Labyrinth bearing seals can offer cost advantage
By David Orlowski, Inpro/Seal Co., Rock Island, IL.
Labyrinth bearing isolators positively seal without contact.
Lip seals consume, on average, 147 W of power.
Life-cycle costs for labyrinth isolators are less than for lip seals.
It is argued that only contact seals, including lip seals and face seals, are able to hermetically seal a bearing enclosure. Hermetic sealing is defined as an absolute prevention of voluntary transfer of air or vapor in or out of a bearing enclosure that can only be accomplished with a contact-sealing device.
Whether or not this degree of sealing is necessary is open to debate. Labyrinth bearing isolators have been refined to the point of positively sealing while the equipment is at rest and dynamically sealing when the equipment rotates. In any event, the arguments are theoretical in nature and in practice the empirical data is at best inconclusive.
Both lip and face seals hermetically seal bearing housings for a time, but both are subject to wear and deterioration at a fraction of the design life of the bearings they are supposed to protect.
Compact, inexpensive face seals, sometimes powered by permanent magnets, were applied to pump bearing housings primarily because of their ability to hermetically seal the bearing environment (Fig. 1).
Fig. 1. Face seals can be energized by magnets.
With the exception of difficult-path, simple labyrinth isolators in API pumps and steam turbines, there was no known viable alternative to long-established methods of sealing bearing enclosures.
Compound labyrinth isolators function in place of a common lip seal and are non-contacting, non-wearing, and do not generate heat.
Labyrinth isolators cost at least ten times more than a rubber lip seal, making whole plant conversions difficult to justify. Conventional wisdom points out that for the price of a labyrinth isolator, one could buy a dozen or so lip seals. Rarely would anyone rationalize the periodic change-out of a lip seal, since it was almost never done.
Grooving of the shaft by a lip seal can be repaired by installing a thin, specially made sleeve. Power consumption, while the lip or face seal was sealing, was never thought to be important.
Contact seals consume power
According to industry sources, the electric power consumed by contacting radial lip seals, and other contact seals, can be substantial.
Although a lip seal’s duration of contact with the shaft is short lived (the mean life is 1800 hr); it has frictional drag and an average power consumption rate of about 147 W. When the lip seal ceases to contact the shaft, its usefulness as a bearing and lubricant protection device also comes to an end.
For maximum performance, lip seals depend on a controlled oil film at the contact point between the sealing lip and the shaft. The shaft can be neither too rough nor too smooth. Machining marks on the shaft can cause almost instant leakage into or out of the bearing enclosure (Fig. 2).
Fig. 2. Lip seals cannot tolerate shaft imperfections.
Face seals also depend on a constantly replenished lubrication film between their optically flat sealing surfaces.
Do the math
An average of about 38 million lip seals are installed each year. The mean operating life of a lip seal is approximately 20% of the 8760 hours in a year, so that 20% of the yearly installation of lip seals is engaged with shafts and actually sealing at one point in time.
This would put 7.6 million lip seals per year sealing and gripping shafts and each consuming an average of 147 W of power. Even these conservative estimates show 1117 mW of power to be wasted in an ineffective bearing sealing strategy.
There is a very small amount of data produced showing the magnitude of power consumption relegated to contacting face seals, but estimates put that number equal to or greater than that of lip seals.
If all 38 million lip seals were engaged and sealing, the total power consumption would be 5586 mW, or about half of what a large utility, such a Florida Power and Light, produces in 24 states. It may be fortunate that only 20% of the lip seals produced in one year are engaging shafts and sealing at any one time.
Noncontact labyrinth bearing isolators consume zero power in operation and have no finite life limitation. Their performance does not deteriorate with time in service. They are capable of outlasting the bearings that they are designed to protect (Fig. 3).
Fig. 3. Labyrinth isolators have no wearing parts.
Heat generation is not a problem and bearing life is enhanced by running in cool, clean lubricant. Even with these advantages, labyrinth bearing isolators command only a tiny segment of the available market.
Rubber lip seals have their place, but not necessarily in the process industries. Their effectiveness is short-lived and even while they are working, their energy drag is costly. Contact bearing sealing in industrial applications cannot be avoided entirely because of situations including flooded lube conditions such as worm gear housings and vertical down housings where there is a pool of oil above the output shaft protrusion.
Because of user demand, most ANSI pump manufacturers equip their products with metallic, noncontact labyrinth bearing isolators. The API has prohibited the use of lip seals for decades because of their concern for pump reliability in the hydrocarbon processing industry. Pump warrantees have been extended to as much as five years because of field experience with labyrinth bearing protected products.
Severe duty electric motors, that previously had little or no bearing protection, now come equipped with labyrinth bearing isolators because of the IEEE-841 standard. For the ultimate motor, some are buying IEEE-841 motors with NEMA premium efficiency. Warrantees have been extended to five years to take advantage of the new level of motor reliability.
The initial cost of a metal labyrinth bearing isolator is ten times that of a rubber lip seal of comparable size. The time for installation of a labyrinth bearing isolator is about the same as that of a rubber lip seal. Even though the effective life of the rubber seal is a matter of a few months, hardly anyone changes them out when they cease to function. If lip seals were replaced when worn, considering the costs of labor and downtime, they would be much more expensive than labyrinth bearing isolators.
Metal labyrinth bearing isolators, once installed, are considered to be permanent bearing protection. Their performance is measured in hundreds of thousands of hours (years) and their life expectancy meets or exceeds the projected design life of the bearings they isolate and protect.
The initial performance of a labyrinth bearing isolator is the same 3 or 10 yr after installation. If the equipment they protect is ever dismantled for any reason, the labyrinth bearing isolator can be used over and over again.