Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and loads caused by couplings and drive systems. They also keep axial and radial shaft deflections within acceptable limits for the impeller and shaft seal.
Handling all these functions—often in difficult process conditions—bearings are subjected to adverse forces that can potentially reduce their service life and reliability.
Radial shaft seals serve as critical elements within a pump’s bearing and sealing system. Seals will effectively offer a “first line of defense” for bearings in pump applications. They retain lubricants or liquids, exclude contaminants, separate fluids and confine pressure.
With pump bearings and shaft seals continually tested as an integrated system, the industry responded to the many challenges with solutions engineered to withstand even the most difficult operating conditions.
Bearing Operating Conditions
Centrifugal pump bearings are typically subjected to high axial loads, marginal lubrication and high operating temperatures and vibration—all while attempting to minimize friction. Friction, if uncontrolled, can result in power loss, excessive heat generation, increased noise and/or wear, and premature bearing failure.
These influences can dramatically impact the service life and reliability of bearings and pumps. Bearings (types, designs and arrangements) should be evaluated in the context of their anticipated operating environment.
For example, angular contact ball bearings are usually specified as pump thrust bearings to support axial loads created by the hydraulic forces in a pump. Each variation will carry particular benefits for an application.
Angular Contact Ball Bearings
Single-row, 40-degree angular contact ball bearings represent the most popular American Petroleum Institute (API) pump thrust bearings currently in service. They are generally used in moderate-speed centrifugal pumps in which high thrust loads can be anticipated.
The bearings are normally mounted in back-to-back paired arrangements to accommodate reversing thrust loads and to provide adequate shaft support. This promotes long mechanical seal life.
Versions designed with robust, machined brass cages can run particularly well in applications in which thrust loads vary greatly during operation and periods of ball skidding are likely.
These bearings also resist destructive vibration forces when cavitation occurs. Double-row, angular contact ball bearing arrangements are used extensively as the primary thrust bearing in American National Standards Institute standard centrifugal pumps and some older API-style pumps.
The most effective types feature a Conrad-design; Annular Bearing Engineer’s (ABEC) 3 (P6) precision tolerances; 30-degree contact angle per row; one-piece, heat-treated pressed steel cages; and multiple sealing options.
Because their contact angles diverge outwardly, the bearings exhibit greater rigidity and increased resistance to misalignment.
Design variations of these types are increasing, including:
- Steeper 40-degree contact angles to deliver increased thrust capacity
- Machined brass cages to impart robust performance in heavy-duty and poor lubrication conditions
- Reduced axial internal clearances to promote load sharing between the two rows of balls and a reduced possibility of skidding in the inactive ball set
- ABEC-3 (P6) tolerances to gain better control of the bearing’s mounted condition and promote smoother bearing operation
Specialized angular contact ball bearing sets are designed for centrifugal pumps with heavy thrust loads that are not reversing or those that only reverse periodically. High-performance matched sets of 40-degree and 15-degree angular contact ball bearings can provide improved robustness in high thrust load conditions by reducing the susceptibility of ball skidding in the inactive bearing.
For pumps with minimal thrust, an alternative arrangement of paired 15-degree angular contact bearings would be appropriate.
The primary advantage of these sets is that the 15-degree bearing offers considerably less internal clearance compared with a 40-degree bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while delivering additional radial stiffness to maintain the integrity of a pump’s shaft and seals.
Split inner-ring angular contact ball bearing arrangements are commonly used in vertical pumps to handle the primary thrust load. They can also be used in horizontal arrangements if the loading is such that the split inner ring bearing does not support radial load on its own.
This bearing set integrates a split inner-ring ball bearing or four-point contact ball bearing to accommodate thrust loads in either direction, matched with a single row 40-degree angular contact ball bearing. Because two bearings acting in tandem share the thrust load, this arrangement provides an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the backside of the split inner-ring bearing.
Other bearing solutions for pumps benefit from advanced materials. Hybrid bearings are equipped with balls or rollers made of silicon nitride, a ceramic material with characteristics that make it a good choice for pump applications requiring high speed, high stiffness and electrical insulation.
Bearings with injection-molded, glass-reinforced polyetheretherketone (PEEK) combine strength and flexibility with resistance to high temperature, chemicals and wear. PEEK cages are ideal for pumps in which the bearings are lubricated with light hydrocarbons, liquefied gases (cryogenic) or fire-safe hydraulic fluids and will often be integrated with hybrid bearings or with super-tough stainless steel rings. The combination offers protection from damage caused by poor lubrication and exposure to chemicals, creating reliable, robust and low-energy pump designs.
Sealing the System
Bearing seals in centrifugal pumps must prevent potential contamination and loss of effective lubrication or lubricant properties. Without effective sealing, contaminants (solid or otherwise) may infiltrate the area of a bearing, invade the lubricant and bearing, and shorten bearing life.
Solid contaminants (depending on the particle size, hardness and brittleness) will produce either indentations or wear on bearing surfaces, while the ingress of water can adversely affect the lubricant efficiency. In addition, if a bearing loses lubrication because of seal failure, dry-running operation can ultimately lead to bearing failure.
Different seal designs and materials are available to accomplish specific tasks and functions in centrifugal pump applications. While the proper sealing choice for centrifugal pumps will depend on the application’s demands and operating conditions, dynamic radial shaft seals will typically be applied to create the necessary barrier between surfaces in relative motion (one usually stationary while the other rotates). These seals usually incorporate a steel or elastomer shell, which is bonded to the sealing material. This enables the requisite interference fit of the seal in the housing bore to be maintained (as well as facilitating proper installation). An elastomer sealing lip installed against the shaft can help exclude contaminants without increasing friction.
The lip provides dynamic and static shaft sealing and features a sealing edge formed by pressing, cutting or grinding. Most oil-based sealing lips are made from a formulation of nitrile rubber. Other materials have been introduced for use with fuels, industrial fluids and highly-compounded lubricants.
When constant pressure differentials are encountered in pump applications, pressure profile seals—in which the seal cavity is pressurized—can counterbalance the conditions. When a seal is exposed to pressure, the radial load of the sealing lip increases, which increases the sealing lip/shaft contact area, resulting in additional friction and elevated underlip temperatures. Some pressure profile seals have been designed to withstand pressure differentials of 50 psi at speeds up to 1,000 feet per minute.
As an added precaution when a pressure differential occurs across a seal, a shoulder or retaining ring should be used at the low-pressure side of the seal to prevent it from being pressed out of the housing bore.
Because of their design and often harsh operating conditions, seals will routinely experience a shorter life than the components they protect. When seals require replacement, users should not automatically replace them with the same design. For example, if an oil analysis shows a higher than anticipated ingress of contaminants, upgrading the entire sealing arrangement (perhaps using a more chemically resistant material or adding elements to bolster the system) may be needed. In cases of excessive wear, non-contact labyrinth-type seal designs may be appropriate.
Viewed as a system, optimized bearings and seals—and the implementation of best-practice maintenance procedures—can make a difference in boosting pump performance, reliability, service life and production.