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.
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 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.
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).