As oil breaks down, a variety of issues can arise, including gelation. This is when the oil is no longer fluid and resists flow. As you can imagine, this leads to poor lubrication and potential machine failure. By understanding what causes gelation and which oils are most prone to this process, you can help to prevent it and ensure your equipment will run as efficiently as possible.
According to ASTM International, oil gelation is defined by a rheological condition of an oil characterized by a marked increase in the flow resistance over and above the normal exponential increase of viscosity with decreasing temperature, particularly at lower shear stresses and temperatures. ASTM D5133 is one method that can be used to analyze the tendency of oil gelation. In this test, a candidate oil is heated and then gradually cooled while the viscosity is measured at different temperatures.
The gelation index is a result of this test. As oil is cooled, the viscosity goes up. This value can be plotted on a chart. The slope of the viscosity line versus temperature is analyzed for changes. If the slope rapidly increases at a certain temperature, this should be noted. Once the test is completed, the full chart can be analyzed and the gelation index developed. The temperature at which the viscosity rapidly thickens is known as the gelation index temperature.
This information becomes important as temperatures decline, especially if the equipment operates in a cold environment such as in a freezer or outside in cold climates. Oils most prone to gelation are typically motor oils, particularly those with a higher wax content than others (paraffinic base oils). This phenomenon occurs in colder conditions or in situations where the oil has experienced a gradual cooling. As the viscosity increases due to lower temperatures, certain contaminants or conditions can be achieved in which the viscosity rapidly increases and the oil gels.
The American Petroleum Institute (API) has established standards for the gelation index in motor oils. In most cases, the maximum acceptable gelation index is 12, with a maximum viscosity of 40,000 centipoise. There is a certain point when an oil simply can’t be pumped due to either gelation or to viscosity that is too high. When viscosity increases, a limited flow condition can occur. This is when the volume of oil being pulled through the pump is lower than what is needed to adequately lubricate the motor.
If the oil gels or the viscosity becomes too high, it can lead to another condition known as air binding. In this condition, an air void is created within the oil in the sump. The oil is too thick to fill the void, and thus the pump just pulls in air. This adversely affects the health of the equipment, as it can lead to boundary conditions, excessive wear and ultimately premature failure.
Other methods can also be used to test an oil’s cold-temperature characteristics. ASTM D3829 is the standard test method for predicting the borderline pumping temperature of engine oil. With this test, the goal is to identify the temperature at which an engine oil can no longer be pumped. The test results can indicate whether a candidate oil remains fluid enough at certain temperatures or if a different oil should be selected.
Gearsets are another area where the gelation or cold-temperature characteristics of an oil become important. Gear oils generally have a high initial viscosity, which leads to a much higher viscosity at low temperatures. Studies on gear oils lubricating wind turbine gearboxes have shown that these oils can become quite cold at several hundred feet above the ground and in cold climates. The cold temperatures coupled with moisture contamination resulted in the formation of gels in some of the in-service gear oils. This condition can be just as detrimental to the health of the gearbox as the motor oil condition discussed previously.
Several factors should be taken into account when determining how well a lubricant will work at colder temperatures and the probability of gelling at these temperatures. These include the base oil, wax content, pour point and the base oil’s refining process. All of these will have a marked impact on gelation and the lubricant’s cold-temperature characteristics.
If your equipment is operating in extreme cold temperatures, you should consider the base oil used in the lubricant. Mineral base oils have a wide operating temperature range but are often discarded in favor of comparable lubricants with a synthetic base oil. Synthetic oils generally have a higher viscosity index, which means they will remain more fluid in cold conditions and thicker in hotter temperatures.
For machines that require mineral oils, take note of the API base oil category or how refined the base oil is. Crude oils from the ground naturally have a bit of wax in them, which can negatively affect the oil’s tendency to gel in cold temperatures. The majority of this wax can be removed through refining. During the dewaxing process, the wax content is reduced or the wax structure is converted to a different structure with better properties. The cold-temperature characteristics are also improved. Typically, the more refined a base oil, the higher the viscosity index and the better the low-temperature properties.
API base oil Groups II and III have lower volatility and lower pour points. When in doubt about which API group a particular base oil falls into, contact the oil manufacturer or consult the technical data sheets.
An oil’s pour point is another property that should be analyzed before selecting a lubricant to be used in cold environments. The pour point is the temperature at which the oil will no longer flow due to gravity. As an oil is cooled, waxes remaining in the oil begin to crystalize and congeal together, making the fluid more solid until it stops flowing. Even oils that are virtually free of wax will have a pour point associated with them. If choosing a lubricant for a machine that will operate in extremely cold environments and there are several oils with the same properties except for the pour point, pick the one with the lowest pour point to avoid issues stemming from reduced flow at cold temperatures.
Synthetic base oils are synthesized from different compounds and mostly have no waxes in them. They also have a lower pour point than mineral oils and are often selected for cold environments due to their higher viscosity index and lower pour point. However, synthetic base oils are still at risk of gelation if they become contaminated with certain contaminants such as water and glycol. Routine oil analysis should be performed to look for these common culprits.
When it is impossible to find an oil that stays thin enough in cold environments, a common solution to avoid the pitfalls of restricted flow or excess viscosity is to install a lube oil heater. These types of heaters can keep oil warm enough to allow it to flow as well as reduce the overall system pressure when the oil would otherwise be too thick to pump. Low-wattage heaters, electric blankets and steam piping are popular heating accessories designed to help the oil remain at a consistent temperature. If you are planning to utilize a heater, make sure it doesn’t heat the oil too much, as this can degrade the oil and shorten its life. If steam is used, the oil should be routinely inspected for water ingression.
The tendency of an oil to gel at specific temperatures and with certain contaminants isn’t always a bad thing. In fact, this property has been used in cleaning up spilled oil, particularly oils spilled in large bodies of water. The Environmental Protection Agency employs “gelling agents” to form gels in spilled oil while not reacting with the water. These agents are blended into the oil slick through mechanical agitation or through the action of the waves in the body of water. Once the oil gels, the agents can then be easily removed by skimming or any other form of separation.
While not all oils perform the same way with contamination and cold temperatures, most problems associated with the thickening of lubricants can be avoided with a good contamination control program and by selecting the right lubricant for the application. Routine oil analysis can help detect issues related to oil gelation before any significant machine damage occurs. If temperature-control devices such as heaters must be used, inspect them often for any signs of faults. Also, check the oil temperature to avoid overheating. With proper attention and care, the lubricants you use in cold temperatures can provide a long service life with few if any difficulties.
|36%||of lubrication professionals have seen the effects of oil gelation in machines at their plant, based on a recent survey at MachineryLubrication.com|