Table of Contents

    1. Valve positioner (valve controller)

       1.1 Structure

       1.2 Valve Positioner types

       1.3 The principle of operation

       1.4 Valve Positioner principle

       1.5 Applicable varieties

    2. Pneumatic valve positioner

       2.1 Working principle

       2.2 Structural principle

1. The valve positioner (valve controller) valve positioner according to the structure: pneumatic valve positioner, electrical valve positioner and intelligent valve positioner, is the main accessory of the control valve, usually used in conjunction with the pneumatic control valve, it accepts the output signal of the regulator, and then to its output signal to control the pneumatic control valve, when the control valve action, the displacement of the valve stem and through the mechanical When the regulating valve is operated, the displacement of the valve stem is fed back to the valve positioner through the mechanical device, and the valve position status is transmitted to the upper system through the electric signal.
1.1 Structure

Valve positioner according to its structural form and working principle can be divided into pneumatic valve positioner, electric - gas valve positioner and intelligent valve positioner.

Valve positioner can increase the output power of the regulating valve, reduce the transmission lag of the regulating signal, accelerate the moving speed of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of the unbalanced force, so as to ensure the correct positioning of the regulating valve.

Fisher DVC6200 Valve Positioner

1.2 Valve Positioner types
1.2.1 The valve positioner is divided into pneumatic valve positioner, electrical valve positioner and intelligent valve positioner according to the input signal.

(1) pneumatic valve positioner input signal is a standard gas signal, for example, 20 ~ 100kPa gas signal, its output signal is also a standard gas signal. (2) The input signal of electrical valve positioner is standard current or voltage signal, for example, 4~20mA current signal or 1~5V voltage signal, etc. Inside the electrical valve positioner, the electrical signal is converted into electromagnetic force, and then the output gas signal is output to the toggle control valve. (3) Intelligent electrical valve positioner converts the current signal output from the control room into the gas signal to drive the control valve, and according to the friction force of the valve stem when the control valve is working to offset the imbalance force generated by the fluctuation of medium pressure, so that the valve opening degree corresponds to the current signal output from the control room. And intelligent configuration can be set to set the corresponding parameters to achieve the purpose of improving the performance of the control valve.

1.2.2 According to the direction of action can be divided into one-way valve positioner and two-way valve positioner. Unidirectional valve positioner for piston type actuator, the valve positioner only a direction of action, bidirectional valve positioner role in the piston type actuator on both sides of the cylinder, in both directions of action.

1.2.3 According to the valve positioner output and input signal gain symbol is divided into positive-acting valve positioner and reverse valve positioner. Positive valve positioner input signal increases, the output signal also increases, so the gain is positive. Reaction valve positioner input signal increases, the output signal decreases, therefore, the gain is negative.

1.2.4 According to the valve positioner input signal is an analog signal or digital signal, can be divided into ordinary valve positioner and fieldbus electrical valve positioner. Ordinary valve positioner input signal is analog air pressure or current, voltage signals, fieldbus electrical valve positioner input signal is the fieldbus digital signal.

1.2.5 According to whether the valve positioner with CPU can be divided into ordinary electrical valve positioner and intelligent electrical valve positioner. Ordinary electrical valve positioner does not have a CPU, therefore, does not have the intelligence, can not deal with the intelligent operation. Intelligent electrical valve positioner with CPU, can handle the relevant intelligent operation, for example, can be forward channel nonlinear compensation, etc., fieldbus electrical valve positioner can also be with PID and other functional modules to achieve the corresponding operation.

1.2.6 according to the feedback signal detection method can also be categorized.

For example, the valve positioner with mechanical linkage detection of valve position signals: the valve positioner with Hall effect detection of displacement detection of stem displacement: electromagnetic induction method of detection of stem displacement of the valve positioner and so on.

1.3 The principle of operation
Valve positioner is the main accessory of the control valve. It will stem displacement signal as the input feedback measurement signal, the controller output signal as a set signal, for comparison, when the two have deviation, change the output signal to the actuator, so that the actuator action, the establishment of a one-to-one correspondence between the amount of stem displacement and the controller output signal. Therefore, the valve positioner consists of a feedback control system with the stem displacement as the measurement signal and the controller output as the setting signal. The control system's manipulation variable is the valve positioner to actuator output signal.

1.4 Valve Positioner principle
(1) Used in important adjustment systems with high requirements on adjustment quality to improve the positioning accuracy and reliability of adjustment valves.

(2) Used in applications where the pressure difference between the two ends of the valve is large (△p>1MPa). Increase the output force of the actuator by increasing the pressure of the gas source to overcome the unbalanced force generated by the liquid on the spool and reduce the travel error.

(3) When the regulated medium is high temperature, high pressure, low temperature, toxic, flammable, explosive, in order to prevent external leakage, the packing is often pressed very tightly, so the friction between the valve stem and packing is larger, at this time with the positioner can overcome the time lag.

(4) The regulated medium is viscous fluid or contains solid suspensions, the positioner can overcome the resistance of the medium to stem movement.

(5) Used for large caliber (Dg>100mm) regulating valve 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 be used to overcome the lag in the transmission of the control signal and improve the reaction speed of the valve action.

(7) Used to improve the flow characteristics of the regulator valve.

(8) A regulator to control two actuators to implement the split-range control, two positioners can be used, respectively, to accept the low input signal and high input signal, then an actuator low-range action, another high-range action, that is, constitutes a split-range regulation.

1.5 Applicable varieties

Commonly used actuators are pneumatic actuators, electric actuators, straight stroke, angle stroke. Used to open and close various types of valves, wind panels, etc. automatically and manually.

2. Pneumatic valve positioner

Masoneilan Svi II Pneumatic valve positioner

2.1 Working principle

Pneumatic valve positioner is one of the important accessories and fittings of pneumatic control valve, and plays the role of valve positioning.

Pneumatic valve positioner is working according to the principle of torque balance, when the signal pressure P1 through the bellows 2 increase, make the main lever 3 around the pivot point of the rotation, so that the nozzle stopper 9 close to the nozzle, the nozzle back pressure amplified by the one-way amplifier 8, through the actuator membrane chamber pressure increases, so that the valve stem downward movement. And drive the feedback lever rotating around the pivot point, the feedback cam also made counterclockwise rotation, through the roller to make the deputy lever 4 rotating around the pivot point, and the feedback spring stretch, the spring tension on the main lever 3 and the signal pressure used in the bellows to achieve torque equilibrium, the instrument reaches the equilibrium state. The valve position of the actuator is maintained at a certain opening degree, and a certain signal pressure corresponds to a certain valve opening degree. The above mode of action is positive acting, if you want to change the mode of action, just flip the cam, A direction into B direction, etc., can be. The so-called positive-acting positioner is that the signal pressure increases, the output pressure also increases; the so-called reverse-acting positioner is that the signal pressure increases, the output pressure decreases. A positive-acting actuator can realize the action of a negative-acting actuator as long as it is fitted with a negative-acting positioner; on the contrary, a negative-acting actuator can realize the action of a positive-acting actuator as long as it is fitted with a negative-acting positioner.

Pneumatic valve positioner

2.2 Structural principle
Pneumatic valve positioner receives weak electrical signals such as 4-20mA from controller or control system, and delivers air signals to pneumatic actuator to control valve position.

It is used in conjunction with pneumatic control valve to form a closed-loop control loop. The DC current signal given by the control system is converted into an air signal to drive the regulating valve and control the action of the regulating valve. At the same time, according to the opening degree of the control valve for feedback, so that the valve position can be correctly positioned according to the system output control signal.

2.3 Main functions

Pneumatic valve positioner and pneumatic actuator together constitute an automatic control unit and a variety of control valves connected to the debugging and installation, combined with pneumatic control valves. Used in various industrial automation process control fields.

3. Electro-Pneumatic Valve Positioner

Fisher 3582i Electro-Pneumatic Valve Positioner

As DCS is now used more and more in the field, many of the controllers are used in the central control system controller, so the central control to the scene are 4-20mA electrical signals to the scene and the need for valve action faster.
Although the valve positioner from the initial gas / gas valve positioner, electric / gas valve positioner development to the current digital valve positioner, regional bus valve positioner, but their basic principles and main functions have not changed significantly.

3.1 Valve positioner in the basic self-control components - electric / gas converter principle
With the development of instrumentation technology, the field of pneumatic instrumentation has been gradually occupied by electric instrumentation and computer control, and now only in some special occasions are still using pneumatic instrumentation, as the instrumentation in the valve accessory “positioner” by the original pneumatic valve (P / P) positioner gradually by the electric / gas (E / P) valve positioner instead of. The “positioner” has also been gradually replaced by electric/pneumatic (E/P) valve positioners from the original pneumatic valve positioners.
So in the electric / gas valve positioner input electrical signals are converted into gas signals? Let's take the SAMSON 6111 type electric/pneumatic converter as an example to introduce its working principle (as picture 1):

Picture 1 

Pneumatic power amplifier (8) in the design; choose the appropriate spring force (8.2), so that when the input signal is 0 mA to maintain the output PA in the 100 mbar, so that the output pressure through the constant throttle orifice (8.4) so that the nozzle (7) inside a certain back pressure.

SAMSON's 4763 electro-pneumatic valve positioner (Picture 2) as an example, the main components of the positioner is shown in Picture 2.

Picture 2

1. Feedback lever (1) 2. Feedback spring (6) 3. Feedback bellows (7) 4. Pneumatic power amplifier (7 lower) 5. Electric/pneumatic converter (21)

3.3 Valve Positioner operating principle

Picture 3 

Input signal ↑ → air pressure at point Pe ↑ → connecting rod (9) in the feedback air box moves to the left → compression spring (6), baffle plate (10.2) moves closer to the nozzle (10.1) → output air pressure ↑ → valve stem (for air-open valve) ↑ → compression spring (6) → connecting rod (9) in the feedback air box moves to the right → baffle plate (10.2) moves away from the nozzle (10.1) → output air pressure (Pst) ↓. When the force of the feedback spring is balanced with the force of the feedback bellows, the valve position is maintained at the position corresponding to the input signal.

B) Positioning

Valve stem ↑ due to change in process conditions → compression of actuator spring → compression spring (6) → rightward movement of connecting rod (9) in the feedback bellows → baffle (10.2) leaves nozzle (10.1) → output air pressure ↓ → spring force downward from actuator returns valve position to its original place.

4. Debugging

5. Fault summary

(1) Failure of solenoid assembly, it is recommended to change the solenoid assembly.

(3) Pneumatic amplifier baffle zero point adjustment is too high, baffle away from the nozzle.

(5) Wrong air line connection (including amplifier).

2. There is no input signal from the valve positioner but the output signal is always maximum.

(2) Nozzle clogging.

Will lead to damage to the membrane head of the regulator valve, air leakage, resulting in an input signal but the regulator valve action slow failure, so that the regulator valve can not achieve the effect of timely regulation, treatment methods to check the membrane chamber, replace the diaphragm.

6. Conclusion