How are instrument function symbols represented in a PID diagram?
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On the gas pipeline, it is necessary to perform a subtraction calculation using the values from the pressure transmitters at the upstream and downstream sections as well as the flow meter; if the difference is significant, the upstream shut-off valve is closed. Should the sign of this subtraction calculation be indicated next to both instrument tags, or should a new tag be created and the subtraction indicated next to that new tag?
It is denoted by *Y—***, where Y represents calculation and conversion. For greater clarity, calculation symbols can be placed next to the graphic symbols, such as plus, minus, multiplication, division, square root, selection, etc.
A PID is a process flow diagram for instruments, showing only the most basic logical symbols, thereby indicating to those who view it that there is a certain logic involved. However, no special indication is given for the specific logic, which is often shown in the interlock logic diagram.