Cooling water is one of the most essential utilities in a chemical plant. Although most of the water used in the plants is chemically treated, equipment failures and problems still occur. The delicate balance between corrosion inhibition, deposition management and microbiological control of the cooling water requires the close attention of the water treatment and corrosion personnel. The tightening of environment regulation, the increasing cost of securing water resource and the disposing of blow-down water also make the work more challenging. Included in the presentation are a few case histories of cooling water related equipment failures. They help demonstrate
Plant people always have a love and hate relationship with their cooling water. A plant cannot run without it, but it also causes many problems and headaches. Chlorine is the most common micro-biocide used in this Bayer Texas Gulf Coast Polymers plant. Due to the discovery of Legionnaire bacteria a few year back, the free chlorine (or hypochlorite) level is normally kept at relatively high levels of 0.5 ? 1 ppm. At the same time, the pH is kept at a range of 7.0 to 7.3 to minimize the scaling problems that are common in many operation units. For corrosion control, the water-treatment company for the plant uses polyphosphate, and copolymer is used for scale control. Despite a very conscientious effort to monitor and control the treatment chemical levels, the corrosion rate on carbon steel heat exchangers still ranges from<2 mpy to 10 mpy, and heat exchangers still need to be pulled regularly to remove the scale build-up. These are accepted as regular maintenance activities. However, premature failures still happen to equipment that use cooling-water, and some of them are not directly related to the effectiveness of the water treatment program. Some examples are discussed in the following case histories.
The tubesheets and the internal surface of the heads of an ammonia refrigeration condenser were found to have significant damage (i.e. spalling) on the thick-film epoxy type lining. In the exposed areas, there were general corrosion in the tube-totubesheet next to the seal welds (Figure 1), and general corrosion in the base metal (Figure 2). The condenser has carbon steel (CS) tubesheets and heads but 316L stainless steel (SS) tubes. Process on the shell side is ammonia at 200?? 250? F, and tower water runs in the tube side. It has been in service for 5 years.
Cooling tower water is corrosive to carbon steel equipment and the corrosivity depends on the water treatment, process conditions and outside environment. The corrosion rate of CS in tower water in this plant is estimated to be as high as 10 mpy. In this case, the corrosion rate was also enhanced by the galvanic effect of the stainless steel tubes and the carbon steel tubesheets and heads. Coatings and sacrificial anodes are normally used for corrosion protection on carbon steel HE. However, coatings can be damaged by various reasons like poor application, high process temperature, or erosion. In this case the high operating temperature most likely had caused the coating failure. Since there were no anodes to protect the tubesheets and heads, corrosion started as soon as the coating failed.
The problem was solved by weld cladding the tubesheet surface with 316L SS, and the installation of magnesium alloy sacrificial anodes.