(90c) Severe Erosion In A Charge Gas Compressor

During a plant turnaround in 2009, charge gas compressor cases were opened for inspection.  Significant damage was found on the shafts under the labyrinth seals, rotors, and third case balance piston.  The severe wear was caused by contact between the polymer coated rotating parts and the stationary components.  The polymer formed was believed to be derived from styrene and butadiene in the cracked gas.  While these polymers are generally “soft” in nature, harder materials can become trapped in the polymer layers, leading to an abrasive surface.

The possibility of this severe erosion has been referred to in literature and similar events have been previously documented with lesser impacts.  Operating temperature has a profound effect on the amount of polymer fouling.  Literature states that styrene and butadiene polymerization rates double with each 10^oF temperature rise from 170^oF to 220^oF.  As a result of a 2004 plant expansion, the charge gas compressor discharge temperature increased.  The most severe damage occurred inside the fourth and fifth stages where temperatures were the highest.  Fouling negatively impacts the efficiency of the stages due to the changes in flow characteristics.  The fourth and fifth stage polytropic efficiencies decreased from 2004 to 2009.  Turbine steam usage was increased to achieve the desired compressor capacity as a result of the lower performance.

During the period between 2004 and 2009, a compressor rotor wash program and anti-foulant addition regimen was followed to reduce fouling and remove build-up.  Vibration monitoring equipment did not indicate any significant vibration excursion during the period, which indicates the damage was continuous in nature with no single significant event.

As a result of observing the significant damage, changes were made to the wash oil addition system. The addition of wash nozzles on the cases to directly wash certain rotors and a more robust automatic delivery system to improve consistency of wash oil rates occurred.  Compressor parts were changed, including tungsten carbide shaft sleeves under the labyrinth seals, and case wear pads.  Interstage coolers were also cleaned to remove polymer build-up to help control operating temperatures.