【Common Issues in Haichuan Pipeline Design】--Design Principles for Safety Valves and Their Pipelines

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During the design, installation, and inspection of pipelines, various minor 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 design of safety valves (PSV/PRV) and their associated discharge piping systems is a crucial aspect of safety in chemical process and mechanical design. Its primary goal is to ensure stable discharge at the designed rate in cases of overpressure, while preventing valve chatter and damage to the piping structure. The following are the key design principles for safety valves and their piping in the chemical and petrochemical industries (referencing standards such as API RP 520/521, GB 50316, and GB 150): 1. Principles for the placement and selection of the safety valve itself: Installation location and orientation: The safety valve should be installed vertically at the highest point of the gas space of the equipment to be protected (such as pressure vessels, towers, or reactors), or as close as possible to the equipment itself. For pipelines transporting fluids over long distances, they should be installed at nodes where pressure may build up. Backpressure adaptation selection: Conventional type: Suitable for systems where the additional backpressure is stable, and the total backpressure (additional backpressure + exhaust backpressure) generally does not exceed 10% of the set pressure. Bellows type: Used when the back pressure fluctuates significantly, or when the back pressure accounts for 10% – 50% of the set pressure. The bellows help to eliminate the impact of back pressure on the valve’s opening pressure, as well as to isolate corrosive fluids from the springs. Pilot-operated: Suitable for applications where the operating pressure is very close to the set pressure (for example, when it reaches 90%–95% of the set pressure), or where the back pressure is extremely high (up to over 70%). Avoid vibration and pulsation: It should not be installed in areas of the exhaust piping of reciprocating compressors or pumps where there is severe pulsation; a buffer distance or buffer tank must be provided. II. Design specifications for inlet piping: The design of the inlet piping is directly related to whether the safety valve can open properly and to avoiding frequent operation. Pressure drop limit (core principle): At the maximum discharge rate, the irreversible pressure loss in the valve inlet pipeline (frictional loss + local losses) must be strictly kept within 3% of the valve’s set pressure. Excessive pressure drop can cause the valve disc to oscillate at high frequency between the open and closed positions (frequency hopping), damaging the valve and reducing its discharge capacity. Pipe diameter requirement: The nominal diameter (DN) of the inlet pipe must not be smaller than the nominal diameter of the safety valve inlet. If the inlet pressure drop exceeds the 3% limit, increasing the inlet pipe diameter should be given priority. Direction and length: Piping should be as short and straight as possible. Try to avoid using elbows; if they must be used, long-radius elbows should be employed to reduce the local resistance coefficient. Avoid liquid accumulation: In systems that handle gases or vapors, the inlet pipelines should be free of \"pockets\" to prevent the accumulation of condensate, which could impede drainage or cause water hammer. III. Design Specifications for Discharge Piping: The complexity of outlet piping systems lies in dealing with two-phase flow, reaction forces, and backpressure control. Pipe diameter and expansion: The diameter of the discharge pipe must not be smaller than the diameter of the safety valve outlet flange. Since the gas expands and its pressure drops during release, the outlet pipe usually needs to be enlarged to limit the flow velocity (typically kept below 0.75 Mach) and reduce the back pressure at the discharge point. Backpressure verification: It is necessary to check the exhaust backpressure (Built-up Backpressure) generated by the exhaust piping system at the maximum discharge rate. The design of the header must take into account the cumulative effect when multiple safety valves activate simultaneously. Drainage and guidance: If discharge is to the atmosphere, the outlet of the exhaust pipe should be at least 3 meters above the highest adjacent operating platform, and it should be cut at a right angle or an inclined angle (usually to prevent rain from entering). A weep hole must be provided at the bottom to allow rainwater or condensation to drain away. If connected to a closed drainage network (such as a flare system or an RTO/acid gas treatment system), the discharge pipeline should have a slope of not less than 1:100, and it should connect to the top of the manifold at an angle of $45^\circ$ or $90^\circ$ in the direction of flow, in order to prevent liquid from within the manifold from flowing back into the safety valve. Thrust Force: A massive dynamic thrust force is generated at the outlet elbow at the moment of release. Stress analysis must be conducted for pipeline design, and supports of sufficient strength must be installed (usually fixed anchors or guide supports are required). IV. Isolation valves and safety interlocks: Under normal conditions, shut-off valves should not be installed at the inlet and outlet of safety valves. However, when continuous production must be maintained and online maintenance is required, strict engineering principles must be followed: redundant configuration (1 in use and 1 as backup): When dual safety valves are installed, the shut-off valves at the inlet and outlet must be equipped with mechanical interlocks (such as Key Interlock) or three-way changeover valves to ensure that at least one safety valve remains fully operational at all times. Seal management: The inlet and outlet isolation valves must be full-bore ball valves or gate valves, and strict CSO (Car Seal Open) management procedures must be followed. The valve stems should be installed horizontally to prevent the valve discs from falling off and blocking the passage.