Question - Why to use P&ID? 
          
        
        Answer - 
        
To describe all plant and equipment. This indicates the flows in a plant and the sensors or other devices in question. It gives names and parameters to each sensor and other equipment.
It is a pictorial image of the processing facilities:
- Key piping and instrument details
 - Control and shutdown schemes
 - Safety and regulatory requirements
 - Basic start up and operational information