The control valve positioner is a controller based on microprocessor technology. It uses digital technology for data processing, decision making, and a two-way communication controller. With the development of smart meter technology, microelectronic technology is widely used in traditional meters, dramatically improving the functions and performance of the meters. As a result, intelligent valve positioners have also become mainstream products.
1. Principle of the valve positioner
According to the structure, a valve positioner is divided into a pneumatic valve positioner, electric-gas valve positioner, and intelligent valve positioner. The valve positioner is the main accessory of the regulating valve. It is usually used in conjunction with a pneumatic regulating valve. It can accept the output signal of the regulator and then use its output signal to control the pneumatic regulating valve. When the regulating valve moves, the displacement of the valve stem is fed back to the valve positioner through a mechanical device, and the valve position is transmitted to the upper system through an electrical signal.
2. Structure of the valve positioner
Valve positioners can be divided into pneumatic valve positioners, electric-pneumatic valve positioners, and intelligent valve positioners according to their structural form and working principle.
The valve positioner can increase the output power of the regulating valve. It can reduce the transmission lag of the regulating signal, speed up the movement of the valve stem, improve the linearity of the valve, overcome the friction of the valve stem and eliminate the influence of unbalanced forces, thus ensuring the correct positioning of the regulating valve.
3. Classification of valve positioner
3.1 Classification by input signal
According to the input signal, valve positioners are classified into pneumatic valve positioners, electric-gas valve positioners, and intelligent valve positioners.
3.1.1 Pneumatic valve positioners
The input signal of the pneumatic valve positioner is a standard gas signal. So, for example, a 20~100kPa gas signal, its output signal is also a standard gas signal.
3.1.2 Electrical valve positioners
The input signal of an electrical valve positioner is a 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 gas signal is output to the toggle control valve.
3.1.3 Intelligent electrical valve positioner
An intelligent electrical valve positioner converts the current signal output from the control room into a gas signal to drive the control valve. According to the valve stem friction, when the control valve is working, offset the unbalanced force caused by fluctuations in medium pressure. So that the valve opening corresponds to the current signal output from the control room and can be intelligently configured to set the corresponding parameters to improve the control valve’s performance.
3.2 Direction of action classification
The direction of action can be divided into one-way valve positioners and two-way valve positioners. One-way valve positioners are used in piston actuators where the valve positioner acts in one direction only. Bi-directional valve positioners work on both sides of the piston actuator cylinder and work from both directions.
3.3 Classification of output and input signals
Valve positioners are divided into positive acting valve positioners and reverse acting valve positioners according to the gain sign of the valve positioner output and input signal. When a valve positioner’s positive acting input signal increases, its output signal also increases, so the gain is positive. When the input signal of a counter-acting valve positioner increases, the output signal decreases, so the growth is negative.
3.4 Analogue or digital signal classification
The valve positioner input signal is an analog signal or digital signal, which can be divided into a standard valve positioner and Fieldbus electrical valve positioner. The input signal of the common valve positioner is an analog air pressure or current or voltage signal. In contrast, the input signal of the Fieldbus electrical valve positioner is a digital signal from the Fieldbus.
3.5 With or without CPU
Valve positioners can be divided into standard electrical valve positioners and intelligent electrical valve positioners according to whether they have a CPU. Ordinary electrical valve positioners do not have a CPU and therefore are not competent and cannot handle the relevant, intelligent operations. Smart electrical valve positioners have a CPU and can take the relevant intellectual processes, for example, non-linear compensation of the forward channel, etc. Fieldbus electrical valve positioners can also have function modules such as PID to realize the corresponding operations.
3.6 They can also be classified according to the detection method of the feedback signal.
For example, we have valve positioners that detect the valve position signal using a mechanical linkage. We have valve positioners that see the displacement of the valve stem using Hall effect displacement detection. We have valve positioners that detect valve stem displacement using electromagnetic induction.
4. Positioner principle of action
4.1 Positioners are used in critical regulating systems where high-quality regulation is required. It is used to improve the positioning accuracy and reliability of the regulating valve.
4.2 The positioner is used when the pressure difference between the valve’s two ends is significant (Δp>1MPa). It can increase the output force of the actuator by increasing the pressure of the air source. It also overcomes the unbalanced force generated by the liquid on the valve spool to reduce the travel error.
4.3 When the regulated medium is high temperature, high pressure, low temperature, toxic, flammable, and explosive, we often press the packing very tightly to prevent external leakage. The friction between the valve stem and the filling is significant. At this time, the positioner can be used to overcome the hysteresis.
4.4 When the regulated medium is viscous fluid or contains solids in suspension, we use the positioner to overcome the medium’s resistance to the movement of the valve stem.
4.5 Positioner is used for large diameter (Dg>100mm) regulating valve. It can increase the output thrust of the actuator.
4.6 When the distance between the regulator and the actuator is above 60m, the positioner can be used to overcome the transmission lag of the control signal and improve the action response speed of the valve.
4.7 We use positioners to improve the flow characteristics of regulating valves.
4.8 When a regulator controls two actuators for split-range control, we use two positioners. First, they accept low input signal and high input signal, respectively, then one actuator joint range action, the other high range action, that is, constitute a split range regulation.
The positioner acts as the brain of the control valve. Therefore, the valve positioner can play a decisive role in the entire control valve’s control performance and field function. It is compared with the traditional electric-gas conversion valve positioner, intelligent valve positioner in the control valve control accuracy, response speed, and functional expansion. At the same time, it also improves the level of valve automation control, and other aspects have more extensive application and market prospects. Therefore, intelligent valve positioners are regarded as the future direction of valve positioner development.