Before the invention of positioners, control valves were operated through direct instrument air supply to the actuator diaphragm through an I-P converter. The other technique was to directly supply air through the pneumatic controller.
The basic purpose of a valve positioner is to adjust air pressure driving the actuator of the valve and position the valve stem correctly. They receive command signals from the controller, amplify the air signals and feed them to the valve actuator to move the valve spindle into the process set point.
Depending on the type of construction and principle of working, positioners are classifed into main 3 types
• Pneumatic positioners
• Electro-pneumatic positioners
• Smart positioners
Pneumatic Positioners : A pneumatic positioner works on the force-balance principle. The input air signal is relayed and the output signal generated and passed on to the actuator for spindle movement. Pneumatic positioner amplies the signals received from I-P and given to Actuator operation. The I/P converter works on the principle of flapper nozzle to signal output pressure proportional to output current. A 4-20 mA signal generates output air signal of 3-15 psi. This air signal of 3-15 psi is fed to the pneumatic positioner. Like in a game of tug-of-war, where the rope does not deviate from its original position when the two opposing teams exert the same amount of force, the position of the valve stem is also fixed at set point when the input and output signal match.
Electro-pneumatic Positioners : In Electro-pneumatic or E/P positioners, the function of the I/P is built into the positioner. As the I/P function is inbuilt, the analogue signal 4-20 mA is directly transmitted to the positioner. Here, force to open and close the nozzle is balanced by valve stem position and by input current signal. Balanced beam movement causes the position of the nozzle to change and required air signals for the pneumatic actuator get generated.
Smart Positioners: Smart Positioners are micro-processor based digital valve controllers. These positioners convert current signal to pressure signal to operate the valve as per the process requirements. Digital positioners offer many additional features as compared to conventional positioner types like pneumatic and electro-pneumatic. This is beneficial for accurate process control, economical operation and overall health of the valve, to prevent any break down maintenance.
As we can see all around us, digital technology is gaining popularity in every field, and swiftly replacing conventional methods. The same applies to positioners. Smart positioners are equipped with many technical features which are beneficial in operation at plants. Smart positioners are also available in different types, depending on construction and working principle.
• Contact type
• Non contact type
• Linear type
• Rotary type
• Double acting
• Single acting
Benefits of using smart positioners with digital technology
• They are equipped with the latest micro-processor based technology
• They deliver high accuracy with low hysteresis
• Less air consumption as compared to conventional positioners resulting in a huge saving on compressed air generation and supply
• Available with different communication protocols like HART, FF, Profibus, etc
• Auto valve calibration eliminates the need of manual intervention
• Advanced diagnostic features to help plan preventive maintenance