I. Valve Positioner . Valve positioners are classified by structure into pneumatic valve positioners, electric valve positioners, and intelligent valve positioners. They are the main accessories of control valves and are usually used in conjunction with pneumatic control valves. They receive the output signal from the controller and then use their output signal to control the pneumatic control valve. After the control valve is activated, the displacement of the valve stem is fed back to the valve positioner through a mechanical device, and the valve position status is transmitted to the upper system through an electrical signal.
(a) Structure
Valve positioners can be classified into pneumatic valve positioners, electro-pneumatic valve positioners, and intelligent valve positioners according to their structure and working principle.
Valve positioners can increase the output power of control valves, reduce the occurrence of signal transmission lag, accelerate the movement speed of valve stems, improve valve linearity, overcome valve stem friction, and eliminate the influence of unbalanced forces, thereby ensuring the correct positioning of control valves.
(II) Classification of Positioners
1. Valve positioners are classified into pneumatic valve positioners, electric valve positioners, and intelligent valve positioners according to their input signals.
(1) The input signal of a pneumatic valve positioner is a standard pneumatic signal, such as a 20~100kPa pneumatic signal, and its output signal is also a standard pneumatic signal. (2) The input signal of an electric valve positioner is a standard current or voltage signal, such as a 4~20mA current signal or a 1~5V voltage signal. The electric valve positioner converts the electrical signal into electromagnetic force and then outputs a pneumatic signal to the control valve. (3) The intelligent electric valve positioner converts the current signal output from the control room into a pneumatic signal to drive the regulating valve. Based on the friction force of the valve stem when the regulating valve is working, it offsets the unbalanced force generated by the fluctuation of the medium pressure, so that the valve opening corresponds to the current signal output from the control room. It can also intelligently configure and set the corresponding parameters to improve the performance of the control valve.
2. Based on the direction of action, valve positioners can be divided into one-way valve positioners and two-way valve positioners. When a one-way valve positioner is used in a piston actuator, the valve positioner operates in only one direction. A two-way valve positioner operates on both sides of the piston actuator cylinder, functioning in both directions.
3. Based on the gain signs of the input and output signals, valve positioners are classified into positive-acting valve positioners and negative-acting valve positioners. For a positive-acting valve positioner, an increase in the input signal results in an increase in the output signal; therefore, the gain is positive. For a negative-acting valve positioner, an increase in the input signal results in a decrease in the output signal; therefore, the gain is negative.
4. Based on whether the valve positioner's input signal is analog or digital, it can be divided into ordinary valve positioners and fieldbus electric valve positioners. Ordinary valve positioners use analog air pressure, current, or voltage signals as input signals, while fieldbus electric valve positioners use digital signals from the fieldbus.
5. Based on whether they have a CPU, valve positioners can be divided into ordinary electric valve positioners and intelligent electric valve positioners. Ordinary electric valve positioners do not have a CPU, therefore they lack intelligence and cannot handle related intelligent calculations. Intelligent electric valve positioners have a CPU and can handle related intelligent calculations, such as performing nonlinear compensation for the forward channel. Fieldbus electric valve positioners can also include PID and other functional modules to perform corresponding calculations.
6. They can also be classified according to the detection method of the feedback signal.
For example, valve positioners that detect valve position signals using mechanical linkages; valve positioners that detect valve stem displacement using the Hall effect; and valve positioners that detect valve stem displacement using electromagnetic induction.
(III) Working Principle
A valve positioner is a key accessory for controlling valves. It compares the valve stem displacement signal as the input feedback measurement signal with the controller output signal as the setpoint signal. When there is a deviation, it adjusts the output signal to the actuator, causing the actuator to move. This establishes a one-to-one correspondence between the valve stem displacement and the controller output signal. Therefore, the valve positioner constitutes a feedback control system with valve stem displacement as the measurement signal and the controller output as the setpoint signal. The manipulated variable of this control system is the output signal from the valve positioner to the actuator.
(iv) Working principle of the positioner
(1) Used in important control systems with high requirements for control quality, to improve the positioning accuracy and reliability of control valves.
(2) Used in situations where the pressure difference between the two ends of the valve is large (△p>1MPa). By increasing the air source pressure, the output force of the actuator is increased to overcome the unbalanced force generated by the liquid on the valve core and reduce the stroke error.
(3) When the medium being regulated is high temperature, high pressure, low temperature, toxic, flammable or explosive, the packing is often pressed very tightly in order to prevent leakage to the outside. Therefore, the friction between the valve stem and the packing is large. At this time, the positioner can overcome the time delay.
(4) When the medium being regulated is a viscous fluid or contains suspended solids, a positioner can be used to overcome the resistance of the medium to the movement of the valve stem.
(5) Used for large-diameter (Dg>100mm) regulating valves to increase the output thrust of the actuator.
(6) When the distance between the regulator and the actuator is more than 60m, the positioner can overcome the transmission lag of the control signal and improve the valve's action response speed.
(7) Used to improve the flow characteristics of the regulating valve.
(8) When a regulator controls two actuators to perform split-range control, two positioners can be used to receive low input signals and high input signals respectively. Then one actuator will move at low range and the other will move at high range, thus forming a split-range regulation.
(v) Suitable varieties
Common actuators are divided into pneumatic actuators and electric actuators, and are further divided into linear stroke and rotary stroke. They are used to automatically or manually open and close various valves, dampers, etc.
II. Pneumatic Valve Positioner
(I) Working Principle
Pneumatic valve positioners are important accessories and components of pneumatic control valves, serving to position the valve.
The pneumatic valve positioner operates on the principle of torque balance. When the signal pressure P1 supplied to the bellows 2 increases, the main lever 3 rotates around the fulcrum, causing the nozzle baffle 9 to move closer to the nozzle. The nozzle back pressure, amplified by the unidirectional amplifier 8, increases the pressure supplied to the diaphragm chamber of the actuator, causing the valve stem to move downwards. This drives the feedback rod to rotate around the fulcrum, and the feedback cam also rotates counterclockwise. This rotation, via rollers, causes the secondary lever 4 to rotate around the fulcrum, stretching the feedback spring. When the spring's tension on the main lever 3 and the force exerted by the signal pressure on the bellows reach torque balance, the instrument reaches a balanced state. The valve position of the actuator is maintained at a certain opening degree; a certain signal pressure corresponds to a certain valve opening degree. This is the direct-acting mechanism. To change the action, simply flip the cam, changing direction A to direction B, etc. A direct-acting positioner means that as the signal pressure increases, the output pressure also increases; a reverse-acting positioner means that as the signal pressure increases, the output pressure decreases. A direct-acting actuator can achieve the action of a reverse-acting actuator simply by installing a reverse-acting positioner; conversely, a reverse-acting actuator can achieve the action of a direct-acting actuator simply by installing a reverse-acting positioner.
(II) Structural Principles
A pneumatic valve positioner is a device that receives weak electrical signals (4-20mA) from a controller or control system and sends air signals to the pneumatic actuator to control the valve position.
It is used in conjunction with a pneumatic control valve to form a closed-loop control circuit. It converts the DC current signal from the control system into a pneumatic signal to drive the control valve, thus controlling its operation. Simultaneously, it provides feedback based on the valve's opening degree, ensuring the valve position is correctly positioned according to the control signal output by the system.
(III) Main Functions
Pneumatic valve positioners and pneumatic actuators together form an automatic control unit. After connection, debugging, and installation, they are combined to form a pneumatic control valve. This is used in various industrial automation process control applications.
III. Electric Valve Positioner
As DCS is increasingly used in the field, many controllers are now using controllers from the central control system. Therefore, the signals from the central control system to the field are 4-20mA electrical signals, and the valves need to operate relatively quickly at the field.
Although valve positioners have evolved from the initial pneumatic/pneumatic valve positioners and electric/pneumatic valve positioners to the current digital valve positioners and area bus valve positioners, their basic principles and main functions have not changed significantly.
(I) Introduction to basic self-control components in the positioner -- Principle of electro-pneumatic converter
With the development of instrument technology, the field of pneumatic instruments has been gradually occupied by electric instruments and computer control. Now, pneumatic instruments are only used in some special occasions. As a valve accessory in instruments, the "positioner" has also been gradually replaced by electric/pneumatic (E/P) valve positioners instead of the original pneumatic valve (P/P) positioners.
So how is the electrical signal input into the electro-pneumatic valve positioner converted into a pneumatic signal? We will take the SAMSON6111 electro-pneumatic converter as an example to introduce its working principle (see Figure 1):
Figure 1 Function Diagram of 6111
When designing the pneumatic power amplifier (8), a suitable spring force (8.2) is selected so that when the input signal is 0mA, the output PA is kept at 100mbar. In this way, the output pressure is passed through the constant throttling orifice (8.4) so that there is a certain back pressure in the nozzle (7).
When the input signal increases; the energized coil (2) cuts the magnetic lines of force of the permanent magnet (3) and generates an upward force → the baffle (6) moves closer to the nozzle (7) to increase the back pressure (PK) → the diaphragm (8.3) ↓ → the valve core (8.5) opens → the output PA ↑.
When the input signal decreases; the baffle (6) leaves the nozzle (7) → the back pressure (PK) decreases → the diaphragm (8.3) increases under the action of the output pressure (PA) → the valve core (8.5) closes → the output pressure is released through the valve core (8.5).
When PA and PK are balanced, the output pressure remains unchanged; at this time, the force generated by the electrical signal in the coil (2) is also balanced with the back pressure (PK).
In this way, the input electrical signal is converted into a pneumatic signal.
(II) Composition of the Positioner
Taking SAMSON's 4763 electro/pneumatic valve positioner (Figure 1A) as an example, the main components of the positioner are shown in Figure 2.
Figure 2
1. Feedback rod (1) 2. Feedback spring (6) 3. Feedback bellows (7) 4. Pneumatic power amplifier (lower part of 7) 5. Electro-pneumatic converter (21)
(III) Working principle of the positioner
1. Simulated Positioner
Let's take the SAMSON 4763 positioner as an example (refer to Figure 3). We assume the control valve is FC (air-to-open) and the positioner is positive-acting.
A) The valve position moves proportionally to the input signal.
Input signal ↑ → Pe point air pressure ↑ → Link (9) in the feedback box moves to the left → compressing spring (6), baffle (10.2) approaches nozzle (10.1) → output air pressure ↑ → valve stem (for air-opening valve) ↑ → compressing spring (6) → Link (9) in the feedback box moves to the right → baffle (10.2) moves away from nozzle (10.1) → output air pressure (Pst) ↓. When the force of the feedback spring is balanced with the force of the feedback box, the valve position remains at the position corresponding to the input signal.
B) Positioning
When the input signal remains unchanged:
Due to changes in process conditions, the valve stem ↑ → compresses the actuator spring → compresses the spring (6) → the connecting rod (9) in the feedback air box moves to the right → the baffle (10.2) leaves the nozzle (10.1) → the output air pressure ↓ → the downward spring force of the actuator causes the valve position to return to its original position.
Due to changes in process conditions, the valve stem ↓ → the actuator spring is released → the spring (6) is released → the connecting rod (9) in the feedback air box moves to the left → the baffle (10.2) moves closer to the nozzle (10.1) → the output air pressure ↑ → the actuator moves upward and the valve position returns to its original position.
(iv) Debugging
When the valve is adjusted to zero, input a current signal (typically 4mA) to the control valve. If the valve position pointer is not at zero, open the valve positioner and slowly adjust the zero-adjustment knob until the pointer indicates zero. When adjusting the range, adjust the setpoint so that its output value reaches the required air pressure value for the valve. After zero-adjustment, input 8mA, 12mA, 16mA, and 20mA signals to the control valve respectively. The valve position pointer should point to 25%, 50%, 75%, and 100% respectively. If the indication is inaccurate, fine-tune the zero-adjustment knob to achieve accurate indication. If the effect is still not satisfactory, the stroke is insufficient. Loosen the range adjustment locking screw and rotate it. After adjustment, tighten the locking screw. By repeatedly adjusting the zero point and range, ensure the actuator stroke is within the tolerance range.
(V) Fault Summary
1. The valve positioner has an input signal but no output signal.
(1) The electromagnet assembly is faulty, and it is recommended to replace the electromagnet assembly.
(2) The gas supply pressure is incorrect. It is recommended to check the gas source pressure.
(3) The zero point of the pneumatic amplifier baffle is adjusted too high, and the baffle is far away from the nozzle.
(4) Gas passage blockage.
(5) Incorrect gas path connection (including amplifier).
(6) The positive and negative terminals of the input signal line of the electric/pneumatic positioner are reversed.
2. The valve positioner has no input signal, but the output signal is always at its maximum.
(1) The zero point of the pneumatic amplifier baffle is adjusted too low, and the baffle is too tight on the nozzle.
(2) Nozzle blockage.
(3) The output pressure is slow or abnormal.
This can cause damage to the diaphragm head of the control valve and air leakage, resulting in a malfunction where the control valve operates slowly despite having an input signal, preventing the control valve from achieving timely regulation. The solution is to check the diaphragm chamber and replace the diaphragm.
3. Poor linearity of the positioner
(1) The feedback cam or spring is not selected properly or the direction is incorrect.
(2) The feedback linkage mechanism is not installed properly or is stuck in some positions.
(3) There are foreign objects in the nozzle or baffle.
(4) There is a slight leakage of back pressure.
(VI) Conclusion
The compressed air supplied to the valve positioner must be filtered and clean. Any parts disassembled during operation must be properly secured after adjustment. Avoid subjecting the magnetic device to vibration or excessive force, as this can damage parts and reduce performance. Minimize the number of times the nozzle baffle device is disassembled.
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