Oil mist and air-oil. Here, oil is atomized and carried by an air stream to the bearing. Among all pump bearing lubrication approaches, this one generates the least amount of friction (allowing rotational speed to be based on the bearing design instead of lubrication limitations) and creates a positive pressure within the bearing housing (fending off invasive contaminants).
Regardless of lubrication method, always specify lubricant according to the demands on vertical shafts and resistance to solids, pressure, temperatures, loads and chemical attack. Where pump locations may be difficult to access, fully automatic systems can be integrated to enable timely, proper and effective delivery.
Sealing the system
Bearing seals in centrifugal pumps handle four crucial tasks. They retain lubricants or liquids, exclude contaminants, separate fluids and confine pressure. So, the seals have their work cut out for them. Solid contaminants, depending on particle size, hardness and brittleness, will produce either indentations or wear on the bearing surfaces; water will affect the efficiency of the lubricant; and contamination in the lubricant can dramatically reduce bearing life.
When a seal fails, contamination can infiltrate the bearing area, entering the lubricant and then the bearing. In addition, loss of lubricant from the bearing can lead to dry-running operation, which ultimately can cause bearing failure.
The choice of seal for centrifugal pump bearings depends on the unique demands and operating conditions of the application. Keep in mind, though, that the bearing and sealing arrangement represent an integrated system.
Dynamic radial seals generally are the best choice for centrifugal pumps. These seals create the barrier between surfaces in relative motion (one usually stationary while the other rotates).
Typically, radial shaft seals incorporate:
Steel or elastomer shell. This is bonded to the sealing material and enables the requisite interference fit of the seal in the housing bore to be maintained (as well as facilitates proper installation); and
Elastomer sealing lip. Usually installed against the shaft, this provides dynamic and static sealing against the shaft and features a sealing edge formed by pressing, cutting or grinding. Most sealing lips are made from a formulation of nitrile rubber.
However, materials specifically for use with fuels, industrial fluids and highly compounded lubricants are available.
Seal selection ultimately must be based on a thorough review of application parameters and environmental factors. Particularly in pump applications, for example, seals will be exposed to relatively constant pressure differentials — making pressure seals (in which the seal cavity is pressurized) the preferred choice.
Due to the nature of their design and the harsh operating environment, seals usually provide a much shorter life than the components they protect. So, don’t fall into the common but unfortunate habit of scheduling seal replacement only at intervals dictated by requirements of other components such as bearings. Many bearing failures can be prevented if seals are replaced when the first signs of wear or leakage have been detected.
To get an inkling of how long seals should last, consider the amount of contamination a seal will encounter, cycle times, speed and heat. Of course, contamination isn’t the only reason for premature seal failures. Others culprits can include:
- poor selection;
- improper installation, resulting in leakage; and
- change of lubricant, causing adverse reactions in common sealing materials.
To head off problems, institute a rigorous seal inspection and replacement program.
Don’t automatically replace a failing seal with another of the same design. For example, if an oil analysis shows higher-than-anticipated ingress of contaminants, it may be time to upgrade the entire sealing arrangement (perhaps using a more chemically resistant material or adding elements to bolster the sealing system). Or, in cases of excessive wear, it may make sense to switch to a non-contact labyrinth seal design.