A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. In automatic control terminology, a control valve is termed a "final control element". The opening or closing of automatic control valves is usually done by electrical , hydraulic or pneumatic actuators.
Choose valves appropriate for the application
Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening.
Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves required high pressure supply and return lines for the hydraulic fluid.
The pneumatic control signals are traditionally based on a pressure range of psi 0. Electrical control now often includes a "Smart" communication signal superimposed on the mA control current, such that the health and verification of the valve position can be signalled back to the controller. An automatic control valve consists of three main parts in which each part exist in several types and designs:. The modes of failure operation are requirements of the failure to safety process control specification of the plant.
Types of Control Valves
In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed. The fundamental function of a positioner is to deliver pressurized air to the valve actuator, such that the position of the valve stem or shaft corresponds to the set point from the control system. Positioners are typically used when a valve requires throttling action.
A positioner requires position feedback from the valve stem or shaft and delivers pneumatic pressure to the actuator to open and close the valve.
The positioner must be mounted on or near the control valve assembly. There are three main categories of positioners, depending on the type of control signal, the diagnostic capability, and the communication protocol: pneumatic analog and digital.
Processing units may use pneumatic pressure signaling as the control set point to the control valves. Pressure is typically modulated between In a common pneumatic positioner the position of the valve stem or shaft is compared with the position of a bellows that receives the pneumatic control signal.
List of valves
When the input signal increases, the bellows expands and moves a beam. The beam pivots about an input axis, which moves a flapper closer to the nozzle. The nozzle pressure increases, which increases the output pressure to the actuator through a pneumatic amplifier relay. The increased output pressure to the actuator causes the valve stem to move.
Stem movement is fed back to the beam by means of a cam.
Control Valve Arrangement
As the cam rotates, the beam pivots about the feedback axis to move the flapper slightly away from the nozzle. The nozzle pressure decreases and reduces the output pressure to the actuator.
Stem movement continues, backing the flapper away from the nozzle until equilibrium is reached.
When the input signal decreases, the bellows contracts aided by an internal range spring and the beam pivots about the input axis to move the flapper away from the nozzle. Nozzle pressure decreases and the relay permits the release of diaphragm casing pressure to the atmosphere, which allows the actuator stem to move upward. Through the cam, stem movement is fed back to the beam to reposition the flapper closer to the nozzle.
When equilibrium conditions are obtained, stem movement stops and the flapper is positioned to prevent any further decrease in actuator pressure. Most modern processing units use a 4 to 20 mA DC signal to modulate the control valves.
Principles of Operation
The pneumatic output signal provides the input signal to the pneumatic positioner. Otherwise, the design is the same as the pneumatic positioner . This type of positioner is a microprocessor-based instrument. The microprocessor enables diagnostics and two-way communication to simplify setup and troubleshooting. The microprocessor performs the position control algorithm rather than a mechanical beam, cam, and flapper assembly.
This pressure is routed to a pneumatic amplifier relay and provides two output pressures to the actuator. With increasing control signal, one output pressure always increases and the other output pressure decreases. Double-acting actuators use both outputs, whereas single-acting actuators use only one output.
The changing output pressure causes the actuator stem or shaft to move. Valve position is fed back to the microprocessor.
The stem continues to move until the correct position is attained. In addition to the function of controlling the position of the valve, a digital valve controller has two additional capabilities: diagnostics and two-way digital communication.
Automatic calibration and configuration of positioner. Real time diagnostics. Reduced cost of loop commissioning, including installation and calibration.
Use of diagnostics to maintain loop performance levels. Improved process control accuracy that reduces process variability.
Different Types of Valves used in Piping – A Complete Guide of Pipe Valves
A huge variety of valve types and control operation exist. However, there are two main forms of action; the sliding stem and the rotary action.
The most common and versatile types of control valves are sliding-stem globe, V-notch ball, butterfly and angle types. Their popularity derives from rugged construction and the many options available that make them suitable for a variety of process applications.
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