Process plants consist of hundreds, or even thousands, of control loops all networked together to produce a prod-uct to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, level, temper-ature, etc. within a required operating range to ensure the quality of the end product. Each of these loops receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides distur-bances that influence the process variable.

To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller then processes this information and de-cides what must be done to get the process variable back to where it should be after a load disturbance oc-curs. When all the measuring, com-paring, and calculating are done, some type of final control element must implement the strategy selected by the controller.

The most common final control ele-ment in the process control industries is the control valve. The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical com-pounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.

Many people who talk about control valves or valves are really referring to a control valve assembly. The control valve assembly typically consists of the valve body, the internal trim parts, an actuator to provide the motive pow-er to operate the valve, and a varietyof additional valve accessories, which can include positioners, transducers, supply pressure regulators, manual operators, snubbers, or limit switches.

Whether it is called a valve, control valve or a control valve assembly is not as important as recognizing that the control valve is a critical part of the control loop. It is not accurate to say that the control valve is the most im-portant part of the loop. It is useful to think of a control loop as an instru-mentation chain. Like any other chain, the whole chain is only as good as its weakest link. It is important to ensure that the control valve is not the weak-est link.