Preventing bearing lube oil from contaminating the seal faces is a key element of seal reliability. Provisions are often necessary to accomplish this when the bearing and seal are in close proximity, especially if the span between the bearing and seal is contained within a housing. A restrictive seal in conjunction with inert gas purging (separation gas), are typically used to form a barrier for the gas seal assembly (see Figure 200-53). The restrictive seal is usually a radial clearance seal in a lantern ring arrangement (separation gas enters between a pair of close clearance seals), and may either be a labyrinth or close clearance carbon ring design. A labyrinth design will typically consume in the range of 5 SCFM of separation gas per machine end, while a carbon ring can reduce this rate by at least one half.
When available, nitrogen is preferred as the separation gas for compressors in combustible gas services. Although air has been used in some applications, it has the potential for creating combustible mixtures in the cavity between the separation seal and the gas seal (the outer seal vent area). At present, excess purge gas (25 or more SCFM) either to the separation seal or directly to the outer seal vent cavity is a solution used by at least one compressor OEM. Membrane units that generate nitrogen from an air supply may provide an acceptable alternative solution provided the membrane system is sized and designed to achieve the proper nitrogen purity.