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Article source: Publication date: 2016-3-3 10:03:32. It discusses the principles for the placement of safety valves and the requirements for piping design, and provides a detailed analysis of the key aspects involved in such design, including the installation location of safety valves, the piping design for their inlets and outlets, as well as the installation of shut-off valves and bypass systems. Safety valves are the best overpressure protection devices for pressure-bearing equipment, vessels, and pipelines. When the medium… It discusses the principles for the placement of safety valves as well as the requirements for piping design, and provides a detailed analysis of the key aspects involved in design, including the installation location of safety valves, the piping design for their inlet and outlet ports, and the installation of shut-off valves and bypass systems. Safety valves are the best overpressure protection devices for pressure-bearing equipment, containers, and pipelines. When the pressure of the fluid exceeds the allowable level, the safety valve opens automatically and releases the fluid in its entirety, preventing further increases in pressure and thus protecting the equipment as well as the personnel operating it ； When the pressure drops to the specified value, the safety valve closes automatically in a timely manner, preventing further discharge of the medium and reducing losses. Since safety valves are automatic valves, they are often used as the final protection device for pressurized equipment. In this sense, its function cannot be replaced by other protective devices. This article was compiled by Shanghai Wuyue Pump Valve Manufacturing Co., Ltd. A company specialized in the research, development, and production of spring-loaded safety valves. For more technical questions and articles related to safety valves, please visit the official website of Shanghai Wuyue. I. Installation of safety valves Safety valves should be installed vertically and as close as possible to the equipment or pipelines they are meant to protect. If it is not possible to arrange it at a close distance, the total pressure drop in the pipeline between the protected equipment or pipe and the safety valve inlet shall not exceed 3% of the set pressure value of the safety valve, or 1/3 of the maximum allowable opening/closing pressure difference (whichever is smaller). A too large pressure drop can cause the safety valve to activate frequently. Common methods used in engineering practice to reduce pipeline pressure drop include decreasing the overall pipeline pressure drop by appropriately enlarging the diameter of the safety valve inlet pipe, using elbows with large radii, and reducing the number of elbows. The installation location of the safety valve should primarily take into account ease of maintenance. Therefore, a maintenance platform should be provided at the safety valve location; this facilitates piping work as well as enables regular inspection, maintenance, and calibration of the safety valve. When installing safety valves with heavy weights, the possibility of lifting them after removal must be taken into account; when necessary, lifting rods should be provided, as well as space reserved for inspection and maintenance. In engineering practice, it is common to install safety valves at the top of the pipe rack. In this way, it is higher than the vent main on one hand, and the safety valves are centrally located on the top layer of the pipe rack for easy inspection and maintenance. When it is difficult to install it on the container body for special reasons, it can be considered for installation on the outlet pipeline. However, the pipeline between the valve installation site and the container should be free of sudden bends or local reductions in cross-section, etc., in order to prevent an increase in pipeline resistance and the accumulation of debris that could lead to blockages. II. Design of the inlet piping for safety valves The inlet piping for safety valves should be short and straight, and long-radius elbows are preferred. The pipeline must have a slope of at least 5%, with the slope pointing toward the system to be protected ； It is necessary to determine whether compensation is required for the inlet pipeline within its operating temperature range. The inlet piping should avoid bag bends as much as possible; if this cannot be avoided, a drainage pipe that ensures continuous flow of the viscous substances at the lowest point of the bag bend should be connected to the same pressure system. If the condensate tends to thicken or solidify, this drainage pipe must be insulated ； For non-condensable media, there is an easily accessible drain valve at the lowest point of the bag bend. The minimum cross-sectional area of the inlet duct for the pipeline shall not be smaller than the cross-sectional area at the inlet of the safety valve. For high-pressure and high-displacement applications, the inlet pipe should have a sufficiently large radius of curvature at the inlet ； Or it may have a conical channel, whose inlet cross-sectional area is approximately twice that of the outlet cross-sectional area. The inlet pipes must have sufficient strength, with appropriate additional support brackets, to withstand the stresses resulting from the combined effects of medium pressure, temperature, and the reaction forces generated by valve discharge ； At the same time, prevent the vibration of the equipment from being transmitted to the safety valve, thereby affecting its sealing. III. Design of the outlet piping for safety valves The design of the outlet pipeline for safety valves should take into account that the back pressure should not exceed a certain value set by the safety valve’s rated pressure. For standard spring-loaded safety valves, the back pressure shall not exceed 10% of the valve’s set pressure value. For the safety valve outlet pipeline in liquid-phase media, it should be as high as possible above the vent main, primarily to prevent backflow of condensate from affecting the proper operation of the safety valve. When the medium discharged into the vent main or flare main contains condensate or condensable gases, the outlet of the safety valve should also be above the main. Otherwise, automatic drainage measures should be considered. In engineering practice, it is common to connect the liquid collection tank via a drain pipe, and then use a pump or gas purge to send the liquid to a separation tank for centralized treatment. The outlet pipe of the safety valve connected to the flare main pipe should be attached at a 45° angle to the top of the discharge main pipe, in line with the flow direction of the medium, so as to prevent condensate in the main pipe from flowing back into the branch pipes and to reduce the back pressure on the safety valve. When materials are discharged, the outflow of these materials exerts a certain force on the discharge pipeline, which is then transmitted to the safety valve. The forces and moments resulting from this can have adverse effects on the device’s connections as well as the inlet and outlet pipes of the safety valve, and may even lead to leaks at the seal of the safety valve, causing accidents. Therefore, the reaction force at the outlet of the safety valve must be calculated, and appropriate support structures must be installed by taking into account factors such as its own weight, vibration, wind loads, and thermal expansion and contraction. IV. Installation of isolation valves and bypass valves before and after the safety valve 1. Installation of isolation valves before and after the safety valve In industrial production, it is not always necessary to stop operations when replacing a safety valve; therefore, in order to maintain production, most companies install isolation valves in front of the safety valve. Generally, a single-plate gate valve should be used, sealed with lead, and the valve stem should be installed horizontally to prevent the valve plate from sliding down if the pin connecting the valve stem and the valve plate corrodes or loosens. When the safety valve reaches the end of its calibration period or gets damaged after opening, the isolation valve at the inlet of the safety valve can be closed, and the safety valve can be removed for calibration or replacement. Based on practical production experience, the main situations in which a shut-off valve is installed in advance are as follows: 1.1 The safety valve experiences leakage during normal operation, and it cannot return to its original position after opening due to overpressure. 1.2 It facilitates monitoring of safety valves by technical supervision departments during production. 1.3 It directly affects the start-up and shutdown of the production system in terms of critical equipment and processes, causing economic losses to the enterprise. 1.4 Equipment and apparatus for producing or storing polluting or hazardous gases and liquids. For safety valves that discharge into a flare or a closed system, even if there is no need to install a bypass on the outlet pipe, a shut-off valve is usually installed on that outlet pipe and sealed with lead, so that it can be used to isolate the flare or closed system in case of a malfunction of the safety valve; otherwise, the entire flare or closed system would have to be shut down. 2. Installation of the safety valve bypass The installation of a safety valve bypass is not necessary. The purpose of setting up a safety valve bypass is to enable the equipment and systems protected by the safety valve to have their safety valves repaired without having to shut down. Only the safety valves of those important devices that require online maintenance and cannot stop operating need to be equipped with bypasses. If the equipment and systems protected by the safety valve can be shut down for maintenance in the event of a failure of the safety valve, then a bypass is not necessary. Another function of the bypass is to allow steam purging of the equipment and systems during startup and shutdown, by opening the bypass to direct the purge flow to the flare system. The safety valve bypass is installed outside the shut-off valve on the inlet and outlet pipelines of the safety valve (with the shut-off valve situated between the bypass and the safety valve), and it generally has the same diameter as the inlet pipeline ； It is best to install a pressure gauge on the pipeline of the system near the safety valve, so that the system pressure can be monitored at any time. V. Conclusion This article discusses the principles for the placement of safety valves as well as the requirements for piping design. Safety valves are among the most common and important protective devices in modern pressure-containing systems, and they play a crucial role in ensuring the safe operation of such systems and equipment. Therefore, sufficient attention should be paid to their placement and piping design in practical applications going forward.