Q. Corrosion Assessment for MVC Evaporator System?
Typical evaporator systems in SAGD plants operate under extremely severe corrosive conditions due to high chloride content, elevated temperature, and very high levels of TDS. Severe pitting and CSCC are major corrosion concerns in these systems.
The evolved gas from the brine contains mostly water vapour and non-condensable gases, which is very corrosive. Compressor metallurgy shall be upgraded to prevent corrosion due to these corrosive gases.
Evaporator's tubes are subject to severe corrosion due to CSCC and pitting.
Distillate is also considered a corrosive fluid. Pitting is the primary damaging mechanism in distillate service.
The general corrosion prevention philosophy in evaporation system is to upgrade the metallurgy. The use of coatings and non-metallic materials are not applicable due to the high service temperature.
Other service conditions that need to be considered in material selection are short term conditions like start-up, shutdown, and cleaning operations. During these conditions oxygen is present at concentrations significantly higher than normal operation. Cleaning operations can be worse case from corrosion points of view as very low pH and high level of dissolved oxygen are concern. Corrosion is most likely to occur during off-design condition rather than normal operation of the package.
Up to 60,000ppm of chloride is the maximum threshold concentration above which upgrading to 6Mo steel is necessary. Also depending on the chemistry of water, for produced water this limit may be lowered to around 40,000ppm; however for deaerated water, like the water used for boilers, the higher margin can be tolerated before upgrading to higher alloys.