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Particles present in the process gas can contaminate the primary seal when seal gas flow across the process side labyrinth is insufficient, or if inadequate filtration or conditioning is provided for seal gas to produce the required quality of gas. As a consequence particles can enter the gas grooves on the rotating face and the sealing gap.
This article is an excerpt from the paper, "Ensuring reliability of compressor gas seals during long periods of pressurized hold" by Glenn Schmidt and Daniel Goebel of EagleBurgmann at the Middle East Turbomachinery Symposium 2013.
If the size of the particles is small enough, they will blow through the seal. When larger particles are present they get trapped inside the grooves or gap causing negative affects to the sealing behavior or seal reliability. Besides the sealing gap, particles can also block the dynamic sealing element. The dynamic sealing element is an O-ring or elastomer free sealing device between the non-rotating seal face and the balance sleeve. The non-rotating seal face must slide together with the dynamic sealing element on the balance sleeve axially compensating for axial position or movements of the compressor shaft in relation to the seal housing.
The non-rotating face must also move freely to adjust for any movements resulting from the normal seal behavior. If the dynamic sealing element is prevented from moving freely to adjust for axial movements, the seal gap will be affected leading to high seal leakage or primary seal failure. In order to avoid the above-mentioned scenarios, a supply of clean gas must be provided to the primary seal whenever the compressor is pressurized or in operation.
A reliable clean gas flow ensures no contamination from the process gas will enter the primary seal. In the next steps, proper filtration must be selected. Typically, filter elements are selected to remove particles as small as 3 µm and sometimes even 1 µm. This ensures larger particles than the seal gap will tolerate are removed producing a clean quality gas for the primary seal. Seal gas filters have high alarms on differential pressure to identify when filter elements require replacement.
Dual filter housings are used so replacing filter elements can be completed during compressor operation without interrupting seal gas flow. Filters supplied with seal gas systems have limitations on the volume and size of liquids and particles they can manage. Higher levels of contaminants in the seal gas will require additional filtration. This filtration is called pre-filters and typically contains some type of liquid knock out.