At the heart of most power generation equipment is a bearing system, the failure of which can result in costly downtime and repair.
Whether it is a pulverizer, turbine, conveyor or other type of equipment, a bearing system is used to support the rotating elements and related loads and to reduce power losses due to friction.
Ball and roller bearings typically fail due to surface fatigue. Either the bearings or the raceways begin to pit, resulting in vibration and audible noise. As the rolling elements continue to degrade, fatigue increases until fracture, destroying the bearing system and sometimes damaging connected components. A standard method of predicting bearing failure is the “L10 life,” which is based on the assumption that 90 percent of a random sample can be expected to meet or exceed a specified number of revolutions at a given size and load.
However, most bearing systems fail to meet their predicted life due to factors other than fatigue. Analysis reveals that only a small percentage of such failures are the result of fatigue alone, which means most either can be prevented or the service life of the bearings can be extended. This can be achieved by using a sealing system to retain lubricant and prevent the ingress of foreign material. Common sealing devices for rotating equipment include compression packings, labyrinth seals, mechanical face seals, radial lip seals and hybrid combinations. In recent years non-contact labyrinth seals, or bearing isolators, have increasingly replaced traditional sealing technology, specifically radial lip seals.
Radial Lip Seals
Radial lip seals were originally designed to maintain direct contact with the sealing surface, whereas the specialized geometries of today’s lip seals provide hydrodynamic sealing. These seals float on a thin meniscus of oil, allowing lubricant to recirculate under the lip and back into the bearing system to reduce friction and abrasion. This oil film is typically 0.00018” (0.0046mm) thick (See Figure 1).
To achieve hydrodynamic sealing it is necessary that the shaft be properly prepared. Most radial lip seals require a shaft surface finish of 10 μin to 20 μin (0.25 μm to 0.50 μm) Ra. The shaft also must have a minimum surface hardness of 30 Rockwell C (RC) to prevent grooving resulting from leakage. Single lip seals are unidirectional either retaining lubricant or excluding debris, but not necessarily both. The seal shown in Figure 1 will only retain oil. To exclude debris in a light-duty environment, a seal with a dust or scraper lip may be used. Heavily contaminated environments call for seals with positive excluder lips.
Although radial lip seals are designed to ride on a meniscus of oil, they come into direct contact with the shaft during startup and shutdown, resulting in power losses. As hydrodynamic sealing is achieved, power loss is reduced. But the period of direct contact between the shaft and seal leads to abrasion and eventual failure of the seal. Therefore, it is important to take into account the friction and abrasion properties of the sealing material, as well as operating temperature, pressure, misalignment and runout, bore condition and other factors.
As the need for energy efficiency has increased, so has the use of non-contact seals. The most common of these are labyrinth seals, which use circuitous pathways to prevent both the escape of lubricants and ingress of contaminants. These types of seals have a stationary stator, with one or more inside diameter grooves, which is mated to the application housing. A dynamic rotor, in turn, mates to the shaft and has one or more protrusions, sometimes referred to as teeth or knives that run inside the grooves of the stator (See Figure 2).
The operating principle of labyrinth seals is based on the statistical motion of a particle on either side of the labyrinth. The more complex the pathway, the less likely the particle will be able to migrate from one side of the labyrinth to the other. Early labyrinth seals were considered only for applications allowing some degree of leakage. Today’s labyrinth seals have evolved into hybrids called bearing isolators which combine basic labyrinth technology with other methods of retention/exclusion, such as centrifugal force, pressure differential and drain- back to provide a higher level of sealing performance.
Before selecting a sealing solution for a particular piece of rotating equipment, the performance of the equipment must be measured both in terms of reliability, or the probability it will perform its intended function within stated conditions for a specified period of time, and maintainability, or the probability it will be retained in or restored to such condition.
Reliability is quantified by comparing the equipment’s productive time to the number of failures occurring within a specified period. The most common method of measuring reliability is mean time between failures (MTBF), which is calculated by dividing the total productive time by the total number of failures
MTBF = PT ÷ N
where MTBF = Mean Time Between Failures; PT = Productive Time; N = Number of Failures That Occur During Productive Time.
The result is the average time the equipment performed its intended function between failures.
Maintainability, or mean time to repair (MTTR), is similarly calculated by dividing total repair time by the number of failures
MTTR = RT ÷ N
where MTTR = Mean Time to Repair; RT = Total Repair Time; N = Number of Failures
To improve MTBF, the life of the operating components must be extended. Therefore, improving a bearing system’s ability to retain lubrication and prevent contamination will not only extend its service life, but also reduce MTBF and associated costs. Improving seal life requires an understanding of failure modes including thermal degradation, excessive wear due to abrasion, lack of lubrication, chemical degradation and changes in physical properties while in service.
In typical applications there are periods of dry running, particularly during start up. This increases under-lip temperature and may cause the seal lip to become hard and brittle. When this occurs, the seal can no longer follow the eccentricities of the shaft resulting in leakage. Direct contact between the seal and shaft also contributes to abrasion of the seal material. This abrasion will eventually decrease the radial cross-section of the seal to the extent that it can no longer make complete contact with the shaft, again resulting in leakage. In addition, exposure to chemicals incompatible with the seal material may result in abnormal swelling. Over time the physical properties of the elastomer— including hardness (durometer), tensile strength, elongation, volume, wear width and Taber wear factor—may change significantly, also resulting in leakage.
The factors that contribute to the failure of seals are directly related to the properties of the materials used to manufacture them. Due to their resiliency, elastomers are typically used for contact seals, although some thermoplastic materials are also used. Common sealing materials include acrylonitrile butadiene (Buna-N, NBR), hydrogenated nitrile rubber (HNBR), fluoroelastomer (FKM, Viton), silicones and polytetrafluoroethylene (PTFE, Teflon). To significantly improve MTBF, specially engineered sealing materials have been developed to offer high abrasion and chemical resistance, low wear and improved retention of physical properties.
The most common method of measuring a material’s resistance to abrasion is the Taber wear test (ASTM D4060), which subjects a precisely weighed specimen to the action of two abrasive wheels applied at a specific pressure. Afterward, the test specimens are reweighed to determine how much material was lost in milligrams per 1,000 cycles. The lower the value, the higher the abrasion resistance. Typical nitrile rubbers have a Taber wear factor of 500 mg loss/1,000 cycles or greater, whereas the specially engineered NBR loses 145.5 mg for a 73 percent improvement. Similarly the resistance of engineered HNBR is 65 percent higher and that of engineered FKM 90 percent higher (See Table 1).
A material’s ability to retain its physical properties over time can be determined by heat aging and ASTM 903 oil immersion testing. This testing requires that properties such as hardness, tensile strength, elongation and volume be benchmarked and retested after a specified time of exposure. Exposed to chemicals or heat, all elastomers undergo some change; however, the objective is to have as little as possible. This can be difficult since the formulation and processing required to maintain certain properties may compromise others, often resulting in a trade-off.
Changes in the properties of a sealing material during service have a significant effect on overall performance. If the material shrinks, for example, the necessary interference between shaft and seal will decrease, adversely affecting the seal’s ability to accommodate shaft misalignment and to retain lubrication. Conversely if it expands, interference will be excessive, reducing the hydrodynamic effect and increasing temperatures.
Engineered elastomers have significantly better in-service property retention. For example, general service grade HNBR can undergo up to 25 percent degradation in tensile strength, compared with just 3 percent for its engineered counterpart.
In summary, radial lip seals can provide consistent, reliable performance in most applications, however there are disadvantages. Elastomer materials can be aggressive when in contact with a soft shaft material. If the surface hardness of the shaft is less than 30 RC, grooving can eventually result in a leak path for lubricant or point of entry for contaminants. Radial lip seals can also induce power losses in a system due to their drag force against the shaft.
In conveyance systems, for example, where multiple idle rollers may be driven from a primary source, frictional losses from sealing devices can be significant. Although such losses may be small relative to the power consumption of the total system, demands for greater energy efficiency may prompt users to seek alternative sealing methods. Moreover, radial lip seals only seal media on one side of the seal and cannot run dry. For these and other reasons bearing isolators are increasingly replacing them in many applications. (See Table 2).
As noted, bearing isolators have evolved from labyrinth seals. Recent advances in design have led to the development of unitized bearing isolator seals, consisting of a rotor and a stator. O-rings are used to mate the moving rotor to the shaft and the stationary stator to the housing bore. The use of low-friction materials to unitize the rotor and stator serves to minimize internal wear and extend service life. Properly specified and installed, these types of seals can outlast the bearing systems they are designed to protect (See Figure 3).
The greater energy efficiency of bearing isolators compared with traditional oil seals has been confirmed by tests in wind power applications conducted by Chitren and Drago. These tests showed that a typical oil seal consumes 285 watts during normal operation, spiking to 670 W during startup. Under similar conditions a bearing isolator consumed 120 W during normal operation and 150 W during startup. Other data shows the service life of bearing isolators to be up to 65 times that of traditional oil seals.
Improving Mean Time to Repair
The most time-consuming aspect of seal maintenance and repair is disassembling and properly reassembling the equipment, including pillow blocks, motor and pump housings and other components. One-piece seals must be installed over the free end of a shaft, and all attached components removed from the assembly. The advent of split seal designs allows users to install seals without having to completely disassemble the equipment, dramatically reducing maintenance time.
Split radial lip seals have a single split point so they can be opened along the axis of rotation to allow easy assembly over the shaft. However, some split seals include a garter spring which needs to be assembled around the shaft and onto the seal. This can be cumbersome and even a possible source of equipment failure if the spring becomes dislodged during installation of the seal into the housing bore. Superior split lip seal designs include a molded-in finger spring eliminating the need for a garter spring and helping distribute the load evenly around the shaft (See Figure 4).
|Figure 4 Split Seal with Finger Spring|
Basic split oil seal technology has existed for a numbers of years. However, applying it to bearing isolators is a recent innovation. A common problem with split bearing isolators is that the clearances between the rotor and stator have to be set manually at the time of installation. Recently, however, fully unitized split bearing isolators have become available (See Figure 5).
Figure 5 Unitized Split Bearing Isolator
These unitized seals come in two halves for simple installation around equipment shafts using alignment pins and internal bolts to lock the two halves together. The seals can then be manually pressed into the housing to complete the installation, saving hours of downtime.
Extending the service life of equipment is a complex process, requiring breaking it down into systems, systems into sub-systems and sub-systems into components. Because bearing systems are such a critical element in power generation equipment, focusing on improving their longevity will have a profound effect on overall performance. Tools such as mean time between failures and mean time to repair provide the necessary metrics to gauge system performance. Innovations such as advanced sealing materials, bearing isolators and split seal designs can help improve these indicators and meet requirements for greater energy efficiency.
Author: David C. Roberts, P.E., is an industrial engineer with Garlock Sealing Technologies.
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