Secondary sealing elements (different than the secondary gas seal in a dual arrangement) provide sealing between the seal assembly and the compressor, as well as between various seal components. Typically, elastomeric o-rings are used as the secondary sealing elements, although other seal types are used to address specific problems. Most secondary sealing elements are static (once parts are assembled, there is no movement of the parts that form the joint). As with other machinery applications, it is important to select materials that are compatible with the normal and potential gas streams seen by the seals. In addition, high pressure applications must be evaluated for the potential of extrusion and explosive decompression (the latter is a function of sealing pressure, gas composition and compressor system decompression rate). High pressures may require the use of high Durometer elastomers or polymer (such as PTFE) materials. The polymer seals often use metallic springs to provide the proper contacting or energizing force.
In addition to static secondary sealing elements, there are also dynamic secondary sealing elements, which seal the moving joints between the stationary seal faces and their retainers or housings. The stationary seal faces must move axially to accommodate lift-off, gas film thickness changes and axial movement or thermal growth of the rotor. The dynamic capability of the stationary face secondary seal is another critical performance and reliability aspect of dry gas seals, since sticking or hang-up of this seal can result in either excessive leakage or damaging face contact. Potential design options to minimize seal hang-up include spring energized polymer seals or spring energized o-rings, both of which reduce o-ring contact forces (spring energized o-rings are shown behind stationary seal faces on prior seal arrangement drawings). Some spring energized designs are also claimed to provide at least some degree of reverse pressurization tolerance. Although this may be a benefit for some applications, at this time, there is insufficient data and experience to support relying on this feature to eliminate or even reduce measures for preventing reverse pressurization.
Installation and maintenance should always be considered in the secondary sealing element joint design, especially those between the seal housing and compressor casing, and the seal sleeve and shaft. Optimum o-ring placement and tapered diameter changes can minimize or eliminate the potentially damaging action of sliding orings across components during installation, as well as reduce potential for o-rings falling out of ID grooves during installation. Besides sealing, o-rings between the shaft and seal sleeve may also serve to center the rotating parts of the seal on the shaft. Since seal sleeves are part of the seal assembly, and must have sufficient clearance for a cold slip fit, o-rings can act as a low friction centering device. More critical applications may require metallic centering devices.