Causes of failure of stainless steel heat exchanger before RO and its protection

Abstract: two sets of plate and frame type 316 stainless steel heaters of chemical make-up water system of Jiaxing Power Plant Phase II project were damaged and failed successively in a short period of time After analyzing the operation conditions, failure macroscopic characteristics, corrosion product characteristics and material chemical composition of heat exchanger, the conclusion of stress corrosion cracking was obtained, and the corresponding preventive measures were put forward Key words: stainless steel; heat exchanger; chloride ion; stress corrosion cracking Jiaxing Power Plant Phase II project chemical make-up water system is equipped with 2 × 130t / h reverse osmosis pre desalination system and ion exchange system to provide high-quality desalted water for the boiler Before the reverse osmosis system, mechanical filter is designed to coagulate and filter the reverse osmosis water, The process flow is as follows: the make-up water supply system was put into operation in September 2004 and began to leak in December 2004 Two sets of plate and frame type 316 stainless steel heaters were damaged and failed successively in a short time, which affected the normal water production of the reverse osmosis system, What causes the failure of the heat exchanger in a short time? Based on the analysis of the operation conditions of the heat exchanger, the macroscopic characteristics of failure, the characteristics of corrosion products and the chemical properties of materials, the causes of stress corrosion of the heat exchanger are analyzed, and the conclusion of stress corrosion fracture is drawn And put forward the corresponding preventive measures 1 Brief introduction of equipment operation condition: the failure heat exchanger plays the role of heating reverse osmosis water inlet in the desalination system The heat exchanger is a plate structure, and the heating source is the auxiliary steam of the unit After the temperature and pressure reduction device, the temperature of the heating steam is reduced to about 150 ℃, and the pressure is about 0.3-0.4mpa The plate of the heater is made of 316 stainless steel (0Cr17Ni12Mo2) corrugated plate, with a thickness of about 1mm One side of the plate runs steam and the other side runs water, During operation, the water pressure is about 0.5-0.6mpa, the inlet water temperature is about 0-15 ℃, and the outlet water temperature after heating is less than or equal to 25 ℃ The water side and the steam side are sealed and isolated by rubber rings The water on the water side is the effluent after the raw water pretreatment The raw water pretreatment is coagulated and clarified by adding polyaluminium chloride into the raw water After the pretreatment, sodium hypochlorite is added for sterilization and disinfection The chlorine ion content of the raw water is about 80-130mg / L on average throughout the year, After pretreatment, the chloride content is higher The chloride ion in the heating steam is about 1 μ g / L It can be seen that the content of chloride ion in the water side is much higher than that in the steam side 2 Analysis of failure characteristics and corrosion products of heat exchanger Through the appearance inspection of the heating plate of the failed heat exchanger, it is found that the steam side of the heating plate is silver gray as a whole, and there are a small amount of white scale deposits at the corrugated plate at the water side and steam side inlet of the heat exchanger The scale is very hard, and there are corrosion points and cracks under the scale The cracks develop from the steam side, and the cracks are dendritic, Zui is 35mm long See Fig 1 for macro morphology There is local pitting in the middle of the steam side, and there are corrosion spots and crevices in the middle of the steam side, with a depth of about 1mm; while the overall color of the water side of the heating plate is light rust yellow, and the overall water side has no corrosion spots.