The causes, hazards and treatment measures of lubricating oil foam in wind turbine gear box

December 22, 2023

The causes, hazards and treatment measures of lubricating oil foam in wind turbine gear box

The cause of excessive foam in lubricating oil can be divided into two categories according to its source: the cause of the lubricating oil itself and the cause related to the equipment.

The oil is in contact with the air in the environment, and the air will naturally dissolve into the oil. How much air can be dissolved in the lubricant depends on the temperature, pressure, and the solubility of the lubricant itself to the air. At atmospheric pressure, air in mineral oil generally accounts for 9% of its volume. With the increase of pressure, the amount of air dissolved in the lubricating oil also increases; When the pressure drops, part of the air is separated from the lubricating oil, reaching a new dissolution equilibrium. The separated air is surrounded by an oil film to form a bubble.

In addition, the air separation in the lubricating oil is also affected by temperature changes. Excessive foaming may be caused by poor anti-foaming performance of new oil after production, failure of Anti-foaming Agent during storage, or anti-foaming agent being filtered out by filter element. Lubricating oil in the process of aging deterioration, resulting in a decline in the surface tension of the oil, will also lead to foam. Lubricating oil contains a variety of additives with different properties, most of these additives are polar compounds, with surface active effect, to promote the oil to produce foam.

The gearbox in operation will generally part of the air into the lubricating oil, forming bubbles.

Common causes:

1) Suction tubing, oil pump valves and seals leak resulting in air inhalation;

2) The position of the return pipe is improper, and the oil flow is agitated and involved in the air;

3) The gear box is contaminated, such as water, other oils, liquids, chemicals and impurities;

4) Mixed with air when there is air in the pipeline or canned;

5) Low oil temperature, large viscosity, gas resistance occurs in the pumping oil supply system.

When the lubricant produces foam in use and is difficult to disappear, it usually has the following hazards:

(1) Increase the volume of oil products. Foam can not be eliminated in time to increase the volume of lubricating oil, overflow from the tank breathing hole, injection pipe; Or the oil level indicator will show false liquid level, resulting in insufficient oil supply, friction pair insufficient oil supply, will cause wear or sintering.

(2) Influence power transmission. On the one hand, it increases the compressibility of lubricating oil and affects the pressure transmission; On the other hand, it produces gas resistance, resulting in insufficient oil supply and affecting power transmission.

(3) The heat dissipation property becomes worse. The large amount of air contained in the oil affects the cooling effect of the lubricating oil and the heat dissipation effect of the machinery.

(4) Accelerated oxidation metamorphism. Increase the contact area between lubricating oil and air, accelerate the oxidation deterioration of oil products and the sludge deposition at the bottom of the tank.

(5) Cause cavitation of equipment. The foam carries a lot of air into the lubricating oil, which will cavitation the equipment.

The method of reducing foam in lubricating oil can be divided into two aspects, that is, physical mechanical anti-foam and chemical additive anti-foam. The overall idea is to prevent the formation of bubbles, reduce the tendency of bubbles to "bubble suppression" and eliminate the generated bubbles, reduce the stability of the foam "bubble".

There are many reasons affecting the foam of lubricating oil from the physical and mechanical point of view, such as the liquid level of lubricating oil, the change of oil temperature, the structure of the gear, the speed of the shaft and the sealing of the gear box, but most of these factors are determined by the actual working conditions, and it is difficult to eliminate the foam. From the perspective of daily operation and maintenance, it is feasible to control lubricating oil cleanliness, oil temperature change and liquid level height.

The cleanliness of lubricating oil is determined according to ISO4406 standard. There are many sources of impurities in the gearbox lubricating oil, and the control of impurities into the gearbox can be considered from the two aspects of the gearbox and the lubricating oil. For gearboxes at the wind turbine site:

1) When the gear box enters the factory, ensure that all the residual oil after the factory test is discharged;

2) Check the gear box filter and filter element regularly, if there are impurities to clean, if there is a rupture should be replaced in time;

3) Check whether the desiccant in the air filter is effective, and replace it in time if the effect is lost.

For lubricants at the wind turbine site:

1) Sampling of new oil, the appearance of lubricating oil should be observed when entering the factory, usually clear, transparent, no solid suspended matter, detection indicators mainly include viscosity, cleanliness, water content (no more than 0.03% mass fraction), etc., if not up to standard need to be returned to the factory for treatment;

2) The required lubricant cleanliness level needs to be agreed between wind turbine manufacturers, gearbox manufacturers and bearing manufacturers. During continuous operation, the stable cleanliness level of the gearbox lubricating oil shall not be lower than -/17/14, if it is not up to the standard, replace the new lubricating oil or replace the filter element.

The environment and the operation mode of the equipment are the main reasons for the rapid change in the temperature of the lubricating oil, and the temperature and viscosity are two factors that affect each other, usually the lubricating oil used in the gearbox of the wind turbine has a large viscosity, when the temperature rises and the viscosity drops to a certain range, the lubricating oil is most prone to foam. Too large or too small a viscosity range will reduce the foaming tendency and foam stability of the lubricating oil. When the viscosity is small, the liquid on the surface of the bubble film is easily lost, and the bubble wall becomes thin, resulting in the bubble bursting. When the viscosity is too large, it is not easy to produce foam, and even if the foam is formed, it is difficult to float to the surface of the lubricating oil. The heating and cooling devices can be added to the wind turbine site according to the actual situation to control the oil temperature and prevent the foam from the rapid change of the lubricating oil temperature.

Too high or too low oil level is easy to bring air into the lubricating oil, in order to ensure that the transmission system in the gear box is fully lubricated, the liquid level is generally not allowed to be lower than the minimum bearing roller or the index circle radius of the relevant gear, taking into account the splash lubrication situation, while ensuring the amount of oil bath lubrication, the liquid level must also be higher than the top circle of the high-speed pinion, should be regularly observed the oil mark, Ensure correct oil level.

Chemical anti-foaming method includes adding chemicals that react chemically with foamed substances or dissolve foamed substances and adding anti-foaming agents, etc., usually adding anti-foaming agents to oil products has the best effect and the method is simple, so it is widely used at home and abroad.

The local interaction between the anti-foaming agent with low surface tension and the directional bubble film leads to the destruction of the mechanical balance of the bubble film and the collapse of the bubble film. The specific microscopic process of defoaming is relatively complex, and there are many theories. The current mainstream view is that the antifoaming agent particles attached to the surface of the bubble film reduce the surface tension of the liquid film, and under the action of the surface tension, the foam bursts.

In addition, according to the Young-Laplace formula, the additional pressure is inversely proportional to the radius of curvature, and the pressure in the small bubble is greater than that in the large bubble. Therefore, the small bubble will exhaust to the large bubble through the liquid film, and the large bubble will become larger and the liquid film will further become thinner, and finally burst. The permeability of anti-foam agent enhances the permeability of air to the bubble film, accelerates the combination of foam, and thus achieves the purpose of bubble breaking.

Anti-foam agents in wind turbine gear box lubricating oil can be divided into three types, namely silicon-containing anti-foam agents, non-silicon-containing anti-foam agents and compound anti-foam agents, and the performance difference of different types of anti-foam agents is significant.

Therefore, considering that the wind turbine gearbox lubrication system is equipped with a filter device, in addition to meeting the anti-foam performance requirements, the selection of anti-foam agent should also pay attention to the dispersion diameter of anti-foam agent particles in the lubricating oil to prevent the anti-foam performance of the oil from being filtered.

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