FINAL CONTROL
ELEMENTS
The final control element in process systems is typically a pneumatically actuated control valve, which regulates the flow of fluid and provides power to translate the controller's output to the process. Pneumatics is used due to its popularity and low operating pressures, making it suitable for safe operation in oil and gas facilities.
The major parts of
a control valve are the actuator and the valve body assembly. There are several
types of body design, flow characteristic, actuator type, and trim design.
Control valves are expected to respond to a control signal to keep a process
variable steady.
There are several main types of control valves based on body design: sliding stem control valves, globe bodies, single port, double port, three-way, angle valves, and diaphragm valves. Globe valves are the most common type in use today, with single port valves providing tight shutoff but may have high unbalanced forces on the plug requiring large actuators. Double port valves balance the forces acting on single port valves, have higher flow capacities, and require smaller stem forces. Three-way valves blend or divert flowing streams, with total flow proportioned only and not controlled. Angle valves are often single-ported and can handle sludge and erosive materials.
Diaphragm valves
consist of a body, bonnet, and flexible diaphragm, also known as Saunders-Type
valves. These valves are suited for slurries and viscous fluids, have high
capacity, and are relatively low in cost. The diaphragm seals the working parts
of the valve from the process fluid and is the only wearing part of the valve.
However, the Saunders-Type valve exhibits poor control characteristics and has
a low turndown ratio.