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What Is a Control Valve and What Is Its Purpose?
A control valve is a type of valve used to regulate the flow, pressure, level, or even direction of a fluid based on the needs of a process. Control valves are available in various designs, such as ball valves, butterfly valves, diaphragm valves, and more.
Control Valve Applications
Most of us want to control various process parameters such as temperature, pressure, and level. To achieve this, it is clear that you need a controller, such as a Programmable Logic Controller (PLC).
Some sensors and transmitters also need to send data to the PLC or DCS. Finally, you need a device (often called the “final control element”) to execute the commands from the PLC.
Actuators include pumps, heaters, control valves, and similar devices.
Typically, these control valves are not used in fully open or fully closed positions. Proportional control valves with specially designed throttling plates are used to restrict flow or regulate flow rate. This allows you to, for example, control the heating time of a furnace, the filling rate of a tank, or reduce the pressure in a line.
Control Valve Components
Body: The part through which the fluid passes and comes into contact is called the body.
Actuator: The actuator can be electric or pneumatic and allows the valve to open and close using auxiliary power.
Trim elemanlar: Sit, klepe ve klepe mili olarak adlandırlır bu kısımlar paslanmaz malzemeden oluşup akışganlar temas eden kısımlardır ve farklı uygulamalarda farklı malzeme çeşitleri kullanılabilir.
Positioner: Typically used to send commands to a pneumatic actuator, positioners come in types such as with or without feedback, self-calibrating, or HART protocol compatible. The main types are electro-pneumatic positioners, pneumatic-pneumatic positioners, and digital positioners.
Application Examples of Globe Control Valves with Positioners
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Temperature Control:
For example, to lower the temperature or maintain it at a desired level, you should connect a measurement device on the pipeline (such as a PT100) that provides a digital output to a controller via cable. The controller processes the digital information from the temperature sensor and sends a 4–20 mA signal to the electro-pneumatic positioner on the control valve. Thanks to this signal, the control valve moves in the throttling or opening direction.
Control Valve
PT 100
Controller
- Flow Control: The digital data from a flow meter is transmitted to the controller via cable. The controller determines whether the flow is too low or too high. If the flow is low, the controller sends a signal to the positioner to open the valve. If the flow is high, it sends a signal to the positioner to close the valve. This process continues until the flow reaches the set value on the controller.
Control Valve
Flow Meter
Controller
- Pressure Control: The digital pressure data transmitted by a pressure sensor (transmitter) is sent to the controller via cable. If the pressure is below the desired value, the controller sends a signal to the positioner to open. If the pressure is above the desired value, the controller sends a throttling signal to the positioner. Until the pressure values set on the controller are reached.
Control Valve
Transmitter
Controller
How Does a Positioner Work?
A positioner converts the 4–20 mA DC signal received from a PLC (Programmable Logic Controller) into air pressure. It transforms the digitally received signal into proportional pneumatic output, allowing proportional control of the actuator. In short, an electro-pneumatic positioner converts an electrical current (I) into pneumatic pressure (P) using special internal mechanisms.
What Is the Difference Between a Positioner and an I/P Converter?
I/P converters can control valves without the need for a controller, but their accuracy is not very high. For this reason, they are not commonly preferred.
Since they are not integrated with any positioner, this means there is no feedback signal available.
Key Considerations for Selecting a Control Valve
When selecting a control valve, the pressure, temperature, flow rate, and type of fluid must be carefully considered, and the selection should be made by experts. Internationally recognized sizing and calculation software is used for this purpose. If these programs are not used and the valve is incorrectly selected, the control valve may experience cavitation, causing vibration, excessive noise, and rapid deformation.
All the conditions mentioned above should be considered to ensure the valve operates with longevity and high precision.