The performance and service life of a centrifugal pump is affected by factors ranging from the system’s design and critical components to the many operating conditions encountered in an application. But regardless of how closely a pump may appear to conform to application requirements at the outset, unanticipated problems can eventually develop and equipment downtime may follow. In these cases, operators should consider whether upgrades to components—rather than a simple fix—make sense. Appropriate upgrades to existing pumps can enhance reliability and improve productivity.
Rolling bearings represent one example of how next-generation design, engineering and materials can optimize pumps in service.
In general, bearings for rotating machinery support shaft loads, reduce friction with rolling elements, and provide shaft location and system rigidity. For centrifugal pumps, bearings support hydraulic loads imposed on the impeller and shaft, as well as loads created by couplings and drive systems. Bearings also keep shaft axial and radial deflections within acceptable limits for a pump’s impeller and shaft seal.
Pump operators can choose from many potential bearing upgrades. The following list outlines several options, each offering distinct application advantages:
- the use of ceramic material for rolling elements and super-tough steel for bearing rings
- specialized wear-resistant coatings
- dissimilar opposing contact angles for rolling elements to withstand potential damage resulting from minimal axial loads
- optimized energy-efficient bearing designs
Making Material Changes
Bearings in especially tough pump applications, such as oil and gas and hydrocarbon process industries, typically must accommodate contaminated and corrosive environments, inadequate lubrication conditions, high and low loads, and high or cryogenic temperatures. At the same time, they must provide a high degree of service, availability and safety.
These demands can test the limits of conventional all-steel bearings. As an upgrade, hybrid bearings that integrate rolling elements made of bearing-grade silicon nitride can substantially improve reliability and robustness. Such bearings are dimensionally interchangeable with similarly sized all-steel bearings, eliminating the need to reconfigure or otherwise alter pump equipment.
An engineered ceramic material, bearing-grade silicon nitride offers a uniform and clean microstructure that is extremely hard and 40 percent less dense than bearing steel. As a result, rolling elements weigh less and exhibit lower inertia, which reduces stress on a bearing’s cage during rapid starts and stops and significantly lowers friction at high speeds. Lower friction translates to cooler running and longer lubricant service life.
In addition, silicon nitride demonstrates a higher modulus of elasticity than steel, which promotes increased bearing stiffness and longer bearing service life in contaminated environments. The lower thermal expansion for silicon nitride rolling elements allows for more accurate preload control and less likelihood of excessive preloading when temperature gradients exist within the bearings.
As an alternative to compensate for extreme conditions, hybrid bearings are available with high-performance stainless steel rings instead of rings made from conventional steel to promote superior corrosion resistance and to compensate for high temperatures.
Hybrid bearings also can improve bearing reliability and service life in other ways. For example, smearing may result in bearings that face insufficient or improper lubrication conditions, high speeds and light loads, and/or sudden starts and stops. Smearing is surface damage to a bearing that results from sliding between the all-steel rolling elements and rings. Smearing will not occur between silicon nitride and steel, enabling hybrid bearings to last longer in applications that have severe dynamic or improper lubrication conditions.
Resisting Friction & Wear
To overcome several common causes of bearing failures, a low-friction, wear-resistant carbon coating can be applied to the bearing’s rolling elements and inner ring raceways.
The coated bearing surfaces retain the toughness of the underlying material—the coating is actually harder than steel—while adopting the hardness, improved friction properties and wear resistance of the coating.
Unique coating properties also promote high-speed capabilities and will tolerate contamination more effectively than other bearing types.
Coated bearings as upgrades can withstand many severe operating conditions, including risks of smearing, insufficient lubrication film, sudden load variations, light loads, rapid speed changes, vibration and oscillations, and high operating temperatures. The anticipated outcomes of these capabilities include increased reliability, extended service life, and reduced chances of premature bearing failure resulting from friction, wear and related factors.
Turning to Contact Angles
Typically, 40-degree matching contact angles for pump bearing rolling elements have become the norm for paired single-row angular contact bearings in American Petroleum Institute (API) pumps. For conventional double-row bearings used in American National Standards Institute (ANSI) pumps, matching 30-degree contact angles are typical.
Particularly for the 40-degree paired single-row bearings, a change in the opposing contact angle can benefit many centrifugal pump applications by promoting smoother operation and longer service life in the way they support loads.
Pump bearing arrangements engineered with dissimilar opposing contact angles—a combination of 40-degree and 15-degree—can carry higher thrust loads with the 40-degree bearing without unloading the opposed 15-degree bearing. Greater axial loads can be applied, and the bearing’s operating temperature will decrease.
These 40/15-degree angular contact ball bearing arrangements are ideal for applications where the axial load is high in one direction and does not change direction during operation. The bearing sets can accept momentary reversals in axial load, including those that occur during pump startups and stops.
In centrifugal pumps with light thrust loads and predominantly radial loads—such as double-suction impeller pumps or pumps with closed impellers—bearing arrangements featuring only 15-degree contact angles offer yet another upgrade alternative.
Compared with conventional 40-degree matched bearing arrangements, the design of these 15-degree bearings promotes cooler running, substantially reduced vibration and extended service life in applications with high radial loads.
Generating Energy Efficiencies
Reducing energy consumption in a system remains a top priority in today’s marketplace, and certain bearings have been introduced to help users reach this goal. One of these bearing families includes two types commonly used in centrifugal pumps: deep-groove ball bearings and double-row angular contact ball bearings. Both types have been internally modified to decrease energy consumption by at least 30 percent compared with standard International Organization for Standardization (ISO) products while maintaining the service life and load-carrying capacity of conventional versions.
Relevant energy-saving technical improvements to deep-groove ball bearings for pumps include optimized internal geometry, newly designed polymer cages, and the use of lower-friction grease for sealed or shielded variants. For the double-row angular contact ball bearings, proprietary technical improvements that deliver energy savings include optimized internal geometry, steel shields on both sides and unique low-friction grease.
Both types of bearings, which are dimensionally interchangeable with conventional bearings, have been highly engineered to reduce friction and, in turn, decrease energy use.
As these innovations demonstrate, pump bearing design and engineering will continue to evolve as operators seek to improve the performance of existing pumps and take advantage of advanced technologies for minimizing problems and maximizing output.