Pumps & Systems, February 2008
Maintenance and repair functions constitute a significant component of pumping system life cycle costs, especially in terms of energy consumption. The proper selection and lubrication of bearings can help bring economies and efficiencies into line.
As with all critical assets, centrifugal process pumps encounter enormous pressures from demands for improved productivity, optimized performance and sustained reliability. Prevailing trends in the process pump industry swirl with requirements to reduce energy consumption, increase service life, reinforce robustness, minimize maintenance and comply with stringent safety and environmental regulations.
Energy-related numbers (tallied by the Hydraulic Institute) tell a compelling story: Five percent of all industrial energy is estimated to be consumed by pumps; in certain industrial plant operations, pumps can account for up to 25 percent of energy usage; 20 percent of the world's electrical energy demand can be attributed to pumping systems; and fully 45 percent of the total cost of pump ownership can be linked to energy consumption.
In striving to stem the rising tide of costs and raise desired levels of reliability, pump users would do well to take a close look at their maintenance and repair practices with particular attention to pump bearings. Overall, maintenance and repair functions constitute a significant component of pumping system life cycle costs, especially in terms of energy consumption. The proper selection and lubrication of bearings can help bring economies and efficiencies into line.
Getting an Angle on Thrust Bearings
Bearings in centrifugal pumps support hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and induced loads due to couplings and drive systems. They further keep the shaft axial and radial deflections within acceptable limits for the impeller and shaft seal.
A variety of typical conditions tend to put bearings continually to the test. Given the often difficult process conditions, bearings in pumps often will be subject to high axial loads, marginal lubrication, high operating temperatures and vibration, all while they attempt to minimize friction. (Friction, if uncontrolled, can result in loss of power, excessive heat generation, increased noise and/or wear and early bearing failure.)
All these influences can dramatically impact the service life and reliability of bearings and, in turn, pumps. First and foremost, bearings (types, designs and arrangements) should be evaluated in the context of their anticipated operating environment.
The industry has responded to the challenges with solutions engineered to satisfy even the most difficult centrifugal pump conditions. As an example among thrust types (to support axial loads created by hydraulic forces in the pump), various versions of angular contact ball bearings can prove suitable, depending on the application.
Single-Row Angular Contact Ball Bearings
Single-row 40-deg angular contact ball bearings are the most popular API pump thrust bearings in service today and generally are used in moderate-speed centrifugal pumps where high thrust loads can be expected.
Variations designed with robust machined bronze cages can run particularly well in applications where thrust loads vary greatly during operation and periods of ball skidding are likely. These bearings will resist destructive vibration forces when cavitation occurs. The bearings are normally mounted in back-to-back paired arrangements to accommodate reversing thrust loads and to provide adequate shaft support to promote long seal life.
Particular attention must be given to the internal clearance variant selected so that when mounted and at operating temperature, the bearings have enough residual internal clearance to operate cool, yet not excessive clearance that promotes skidding of the inactive bearing.
Double-Row Angular Contact Ball Bearings
These arrangements are used extensively as the primary thrust bearing in ANSI standard centrifugal pumps and some older API style pumps. The most effective types feature a Conrad-design; ABEC-3 precision tolerances; a 30-deg contact angle per row; one-piece heat-treated pressed steel cages; and multiple sealing options. Since their contact angles diverge outwardly, the bearings exhibit greater rigidity and increased resistance to misalignment. As with paired single-row angular contact ball bearings, the need for normal internal clearance or greater than normal (C3) internal radial clearance is defined by the operating conditions.
Design variations are becoming more popular. Examples include steeper 40-deg contact angles to deliver increased thrust capacity; machined brass cages to offer 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; and ABEC-3 (P6) tolerances to contribute better control of the bearing's mounted condition and smoother bearing operation.
Specialized Angular Contact Ball Bearing Sets
High-performance matched sets of 40-deg and 15-deg angular contact ball bearings exist to provide improved robustness in high thrust load conditions by reducing the susceptibility of ball skidding in the inactive bearing. These designs are intended for centrifugal pumps with heavy thrust loads that are not reversing or reverse only periodically. For pumps with minimal thrust, an alternative arrangement of paired 15-deg angular contact bearing versions should be utilized.
The primary benefit of these sets is that the 15-deg bearing is designed with considerably less internal clearance than the 40-deg bearing, making it less susceptible to centrifugal and gyroscopic forces producing ball sliding and shuttling, while providing additional radial stiffness to maintain integrity of the shaft and seals.
Split Inner Ring Angular Contact Ball Bearing Sets
This bearing set encompasses a split inner ring ball bearing, or four-point contact ball bearing, which is designed to accommodate thrust loads in either direction matched with a single row angular contact ball bearing (40-deg). This arrangement is commonly used in vertical pumps to handle the primary thrust load but can also be utilized in horizontal arrangements, provided the loading is such that the split inner ring bearing does not support radial load on its own.
Because two bearings acting in tandem share the thrust load, this solution offers an extremely high thrust-carrying capacity. Reversing thrust load can be accommodated on the back side of the split inner ring bearing. These two-bearing sets behave like "triplex" sets with the added advantage of saving space and costs.
Delivering the Proper Lubrication
Proper lubrication for pump bearings is essential for reliable service, especially given some industry findings that show improper lubrication can account for more than 30 percent of bearing failures.
Good lubricants primarily provide a separating film between a bearing's rolling elements, raceways and cages to prevent metal-to-metal contact and undesired friction that otherwise would generate excessive heat that could cause wear, metal fatigue and potential fusing of the bearing contact surfaces. Adequate lubrication for bearings further acts to inhibit wear and corrosion and help guard against contamination damage.
Among the common methods for effective lubrication of pump bearings:
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Grease. Grease is easy to apply, can be retained within a bearing's housing, and offers extra sealing protection. Depending on the rotational speeds and operating temperatures, relubrication may be required to combat short grease lives. As an attractive alternative when the operating conditions allow, sealed "greased-for-life" bearings have been developed to eliminate the need for relubrication and related maintenance tasks.
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Oil bath. This option establishes an oil level at the center of the bearing's bottom rolling element and represents the comparative baseline of bearing friction among the lubrication methods. Best results over time can be achieved using a constant-level oiler.
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Oil ring. In this method, an oil ring is suspended from the horizontal shaft into an oil bath positioned below the bearings. The rotation of the shaft and ring flings oil from the bath into the bearings. The reduced oil volume in the bearing reduces the viscous friction in the bearing system to allow higher shaft speeds and better cooling.
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Oil mist and air-oil. In this case, oil is atomized and carried by an air stream to the bearing. Among all pump bearing lubrication approaches, this process 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 (preventing invasive contaminants).
Regardless of the lubrication method, users should always specify lubricant according to the demands on vertical shafts and resistance to solids, pressure, temperatures, loads and chemical aggression. Where pump locations may be difficult to access, fully automatic delivery systems can be integrated to enable timely, proper and effective delivery.
Thinking "Outside the Pump"
Beyond suitable pump bearing selection and lubrication, these additional actions are recommended to further optimize pump performance:
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Align systems accurately with precision tools to minimize vibration and potential bearing damage.
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Conduct "energy audits" to identify opportunities for improved efficiencies in operation and costs.
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Implement proactive, strategic reliability practices (including condition monitoring) to detect and remedy problems before they can escalate.
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Introduce online systems with sensors for round-the-clock condition monitoring.
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Involve operators in pump condition monitoring.
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Promote cross-functional teamwork, communications, and feedback on pump conditions and potential corrective actions.