Last Updated on February 25, 2024 by Ali Hamza
A positioner needs to be set based on the desired set point for a process variable, such as flow or pressure, to adjust the position of a control valve. In this article, we will discuss the details of a control valve positioner.
The Control valve positioner
When the valve stem reaches a balance with the process variable instrument controller’s output signal, the valve positioner increases or decreases the air load pressure pushing the actuator.
To control valves with accuracy and rapidity, valve positioners are used when no errors or hysteresis can be tolerated. Slider positioners for linear sliding stem control valves are generally mounted on the top or side yoke of the pneumatic actuator. In contrast, rotary control valve positioners are usually mounted at/near the end-of-shaft.
The feedback for these two basic designs is provided to the controller by mechanical linkages mounted directly to the stem of the valve. Whenever a change in position is signaled to the positioner by the process controller, the feedback linkage communicates this change back to the positioner, confirming that it has taken place and providing a sense of its magnitude.
Types of Control valve positioner
In addition to pneumatic positioners, there are electronic positioners, electro-pneumatic positioners, and digital positioners.
The Digital control valve positioner
Similarly, a Digital Valve Positioner receives a signal from a controller (usually 4-20mA, HART, PROFIBUS, or Foundation Fieldbus); however, it’s a digital signal instead of an analog one. A corresponding pneumatic signal then triggers a pneumatic valve actuator.
Microprocessors are also used in an EP Positioner rather than mechanical beams, cams, and flappers to convert the control signal and control the position.
It allows the user to perform more advanced functions by using the electrical input. The “smart positioners” also notify the user when valve performance, diagnostics, and maintenance need to be performed.
The Pneumatic control valve positioner
An input signal from a pneumatic device opens or closes valves by sending pressure. They use a flapper/nozzle system that requires compressed air for everything to work correctly. In order for the controller’s data to be received, a sign of 3-15 pounds per square inch must be present. A bellows expands or compresses in response to an input signal.
Moving the bellows can move the flapper assembly and beam measuring valve stem feedback. This process involves the flapper moving towards or away from the nozzle. The relay controls the air output to the valve actuator by increasing or decreasing its voltage. Through the beam, the valve stem sends feedback when it moves. The valve will be positioned appropriately when the air output decreases or increases according to the input signal.
Electric control valve positioner
Positioning valves with electric actuators involves the transmission and reception of electrical signals. A direct current (DC) motor can be actuated in three ways: using direct current (DC), alternating current (AC), and three-phase alternating current (AC).
Electro-pneumatic control valve positioner
Analog signals, typically 4-20 milliamps, are sent from a controller to electro-pneumatic positioners. When an electric signal is applied to a coil, it produces an electromotive force. In response to this force, the flapper moves, increasing or decreasing the airflow to the actuator of the valve. As well as providing feedback, the valve stem and positioner enable the valve actuator to be adjusted as needed.
Working of positioner valve
Pneumatic actuators with linear control valves typically have valve positioners installed on the yolk or top casing. An actuator positioner is installed on top or the side of an actuator on rotary control valves. Typically, it is installed along with valve stems and actuator stems. A variety of actuators can be installed, depending on their type.
When the positioner is attached to the actuator, it can measure the stem travel for linear valves or the degree of rotation for rotary valves. As well as changing the valve position according to the instrument controller input signal, this also changes the instrument’s part.
Positioners receive electrical or pneumatic signals from the instrument controller when the process variable varies from the set point. This way, the valve can be opened or closed based on the pneumatic output from the actuator. As long as the desired set point is not reached, this process continues.
Purpose of control valve positioner
The following circumstances warrant the consideration of a positioner:
- In the case of single seated valves, when the valve pressure drop at maximum flow rate exceeds 5 bar; In matters of double seated valves, when the valve pressure drop exceeds 10 bar;
- Varying differential pressure within the fluid will cause the plug position to change;
- The use of wide throttling ranges is required when controlling.
- An actuator that is not linear can be linearized;
- In cases where precise valve positioning is required.
- The valve’s response should be sped up. Using more excellent airflow and higher pressure, the positioner adjusts the valve position;
- An actuator or valve can close against more pressure by increasing its pressure rating.
- Solids in suspension are handled by valves when handling sludge or sludge.
Benefits of control valve positioner
A minimum amount of friction is observed, which causes hysteresis and headband.
The valve stem packing suffers from friction, which increases hysteresis and deadband, reducing productivity.
· Accurately controls the system.
Through the use of signals, valve positioners improve and refine fluid system control. An actuator’s spring rate range can vary by as much as 8 percent, but this precise control helps to compensate for this variation.
· Flexible configurations and functions
The positioners are used to control valves by placing a distance between the controller and the valve and using diaphragms or pistons. The flow characteristics can also be changed when necessary, as well as switching between direct and reverse control actions. It is even possible to use one controller to control two valves simultaneously (i.e., split ranging).
· Variable pressures do not affect valve position.
It can indicate control loop instability if the differential pressure across the valve varies. The position of a valve can be stabilized with the help of a positioner.
· Accurate control is improved
A valve positioner provides more precise control than a single actuator because it can determine the exact position of the valve. Additionally, positioners increase accuracy in valve strokes at the bottom, where errors are more likely to occur.
FAQs (Frequently Asked Questions)
In what ways does a positioner increase the response time of a control valve?
Valve positioners provide “stem position feedback” in the control loop by causing a change in controller SIG output to change positioner LOAD output, resulting in valve stem movement. After travel, the mechanical feedback linkage “confirms” that it has occurred.
How does a valve positioner with an actuator work?
Using a positioner as an amplifier or booster for signals is possible. To ensure that the valve reaches the desired position, the actuator diaphragm receives a low-pressure air control signal and multiplies that using its higher pressure input.
In instrumentation, what does a positioner do?
By increasing and decreasing the air load pressure driving the valve’s actuator, the Control Valve Positioner ensures the valve’s stem is balanced with the process variable instrument controller’s output signal.
What are the ways in which positioners improve safety?
Positioners eliminate those hazards by eliminating chains, straps, hoists, and cranes from the assembly and welding process. Elements and assemblies are raised and positioned to the optimal height, rotation, and working position by positioners.