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Chen Shiliang. Affiliation: Xianyang Branch of China Shenhua Coal-to-Oil Chemical Co., Ltd., Xianyang, Shaanxi 712000. Please indicate the author’s copyright when reproducing this article. [Abstract] In response to the issue of water ingress into the low-pressure nitrogen pipeline network in coal gasification units, equipment and pipelines prone to such intrusions were inspected, and the causes of water ingress were analyzed: After heating was installed on the outlet pipeline at the bottom of the accident burner cooling tank (V2), the sealing performance of the valves declined, resulting in internal leakage. This caused water from V2 to enter the main nitrogen pipeline, thereby raising the nitrogen dew point. The large flow rate of low-pressure nitrogen entering the accident flare, combined with its flowing state, led to heat absorption due to expansion, which reduced the temperature of the nitrogen and caused the supersaturated nitrogen to condense into liquid water. Measures such as cutting off the water supply to the nitrogen pipeline network, installing drain valves on the main nitrogen pipelines, and increasing the nitrogen discharge volume were taken to treat the inlet nitrogen pipeline network, which effectively reduced its drying time. Meanwhile, the nitrogen pipelines used by the on-line analyzers were modified, thereby completely eliminating the risk of failures in these analyzers due to water accumulation at low points. It was also pointed out that the design flaw of the remote level gauge on V2 prevents real-time monitoring of its liquid level, posing a significant safety risk. In the coal chemical industry, nitrogen is often used to displace flammable, explosive, toxic, and hazardous gases within facilities. It is also employed for equipment sealing, maintaining pressure, protecting instrumentation components, and as a power source. Strict requirements are placed on its water content, with it generally needing to be kept below 2 ×10‑6. If water enters the nitrogen pipeline network during operation, it can pose a threat to the safe production of the entire plant. In winter, the pipelines may even freeze and become blocked, resulting in the paralysis of the nitrogen pipeline network. When the system needs to be shut down due to an emergency, nitrogen cannot be used for processes such as displacement, which creates significant safety risks and may even lead to serious accidents such as equipment damage, fires, and explosions. Therefore, it is of great significance to analyze and resolve the problem of water ingress into nitrogen pipelines. http://www.huoyumi.com/d/file/news/industry/2017-06-14/fb8219b131db36fdb043e91a57e7c286.gif 2. 3.1 Inspection of the low-pressure nitrogen pipeline connected to the high-pressure flash tank: There are two stop valves installed on the low-pressure nitrogen pipeline connecting to the high-pressure flash tank, and both of these valves are in the closed position. The temperature of the valve bodies as well as the low-pressure nitrogen pipeline downstream of the valves is 2 °C (which is higher than the ambient temperature of –6 °C), far lower than the temperature of the fluid inside the high-pressure flash tank, which is 160 °C. If there were any slight internal leaks in these valves, combustible gases such as CO, H2, and NH3 would appear in the nitrogen pipeline. Samples taken downstream of valve X4 showed that no such combustible gases were present in the pipeline. Furthermore, the inlet of the low-pressure nitrogen pipeline on the high-pressure flash tank is located below the liquid level of the tank; in the event of a leak, the water in the nitrogen pipeline will take on a color. The water discharged from behind valve X4, on the other hand, is colorless, odorless, and clear, which indicates that the black water or high-pressure flash gas (with a water content of 99%) inside the high-pressure flash tank has not entered the low-pressure nitrogen network. The higher temperature of the valve body and the nitrogen pipeline downstream of the valve is caused by the thermal radiation from the high-pressure flash tank. http://www.huoyumi.com/d/file/news/industry/2017-06-14/08c78f3c2f2539cae5a22b0e1ace03e7.jpg 2. 3. Inspection of the low-pressure nitrogen pipeline in V2: Within the accident burner cooling water tank (V2), in order to maintain the flow rate of water used to cool the burner during emergency situations, nitrogen is used to pressurize the area at the top of V2 to 0.45 MPa. The inlet for the nitrogen pipeline is located at the top of V2; the magnetic level gauge on site indicates that the liquid level in V2 is 2.3 m (the total height of V2’s tank is 2.6 m). The bypass valve of the safety valve SV1 at the top, as well as the make-up water valve X7 and its drain valves XV1 and X6, are all closed. Valve X5 is open to 1/2 of its capacity; therefore, the pressure inside V2 is the same as the pressure in the nitrogen pipeline network. Since V2 is located at the highest point of the gasification unit, if the level in V2 reaches its maximum, water will, under the force of gravity, flow through the check valve (internal leakage) and valve X5 into branch pipe 1, and from there into the horizontal nitrogen pipeline. The horizontal pipeline and the various branch pipes are arranged in a vertical U-shape, with the horizontal pipeline situated at the lowest point of this U-shape; as a result, the horizontal nitrogen pipeline acts as a water collection vessel, which causes the dew point of the nitrogen gas within it to rise (the dew point of the nitrogen gas was measured at –14.8 °C on January 30, 2016), leading to a high water content. After closing the X5 valve and disconnecting the flange behind it, it was found that the V2 level was full and a large amount of water was flowing out. The flow rate remained constant at 0.03 m3/h. This indicates, on one hand, that the magnetic flap level gauge LG1 is damaged, resulting in distorted readings of the V2 level; on the other hand, it highlights the safety risks associated with the lack of a remote-level measurement device for V2. Thus, when valves US2, US3, US4, and US5 were opened, a small amount of water flowed out, with the amount decreasing sequentially; when valve X3 was opened, no water flowed out, indicating that there was not much water accumulated in the nitrogen line, and that the water did not enter branch pipe 2 as a result of the line being filled with water. The V2 and process gas online analyzers are located on Branch 1 and Branch 2, respectively. Since the nitrogen flowing toward the emergency flare at the top of Branch 2 is in motion (with a flow rate of around 1,900 m3/h), there is a significant pressure drop in the nitrogen pipeline of diameter 2″. The expansion of this low-pressure nitrogen absorbs heat, causing its temperature to drop, which in turn leads to the condensation of saturated nitrogen into condensate water. This water flows down along the pipe walls and accumulates at the low points of the utility stations on each floor. Additionally, the high flow velocity of the nitrogen stream can carry small liquid droplets, which also flow down along the pipe walls and accumulate at those low points. When US1 was opened, it was found that the pipeline at US1 was blocked, with no nitrogen or condensate being discharged. US1 was heated using steam at 0.7 MPa for 12 hours, but still no water or nitrogen was discharged. Analysis suggests that this is mainly because US1 is located at the lowest point of branch pipe 2; due to its long-term disuse, a large amount of rust deposits accumulated there, blocking the nitrogen pipeline at US1. As a result, X4 became the relatively lower point of branch pipe 2, and more and more water accumulated there, eventually entering the process gas online analyzer via X4. At the low-point drain in the low-pressure nitrogen pipeline at the utility station on each floor of branch pipe 4, no water is discharged; only nitrogen is present. This is mainly because there is no flow of low-pressure nitrogen within branch pipe 4, hence no pressure drop occurs. The dew point of the nitrogen does not rise, and no supersaturated nitrogen is formed, so no condensate water is generated. 3 Treatment Measures 3.1 Cut off the water source entering the nitrogen pipeline network. The layout of the low-pressure nitrogen pipeline network after the technical modification is shown in Figure 1 (including the dashed lines). Valve X5 was closed, and its flange at the outlet side was disconnected, thereby completely cutting off the source of water entering the nitrogen pipeline network. A blind plate was installed behind Valve X7 to prevent internal leaks that could occur due to a decline in its sealing performance over time, and the amount of nitrogen discharged was increased in order to reduce the water content within the nitrogen pipeline network. On January 31, 2016, the nitrogen dew point measured behind Valve X4 was –17.9°C, indicating that the water content in the low-pressure nitrogen had decreased, suggesting an improvement in the situation. In addition, replace valves X6 and XV1, repair the magnetic flap level gauge LG1, and strengthen the inspection and maintenance of the level gauges. The above measures have completely eliminated the risk of water ingress into the nitrogen pipeline network. 3.2 Guiding drain valves installed on the horizontal main pipes: As can be seen from Table 2, on February 3, 2016, the nitrogen dew point measured behind valve X4 was –17.5 °C, indicating that there was still water present in the horizontal main pipes. Increasing the flow rate of nitrogen to dry the nitrogen pipeline proved effective, but it required a considerable amount of time. Therefore, guiding drain valves X1 and X2 were added at both ends of the horizontal main pipes (as indicated by the dashed lines in Figure 1) to increase the area through which nitrogen flowed, thereby reducing the drying time. After opening X1 and X2, approximately 6 m3 of water was discharged. Once discharge was complete, freezing and blockage occurred at both the X1 and X2 valves, resulting in no nitrogen being released from those valves, which indicates that ice may already be present in the nitrogen main pipe. Steam at 0.7 MPa was used to heat and deice valves X1 and X2, allowing them to be cleared. On February 4 and February 5, 2016, the nitrogen dew point was measured behind valve X4, at –23.5 °C and –24.2 °C respectively, indicating that the moisture content in the nitrogen had decreased. However, this value was still quite different from the dew point of nitrogen that meets the required standards. The dew point becomes closer to the acceptable value when the temperature and pressure of the nitrogen discharged from valve X4 are similar to those of the nitrogen supplied by the air separation unit. On February 6, 2016, the dew point of nitrogen measured behind valve X1 was –3.4 °C; the water content there was higher than that behind valve X4. This is likely due to the lower nitrogen flow rate at valve X1, which allowed the sublimation of ice in the pipes to reach equilibrium with the water content in the nitrogen, whereas the higher nitrogen flow rate in branch pipe 2 resulted in a lower water content. This indicates that a high nitrogen discharge rate from the low-pressure nitrogen pipeline needs to be maintained for an extended period in order to dry the nitrogen to acceptable levels (with a dew point reduced to –71.4 °C). On February 14, 2016, the dew point of nitrogen measured at valve X4 was –71.4 °C, which is the same as the dew point of the nitrogen supplied from the air separation unit; this indicates that all water in the low-pressure nitrogen pipeline network has been completely removed. 3.3 Modify the nitrogen pipeline for the on-line process gas analyzer: The nitrogen supply for this analyzer will now be taken from behind valve X3, instead of from behind valve X4 (as indicated by the dashed line in Figure 1). Additionally, a drain line will be added at the lowest point located behind valve X3 and before the on-line process gas analyzer, to facilitate purging, exhaust, or drainage. This measure aims to completely eliminate the risk of water accumulation due to the location of valve X4 at a low point, which could cause failures in the on-line process gas analyzer. 4 Conclusions (1) Through the analysis and investigation of the low-pressure nitrogen pipeline network, it was found that internal leakage in the outlet valves of V2 (X6, XV1) caused water to enter the main low-pressure nitrogen pipeline via valve X5 once V2 became full of liquid, thereby raising the nitrogen dew point. (2) The high flow rate of nitrogen maintaining the pressure of the emergency flare in branch pipe 2 results in a large pressure drop; the expansion of low-pressure nitrogen absorbs heat, causing its temperature to drop, which in turn leads to the condensation of supersaturated nitrogen into condensate water. The accumulation of large amounts of this water causes malfunctions in the on-line process gas analyzer. (3) By taking measures such as cutting off the water supply, increasing nitrogen emissions, and adding discharge drains on the horizontal main pipes, the drying time of the inlet nitrogen pipeline network was reduced, and the nitrogen dew point was lowered from a maximum of –3.4°C to the acceptable value of –71.4°C. (4) The nitrogen pipeline for on-line process gas analyzers was modified, **reducing the risk of water ingress. (5) The design flaw of the remote level gauge on V2 results in the inability to monitor the liquid level of V2 in real time, posing a significant safety risk.