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Issues that are easily overlooked in pressure pipeline design Since the Ministry of Labor issued the \"Regulations on the Safety Management and Inspection of Pressure Pipelines,\" attention has been paid to problems related to pressure pipelines by various parties. Since pressure pipelines are a key component in petrochemical production plant projects, to ensure their safe operation during production, strict management must be implemented at all stages such as design, manufacturing, installation, inspection, and operation; among these, sufficient attention must first be paid to the design phase. 1. Pressure pipelines refer to special types of equipment used in production and daily life that may pose significant risks such as fires, explosions, or poisoning. Pipes such as steam pipes, pipes for toxic, flammable, and explosive media, gas and natural gas pipes, long-distance oil and natural gas pipelines, and pipes with a gas pressure of 0.1 Mpa or higher are all considered pressure pipes ； If the medium inside the pipe is one that can easily cause combustion, explosion, or severe corrosion, such a pipe is still considered a pressure pipe, even at atmospheric pressure. It is evident that pressure pipelines are widely used; if not managed properly, they can easily lead to casualties and economic losses. Therefore, pressure pipelines are classified as special equipment alongside boilers and pressure vessels, and are subject to **safety supervision. 2. The design of pressure pipelines must not only meet the design requirements specified in the Piping and Instrumentation Diagram (P&ID) and comply with relevant codes, standards, and regulations; careful consideration must also be given to the details of the design. Otherwise, safety accidents are likely to occur. The following are some details that I have identified through my practical work as being easily overlooked in the design of pressure pipelines. 2.1 Air and inert gases at room temperature can be discharged on-site, but when the pipelines are indoors, it is best to lead them outside for discharge. Steam and other flammable, explosive, or toxic gases should be discharged into a flare or into the atmosphere, depending on factors such as their volume and density. When the exhaust pipe discharges into the atmosphere, care must be taken to ensure that its outlet is not directed toward platforms, stairs, cables, etc. Water discharge can be directed to nearby sink drains or gutters, while the discharge of other liquid substances must be routed to designated drainage systems. In particular, for Class B fluids, a sealed discharge system must be introduced. For flame arresters on pipelines carrying flammable and explosive gases, they should be installed near the vent. The flame arrester on the exhaust pipe of outdoor containers must be at least 500 mm away from the device’s outlet. The exhaust pipes of indoor containers need to extend outside the roof; the flame arrester should be placed on or near the roof to facilitate installation and maintenance. The distance between the flame arrester and the exhaust outlet must meet the maximum safety clearance required for the flame arrester. 2.2 Generally, a filter is installed at the inlet of pumps to prevent impurities from entering the equipment body and causing damage to its interior. Many people overlook the requirements regarding the core-pulling distance of filters when designing inlet pipelines. Both T-type and Y-type filters involve the issue of the core extraction space, except for basket filters that are of the lifting type. Some T-type filters also have a drain port, and the core-extraction side must face downward. Apart from the fact that the Y-type filter can be offset by 450, when arranging the equipment, the pump foundation should be raised appropriately so that there is enough space to smoothly remove the filter. When the medium entering the pump’s inlet flows into the pump by gravity due to a height difference created by other equipment, it is necessary to consider not only raising the pump foundation but also any changes to the foundations of these related pieces of equipment. 2.3 Safety valves prevent pressurized components such as equipment or pipelines from exploding due to pressure exceeding their design limits, which can be caused by fires, operational failures, or interruptions in water or power supply. When the pressure inside the device reaches a predetermined value, the safety valve relief mechanism activates to release the pressurized medium. Therefore, the design location of the safety valve should be as close as possible to the equipment or pipeline that is being protected. It is essential to ensure that the pressure drop in the pipeline between the outlet of the equipment under protection and the inlet of the safety valve does not exceed 3% of the valve’s set pressure; otherwise, the pressure at the inlet of the safety valve will not accurately reflect the pressure inside the equipment under protection, which can affect the proper functioning of the safety valve and create safety hazards. When a shut-off valve is installed on the inlet pipeline of the safety valve, it should be sealed with lead; the valve stem must be installed horizontally to prevent corrosion or loosening of the pin that connects the valve stem to the valve disc, which could cause the valve disc to drop and close the shut-off valve. Closing the shut-off valve is equivalent to disabling the protective function of the safety valve, thereby creating safety hazards. When releasing pressure, a safety valve generates a significant reverse force; therefore, the outlet pipe of the safety valve must be equipped with a fixing frame or guide frame to prevent this reverse force from damaging the device’s piping, which could lead to economic losses and safety incidents. This situation also applies to cases where certain devices are fixed to pipes, such as fire monitors or fire hydrants; some of these devices are secured to the pipes using flanges. When the equipment is turned on, a reaction force is also generated; therefore, the pipe supports closest to the equipment need to be reinforced to prevent damage to them. 2.4 A piping system consists of pipes, fittings, instruments, and other components. Pipes with a diameter larger than DN300 are generally welded steel pipes, and fittings of the same size are also usually made up of two connected parts. In other words, such pipes and fittings already have a weld on them. In the welding of pipes to pipes or pipes to fittings, the pipes can be rotated to avoid the alignment of the two longitudinal welds. However, in engineering design, it is common for pipe fittings to be welded to one another. If the alignment of the joints results in two longitudinal welds being on the same line, this creates cross-welds together with the circumferential welds, and such a situation is absolutely not allowed in engineering. Since cross welds cause stress concentration, fatigue can occur over time under high pressure, posing significant safety risks. Since, during the design phase, the designers are unaware of the production methods used by the future manufacturers of the fittings, as well as the locations of the longitudinal welds used to join the fittings together, it is difficult to determine whether there are already welds in those joints or whether cross-welds will be formed. Therefore, for pipe fittings larger than DN300, it is advisable to include a straight section between the fittings. Although this adds an additional weld seam, it is still better than facing the awkward situation in construction where the equipment is already in place and the fittings require cross-welds with no space left for adding a straight section. 2.5 Pipes for transporting flammable and explosive liquids or gases generate static electricity due to the flow of material through the pipes as well as during the process of material entering and leaving containers via these pipes. The accumulation of a certain amount of static electricity can cause static electricity-related failures (where production systems malfunction due to static electricity, leading to reduced productivity, poor product quality, and even failure or damage), as well as static electricity-related disasters (such as fires, explosions, electric shocks caused by static discharge, and secondary accidents resulting therefrom). Especially for chemical plants that operate under high temperature and pressure, and are prone to fire, explosion, or contain toxic substances, ensuring static electricity safety and preventing static-related failures and disasters is a prerequisite for maintaining normal production operations. A proper pipeline design can prevent the generation of local static electricity. For example, pipes used to transport flammable liquids to storage tanks generate static electricity when the liquid flows by free fall; however, inserting the pipe below the liquid surface or allowing the liquid to flow slowly down the container walls can prevent the generation of static electricity ； The hose fittings are made of a metal that does not produce sparks when impacted (brass or aluminum), which also helps to prevent the generation of static electricity. In areas where static electricity cannot be avoided, it is necessary to design static electricity grounding points for the pressure pipelines; the total grounding resistance should generally not exceed 10 ohms. Static electricity grounding points are usually installed in the following three locations: 1. At the points where pumps, filters, buffers and other equipment are connected, as these are areas where static electricity levels change, and they also provide convenient locations for grounding. 2. Large pipelines transporting flammable and explosive media are grounded at their starting points, ending points, and at each junction. 3. On pipe galleries, static grounding is typically provided at the pipe supports every 80–100 meters. Metal components on non-conductive pipe sections must be grounded, especially the metal joints in between, to prevent static electricity buildup and resulting electrostatic discharge. The contact resistance between the flanges should not exceed 0.03 ohms, and this requirement is met once the flange bolts are properly tightened. When special requirements exist, metal bridging must be added. 2.6 The characteristic of petrochemical plants is that many devices, pipelines, and instruments are interconnected to ensure normal operation. In the northern regions, especially during winter, the significant heat loss from equipment and pipelines causes materials with higher freezing points to easily solidify, form deposits on the walls, or experience an increase in viscosity. Some facilities shut down during winter without draining the water from their equipment and pipelines, and when they restart in spring, many of the equipment, pipelines, and fittings crack due to freezing. For pipelines where the temperature of the medium may be below the freezing point, steam or other heat sources are used for internal heating or jacket heating to prevent condensation and freezing. In piping design, efforts should be made to avoid areas with dead ends or blind spots; if this cannot be avoided, backflow prevention measures must be implemented. Moreover, backwashing also requires heating, because if the liquid collection point at the backwashing site freezes, it becomes useless. In the process flow, anti-freezing measures must be considered for equipment or pipelines that are used during operation, for backup equipment or pipelines, as well as for pipelines that are used intermittently. And when not in use, empty the medium inside. A bypass line should be added to the circulating water pipeline, and it is preferable for this bypass line to be equipped with heat tracing. If a heat exchanger needs to be repaired, after closing the inlet and outlet valves for the circulating water, the bypass valve is opened to keep the water flowing in the pipeline, which also serves as a measure to prevent freezing. Preventing pipe freezing in winter is a rather complex issue; inadequate measures to prevent freezing can lead to disastrous consequences, with losses that are often unpredictable. The above-mentioned points are some of the minor aspects that we often encounter in pipeline design work. If insufficient attention is paid to these details during the design process, it can often lead to serious accidents. Due to the special nature of petrochemical plants, there are many other aspects related to personal safety and protection in the design of pressure pipelines. Only by strictly adhering to relevant standards and specifications during the design process, by enhancing our awareness of details pertinent to safety, and by handling them with due care, can we design safe and compliant chemical pipelines.