Thinking About a Change?

A three-step approach can simplify pump bearing lubrication, reducing the number of oil sump-related lubrication changes --- and extending equipment life

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Although many types of lubrication methods are available for bearings in horizontal process pumps, the most widely used method is still oil sump lubrication.

The most common form of bearing lubrication is direct contact. As the shaft rotates, the rolling elements in the bearing ," typically steel balls ," make contact with a controlled level of oil.

Although some debate exists regarding the most effective depth of contact, the amount of contact between the rolling element and the oil is not generally considered a specific measurement. The most important considerations are speed, oil viscosity and load.

It is critical that an effective oil film be maintained between the rolling element and the race of the bearing. Only enough contact between the bearing and the oil surface as necessary to "load" the bearing with lubricant is required.

If the lubricant level is too high or too low, excessive heat will be generated, accelerating oil degradation and shortening bearing life. When the oil level is too high, a condition known as "churning" occurs. Similar to what occurs through the use of an egg beater, air is "whipped" into the oil. This, along with the induced heat, increases the oxidation rate, shortening the effective life of the oil.

When the oil level is too low, contact is not sufficient to lubricate the bearing or act as a heat sink to carry away the normal heat levels generated by the bearing.

Critical lubrication elements

The most critical elements of lubrication are quality and quantity. Without one, the other is affected negatively. The proper quantity of low-quality oil is no better than an insufficient quantity of high-quality oil. Plants should select the best oil for the application.

Quality. In basic terms, lubrication quality can be looked at in two ways: how the lubricant can become contaminated and how the lubricant can degrade. Although contamination is widely recognized for its effect on the quality of oil, degradation can be just as damaging to equipment. The leading causes of contamination are particulate matter, moisture, incompatible fluids and air entrainment. The leading causes of degradation are oxidation, heat and regular use.

Quantity. The proper quantity of oil is perhaps even more important than the quality of the oil. Oil sump lubrication does not require a specific level to be maintained for proper bearing "loading." However, if the oil level in the sump reaches critically low or high points, damaging conditions could occur. In a low-level operating condition, the bearing will not receive enough lubricant for proper film strength ," a precursor to surface contact, skidding and possible catastrophic failure. In a high-level operating condition, "churning" of the lubricant will occur, accelerating the oxidation rate as a result of excessive air and elevated temperatures.

 

New on-line monitoring devices measure water contamination by percentage or SRH.

 

Specially designed containers can decrease the likelihood of contaminated oil introduction.

Poor lubrication: the culprits

Contamination, oil degradation, oil starvation and excessive lubrication all can adversely affect oil sump lubrication.

Contamination. Particle contamination is possibly the most well known form of lubricant contamination. This form is considered the cause of component part wear, silting and surface fatigue. In a study performed by the National Research Council of Canada, nearly 85 percent of contaminant-related wear of components and surfaces was found to be particle induced. To make matters worse, particle contamination can create more particles ," and more wear.

Lower particle counts significantly extend the life expectancy of equipment. By reducing contamination levels from ISO 21/18 to ISO 11/8, plants can extend the life of a 50-gallon-per-minute (gpm) pump by a factor of seven.

Particle contamination can occur from ingress from the surroundings, improper cleaning of the bearing housing during maintenance cycles, or corrosion products from the high water content in the oil.

Water contamination can cause several problems. Because each type of oil has its own unique "safe" water level, the common practice of measuring parts per million (ppm) is not conclusive.

Significant differences exist among oils, beginning with mineral and synthetic bases. Additive packages, commonly referred to as "ad-pacs," also can make a difference in how much water an oil can hold before phase separation occurs ," and free water forms. Temperature also plays a major role in how much water oil can hold. Damaging levels of water, or "free water," begin to occur in some mineral-based oils between 400 ppm and 500 ppm at 140 Degrees F. Free water might form at 200 ppm at 125 Degrees F in the same oil.

Therefore, it is important to know the saturation point of an oil at a given temperature to begin to determine a valuable setpoint for effective lubrication maintenance. Sponges provide a simple way to illustrate this. Different sponges can hold varying amounts of water. A dense-cell sponge can hold more water than an open-cell sponge, even though both have the same cubic volume.

Oil degradation. The primary causes of oil degradation are high heat, air entrainment and the mixing of incompatible fluids. Increased viscosity (thickening) is one of the results of this degradation. This usually happens over time, and varies by the combination of these elements.

Viscosity is the single most important property of a lubricant. To more fully understand the significance of viscosity, it is necessary to understand how a lubricant works. The primary functions of a lubricant are to reduce friction and wear. To perform these functions, a protective oil film is required. The three basic oil film conditions are referred to as full film, elastohydrodynamic (EHD) and boundary layer.

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