In reciprocating compressors the compression chamber is sealed against the distance piece by means of a piston rod packing. 100% sealing efficiency is technical not possible even in case of piston rod packings with a buffer gas system there is a possibility that process gas is able to enter the first distance piece.
Therefore, when compressing flammable, explosive or toxic gases the compressor layout usually includes a double distance piece and the two distance pieces are separated by so-called intermediate packing. This avoids the formation of a dangerous gas mixtures. Intermediate packings can be classified into purge, non-purged and buffered versions.