【Common Issues in Haichuan Pipeline Design】The liquid pocket problem in the flare main pipe

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During the design, installation, and inspection of pipelines, various small issues that have been overlooked or not taken into consideration are often identified. These issues are collected and organized into a series; this list is being updated continuously. Everyone is welcome to actively participate in discussions. This is a compilation post on common problems in Haichuan pipeline design – it’s still being updated, and discussions are welcome: https://bbs.hcbbs.com/thread-5705828-1-1.html ------------------------------------------------------------------ The issue of liquid pockets in the flare header. Read more in the novel reader. The issue of liquid pockets in the flare header: Flare headers and pressure relief systems serve as the last line of defense against sudden overpressure and leaks in factories. Therefore, the efficient design, management, and maintenance of these systems are crucial for the safe operation of the factory throughout its entire lifecycle. The pipeline design of the flare header is a core component of this system, and any possible phase changes must be taken into account to address the multiphase flow conditions within the flare header. Liquid entrainment into the flare tower may affect the overpressure protection effect, especially when condensation occurs after the liquid separation drum (KO drum). Therefore, a properly designed flare header and integrity assurance play a key role in the overall reliability of the system. Bag-shaped sections in flare pipelines pose potential safety hazards, including fluid accumulation, deposit buildup, and retention of corrosive fluids. These factors can impede the flow of fluids during emergencies; they may also give rise to slug flow, ultimately leading to system failure and leakage incidents. Once the design enters the detailed engineering phase, the opportunities to modify or optimize the piping system design decrease; otherwise, it will result in significant change orders and project delays. This article focuses on areas where significant benefits can be achieved as early as the Feasibility, Engineering, and Design (FEED) stage of a project. In practice, such liquid pockets are commonly found in the flare header of certain facilities, which may be due to limited height of the flare tower or to control over costs related to pipeline support and structure. However, this practice contradicts the best practices and guidelines in international standards, which prohibit the presence of liquid pockets in the pipes of torch systems. Torch main pipe design: The standard layout of the torch system piping is a crucial aspect of plant design, and therefore it needs to be planned at the early stage of engineering design. This helps to avoid unexpected situations and major changes during the detailed design phase, which could significantly affect costs and schedules. In the flare system, the flare knockout drum (FKOD) is used to separate liquids from gases/vapors. Torch towers are not designed to handle large volumes of liquid; therefore, liquid separators are crucial for mitigating the risk of “fire rain”. Ideally, the main flare liquid separation tank should be as close as possible to the flare tower (distance ≤ 100 meters) to ensure maximum liquid separation. Standard API 521 specifies that all branch pipes and main pipes must maintain a slope of 21 millimeters over a distance of 10 meters (i.e., a slope of 0.21%). The slope direction should face the liquid separation tank to avoid the formation of low points. The typical slope requirements are shown in Figure 1.