Flowsheet symbols Instruments

Figure 1-20B. Commonly used instruments for process instrumentation flowsheets. Adapted by permission, ISA Std. ANSI Y32.20—1975, ISA S5.1—1973, Instrumentation Symbols and Identification," Latest edition, 1984.

Rather less freedom is allowed in the construction of mechanical flowsheets. The relative elevations and sizes of equipment are preserved as much as possible, but all pumps usually arc shown at the same level near the bottom of the drawing. Tabulations of instrumentation symbols or of control valve sizes or of relief valve sizes also often appear on P I diagrams. Engineering offices have elaborate checklists of information that should be included on the flowsheet, but such information is beyond the scope here. 

Figure 3.4. Symbols for control elements to be used on flowsheets. Instrument Society of America (ISA) publication no. S51.5 is devoted to process instrumentation terminology.

The symbols used to show the equipment, valves, instruments, and control loops will depend on the practice of the particular design office. The equipment symbols are usually more detailed than those used for the process flowsheet. A typical example of a P and I diagram is shown in Figure 5.29. 

In order for flowsheets and P IDs to be understood by people with different job responsibilities such as plant designers, process engineers, instrumentation specialists, and vendors, it is useful to use standardized symbols and conventions on the flowsheets. Standards concerning instrumentation symbols and flowsheet conventions have been developed by technical societies, such as the International Society of Automation (ISA). However, individual companies often use different or additional symbols for particular processes. 

Additional information concerning instrumentation symbols and flowsheets is available from ISA (1992) and the Instrument Engineer s Handbook (Liptak, 2003). 

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