Pay attention to the hydrostatic test of stainless steel storage tanks

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Pay attention to the hydrostatic test of stainless steel storage tanks: A newly built 304L stainless steel storage tank started operation and leakage occurred immediately at its bottom. After cleaning, inspection revealed numerous rust spots and small nodules on the bottom, along with micro-erosion pits the size of pinholes. There are approximately a thousand pits at the bottom, and about 50 of these pits have completely penetrated through the 6.4 mm thick plate. A new base was installed, and the pitting in the side walls and horizontal welds was ground off and rewelded. The characteristics of the corrosion damage indicate microbial corrosion. A hydrostatic test was conducted 6 weeks ago, using untreated river water from the nearby area; it contained approximately 200 ppm of chlorides and had moderate hardness. After the pressure test, the water was not drained and remained in the tank for 6 weeks. Comment: Pressure testing of equipment is an important aspect of corrosion control. Because the water used for pressure testing is also a corrosive agent for the equipment materials. To prevent corrosion of the equipment caused by pressure testing, it is necessary to pay attention to the quality of the water used for testing. This water should be treated to remain clean and free from any substances that could cause severe corrosion to the equipment. Second, it is important to ensure that the pressure testing procedure is carried out correctly. For stainless steel equipment, the key is to prevent the surface from being contaminated by chlorides. Because chlorides can cause various types of localized corrosion in stainless steel. Although the overall corrosion rate of stainless steel by tap water is low, tap water contains high levels of chlorides, making it unsuitable for pressure testing. Each factory has specific regulations regarding the allowable chloride content in the water used for pressure testing different stainless steel equipment. More importantly, after pressure testing, water should not remain in the equipment; it must be drained immediately and the surface dried. Otherwise, it can lead to the initiation of pitting. Furthermore, during the evaporation of water, chlorides are concentrated, creating a harsh corrosive environment for stainless steel equipment, which may suffer corrosion and damage even before it is put into use.