same limitation applies to IFT. Oil replacement will not produce lasting IFT improvement when the root cause is byproduct saturation of the solid insulation.
• Hot oil cleaning is the appropriate corrective action. Circulating oil heated to its aniline point through Fuller’s Earth columns removes byproducts from both the oil and the paper over multiple passes.
• Pass count should reflect contamination severity. Units with unacceptable test results or deferred maintenance require additional recirculation passes to achieve full remediation.