【Common Issues in Haichuan Pipeline Design】--Typical Diagrams for Safety Valve Venting Design

Thread Content

This post was last edited by HaiChuan LaoYu on 2026-5-2 at 13:59. During the design, installation, and inspection of pipelines, various minor issues that have been overlooked or not taken into account are often discovered. These issues are collected and organized into a series; the 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 ------------------------------------------------------------------ Typical diagrams for safety valve venting design. I. Stress on the safety valve outlet pipe: When installing the safety valve outlet pipe, attention must be paid to the stress exerted by the outlet pipeline on the inlet pipeline, as well as the discharge backpressure. In many cases, the outlet of safety valves leads directly to a discharge pipe; for such valves, there is no need to consider the effects of thermal and mechanical stresses, and only the reaction force resulting from discharge needs to be calculated. To offset part of the exhaust reaction force, supports can also be added in the opposite direction to the exhaust flow in order to counteract a portion of it. The requirement for the limit on the discharge backpressure during the emission process is generally less than 25%. Factors such as the installation of shut-off valves at the outlet, the size of the discharge pipeline, whether the pipeline diameter is enlarged, and whether the medium undergoes phase changes can all affect the discharge backpressure. These aspects need to be thoroughly considered during the design of the process system, and they should be used as input data for the selection of safety valves. II. Accumulation of liquid at the outlet of the safety valve: To deal with any liquid that may accumulate during the discharge process (which could be the liquid being discharged or rainwater), a drainage pipe should be installed at a lower position along the discharge line, in order to prevent liquid from accumulating downstream of the pressure relief device. The installation of this drainage pipe should not affect the safety valve. If a shut-off valve that remains closed under normal conditions is installed at the outlet of the drainage pipe, an observation port should also be added to check whether liquid has accumulated in the discharge line (this also helps determine whether the safety valve has activated and is leaking). III. Process installation requirements for safety valves: To protect the equipment, it is best to install the safety valve as close to the equipment as possible. For example, from a safety perspective, it is better to install a safety valve directly on the container than on the outlet vapor pipeline. PSV for container vapor release – should be installed in the vapor space above the HHL. If the equipment is equipped with a demister, and since the demister may become clogged, the PSV should be installed on the container below the demister. If the safety valve is installed on the demister, the equipment will already be under overpressure or even damaged by the time the safety valve activates. A safety valve can be installed on the vapor pipeline at the outlet of the equipment. In this case, pay attention to the 3% voltage drop limit. For the inlet, ensure that the total pressure drop across the inlet is less than 3% of the set pressure of the safety valve. This requirement is to avoid frequent opening and closing of the safety valve once it reaches its set pressure. Since the reset pressure of a typical safety valve is 97% of the set pressure, once the valve opens, the pressure drop caused by the flow of the medium results in the pressure at the valve inlet dropping to the reset pressure, causing the valve to close. However, once it closes, the flow of the medium in the inlet pipe stops, and the pressure rises again to the opening pressure, prompting the valve to open once more. The inlet pipe of the safety valve should slope downward step by step toward the container to prevent liquid accumulation. The presence of pockets in the inlet pipe can lead to liquid accumulation; one of the consequences is that the hydrostatic pressure causes a deviation between the starting pressure and the set pressure. The isolation valve on the inlet pipeline (if any) should be LO ； The isolation valve on the inlet line of the backup safety valve should be of the LC type. The isolation valve at the outlet pipeline (if any) shall be LO. The purpose of installing isolation valves on inlet and outlet pipelines is to allow the safety valve to be removed for inspection or maintenance. However, if such a need indeed exists, it is best to install a backup safety valve to prevent the equipment from losing its protective layer. If there is no requirement for removal online, try to avoid installing shut-off valves for import and export. If discharged into the atmosphere, the outlet pipe does not require a shut-off valve, and a drain hole should be provided at the lowest point of the discharge pipe to allow liquid to drain out. Special attention should also be paid to the valve seating force, and proper pipe support must be provided. The design Mach number for the outlet pipeline should be less than 0.7, and the pressure drop in the outlet pipeline determines the selection of the safety valve. If the outlet is a two-phase flow, care should be taken regarding plug flow; it is advisable to indicate two-phase flow on the P&ID to remind pipeline engineers to reinforce the pipe supports. The outlet pipeline must be connected to the flare collection pipe step by step at lower levels, with a diagonal connection in the flow direction at the top. In my opinion, the purpose of installing a bypass is mainly to relieve pressure in the system; therefore, if the system already has other vents or pressure-relief points, a bypass is not necessary. 4. Typical configurations for safety valve vent pipes. Note: × indicates the pipe rack support points. The type of pipe rack support is determined by the designer; the dimensions indicated are for reference only. An inlet shut-off valve for the safety valve may be required or not, depending on process requirements. The direction of the valve handle is determined by the designer. For vent safety valves, it is advisable to avoid too many bends in the vent piping (the fewer the better, as more bends increase the discharge backpressure). The following two configurations are not ideal, as they not only increase the discharge backpressure but also raise the bending moments resulting from the discharge forces. The exhaust pipe does not require a horizontal section at the back; it can be drained directly. The increased horizontal section greatly increases the bending moment generated during the discharge process, which can very easily lead to excessive stress at the inlet of the safety valve and result in its failure. VI. Requirements for discharge from safety valves Additionally, in principle, the discharge outlet must not be directed at the equipment or areas where people may be present. For exhaust pipes that vent upward, some users also install a small rain cover at the outlet; however, this additional component may increase the back pressure during exhaust. The exhaust force is usually very high, and it is likely to blow the rain cover away directly; therefore, an increase is not recommended. Liquid safety valves rarely discharge directly; if discharge is necessary, it is best to direct the flow downward into ditches or similar locations, taking into account the potential for splashing resulting from such discharge.