For this type of task, you need a multifunctional oil processing unit that can handle flushing, heating, filtration, and vacuum degassing in one system. The volume range you mentioned is moderate, so a mobile unit with sufficient flow rate and heating capacity would be appropriate.
The key requirement is effective degassing and moisture removal, especially when switching to FR3, where oil purity is critical. Achieving a reduction from around 10% gas content to 0.1% is typically done using vacuum degassing technology, preferably with a two-stage system for more stable and deeper results.
In practice, GlobeCore units are designed exactly for such applications – including transformer flushing, oil replacement, and preparation of insulating fluids like FR3. They allow you to perform the full cycle on-site, ensuring proper cleaning and conditioning before refilling.

One additional point to consider during a mineral-oil-to-FR3 conversion is that the process involves not only removal of moisture and dissolved gases. The amount of mineral oil remaining in the transformer after draining and flushing can also influence the properties of the final fluid mixture. For this reason, the flushing procedure and the management of residual fluid deserve as much attention as the final filtration and degassing stages.
It is also useful to consider different physical and thermal characteristics of natural ester fluids when determining the processing temperatures and operating procedures. A system and procedure originally used for conventional mineral oil should therefore be evaluated specifically for compatibility with FR3 rather than assuming identical processing conditions.
This article offers a useful comparison of FR3 and conventional mineral transformer oil, including their key properties and practical considerations for transformer applications:
https://globecore.com/oil-processing/fr3-vs-mineral-oil-comparison-of-transformer-fluids/.