CONTROLLERSProcess ControlsManual push buttons. These buttons are located below each end of the output meter andare used in the manual mode of operation. Buttons are labeled to indicate their effect onthe final control element. The labels are "open-close" for valves and "slow-fast" forvariable speed devices. The left push button decreases the output signal. The right pushbutton increases the output signal. Either button can be depressed at two differentpositions, half-in and full-in. At the half-in position, the output signal changes slowly.At the full-in position, the output signal changes about ten times faster.Mode indicating lights. Located directly below the manual push buttons, these lightsindicate the operating mode of the controller. When in manual mode, the left light,labeled "M", will be lit; when in the automatic mode, the right light, labeled "A", will belit.Mode selection buttons. Located directly under each mode indicating light, each buttonwill select its respective mode of control. If the button below the "M" mode light isdepressed, the controller will be in the manual mode of operation; if the button below the"A" mode light is depressed, the controller will be in the automatic mode of operation.As previously discussed, a particular plant will probably have controllers different from the twodescribed here. Although most information provided can be generally applied, it is extremelyimportant that the operator know the specific plant’s controllers and their applications.Final control elements are devices that complete the control loop. They link the output of thecontrolling elements with their processes. Some final control elements are designed for specificapplications. For example, neutron-absorbing control rods of a reactor are specifically designedto regulate neutron-power level. However, the majority of final control elements are generalapplication devices such as valves, dampers, pumps, and electric heaters. Valves and dampershave similar functions. Valves regulate flow rate of a liquid while dampers regulate flow of airand gases. Pumps, like valves, can be used to control flow of a fluid. Heaters are used tocontrol temperature.These devices can be arranged to provide a type of "on-off" control to maintain a variablebetween maximum and minimum values. This is accomplished by opening and shutting valvesor dampers or energizing and de-energizing pumps or heaters. On the other hand, these devicescan be modulated over a given operating band to provide a proportional control. This isaccomplished by positioning valves or dampers, varying the speed of a pump, or regulating thecurrent through electric heater. There are many options to a process control.Of the final control elements discussed, the most widely used in power plants are valves. Valvescan be easily adapted to control liquid level in a tank, temperature of a heat exchanger, or flowrate.IC-07 Page 52 Rev. 0
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