Upstream Pumping

Bearing Selection: Materials to Meet Pump Demands

Pumps in the oil and gas industry are being tasked with increased efficiency, production and operating life. This reflects demands for pumps to operate at higher temperatures, loads and 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. So the performance of fluid film bearings must keep pace with increasing operational demands.

Bearing life can directly affect pump production, while bearing performance can influence pump efficiency. Thrust bearings usually 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 stiffness and damping for smooth rotor operation. Low friction, minimal wear, low power consumption and long life are differentiating characteristics for fluid film bearings.

Options for bearing material include babbitt, bronze, polymer and ceramic/cermet. These materials are ordered by increasing temperature and load capabilities, although each have additional characteristics that affect which would provide the longest life and optimum performance. Thrust bearing designs and materials are presented, but most of the material performance characteristics apply to journal bearings. The design will also affect the bearing’s performance and life.

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Figure 1. Load capacity at increasing temperatures for similar ceramic and polymer center pivot tilt pad thrust bearings (Courtesy of Waukesha Bearings)

A Baseline for Performance
Whitemetal or babbitt, in particular tin-based alloys of this class of bearing materials, is widely used for fluid film bearings in rotating machinery, including pumps. When fluid film bearings operate 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.

Typical industry practice is to limit the maximum operating temperature of babbitt bearings to 130° C (266° F) because babbitt loses strength at elevated temperatures. Babbitt also has a relatively low fatigue strength. This usually only becomes a problem with significant dynamic loading.

Extended Temperature
When bearing environment temperatures increase beyond the limits of babbitt but loads remain relatively low, bronze bearing surfaces can provide an 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 megapascals (MPa) (290 pounds per square inch [psi]). Several design variations for bronze thrust bearings are available — from a monolithic cast to tilt pad designs.

Because of bronze’s relatively low embedability compared to babbitt, it provides very little acceptance of contamination in the lubricating oil, which can lead to bearing and shaft damage. 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 are 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, engineered polymer bearing designs 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, therefore, a 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 (1,160 psi) and lubricating oil operating temperatures up to 200° C (392° F).

High Loads at Increasing Temperatures
New pump technologies, such as steam-assisted gravity drainage (SAGD), are 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. These materials have greater temperature, load and abrasive resistance capabilities than polymer designs. Ceramic/cermet surfaces retain strength at lubricating oil temperatures of 300° C (572° F) and higher, but polymers start to lose strength above 200° C (392° F) (see Figure 1).

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. One ceramic/cermet tilt pad design has a rated unit load capacity of 8 MPa (1,160 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 can meet both high temperatures and loads in normal operating conditions and the demands of upset conditions. With their superior strength and hardness, ceramic/cermet bearings can operate with abrasives in case a mechanical seal is damaged and lubrication oil becomes contaminated.

For ceramic/cermet bearings to operate reliably, the mating rotor or thrust collar must be made of equally hard material. Development work on ceramic bearing technology has shown that not all ceramic/cermet mating surfaces are the same. Materials can 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. The reliable operation of these pairings is typically validated by in-house testing and field trials.

The Complete Design
Research and development has demonstrated that material alone does not necessarily improve performance. Mechanical design is vital. Consideration must be given to material selection as well as bearing pad design and manufacturing tolerance. Conditions such as environmental temperatures and loads must be examined.

When all material options have been exhausted, design options can improve load-carrying capacity for a given bearing material. For example, in tilt pad bearings, an off set pivot design can carry 20 to 25 percent additional load within the same envelope compared to a center pivot design.

To meet the new exacting application requirements, bearing research continues. Ongoing tests evaluate existing and new bearing materials and mechanical designs at higher temperatures, loads and speeds than traditional applications.

Sriram Venkatesan is a development engineer with Waukesha Bearings, headquartered in Pewaukee, Wisconsin. His primary responsibilities are to conceptualize and develop new products, processes and design tools for fluid film bearings and related technologies.