Is the control valve a selector valve or a bypass valve?

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In industrial automation and fluid control systems, the configuration and selection of control valves are crucial. Especially regarding the choice between the handwheel of the control valve and a bypass, many people get confused: when is a handwheel needed, and when is a bypass required? Handwheel or bypass? The handwheel of the control valve and the bypass should be considered as a whole; generally, if there is a bypass, there is no need for a handwheel, and if there is a handwheel, there is usually no need for a bypass. Based on the requirements of HGT20507, SHT3005, HGT20559, YPU361, as well as on-site operation and usage conditions and operations under special circumstances, the general principles are outlined as follows: 01 Conditions in which bypasses and handwheels should not be installed: for use in safety interlocks (i.e., SIS) ； Control valve for compressor anti-surge protection ； Installed in restricted, hazardous areas ； During production, stopping the operation can be done at will; a bypass should be provided in situations where there is no handwheel. In case of a failure of the control valve, it must be replaced or repaired promptly while the system is still in operation ； Regulating valves that provide continuous control require the assistance of bypass valves for regulation and control during the initial operation phase, when there are large fluctuations in the flow rate of the medium, or during maintenance of the regulating valves, in order to ensure a stable and continuous operation of the process. 03 Condition with a handwheel, without a bypass: precise control (such as ratio) ； Key valves in sequential control ； A handwheel mechanism should be provided in applications with large diameters where it is necessary to limit the valve opening using a handwheel. For operating conditions that allow either a bypass or a handwheel, a handwheel should be preferred. In particular, the use of a handwheel should be considered first in the following operating conditions: intermittent operation, presence of a backup system, reduction of leaks (for highly toxic, corrosive, or self-igniting substances), reduction of deposits (including solid particles, slurries, crystals, heat-sensitive substances, and polymers), pressure difference ≥ 4 MPa, pipe diameter ≥ DN80, and for special control valves. Therefore, if the project/design institute insists on including handwheels, special valves can be managed by locking them; for control valves that do not have handwheels, standard handwheels can be purchased and placed in a fixed location (it is understood that these handwheels are generally not used). Relevant standards: 1) HG/T 2050711.9.7 The installation of handwheel mechanisms shall meet the following requirements: 1 Control valves without bypass systems must be equipped with handwheel mechanisms. 2 The process production safety interlock, which is used to control the emergency shut-off valve, shall not be equipped with a handwheel mechanism. 3 Handwheels should not be used for the mechanical positioning of valves. 11.2.5 The selection of the anti-surge control valve for compressors shall meet the following specific requirements: 1 The anti-surge control valve should be equipped with solenoid valves and limit switches; a valve position transmitter may be included depending on actual conditions, but a handwheel mechanism should not be used ； 2) SH/T 300510.1.12 b) (Control valves): Handwheel mechanisms are suitable for use in the following situations: 1) Control valves that do not have shut-off valves or bypass valves ； 2) Handwheels cannot be used for the mechanical positioning of valves; except in special cases, no other methods are suitable for mechanical positioning ； 3) Special control valves. 10.3.6.12 (Isolation valves) The selection of the handwheel for the pneumatic actuator shall comply with the following requirements: a) The handwheel should preferably be of the side-mounted type ； b) The manual and automatic positions of the handwheel should be clearly marked, and the automatic position should be equipped with a locking mechanism ； c) The handwheel shall enable the actuator to be operated without power and under the design conditions of the valve’s maximum load ； d) The opening/closing direction of the valve shall be clearly indicated on the handwheel, which must not be used as a mechanical travel limiter. 10.3.7.10 A handwheel is standard equipment for electric actuators. The design of the clutch mechanism shall ensure that motor operation takes precedence over handwheel operation; whenever the motor starts, handwheel operation shall be automatically disengaged. 3) GB/T 508925.8.21.3 (Control valves): The selection of handwheels shall comply with the following provisions: 1) Emergency shut-off valves used for process safety interlocks, or control valves installed in hazardous areas where access is prohibited, shall not be equipped with handwheel mechanisms; control valves intended for other functions and without bypass systems shall be equipped with handwheel mechanisms ； 2) A handwheel mechanism should be provided for large-diameter valves and those where the valve opening needs to be restricted using a handwheel. 6.7.2 Control valves should be equipped with upstream and downstream isolation valves as well as bypass valves. The installation of these isolation and bypass valves shall meet the following requirements: 1 When a control valve is equipped with a bypass, isolation valves shall be installed upstream and downstream of the control valve, and a bypass valve shall be provided for the bypass. 4) SH/T 3104-2013 9.4.1 Control valves and on-off valves that meet one of the following conditions may not require isolation valves or bypass valves: a) Used for sequential control ； b) Used for emergency stop interlock ； c) Control valves with handwheels under the following conditions: for non-hazardous media (water, air, nitrogen, etc.) where the control temperature is not higher than 225°C and the pressure is not greater than 0.1 MPa ； The process pipelines can be shut down individually. d) Butterfly control valves with a diameter greater than 350 mm ； e) Three-way control valve ； f) Steam control valve of the steam turbine pump driven by a standby motor ； g) Locations where leaks of hazardous substances (hydrofluoric acid, phenol, etc.) need to be reduced. 5) HG 20559-1993 Specifications for the engineering design of process systems in chemical plants – Basic unit configurations for flow meter systems and control valve assemblies – 3 Basic unit configurations for control valve assemblies 3.0.1.1(2) Control valve assembly II: A control valve assembly without bypasses; its compact layout helps reduce costs. Control valves without bypasses shall be equipped with a handwheel. Bypasses and bypass valves may not be required in the following situations: a. Production requirements dictate a reduction in leaks of hazardous media. b. A paste that is difficult to rinse off. c. Intermittent passage will result in precipitation. d. Usually used for control valves with a nominal diameter DN≥80. e. Control valve bank for batch operation. 6) SH/T 31845.4.3.6 Electric actuators shall be equipped with handwheels. YPU361 5.2.2.5 Control valves f) All control valves shall be equipped with intelligent, self-diagnostic functionally capable positioners; the Fieldbus Foundation protocol or a 4–20mA signal combined with the HART protocol shall be used, and the positioners shall be from brands that adhere to the Wanhua standard framework. The actual valve opening degree should be displayed on the DCS. For valves in ordinary control loops, their valve position signals can be directly read by the DCS via HART communication. For valve position signals used in interlocks and complex controls, valve position transmitters must be installed. For control valves included in the equipment, a requirement for displaying the actual valve opening degree must be specified; if this cannot be achieved, a separate report explaining the reason is required. It should be determined whether a handwheel shall be provided for the control valve, in accordance with the requirements of the patent holder and the process specifications. p) General principles for selecting cut-off valves: For diameters of 8″ or less, straight-through ball valves are generally chosen; for diameters larger than 8″, butterfly valves are typically used. In special cases, the selection should be based on the recommendations of the manufacturer or process requirements. SIS shut-off valves are not allowed to have handwheels. 7) GB 50074-2014 9.1.12 Valves on process pipelines should be made of steel. The selected electric or pneumatic valve should have a manual operation function. For valves with a nominal diameter of 600 mm or less, the time required to manually close the valve should not exceed 15 minutes ； For valves with a nominal diameter greater than 600 mm, the time required to manually close the valve should not exceed 20 minutes. Explanation of the provision: 9.1.12 Steel valves have superior properties such as tensile strength and toughness compared to cast iron valves. The use of steel valves is much safer in preventing valve freezing, cracking, water hammer, and other external mechanical damages compared to cast iron valves. To ensure safety, steel valves are now widely used in pipelines for flammable and combustible liquids in the petrochemical industry. In terms of price, steel valves are not much more expensive than cast iron valves. In light of this, this provision stipulates that \"valves on process pipelines shall be steel valves.\" One of the lessons learned from a oil depot fire that occurred in 2010 was that, after the power supply system was damaged, the electric valves installed on the pipelines for inflowing and outflowing oil in the storage tanks could not be shut down manually quickly, which led to an expansion of the scale of the accident. The time limit specified for manually shutting down the valves is intended to prevent similar situations from occurring. 8) GB 50160-2018 7.2.15 The extraction pipes for liquefied hydrocarbon equipment shall be equipped with isolation valves near the base of the equipment. When the distance between a liquefied hydrocarbon unit with a volume of over 50 m3 and its extraction pump is less than 15 m, such a shut-off valve shall be a remotely operated valve with a manual function; the distance between the local operation button of the remotely operated valve and the extraction pump shall not be less than 15 m. Explanation of the provision: 7.2.15 This provision is designed to allow for the timely shutdown of material flow in the event of a fire in the downstream equipment directly connected to the tank. For example, if an accident and fire occur in the pump located downstream of the liquefied hydrocarbon tank in a plant’s product refining unit, it becomes impossible for personnel to approach the pump in order to shut off the isolation valve. Moreover, there is no isolation valve on the pipeline near the base of the tank between the pump and the tank, which results in the liquefied hydrocarbon in the tank being completely burned out before the fire can be extinguished, causing significant losses. APl Std 2510 \"Design and Construction of Liquefied Petroleum Gas (LPG) Facilities\" stipulates that isolation valves on liquefied hydrocarbon pipelines should be located as close as possible to the tanks, preferably at the tank wall nozzles. For ease of operation and maintenance, the installation location of the shut-off valve should be easily accessible. When the volume of a liquefied hydrocarbon tank exceeds 10,000 gal (≈38 m3), all isolation valves located on pipes below the highest liquid level in the tank must be able to close automatically or via remote control within 15 minutes of a fire breaking out. The control system for the shut-off valve shall be protected against fire, and the shut-off valve shall be operable manually. 7.2.17 Pipelines transporting flammable gases, liquefied hydrocarbons, and flammable liquids shall be equipped with emergency shut-off valves within the enclosure at their entry and exit points to petrochemical plants. The emergency shut-off valve should have automatic and manual shutdown functions. 9) According to the explanatory notes in 5.8.3 of GB 51283-2020, a handwheel can be installed on the fire emergency shut-off control valve; this mechanism must enable manual operation of the actuator even in the absence of power and under the maximum load condition of the valve. 10) IEC 61511-1-2016 specifies strict management procedures and measures. Bypass: An action or facility intended to prevent all or part of the SIS functions from being executed. The design of the SIS should minimize the need for operators to select certain options or to bypass the system when hazards are present. If operator action is required as part of the design, then measures should be included to protect against operator errors. Bypass switches or mechanisms must be protected to prevent unauthorized use (for example, through key locks or passwords, in combination with effective management controls). Bypass: An action or facility that prevents all or part of the SIS functions from being carried out. The design of SIS should minimize the need for operators to select options, as well as the need to bypass the system in case of danger. If the design indeed requires the operator’s actions, it should include mechanisms to prevent the operator from making mistakes. The bypass switch or device should be protected to prevent unauthorized use (for example, by combining a key lock with an effective management control).  Note: The principle of the bypass for flow meters is similar to that of valves; according to SH/T 3104 7.1.4, flow meters should be equipped with bypass pipelines as required by the process. According to the explanatory notes, the purpose of installing a bypass pipe for the flow meter is to ensure that flow measurement is not affected during the removal and maintenance of the flow meter. Therefore, a flow bypass pipe should be installed in locations where continuous flow measurement is required, while it may not be necessary in locations where flow is measured intermittently; in such cases, maintenance personnel can repair the flow meter during the intervals between measurements.