Fluid Film Bearing Material Advances
By Sriram Venkatesan | January 2017
Bearing life can impact pump production while bearing performance can affect pump efficiency. This article examines how advances in fluid film bearing materials allow pump manufacturers and users to continue to improve operation.
Pumps in the oil and gas industry, be they new or existing equipment, are being tasked with increased efficiency, production and operating life. This in turn creates demands for pumps to operate at higher temperatures, higher loads, higher speeds and in harsher environments. Fluid film bearings are critical for supporting centrifugal pumps’ rotating components and transmitting loads to the machine frame and ground. The performance of fluid film bearings must therefore keep pace with the increasing operational demands.
Bearing life can directly impact pump production, while bearing performance can affect pump efficiency. Thrust bearings are typically used to carry axial loads imposed on rotating components due to the pressure differential of the working fluid as it moves through the pump stages. Journal bearings carry radial loads and provide the necessary stiffness and damping for smooth operation of the pump rotor. Low friction, minimal to no wear, low power consumption and long life are all differentiating characteristics for fluid film bearings.
Figure 3. Load capacity at increasing temperatures for similar ceramic and polymer center pivot tilt pad thrust bearings.
Depending on the particular pump needs, pump manufacturers and operators have several options for bearing material, including babbitt (tin-based), bronze, polymer and ceramic/cermet. These materials will be discussed in order of increasing temperature and load capabilities, although each has additional characteristics that will affect the choice for longest life and optimum performance. For the sake of comparison, thrust bearing designs and materials are presented here, but the majority of the material performance characteristics apply to journal bearings as well. It should also be noted that material alone does not determine performance; significant consideration must be given to the overall engineering analysis and design of the bearing.
A baseline for performance
Whitemetal or babbitt, in particular tin-based alloys of this class of bearing materials, has long been and continues to be widely used for fluid film bearings in rotating machinery, including pumps. When fluid film bearings are operating as designed, the rotating and stationary surfaces are separated by a thin film of lubricant, reducing wear and power loss compared to metal-to-metal contact. For start-up and off-design conditions, babbitt is a ‘soft’ metal that exhibits excellent conformability, compatibility and embedability. These are all desirable characteristics for efficient fluid film bearing operation and long life.
Operating temperatures for babbitt are limited by the melting point of tin (232°C [450°F]), but a babbitt bearing lining loses strength at a much lower temperature. It is typical industry practice to limit the maximum operating temperature of babbitt bearings to 130°C (266°F). Babbitt also has a relatively low fatigue strength. This usually does not present a problem in rotating pump applications unless there is significant dynamic loading on the bearing. In extreme cases of dynamic loading, low fatigue strength can limit the life of the bearing as fatigue failure of the bond causes the babbitt to separate from the backing material.
Extended temperature capabilities
When bearing environment temperatures increase beyond the limits of babbitt but loads remain relatively low, bronze bearing surfaces can provide additional safety margin at the higher temperature. Successful operational experience of bronze thrust bearings exists at lubricating oil temperatures beyond 150°C (302°F) with unit loads (bearing load divided by bearing surface area) below 2 MPa (290 psi). Several design variations for bronze thrust bearings are available depending on the application requirements. These range from a monolithic cast design to tilt pad designs, including Deﬂection Pad and Flexure Pivot bearings.
Due to bronze’s relatively low embedability compared to babbitt, however, it provides very little acceptance of contamination in the lubricating oil, which can lead to bearing and shaft damage. In addition, bronze’s conformability characteristics are inferior to those of babbitt, making bronze bearings more sensitive to misalignments.
Higher load capacity
For higher loads and improved reliability in the expanding pump operating envelope, polymer bearings have emerged as an effective solution. Like babbitt, polymers exhibit good embedability and conformability, preventing damage to the shaft when small amounts of debris enter the lubricating film or when contact between the stationary and rotating parts is imminent. With clean process fluid lubrication (like water), engineered polymer bearing designs have been proven to operate reliably on a relatively thin lubricant film and have good compatibility when running against a steel rotor.
With oil lubrication, engineered polymer bearings have higher strength at elevated temperatures and higher fatigue strength than babbitt, as well as higher operating temperatures than babbitt and bronze. This gives polymer bearings a higher load-carrying capacity, which enables bearing size reduction compared to a metallic bearing and, thus, reduction in power loss.
On electric submersible pumps (ESPs), polymer thrust bearings are widely used in the motor and seal/protector sections with unit loads up to 8 MPa (1160 psi) and lubricating oil operating temperatures up to 200°C (392°F). Polymer is also an electrical insulator and prevents stray shaft currents from transmitting through the bearing, providing an added benefit in the motor section of an ESP string without requiring an additional layer of insulating material between the bearing and its housing.
Figure 4. Ceramic/cermet thrust bearings for SAGD applications.
High loads at increasing temperatures
New pump technologies such as steam-assisted gravity drainage (SAGD) are now pushing pump operation beyond the limits of polymer. ESP seal environment temperatures are rated at 270°C (518°F) for SAGD applications, making it necessary for pump manufacturers and operators to consider ceramic/cermet bearing materials, which have greater temperature, load and abrasive resistance capabilities compared to polymer designs. Ceramic/cermet surfaces retain strength at lubricating oil temperatures of 300°C (572°F) and higher, whereas polymers start to lose strength above 200°C (392°F) (see Figure 3).
For ESPs operating in SAGD applications, ceramic/cermet thrust bearings can provide a drop-in replacement for existing polymer bearings to increase the operating envelope. The ceramic/cermet tilt pad design of the author’s company has a rated unit load capacity of 8 MPa (1160 psi) at an oil bath temperature of 300°C (572°F) and has accumulated several years of field experience in SAGD applications. Test results in one pump series have shown excellent bearing performance even above the rated temperature and load.
Ceramic/cermet bearings are positioned to meet not only high temperatures and loads in normal operating conditions but also the demands of upset conditions. With their superior strength, particularly at higher temperatures, and superior hardness, ceramic/cermet bearings have the ability to operate with abrasives in case a mechanical seal is damaged and lubrication oil becomes contaminated. Given the high cost of a well’s down-time and high equipment workover costs, this capability can be invaluable.
For ceramic/cermet bearings to operate reliably under any conditions, the mating rotor or thrust collar must be of equally hard material. Development work on ceramic bearing technology has shown that not all ceramic/cermet mating surfaces are the same, however. It is possible for materials to be incompatible even when the mating surface is hard-faced. For good compatibility, the correct grades of material must be selected for both the rotating and stationary components of the bearing. The reliable operation of these pairings is typically validated first by in-house testing, then by field trials.
The complete design
It is clear that research and development has further demonstrated that material alone will not necessarily improve bearing performance. Mechanical design is vital to reliable performance. Consideration must be given not only to material and material grade selection, but also to bearing pad design and to manufacturing tolerances. Conditions such as environment temperatures and loads must be examined as part of the operating duty cycle.
When all material options have been exhausted, various design options can also improve load-carrying capacity for a given bearing material. For example, in tilt pad bearings an offset pivot design is capable of carrying 20% to 25% additional load within the same envelope compared to a center pivot design.
To meet the next generation of exacting application requirements, bearing research continues. Tests are ongoing to evaluate the durability and performance margins of existing and new bearing materials and mechanical designs at ever higher temperatures, loads and speeds, in abrasive environments and in a variety of lubricating fluids. Whether applied in ESPs, process-lubricated pumps, subsea pumps, or other pump environments, these advances in bearing technology will allow pump manufacturers and users to continue to improve operation.
Senior development engineer Waukesha Bearings
W231 N2811 Roundy Circle East, Suite 200 Pewaukee, WI
Tel. +1 (262) 506-3000