How to displace nitrogen

Since nitrogen is a gas, as long as the pressure is higher than the water pressure built up in the pipes, it can blow out the accumulated water. It’s mainly a matter of stress, haha. . . . . .

The pressure in the gas system is very low, usually around 2 atmospheres, while the pressure of nitrogen is high, reaching over 6 atmospheres. Therefore, it’s definitely no problem to use nitrogen to purge water from the gas pipes.

In our company, the pressure of nitrogen is generally maintained at 5–6 kilograms; high-pressure nitrogen is kept above 20 kilograms. Low-pressure nitrogen is used for production, while high-pressure nitrogen is employed to prevent accidents by replacing the raw gas in the system, thus avoiding catalyst poisoning and deactivation due to carbon buildup.

The nitrogen pressure is sufficient, and it can prevent explosions caused by the accumulation of flammable gases.

The pressure of nitrogen is generally maintained at 5–6 kilograms, which is used to displace air in pipelines at normal pressure; this pressure is sufficient and the process is quite simple. The entire system can also be pressurized with nitrogen and then depressurized.

For combustible gas pipelines, it is essential to dilute and displace the air with an inert gas prior to any maintenance work. Nitrogen is the most commonly used and cost-effective gas for this purpose; generally, its pressure can reach 4–6 kilograms per square centimeter, which is more than sufficient to remove water from the pipelines. I have a paper on this topic here; I’m not sure if it can be posted, so if not I’ll create a new post. I hope it will be useful to you. Abstract: For newly built or pipelines that require maintenance, this paper introduces several methods of displacing air with nitrogen before they are put into operation. It explains the working principles and implementation steps of these methods, compares their advantages and disadvantages, and determines the appropriate scenarios for each method. Additionally, it discusses and summarizes the methods for determining operational parameters such as nitrogen injection temperature, pressure, location, volume, and the speed at which natural gas is introduced. The conclusions drawn serve to ensure the safe operation of natural gas pipelines. Suggestions and recommendations are put forward regarding the methods for determining the optimal replacement process and operating parameters.

After purging with nitrogen, analyze the oxygen content; it must be less than 5%!

It’s on the 10th floor. I don’t know if you’re the elder brother or sister; could you please help by sending it? If not, please send it to my email address: wshqiang32248@yahoo.com.cn. Thank you so much!

It’s here: http://bbs.hcbbs.com/viewthread.php?tid=114836&pid=466930&page=1&extra=page%3D1

It’s not definitely 5%, right? Is it depending on the requirements for the process gas? What about flammable and explosive process gases? Should the oxygen content also be required to be less than 5%?

Nitrogen purging is a common method for preventing explosions; due to its inertness in chemical reactions, industrial pipelines generally need to be purged before any hot work is carried out, and a positive pressure is sufficient

In accordance with the safety management requirements of the former Ministry of Chemical Industry, the oxygen content is normally not more than 0.5%.

After the construction and installation of gas pipelines are completed, it is necessary to clean out any debris inside the pipes. The most common methods for this purpose are the pigging method and the gas-pressure blasting method. The pigging method is generally suitable for transmission pipelines with uniform diameters and long lengths, while the gas-pressure blasting method is more often used in urban low- and medium-pressure networks where the pipelines are not very long, their diameters may vary, and there may even be branch lines. Although some areas attempt to use the pigging method to clean urban pipelines as thoroughly as possible, operational complexities arise due to various constraints; if a blockage occurs during cleaning, it is necessary to disassemble the equipment for backwashing or even cut the pipe to remove the blockage. As a result, this method cannot be widely applied in urban pipeline cleaning. In contrast, the gas-pressure blasting method is simpler and does not have these drawbacks, which is why it continues to be widely used in cleaning urban pipelines to this day. The method of using sO100 gas pressure to clean pipes has always had one drawback: in the section of the pipe at the inlet end, due to the low flow velocity of the gas, debris inside – especially those with small size and high density, such as stones and steel shrapnel – often cannot be completely removed. Moreover, the longer the pipe that is being cleaned, the greater the length of that section at the inlet end that remains uncleaned. When using the pressurized blasting cleaning method for pipeline cleaning, the flow velocity of gas inside the pipeline varies. According to Boyle’s law and Mariotte’s law: for a fixed amount of gas at a constant temperature, the volume is inversely proportional to pressure before and after a change in the gas state. That is, PI/P2 = V2/V1 or PI·V1 = P2·V2. In the case of pressurized blasting, the state change of the gas occurs in one direction within a pipe of constant diameter; therefore, the above formula can be simplified to: P1/P2 = V2/V1 = SL2/SL1 = L2/L1. Thus, V = P1·L1/P2 = (L2 – L1)/t = (P1·L1/P2 – P2·L1/P2)/t = L1. Here, P1 and P2 represent the different pressures applied to the gas inside the pipe (in Mpa); V1 and V2 are the volumes of the gas at those different pressures (in m3); S is the cross-sectional area of the pipe (in m2); L1 and L2 are the distances from a certain section of the pipe to its end at those different pressures (in m); t is the time taken for the pressure to change (in s); and V is the flow velocity of the gas at that section (in m/s). At any given moment, the pressure differences across different sections of the pipe are not very large. As can be seen from the formula, the flow velocity of the gas at each section is proportional to the distance from that section to the end of the pipe. The relationship between the flow velocity and various sections of the pipe is shown in the diagram below. It can be observed from the diagram that the flow velocity of the gas at the inlet is the lowest, while it is the highest at the outlet. The standards require that the actual flow velocity of the gas inside the pipe be no less than 20 m/s, and this refers to the average flow velocity of the gas. Based on this, the actual flow velocity at the outlet must be at least 40 m/s. When the flow velocity at the outlet is 40 m/s, the flow velocity of the gas from the inlet to the middle part of the pipe is less than 20 m/s. To increase the flow velocity in this section of the pipe, it is necessary to increase the pressure further. The higher the pressure, the shorter the length of the pipe segment where the flow velocity cannot reach 20 m/s at the inlet. However, each section of the pipe has its own designed operating pressure. The standards specify that the maximum pressure during pipe cleaning must not exceed the designed operating pressure. Therefore, pressure increase is not unlimited; it can only go up to the designed operating pressure. It is clear that simply increasing the pressure is not the key to solving the problem. Thinking in a different way, if one end of the inlet in the pipeline is already clean, then wouldn’t the weaknesses mentioned above no longer exist when using the pressurized blasting method to clean the pipeline? Based on this idea, we improved the pressurized blasting method for pipeline cleaning. Through multiple trials, we found that using this improved method not only ensures thorough cleaning but also allows for an increased cleaning length for the pipeline. Regulations stipulate that the cleaning length for each section of pipeline should not exceed three kilometers; since our improved method involves cleaning each section of the pipeline in multiple stages, the total length still needs to be kept within three kilometers. The improved method involves the following process as shown in the diagram below. The specific steps are as follows: An air inlet is installed at the starting end of the pipeline. Once the pipeline has been constructed to a certain length a, purging of the first section of the pipeline begins. The exact length of this first section is determined based on the design working pressure and pipe diameter; for medium-pressure piping systems, the value of a can be referred to in the table below. Design working pressure (Mpa), Pipe diameter (mm), Length of the initial section to be cleaned (mm), Number of cleaning cycles (reference values): 0.3–0.4: 400; 400: 5–7; 400: 8–7; 300: 4–6. For pressures of 0.2 and below: 300; 300: 5–8: 2000; 300: 5–8: 1000 and below: 200. For lengths of 4–6, the cleaning pressure for the initial section is set at the design working pressure. After several cleaning cycles until the section is thoroughly clean, the inlet is left unchanged while the outlet is removed, and pipe welding continues. Once the total length b reaches two to three times the original length a (b=2a or 3a), the outlet is reinstalled for a second cleaning cycle. Since the initial section has already been cleaned, an appropriate pressure is selected; two to three cycles are sufficient to clean the pipe completely. Again, the inlet remains unchanged, the outlet is removed, and welding continues. When the total length c reaches two to three times the length b from the previous cleaning cycle (c=2b or 3b), a third cleaning cycle begins. This process is repeated continuously. If the pipe is not very long (e.g., within ten kilometers), a booster device with suitable exhaust volume and pressure can be used to carry out the work to completion. When using this method for construction, the following points should be taken into consideration: First, before constructing the initial section of the pipeline, it is necessary to clean each pipe thoroughly, in order to minimize the amount of debris inside the pipes at the inlet end. This is because there is still the issue of low gas flow velocity at the inlet end when cleaning the initial section of the pipeline. II. During construction, it is advisable to use the higher elevation end as the starting point of the project. This is done to prevent backflow that may occur when water enters the pipes due to various force majeure factors. This is especially true for pipes that are not equipped with condensate tanks for transporting dry gas, as backflow resulting from water intrusion makes it very difficult to clean these pipes thoroughly. III. During the construction process, facilities such as valves designed for the pipes can be installed later; first, the valve chambers should be built, and once the pipe installation is complete and the pipes have been cleaned, those facilities can be installed all at once. IV. If there are branch pipes, when construction reaches the designed position, a tee can be installed and construction can proceed simultaneously; the branch pipes should also be purged along with the main pipeline. The valves on the branch pipes should be installed only after construction is completed and the pipes have been thoroughly purged. The improved pipe cleaning method features only one air inlet, and as the pipe extends, the length of pipe that needs to be cleaned increases exponentially. Although the time required for pressurization increases when the pipe gets longer, the increased volume of air inside the pipe results in a longer cleaning time, which in turn reduces the number of cleaning cycles needed. Compared to cleaning the pipe in sections, not only is the workload reduced, but the cleanliness of the pipe also improves. Colleagues in the gas industry might want to give this method a try in their future projects.

For flammable and explosive gases, the required oxygen content after displacement is usually less than 0.5%, as is the case with acetylene, for example

Here, the specified oxygen content must be less than 0.2%. Nitrogen is a protective gas that is used in many applications; it’s not only used for purging pipelines but also to break vacuums. In the burner plate systems of vaporization furnaces, it is used to create pressure, thereby preventing the burner plates from being damaged in the event of a power outage. Wait: lol

The former Ministry of Chemical Industry specified 0.2%, but I believe that using water for displacement in gas pipelines has many advantages: even if dead ends are formed, a good water seal is maintained, and the risk associated with hot work is minimized. As for using nitrogen to blow out any remaining water, it is essential to ensure the quality of the nitrogen; the nitrogen pipeline must be connected hermetically to the natural gas pipeline to prevent air from entering. The pressure should be increased gradually, from low to high, until all the water is blown away.