What steps are involved in transformer oil processing and purification?

Processing usually includes pre filtration to remove solids, heating to reduce viscosity, vacuum dehydration to remove dissolved and free water, vacuum degassing to remove air and fault gases, and fine filtration to improve cleanliness. Some processes include adsorption or clay treatment to reduce acidity and polar compounds, and inhibitor adjustment if required. Oil is then cooled, sampled, tested, and transferred using sealed lines to avoid re contamination. Final acceptance is based on standards for BDV, moisture, acidity, interfacial tension, and resistivity.