Symbols and layout

Standard symbols for instruments, controllers and valves are given in the British Standard BS 1646. 

Austin (1979) gives a comprehensive summary of the British Standard symbols, and also shows the American standard symbols (ANSI) and examples of those used by some process plant contracting companies. 

When laying out the diagram, it is only necessary to show the relative elevation of the process connections to the equipment where these affect the process operation for example, the net positive suction head (NPSH) of pumps, barometric legs, syphons and the operation of thermosyphon reboilers. 

Computer aided drafting programs are available for the preparation of P and I diagrams, see the reference to the PROCEDE package in Chapter 4. 

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