Cryogenic liquid storage tank

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2. Inspection of cryogenic liquid storage tanks: When the storage tank is in working condition, there are potential dangers such as leakage, overpressure, and explosion. If the hidden dangers before these accidents are not discovered and dealt with in time, it will develop into a serious accident. Therefore, it is very important to formulate a complete inspection system and implement it carefully to ensure the safe operation of the storage tank. Daily inspection of storage tanks mainly includes the following contents: 1. Check whether the valves and pipelines are leaking, and whether the shell is frosted or sweating. 2. Whether all valves are in normal opening and closing state. 3. Whether the instruments (liquid level gauge, pressure gauge) are working properly, and whether the DCS display parameters are consistent with the on-site primary gauge. 4. Check whether the tank pressure is normal. When the pressure is close to or equal to the maximum pressure, the vent valve needs to be opened to relieve the pressure. 5. Whether the liquid filling rate exceeds 95%. 6. For normal pressure powder insulated storage tanks, is the sealing gas normal? (50mmH2O) 7. It is strictly prohibited to place flammable, explosive items and all debris near the liquid oxygen storage tank. 8. Fireworks are strictly prohibited near liquid oxygen storage tanks. 9. Test the acetylene and total hydrocarbon content in the liquid oxygen in the storage tank at least once a week. The acetylene content must not exceed 0.1×10-6 (v/v). If it exceeds, the liquid oxygen must be discharged in time for replacement. 10. Whether the liquid oxygen storage tank is well grounded. 11. If it is not to be stopped for a long time, make sure there is a certain amount of liquid in the tank to avoid re-cooling the replacement tank and using it for a long time. 12. Check whether the legs are damaged, whether the foundation sinks, tilts, or cracks, whether the fastening bolts are intact, and whether the tank body is deformed. 13. Regularly check the vacuum degree of the storage tank. 3. Annual inspection of cryogenic liquid storage tanks: Check content: 1. Original data review ⑴ Review of the product’s factory technical documents, including material certificates and re-inspection reports of the inner and outer cylinders, stainless steel welding process assessment reports, weld flaw detection reports (including flaw detection reports for welding repair parts), hydraulic pressure test reports, air pressure (tightness) test reports, helium leakage test reports, evaporation rate test reports and vacuum degree test reports, etc. ⑵, Review the storage tank operation records, and ask the management and operators of the equipment. Is there any significant change in pressure during operation? Whether the safety valve jumps, whether the evaporation volume increases, etc. 2. External inspection In addition to external inspection according to the requirements of general pressure vessels, the following contents should also be mainly inspected:: (l) Whether the opening and closing of various valves are normal ； ⑵, pressure gauges, liquid level gauges and other safety accessories are inspected according to regulations, and whether they are used within the calibration period ； ⑶, whether there are leaks at the connections between containers, pipes and pipe valves ； ⑷. Whether there is "sweating" or "frost" on the outer surface of the storage tank (especially the outer surface of the top and bottom of the outer cylinder)." ； ⑸, damage to the legs, sinking, tilting, cracking of the foundation, and the integrity of the fastening bolts. 3. Wall thickness test: Select 4 to 8 points on the outside of the outer cylinder for wall thickness testing to determine the minimum wall thickness H ； At the same time, measure the diameter D2 of the outer cylinder from the outside of the outer cylinder. According to the recorded inner diameter D1 of the original data review, according to the formula (D2-H-D1)/2<300mm 4. Inspect the inner surface of the endoscope inner cylinder to check the corrosion of the inner surface of the inner cylinder. Conduct focused inspections on suspicious areas. 5. Surface flaw detection (MT or PT) Surface flaw detection is performed on the outer surface of the fillet welds of all outer cylinder nozzles, and penetrant flaw testing (PT) is performed on the parts that cannot be subjected to magnetic particle testing (MT). 6. Vacuum degree test: In the cold state, when the tested vacuum degree reaches 16Pa or the safety valve jumps frequently or the inner cylinder is abnormally pressurized, the vacuum needs to be re-evacuated. 7. Air pressure test: Carry out air pressure test on the inner cylinder of the storage tank. The test pressure is 1.2 times the working pressure. The specific steps shall be carried out in accordance with the relevant provisions of the "Regulations". Conduct an air pressure test on the inner cylinder to check its strength on the one hand, and test the sealing performance of the inner cylinder on the other hand, and determine whether there is leakage in the inner cylinder through the display of the pressure gauge. The above inspection plan is developed for the situation where the cover is not opened or it is deemed unnecessary to open the cover after reviewing the original data, external inspection and vacuum test. Because the design life of this kind of container is generally 15 years, the storage medium basically does not corrode the inner cylinder, and it is guaranteed by higher NDT requirements. Under normal use conditions, there will generally be no major problems after the equipment is put into use for 5-10 years. However, if major problems affecting the normal operation of the equipment are discovered during operation, the cover must be opened, or if the vacuum still fails to meet the requirements after re-evacuation, it means there is a leak and the cover must be opened for maintenance. In addition to the above five contents, the following contents should be added to the inspection plan: (l) Conduct ultrasonic flaw testing (UT) on the inner cylinder ring and longitudinal welds, and conduct penetrant testing (PT) on the inner surfaces of the inner cylinder ring and longitudinal welds. ①Mainly focus on the inner cylinder butt weld repair parts and T-shaped welds. The proportion of UT and PT flaw detection shall be randomly checked at the proportion of 20% of the butt weld length. ； When the above flaw detection still cannot find the cause of the problem, the flaw detection ratio of butt welds UT and PT is increased to 100% ； ②Conduct random inspections of more than 20% of UT and PT on the base material itself, especially the key suspicious parts considered by the inspector after reviewing the original data. ； In addition, the inspector can increase the proportion of flaw detection according to the actual situation of on-site inspection. (2) Conduct 20% ultrasonic flaw testing (UT) on the butt weld of the outer cylinder, and conduct 20% magnetic particle testing (MT) on the outer surface of the butt weld of the outer cylinder. ； All T-shaped weld parts of the outer cylinder and the parts with defective welds of the outer cylinder recorded in the original data shall be included. (3) Carry out the hydraulic pressure test of the inner cylinder according to the manufacturing requirements mentioned above, and the test pressure is 1.25 times the design pressure. ； Inner cylinder air tightness test, the test pressure is the design pressure ； Jacket air pressure test, the test pressure is 0.2MPa, and the pressure is maintained for 4 hours ； Helium leak detection test ； Vacuum degree test and evaporation rate test. Self-consumption measurement: Storage tank technical characteristics requirements: The daily evaporation rate is 0.5%, which is another important indicator. If the daily evaporation rate is too high, it will reduce work efficiency and waste raw materials. It can also determine whether there is leakage in the inner cylinder. The specific detection methods are as follows: Add more than 50% low-temperature liquid to the inner tank, open the vent valve, close the valves except the pressure gauge and liquid level gauge, and wait for thermal balance for 48 hours. Then install a rotameter on the vent valve mouth and measure the flow rate once an hour. After several hours, obtain the stable gas flow value, and use the following formula to calculate the daily evaporation rate Q% Q＝Q1/c×t/V×100%≤0.5% Where: Q1-- Stable gas flow value m3/hc-- Gas-liquid volume ratio under quasi-state, liquid oxygen c= 800 t-- Stable gas time ； V - Effective volume of the measured storage tank 4. Analysis and treatment of common problems in the maintenance of cryogenic liquid storage tanks: The maintenance of vacuum in the interlayer of the storage tank is the guarantee of the thermal insulation performance of the storage tank and the fundamental guarantee of the normal operation of the storage tank. After the storage tank is put into use, common problems are often related to the degree of vacuum maintenance. ⑴There is obvious large area of ​​"sweating" and "frost" outside the storage tank. It may be caused by pipeline leakage in the interlayer of the storage tank, failure to fill the pearlescent sand, or other reasons that cause the interlayer vacuum to be damaged. This requires inspection and repair, or leak detection, or replenishing pearlescent sand, and re-vacuuming. ⑵, the pressure in the tank inner cylinder increased abnormally, and the safety valve tripped. It may be caused by the following three reasons: a. The vacuum degree of the storage tank interlayer is destroyed ； b. The inner cylinder boosting valve fails and the boosting valve needs to be repaired or replaced. ； c. The leakage part of the lower part of the interface is at the welding point between stainless steel and carbon steel shell, or at the connection point between copper pipe and stainless steel inner cylinder, that is, at the dissimilar welding joint. Electrochemical corrosion is mainly caused at the dissimilar welding joint. (3) The evaporation volume becomes larger and the vacuum degree becomes smaller. It may be due to the pearlescent sand outgassing. Pearlescent sand has certain particle size and temperature requirements when filling. When the particle size and temperature of the pearlescent sand are inappropriate, after running for a period of time, the pearlescent sand will release water vapor, which will reduce the vacuum degree and increase the evaporation amount. (4) The top of the outer cylinder of the storage tank is "sweating". It may be caused by pearlescent sand gathering in the lower part. After being put into use for a period of time, the pearlescent sand sinks, forming a space on the top of the container, and the local insulation effect is significantly reduced, causing the container to run cold. In this case, if the evaporation volume is large, the top of the jacket outer cylinder can be dug out and pearlescent sand added. Introduction to vacuum storage tank repair technology (addition of pearlescent sand, pressure test, vacuuming): 1. First of all, it is very important to choose a reliable and high-performance vacuum pump. We choose a 2X--70 two-stage rotary vane vacuum pump with a pumping speed of 70 liters/second and a vacuum degree of 10-3 Torr (0.133Pa). The use effect is good. In order to shorten the pumping time, a tandem pump can be used, that is, a Roots back-stage pump can be added. 2. The accuracy of vacuum measurement is the key to the success of vacuuming. Generally, vacuum storage tanks are randomly installed with a vacuum gauge. A thermocouple vacuum gauge can be used to directly measure the vacuum degree at any time. However, the vacuum gauge is easily contaminated, resulting in a large error in the measurement value. A thermocouple vacuum gauge has been used to measure many vacuum storage tanks. Most of the measured values deviated greatly, and there is no reference price. value, the author recommends the use of a portable rotary dial vacuum meter. It is designed based on Boyle-Malott's ideal gas constant temperature compression law. Therefore, its scale is calculated based on the instrument. The measurement readings have the meaning of "absolute accuracy". It is easy to use, flexible, and has a large measurement range. 3. According to the type of vacuum interface of the storage tank, make a matching connection device. The vacuum valve type is relatively simple. It can be directly connected to the flange, and then the valve can be opened and closed. ； The blocking type is more complicated and requires a complex set of connecting devices. ； Includes Huff, push-pull rod with screw head, lined with gasket to guarantee no leakage. 4. Vacuuming steps: ⑴. Connect the vacuum joint, slowly fill in low-pressure dry nitrogen with the connecting pipe connected to the pressure gauge (nitrogen from the oxygen generator device is best), and stop inflating when the interlayer pressure reaches atmospheric pressure. ⑵1. Open the explosion-proof membrane and check the filling condition of the interlayer pearlescent sand. If the pearlescent sand sinks seriously, it should be replenished. The basic requirement for filling is uniformity and fullness, so as to ensure the thermal insulation effect. ⑶, The explosion-proof membrane is customized with a blind plate and clamped, the interlayer is pressurized to 0.2MPa, and an air tightness test is performed. If it does not leak for 12 hours, it is qualified. ⑷, interlayer pearlescent sand for drying (because pearlescent sand is a highly hygroscopic material): Dry and anhydrous nitrogen or air is introduced from the vacuum interface and flows out from the explosion-proof membrane. ⑸, fill with dry nitrogen or CO2 gas for replacement. This is because a large amount of non-condensable gases (such as H2, etc.) are adsorbed in the insulation material, and these gases are continuously released under vacuum, thus reducing the vacuum degree of the adiabatic space and the insulation performance. These gases can be replaced by CO2 or N2. The main reason is that the CO2 or N2 molecules in the viscous flow state when vacuuming is started can easily collide with the non-condensable gas. * Roll it out. ⑹. After connecting the vacuum pump, you can start vacuuming and heat the inner tank at the same time. This is because most of the thermal insulation materials are porous media, which have strong air suction properties and large air extraction resistance. Therefore, using heated air outlet is an important part of vacuuming the insulator. Heating and drying can also reduce the humidity in the insulation material, and the moisture adsorbed in the insulation material is difficult to extract. Therefore, heating and degassing is an important process to speed up gas extraction. When the vacuum degree is 0.5~6Pa, the pump can be stopped and vacuuming is completed. 5. When vacuuming, you need to pay attention to the following aspects:: ⑴, The connection should be tight, apply vacuum grease well, and there should be no leakage. (Vacuum grease is important to ensure tightness). ⑵The vacuum pump must run continuously without interruption. If the pump stops due to accidents, power outages, etc., it must be immediately disconnected from the vacuum layer of the storage tank. Otherwise, the vacuum pump oil will flow back into the interlayer, causing a pollution accident. ⑶. When the vacuum pump is running, pay attention to whether the cooling water is interrupted and the temperature of the pump oil does not exceed 40°C. ⑷When the weather is cold, vacuuming should be done indoors, otherwise the outdoor temperature will be too low, which will affect the vacuuming effect. ⑸, Pay attention to the maintenance and maintenance of the vacuum pump. The vacuum pump should be kept clean at all times and no other objects should be placed on the pump. In addition, the oil must be changed every time (use 1# vacuum pump oil). This is because during vacuum operation, the interlayered pearlescent sand fine foam can easily be sucked into the vacuum pump cylinder. If the oil is not changed and cleaned in time, the next vacuuming will affect the vacuum degree. It is best to change the oil after the oil temperature rises. Pay attention to the amount of oil added. Too much oil will cause undesirable phenomena such as difficulty in starting, oil return, and oil injection. Less oil will affect the vacuum degree. ⑹. When using a McFarland vacuum timer, you need to slowly rotate it to an upright position. After the measurement is completed, you must immediately return to the original recumbent position. ⑺. To prevent the vacuum degree from decreasing, it is strictly forbidden to hit or hit the storage tank during installation and transportation, and pay special attention to the explosion-proof device of the outer cylinder and the vacuum valve not to hit each other. ⑻. If the air tightness test fails, the leakage point should be dealt with first and then vacuumed.