When making plant additions or modifications, it is sometimes best to continueusing the existing symbology familiar to plant personnel. However, new facilities should use modern ISA symbology to ensure clear communications with installation personnel.
An ISA symbol exists or can be adapted for almost any instrument. Equipment symbols are another matter. It is often necessary to elaborate on ICM-EF-824A for complex machinery such as compressors, multistage pumps, and materials handling equipment.
All project-specific symbols and other unusual symbology should be clearly recorded either on the project P&ID symbol drawings (if incorporated as project drawings) or in the notes column of the P&IDs themselves. Such symbols must also
be used consistently throughout the project. This is vitally important because P&IDs may be the only drawings available to those unfamiliar with a particular project or facility, such as engineers involved in facility additions and modifications. Often what is regarded as a universal “standard” symbol by one organization is found to be different elsewhere.