Pressure test of air separation unit
Generally, clean water is used as the test medium for pipeline pressure test. For stainless steel pipelines, the chloride ion content in the water shall not exceed 25×10-6. When the pipeline design pressure is less than or equal to 0.6MPa, gas can also be used as the test medium, but effective safety measures should be taken.
For this part of low-temperature pipelines with a design pressure less than or equal to 0.6MPa, choose gas pressure test. The gas pressure test temperature must be higher than the brittle transition temperature of the metal material, and the test pressure is 1.15 times the design pressure. For low-temperature pipelines with a design pressure greater than 0.6MPa, clean water is used for testing, and the test pressure is 1.5 times the design pressure. During the test, the ambient temperature should not be lower than 5℃.
When the ambient temperature is lower than 5℃, antifreeze measures should be taken. After the pressure test is qualified, it is purged with compressed air. However, due to the difficulty of on-site drying, construction progress and process reasons, the design agrees to change the low-temperature pipeline water pressure test to gas pressure test, but the consent of the construction unit must be obtained, and it must be ensured that the pressure test plan has practical safety measures. The pressure test plan must be reviewed by the safety department and technical department of the construction unit and approved by the technical person in charge.
Compared with the water pressure test, the gas pressure test has many advantages such as easy test medium source, short pressure test time, easy drying, shortened construction time, and reduced construction procedures. However, the fatal disadvantage of gas pressure test is poor safety and great potential for accidents. This is because during the gas pressure test, the energy storage in the system is much larger than that of the water pressure test. Once the pipeline ruptures, the energy released by the gas explosion of the same volume and pressure is much greater than that of the liquid, and the scope of harm is wide.
Therefore, when selecting gas pressure test, special construction technical measures must be formulated, and they can only be implemented after the consent of all parties in the project management.
The design points of low-temperature pipeline materials outside the cold box of the air separation unit are as follows:
1) Austenitic stainless steel is recommended for low-temperature pipeline materials.
2) The material and structure of low-temperature valves should meet the requirements of low temperature resistance. The low-temperature valve adopts a long-neck bonnet structure. For low-temperature valves with double valve seats, pressure relief measures should be set.
3) The cold insulation material of the low-temperature pipeline should have good thermal insulation performance, and the vacuum insulation pipe should ensure the smooth construction and installation.
4) Low-temperature pipeline pressure testing usually uses water pressure testing. When using air pressure testing, effective safety measures should be taken.
