Process Control Elements in the Loop
A simple process control loop consists of three elements, the measurement, the controller and the final control element.
Measurements have got to be one of the most important equipment in any processing plant. Any decision made on what the plant should do is based on what the measurements tell us. In the context of process control, all controller decisions are similarly based on measurements.
With the advent of computers, it is now possible to do inferential measurements, meaning telling the value of a parameter without actually measuring it physically. It should however, be remembered that inferential measurement algorithms are also based on physical measurements. Therefore, rather than rendering measurements redundant, they have made measurements all the more important.
Final Control Elements
Final control elements can refer to three things, control valves, variable speed drives and dampers. In any chemical process plant, more than 90% of the time, a final control element is a control valve. The issues relating to final control elements will be most relevant to control valves although they are applicable to a large extend to dampers and in some cases variable speed drives.