Standards - Piping and Instrumentation Diagram (P&ID)

A P&ID is a piping & Instrumentation diagram which is a graphical representation of the functional relationships between the equipment, instruments and piping system of a process.

What is a PFD

A process flow diagram (PFD) is a diagram used to show the flow of materials and the main equipment in a process. In general, only the equipment that is in direct contact with the effluent and chemicals is shown. The PFD therefore shows conceptually and without detail the relationships between the main elements of the system (global view). In addition to the major equipment, the incoming and outgoing streams are shown with their flow rates and names.


Here is an example of a PFD for an osmosis system:


What is a P&ID

A P&ID is a piping & Instrumentation diagram which is a graphical representation of the functional relationships between the equipment, instruments and piping system of a process. It is therefore a more detailed diagram than the PFD, and therefore more precise and complete. All the elements of the process are shown on the P&ID according to their symbol, namely

  • equipment (tank, pump, osmosis vessel, etc.);
  • Instrumentation (flow meter, transmitter, pressure, manometer, probes, etc.)
  • Control elements (alarms, signals, etc.);
  • All valves (manual, automatic, control, sampling);
  • Process piping;
  • Interconnection reference;
  • Vents;
  • Drains;
  • Special fittings;
  • Sampling lines;
  • Equipment and line identification;
  • Flow directions;
  • Control inputs and outputs.

The P&ID is not an isometric drawing of the plumbing and is not a physical 2D representation of the process. The spatial positioning of the equipment is shown by the general arrangement drawings which are usually taken from the 3D model of the system. However, the inlets and outlets of a tank are represented at least on the correct walls and shown to indicate their positioning (top, middle or bottom of the tank).


  1. Inflows start from the left. Outflows leave from the right. Both are vertically aligned whenever possible.
  2. Valves are indicated horizontally with "head up" whenever possible. When they are to be shown on a vertical line, they are rotated 90 degrees counterclockwise, i.e., the actuator, if present, is on the left and the text on the right. The text is also rotated to follow the direction of the symbol. Clockwise rotation is not allowed, regardless of the direction of flow. When the symbol indicates "one way", it is mirrored on its vertical axis and the text remains unchanged.
  3. Whenever possible, elements are arranged to flow from left to right only in accordance with the logic of the process. Circuits are allowed as long as the logic remains clear and consistent. For a very linear process that requires more than one page width, it is best to arrange it in two "rows" on the page. Page change arrows can be used to avoid a long line connecting the two sections of the diagram.

Legend - Notes on Interpretation and Tagging

  1. P&ID tags must have an unwritten project prefix (which we write on the physical tags, but not on the P&IDs or anything other than a note to that effect). For example: SO12345-AE-101
  2. The P&ID tags will have 3 digits and will be unique within the project. For example, you cannot have a valve with the number 101 and a pump with the number 101. This means that the numbering is sequential in the correct sequence (see point 4 below), so you can have PT-101, PT-102, AE-103, PT-104, etc. Note that we cannot have a PT-103 as was sometimes the case when the numbering was by instrument/equipment type.
  3. The only exception to the above rule is when the P&ID tags are explicitly identical (other than the type identifier (AE, TE, AV, etc.)) because they are all for related items in a control loop. For example, an AV-101 automatic valve with an SV-101 solenoid (NB: the AV and SV designations may be incorrect based on our current legend and are only given as an example).
  4. The following series of identifiers will be used (this, in particular, must be reviewed for logic):
  5. 000-099: Equipment ;
  6. 100-199: Instrument loops;
  7. 200-399: Manual valves;
  8. 400-499: Automatic valves (note that control valves will share the instrument loop number);
  9. 500-599: Sample points; and
  10. 600-999: Lines.

in FAQ
What is pressure drop when it comes to piping?
Pressure drop refers to the loss of pressure within the piping system used for the movement of fluids.