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.
The design of a STASSKOL intermediate packing is similar to the design of a piston rod packing. Leakage of lubrication oil into the distance piece is prevented by the use of suitable sealing elements. STASSKOL manufactures intermediate packing in several concepts.
The STASSKOL standard is a purged version. This packing consists of a basic case with two covers in a modular desing. An additional chamber ring can be installed between basic case and cover to mount a second leakage sealing, together with O-rings serves for the sealing if intermediate packing. The standard intermediate packing also has a connection for purge gas and additionally a labyrinth ring with a thread groove at the inside diameter. Generally, nitrogen gas is used as a purge agent.
MODE OF OPERATION
At the standard version of the intermediate packing the thread inside of the labyrinth ring will provide the rotation of purge gas around the reciprocating piston rod. This construct avoids gas leakages into the second distance piece or even the crank case.
In general nitrogen is used. The intermediate packing is designed to fit the purpose of leading the purge gas flow in the compressor distance piece from where it will lead to the vent line.
A change of both the wear parts as well as the complete intermediate packing is possible. In these case the connection between cross head and piston rod must be disconnected. It is vital to ensure that the piston rod thread is covered by a conical mounting sleeve in order to prevent damage to the internal parts.
The outside diameter of the mounting sleeve must not in any case exceed the nominal size of the piston rod.