Process plants consist of hundreds, or even thousands, of control loops all networked together
to produce a product to be offered for sale. Each of these control loops is designed to control a
critical process variable such as pressure, flow,level, temperature, etc., within a required operating range to ensure the quality of the end-product.
These loops receive, and internally create,disturbances that detrimentally affect the process
variable. Interaction from other loops in the network provides disturbances that influence the
process variable. To reduce the effect of these load disturbances, sensors and transmitters collect information regarding the process variable and its relationship to a desired set point. A controller then processes this information and decides what must occur in order to get the process variable back to where it should be after a load disturbance occurs.
When all measuring, comparing, and calculating are complete, the strategy selected by the
controller is implemented via some type of final control element. The most common final control element in the process control industries is the control valve.
A control valve manipulates a flowing fluid such as gas, steam, water, or chemical compounds to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point. Many people who speak of “control valves” are actually referring to “control valve assemblies.”
The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive power to operate the valve, and a variety of additional valve accessories, which may include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches.
It is best to think of a control loop as an instrumentation chain. Like any other chain, the
entire chain is only as good as its weakest link. It is important to ensure that the control valve is not the weakest link.