With the development of the economy, the applications of oxygen, nitrogen, fluorine, carbon dioxide and other gases in industrial, agricultural and daily life are becoming more widespread. These gases are present in liquid form during transport and storage in order to increase efficiency. To liquefy the above gas, it must be cooled and pressurized. In order to facilitate the processing and manufacture of its storage tank, its working pressure is generally set at 0.8 to 1.0 MPa, which is the design pressure of the storage tank. Corresponding to the design pressure, the working temperature of the liquefied gas is determined by its saturated vapor pressure. The operating temperature range is generally based on the saturated vapor pressure of different gases.
-196 ° C or more. The design temperature is typically -196 °C. Since the low temperature liquefied gas storage tank is operated at a low temperature,
In order to make it have better heat preservation, it is generally designed as a vacuum jacket structure of a liner and a casing, that is, a vacuum insulation type. The jacket material is 16MnR or Q235A; the liner material is generally considered for its low temperature resistance.
OCr19Ni9; maintains a vacuum of 1 to 3 Pa or more in the jacket and fills the insulating material Zhu Guangsha. The general requirements for Zhu Guangsha are: the particle size is in the range of 0.1 to 1.2 mm, the bulk density is in the range of 30 to 60 kg/m3, and the weight ratio of humidity is not more than 0.3%. In order to fix the inner casing, a pulling member is used between the inner wall of the outer casing and the outer wall of the inner casing to connect the two together, so that the inner tank does not vibrate and deflect during transportation and movement. In order to ensure the sealability, such tanks generally do not have manholes and inspection holes.
The boiling points of oxygen, nitrogen and argon are very close, so the design pressure and temperature of liquid oxygen, liquid nitrogen and liquid argon containers are almost the same, their liner design pressure is 0.8-1.6 MPa, and the design temperature is -196 °C; Shell design pressure is 0
. 1 MPa, the design temperature is normal temperature. Due to the low design temperature, according to the Pressure Vessel Safety Technology Supervision Regulations,
Such pressure vessels belong to class III pressure vessels when the volume is greater than 5 m3. The welding of the product must be carried out by a welder with the appropriate qualification in accordance with the qualified welding procedure. Due to the different design requirements of the liner and the outer casing,
Its manufacturing and inspection requirements are also different. During the manufacturing process, the inner joints of the A and B welds are 100% RT-checked.
GB4730-94 grade II qualified; the production of welding test panels, the mechanical properties of the weld. If the design temperature is lower than -100 °C, a low temperature impact test is also required. Shell A, B welds 20% RT inspection, GB4730-94III qualified (the last ring seam can be checked by UT, GB4730-94I qualified). After the product is finished, the inner tank shall be tested for pressure with suitable medium according to the design requirements. The jacket shall be tested for air tightness and vacuum, and the pressure holding time shall not be less than 4 hours.
The low temperature liquefied gas storage tank and the normal temperature liquefied gas storage tank have different working states, and the working pressure is determined by the operating temperature thereof, and it is much lower than the normal ambient temperature, so that the environment continuously transfers heat to the tank. The low-temperature liquefied gas vaporizes after it absorbs heat, so that the pressure inside the tank rises. In order to maintain its working pressure, the gas must be vented or cooled by a refrigerator. The vacuum adiabatic liquefied gas storage tank uses vent gas. Maintain its pressure. There are two ways to vent the gas. One is to continuously release the gas under normal working conditions, such as using the unit's cryogenic liquid storage tank; the other is to discharge through the safety relief system, such as the cryogenic liquid tank truck during transportation. Safety relief. Therefore, the working state of the vacuum adiabatic low-temperature liquefied gas storage tank is a heat balance state, which is maintained by the bleed gas. It is obvious that the better the heat preservation performance of the storage tank or the larger the working pressure is allowed to rise, the more gas is required to be vented. less.
It has been found that the main problem in the low-temperature liquefied gas storage tank during the use is the insulation problem of the jacket. The insulation problem can be roughly divided into two aspects. On the one hand, the leakage of the liner or the casing is caused by various reasons. The vacuum of the jacket is reduced; on the other hand, the venting of the Zhuguang sand causes a decrease in the degree of vacuum or the sinking of the cinnabar causes the space at the top of the container to form a space to reduce the thermal insulation performance. All of the above will reduce the insulation performance of the jacket, and the pressure inside the tank will increase. In view of the above situation and the fact that no manhole can not be internally checked,
According to the relevant requirements of the Pressure Vessel Safety Technology Supervision Regulations, the following inspection schemes are proposed:
1 External inspection (1) Check the corrosion of the outer surface of the outer casing. For the severely corroded parts, the ultrasonic thickness gauge should be used for thickness measurement.
(2) Check the outer surface of the casing for mechanical damage and whether the foundation is sunk.
(3) Check the outer surface of the outer casing for frost or sweat, and whether the vacuum degree is lowered during use.
(4) Check if the pressure gauge, safety valve and rupture disc are within the validity period.
阎 Check that the valve is intact and that the pipe is damaged or bent during use.
2 Internal inspection (1) Vacuum inspection inside the jacket: Connect the vacuum gauge to the vacuum tube located at the lower part of the storage tank.
Open the tank vacuum valve, the reading of the vacuum gauge should be less than 10Pa, otherwise you need to re-vacuate to improve the insulation performance of the jacket layer.
(2) Determination of self-consumption: According to the relevant technical requirements, the daily evaporation rate of the storage tank should be less than 5%, otherwise it means that the insulation performance of the jacket layer is degraded or the tank body is leaking. The specific detection method is as follows: the inner tank is added with more than 50% of the cryogenic liquid, and the deflation valve is opened. Except the pressure gauge and the liquid level gauge valve are opened, the other valves are closed to maintain the heat balance.
48h, then measure the steady gas flow rate with the flow meter on the vent valve nozzle, and calculate the mouth evaporation rate H by the following formula
H=24Q/CN×100%
Where: Q—gas flow value m3/h
Gas-liquid volume ratio under C-standard state
Effective volume m3 of V tank
3 Pressure test The pressure test of the container is generally carried out with gas, and the test pressure is 1.15 times of the design pressure. The test method is carried out in accordance with the relevant requirements of the Pressure Vessel Safety Technical Supervision Regulations. The vacuum jacket air pressure test pressure is 0.
2MPa, the pressure holding time is not less than 4h.
The above external inspections are generally performed at least once a year, internal and external inspections are performed every 3 to 6 years, and mobile storage tanks are every 6
A pressure test is conducted once a year. The fixed storage tank shall be tested for at least one pressure test every two internal and external inspection periods.
Through the above inspections, if problems affecting safety or normal use are found, repairs shall be made and repairs shall be carried out in accordance with the following principles.
(1) Due to the grading of Zhu Guangsha deflation or Zhuguang chopping, the vacuum should be filled, and the vacuum should be filled after the Zhuguang sand is filled.
And make the vacuum meet the relevant requirements.
(2) When the outer casing is affected by mechanical damage, local corrosion and cracks, it should be ground and removed, and then repaired. After repairing, the excess part should be polished and eliminated, and the smooth transition.
(3) If the liner leaks and must be repaired, a repair plan should be prepared and reported to the local boiler pressure vessel safety supervision department for approval. Due to the need to remove the outer casing during rework, it is difficult and technically demanding. It is recommended to be handed over to the manufacturing unit.
(4) If the material is deteriorated or the large area is severely corroded, it should be disposed of.
Technical Service Consulting Forest Manager