Knowledge about valve actuators!

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This post was last edited by yinkuilin6868 on 2015-12-21 10:14. The electric actuator for valves is an essential driving device for enabling programmed, automatic, and remote control of valves; its movement can be controlled by the amount of travel, torque, or axial thrust. The operating characteristics and utilization rate of valve actuators depend on the type of valve, the operating specifications of the actuator, and the position of the valve in the pipeline or equipment. 1. An electric actuator generally consists of the following components: a dedicated motor, which is characterized by high overload capacity, high starting torque, and low moment of inertia, as well as the ability to operate for short periods in a intermittent manner; a reduction mechanism, used to lower the output speed of the motor; a stroke control mechanism, used to adjust and precisely control the opening and closing position of the valve; a torque limitation mechanism, used to regulate the torque (or thrust) so that it does not exceed a predetermined value; a manual/electric switching mechanism, used to enable manual or electric operation; and an opening indicator, used to show the position of the valve during its opening and closing process. 2. The type of electric actuator should be selected based on the valve type: Angular stroke electric actuators – These actuators have an angular range of less than 360 degrees, and they are suitable for use with butterfly valves, ball valves, plug valves, etc. The rotation of the output shaft of the electric actuator is less than one full cycle, that is, less than 360 degrees; usually, 90 degrees is sufficient to control the opening and closing of the valve. Such electric actuators are further divided into direct-connected type and base-crank type, depending on the installation interface method. Direct connection type: refers to the configuration in which the output shaft of the electric actuator is directly connected to the valve stem. Base-crank type: refers to the configuration in which the output shaft is connected to the valve stem via a crank. Multi-turn electric actuators – Multi-turn electric actuators (with an angle of rotation greater than 360 degrees) are suitable for gate valves, globe valves, etc. The rotation of the output shaft of an electric actuator exceeds one full cycle, that is, more than 360 degrees; generally, multiple rotations are required to control the opening and closing of the valve. Straight stroke – Straight stroke (linear motion) – suitable for single-seat control valves, double-seat control valves, etc. The motion of the output shaft of the electric actuator is linear, not rotational. The control mode of the electric actuator is determined based on the requirements of the production process control. Control modes for electric actuators include on-off type (open-loop control). On-off type electric actuators are generally used to control whether a valve is open or closed; the valve is either in its fully open position or fully closed position, and such valves do not require precise control over the flow rate of the medium. It is particularly worth mentioning that switch-type electric actuators can also be divided into split-type and integrated types depending on their structural design. This must be specified during the selection process; otherwise, mismatches such as conflicts with the control system can often occur during on-site installation. 1. Split structure (commonly referred to as the standard type): The control unit is separated from the electric actuator, and the electric actuator alone cannot control the valve – a separate control unit is required for control, which is usually provided in the form of a controller or control cabinet. The disadvantage of this structure is that it makes it difficult to install the entire system, increases wiring and installation costs, and is prone to failures; when a failure occurs, it is hard to diagnose and repair it, resulting in an unsatisfactory cost-performance ratio. II. Integrated structure (usually referred to as a monolithic type): The control unit and the electric actuator are combined into one unit, allowing for local operation without the need for a separate control unit, while remote operation can be achieved by simply sending the relevant control signals. The advantage of this structure is that it facilitates the overall installation of the system, reduces wiring and installation costs, and makes it easy to diagnose and resolve faults. However, traditional integrated structure products also have many shortcomings, which led to the development of intelligent electric actuators. Regulating-type (closed-loop control) electric actuators not only possess the functions of switch-type integrated structures but can also precisely control valves in order to regulate the flow rate of the medium. 1. Types of control signals (current, voltage). The control signals for adjustable electric actuators are usually current signals (4~20mA, 0~10mA) or voltage signals (0~5V, 1~5V); it is necessary to determine the type of control signal and its parameters when selecting such actuators. Second, there is the issue of operating mode (on-demand type, off-demand type). The operating mode of regulated electric actuators is generally normally open (taking 4–20mA control as an example; in this case, a 4mA signal corresponds to the valve being closed, while 20mA corresponds to the valve being open). Another mode is normally closed (again using 4–20mA control as an example: here, a 4mA signal corresponds to the valve being open, and 20mA corresponds to the valve being closed). Thirdly, there is signal loss protection. Signal loss protection refers to the mechanism by which, in the event that a control signal is lost due to faults in the lines or other components, the electric actuator operates the control valve to a predetermined safe position; common safe positions include fully open, fully closed, or maintaining the current position. Electric actuators for valves can be classified according to their operating environment and explosion-proof requirements, into ordinary types, outdoor types, flameproof types, and outdoor flameproof types. The output torque of the electric actuator is determined based on the torque required to open and close the valve. The torque needed for valve operation determines the appropriate output torque for the actuator, and this is usually specified by the user or chosen by the valve manufacturer itself. As a manufacturer of actuators, the responsibility lies solely in ensuring the correct output torque; however, the torque required for normal valve operation is influenced by factors such as the valve’s diameter and operating pressure. Due to differences in manufacturing precision and assembly techniques among various manufacturers, valves of the same specification may require different torques. Even valves produced by the same manufacturer can have varying torques. If the torque of the actuator chosen during selection is too low, it will prevent the valve from opening and closing properly; therefore, it is essential to select an actuator with an appropriate torque range. 4. Criteria for selecting the valve actuation device: Operating torque: The operating torque is the most important parameter when choosing a valve actuator; the torque output by the actuator should be 1.2 to 1.5 times the maximum torque required to operate the valve. Operating thrust: There are two types of main structures for valve actuator units: one type does not have a thrust disc and delivers torque directly ； Another approach is to use a thrust disc, with the output torque being converted into output thrust through the valve stem nut in the thrust disc. Number of rotations of the output shaft: The number of rotations of the output shaft of the valve actuator is related to the nominal diameter of the valve, the pitch of the valve stem thread, and the number of thread turns. It should be calculated using the formula M = H/ZS (where M represents the total number of rotations required by the actuator, H is the opening height of the valve, S is the pitch of the valve stem drive thread, and Z is the number of thread turns on the valve stem). Valve stem diameter: For multi-turn straight-stem valves, if the maximum valve stem diameter allowed by the electric actuator is too large to fit the valve stem of the corresponding valve, it is not possible to assemble an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric actuator must be larger than the outer diameter of the valve stem of the straight-stem valve. For some rotary valves and stem-type valves among multi-rotary valves, although there is no need to consider the issue of the valve stem diameter fitting through, the dimensions of the valve stem diameter and the keyway should still be taken into full account during selection to ensure proper operation after assembly. Output speed: If the opening and closing speed of the valve is too fast, water hammer can occur. Therefore, the appropriate opening and closing speed should be selected based on different operating conditions.