A user of DEAR air separation reflected that when the air separation plant was put into operation, a large liquid nitrogen storage tank was equipped for emergency backup when the air separation was stopped or export of liquid products. The value of oxygen content in nitrogen product is designed to be less than 3ppm (i.e., < 3 ×10-6) due to the high requirement of users for nitrogen purity. But in fact, the oxygen content of liquid nitrogen in the tanks had been found more than 5ppm. The substandard purity of liquid nitrogen not only affects the export of products, but also directly affects the emergency backup of liquid nitrogen backup system.
On-site observation by DEAR air separation technician: in the process design, when the air separation plant is started or shut down, the liquid nitrogen feeding valve HV3207 is closed before the liquid nitrogen purity is qualified, and it is forbidden to enter the storage tank. In theory, there is no possibility that the liquid nitrogen purity is not up to the standard. But in fact, in the process of start-up or shut-down, the phenomenon of high liquid nitrogen oxygen content in the storage tank has occurred for many times.
Through the actual observation and analysis, it is found that the reasons for the excessive content of liquid nitrogen and oxygen are as follows:
1.HV3207 valve has internal leakage. When the plant is started orshut down, the operator often wrongly thinks that the process of liquid nitrogen entering the storage tank has been cut off, and the manual valve 7306 of liquid nitrogen entering the tank has not been closed in time, resulting in unqualified liquid nitrogen entering the storage tank;
2.During the plant start-up orshut-down, the working condition fluctuates greatly, and the range of the pressure controller of the gas-liquid separator is too small to be adjusted. Once the pressure of the gas-liquid separator is lower than the pressure of the upper column, the impure gas nitrogen of the upper columnflows back into the gas-liquid separator, which reduces the purity of the liquid nitrogen;
3.The pipeline of liquid nitrogen entering the storage tank is not completely replaced, which makes the residual oxygen components in the pipeline enter the storage tank.For theabove situation, the user is required to take the following measures: standardize the operation of valve 7306, close valve 7306 immediately and slightly open valve 7320 when HV3207 valve is closed automatically due to unqualified liquid nitrogen purity during equipment start-up or shut-down, to prevent unqualified liquid nitrogen from entering the storage tank; expand the range of pressure controller of gas-liquid separator, and switch valve PV3207 to manual mode and turn down in time when plant start-up or shut-down, to ensure that the pressure in the gas-liquid separator is not lower than the upper column pressure; completely replace the pipeline of liquid nitrogen into the tank.
Through the improvement of operation method, in the later operation, the occurrence of nitrogen purity exceeding the standard is avoided. The quality of liquid nitrogen products are guaranteed to be exported and stored.
