The valve actuator is the mechanism for opening and closing valves. Manually operated valves require a person present to adapt them using direct or directed mechanisms attached to the valve stem. Power-operated actuators, using gas pressure, hydraulic or electric pressure, allow the valve to be adjusted remotely, or allow the rapid operation of the large valve. Power-operated valve actuators can be the last element of an automatic control loop that automatically regulates multiple streams, levels or other processes. The actuator may be only to open and close the valve, or may allow the middle position; some valve actuators include switches or other means of indicating the position of the valve from a distance.
Used for industrial valve automation, actuators can be found in all types of process plants. They are used in wastewater treatment plants, power plants, refineries, mining and nuclear processes, food factories, and pipelines. The valve actuator plays a major role in automating process control. Valves to be automatic vary both in design and dimensions. The diameter of the valve ranges from a tenth of an inch to several feet.
Video Valve actuator
Jenis
There are four common types of actuators: manual, pneumatic, hydraulic, and electric.
Manual
Manual actuators use levers, gears, or wheels to move the valve stem. The manual actuators are powered by hand. Cheap manual actuators, usually self-contained and easy to operate. However, some large valves can not be operated manually and some valves can be placed in remote, toxic or hostile environments that prevent manual operation. As a security feature, certain types of situations may require faster operation than manual actuators that can provide for closing valves.
Pneumatic
Air pressure (or other gas) is a power source for pneumatic valve actuators. They are used on linear valves or quarter turns. Air pressure works on bellows pistons or diaphragms that create a linear force on the valve stem. Alternatively, a quarter-turn of the propeller actuator produces torque to provide rotating motion to operate a quarter-turn valve. Pneumatic actuators can be arranged into closed springs or springs, with air pressure overcoming springs to provide movement. Double acting actuators use air applied to different niches to move the valve towards opening or closing. Central compressed air system can provide clean, dry, and compressed air required for pneumatic actuators. In some types, for example, the regulator for compressed gas, the supply pressure is provided from the process gas stream and the waste gas either being released into the air or discharged into a low pressure process pipeline.
Hydraulics
The hydraulic actuator converts the fluid pressure into motion. Similar to pneumatic actuators, they are used on linear valves or quarter turns. The fluid pressure acting on the piston provides a linear boost for the gate valve or globe. The quarter-torque actuator produces torque to provide rotating motion to operate a quarter-turn valve. Most types of hydraulic actuators can be supplied with unsafe features to close or open the valve in an emergency. Hydraulic pressure can be supplied by an independent hydraulic pressure pump. In some applications, such as water pump stations, process liquids can provide hydraulic pressure, although the actuator must use a material compatible with the liquid.
Electricity
The electric actuators use electric motors to provide torque to operate the valves. They are calm, non-toxic and energy efficient. However, electricity must be available, which is not always the case, they can also operate with batteries.
Spring
Spring-based actuators retain springs. Once anomalies are detected, or power is lost, the spring is released, operating the valve. They can only operate once, without repeated, and are so used for one purpose of use such as an emergency. They have the advantage that they do not require a strong power supply to move the valves, so they can operate from limited battery power, or automatically when all power has been lost.
Maps Valve actuator
Actuator movement
A linear actuator opens and closes a valve that can be operated through a linear force, a type sometimes called a "rising rod" valve. This type of valve includes a world ball valve, an elevated ball ball valve, a control valve and a gate valve. The two main types of linear actuators are the diaphragm and the piston.
The diaphragm actuator is made of a round piece of rubber and is squeezed around the edges between two sides of the cylinder or the chamber allowing air pressure to enter either side of the rubber piece pushing one direction or the other. A rod is connected to the center of the diaphragm so that it moves when pressure is applied. The rod is then connected to the valve rod that allows the valve to experience a linear motion so that it opens or closes. The diaphragm actuator is useful if the medium supply pressure and the required valve travel and drive are low.
Piston actuators use pistons that move along the cylinder. The piston rod conveys force on the piston to the valve stem. The piston actuator allows for higher pressures, longer travel spans, and higher thrust forces than diaphragm actuators.
Spring is used to provide the behavior specified in case of power loss. This is important in safety related incidents and is sometimes a driving factor in the specification. An example of power loss is when an air compressor (the main source of compressed air that provides fluid for the actuator to move) is dead. If there is a spring inside the actuator, it will force the valve open or closed and will remain in that position while the power is restored. An actuator can be determined to "fail open" or "fail to approach" to describe his behavior. In the case of electric actuators, the lost power will maintain stationary valves unless there is a backup power supply.
The typical representative of the valves to be automated is the plug-type control valve. Just as the plug in the tub is pressed into the drain, the plug is pressed into the seat of the plug by the movement of the stroke. The medium pressure acts on the plug while the thrust unit must provide the same amount of impulse to be able to hold and move the plug against this pressure.
Features of electric actuators
Motor (1)
Asynchronous three-phase asynchronous motors are mostly used as propulsion, for some applications a single phase AC or DC motor is used. These motors are specially adapted for valve automation because they provide a higher torque than a stop than a comparable conventional motor, the requirement required to release the sticky valve. Actuators are expected to operate under extreme ambient conditions, but they are generally not used for continuous operation due to the overheating of motor heat can be excessive.
Boundary and torque sensor (2)
The signal switch limit when the end position has been reached. The transition torque measures the torque present in the valve. When it exceeds the set limit, it is marked the same way. Actuators are often equipped with remote position transmitters that indicate the position of the valve as a continuous current signal or voltage of 4-20mA.
Gearing (3)
Often gearing worms are used to reduce the high output speed of electric motors. This allows a high reduction ratio in the gear stage, leading to the desired low efficiency for the actuator. Therefore, self-locking gearing prevents unintentional and unwanted valve position changes by acting on the valve closure element.
Valve installation (4)
The valve attachment consists of two elements. First: Flanges used to connect actuators to partners on the side of the valve. The higher the torque to be transmitted, the greater the flange required.
Second: The type of output drive is used to transmit torque or drive from the actuator to the valve shaft. Just as there are many valves there are also many valve attachments.
Dimensions and valve fitting designs and valve attachments are set in EN ISO 5210 standards for multi-turn actuators or EN ISO 5211 for mid-roll actuators. The valve attachment design for linear actuators is generally based on DIN 3358.
Manual operation (5)
In their basic version, most electric actuators are equipped with handwheels to operate the actuators during commissioning or power outages. Handwheel does not move during motor operation.
The electronic torque restriction switch does not work during manual operation. Mechanical torque dividers are commonly used to prevent excess torque during manual operation.
Actuator control (6)
Both actuator signals and DCS operation commands are processed inside the actuator controls. This task can in principle be assumed by an external control, such as a PLC. Modern actuators include integral controls that process signals locally without delay. The controls also include the switchgear needed to control the electric motor. This may reverse the contactor or thyristor which, being an electrical component, is not subject to mechanical wear. Controls using switchgear to turn on or off an electric motor depends on the signal or command. Another task of actuator control is to provide DCS with a feedback signal, ie. when it reaches the position of the valve tip.
Power connection (7)
The cable supplies the motor and signal cable to transmit the commands to the actuator and sends the feedback signal on the actuator status connected to the electrical connection. Electrical connections can be designed as separate terminal connectors or plug/socket connectors separately. For maintenance purposes, cables should be easily disconnected and reconnected.
Fieldbus Connection (8)
Fieldbus technology is increasingly being used for data transmission in process automation applications. Electrical actuators can be equipped with all common fieldbus interfaces used in process automation. A special connection is required for fieldbus data cable connections.
Function
Auto-shutdown in end position
After receiving the operation command, the actuator moves the valve toward OPEN or CLOSE. Upon reaching the end position, automatic switch procedure begins. Two different lethal mechanisms can be used. The control turns off the actuator as soon as the specified set point has been reached. This is called the seat boundary. However, there is a type of valve whose locking element must be moved at the end position on a specified force or specified torque to ensure that the valve seals closely. This is called torque seating. The controls are programmed to ensure that the actuator is turned off when it exceeds the set torque limit. The end position is marked by a limit switch.
Security function
Torque shifts are not only used for torque seats in the final position, but also serve as overload protection over the entire journey and protect the valve against excessive torque. If excessive torque works on the closing element in the middle position, ie. because the trapped object, the torque switch will trip over when it reaches torque. In this situation the end position is not marked by a limit switch. Therefore the control can distinguish between normal operating torque switches that tripped in one end position and turned off in the middle position due to excessive torque.
Temperature sensors are required to protect the motor from overheating. For some applications by other manufacturers, increased motor currents are also monitored. Thermoswitches or PTC thermistors embedded in motor coils are largely reliable to fulfill this task. They travel when the temperature limit has been exceeded and the control turns off the motor.
Process control function
Due to the increasing decentralization in automation technology and the introduction of micro processors, more and more functions are transferred from DCS to field devices. The volume of data to be transmitted is reduced, especially by the introduction of fieldbus technology. Electrical actuators whose functions have been greatly expanded are also influenced by these developments. The simplest example is position control. The modern positioner is equipped with self-adaptation ie positioning behavior is monitored and kept optimized through controller parameters.
Meanwhile, the electric actuator is equipped with a complete process controller (PID controller). Especially for remote installation, e.g. control flow to the elevated tank, the actuator may receive PLC tasks which otherwise have to be installed in addition.
Diagnosis
Modern actuators have extensive diagnostic functions that can help identify the cause of failure. They also recorded operating data. The study of already logged data allows the operation to be optimized by changing the parameters and wear actuators and valves to be reduced.
Types of tasks
Open-close tasks
If the valve is used as a breaker valve, it will open or close and the center position is not held...
Positioning tasks
The specified middle position is approached to set the static flow through the pipe. The same running time limit as in the open-close task is applicable.
Modulate task
The most distinctive feature of closed-loop applications is that changing conditions require frequent adjustment of actuators, for example, to set a certain flow rate. Sensitive closed-loop applications require adjustments within a few seconds intervals. Demands on actuators are higher than open-close or task positioning. The actuator design should be able to withstand a high start count without reducing the accuracy of the controls.
Service conditions
The actuator is determined for the desired life and reliability for a set of service-specific application conditions. In addition to the static and dynamic load and response time required for the valve, the actuator must withstand the temperature range, corrosion environment and other conditions of the particular application. Actuator valve applications are often associated with safety, therefore factory operators place high demands on device reliability. Failure of actuators can cause accidents in process-controlled plants and toxic substances can leak into the environment.
Process control factories are often operated for decades that justify the higher demands given in the life of the device.
For this reason, actuators are always designed in high enclosure protection. The producers put a lot of work and knowledge into corrosion protection.
Encoslosure protection
The type of enclosure protection is determined in accordance with IP code EN 60529. The basic version of most electric actuators is designed for the second highest IP 67 enclosure protection. This means they are protected against ingress of dust and water during immersion (30 minutes). at max. water head 1 m). Most actuator manufacturers also supply the device to IP 68 enclosure protection that provides maximum immersion protection. water head 6 m.
About ambient temperature
In Siberia, temperatures up to -60 ° C can occur, and in the technical process plant 100 ° C can be exceeded. Using the right lubricant is essential for full operation under these conditions. Fat that can be used at room temperature can be too dense at low temperatures for actuators to overcome barriers inside the device. At high temperatures, this fat can melt and lose its lubricant power. When measuring the actuator, the ambient temperature and proper selection of lubricants are essential.
Blast protection
Actuators are used in applications where a potentially explosive atmosphere can occur. These include refineries, pipelines, oil and gas exploration or even mining. When a gas-air mixture-a potentially explosive gas or dust-gas mixture occurs, the actuator should not act as a source of ignition. The heat surface on the actuator as well as the sparks created by the actuators should be avoided. This can be achieved with a fireproof enclosure, where housing is designed to prevent sparks from leaving the housing even if there is an explosion in it.
Actuators designed for this application, which are anti-explosive devices, shall be qualified by a notified body. The explosion protection is not standardized around the world. Within the EU, ATEX 94/9/EC applies, in the US, NEC (approval by FM) or CEC in Canada (approval by CSA). Explosion-proof actuators must meet the design requirements of these directives and regulations.
Additional use
Small electric actuators can be used in a wide range of assembly, packaging and testing applications. Such actuators can be linear, rotary, or a combination of both, and can be combined to do the work in three dimensions. Such actuators are often used to replace pneumatic cylinders.
References
Source of the article : Wikipedia
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