When the purity of the product decreases or the output decreases during the operation of the air separation plant, people will think of whether the heat exchanger will leak. Generally, the decrease of product purity is due to the leakage of high pressure air into the low pressure system; the decrease of production is due to the leakage of products into the cold box. However, the mutual leakage of air between the compartments affects both the output of oxygen and the purity of nitrogen. Now several judgements are introduced as follows:
1) high pressure air leaks into the oxygen Interlayer: analyze the oxygen purity of the condensing evaporator and the oxygen purity of the oxygen tube (which can be taken from the oxygen flow tube), and judge according to the purity difference between them. The purity of the two is the same, indicating that the heat exchanger does not leak; otherwise, it leaks. A small amount of air leakage will not affect the purity of the condensing evaporator, and the low pressure will not rise; a large amount of air leakage, there is a significant difference in purity between the two, at the same time, the low pressure increases, high and medium pressure decreases. In addition, the liquid oxygen purity and the gas oxygen purity of the condensing evaporator will be obviously unbalanced.
2) high pressure air leaks into the nitrogen Interlayer: analyze the purity of nitrogen in the upper tower and out Tower and the purity of nitrogen outlet tube (sampling from the nitrogen flow meter tube), and identify them according to the purity difference between them. If the purity of the two is consistent, it means that it is not the heat exchanger leak, but other reasons; if there is a purity difference, it may be the heat exchanger leak. However, the leakage of liquid supercooler may also lead to the decrease of nitrogen purity, which needs further examination. 150 m with acetylene adsorber3/H air separation equipment. You can open the joint-3 valve, close the joint-1 valve and the joint-2 valve to eliminate pressure. If the purity difference disappears, it may be the leakage of liquid supercooler; if the purity difference remains unchanged, the possibility of the leakage of heat exchanger is high. A small amount of air leakage will not affect the pressure, while a large amount of air leakage will not only affect the purity, but also reduce the pressure of high pressure and medium pressure.
3) high pressure leakage into the distillation Interlayer: a small amount of air leakage is difficult to be found during operation, and only when it leaks to a certain extent can the nitrogen be pure
Degree drop, high pressure, medium pressure drop, low pressure rise.
4) high pressure air leakage into the cold box insulation layer: leakage occurs due to pipe plate welding, pipe cracking, flange loosening and other reasons. It will reduce the pressure of high pressure air, the output of oxygen and nitrogen and affect the purity. The Pearl sand will blow out from the cold box.
5) leakage of oxygen, nitrogen or distillation Interlayer: If the oxygen, nitrogen or distillation of the product leaks into the insulation layer, the output of oxygen and nitrogen will decrease or the flow of distillation will decrease. In serious cases, the Pearl sand will blow out. If the bottom of the heat exchanger leaks, it will make the cold box "sweat" or frost.
6) oxygen leaks into the nitrogen Interlayer: When the foreskin of the oxygen and nitrogen interlayer is de-welded, since the pressure of the oxygen interlayer is slightly higher than that of the nitrogen interlayer, oxygen will leak into the nitrogen, which will decrease the purity of the nitrogen, the output of oxygen decreases while the purity of oxygen can be adjusted to the requirement.
The above analyses are the preliminary judgement in the operation of the air separation tower. In order to confirm the accuracy of the preliminary judgment, further analysis and inspection are needed:
1) stop the air separation Tower, heat and blow it off thoroughly, close all the valves on the distillation tower, and prepare the distillation tower to do the preliminary air tightness test.
Introduce high pressure air into the heat exchanger. When the high pressure reaches the highest working pressure, stop the pressure for 1 hour. If the high pressure is converted by temperature, the pressure will not drop and the low pressure will not rise either, it can be said that the high pressure does not leak into the low pressure; if the high pressure decreases, the low pressure rises obviously, then the high pressure air leaks into the low pressure; if the high pressure decreases, the low pressure does not rise, the reason of high pressure drop should be checked carefully. If the reason for the high pressure drop cannot be found, check the leakage of the valve and flange of the low pressure system. If necessary, blind plate should be added to the flange of the low-pressure safety valve of the outlet pipe of oxygen, nitrogen and distillation. The air leakage of the low-pressure system should be eliminated first, and the pressure test of the low-pressure system should be conducted. It is qualified to stop the pressure for 4 hours without leakage. Then raise the high pressure system to the working pressure. If the high pressure drops and the low pressure remains unchanged, it is possible that the high pressure leaks into the cold box. If the pressure of low pressure can not stop or the pressure drops more than 0.005MPa after stopping for 4 hours, and no leakage can be found outside, it is possible that the low pressure system in the cold box leaks, and Pearl sand should be taken out for inspection.
2) after extracting the Pearl sand, maintain the working pressure on the high, medium and low pressure system and conduct a comprehensive leak detection.
First eliminate the air leakage on the surface, and eliminate the leakage of the high-pressure insulation layer to the cold box and the leakage of the foreskin of the oxygen, nitrogen and distillation layer in the low-pressure system. Then try again the high pressure and low pressure to determine whether the high pressure leaks to the low pressure. If the purity of nitrogen decreases, the medium pressure should be increased to the working pressure to see if the low pressure rises. If the medium pressure leaks to the low pressure, check whether the leakproofness of the medium pressure throttle and the liquid supercooler leak. If the purity of oxygen decreases, check whether the condensation evaporator leaks.
3) after the confirmation of high pressure leakage to low pressure, the leakage position should be determined specifically.
Unload the heat exchanger, install the stuffing plate, pressure gauge and vent valve on the inlet and outlet of oxygen, nitrogen and distillation respectively, and then slowly boost the high pressure. If the pressure of the low-pressure interlayer rises, it indicates that the interlayer leaks. Stop the high pressure test, dismantle the inlet flange and collector flange of the interlayer heat exchanger, test the interlayer to the working pressure through nitrogen, and check whether the high pressure pipe leaks, determine whether the first heat exchanger leaks or the second heat exchanger leaks. If the first heat exchanger leaks, the tube of the high-pressure inlet tube plate should be plugged with rubber plug, and the Middle collector tube should be checked with soap water to determine the leaking tube and mark it. Then block it with rubber plug, find the leaking pipe on the inlet pipe board and mark it. If the second heat exchanger leaks, firstly block the tube of the end collector with rubber plug, and find out the leaking tube in the middle collector. Then use rubber plug to block the leaking pipe, and find out the leaking pipe at the end collector. After finding the air leakage pipe, both ends of the pipe are blocked with copper awl, and then covered with tin welding. The blocked pipe should not be more than 15% in case of reducing the performance of the device.
4) when the oxygen barrier gradually rises to the working pressure, check whether the nitrogen and distillation barrier are discharged by gas.
In the process of boosting pressure, due to the deformation of the foreskin and the change of the volume, it is normal that a small amount of bubbles will be discharged, not necessarily the air leakage. As time goes on, bubbles disappear and the pressure of oxygen interlayer does not drop, which proves that there is no air leakage. If the pressure of the oxygen partition continues to drop, nitrogen or the bubbles of the distillation partition are constant, or the pressure difference of the water column can be measured after the "U" shaped pipe is connected, it means that the oxygen leaks into the partition. The foreskin and the winding pipe should be removed for welding.
5) after the heat exchanger is repaired, high-pressure pressure test should be conducted to check whether the oxygen, nitrogen and distillation compartments leak.
It is normal for high pressure to have bubbles in the low-pressure interlayer during the process of rising pressure and at the initial stage of stopping pressure. This is caused by the change of volume and temperature, which can disappear after stopping the pressure for half an hour. If the high pressure drops and bubbles continue, there may be air leakage parts, which should be further eliminated.
When internal leakage occurs to the heat exchanger, the manufacturer should send professionals to repair it.
