Because more and more factories use automatic control, manual operation is replaced by mechanical or automation equipment, the executive body is required to act as an interface between the control system and the valve mechanical movement, and the implementing agency is required to enhance the work safety performance and Environmental protection performance. In some hazardous situations, automated actuator installations reduce personnel injuries. Some special valves require emergency opening or closing under special circumstances, and valve actuators can prevent the danger from further spreading while minimizing plant damage. For some high-pressure large-diameter valves, the required actuator output torque is very large, then the need for implementing agencies must improve the mechanical efficiency and the use of high-output motor, so smooth operation of large-diameter valve.
First, the valve and automation
In order to successfully automate the process, it is of utmost importance to ensure that the valve itself can meet the specific requirements of the process and the medium in the pipe. Usually the production process and process medium can determine the type of valve, the type of spool, and the structure and material of the trim and valve.
After the valve is selected, the automation needs to be considered next, namely the choice of actuator. Actuators can be easily considered in two basic types of valve operation.
1. Rotary valve (single rotary valve)
Such valves include: plug valves, ball valves, butterfly valves and damper or baffle. This type of valve requires a torque that has been required 90-degree rotation of the implementing agencies
2 multi-turn valve
This type of valve can be a non-rotating lift stem or a non-lifting stem, or they require a multi-turn operation to actuate the valve to the on or off position. Such valves include: straight-through valve (stop valve), gate valve, knife valve and so on. As an alternative, a linear output pneumatic or hydraulic cylinder or membrane actuator also drives the valve.
Second, the valve and implementing agencies
The most widely defined definition of an actuator is: A drive that provides linear or rotational motion that uses some kind of drive energy and operates under some sort of control signal.
Actuators use liquid, gas, electricity or other energy sources and convert them into drive action through motors, cylinders or other devices. The basic actuator is used to drive the valve to the fully open or fully closed position. Actuating and control valve actuators enable precise positioning of the valve to any position. Although most actuators are used for on-off valves, today's actuators are designed to go beyond the simple switching functions of position sensing, torque sensing, electrode protection, logic control, digital communication modules And PID control module, etc., and these devices are all installed in a compact housing.
There are currently four types of actuators, which can use different drive sources and can operate various types of valves.
1. Electric multi-turn actuator
Electric-driven multi-turn actuators are among the most commonly used and most reliable types of actuators. Use single-phase or three-phase motors to drive the gear or worm gear to finally drive the stem nut. The stem nut causes the stem to move to open or close the valve.
Multi-turn electric actuators can quickly drive large-size valves. In order to protect the valve from damage, the limit switch installed at the end of the valve stroke will cut off the power of the motor. At the same time, when the safety torque is exceeded, the torque sensing device will also cut off the power of the motor. The position switch indicates the valve switch status, The handwheel mechanism with the clutches is installed to operate the valve manually in the event of a power failure.
The main advantage of this type of actuator is that all the components are housed in a single housing that integrates all the basic and advanced features in this water, dust and explosion proof enclosure. The main disadvantage is that in the event of a power failure, the valve can only remain in place and the valve can only be in a fail-safe position if the backup power system is used (fault open or fault closed)
2. Electric single rotary actuator
This actuator is similar to the electric multi-turn actuator, the main difference is that the actuator is the final output is 1/4 or 90 degrees of movement. The new generation of electric single rotary actuators combines the complex functions of most multi-turn actuators, such as parameterization and diagnostics using a non-intrusive, user-friendly operator interface.
The compact single-turn actuators can be mounted on small-size valves, usually with output torques up to 800 kg / m and small power requirements, allowing them to be installed with batteries for fail-safe operation.
3. Fluid-driven multi-turn or linear output actuator
Often, this type of actuator is used to operate through valves (shut-off valves) and gate valves, which use either pneumatic or hydraulic actuation. Simple structure, reliable, easy to implement fail-safe operation mode.
Normally people use electric multi-turn actuators to drive gate and stop valves. Hydraulic or pneumatic actuators are only considered when there is no power supply.
4. Fluid driven single rotary actuator
Pneumatic, hydraulic single rotary actuators are very versatile, they do not require power and simple structure, reliable performance. They are used in a wide range of areas. Usually output from a few kg / m to tens of thousands of kg / m. They use the cylinder and gear to convert linear motion into right angle output, the transmission usually have: fork, rack and pinion, lever. The rack and pinion outputs the same torque over the full range of travel. They are ideally suited for small-sized valves, with high efficiency for the fork and high torque output at the start of the stroke, making them ideal for large-diameter valves. Pneumatic actuators are generally installed solenoid valve, positioner or position switch and other accessories to achieve the valve control and monitoring. This type of actuator is easy to implement in fail-safe mode of operation and the following elements must be considered when choosing a suitable valve actuator type and size:
Drive energy
The most commonly used driving energy is the power source or the fluid source. If the power source is selected as the driving energy, the three-phase power source is generally selected for the large-size valve, and the single-phase power source is used for the small-sized valve. General electric actuator can have a variety of power types to choose from. Sometimes optional DC power supply, this time by installing the battery power failure safe operation. There are many types of fluid sources. First, they can be different media such as compressed air, nitrogen, natural gas, hydraulic fluid, etc. Second, they can have various pressures. Third, actuators have various sizes to provide various output torque.
2 valve type
When choosing a valve actuator, you must know the type of valve so that you can choose the correct actuator type. Some valves require multi-turn drives, some require single-turn drives and some require reciprocating drives, which affect the choice of actuator type. The multi-turn pneumatic actuator is more expensive than the electric multi-turn actuator, but the reciprocating linear stroke pneumatic actuator output is cheaper than the electric multi-turn actuator.
For the 90-degree valve rotation, such as: ball valve, butterfly valve, plug valve, the valve is best obtained through the valve manufacturers.
3 torque size
For 90-degree valve rotation, such as: ball valve, butterfly valve, plug valve, the best valve manufacturer through the valve to obtain the appropriate size of the valve, most of the valve manufacturers by testing the valve at rated pressure valve required operating torque, they will this torque Available to customers. In the case of multi-turn valves, these valves can be divided into: Reciprocating (Lifting) Movement - Stem Does Not Rotate, Reciprocation - Stem Rotation, Non-Reciprocating - Stem Rotation, Valve Stem Must Be Measured Diameter, Stem Connection Thread Size to Determine Actuator Size.
Third, the executing agency selection
Once the type of actuator and the required drive torque for the valve have been determined, the data sheet or selection software provided by the actuator manufacturer can be used for selection. Sometimes consider the speed and frequency of valve operation.
Fluid-driven actuators can adjust the travel speed, but the three-phase power of the electric actuator has only a fixed travel time. Some small specifications of the DC electric single-rotary actuator can adjust the travel speed.
Switch control
The biggest advantage of an automatic control valve is the ability to operate the valve over a long distance, which means that operators can sit in the control room to control the production process without having to manually open and close the valve on site. People only need to lay some pipelines to connect the control room and the implementing agency, and the driving energy directly energizes the electric or pneumatic actuators through the pipeline, and usually uses the 4-20mA signal to feedback the position of the valve.
2. Continuous control
If the actuator is required to control parameters such as level, flow or pressure in the process system, this is a job requiring frequent actuator action and a 4-20mA signal can be used as the control signal, however, this signal may be as frequent as the process change. If you need a very high-frequency action actuator, only to select a special start and stop frequently adjustable actuator. When multiple actuators are required in a single process, the individual actuators can be connected by using a digital communications system, which greatly reduces installation costs. Digital communication loops deliver instructions and gather information quickly and efficiently. Currently there are a variety of communication methods such as: FOUNDATIONFIELDBUS, PROFIBUS, DEVICENET, HART and PAKSCAN designed for valve actuators. Not only do digital communications systems reduce capital costs, they also collect a wealth of valve information that is valuable for predictive maintenance of the valve.
Predictive maintenance
The operator can use the built-in data memory to record the data measured by the torque sensing device during each actuation of the valve. These data can be used to monitor the status of the valve, indicate whether the valve needs servicing, or use this data to diagnose the valve.
For the valve can diagnose the following data:
(1) valve seal or packing friction
(2) stem, valve bearing friction torque
(3) seat friction
(4) the friction in the valve operation
(5) spool of the dynamic force suffered
(6) stem thread friction
(7) stem position
Most of the above data exists for all types of valves, but with a different emphasis, for example: For butterfly valves, the friction in the valve operation is negligible, but for the plug valve this force is large.
Different valves have different torque operating curves, for example: for wedge gate valves, the opening and closing torques are very large, the other trip only filler friction and thread friction, closed, the hydrostatic pressure on the gate increased Seat friction, the final wedge effect so that the torque rapidly increased until closed in place. Therefore, the change of torque curve can predict the fault that will occur and provide valuable information for predictive maintenance.
4 intelligent frequency control
Actuators in the course of their work, due to the frequent start of the motor, resulting in changes in the rated frequency of work through the intelligent frequency control allows the frequency to reach the rated value.
For example: due to resistance or external causes, the motor slows down, resulting in the actuator stroke control error, the use of intelligent variable frequency control, you can change the input speed, so that the implementing agencies more reliable and stable.
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