How Valve-Based Control Systems Operate
Process plants consist of hundreds or thousands of control loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable such as pressure, flow, temperature, level, 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 disturbances that influence the process variable.
To lessen 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 decides what must be done to get the process variable back to where it should be after a load disturbance occurs. Once all the measuring, comparing and calculating are done, some type of final control element must implement the strategy selected by the controller.
Control valves automatically regulate pressure and/or flow rate. They are available for any pressure. If different plant systems operate up to, and at pressure/temperature combinations that require Class 300 values, sometimes, all control valves chosen will be Class 300 for interchangeable functions. However, if none of the systems exceed the ratings for Class 150 valves, this is not necessary.
Globe valves are normally used for control, and their ends are usually flanged for ease of maintenance. Depending on their type of supply, the disk is moved by a hydraulic, pneumatic, electrical or mechanical actuator. The valve modulates flow through movement of a valve plug in relation to the port/ports located within the valve body. The valve plug is attached to a valve stem, which in turn, is connected to the actuator.
The most common final control element in the process control industries is the control valve. The control valve manipulates a flowing liquid 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.
Control valves may be the most important, but sometimes the most neglected, part of a control loop. The reason is usually the instrument engineer’s unfamiliarity with the many facets, terminologies, and areas of engineering disciplines such as fluid mechanics, noise control, piping and vessel design that can be involved depending on the severity of service conditions.
Any control loop usually consists of a sensor of the process condition, a transmitter and a controller that compares the “process variable” received from the transmitter with the “set point”. The controller, in turn, sends a corrective signal to the “final control element”, the last part of the loop and the “muscle” of the process control system. While the sensors of the process variables are the ‘eyes’, the controller the ‘brain’, the final element is the ‘hands’ of the control loop. This comes about, in part, due to our strong attachment to electronic systems causing some neglect in the proper understanding and proper use of the all-important hardware.