What are the lubrication methods for centrifugal pump bearings?

In the case of centrifugal pumps, the bearings carry the loads associated with pumping, while lubrication is the process by which friction and energy losses are minimized in the bearings. Without proper lubrication, the heat generated by friction will cause premature wear or failure of the bearings. The most important qualities of a lubricant are its heat resistance and viscosity, which determine its most ideal application area and method. In order to properly lubricate a bearing, a protective film must be maintained at all times during operation.

Figure 1: Manual grease application
For horizontal pumps with rolling element bearings, lubrication is typically either grease or oil. For grease lubrication, grease is packed into the bearings and periodic re-lubrication is usually required (see Figure 1). Oil lubrication has a variety of methods, but like grease, maintenance is required to ensure lubrication properties are maintained. Grease lubrication is typically used for low horsepower (power) applications. Grease lubrication prevents moisture/water ingress, has a simpler maintenance process, and may be more suitable for stop-start applications.

Figure 2: Oil-bath lubricated bearing soil-bath lubrication (see Figure 2)
Oil-bath lubrication can be accomplished by maintaining the oil level just above the outer race of the bearing, or by splash lubrication with one or more oil rings rotating on the shaft and splashing oil from the reservoir onto the bearing. In these types of lubrication, it is critical to maintain an optimum level of lubricating oil in the bearing housing. Too much oil can result in a reduced ability of the outer bearing race or oil ring to distribute oil to the bearing, oil foaming, and excessive friction. Too little oil can result in the bearing not receiving adequate lubricant and can lead to rapid failure. Thin oil lubrication can also be accomplished by forcing atomized oil (i.e., oil mist) into the bearing housing.

Figure 3: Pure oil mist lubrication (left) and purge oil mist lubrication (right)

Oil mist lubrication can be divided into pure oil mist lubrication and purge oil mist lubrication (see Figure 3). Both involve the same main process, in which a continuous, constant oil mist is applied directly to the bearing while it is in operation. However, purge oil mist lubrication uses an oil reservoir inside the bearing housing to form a wet oil pool. The advantages of pure oil mist lubrication are that it can be used at lower operating temperatures and consumes less oil than oil bath or splash lubrication, while also generating fewer wear particles in the bearing. Purge oil mist lubrication can also be used at lower operating temperatures than oil bath/splash lubrication, and the advantages of purge oil mist lubrication compared to pure oil mist lubrication are that the oil level is clearly visible and maintenance is easier.

There is another lubrication method, "pressure oil lubrication". API 610 standard stipulates that if the energy intensity of the pump (i.e. the product of the pump's rated power kW and rated speed r/min) is 4 million or more, liquid radial sliding bearings and thrust bearings should be used. In this case, the pump bearings need to be lubricated with pressure oil. I will not go into details here.