Corrosion and protection of heat exchanger in the conversion process
our plant adopts the pressurized conversion process for synthetic ammonia, with the working pressure of 1.2MPa. The process before and after the first heat exchanger (hereinafter referred to as "Jiajiao") is roughly as follows: half water gas is heated and humidified through direct contact with hot water through the saturation tower, and then only some superheated steam is added through the mixer and directly enters the lower head of Jiajiao, and then indirectly exchanges heat through the heat change and air exchange between pipes, The heated and humidified semi water gas from the saturation tower in the process has not been separated from gas and water
in 1985, when the 25kt/a synthetic ammonia was expanded, a stainless steel cored nail was added. The Jiajiao was out of operation in 1999 due to the corrosion of the lower shell (the high heat and high humidity shift gas converted by the shift furnace on the shell side), but the stainless steel core was intact. According to the electrochemical principle, this is a typical galvanic corrosion in electrochemical corrosion, that is, the active carbon steel shell is corroded, and the corrosion part is at the lower part of the shell (cylinder), that is, the ventilation outlet after cooling. When excessive steam is added due to the pursuit of too high CO conversion rate during operation and the steam after gasification of chilled water in sections II and III of the shift furnace is cooled, it is completely possible to make the temperature of the change gas lower than the dew point at the lower part of the A-line. Acidic components such as H2S, CO2 and residual O2 in the medium provide conditions for electrochemical corrosion, thus forming acid batteries and generating chemical corrosion. In 1999, the company replaced a new one with the same model and material, which must be equipped with a separate oil return pipe material. In October 2001, the tube bundle and tube plate (both made of 1Cr18Ni9Ti) were corroded and could not continue to operate. The corroded parts were in the lower tube plate and the lower part of the tube bundle. The members of the wood plastic composites Professional Committee of China Green Building Materials Industry Development Alliance nearly include all backbone enterprises and organizations engaged in the R & D and production of new biological materials (wood plastic, a high-power density heat-resistant and high-strength aluminum alloy material for military and marine equipment) composites and their equipment. This paper analyzes the corrosion phenomenon of stainless steel internals of heat exchangers, and strives to find out the causes of corrosion and protection methods
1 a-cross corrosion
1.1 a-cross structure and material
this damaged heat exchanger is a typical fixed tubesheet heat exchanger. The upper and lower tubesheets and tube bundles are made of 1Cr18Ni9Ti, the shell has expansion joints, and the shell barrel and head are made of carbon steel. In order to prevent the air flow from eroding the tube bundle, the upper and lower air inlet and outlet ends of the shell side are equipped with annular air guide baffle rings
1.2 Nail Cross corrosion
see Figure 1 for Nail Cross corrosion parts. See Figure 2 for the operating environment with the continuous improvement of social development and people's living standards.