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How to read P&IDs
Instrumentation detail varies with the degree of design complexity. For example, simplified or conceptual designs, often called process flow diagrams, provideless detail than fully developed piping and instrumentation diagrams (P&IDs). Being able to understand instrumentation symbols appearing on diagrams means understanding ANSI/ISA’s S5.1-1984 (R 1992)Instrumentation symbols and identification standard. S5.1 that defines how each symbol is constructed using graphical elements, alpha and numeric identification codes, abbreviations, function blocks, andconnecting lines.
Deciphering symbols ISA S5.1 defines four graphical elements—discrete instruments, shared control/display, computer function, and programmable logic controller—and groups them intothree location categories (primary location, auxiliary location, and field mounted). Discrete instruments are indicated by circular elements. Shared control/display elements are circles surrounded bya square. Computer functions are indicted by a hexagon and programmable logic controller (PLC) functions are shown as a triangle inside a square. Adding a single horizontal bar across any of the fourgraphical elements indicates the function resides in the primary location category. A double line indicates an auxiliary location, and no line places the device or function in the field. Deviceslocated behind a panel-board in some other inaccessible location are shown with a dashed horizontal line Letter and number combinations appear inside each graphical element and letter combinations aredefined by the ISA standard. Numbers are user assigned and schemes vary with some companies use of sequential numbering, others tie the instrument number to the process line number, and still others adoptunique and sometimes unusual numbering systems. The first letter defines the measured or initiating variables such as Analysis (A), Flow (F), Temperature (T), etc. with succeeding letters defining...