Multiple lip seals are commonly used in centrifugal pumps and positive displacement pumps. There are many variations, but one version that has been particularly effective is a triple lip seal arrangement. The key feature is the third outboard lip seal element, which can be used for several functions. Sealed media can be compartmentalized, providing the opportunity to apply any of the API piping plans based on the type of media being sealed. For instance, it can serve as an excluder or a secondary seal in a quench gland design for media that crystallizes or hardens with exposure to ambient temperature and pressure. Unlike a mechanical face seal, there are no rotating parts, and all internal components are not just replaceable, but replaceable on-site by in-house or field maintenance staff. The design of a triple lip seal yields a pressure-actuated sealing element with a membrane-like body, which can be manufactured from many polymer types such as filled or virgin polytetrafluoroethylene (PTFE), polyetheretherketone (PEEK), and polyurethane. Other triple lip seals are also designed and manufactured in various elastomeric and elastomeric energized PTFE configurations (see Figures 1 through 4). In this article, we will focus our discussion on the most versatile sealing element material—filled or virgin PTFE.
Sealing Sense
Fluid Sealing Association, Ergoseal
08/03/2017
(clockwise) Figure 1. PTFE triple lip seal. Figure 2. PTFE triple lip seal. Figure 3. Elastomeric triple lip seal. Figure 4. PTFE triple lip seal (Graphics courtesy of FSA)
Figure 5. Variables for lip seal heat transfer
In designing the lip seal membrane, the length of the heat transfer area must be minimal. A larger area will result in an increased amount of heat generated. The larger the temperature gradient between T1 and T2, the greater the heat transfer rate will be. As the length increases, the ability to remove heat is less efficient, and in turn, the sealing surface area reaches higher temperatures.
Another key performance parameter that affects lip seal performance is the pressure velocity (PV) capability of the lip seal material. The performance of a lip seal is directly related to the heat at the sealing surface. Because wear rate increases with higher heat based on material tensile strength, it is important to understand the major factor influencing heat generation. The PV relationship best explains heat generation and wear on the seal:
Figure 6. Filled PTFE element (membrane)
where b reflects lip seal concentricity; DP is the pressure differential across the lip seal; Pi is the pressure due to seal and sleeve interference; and Vm is the velocity at the sealing surface diameter. As pressure and velocity increase, frictional forces result in higher interfacial temperature and thus higher wear rate. Therefore, high PV capability requires the use of a lip seal material with a low coefficient of friction (see Figure 6).
Figure 7. Triple lip seal cartridge (conventional).
Figure 8. Triple lip seal cartridge with intermediate lubricant (conventional).
Figure 9. Triple lip seal cartridge with intermediate lubricant (dual seal configuration).
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