Ascending Efficiency: Rising Stem Valves in Critical Process Control
Nov 21,2023Read More
Valve selection is a relatively complex and extremely important work. The selection of the actuator is an important part of the valve selection process, and this part determines the stability of the valve during operation to a large extent. . If you want to choose a valve electric actuator, you must clearly control key parameters such as valve type, production process, use environment and valve output torque.
Ⅰ. Select the actuator according to the valve type
1. Rotary stroke electric actuator (rotation angle <360°) is suitable for butterfly valve, ball valve, plug valve, etc.
a) Direct connection type: refers to the form in which the output shaft of the electric actuator is directly connected to the valve stem.
b) Base crank type: refers to the form in which the output shaft is connected to the valve stem through a crank.
2. Multi-turn electric actuators (angle>360°) are suitable for gate valves, globe valves, etc.
3. Straight stroke (linear motion) is suitable for single-seat control valve, double-seat control valve, etc.
Ⅱ. Select the control mode according to the production process
1. Switch type (open loop control)
The switch-type electric actuator generally realizes the opening or closing control of the valve. The valve is either in the fully open position or in the fully closed position. Such valves do not need to precisely control the flow of the medium. It is particularly worth mentioning that the switch-type electric actuator can be divided into a split structure and an integrated structure due to different structural forms. This must be explained when selecting models, otherwise there will often be mismatches such as conflicts with the control system during on-site installation.
a) Split structure (usually called ordinary type): the control unit is separated from the electric actuator, the electric actuator cannot control the valve alone, and the control unit must be added to realize the control. Generally, the external control is carried out in the form of a controller or a control cabinet. Matching. The disadvantage of this structure is that it is inconvenient for the overall installation of the system, increases wiring and installation costs, and is prone to failures.
b) Integrated structure (usually called integral type): The control unit and the electric actuator are packaged into one body, which can be operated on the spot without external control unit, and can be operated remotely only by outputting relevant control information. The advantages of this structure are that it is convenient for the overall installation of the system, reduces wiring and installation costs, and is easy to diagnose and troubleshoot. However, the traditional integrated structure products also have many imperfections, so intelligent electric actuators have been produced.
2. Modulating type (closed loop control)
The regulating electric actuator not only has the function of the switch-type integrated structure, but also can precisely control the valve and adjust the flow of the medium.
a) Control signal type (current, voltage), the control signal of regulated electric actuator generally includes current signal (4~20mA, 0~10mA) or voltage signal (0~5V, 1~5V), which should be clearly defined when selecting models. Control signal type and parameters.
b) Working form (electric open type, electric close type), the working mode of regulating electric actuator is generally electric open type (taking 4-20mA control as an example, electric open type means that the 4mA signal corresponds to the valve closing, 20mA The corresponding valve is open), and the other is the electric close type (taking 4-20mA control as an example, the electric open type means that the 4mA signal corresponds to the valve open, and the 20mA corresponds to the valve close).
c) Loss of signal protection. Loss of signal protection means that when the control signal is lost due to faults such as lines, the electric actuator will control the valve to open and close to the set protection value. The common protection values are fully open, fully closed, and remain in place. three conditions.
Ⅲ. Use environment
According to the use environment and explosion-proof grade requirements, the electric device of the valve can be divided into ordinary type, outdoor type, explosion-proof type, outdoor explosion-proof type, etc. Generally, the dustproof and waterproof actuator grades are mainly IP65, IP67, IP68 and explosion-proof grades ExdIIBT4, ExdIIBT6, ExdIICT5.
Ⅳ. Output torque
The torque required to open and close the valve determines the output torque of the electric actuator, which is generally proposed by the user or selected by the valve manufacturer. As the actuator manufacturer, it is only responsible for the output torque of the actuator. The torque is determined by the valve diameter, working pressure and other factors, but due to the differences in the processing accuracy and assembly process of valve manufacturers, the torque required for valves of the same specification produced by different manufacturers is also different, even if the same valve manufacturer produces the same specification The valve torque is also different. When the torque selection of the actuator is too small, it will cause the valve to be unable to open and close normally. Therefore, the electric actuator must choose a reasonable torque range.
Ⅴ. The basis for the correct selection of valve electric device
Operating torque: The operating torque is the most important parameter for selecting the valve electric device. The output torque of the electric device should be 1.2 to 1.5 times the maximum operating torque of the valve.
Operating thrust: There are two main structures of the valve electric device: one is not equipped with a thrust plate, and the torque is directly output; the other is equipped with a thrust plate, and the output torque is converted into an output thrust through the valve stem nut in the thrust plate.
The number of rotations of the output shaft: the number of rotations of the output shaft of the valve electric device is related to the nominal diameter of the valve, the pitch of the valve stem, and the number of thread heads. It should be calculated according to M=H/ZS (M is the total rotation that the electric device should meet. The number of turns, H is the opening height of the valve, S is the thread pitch of the valve stem drive, and Z is the number of stem thread heads).
Stem diameter: For multi-turn rising stem valves, if the maximum stem diameter allowed by the electric device cannot pass through the valve stem of the equipped valve, it cannot be assembled into an electric valve. Therefore, the inner diameter of the hollow output shaft of the electric device must be larger than the outer diameter of the valve stem of the rising stem valve. For part-turn valves and dark-stem valves in multi-turn valves, although the passage of the diameter of the valve stem does not need to be considered, the diameter of the valve stem and the size of the keyway should also be fully considered when selecting, so that the valve can work normally after assembly.
Output speed: If the opening and closing speed of the valve is too fast, it is easy to produce water hammer. Therefore, the appropriate opening and closing speed should be selected according to different use conditions.
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