Piping and Instrument Diagram (P&ID)

The Piping and Instrumentation Diagram is the road map for completing Piping Layout/Arrangement Drawings. The Piping and Instrumentation Diagram (P&ID) is developed from the Process Flow Diagram (PFD) and is a schematic drawing representing the process equipment, showing nozzles, all pipes and in-line fittings (valves, tees, reducers, strainers, sight glasses, drains, vents, sample points, control valves etc.) instruments and control loops (but not instrument piping), and some electrical data such as tracing and motor drives.

Larger plants will have a separate Utility Diagram which shows all utility lines servicing process lines and equipment. Utility lines would consist of steam, condensate, water, air and possibly inert gas etc. In a small plant all process and utility piping would be included in the P&ID .

The piping and instrument diagram (P&ID) provides a schematic representation of the piping, process control, and instrumentation which shows the functional relationships among the system components. The P&ID also provides important information needed by the constructor and manufacturer to develop the other construction input documents (the isometric drawings or orthographic physical layout drawings).

P&ID SHOULD show the following :

• Instrumentation and designations 
• Mechanical equipment with names and numbers 
• All valves and their identifications 
• Process piping, sizes and identification 
• Miscellaneous - vents, drains, special fittings, sampling lines, reducers, increasers andswagers 
• Permanent start-up and flush lines 
• Flow directions 
• Interconnections references 
• Control inputs and outputs, interlocks 
• Interfaces for class changes 
• Seismic category 
• Quality level 
• Annunciation inputs 
• Computer control system input 
• Vendor and contractor interfaces 
• Identification of components and subsystems delivered by others 
• Intended physical sequence of the equipment 

       

P&ID SHOULD NOT show the following :

• Instrument root valves 
• control relays 
• manual switches 
• equipment rating or capacity 
• primary instrument tubing and valves 
• pressure temperature and flow data 
• elbow, tees and similar standard fittings 
• extensive explanatory notes 

The P&ID provides direct input to the field for the physical design and installation of field-run piping. For clarity, it is usual practice to use the same general layout of flow paths on the P&ID as used on the system flow diagram. The P&ID ties together the system description, the system flow diagram, the electric control schematic, and the control logic diagram. It accomplishes this by showing all the piping, equipment, principal instruments, instrument loops, and control interlocks. The P&ID contains a minimum amount of text in the form of notes (the system descriptions minimize the need for text on the P&ID). The first P&ID in the set for the job should contain a legend defining all symbols used; if certain symbols are defined elsewhere, it may be appropriate to only reference their source. The P&IDs are also used by the start-up organizations for preparing flushing, testing, and blowout procedures for the piping system and by the plant operators to operate the system. The correctness and completeness of the SD, SFD, and P&ID drawings are crucial to the success of the start-up program.

P&IDs play a significant role in the maintenance and modification of the process that it describes. It is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect. During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as the hazard and operability study (HAZOP).