By Neil Hoehle, Director of Technology, Inpro/Seal
Pumps and coupled rotating equipment are essential to industrial production. Without them, the many processes dependent upon the movement of fluids cease and production stops. Practiced methods of increasing rotating equipment reliability, such as preventative maintenance and condition monitoring, have resulted in dramatic improvements in unscheduled downtime over the years. Reliability has also been engineered into rotating equipment. These technologies need to keep pace with the demands put on pump operations.
Design Essentials
The heart of rotating equipment is the bearings. Without bearings, rotating equipment does not rotate. Therefore, bearing protection should be an important milestone in the equipment design process. Equipment design engineers should give careful thought to how those all-important bearings will be protected from both lubricant loss and contamination; the two major causes of bearing failure.
There are two broad choices when it comes to bearing protection: contact seals or non-contact seals. Contact seals rely solely on contact to perform the sealing function. Non-contact seals rely on a close clearance and difficult pathway to perform the sealing function. That is not to say that non-contact seal components never touch; they often do, in order to comply with conditions such as axial shaft movement. This contact though, has nothing to do with the performance of their sealing function.
Contact Seals
Contact seals have the distinct advantage of being able to seal against differential pressures, such as those induced by a static level or head of lubricant. Although pump bearing housings are commonly designed with splash lubrication (where the static level of lubricant is fully below the horizontal shaft), flooded housings (with static lubricant levels at least partially covering the shaft) are sometimes necessary. In those cases, contact seals are the logical choice.
The most common type of contact seal is the lip seal. Lip seals are typically elastomeric radial structures compressed against the shaft, often with the aid of a circumferential spring. While they are inexpensive and readily available, lip seals have a short life. Their rapid wear rate can result in shaft wear and damage.
Magnetic seals utilize magnetic force (in the place of springs) to load two optically ¬ at faces against each other. To those familiar with mechanical seals, magnetic seal face loading is usually far less, and because the pressures being sealed are likewise far lower than those sealed by mechanical seals, magnetic seals are often unbalanced. PTFE compounds and stainless steel are the most common face materials, used to minimize the friction between the seal components. Magnetic seals, along with select non-contacting seals, are an effective sealing solution for oil mist lubrication systems in pumps.
While lip seals and magnetic seals both perform the same function, the magnetic seal has the benefit of not making direct, frictional contact with the shaft, avoiding shaft wear and grooving. Lip seals, however, can often seal against higher differential pressures. For most flooded shaft conditions in rotating equipment, the magnetic seal, although more expensive than lip seals, will prove the superior choice.
Both contact seal types consume energy, generate heat, and have a finite life expectancy. The ability to predict life expectancy of contact seals, given all the variables of operating conditions, remains elusive and unknowable.
Non-Contact Seals
Non-contact seals come in three categories: simple gap seals, labyrinth seals, and compound labyrinth seals, commonly called bearing isolators.
Gap seals utilize a close clearance between the rotating shaft and stationary housing to block anything larger than the gap. They offer minimal bearing protection and are only suitable for grease- lubricated applications.
Traditional labyrinth seals add a labyrinth (that is, a difficult path) to the gap seal. This further hinders anything passing through the seal, but only marginally improves the performance of the gap seal. Traditional labyrinth seals are also usually one-directional, making them an impractical choice where both contaminant exclusion and lubricant retention are required.
The only non-contact seal that provides protection against both lubricant loss and bearing contamination is the bearing isolator. Bearing isolators utilize both static and dynamic components to provide an effective seal. Due to their non-contacting nature, they offer essentially infinite life, consume virtually no energy, and, when properly applied, significantly add to rotating equipment reliability. With the extreme reliability of bearing isolators, rotating equipment has fewer potential failure modes.
The disadvantage of bearing isolators is that they generally do not form an effective seal against static levels of lubricant or differential pressures. However, there are some caveats to that broad statement. Bearing isolators can be applied in ¬ ooded applications if the lubricant collected in the labyrinth can be drained out and routed back to the bearing housing through some mechanism, such as the return line of a forced-feed lube system. The key to these applications is balance; the bearing isolator must be able to drain lubricant faster than the lubricant accumulates in the labyrinth grooves. Where that balance is possible, the bearing isolator can offer all the advantages of infinite life and no energy consumption for flooded applications. Proper consultation with a bearing protection specialist is strongly advised for these applications.
Non-contact seals come in three categories: simple gap seals, labyrinth seals, and compound labyrinth seals, commonly called bearing isolators.
Gap seals utilize a close clearance between the rotating shaft and stationary housing to block anything larger than the gap. They offer minimal bearing protection and are only suitable for grease- lubricated applications.
Traditional labyrinth seals add a labyrinth (that is, a difficult path) to the gap seal. This further hinders anything passing through the seal, but only marginally improves the performance of the gap seal. Traditional labyrinth seals are also usually one-directional, making them an impractical choice where both contaminant exclusion and lubricant retention are required.
The only non-contact seal that provides protection against both lubricant loss and bearing contamination is the bearing isolator. Bearing isolators utilize both static and dynamic components to provide an effective seal. Due to their non-contacting nature, they offer essentially infinite life, consume virtually no energy, and, when properly applied, significantly add to rotating equipment reliability. With the extreme reliability of bearing isolators, rotating equipment has fewer potential failure modes.
The disadvantage of bearing isolators is that they generally do not form an effective seal against static levels of lubricant or differential pressures. However, there are some caveats to that broad statement. Bearing isolators can be applied in flooded applications if the lubricant collected in the labyrinth can be drained out and routed back to the bearing housing through some mechanism, such as the return line of a forced-feed lube system. The key to these applications is balance; the bearing isolator must be able to drain lubricant faster than the lubricant accumulates in the labyrinth grooves. Where that balance is possible, the bearing isolator can offer all the advantages of infinite life and no energy consumption for flooded applications. Proper consultation with a bearing protection specialist is strongly advised for these applications.
Protection Against EDM
Bearing isolator designs can also be combined with shaft grounding features to protect against potential electrical damage. As energy conservation continues to be paramount, variable frequency drives (VFDs) have become increasingly popular. Unfortunately, VFDs induce high frequency voltages on the shaft that seek a path to ground, typically through the bearings. This is commonly referred to as electrical Discharge Machining (EDM). EDM causes fusion craters, pitting, frosting, and fluting on the bearings, leading to premature bearing failure. Often thought of purely as an electric motor problem, shaft currents induced by VFDs can migrate to coupled equipment, causing damage to bearings and even the mechanical seal. Pump bearings are not immune to EDM damage. While contamination remains the largest cause of premature bearing failure in pumps, rotating equipment design engineers should also seriously consider protection against stray currents in the selection of bearing housing seals.
Many rotating equipment design engineers already possess a strong understanding of bearing protection fundamentals, but in a world where reliability is ever more important, continuing education on potential risks and available technologies from bearing sealing specialists cannot help but add value.
The bottom line: In rotating equipment, the best place to target increased reliability is still in the bearing protection. No longer a last-minute catalog selection, bearing protection a critical milestone decision in the rotating equipment design process.
About the Author
Neil Hoehle is the Director of Technology for Inpro/Seal, LLC. Joining Inpro/Seal in August of 1981, he has spent over 40 years in the design and development of bearing housing seals. He has over 20 related patents, and is presently concentrating on new product development. Neil holds a Bachelor’s degree from Western Illinois University and an MBA from the University of Iowa. He may be reached at nhoehle@doverprecision.com.