- Bearings can only function reliably when the entire rotating system and adjoining components operate as designed.
- The correct specification of seals consistent with operating conditions can help to maximize component service life and keep centrifugal pumps up and running as intended.
- The process of selecting seals to protect bearing arrangements in centrifugal pumps universally will be governed by the application’s particular parameters and operating environment.
Bearings play vital roles in centrifugal pumps by supporting the mass of the impeller and rotating shaft, the hydraulic loads imposed on the impeller, and loads due to couplings and drive systems. They further keep axial and radial deflections of shafts within acceptable limits for the impeller and shaft seal. Ultimately, the service life of bearings will directly impact the reliability and service life of a centrifugal pump.
Pump & Turbo
Brian P. Dahmer, application engineering manager at SKF USA, will be leading a tutorial, “Bearing Maintenance Practices to Ensure Maximum Bearing Life,” on Oct. 2 at the 29th Pump and 42nd Turbomachinery Symposia in Houston.
Bearings as key and complex machine components, however, should always be viewed as part of a larger system dependent upon all other elements. Bearings can only function reliably when the entire rotating system and adjoining components operate as designed.
A notable example can be found in bearing housing seals. Wherever there are bearings in rotating machinery, including pumps, there are seals. Should bearing housing seals become damaged or otherwise compromised, solid or liquid contaminants can find openings to infiltrate both the lubricant and bearings, leading to potentially dire consequences impacting both the cleanliness and integrity of the lubricant and the life of the bearing. And, while less likely, when lubricant is lost due to seal failure, dry-running operation can lead to premature and rapid failure of bearings and, in turn, the pump.
The correct specification of seals consistent with operating conditions can help to maximize component service life and keep centrifugal pumps up and running as intended.
Turning to contamination
Figure 1. Over-rolling contaminant particles with high-contact pressure conditions will result in dents in the raceway.
While bearings are manufactured from hardened steel, they nevertheless will be susceptible to contamination. The reason is very high contact pressures present in the small contact zones between a bearing’s rolling elements and raceway. Pressures of around 200,000 psi between the ball and the race in an angular contact ball bearing are not uncommon. Over-rolling contaminant particles with such high contact pressure conditions will result in dents in the raceway (Figure 1).
Particulate contaminants, whether those particles are soft or hard, large or small, will cause some damage to bearings. Even polymer particles, although relatively soft, will extrude when over-rolled and cause large but shallow dents. Harder particles will create smaller but sharper dents. All these dents are certainly detrimental, but the harder the particle, the sharper the dent and the higher the stress concentrations at the edges. Therefore, typically, hard-particle contamination is more damaging than soft-particle contamination, but both types of particles still cause damage.
The presence of contamination can shorten bearing service life in two ways. First, every time a rolling element passes over a dent, contact pressure increases at the edge of the dent. Higher stresses result in shorter fatigue life. The second mechanism is wear. While balls roll in a ball bearing, due to the curvature of the balls and races, there is some sliding that occurs, as well. The sliding portions of the contact, when contamination particles are present, can result in wear of the surfaces. Roller bearings can also exhibit wear from contamination, although this wear may be in different places, such as the ring flanges in addition to the raceways.
Selecting the seals
The process of selecting seals to protect bearing arrangements in centrifugal pumps universally will be governed by the application’s particular parameters and operating environment. Seals should be appropriate for the operating conditions, such as shaft speed, shaft material, temperature, pressure differential across the seal, and other factors, and should be designed to prevent entry of all types of contaminants into the bearing system.
Regardless of application, though, seals serve both at the “dry” and “wet” ends of pumps primarily to retain lubricant, exclude contaminants, separate fluids, and confine pressure.
Among the seal types suited for protecting bearings in centrifugal pumps are lip-type contact seals, labyrinth seals, and magnetically charged face seals.
Lip seals, typically consisting of rubber lips encased in a sheet steel shell or case, can provide excellent protection from external contaminant, but the lip material must be given careful consideration.