DOE-HDBK-1016/1-93Engineering Fluid Diagrams and PrintsENGINEERING FLUIDS DIAGRAMS AND PRINTSExamplesof SimpleInstrumentLoopsFigure 14 shows two examples ofFigure 14 Instrumentation System Examplessimple instrument loops. Figure 14(A) shows a temperature transmitter(TT), which generates two electricalsignals. One signal goes to a board-mounted temperature recorder (TR) fordisplay. The second signal is sent toa proportional-integral-derivative (PID)controller, the output of which is sentto a current-to-pneumatic modifier(I/P). In the I/P modifier, the electricsignal is converted into a pneumaticsignal, commonly 3 psi to 15 psi,which in turn operates the valve. Thefunction of the complete loop is tomodify flow based on process fluidtemperature. Note that there is notenough information to determine howflow and temperature are related andwhat the setpoint is, but in someinstances the setpoint is stated on aP&ID. Knowing the setpoint andpurpose of the system will usually besufficient to allow the operation of theinstrument loop to be determined.The pneumatic level transmitter (LT) illustrated in Figure 14 (B) senses tank level. The outputof the level transmitter is pneumatic and is routed to a board-mounted level modifier (LM). Thelevel modifier conditions the signal (possibly boosts or mathematically modifies the signal) anduses the modified signal for two purposes. The modifier drives a board-mounted recorder (LR)for indication, and it sends a modified pneumatic signal to the diaphragm-operated level controlvalve. Notice that insufficient information exists to determine the relationship between sensedtank level and valve operation.ComponentsWithin every fluid system there are major components such as pumps, tanks, heat exchangers,and fans. Figure 15 shows the engineering symbols for the most common major components.Rev. 0PR-02Page 13
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