- the amount of calcium and magnesium in dry silica clay should be no more than 2.2% and the amount of silic acid should be no less than 32.5%;
- the apparent density of dry grits should be no more than 0.65g/cm3;
- adsorption activity should be no less than 45%;
- the size of the main grit grains should be between 3 and 7 mm and the content of grit grains with a size of 3 mm should not exceed more than 5% of the total volume;
- no foreign impurities; and
- humidity should not exceed 10%.
Transformer oil is used in electric power transformers, high-voltage bushings and oil circuit breakers to transfer heat from the heated parts of the transformer. Additionally, the oil is used as a liquid insulation to help insulate internal conducting parts of the transformer’s windings and core. Transformer oil protects the solid insulation from moisture by filling all the space inside the transformer’s oil storage tank and serves as a diagnostic tool in determining the condition of the transformer. In switching devices, oil is necessary to quench an unwanted and dangerous arc. Since oil filled electric devices are usually operated under heavy load conditions, transformer oil is exposed to a wide range of unfavourable factors such as: (1) temperature extremes; (2) atmospheric oxygen; and (3) flooding. All these factors, either separately or together, will lead to the degradation of the performance characteristics of the insulating fluid and the solid insulation. Transformer oil can degrade quickly based on adverse conditions and consequently will become unable to perform its intended functions. All this may result in serious consequences: (1) accidents; (2) expensive equipment failures; and (3) unplanned outages. Transformer oil samples therefore, should be taken regularly and analyzed to prevent such consequences. The degradation of the rated values of transformer oil requires the oil to be dried, purified, and degassed to remove mechanical impurities, water, gasses and sludge. One of the most effective methods of transformer oil purification is through the use of adsorbents. They are substances that are able to absorb the products of aging and moisture from insulating fluids. Such natural materials as silica clay, bleaching clays, amorphous coal and activated carbon, silica gels, aluminum oxides and zeolites serve as adsorbents. Natural adsorbents are eco-friendly and they are less expensive to obtain than artificial ones. They have a high specific porosity and a high ability to perform adsorption. Due to these characteristics, natural adsorbents are widely used in the industry. Silica clay is a natural material with high stability and high porosity. Additionally, it does not swell when wet and is resistant to acids and alkalis. Silica clay includes finely grained amorphous silica (76-88%) and a mixture of clay, sand and glauconite. This rock may be of white, grey or even black color. Microscopic studies have shown that silica clay is formed from opal grains and the remnants of silicon skeletons of organisms, consolidated by siliceous substances. One of the hypotheses of silica clay origin states that this material resulted from the structural transformations of diatomites and other suchlike materials. Usually, a natural sorbent derives its name from the location of its deposits. The bleaching characteristic of silica clay depends on the silica/alumina ratio: the higher the ratio, the higher the adsorption characteristics. Silica clay is recommended to be stored only in special dry rooms when humidity is controlled. Prior to shipment, the substance grits are exposed to a thermal activation at between 150ºС and 200ºС with a subsequent sieving of small grains with a size of 3 mm on a special sieve. The latter has 16 holes on 1 cm2 of the surface. Silica clay used for transformer oil purification should meet the following requirements: