Valve positioners compare a control signal to a valve actuator’s position and move the actuator accordingly. They are used with both linear valves and rotary valves. Valve positioners are used when the 0.2 to 1 bar pressure in the diaphragm chamber is not able to cope with friction and high differential pressure.
Pneumatic devices send and receive pneumatic signals. They are intrinsically safe and can provide a large amount of force to close a valve.
Single-acting or three-way pneumatic positioners send air to and exhaust air from only one side of a single-acting valve actuator that is opposed by a range spring.
Double-acting or four-way pneumatic positioners send and exhaust air from both sides of the actuator.
Electric valve positioners send and receive electrical signals. There are three electric actuation types: single-phase and three-phase alternating current (AC), and direct current (DC) voltage.
Electro-pneumatic valve positioners convert current control signals to equivalent pneumatic signals.
Digital or “smart” devices use a microprocessor to position the valve actuator and monitor and record data. They are very accurate, use less air than analogue positioners, and allow for online digital diagnostics.
Pneumatic input signal range and maximum supply pressure are measured in pounds per square inch (psi).
Split ranges commonly include 4 – 10 mA and 12 – 20 mA. Two-way, three-way, and four-way splits are available.
There are three types of output actions: direct, reversible, and field reversible.
Direct action devices increase the output signal as the input signal increases.
Reversible action devices decrease the output signal as the input signal increases.
Field reversible products can be switched between direct and reversible action.
