Regular inspection of liquid carbon dioxide low temperature liquid storage tank

Periodic inspection of liquid carbon dioxide low temperature storage tank includes comprehensive inspection and pressure test.
 
1. Inspection of low temperature storage tank system and valves.
 
(1) leakage of interfaces of safety accessories and other accessories, and firm and reliable connection;
 
(2) surface crack of the connection weld between each pipeline system and vacuum interlayer;
 
(3) bumping, blocking, etc. of each pipeline of the storage tank.
 
2. Thermal insulation performance inspection of liquid carbon dioxide low temperature storage tank
 
Vacuum degree test: regularly check the vacuum degree of cryogenic storage tank, grasp its change, and determine the vacuum change factors, so as to facilitate the use and maintenance of low-temperature liquid carbon dioxide storage tank. In cold state, when the vacuum degree tested reaches 16pa or the safety valve takes off frequently and the inner cylinder is abnormally pressurized, it is necessary to re vacuumize to ensure that the evaporation will not increase. Before vacuumizing, drain out the liquid in the tank of low-temperature storage tank, put 70 ~ 80 ℃ dry air or nitrogen into the tank, heat it to normal temperature, then heat it with 150 ℃ dry air for more than 4 hours, and pump it to the specified vacuum degree. Before opening the vacuum valve of the storage tank, the pipeline shall be pre emptied to prevent the moist air from entering the interlayer and affecting the normal evacuation.
 
Low temperature liquid (adiabatic) pressure vessel without vacuum test device on the mezzanine, check the change of daily evaporation rate, and measure the daily evaporation rate of the tank. It is qualified if the measured daily evaporation rate is less than 2 times the fixed daily evaporation rate.
  3. Review of original data and investigation of users' use
 
(1) review of delivery technical documents of liquid carbon dioxide cryogenic storage tank products, including material certificate and re inspection report of inner and outer cylinder, stainless steel welding process qualification report, weld flaw detection report (including flaw detection report of welding repair part), hydraulic test report, air pressure (tightness) test report, helium leakage test report, evaporation rate test report and vacuum degree test report And other information.
 
(2) review the operation records of the low temperature storage tank and pressure vessel storage tank, and inquire the management and operation personnel of the equipment. Whether there is obvious change of pressure during operation. Whether the safety valve takes off, and whether the evaporation capacity increases, etc.
 
(3) operation records (including the frequency and working conditions of the storage tank, whether there is any abnormal situation, etc.);
 
(4) change of daily evaporation rate of pressure vessel of low temperature storage tank, and whether the shell of storage tank is frosted or sweaty.
 
The external inspection, shell structure inspection and corrosion inspection methods of liquid carbon dioxide low temperature storage tank are mainly macro inspection, and other effective inspection methods are used when necessary. Macro inspection is mainly to check whether the appearance, structure and geometric dimension of the pressure vessel meet the requirements of safe use of the vessel. 4. Basic requirements for external inspection are as follows:
 
(1) the surface of butt weld and fillet weld of liquid carbon dioxide jacket (including the near seam area) shall be inspected by naked eye or 5-10 times magnifier.
 
(2) check whether there is deformation, corrosion, mechanical damage and leakage on the surface of low-temperature pressure vessel jacket;
 
(3) check whether there is condensation on the jacket surface of cryogenic storage tank;
 
(4) check whether the equipment foundation sinks, tilts or cracks;
 
(1) the gas used for the test is dry nitrogen or other inert gas;
 
(2) the test gas temperature of low alloy steel pressure vessel shall not be lower than 15 ℃. For pressure vessels made of other materials, the temperature of test gas shall meet the requirements of design drawings;
 
(3) during the air pressure test of low-temperature storage tank, the safety department of the test unit shall conduct on-site supervision.
 
5.The operation process of air pressure test is as follows:
 
(1) slowly increase the pressure to 10% of the specified test pressure and maintain the pressure for 5-10min. If there is no abnormality, the pressure can be increased to 50% of the specified test pressure.
 
(2) if there is no abnormal phenomenon, increase the pressure step by step according to 10% of the specified test pressure until the test pressure is maintained for 30min. Then the pressure shall be reduced to 87% of the specified test pressure, and the pressure shall be maintained for inspection for sufficient time. During the inspection, the pressure shall remain unchanged, and continuous pressurization shall not be used to maintain the test pressure unchanged.