Frustrated by Mechanical Seals?
Are you thinking about replacing mechanical seals? Here's what you should know.
Mechanical seals, invented in the early 1920s, became the go-to rotating shaft sealing method in the 1990s, becoming common in most process industries and applications. Their perceived 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.
They have recently fallen out of favor due to their high cost and sometimes low reliability. Installing these seals on equipment with shaft runout, a scored sleeve, or worn parts almost always leads to premature failure and costly downtime. Many process engineers and reliability supervisors find that the investment return is unrealized and are thinking about switching to lower-cost methods. The promise of infinite life and problem-free running has faded alongside increased knowledge that mechanical seals require both mechanical conditions and plant operations to be near-perfect for long term success.
When to Use Mechanical Seals
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 also require advanced sealing methods. If your process fits this category, then mechanical seals may be the only option. One should refer to OSHA and EPA mandates for exact requirements. Although there are ways to increase their reliability on older or worn equipment where conditions are less than conducive to success, seal life will likely be adversely affected.
Some common applications that generally require mechanical seals:
- Chlorine Dioxide (CLO2)
- Hydrochloric Acid
- Hydrogen Peroxide
- Sulfuric Acid
- Some Condensate applications
Utilizing a precision shaft bearing as close to the rotating element as possible minimizes radial shaft movement from run out, misalignment, or cavitation. The bearing also acts as the replaceable wear surface, reducing stress on the sealing faces inside the mechanical seal. Proper bearings in that role also aid in vibration dampening. Without a bearing supporting the rotating shaft, you are relying on the stuffing box throat with a clearance of over 30 thousandths of an inch (0.030") to maintain your seal faces that are ground flat to within two light bands or approximately 0.0000232 inches. Precision equipment like mechanical seals need additional support.
Properly installed and maintained mechanical seals can be an effective seal for the most corrosive and toxic mediums. Because of their high cost, mechanical seals should last 3 to 5 years between rebuild cycles to fully realize a return on investment.
When not to use mechanical seals
If your medium is outside the parameters of the most hazardous categories, there are more options. Applications currently using a single or double mechanical seal technology can be adapted to more reliable bearing sealing systems.
Some common processes that can gain reliability from removing mechanical seals:
- Agitators and Mixers
- Vertical Turbine Pumps
- General Duty Pumps
- Split Case Horizontal Pumps
- Slurry Pumps
- Outlet Devices
- Gear Pumps
- Condensate Pumps
Mechanical seals, especially single seal systems, struggle when used with high particle density slurries. Typically, the seal is paired with a flush system to keep the slurry particles from entering the seal faces. Issues arise when the water is dirty (river water, for example) or inconsistent in pressure or flow rate. Double seal systems might use a barrier fluid tank to stop slurry contamination. Still, dual seal systems are expensive and are even more complicated than a single seal system resulting in higher rebuild costs. Bearing sealing systems are designed to work in slurry applications where mechanical seals can fall short.
Mechanical Seals are Over-prescribed.
Utilizing Bearing Systems
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 various high compressive strength polymers. These polymers are designed to be chemical and 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 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 hole 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 very close clearances automatically 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 added 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 sealing aids, 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 (patent pending) ExtractPRO™ Technology).
Utilizing Braided Packing
Modern braided packing is made from high-tech synthetic fibers weaved in 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 that allows the shaft to rotate.
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 but also be durable to withstand the rotational wear.
There is a wide variety of packing styles and configurations, and not all packing is the same. There are generic packings that generally get utilized in various common applications. They are effective for a while, then wear out and need to be replaced. More advanced packing is designed for specific applications. When choosing packing consider these parameters: Temperature, Speed, pH, Safety, Medium. An engineered packing can be more expensive by the pound, but it lasts longer and saves on downtime and maintenance hours.