Centrifugal compressor
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Why do centrifugal compressors have a two-stage seal? What is its function?
The primary seal is prone to leakage; additional sealing is required when the medium being transported is a toxic or harmful gas, or a gas that cannot be wasted
Grade 1 sealing medium: gas; Grade 2 isolation lubricant
Those equipped with a secondary seal are generally used in high-pressure, flammable, and toxic environments where zero leakage is required; the secondary seal serves as a protective barrier designed to prevent the leakage of the medium into the atmospheric side in the event of a leak in the primary seal. It is usually connected to a flare. The gas used for isolating lubricating oil is called isolation gas
Most systems use a three-stage air intake system: the primary sealing gas is the process gas at the compressor outlet (high-pressure nitrogen is used initially); some of this gas leaks into the compressor while the rest flows through the primary leakage pipeline. The secondary sealing gas is nitrogen, and any leaks proceed through the secondary leakage pipeline, with an isolator in place to prevent lubricant from entering.