Rotating Shaft Sealing Methods
What are the options for rotating shaft sealing? Below is a list of the 4 more common sealing methods used in rotating shaft sealing.
Mechanical seals, invented in the early 1920s, became the go-to rotating shaft sealing method in the 1950s within the oil and gas industry and continued to gain market share through the 1990s, becoming common in most process industries and applications. Their ability to seal rotating equipment better than the traditional braided packing methods was evident. Over the last 30 years, their usage has become a standard procedure for many application maintenance programs.
Mechanical seals are critical to applications in which the pumped medium is considered toxic, corrosive, or explosive. Compounds classified to be hazardous or fugitive emissions typically use double seal technology.
Modern mechanical seals use ultra-precise, ultra-flat opposing faces, one stationary and one rotating with the shaft. The opposite faces are so precisely paired that they leave a gap measured in microns. The microscopic gap causes the pumped medium to vaporize as it is moved centripetally to the seal edge. The result is a seal that does not measurably leak. It also creates a low friction environment for rotating shafts; low friction at the stuffing box results in lower energy costs for rotating equipment.
While mechanical seals, when installed and maintained correctly, create effective seals, they can be problematic because of their inherent complexity and the need to keep the opposing faces perfectly mated and cooled. Installing these seals on equipment with shaft runout, a scored sleeve, or worn parts can lead to premature failure and costly downtime.
This type of seal's complexity and precision-based nature also leads to a high price compared with other sealing methods. Many seals can be in the $10s of thousands of dollars per application. Because of their high cost, mechanical seals should last 3 to 5 years between rebuild cycles to fully realize a return on investment.
Pros: No measurable leakage. Low-friction design uses less energy.
Cons: Expensive. Complicated. Can be unreliable in anything-but-perfect conditions.
Well Known Mechanical Seals Brands
Flexaseal, John Crane, Flowserve, Eagle Burgmann, AES, Chesterton
In some applications that require a mechanical seal on worn equipment, you can install an o-ring mounted bearing to mitigate shaft movement, which can protect the critical seal faces. (see the SealRyt ORM)
Stuffing box mounted bearings were first introduced in the early 2000s when SealRyt patented the PackRyt® Bearing System. Well known for their high-performance braided packing, SealRyt engineered a load-bearing, close clearance polymer that is machined to custom fit each stuffing box.
Bearings are manufactured from high compressive strength polymers. These polymers are designed to be chemical and high temperature resistant but carry an ultra-low friction coefficient.
Bearings work similar to mechanical seals but without the ultra-precise, micron-level clearances. Bearings stabilize the shaft, reducing runout or wobble, bringing the shaft into concentricity. Bearing systems work in concert with packing or mechanical seals to create a close clearance seal at the bottom of the stuffing box that stabilizes the shaft, then the braided packing is layered into the remaining space and compressed using the gland follower. The combination of shaft stabilization and close clearance fit provided by the bearing allows the packing to seal effectively and longer than packing alone.
Bearings can also incorporate a lantern ring into their design, allowing for a flush channel. The benefit of combining the lantern ring into the bearing is that it prevents movement. In a standard packing/lantern ring/packing (or 2L3) setup, the packing can push the lantern ring past the flush holes resulting in misalignment and overheating. Teflon lantern rings are also easily crushed when doing basic packing maintenance. The joined bearing and lantern ring unit are made from a non-compressible polymer, so once installed, the lantern ring is always aligned.
Additionally, bearings can automatically reduce the amount of flush used in the process. An 85% reduction in flush usage is common. The close clearances throttle the flush down without the need for external flow controls.
Many industries rely on old pump equipment that has outdated stuffing box designs. Many older OEM pumps have placed the flush inlet close to the bottom of the box, which results in dilution and sealing issues. Bearings can be machined with internal channels (patented) to relocate the flush without modification to the stuffing box. By moving your flush location, you can reduce water usage that goes directly into the process. (see the SealRyt Diverter®)
Bearings are effective and long-lasting seals, but there are some drawbacks. The bearing must fit to work properly, which means the stuffing box needs to be measured accurately (How to Measure). Bearings rely on close clearances to prevent shaft movement, but that also means they are a snug fit and can be difficult to install or remove without the proper knowledge and tools. (Note: Ask our engineers about ExtractPRO™ Technology).
Pros: Minimal leakage. Can reduce flush usage by up to 85%. Long life with minimal maintenance.
Cons: Must be accurately measured.
Well Known Bearing Brands
SealRyt Corp. (Patented Product)
Bushings are almost as old as packing. Early engineers looked to space packing out into different configurations that resulted in improved performance. Bushings, such as lantern rings, allowed the ability to transform the dynamics of the stuffing box.
The difference between bearings and bushings is simple: Bearings are designed to bear the load and contact the rotating shaft. Bushings are designed to be a non-contact spacer and, depending on design, to alter flush flow characteristics. So there are two main differences: clearances and materials. Bushing clearances are much larger to ensure that the part DOES NOT come into contact with rotating shaft. Contact with the shaft leads to damage of the bushing material. Bushing material tends to be low durometer materials such as rubber, polyurethane, and other semi-hard plastics such as carbon-filled Teflon.
The utilization of bushings is related to flush control. Bushings allow flush to circulate more freely inside the stuffing box, but they don't limit or regulate flush volume independently. The varied designs on the market can alter the fluid dynamics inside the stuffing box environment. Bushings are used in conjunction with packing. In this configuration, the packing does the brunt of the fluid sealing. There are a few designs of bushings to operate with mechanical seals.
Pros: Creates flush dynamics inside the stuffing box. Registers the flush channel with the flush inlet hole.
Cons: Does not reduce leakage on its own. Not capable of bearing a load.
Well Known Bearing Brands
Chesterton, EnviroSeal (SpiralTrac), John Crane, Garlock, SealRyt Corp.,
Packing has been used for centuries, since the first use of rotating equipment as pumps. Rope packing gets its name from jamming rope around a rotating shaft to be sealed. The term stuffing box is a relic of this early sealing method. It's the most common rotating shaft sealing method because it's relatively inexpensive and reasonably effective.
Packing has come a long way from using organic twisted fiber. Today modern braided packing is made from high-tech synthetic fibers woven into complex patterns that create a square-shaped packing compressed into a stuffing box in layers. The packing gland is then tightened enough to create a seal but allows the shaft to rotate. Essentially, packing seals through both friction as well as hydraulic pressure break-down.
Using braided packing to seal requires the correct combination of material characteristics depending on the temperature, rotational speed, and medium being sealed. The packing should be low friction as it contacts the shaft and be durable to withstand the rotational wear. Packing that has heat conductivity is a huge plus as well.
There is a wide variety of packing styles and configurations, and not all packing is the same. Generic packings generally are utilized in various common applications and in bid situations where low price is considered a requirement. They are effective for a while, then wear out/compress and need to be replaced. More advanced packing is designed for specific applications, higher reliability requirements, and a higher predictive quotient. When choosing packing consider these parameters: Temperature, Speed, pH, Media, and Safety. An engineered packing can be more expensive by the pound, but it lasts longer and saves on downtime and maintenance hours.
Pros: Inexpensive. Easy to replace.
Cons: Leakage is common. Must be continuously maintained.
Well Known Bearing Brands
SealRyt Corp., Chesterton, Garlock, John Crane, Sepco