Oil Degassing
Fluidex 24 December 2024
Oil degassing for the removal of harmful impurities helps extend the service life of both oil products and the components they lubricate. For example, transformer oil should be purified annually. Degassing, dehydration, and regeneration are typically performed every five years. Otherwise, its cooling capacity and fluidity may decrease significantly, potentially resulting in the need for transformer repair.
Classification of Industrial Oils
- Hydraulic oils: used as an energy carrier in hydraulic systems of hoists, cranes, and other equipment.
- Gear oils: developed for use in enclosed gearboxes. They are resistant to oxidation and possess demulsifying and anti-foaming properties, which significantly reduce friction and mitigate the risk of scoring on gearbox components.
- Lubricating oils: designed for lubrication and widely used in transfer cases, angle reducers, and other mechanical systems.
- Compressor oils: used in compressors of various types. Their main function is cooling and lubrication. This group of lubricants includes oils designed for piston and screw compressors.
- Circulation oils: used in units with closed forced lubrication systems.
- Turbine oils: designed for centrifugal turbines and turbochargers. They help reduce wear and friction while ensuring effective heat dissipation.
- Heat transfer oils: used to transfer heat from one system to another. They are applied in heating and cooling systems where efficient heat transfer is required.
- Transformer oils: provide insulation, cooling, and heat dissipation for transformers operating at high voltages and generating significant heat during operation. Transformer oils are typically mineral-based or synthetic oils selected for their electrical insulating properties and their ability to withstand high temperatures and voltages.
Causes of Gas Formation in Transformer Oils
The lifespan of a transformer is significantly longer than the service life of its oil. A transformer can operate for 10 to 15 years without major repairs, while the oil inside typically requires purification after about one year and regeneration after 4 to 5 years. Over time, the oil in transformers undergoes substantial stress during operation, which gradually deteriorates its quality. In order to extend the useful life of oil and prevent premature failure of transformer components, several measures are implemented. These measures include protecting the oil from contact with external air by installing expansion tanks with filters that absorb oxygen and water, as well as removing air from the oil.
Before being filled into a sealed transformer, the oil undergoes degassing to remove dissolved gases. The "total gas content" refers to dissolved gases in oil, which predominantly include nitrogen, oxygen, and carbon dioxide. Air solubility in oil is quite high, with air accounting for approximately 10 to 12% of the oil volume at 77°F (25°C). If the air content of oil reaches saturation levels, it can impair the transformer insulation, possibly leading to failure. Oxygen accelerates the aging of both the insulating oil and the solid insulation; for that reason, oil degassing is performed to ensure that the gas content of oil is reduced to safe levels — typically to 0.1% before filling and 0.2% after the oil has been fed into the transformer.
Gases associated with Specific Defects in Transformer Oil
One of the main negative factors affecting the performance of transformer oil is the dissolution of gases in the oil. Dissolved gases can reduce the dielectric strength of the oil by 20–30%. If gas bubbles form in the oil, this effect becomes even more pronounced, further compromising the insulating properties of the oil and increasing the risk of failure. Temperature fluctuations, intense electric fields, and excessive oil flow through pipes all contribute to the formation of gas bubbles.
When oil containing dissolved gases comes into contact with transformer paper insulation, it accelerates its degradation. In some cases, the impact on the paper insulation can be even more severe than the effect of oxidation products in the oil. While the detrimental impact of dissolved gases on transformer oil is well understood, less is known about the solubility of different substances in insulating liquids. For example, it has been established that the gas content of the oil is directly related to the pressure of gas above the liquid, and that higher temperatures increase the solubility of gases in the oil. However, the solubility of gases can vary significantly between different types of oil, and even within the same oil, gases dissolve at different rates.
Gases are not only absorbed from the surrounding environment, but are also generated during accelerated aging of the insulation. High temperatures in transformers can cause the oil to decompose, forming the gases such as methane, ethane, and hydrogen, all of which are highly soluble in insulating liquids. If the impregnated paper insulation breaks down, the gases such as carbon dioxide, hydrogen, and carbon monoxide may enter the oil. Moreover, partial discharges in the insulation can generate gases within the oil. The main cause of oil oxidation and degradation is the presence of air, so timely degassing can significantly slow down these processes and help maintain oil quality, ultimately extending transformer service life.
Specific gases are associated with certain defects in transformer oil. For example, hydrogen is often present in the case of electrical defects such as partial discharges, sparking, or arcing. Acetylene is associated with sparking, electrical arcing, or temperatures exceeding 1,292°F (700°C). Ethane indicates thermal heating of insulating oil and paper insulation at temperatures up to 572°F (300°C), while ethylene forms when oil and insulation are exposed to temperatures above 572°F (300°C).
Methods of Oil Degassing
Degassing of industrial oils involves mechanical, physical, and chemical methods, each offering its own set of advantages and limitations.
Mechanical methods of degassing, such as centrifugal separation and filtration, rely on separation technologies to remove gases from the oil. For example, centrifugal methods are used in hydrocyclones and centrifuges, where centrifugal forces help separate gas bubbles from the liquid. Filtration, on the other hand, effectively purifies the oil by removing mechanical impurities and can help eliminate up to 90% of dissolved gases. While effective, these methods may require regular maintenance and have limitations in the complete removal of dissolved gases.
Chemical methods involve the use of agents that react with dissolved gases in the oil. While they can be effective, chemical degassing is less commonly used because of potential chemical residues and the need for careful monitoring during application.
Physical methods, such as vacuum degassing, work by lowering the pressure around the oil to allow the escape of dissolved gases. This technique is highly effective and commonly used in oil purification systems, particularly for large-scale oil treatment operations. However, it can be costly due to the need for specialized equipment.
Degassing Systems for Industrial Oils (FLD D)
Conventional methods for dehydration of transformer oils typically ensure degassing to the extent that only traces of gases remain in the insulating liquid, which can be difficult to measure. This oil can absorb gases during transformer operation, especially if the transformer lacks a sealed membrane. Even the presence of an expansion tank or a protective inert gas cushion that is soluble in the oil does not always resolve the issue.
In practice, oil degassing is effectively achieved with the use of FLD D type systems. These systems allow vacuuming of transformers and removal of gases, water, and mechanical impurities from insulating oil. The systems are designed for thermal vacuum dehydration, degassing, heating, filtration, and pumping of transformer and turbine oils. They are widely used at power stations, substations, and other energy enterprises involved in the installation, repair, and maintenance of power transformers and other oil-filled equipment.
Modes of Operation:
- Transformer Heating
- Oil Degassing
- Transformer Vacuuming
Gases cannot only enter the oil from the surrounding environment, but they can also form during the accelerated aging of insulation. This is often caused by excessive temperature in the transformer. When the oil decomposes due to overheating, gases such as ethane, methane, and hydrogen begin to form. These gases are highly soluble in the insulating liquid. If the impregnated paper insulation breaks down, carbon dioxide, hydrogen, and carbon monoxide can also enter the oil. Additionally, gases can form in transformer oil due to partial discharges in the insulating paper.
The FLD D systems provide a reliable solution for maintaining the purity and stability of oils used in industrial systems.
