Determination of the number of inert gas purging cycles for storage tanks
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There is a process tank that, under normal conditions, contains flammable process media. Before it is taken out of service, it needs to be purged with nitrogen in order to reduce the concentration of flammable gases inside the tank to below a certain level. But how is the number of nitrogen displacement cycles determined? Is there a formula? Have any of you sailors encountered this problem?
This ultimately needs to be verified through laboratory analysis; conversely, it determines whether your dry method requires curing.
As mentioned on the 2nd floor, it’s not that fixed； But it is theoretically possible to calculate this: assuming that the initial gas pressure in the device is 0.05 MPa and the volume concentration of the flammable gas is 100%, if N2 is introduced to raise the pressure to 0.25 MPa, the concentration of the flammable gas will be around 20%. If the pressure is then reduced back to 0.05 MPa and raised again to 0.25 MPa, the concentration of the flammable gas will be approximately 4%. After another replacement, it will be around 0.8%. By this method, it is possible to determine the number of replacements required; of course, the actual number of replacements will be higher due to various factors, including: 1. The pressure at the end of N2 introduction during replacement, and the pressure at the end of pressure reduction ； 2. Note on the N2 injection point and pressure release point: (related to gas density) If the gas density is higher than that of N2, it is recommended to place the N2 injection point above the equipment, while the pressure release point should be below the equipment ； 3. It is also related to the gas injection rate, affecting whether the gases are properly mixed ；
This post was last edited by ylb913 on 2016-7-17 at 17:35. Replacing air with nitrogen is something I did 20 years ago, using exactly the method you mentioned. Our low-pressure nitrogen pressure was 6–7 kg; we considered the absolute pressure to be 7 kg, which made it easier to do the calculations. The concept I proposed at that time was to first use nitrogen to purge the air (by turning on the vent), then turn off the vent and allow the system to stand for a while (about half an hour), after which the pressure would be reduced to normal levels. No further venting would occur, followed by pressurizing the system to 6 kg; again, the system would be left to stand for about half an hour, after which the pressure would be reduced to normal levels. As a precaution, this process would be repeated a third time (pressuring the system to half of that level) before concluding the procedure. The oxygen content in the air is 21%; a single pressurization and release reduces the oxygen level to 3%, and a second such process reduces it to 0.4%.
If you reduce the pressure in the tank to 0, then push in 7 kilograms of nitrogen to reach a pressure of 3.5, and then release it; after pushing it to 3.5 and releasing the gas, the process is basically complete
Theoretical calculation: Number of times = ln(Required oxygen concentration/Initial oxygen concentration) / ln(AbsOLUTE pressure after nitrogen addition/Atmospheric pressure)
Thank you. It needs to be solidified, as this is a product tank that requires frequent replacement
Thank you:):handshake
Thank you, that’s great:handshake:handshake
Should it be, at the molecular level: ln(initial oxygen concentration/required oxygen concentration)?