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Check valves are used in many applications within chemical processing plants. Their main function is to prevent the backflow of materials and cross-contamination between them, thereby avoiding accidents that could result from such backflow. Although check valves are not particularly effective at preventing backflow—especially under high-pressure conditions—they still play a very important role in chemical production. How exactly should check valves be installed? How to choose a model? How to install? How to maintain it? We will explain each point from the perspective of compliance, hoping that this consolidation of knowledge will be of some help to everyone. Image 1: Installation (1) HG/T 20570.18-1995 – Valve installation; 2.0.4.6 When public material pipelines may be contaminated due to backflow of process fluids, a check valve should be installed downstream of the isolation valve for those pipelines. 2.0.7.1 Self-acting control valves shall be installed on containers and tanks that require inert gas protection, with check valves connected in series. 2.0.8.3 Each separation tank between stages of the compressor shall be equipped with its own drain valve. When all liquids are directed to a single main pipe, the pressure drop must be calculated to ensure that the pressure at the main pipe is lower than the pressures at each stage, and check valves should be installed at the outlets where the liquids are separated at each stage. 2.0.9.3 Positive displacement pumps (such as reciprocating pumps) usually have built-in check valves at their inlet, so there is no need to install an additional check valve in the pipeline to prevent fluid backflow. The system specialist should check the information provided for the selected pump; if the pump manufacturer does not supply a built-in check valve, such a valve should be added. 2.0.9.3 Vane pumps: Check valves should be installed on the pump outlet pipeline when backflow of the liquid could lead to the following situations: a. The liquid temperature rises by more than 90°C above the normal conveying temperature. b. The combined temperature and pressure of the output fluid exceed the design parameters of the pump casing. c. The impeller may be damaged due to reverse rotation. d. Various variations that are not tolerable in the process operation. 2.0.9.3 For pumps operating in parallel, check valves shall be installed at the outlet of each pump respectively. (2) SH/T 3122-2013 Code for Design of Process Piping in Refining Units 3.3 At the inlet and outlet points of the process piping at the unit boundaries, isolation valves, check valves, drain valves, tee blind plates, and isolation valves shall be installed in sequence. For process pipelines that can both start oil flow and stop it, no check valves are required. 3.4 When utility pipes are connected to pipes and equipment for flammable gases, liquefied hydrocarbons, and flammable liquids, the following requirements must be met: a) For utility pipes that are used continuously, a shut-off valve and a check valve shall be installed at the connection points ； b) For utility pipelines used intermittently, install shut-off valves, check valves, and 8-inch orifice plates at the connections ； c) For the utility pipelines used when the equipment is shut down, install isolation valves and 8-shaped blind flanges at the connections. 9.3 When water or steam is required for the heater tubes, a shut-off valve, a check valve, a drain valve, an 8-shaped blind flange, and another shut-off valve shall be installed in sequence on the water or steam pipeline. 11.5 Check valves should be installed on the outlet pipes of centrifugal pumps and vortex pumps; these check valves are preferably placed between the pump’s outlet nozzle and the first shut-off valve on the outlet pipe. When the pump outlet pipeline has multiple branches, it is advisable to install a check valve on the main pump outlet pipe. 11.7 For centrifugal pumps with a pressure difference between the outlet and inlet exceeding 4.0 MPa, it is advisable to install double shut-off valves and different types of double check valves on the pump outlet pipeline. 12.4 A check valve should be installed on the outlet pipe of the centrifugal compressor, and this check valve shall be located upstream of the shut-off valve. 13.4 A shut-off valve and a check valve shall be installed on the waste steam pipeline of the back-pressure turbine, with the check valve to be installed near the turbine outlet nozzle. 14.6 For equipment using steam during abnormal operations, a shut-off valve, a check valve, an inspection valve, and a shut-off valve shall be installed in sequence on its steam inlet pipe. 18.1 The piping layout of the cooling water system shall meet the following requirements: a) A single main pipe should be used for the pump cooling water; when cooling water of two or more different qualities is used, isolation valves and check valves shall be installed at the points where each type of water connects to the main pipe. c) The seal oil and lubricating oil coolers of the compressor can be cooled by circulating water; fresh water can be used as a backup if necessary, with cut-off valves and check valves installed. (3) GB 50160-2008 Fire protection design standard for petrochemical enterprises (2018 edition). Clause 7.2.7: When connecting utility pipelines to pipelines or equipment carrying combustible gases, liquefied hydrocarbons, and flammable liquids, the following requirements shall be met: 1. Check valves shall be installed on utility pipelines that are used continuously, and shut-off valves shall be installed at their roots ； 2. On utility pipelines used intermittently, a check valve and one shut-off valve, or two shut-off valves, shall be installed; an inspection valve shall be installed between the two shut-off valves. 7.2.11 Centrifugal combustible gas compressors and combustible liquid pumps shall be equipped with check valves on their discharge pipelines. Image 2. Selection of types (1) HG/T 20570.18-1995 Installation of valves. 2.0.9.3 The size of the check valve should be the same as that of the shut-off valve at the pump outlet. 3.0.2.5 The commonly used types are swing type, lift type, and ball type. The swing-type has a larger diameter ratio than the latter two types; it can be installed on horizontal or vertical pipes, and when installed on a vertical pipe, the fluid should flow from bottom to top. The lift-type and ball-type types have smaller diameters and can only be installed on horizontal pipelines. When the inlet of a centrifugal pump is in suction mode, a bottom valve installed at the inlet pipe is also used to keep the liquid inside the pump in place; it functions as a check valve as well. When the container is open, the bottom valve can be equipped with a filter screen. (2) SH/T 3012-2011 Code for Design of Layout of Metal Piping in Petrochemical Industries 10.4.1 Lift-type check valves should be installed on horizontal pipes, while vertical lift-type check valves can be installed on vertical pipes where the medium flows from bottom to top. 10.4.2 Swing check valves should be installed preferentially in horizontal pipes; they can also be installed in vertical pipes where the medium flows from bottom to top. 10.4.3 The foot valve shall be installed at the vertical pipe end of the pump suction pipe. 10.4.4 To reduce the installation height of the pump outlet shut-off valve, a butterfly check valve can be used. When the diameter of the pump outlet does not match that of the connected pipeline, a reducing check valve can be used. Image 3. Installation: DL/T 923-2016 Technical Guidelines for Check Valves Used in Thermal Power Generation. 8.7 Straight-through lift check valves shall be installed on horizontal pipelines, so that the valve disc moves up and down in a direction perpendicular to the horizontal plane. 8.8 Vertical lift check valves should be installed on vertical pipes. 8.9 Swing check valves and butterfly check valves should be installed in horizontal pipes; when installed in vertical pipes, the fluid should flow from bottom to top. 8.10 The opening and closing state of the check valve at the time of installation shall meet the following requirements: a) When welded to the pipeline, the check valve shall be in the open position during welding installation ； b) Connected to pipes via flanges or threads: The check valve should be in a slightly open position ； c) Welded to the pipe and equipped with a non-metallic sealing gasket ； During welding installation, unless otherwise specified, the non-metallic valve disc should be removed first before welding can be carried out. Image IV. Maintenance DL/T 923-2016 Technical Guidelines for Check Valves Used in Thermal Power Generation 9.2 From the time of installation until the end of the valve’s service life, a regular maintenance and inspection system should be established to promptly address any faults and eliminate potential safety hazards. The maintenance tasks should include at least: a) the service life of gaskets and non-metallic seals, as well as checking for leaks ； b) Whether the spring is loose or not, and whether it is operating normally ； c) Check whether there is any jamming, oscillation, or frequent opening/closing of the valve disc ； d) Leakage and wear between the sealing surfaces ； e) Whether there is any dirt accumulation in the valve interior cavity and between the valve disc and the guide surface.