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A final control element is the device manipulated by a control loop to affect the process, principally by means of changing a flow. Final control elements are an essential part of nearly every process control system. Without final control elements, there is no way of controlling the process. We could not change operating points or corfor rect for disturbances. There may be several layers of control loops, but it is usually a flow that a final control element ends up changing in a process. The most notable exceptions are heater or electrode current and mixer speed.
By far, the most common final control element is the control valve, with its attendant positioner, actuator, and other components. Variable speed peristaltic pumps are used for the exceptionally small flows of bench top and pilot plant operations. Variable speed positive displacement pumps are used for small additive and reagent flows in production. For large flows in plants and powerhouses, variable frequency drives and dampers are sometimes used instead of control valves to reduce capital and operating costs.
Axial and centrifugal blowers, fans, and pumps are used for the flow ranges normally associated with gas and liquid streams in industrial plants. A variable frequency drive (VFD), particularly in large utility flow applications, can save energy by the elimination of a control valve and its pressure drop. However, the energy savings is usually overestimated for process streams by not taking into account the service time and efficiency at low flow and the loss in turndown due to static head.
A damper can reduce the cost of the final element or fit in a non-circular duct. Dampers are commonly used in HVAC systems, boilers, furnaces, and scrubbers to manipulate air and vent gas flows. Dampers have a lower pressure drop than a control valve, but generally the performance (e.g., rangeability, resolution, sensitivity, speed, and seal) of a damper is not as good as a control valve. The leakage and limited dynamic response and materials /ruggedness of construction of dampers relegate their application to mostly utility and vent systems.
Valve design, dynamics
The shaft of the actuator and the stem of the internal closure component (plug, ball, or disk) of the control valve are normally separate. The closure component may be...