Raphael Bridon and Olivier Lebigre taught a tutorial at the 40th Turbomachinery Symposium September 2011 in Houston, Texas. Below are excerpts from the tutorial.
All interfaces with the gas seal and its environment are potential pollution paths. Pollution occurs:
On the inboard side if the untreated process gas leaks through the compressor inner labyrinth when the seal gas pressure is lower than the reference pressure
On the outboard side in the compressor bearing oil when the separation seal is damaged or fails, or there is insufficient separation gas (air or nitrogen)
In the seal supply ports in the compressor head if sealing gas is not adequately treated in the gas seal system located upstream of the gas seal or if piping is dirty
In the vent lines if dirt remains or if liquids can be trapped or do not drain.
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.
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
- Particles in the process gas
Pollution can occur because of:
- Poor or non-existent filtration of sealing gas
- Reverse pressurization of the gas seal
- Insufficient flow under the compressor inner labyrinth
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:
- Result from bearing oil leakage through the separation seal or migration along the shaft line
- Be present in the process gas stream if there is no, or an inappropriately designed, coalescer in the filtering system
- Be due to the condensation of the sealing gas; seal gas is most commonly taken from the compressor discharge, filtered and then expands as it passes through the gas seal system components, such as the filter, valves, orifices and gas seal faces; as pressure drops, temperature decreases and could result in the seal gas entering in the liquid phase
- Result from contamination by corrosion inhibitors present in the process piping.
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.
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:
- Between the seal gas components themselves, for instance polymerization of the seal gas
- Between seal gas components and gas seal / gas seal panel components’ materials, such as a reaction between sulfur present in the seal gas stream and nickel present in the rotating ring
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.