Proper selection of packing or mechanical seals for rotating equipment is based on important system variables that lie beyond cost issues. Pay attention to the following considerations if you are responsible for selecting these components. Proper application of these principles can improve the life, dependability and operation of your process and mechanical machinery.
Any piece of rotary equipment (including pumps, mixers, boilers, chillers, and the like) that requires sealing will need occasional part replacement because of the very nature of the system. Don’t consider packing or mechanical seals to be the weakest link or merely a throwaway part. For maximum cost-to-wear benefit, view them as part of a regular investment, preferably one that’s synchronized to the rotary equipment’s major overhaul schedule.
Packing
Using packing rather than mechanical seals has some sensible advantages. Packing usually is easy to install. It’s more cost-effective than a mechanical seal and typically more forgiving of axial shaft movement. However, because a packing’s sealing characteristics are imperfect, small leaks of process liquid should be expected. If these leaks can be tolerated and properly drained, then packing is probably a good choice for your system equipment.
The three common packing materials are graphite fiber, carbon fiber and Teflon (PTFE). Although the terms carbon fiber and graphite fiber frequently are incorrectly interchanged, carbon fiber packing is less expensive and can handle tougher, more abrasive media conditions. Graphite fiber packing is smoother and offers lower friction, which results in better lubricity and heat dissipation characteristics. That’s why graphite packing is typically better than carbon packing for higher shaft speeds.
Teflon packing has excellent chemical resistance characteristics and a lower coefficient of friction than graphite packing. However, because PTFE is a polymer, its sealing capability is limited at high temperatures and pressures. Plastic materials don’t conduct heat or exhibit dimensional thermal stability. A woven blend of carbon- or graphite-fiber materials with a PTFE coating is a versatile way to get the best features and benefits of both packing types.
Mechanical seals
Mechanical seals are preferred over woven packing for several functional and economic reasons. Packing is more prone to leakage. Realize that a single drop of process fluid leaking every second works out to be more than one gallon of fluid in a 24-hour period. In most applications, government regulation has tightened up on these types of emissions and releases.
In addition, safety and expense issues are associated with leaking fluids. Moreover, a lower-friction mechanical seal generally consumes less energy than packing. If properly installed, mechanical seals don’t normally need to be adjusted or changed out for wear as frequently as packing does. On the whole, mechanical seals can handle higher demands than packing without a change out, which correlates to longer times between maintenance calls. The longer the seal lasts, the better the payback on a long-term investment.
Proper selection of a mechanical seal involves three steps: initial design, equipment/system startup, and ongoing review.
Initial design
The cost and lead time for getting the right mechanical seal are frequently the critical considerations. Although price and schedule are important factors of any design and purchasing decisions, the seal selection process shouldn’t stop there. Function also must be considered.
In addition to price and schedule, review the process conditions. Mechanical seal failure, in most cases, is caused by dust, particulates and other contamination, or because of inadequate heat dissipation from insufficient cooling, flushing or quenching of the seal.
Material compatibility issues also must be accounted for because the process fluid normally lubricates the mechanical seal faces. The seal’s metallic body material, gaskets and seal faces must be compatible with the process fluid. An example of incompatibility is the use of Viton, a common seal gasket material, on steam or dilutes caustic applications. The Viton seal gasket could swell and fail quickly because of material incompatibility.
Normal wear and tear occurs because one seal face is held stationary while the other face rotates with the shaft. Ordinarily, the two seal faces are of dissimilar materials to prevent adhesion.
Dimensional issues also can come into play. Putting a mechanical seal in a difficult-to-reach location - such as the bottom of a vertical pump in a well - can produce accessibility problems. When switching from packing to a mechanical seal, remember that the seal must fit within the physical space formerly occupied by the packing.
Startup and troubleshooting
Tolerances are much tighter on a mechanical seal than packing, making it more susceptible to damage during shipment. Upon receipt, closely inspect the seal for damage. Improper mechanical seal installation is common because of the tight tolerances. After initial out-of-the-box inspection, follow the installation instructions carefully.
Prepare for startup by eliminating any possible sources of pipe vibration, pipe strain and equipment misalignment. Minimize the possibility of dry running. Any of these conditions will shorten the mechanical seal’s lifespan quickly.
Successful startup requires correctly priming, venting and lubricating the rotary equipment and seal. During day-to-day operation, surges can occur in process variables including flow, pressure, speed and torque. Given the mechanical seal’s rating and the best efficiency point of the rotary equipment, these atypical states could cause a mechanical seal to fail. If that happens, look for the cause of the problem, not just the symptoms of seal failure. For example, a leaking seal (the symptom) could be the result of a shaft that bent because of continuous water hammer (the root cause of the problem).
Discuss proper day-to-day operation and worst-case scenarios with the equipment manufacturer. Even better would be to discuss them with the mechanical seal manufacturer’s representative.
Ongoing review
The root cause of a mechanical seal failure commonly falls into one of three general categories and it should be your goal to understand and be aware of them:
- Mechanical breakdown: The seal faces open if they get chipped or pitted.
- Thermal breakdown: Thermal destruction from excessive heat buildup caused by poor lubrication; experience unusual wear or warp from heat or pressure.
- Chemical breakdown: Challenging chemical conditions such as polymerization or crystallization.
Root causes for failure sometimes can be determined by appearance. A failure of a mechanical seal caused by chemical attack typically looks crumbly and flaky, and the metal finish will be dull. A failure caused by thermal attack generally exhibits cracks, fractures or blistering. A failure from mechanical breakdown usually shows up as fretting on the shaft, which leads to corrosion. Fretting occurs from relative shaft movement that eats away at the surface. Fretting also can cause seal leaks.
Seals have finite shelf and operating lives. A mechanical seal made with common rubber gaskets, for example, could have a short shelf life because of ozone attack, exposure to direct sunlight or a hot storage environment. Large temperature swings during consecutive hot days can cause a ceramic seat to crack. Re-starting long idled equipment is a serious situation that can cause a seal face to stick and fail.
Failure mode
Failure happens when the seal stops performing its intended purpose — whether prematurely or after an acceptable lifespan. Keep this in mind as the mechanical seal industry asserts that a majority of mechanical seals fail first and wear out later. All seals eventually fail, so it’s good to have a backup plan.
A repeating premature seal failure can only be prevented if there’s a change in the conditions that brought about the problem in the first place. Be aware that the same seal in the same pump working on the same application in another part of the plant might be facing a different set of conditions and issues.
Ask yourself, “What is different here?” The collateral costs associated with repair, parts, labor and lost production time seem invisible for a seal until you face a problem with no quick solution. Thus, mechanical seal failure should be viewed in terms of the complete cost of ownership. Also, like most equipment, seals in some applications simply have more problems and need more time and care than others. Repairs and downtime are costly, so be proactive from the start.
Depending on past experience, your plant might have a good rationale for standardizing on a specific type or configuration of packing or mechanical seal. Extensive familiarity of maintenance staff with one brand or type of packing or seal will decrease labor hours. A price discount on large orders and capability for a quick response to equipment that’s down often make this proactive philosophy a valuable one.
Specific seal types
Although there are many varieties of mechanical seals in the marketplace, they generally can be classified as pusher-type and bellows-type. The pusher seal is ordinarily more cost-effective than the bellows seal, but the bellows seal has some process-specific advantages.
Pusher seals use a dynamic gasket to seal the components. This configuration is more forgiving of shaft deflection, motion and misalignment. However, the dynamic gasket might be sensitive to the process liquids or temperature excursions. For instance, a sign of imminent failure in a pusher seal is Buna-N rubber gasket components swelling and leaking in the presence of some common refrigerants.
Consider using a bellows-type seal for the more aggressive chemical applications and process characteristics. The series of bellows prevent deposit buildup and, thus, protects the rest of the seal.
Another subcategory of mechanical seal is the cartridge design that provides a more foolproof way of achieving effective sealing without requiring precise and subtle installation adjustments. This self-contained, preassembled design is easier to change out than standard mechanical seals, which reduces seal setting errors. Pusher-type and bellows-type seals can be configured as cartridge seals and used for many applications including chemical processing, wastewater and pulp/paper.
Split mechanical seals allow you to install the seal in the rotating equipment without disturbing the shaft alignment.
Dual tandem seals are good for use with hazardous or carcinogenic materials.
Other types of process-specific seals include slurry seals, mixer seals, compressor seals and steam turbine seals. In addition, special-duty seals are available for the specific and unusual application. Collecting the necessary system data helps in selecting the correct seal for an application.
Selecting pusher seals
You need specific data to specify or reorder the correct seal for an application. An example of the data normally required for properly specifying or reordering a single pusher-type mechanical seal includes:
- Shaft size — measure the diameter
- Match the seal head type — use vendor photos, if necessary
- Measure seal head outside diameter
- Determine seal head operating height — measure length of entire assembly of compressed seal both installed and uninstalled (full length)
- Determine seal seat type — use vendor photos, if necessary
- Measure seal seat outside diameter
- Measure seal seat thickness
- Determine materials of construction for gaskets, metallic parts, seats and springs
Learning how to specify the correct packing or mechanical seal is important. Beyond simply accepting what the equipment manufacturer supplies out of the box, understanding the specific process and conditions of your rotating equipment and quantifying the cost of ownership will make for the best fit.
Jeremy Marwil is in the Chemical Department at CH2M HILL in Tempe, Ariz.
Contact him at [email protected] and (480) 731-6179.