Five protective measures for low temperature liquid nitrogen storage tank

1. Material selection of liquid nitrogen storage tank. It has been proved that the higher the strength of the pressure vessel material is, the greater the possibility of stress corrosion. However, the lower strength limit without stress corrosion is related to the content and characteristics of impurities, stress size, operation speed and other factors. In order to prevent the stress corrosion of the low temperature liquid nitrogen storage tank, the best choice should be made under the comprehensive consideration of operation pressure, residual stress, safety and economy Steel with lower strength.

2. Regularly check the liquid nitrogen concentration and moisture content of the liquid nitrogen low-temperature storage tank. If the water content is lower than the critical concentration, timely supplement the water content to keep the moisture content of the liquid nitrogen low-temperature storage tank in the range of 012% - 1%.

3. The reasonable structure and welding process shall be adopted for the liquid nitrogen storage tank. The stress concentration caused by too many and too concentrated welds, asymmetric welds, crossed welds and unreasonable welding sequence shall be avoided in the structure. During the manufacturing of the liquid nitrogen low temperature storage tank, the strong welding shall be avoided to prevent the defects such as undercut and staggered edge, so as to ensure the smooth surface contacting the medium as much as possible.

After manufacturing, annealing heat treatment should be carried out to remove the residual heat stress after welding. Correct PWHT can greatly reduce the residual stress in the manufacturing process and the peak hardness of the HAZ. 4. For the new liquid nitrogen cryogenic storage tank before being put into use, the air inside shall be completely removed; in the process of filling, discharging and maintenance, some measures shall be taken to avoid bringing in any air. For the large liquid nitrogen low temperature storage tank, the ammonia steam shall be continuously condensed, while the non condensable gas is mostly air, which shall be discharged. For the smaller equipment, the air inside the liquid nitrogen storage tank shall be removed by extraction or transpiration 。 In a word, eliminating air pollution in liquid nitrogen tank can effectively prevent stress corrosion.

5. For the newly used liquid nitrogen cryogenic storage tank, the internal and external inspection shall be carried out according to the regulations and periodic inspection shall be carried out. The key inspection shall be carried out for the liquid and gas phase interface, arc striking and striking place, T-joint and other corrosive parts. All welds below the liquid level shall be subject to 100% magnetic particle or ultrasonic flaw detection. If conditions permit, all welds shall be subject to 100% magnetic particle detection.