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Dry gas seals interface with the compressor process inner labyrinth, compressor heads, and journal bearings. All the interfaces of the gas seal with its environment are potential pollution paths.
Below are excerpts from the paper, "Gas seal contamination" presented by Raphael Bridon and Olivier Lebigre of Dresser-Rand at the First Middle East Turbomachinery Symposium.
To avoid gas seal pollution and because of the very thin running gap between rotating and static faces, it is necessary to properly treat sealing gas upstream of the gas seal. Additionally, seal gas quality must be ensured at all times, during all operating sequences such as standby, start-up, running, and shutdown. Sealing gas is usually required to be free of particles 3 microns (absolute) and larger and 99.97 percent free of liquids.
Black powder coming from compressor inner labyrinth[/caption]
Contaminants can be either solids, liquids or gaseous. Foreign particles can be:
Particles from unclean piping (seal supply lines, vent lines)
Particles from corroded piping, compressor or gas seals components
Pollution can occur because of:
Salty deposit on gas seal housing[/caption]
Because the running gap between static and rotating faces is around 5 microns, any particle larger than the gap will cause erosion of the faces leading to an increase in the gas leakage and eventually failure of the gas seal. Where very thin particles are present, particle accumulation and clogging of the rotating seat grooves results in a loss of the lift-off effect and again failure of the gas seal.
Another consequence can be damage of the secondary sealing surfaces and more specifically of the balance diameter.
Liquid pollution can:
Condensates at the gas seal interface will lead to a degradation of the lift-off effect, friction between static and rotating seats, heat generation, parts deformation, O-rings extrusion, thermal shock on the rotating seat and eventually failure of the rotating and/or static rings.
In addition static faces are commonly made of carbon and are therefore subject to blistering due to the porous nature of this material.
Scratched static seat[/caption]
Seal gas is not inherently a pollutant for the seal, assuming it is adequately treated upstream of the gas seal. However, some pollution could occur if a chemical reaction develops:
Scratched dynamic seat[/caption]
As a contact-free sealing device, the gas seal does not require any maintenance of the cartridge itself. However, in operation, some periodic maintenance of the seal is preferred to check for any pollution, check the condition of the carbon rings and replacement of O-ring which have a limited life span. This periodic maintenance is an opportunity to control the condition of operation, the efficiency of the panel and take corrective action if required. As a consequence, providing both gas seal and gas seal system are properly designed, a gas seal failure is always an accident and shall not be considered unavoidable. Thus, a root cause analysis (RCA) of each seal failure shall be conducted.