A brief discussion on the installation and commissioning of control valves

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Introduction: With the advancement of science and technology, control valves used to regulate fluid flow in automated production processes have become widely used across various industries. In thermal and chemical process control systems, control valves, which regulate various quality parameters and safety-related production indicators as the media in the final control process, play a crucial role in stabilizing production, optimizing control, as well as controlling maintenance and repair costs. In the application of control valves, calculation and selection are prerequisites, installation and commissioning are crucial, while use and maintenance serve as the goals. If a control valve is installed improperly or not tuned properly, it will fail to perform its regulating function; in fact, it may even become a burden on the system. 1.1 Basic principles of installation: The installation of control valves shall comply with **relevant standards, and be carried out strictly in accordance with the provisions of the design drawings and design documents. For example, the Quality Inspection and Evaluation Standards for Building Installation Projects, the Construction and Acceptance Specifications for Industrial Automation Instruments, the Construction and Acceptance Specifications for Electrical Equipment Installation Projects, etc ； The equipment required for installation, auxiliary equipment, and main materials shall comply with the relevant provisions of current **standards. 1.2 Inspections before installation: From the time of manufacture until installation, control valves and their accessories are exposed to random vibrations caused by the means of transport during shipping, as well as various impacts during loading and unloading. Additionally, changes in environmental conditions such as temperature and humidity during transportation and storage can also cause changes in the performance of these control valves and accessories. Therefore, it is necessary to conduct a partial performance test before installation. The inspections prior to the installation of control valves mainly include the following aspects: appearance, static characteristics, leakage rate, full-stroke time at no load, pressure resistance, insulation performance, airtightness, and sealing performance. Among these, the first 5 items are mandatory inspection items. Of course, there are also certain requirements for the location where the testing is conducted. For example: The ambient temperature should be between 10 and 35℃ ； The relative air humidity should not exceed 75% ； The venue has sufficient space and is well-lit or has adequate lighting ； There should be no intense vibration sources or strong magnetic field interference in the surrounding area ； The power supply and gas supply meet the requirements, etc. 1.3 Installation Notes When installing control valves, it is necessary to ensure safety, optimize performance, facilitate operation and maintenance, and reduce installation costs. Next, I will discuss the specific issues that need attention during installation in relation to these four points. 1.3.1 Ensuring safety 1) Preventing leaks – The installation process must not allow the control valve to leak. During use, leaks may occur if gaps or micro-holes form in areas such as the filler box and flange gaskets. If the operating conditions of the fluid medium are harsh, such as high temperature, high pressure, or if the fluid is corrosive, damage will increase and the risk of leakage will be greater. There are many ways to prevent leaks; during installation, factors such as the selection of packing, the choice of sealing method, and the use of control valves with good sealing performance are all essential to consider. 2) Installation of relief valves: In any piping design and installation, high-pressure fluid inevitably accumulates when the system is shut down. If the potential energy of this accumulated fluid is significant, relief valves should be installed on both sides of the control valve. This allows the energy of the high-pressure fluid remaining in the control valve to be released after the system is shut down, thereby protecting personnel safety. It also facilitates the maintenance of the control valve. Relief valves that can limit flow rates can also be considered for use. 3) Safe pipelines: Before installing the control valve, all fittings and pipelines must be thoroughly purged, as sand particles, scale, iron filings, and similar substances in the pipelines can damage the inner surfaces of the control valve, especially those areas with strict sealing requirements. When system pressure fluctuations are severe, it is recommended to install buffering devices on the pipelines. If the fluid being discharged is a harmful gas or liquid, the pipeline used for discharging it must be connected to a safe location, or to a specific safe container, so as to protect people from harm and prevent environmental pollution. 1.3.2 Ensuring performance – When installing control valves, every effort should be made to ensure that their performance is not affected. This effect undermines the various factors considered when selecting control valves. 1) Installation of upstream and downstream isolation valves and bypass valves on the control valve: The length of the straight pipe section between the upstream and downstream isolation valves and the control valve should take into account the pipeline resistance and its impact on the flow pattern of the fluid. A long straight pipe section helps to stabilize the flow of fluid after passing through the cut-off valve, enabling smoother flow, reducing the effects of turbulence, and lowering noise levels ； A short straight pipe section means that the fluid enters the control valve before it has stabilized after passing through the shut-off valve, which increases noise. However, a short straight pipe section helps to reduce pipeline resistance and increase the pressure drop across the control valve, thereby reducing distortions in the flow characteristics and facilitating the stable operation of the control system. Therefore, the pros and cons should be weighed and considered comprehensively. Based on experience, there is usually a straight pipe section of 10D to 20D on the upstream side, and a straight pipe section of 3D to 5D on the downstream side (D being the pipe diameter); a flow rectifier should be installed if necessary. When disassembling and repairing control valves, a bypass valve can be used to operate the production process. When the flow rate is too high and cannot be properly regulated using a control valve, as a contingency measure, a bypass valve can also be used in parallel with the control valve to regulate the process. To reduce costs, large-diameter control valves are equipped with handwheel actuators that can be used to operate in place of bypass valves. The bypass valve should be installed in a way that facilitates operation; it, together with the control valve and the upstream and downstream isolation valves, forms the control valve assembly. Therefore, when installing control valves, they should be considered in conjunction with isolation valves and bypass valves, and installation should be completed simultaneously. The nominal diameter of the bypass valve is the same as that of the pipeline, and its pressure rating is also consistent with the process pressure rating. 2) Installation of relief valves, vent valves, and drain valves. To facilitate the disassembly of the control valve, it is necessary to release the pressure before and after the valve before disassembly; the relief valve should be installed between the control valve and the upstream and downstream isolation valves. Vent valves and drain valves are used to remove the non-condensable gases and condensate contained in the fluid. If the fluid being controlled is gas or steam at the time of installation, the drain valve should be installed at the lowest point of the control valve assembly to facilitate the removal of condensate ； When the fluid under consideration is a liquid, to facilitate the discharge of non-condensable gases, the vent valve should be installed at the highest point of the control valve assembly. 3) Connection between the control valve and the pipeline: There are various ways to connect them, such as threaded connection, flanged connection, and welded connection. Threaded connections are used for the installation of small-diameter control valves, and detachable movable connectors must be installed simultaneously ； There are two types of flange connections: the standard flange connection and the clamping connection. The nominal diameter of the pipeline connection flange should match that of the control valve, and its pressure resistance rating should also be consistent with that of the control valve. The allowable deviation in the perpendicularity between the flange surface and the pipeline axis is 1° ； Try to avoid using welding connections. When connecting control valves, no new protrusions should appear inside the connection pipes; for example, gaskets and welds should not protrude within the pipes. At the same time, it should be ensured that the flow direction of the fluid inside the control valve is consistent with the direction of the arrow indicated on the valve body; in special cases, this requirement may not apply. 4) Installation of the actuator: Typically, the actuator is connected directly to the valve body of the control valve. However, in cases where hydraulic actuators or actuators with long stroke lengths are separated from the valve body, it is important to ensure that the linkage rods and other mechanical moving parts of the actuator are flexible, with no signs of looseness or jamming ； The length of the connecting rod is appropriate, ensuring stable, flexible, and reliable operation over the entire range of motion ； Handwheel actuators are available in side-mounted and top-mounted versions; sufficient operating space should be left during installation. The handwheel should operate smoothly, without any jamming or twisting. There should be indications showing the relationship between the direction of handwheel rotation and the valve’s opening degree. The limit devices must be adjusted properly to serve their protective function ； The hydraulic actuator should be installed below the controller; if it must be installed above the controller, the height difference should not exceed 1.0 m. An exhaust valve should be installed at the gas collection point of the piping, and an automatic shut-off valve should be installed near the controller. 5) Installation of the valve positioner: The valve position sensing device of the valve positioner is directly connected to the valve stem or shaft of the control valve; therefore, during installation it is necessary to ensure that the feedback signal can accurately and promptly reflect the valve position and any changes in it. Typically, the valve positioner is supplied together with the control valve, and the manufacturer handles the connection between the two. When valve positioners need to be added for process control, it is essential to ensure that the valve position detection mechanisms of these positioners operate correctly, reliably, and flexibly. The mechanical clearance at the pivot point of the feedback rod should be kept as small as possible. The signal lines of the valve positioners must be connected properly, and the air supply lines, as well as the output and input lines, should be labeled ； The valve position indication signal of the valve positioner should be easy for operators and maintenance personnel to observe.