Introduction

Final control elements typically are automatic valves, but motors or other electric devices can be used. The final control element is the last link in the control loop and is the device that actually makes the change in the process. Automatic valves will open or close to regulate the process. Because they can be controlled from remote locations, they are invaluable in modern processing. An actuator is the device that automates a valve. The actuator controls the position of the flow control element by moving and controlling the position of the valve stem. Actuators come in three basic designs: pneumatic, electric, and hydraulic. Pneumatic (air operated) actuators are the most common. Pneumatic actuators convert air pressure to mechanical energy. They can be found in three designs: diaphragm, piston, and vane. The diaphragm actuator is a dome-shaped device that has a flexible diaphragm running through the center. It is typically mounted on the top of the valve. The center of the diaphragm in the dome is attached to the stem. The valve position (on or off) is held in place by a powerful spring. When air enters the dome on one side of the flexible diaphragm, it opens, closes, or throttles the valve, depending on design. The piston actuator uses an airtight cylinder and piston to move or position the stem. It is commonly found in use with automated gate valves or slide valves. It is used where a lot of stem travel is needed. Vane actuators direct air against paddles or vanes.

Electrically operated actuators convert electricity to mechanical energy. There are two types: solenoid valve and motor-driven actuator. Solenoid valves are designed for on/off service. The internal structure of a solenoid resembles a globe valve. The disc rests in the seat, stopping flow. The stem is attached to a metal core or armature that is held in place by a spring. A wire coil surrounds the upper spring and stem. When the wire coil is energized, a magnetic field is set up, causing the armature to lift and compressing the spring. The armature is held in place until the current stops.

A motor-driven actuator is attached to the stem of a valve by a set of gears. Gear movement controls the position of the stem. Hydraulically operated actuators convert liquid pressure to mechanical energy. The hydraulic actuator uses a liquid tight cylinder and piston to move or position the stem. It is commonly found in use with automated gate valves or slide valves and is used where a lot of stem travel is needed.

References

1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas