Torch collection pipeline system
Thread Content
In chemical projects, the emission of flammable and explosive materials is inevitable both during normal operations and in the event of accidents. To prevent environmental pollution, fires, and explosions, and to ensure the safety of equipment and personnel, it is necessary to design flare systems to handle all such emitted materials. In olefin projects, the flare system mainly consists of the flare gas collection network within the plant area, the main flare network outside the plant area, and the flare unit itself. In various projects, the olefin plant can use its own flare system or share one flare system with other plants ； In different projects, the scope of the flare system varies as well ； The torch systems for some projects belong to olefin plants, while those for other projects are part of storage and transportation or utility plants ； Although the devices to which they belong differ, there are commonalities in the design and operation of both overhead flares and ground flares. 1. The main functions of the flare 1.1. To safely dispose of excess flammable and explosive materials emitted during the normal operation of the facility. 1.2. Materials in the processing unit under abnormal conditions. 1.3. As safety measures for emergency incident handling. 2. Comparison of cold flare collection systems in different projects: Flare collection pipeline systems can be divided into four categories: hot flare emissions (WF), cold flare emissions (DF), hot liquid hydrocarbon emissions (HBD), and cold liquid hydrocarbon emissions (CBD) (different plants use different terms; some use terms such as dry flares and wet flares). The classification is based primarily on the temperature of the emissions after they pass through the system and whether they contain water ； The classification of cold and hot flare collection systems varies across different standards and process packages; the common point is that the criterion regarding the presence of water remains the same (water-containing fluids are sent to the hot flare), but the classification based on the temperature after emission differs.
3. Maximum emission volume of the flare under accident conditions: The design of the flare collection system in the facility is such that it can safely handle the flare gas generated under various operating conditions. Emission levels can range from almost zero to very high levels during emergencies; their composition, temperature, pressure, and emission frequency are the key factors in the design of flare systems. In olefin projects, the flare gas emission volume is typically determined based on impact analyses under utility failures and other types of failures; the specific design criteria will not be detailed here, and relevant documentation can be referred to. With the increase in production scale for olefin projects, emissions under accident conditions also vary significantly. The release of large quantities of flammable and explosive materials not only results in huge economic losses but also poses a significant challenge to the safe operation of the flare system. In early ethylene plants, cold liquid hydrocarbon heaters were directly heated using steam; during emergency releases, the large temperature difference made it very easy for the heat exchangers to leak, posing significant safety risks. In new projects nowadays, methanol is generally used as an intermediate medium, which reduces the temperature difference for heat exchange and effectively lowers the risk of leaks in the heat exchangers. 4. Purpose and significance of the purge gas design for the flare system: To prevent air from entering the flare system and causing an explosion, fuel gas or nitrogen is injected at the ends of various branches in the flare collection system within the facility. The design of different projects varies slightly. Under specific conditions, the torch pipeline system experiences two types of negative pressure. One scenario of negative pressure occurs when high-temperature gas emissions cease and it starts to rain; the temperature of the gas inside the pipes drops significantly, resulting in negative pressure throughout the pipeline network. If there is insufficient sealing water at the flare, air can enter the pipeline system from the flare head. Another negative pressure condition occurs when, due to differences in atmospheric pressure, the exhaust gases with a density lower than that of air are in a state of slow flow or no flow; as a result, every point from the water seal tank to the torch outlet is under a different level of negative pressure. If the amount of water in the seal is insufficient and the system pipelines maintain positive pressure, then the entire torch emission system will experience negative pressure. In this case, air will not enter the pipeline system from the flare tip, but it can enter the system through leakage points in the flare collection system. Injecting purge gas is the only means to prevent torch flashback. Purge gas lines installed in the flare branch pipelines are all equipped with flow-restricting orifice plates. For different projects, the purge gas pipelines have sizes such as DN20, DN25, and DN50. During normal operation, in order to save materials, some facilities reduce the flow rate of the purge gas at the ends of various branches in the flare system. While this reduces the consumption of utility materials, it also increases the risks associated with the flare system.
5. Issues with the flare collection system
5.1. At the bends, pinholes, and weld points of the thinner CBD discharge pipelines, leaks are likely to occur during emergency discharges. During the project construction phase, especially for the finer CBD and HBD emission pipelines, construction workers, in an effort to meet deadlines, use acetylene to heat and bend the joints. This practice damages the properties of the material, making leaks at those joints very likely to occur during emissions. Another approach is to use proper elbows at the bends of thinner pipes, but poor welding quality can also easily lead to leaks. Due to inherent quality issues with the valve body and piping, such as sand holes, material leakage can easily occur even during emergency discharge. 5.2. Leakage at the flange of the flame superheater during commissioning: During the commissioning of the flame heater and the cold flame superheater in the ethylene tank farm of a certain project, leakage occurred at the flange of the equipment due to emissions from the flame. The flange of the liquid-phase pipeline body at the bottom of the cold flare tank in an ethylene project leaked due to low-temperature emissions during an emergency shutdown. 5.3. Low temperature occurs due to insufficient heating during torch discharge. In one project, during its initial start-up after loading materials, the downstream valve of the steam trap for heating the thermal flare tank closed; as a result, the material could not be heated during discharge, leading to freezing and blockage of the thermal flare tank. During the process of using gaseous ethylene to purge the ethylene tanks for torching in a certain project, heating was not carried out in a timely manner, resulting in the temperature at the outlet of the torch heater dropping sharply below 0°C. 5.4. The piping design is unreasonable; during material discharge, locally high pressure affects adjacent systems. In a certain olefins project, the discharge line from the ethylene pump in the ethylene storage area is located very close to the discharge line for the pump seal. During the pre-cooling and discharge operations of the pump, this proximity causes an increase in the pressure on the pump seal, thereby affecting the safe operation of the pump. In a certain olefin project, the flare line for the dry gas seal of the pyrolysis gas compressor is connected too closely to the main flare line of the process system. When the process system discharges gases, the back pressure of the dry gas seal rises sharply, causing the flow rate through the dry gas seal’s flare line to drop to zero instantly. The large fluctuations in the flare flow rate can easily lead to an interlocked shutdown. 5.5. Fire incident in low-pressure flare pipeline: A fire broke out in a low-pressure pipeline at an ethylene plant. The main cause of the accident was leaks in the flare system, allowing air to enter the system. Precautionary measures: All the emission pipelines and flare lines in the system are designed to withstand certain levels of flow. Prolonged excessive emissions or the failure to take necessary measures for emissions over time can pose potential risks; therefore, sufficient attention must be paid to the operation of material emissions within the system ； The nitrogen at each endpoint of the flare line should be regularly inspected to ensure a sufficient flow of nitrogen within the flare line and other systems that have nitrogen protection systems, thereby maintaining the safety of the system. 6. Sources and analysis of abnormal material emissions from the flare system: After the unit starts operating with feedstock, under normal conditions, no material should be emitted from the flare system apart from the purge gas ； However, in actual operation, it is found that there is material emission from the flare system; such abnormal leakage of materials not only affects economic efficiency but also can impact the stable operation of the equipment. When no source of abnormal material emission can be identified, in order to reduce the amount of work required for investigation, samples can be taken at the flare main pipe to analyze the components in the flare gas; by examining the concentrations of these components, the scope of the search can be narrowed down, allowing the emission source to be located more quickly. In actual operation, the main sources of abnormal emissions are as follows: a) Internal leakage in the flare emission valves of the process system: Impurities enter the valve passages during purging, or the sealing surfaces wear out; this situation is very common in newly built facilities. b) The installation direction of the low-temperature valves in the cold flare system is incorrect. c) After the accident was handled, the flare valve did not close properly. d) Emissions from pressure relief and displacement processes during the cleaning of pump strainers, disconnection and reactivation of heat exchangers. e) Sampling and displacement emission.
5.1. At the bends, pinholes, and weld points of the thinner CBD discharge pipelines, leaks are likely to occur during emergency discharges. During the project construction phase, especially for the finer CBD and HBD emission pipelines, construction workers, in an effort to meet deadlines, use acetylene to heat and bend the joints. This practice damages the properties of the material, making leaks at those joints very likely to occur during emissions. Another approach is to use proper elbows at the bends of thinner pipes, but poor welding quality can also easily lead to leaks. Due to inherent quality issues with the valve body and piping, such as sand holes, material leakage can easily occur even during emergency discharge. 5.2. Leakage at the flange of the flame superheater during commissioning: During the commissioning of the flame heater and the cold flame superheater in the ethylene tank farm of a certain project, leakage occurred at the flange of the equipment due to emissions from the flame. The flange of the liquid-phase pipeline body at the bottom of the cold flare tank in an ethylene project leaked due to low-temperature emissions during an emergency shutdown. 5.3. Low temperature occurs due to insufficient heating during torch discharge. In one project, during its initial start-up after loading materials, the downstream valve of the steam trap for heating the thermal flare tank closed; as a result, the material could not be heated during discharge, leading to freezing and blockage of the thermal flare tank. During the process of using gaseous ethylene to purge the ethylene tanks for torching in a certain project, heating was not carried out in a timely manner, resulting in the temperature at the outlet of the torch heater dropping sharply below 0°C. 5.4. The piping design is unreasonable; during material discharge, locally high pressure affects adjacent systems. In a certain olefins project, the discharge line from the ethylene pump in the ethylene storage area is located very close to the discharge line for the pump seal. During the pre-cooling and discharge operations of the pump, this proximity causes an increase in the pressure on the pump seal, thereby affecting the safe operation of the pump. In a certain olefin project, the flare line for the dry gas seal of the pyrolysis gas compressor is connected too closely to the main flare line of the process system. When the process system discharges gases, the back pressure of the dry gas seal rises sharply, causing the flow rate through the dry gas seal’s flare line to drop to zero instantly. The large fluctuations in the flare flow rate can easily lead to an interlocked shutdown. 5.5. Fire incident in low-pressure flare pipeline: A fire broke out in a low-pressure pipeline at an ethylene plant. The main cause of the accident was leaks in the flare system, allowing air to enter the system. Precautionary measures: All the emission pipelines and flare lines in the system are designed to withstand certain levels of flow. Prolonged excessive emissions or the failure to take necessary measures for emissions over time can pose potential risks; therefore, sufficient attention must be paid to the operation of material emissions within the system ； The nitrogen at each endpoint of the flare line should be regularly inspected to ensure a sufficient flow of nitrogen within the flare line and other systems that have nitrogen protection systems, thereby maintaining the safety of the system. 6. Sources and analysis of abnormal material emissions from the flare system: After the unit starts operating with feedstock, under normal conditions, no material should be emitted from the flare system apart from the purge gas ； However, in actual operation, it is found that there is material emission from the flare system; such abnormal leakage of materials not only affects economic efficiency but also can impact the stable operation of the equipment. When no source of abnormal material emission can be identified, in order to reduce the amount of work required for investigation, samples can be taken at the flare main pipe to analyze the components in the flare gas; by examining the concentrations of these components, the scope of the search can be narrowed down, allowing the emission source to be located more quickly. In actual operation, the main sources of abnormal emissions are as follows: a) Internal leakage in the flare emission valves of the process system: Impurities enter the valve passages during purging, or the sealing surfaces wear out; this situation is very common in newly built facilities. b) The installation direction of the low-temperature valves in the cold flare system is incorrect. c) After the accident was handled, the flare valve did not close properly. d) Emissions from pressure relief and displacement processes during the cleaning of pump strainers, disconnection and reactivation of heat exchangers. e) Sampling and displacement emission.
7. In an emergency situation, the pressure in the flare main pipe changes; under normal operation of the facility, the pressure in the flare system is 10 Kpa； In an emergency situation, the system releases a large amount of flares, causing the pressure in the flare system to rise sharply. The temperature of the cold flare tanks drops drastically to below -50°C, and a thick layer of frost forms on the cold flare pipelines on site. This represents a critical test for the flare system. 8. Conclusion: The flame system serves as the last line of defense to ensure safety in case of an accident. However, many projects do not pay sufficient attention to this system during its construction and operation, resulting in various risks. It is essential to focus on identifying risk points at different stages of a project in order to ensure the safe operation of the equipment. References: 1. SH-3009, Design Code for Flammable Gas Emission Systems in Petrochemical Industries. 2. HG/T-20570, Technical Specifications for the Design of Process System Engineering. 3. Edited by Wang Songhan, Technology and Operation of Ethylene Plants. 4. API-521, Pressure-relieving and Depressuring Systems.