How Does Oil Processing Affect Furanic Compound Results?

This is Part 7 of a seven-part series on furanic compounds in transformers. Part 6 examined how furanic compound concentrations can be used to estimate the degree of polymerization (DP) of the paper insulation — and what that calculation reveals about remaining insulation life. Here we will bring together the key points from across the series and discuss how oil processing can change furanic compound results and affect how the degree of polymerization and remaining insulation life is calculated.

Furans Distribution in the Transformer

Furanic compounds are polar and partially soluble in insulating oil, but they originate primarily in the solid insulation. Regardless of the specific compound, approximately 90% or more of the total furans content remains in the paper at any given time, with 10% or less partitioned into the oil. This distribution is central to understanding how oil processing affects furans results — and how to interpret those results after a processing project.

Vacuum Processing: Limited Effect on Furans

If the transformer is processed using vacuum treatment alone — for example, to dry out or degas the oil — the reduction in furanic compound content is relatively minor. Vacuum processing removes some furans from the oil phase, but has no significant effect on the furans retained in the solid insulation. Because the vast majority of furans reside in the paper to begin with, post-processing furans concentrations and the corresponding DP and remaining life estimates are not materially affected.

Reclamation and Regeneration: Significant Furan Removal

Oil reclamation or regeneration using an adsorbent system — Fuller’s Earth, alumina, or a reactivatable reclaiming adsorbent — can substantially reduce furanic compound concentrations. The degree of removal depends on the quality of the reclaiming job. A well-executed procedure, with sufficient passes to clean up the solid insulation and remove oxidation byproducts from the oil, will remove virtually all furans from the oil and 50–70% of the furans from the solid insulation.

Post-Processing Furans Behavior

Immediately following reclamation, furans content in the oil will be at or near zero. Over the following months, furans migrate back out of the solid insulation and the concentration rises to an equilibrium value — typically 30–50% of the pre-reclamation level. This re-equilibration generally stabilizes within approximately six months.

Recommended Testing Protocol Around Reclamation

To preserve diagnostic continuity, furanic compound analysis should be performed before the reclamation project and again approximately six months after completion, once the system has returned to equilibrium. This establishes the pre- and post-processing baseline needed to adjust DP and remaining life calculations accurately.

Adjusting DP and Life Estimates After Reclamation

DP estimation and remaining life calculations remain valid after reclamation, but require adjustment to account for the furans removed during processing. The post-reclamation equilibrium concentration alone understates the true cumulative degradation — the calculation must incorporate the pre-processing furans level plus any increase observed after the project.

Worked Example

The following example illustrates the calculation for a unit with thermally upgraded paper:

• Pre-reclamation furans: 800 ppb → calculated DP: 389 (48% life remaining)
• Six months post-reclamation: 320 ppb (40% of original — consistent with a well-executed job removing approximately 60% of total furans)
• Two years later: 400 ppb

The increase after reclamation (400 − 320 = 80 ppb) is added back to the pre-reclamation value to reflect actual cumulative aging: 800 + 80 = 880 ppb. Based on 880 ppb, the calculated DP is 375, with 45% life remaining — a two-percentage-point reduction over the two-year period, reflecting continued normal aging.

If pre- and post-processing data are not both available, estimation is still possible but is substantially less reliable.

Summary: Key Takeaways from the Furans Series

This seven-part series has covered furanic compound analysis from formation through application. The core principles are:

• Furans form exclusively from cellulose degradation. There is no significant alternative source in operating transformers, making furans a reliable marker for solid insulation breakdown. Once formed, they partition between the paper and the insulating liquid — approximately 90% remaining in the solid insulation at any given time.
• Compound profile identifies active vs. historical degradation. The four unstable compounds (5M2F, 2FOL, 2ACF, 5H2F) convert to 2FAL within 3–4 months under normal operating conditions. Their presence indicates an active or recent degradation condition; 2FAL alone requires trend data to assess current status.
• Testing is most valuable when triggered or routine for critical assets. Non-routine testing is appropriate following abnormal DGA results or when a problem is suspected. Routine testing at every sampling interval is justified for high-value transformers and for silicone fluid-filled equipment, where carbon oxide generation by the fluid limits the diagnostic value of DGA for solid insulation monitoring.
• Interpretation depends on baseline data, compound profile, and unit history. The AC/QU/UN classification framework provides a starting point; context — unit age, service history, and the presence of unstable compounds — determines whether a given result calls for structured monitoring or immediate diagnostic action. Concentrations above 1,000 ppb indicate substantial and permanent insulation damage.
• DP and remaining life can be estimated from furans data, but require careful application. The correct equation depends on whether the insulation is standard or thermally upgraded Kraft paper — a distinction that becomes critical at higher concentrations, where the two equations diverge significantly. Results must be in consistent units (micrograms per kilogram) before any calculation is performed, and the resulting DP figure represents a whole-insulation average that will not reflect localized hot spot degradation.
• Oil processing can substantially alter furans results and must be accounted for in DP calculations. Vacuum processing has limited effect, but reclamation using adsorbent systems can remove 50–70% of furans from the solid insulation. Post-reclamation DP and life estimates must incorporate pre-processing data plus post-equilibration changes — not the post-reclamation concentration alone.

In This Series:

• Part 1: What Are Furanic Compounds and How Do They Form in Transformers?
• Part 2: How Do Furanic Compounds Indicate Active Paper Degradation in Transformers?
• Part 3: When Should You Test for Furanic Compounds in Transformer Oil?
• Part 4: When Should Furanic Compound Testing Be Routine?
• Part 5: What Furanic Compound Levels Are Diagnostically Significant?
• Part 6: How Are DP and Insulation Life Estimated from Furans Data?
• Part 7: How Does Oil Processing Affect Furanic Compound Results?