DOE-HDBK-1016/1-93Engineering Fluid Diagrams and PrintsENGINEERING FLUIDS DIAGRAMS AND PRINTSNote that the symbols alone in Figure 3 do not provide the reader with enough information todetermine whether applying air pressure to the diaphragm opens or closes the process controlvalve, or whether energizing the solenoid pressurizes or vents the diaphragm. Further, Figure 3is incomplete in that it does not show the electrical portion of the valve control system nor doesit identify the source of the motive force (compressed air). Although Figure 3 informs the readerof the types of mechanical components in the control system and how they interconnect, it doesnot provide enough information to determine how those components react to a control signal.Control valves operated by an instrument signal are symbolized in the same manner as thoseshown previously, except the output of the controlling instrument goes to the valve actuator.Figure 4 shows a level instrument (designated "LC") that controls the level in the tank bypositioning an air-operated diaphragm control valve. Again, note that Figure 4 does not containenough information to enable the reader to determine how the control valve responds to a changein level.Figure 4 Level Control ValveAn additional aspect of some control valves is a valve positioner, which allows more precisecontrol of the valve. This is especially useful when instrument signals are used to control thevalve. An example of a valve positioner is a set of limit switches operated by the motion of thevalve. A positioner is symbolized by a square box on the stem of the control valve actuator. Thepositioner may have lines attached for motive force, instrument signals, or both. Figure 5 showstwo examples of valves equipped with positioners. Note that, although these examples are moredetailed than those of Figure 3 and Figure 4, the reader still does not have sufficient informationto fully determine response of the control valve to a change in control signal.Rev. 0PR-02Page 5
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