Dual coil solenoid operated valves (SOV) are suitable for fail fixed action. This system requires both an open and a closed signal to operate. It is an economical fail closed design.
5 micron dust and water filter is strongly recommended on the air inlet to all positioners. On the bottom the air filter is fitted with a water release valve on the bottom
These boost the volume of air to the actuator at the same pressure as the positioner output allowing the actuator to fill and exhaust up to six times faster. This results in a control loop that can be tuned to respond faster. These devices are recommended for control valve actuators larger than 300 mm in diameter.
The exhaust air from the actuator is allowed to bypass the positioner and vent directly from the quick exhaust, this results in faster response then with positioner alone but can lead to some instability. It is recommended for actuators of 250 mm and 300 mm diameter. Two quick exhausts are required per double acting actuator. They are relatively inexpensive compared to fitting volume boosters.
Most modern positioners and actuators are designed to operate on 4 – 10 bar(g) air pressure; they do not need air pressure regulators to work. However should the maximum supply pressure exceed the maximum allowable air pressure it is necessary to fit an air pressure regulator.