- 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.
Nitrile butadiene rubber (NBR), generally referred to as nitrile, is the most common seal lip material and does an excellent job in most applications. However, if temperature is elevated to above 250 °F or if the seal will be exposed to certain chemicals or gases, different materials may be required.
Among these other materials, hydrogenated nitrile butadiene rubber (HNBR) offers the capability to operate at temperatures up to 300 °F and offers higher resistance to heat and to weathering and ozone effects. It is also stronger than NBR, delivering better abrasion resistance.
Fluoroelastomer (FKM) provides the highest temperature resistance of common seal lip materials and is highly resistant to chemicals and gases. However, FKM emits toxic fumes when burned, and care must be taken not to overheat the seals. FKM functions to temperatures of 600 °F, well above the limits for typical bearings and lubricant.
Seal lips may also be made of polytetrafluoroethylene (PTFE), which exhibits excellent chemical resistance and high temperature capabilities. This material can function up to 500 °F but also can be dangerous when combusted. PTFE is highly resistant to media and chemical compounds and possesses the advantage of very low temperature capability.
Regardless of material, seal lips will wear. Depending on conditions, the seal lip may fail well before a bearing fails and seal failure will likely lead to bearing failure. Once signs of seal leakage are present, taking the time to replace lip seals promptly may reduce the frequency of bearing failures. If seals seem to be failing with high frequency, the seal selection, installation process, and adjoining component quality should be examined, rather than just continuously replacing worn seals with new, identical seals. Different seal material or seal design may be necessitated if existing operating conditions continue to result in very short seal lives.
Labyrinth seals can be developed in a variety of forms, including the more common and effective cartridge type labyrinth seals, often referred to as isolator type seals. These include a rotating element mounted on the shaft and a stationary element in the housing. Using nomenclature similar to that of motors, the rotating element is typically referred to as the rotor and the stationary element is typically called the stator.
The rotor provides a flinger effect to throw contaminants off the seal when it is rotating. The axial, or face, labyrinth created by the nesting of the rotor and stator makes it extremely difficult for any type of contamination to pass through the seal. In addition, typically there are ports at the bottom of the seal to allow any contamination entering the labyrinth to exit through these ports.
Bearing isolators can be manufactured from various materials. The most common is brass or bronze. Other materials can be utilized, including PTFE, stainless steel, or aluminum. If needed, the seal can be optimized to the environment and to the types of contamination the isolator seal may encounter.
Magnetically charged face seals may also serve as viable options to seal bearing housings. These seals use magnets to provide the force necessary to keep precision lapped face seal surfaces in contact. Similar to the isolator type seal, one of those face seal surfaces is part of a rotor driven by the shaft, and the other is part of a stationary body attached to the housing. The magnetic force compresses and aligns the mating surfaces so the seal is perfectly adjusted for life.
These seals can prevent ingress of contaminant, including water wash down, and effectively retain lubricant. While there is contact between the face seal surfaces, seal life still will be substantially longer than the life of lip seals.
Troubleshooting seal failures
Despite all the advances in sealing system designs, materials, and performance over the years, seals are not immune to potential failure, many times for reasons other than the seal itself. Picking an inappropriate replacement, improper installation, or switching or mixing lubricant can turn problematic over time. When good seals go bad, the best troubleshooting practice is to ask the right questions and then follow a logical sequence of steps to analyze the failures and take remedial action.
Two important questions will guide the detective work in pinpointing failure causes.
1. How well has the seal performed in the past and is it the correct seal for the application? If there is a history of failures with a particular seal, the culprit may not be the seal itself, unless the seal is not the correct design or the material inappropriate for the application. At the outset of signs of failure, check the seal’s part number and review recommended applications to eliminate the seal itself as suspect. Then, in a process of elimination, attention should focus on all the many influences that can impact seal performance and service life.
The operating conditions should always be scrutinized to determine whether they conform within the optimum range specified for the seal. When seals must endure operating conditions for which they haven’t been designed, failure will surely follow.
For example, when the operating temperature or pressure is greater than the lip material maximum, the seal may exhibit heat cracking, which will be evidenced by a hardened seal lip or fine cracks visible in the seal lip surface. Even excessive surface speeds or insufficient lubrication at the seal lip can eventually lead to heat cracking and damage.
Among other causes for investigation upon seal failure, shaft-to-bore misalignment or dynamic run-out can cause early lip leakage, excessive and uneven lip wear on one side of the seal, or excessive but consistent lip wear all around. Shaft-to-bore misalignment can result from inaccurate machining, shaft bending, lack of shaft balance, or worn bearings, and dynamic run-out is a similar condition where the shaft does not rotate around its true center. If shaft misalignment is suspected, the seal’s lip area with the greatest wear will indicate the direction of the misalignment.
A breakdown in lubrication or improper lubricant also can cause problems down the road. Sometimes heat may be high enough to break down the lubricant, but not enough to harden the seal’s lip. When this occurs, undesired sludge or varnish-like deposits will accumulate on the seal lip, and damage will occur. Using the proper lubricant for the seal and regularly changing the oil are among the best practices to help to avoid lube-related seal failures.
When seals have been improperly installed, they will likely fail in short order. In the case of damage caused by improperly installing with a hammer blow, symptoms will include visible dents on the seal back, a distorted sealing element, or a garter spring that pops out. The seals also can be misaligned in the bore. All will become causes for concern and necessitate seal replacement.
Other factors ranging from possible media intrusion to undue pressure within a seal cavity can adversely impact seal performance. The central message is that users should confirm that the seal has been installed properly, runs within specified operating condition ranges, and benefits from the proper lubricant. Maintenance practices and operating practices also should be retraced for any potential adverse impact to sealing systems.
2. What is the source of the leak? It will be helpful as a reference point to determine whether the leak is in the inner diameter or the outer diameter of the seal. If the leakage can’t be located, ultraviolet dye can be added to the sump or white powder can be sprayed on the area. After operating for 15 minutes, ultraviolet light can be used to highlight the leakage source. In addition, documenting when the leak first occurred may relate the leakage to a change in maintenance or operating procedures.
|Brian P. Dahmer is application engineering manager at SKF USA. Contact him at (267) 436-6517 or firstname.lastname@example.org.|
These observed conditions will help point the way toward a root cause of failure, which could be improper installation, excessive or inappropriate operating conditions, poor lubricant, out-of-spec components, or any number of the other influences already cited that will prevent a seal from performing as intended. When the cause is determined and remedial action is taken, history of failure will be less likely to repeat itself.
Although seals are designed to provide economical and versatile protection for bearings in centrifugal pumps, optimizing a sealing system truly becomes a balancing act. Careful identification of the application requirements, evaluating all conditions, and adopting a holistic approach to seal specification with a system-wide perspective will contribute significantly to how a seal will perform and for how long.