Because of the unique problems experienced with highly viscous fluids that can crystallize to form abrasive particles, as well as those with adhesive tendencies, an alternative to face seals is often preferred. Radial face or lip seals provide many performance advantages when these fluids are being pumped. Traditionally, lip seals have been used to seal low (7-psi to 10-psi) pressures, but newer lip seals designs now can seal 150-psi or greater on rotary equipment.
This relatively new version of the older technology has evolved because of the advancements in sealing element materials. The current lip-sealing devices are primarily designed for positive displacement pumps and centrifugal pumps up to 1750-rpm, but can be applied to other types of rotating equipment as well. They are offered in various designs from single lip seal to multiple lip cartridge versions. There are designs for vacuum and mixer service (incorporated with an expansion joint) that can combat excessive shaft movement from radial, axial, and angular vibration.
Figure 1. Cross section of a typical lip seal.
Lip seals are particularly effective in applications with process fluids such as adhesives, asphalt, polymers, caustics, chocolate, fuels, latex, molasses, oil, resins, soaps, syrups varnish, etc. These can be some of the most difficult pump sealing applications.
High-Pressure Lip Sealing Technology
Any seal alone will not prevent a product's tendency to change state and cause sealing issues. In many cases environmental controls are recommended to circumvent this problem and enhance seal life.
Newer lip seals designs with dimensionally stable sealing elements have only one moving part (the seal sleeve/pump shaft). The seal has no springs to clog and the sealing element does not allow product between the stationary and dynamic sealing surfaces. There is a positive seal at the tip of the sealing element. The lubricity that is required to prevent excessive frictional heat is provided by a smooth hard-face coating on the shaft sleeve (typically chrome oxide) and the inherent lubricity of the lip material itself.
Materials for the sealing elements should have good lubricating qualities, not cold-flow, be chemically resistant and have good recovery (memory) characteristics through a wide temperature range. The memory characteristics can easily be checked by determining how snugly the sleeve is gripped by the lip seal when turning the shaft. If the sleeve is loose and/or easy to turn, the lip seal will probably leak at low pressure. The sealing material also should transfer frictional heat from the rotating surface and should not cause product contamination.
Most high-pressure lip seals with the newer dimensionally stable sealing elements are very sensitive to radial shaft movement, so it is important that the shaft is stable and the seal is mounted to the equipment absolutely concentric and square to the shaft. Correct seal installation can make the difference between three years service life and two weeks, or less.
Environmental Controls Improve Seal Performance
The versatility of the multiple lip high-pressure seal can be enhanced by various environmental controls. Many will not contaminate the process fluid and will offer an indication of when a new repair kit is required.
Some process fluids are heat sensitive, some are oxygen sensitive, and some solidify with moisture. To achieve effective sealing of these products, one must first know what causes each to change state and act accordingly.