Thread Content
Precautions and reasonable suggestions for the design of petrochemical pipelines; safety management during the transfer process. With the continuous development of China’s petroleum industry, the scale of petrochemical plants is increasing, which leads to more stringent requirements for the design of these pipelines. As special equipment for material transportation, petrochemical pipelines serve to connect various devices and related system facilities, enabling petrochemical plants to function as a cohesive whole. They also provide important protection for the safe transport of various fluids. However, in actual pipeline design work, various factors such as different types of pipelines and operating conditions have a significant impact on the safety of pipeline design. Therefore, to design a pipeline that meets the requirements, pipeline designers need to take into account various factors. Special attention must be paid to the precautions in pipeline design, and effective measures should be adopted to ensure the safety and durability of the pipeline design, as well as its safety and rationality. Factors affecting pipeline safety: Corrosion. Corrosion is the degradation of pipeline materials caused by chemical reactions with the substances in the environment surrounding the pipelines, and it has a significant impact on the safety of these pipelines. The forms of corrosion-induced damage to pipelines include stress corrosion, uniform corrosion, local corrosion, and atmospheric corrosion. Due to the different environments in which various pipeline materials are exposed, their corrosion resistance also varies. The environment in which pipelines are located is generally complex and prone to change, and coupled with certain defects in the materials themselves, corrosion becomes even more difficult to predict and control. Physical damage to oil pipelines is mainly manifested in two aspects: low-temperature brittle fracture and high-temperature degradation. Under low-temperature conditions, when the temperature of the pipeline material falls below its brittle transition temperature, this can lead to a decrease in its impact toughness and subsequent brittle fracture. At high temperatures, the properties of pipeline materials undergo adverse changes; phenomena such as creep failure, graphitization in carbon steel and carbonaceous steels, and temper embrittlement can all lead to the weakening and embrittlement of these materials. At the same time, pipeline metal materials are prone to fatigue when exposed to alternating loads or periodic temperature changes, which can lead to metal deformation, damage, and even fracture, severely affecting the safety of petrochemical pipelines. Sealing performance: Currently, the sealing of petrochemical pipelines relies mainly on pipe flange sealing and valve sealing to prevent pipeline corrosion and leakage, thereby ensuring pipeline safety. Flange sealing relies on the combined action of three components: the flange, bolts, and gaskets. The stiffness of the flange, the design of its sealing surface, as well as the properties of the gaskets all have a direct impact on the sealing performance. If the pipeline shifts, it will also affect the flange sealing, thereby impacting the safety of the pipeline. Valve leaks can be categorized into internal and external leaks. Statistics show that leaks from valves and flanges account for approximately two-thirds of all leakages. Such leaks directly affect the safety of pipelines and may even lead to more serious consequences. Therefore, taking effective measures to prevent leaks from valves and flanges is of great significance for the safety of petrochemical plants. Considerations for pipeline design: Proper material selection. The choice of materials for petrochemical pipelines, especially metal materials, has an impact on various aspects such as pipeline safety, maintenance, and technical upgrades. Improper material selection can potentially lead to explosions or other safety incidents; therefore, it is essential to choose the right materials for pipelines. Different gaskets should be used for flange connections made of different materials; metal gaskets are employed for high-grade flanges to withstand higher sealing pressures, and bolts corresponding to flanges with lower hardness should be of high grade as well. The support structure design must be stable; an improper design of the support structure can lead to damage to the pipelines during operation or to damage to the rotating equipment. The design of the support structure is closely linked to that of the pipelines, and it is essential that the support structure design remain stable. When designing pipe racks, try to minimize the use of spring brackets, as they are difficult to install and tend to fail under prolonged operation. During the installation of spring brackets, the positioning pins must not be removed arbitrarily. Along the pipeline of the tower, generally only one load-bearing support is installed; if the load carried by the first support is too large, a second spring suspension bracket is added, with guide supports installed at regular intervals. Hangers generally have a certain turning angle, and designing too many hangers in a long pipeline can affect the stability of the pipeline. The pipeline layout should be reasonable. In the design of petrochemical pipelines, the layout must take into account the requirements related to pipeline construction, maintenance, as well as the arrangement of processes and equipment, in order to ensure a scientific and rational approach. The design margin on the pipe gallery should be between 20% and 30%, with heat-stressed pipes arranged on both sides. To prevent condensate in the main pipe from flowing into the branch pipes and to reduce the back pressure on the safety valve, the outlet of the safety valve should be inserted into the pressure relief main pipe at a 45-degree angle from above, following the flow direction. For jacketed pipes, seamless or seam-welded fittings should be used; the spacing between the butt welds of the inner pipe must be greater than 2 meters, and the outer pipe can be installed only after passing the relevant inspection. The pump inlet pipe should be as short and straight as possible; eccentric tees should be used to prevent gas accumulation and ensure that there are no air pockets or liquid pockets. Pay attention to material inspection: After the pipeline materials arrive at the site, they should be inspected first to ensure their quantity and quality. If any materials are missing, it is necessary to contact the relevant manufacturers immediately to arrange for their provision. Materials that do not meet the required standards should be sent to the appropriate authorities for reinspection, in order to avoid delays and rework caused by insufficient or substandard materials. All metal pipe materials must undergo hydraulic and airtightness tests, while valves are subjected to hydraulic testing and random inspections for adjustment, depending on their safety level. The rationality of process pipeline design: Pipeline design plays an important role in chemical plants. Its design principles remain largely unchanged; aside from the issues mentioned above, it is also necessary to have a good grasp of relevant standards and basic knowledge, as well as a thorough understanding of the process diagrams, in order to meet the requirements of the production process. Breakdown of process pipeline materials and grades: In petrochemical plants, there are significant differences in pressure and temperature between high-pressure systems and low-pressure systems; therefore, when connecting these systems, it is necessary to determine the division points according to the grade of the pipelines that are under lower pressure. Under normal circumstances, the section limits are clearly defined in accordance with the P&ID; in case of special circumstances, corresponding adjustments need to be made based on local conditions to properly define the boundary points and lines. Grade decomposition is generally divided into the following 3 cases: First, the pressure grades are the same, but the materials differ, as shown in Figure 1. Secondly, the materials are the same, but the pressure ratings differ, as shown in Figure 2. Thirdly, the pressure ratings and materials are different, as shown in Figure 3. As can be seen from Figure 1, under normal circumstances, the flanges and gaskets are made of lower-grade materials, while the bolts and valves are made of higher-grade materials ； As can be seen from Figure 2, flanges, gaskets, bolts, and valves must be made of high-pressure materials ； As can be seen from Figure 3, the flanges and gaskets use high-pressure grade material a, while the bolts and valves utilize high-pressure grade material b. Piping design for towers and vessels in plant processes: In petrochemical installations, the piping design for towers and vessels must take into full account their design principles as well as the specific requirements of the process. It is mainly divided into the pipeline layout between the distillation tower and the reflux tank, and the pipeline layout between the distillation tower and the stripping tower. Piping arrangement between the distillation tower and the reflux tank: When the top pressure of the distillation tower is controlled using a thermal bypass, it is essential to ensure that the thermal bypass is short and free of any bottlenecks; therefore, the control valve should be located above the reflux tank, as shown in Figure 4. Piping arrangement between the distillation tower and the stripping tower: Under normal circumstances, there should be a control valve assembly in the distillation tower, and this valve assembly should be installed near the stripping tower to ensure that the control valves have sufficient liquid column, as shown in Figure 5. The piping for two-phase flow should be arranged according to the specific conditions. When designing such piping, the control valves should be placed as close as possible to the container that receives the fluid, in order to reduce the pressure in the pipes and prevent vibrations. Additionally, the pipelines cannot be laid arbitrarily; they must be designed accordingly based on the actual circumstances, as shown in Figure 6. The rationality of pipeline and valve layout: The layout of pipelines and valves should be analyzed from two aspects: the placement of sampling points and the setup of steam purging lines. First, the placement of sampling points. It is necessary to meet the design requirements of the relevant process; it should be installed on the main pipe, in front of the branch, avoiding sampling in dead corners or at the bottom of horizontal pipes, to ensure that the samples collected are representative. Secondly, the installation of the steam purge pipeline. This type of pipeline is quite common in chemical plants; its main purpose is to detect and identify leaks. For different process pipelines, steam purge lines should be connected. The advantage of using this installation method is that if one of the pipes leaks, it is possible to shut off the isolation valve on that pipe without affecting the normal operation of the other pipes or the main steam pipeline. The rationality of the pipeline design for heat exchange equipment: In the process of designing the process pipelines for petrochemical plants, the proper arrangement of pipelines related to heat exchange equipment differs significantly from that of other types of process equipment. There are many complex technical factors involved, and therefore, it is necessary to conduct a thorough examination of these factors when it comes to operating, maintaining such equipment, as well as managing thermal stresses in the pipelines. (1) Regarding counterflow heat transfer. In cold exchange equipment, the cold water pipes are usually arranged with the inlet at the bottom and the outlet at the top, so that in the event of a failure, there is still a certain amount of water remaining in the heat exchanger, preventing it from being emptied. (2) Regarding the minimum distance for installation. During the heat exchange process in cold exchange equipment, to facilitate inspection, a certain clear distance is usually maintained between the pipelines and valve flanges at the inlet and outlet of the heat exchanger and the flanges of the equipment’s head. Generally, this clear distance is set at around 310 mm to ease disassembly. (3) Regarding thermal stress. During the proper layout of heat exchange equipment, the fixing points of the heat exchangers are generally placed at the ends of the tube boxes; moreover, the pipes connecting to the end nozzles must also be carefully considered in light of the displacement caused by the thermal expansion of the heat exchangers. The rationality of the pipeline design for cold exchange equipment: The design of the pipe supports is closely linked to that of the pipes themselves. If the design of the pipe supports is inadequate, it can lead to damage to the pipes during operation, and even cause damage to the equipment as well. (1) Reduce the relative displacement between the pipeline and the supports. As shown in Figure 7: In steam pipes, there are usually drainage tubes adjacent to the main pipe. If point A is used as a support on the branch surface, the vertical pipe sections tend to expand, which prevents the spring from moving downward and can thus cause damage to the equipment. Therefore, in light of such problems, the correct design method should be as shown in Figure 8: a bracket supported on the ground is mounted on the main pipe, and this bracket rises and falls together with the main pipe frame, thereby reducing the relative displacement between the pipe and the bracket. (2) If the pipe supports are properly arranged, waste can also be reduced. Since the springs in these supports lose their elasticity over time, the design of the pipe supports should minimize the use of such spring supports. (3) Laying of pipelines. Pipelines laid along the tower generally have only one load-bearing support, and there should be a welding line between this support and the top of the tower. If the load on one load-bearing support becomes too high, another support can be installed; this additional support should be a spring-supported type, with guide supports placed at regular intervals. When the temperature difference between the backflow line and the tower wall is large, the resulting elongation is also greater. The first support on the horizontal pipe section should be adjusted vertically, and spring supports should be used. When L is large, and there is sufficient relative elongation, guide clamps can also be installed. The rationality of the pipeline design for heat exchangers: The pipeline design for pumps mainly involves the design of the eccentric diameter tube at the pump inlet, the design of the straight section at the pump inlet, and the study of pipeline flexibility. (1) Design of the pump inlet eccentric diameter tube. Proper design of the pump inlet eccentric diameter tube is primarily aimed at ensuring the smooth operation of the pump. If the eccentricity of the pump inlet diameter changes, gas will not converge at the area where the diameter changes, which in turn can lead to cavitation. Therefore, the top-level installation method should be used when setting the eccentricity of the pump inlet, and a drain valve should be installed at the lowest point; this will help to prevent cavitation. (2) Design of the straight pipe section at the pump inlet. The design of the pump inlet pipeline generally takes two factors into consideration: First, since the pump’s inlet is on one side, the supports for the inlet pipe must be adjustable, and the inlet pipe along with the valves should be located at the front side of the pump ； Secondly, air blockages in the pipeline leading to the pump should be avoided; therefore, this aspect needs to be taken into consideration during design. (3) Pipeline flexibility. Since the pump is a rotating machine, the action of pipeline forces can cause misalignment at the pump outlet. To address this issue, when designing the pipeline, it is necessary to ensure that the forces acting on the pump outlet remain within acceptable limits; thermal compensation is particularly important in this regard. Conclusion: The design of process pipelines for petrochemical plants is a complex engineering task that requires not only specialized knowledge from the personnel involved, but also an innovative mindset on the part of the designers. By being able to design pipelines in a way that takes actual conditions into account, it is possible to ensure the safety of these pipelines and, ultimately, the proper operation of the chemical processing equipment.