8 Cheat-sheet Tips on Reading Piping & Instrumentation Diagram (P&ID) Single Line Diagrams

A Guest Post by Siti Naqiah

While assigned to Keppel Seghers, AcePLP engineer Siti Naqiah worked on a Qatar water treatment plant project involving the drafting of Piping & Instrumentation Diagrams (P&ID). She shared her knowledge and experience in a sharing session with other colleagues, and compiled her top tips for reading P & ID single line diagrams.

1) Refer to the Legend

There are a lot of different types of valves or actuators or pumps/compressors (see figure 1). Even though the symbols are universal amongst different consultants, they look very similar, in particular, for valves. In every drawing, there will be many types of valves to place. All valves have different tag numbers when connecting to pipes (see figure 2). Each valve will also be indicated with NO or NC (meaning normally open or normally closed. So you have to take care when placing the valves into the drawings.  Also we offer you to read more on this topic.

You should familiarize yourself with the drawing legend. It’s a good habit to consistently refer to the legend as you draft. You shouldn’t assume that you know, because it could take years of drafting experience to really fully familiarize yourself with these symbols. Without going on-site and experiencing the equipment in a hands-on way, it would be extremely difficult to familiarize with the symbols!


Legends of Valves or Acutators or Pumps or Compressors

Figure 1: Legends of Valves or Acutators or Pumps or Compressors

Valves with tag number

Figure 2: Valves with tag numbers


2) Know your Line Numbers (Pipe Names)

The line number indicates the pipe name, and it is a way for consultants and engineers to identify particular pipe in a standardized way. The example below shows a deconstructed line number and a typical pipe number in a drawing is shown in figure 3.


P&ID Line and Tag Number example

Example 1: P&ID Line and Tag Number


Example of a line number

Figure 3: Example of a line number


3) Pipe Numbers Change after a Branch

This is to indicate a different pipe (see figure 4).

In figure 4, the different colours indicate different pipes. However, in a standard diagram, different colours indicate different systems. As long as the different pipes are within the same system, they are to be illustrated in the same colour.

How to Read the Pipe Line with the Line Number

Figure 4: How to Read the Pipe Line with the Line Number


4) Placement of the Line Number After a Pump/Taper

After a pump/taper (reducer), the line number will change (see figure 5). After a taper (reducer), the pipe diameter will change. For pumps, the diameter and material may stay the same but the pipe will be different after passing through the pump (a piece of equipment).

Placement of Line Number

Figure 5: Placement of Line Number


5) Reading the Flow-Arrow

The aesthetic of the drawing helps in reading the flow from one drawing to the next. This provides a practical way to present continuity amongst the different drawings in a project. See figure 6 for ways to read the flow-arrows.

The indication of the Flow-Arrow

Figure 6: The indication of the Flow-Arrow


6) Presentation of two different materials joined/welded together

Two different materials being joined or welded together can be indicated by a single or double perpendicular line (see figure 7 for examples of this).

Two different materials are joined or welded together

Figure 7: Two different materials are joined or welded together


7) Presentation of double-contained/containment

For pipes which are intended to be used for transportation of chemicals, a layering of two different materials will typically be used to prevent chemical leakage. See figure 8 for indications of double-containment.

Indications of Double-containment

Figure 8: Indications of Double-containment


8) Presentation of Equipment By External Supplier

When the source of equipment is from other suppliers or consultants, a text tag “BY OTHERS” will differentiate this (see figure 9).

Equipment from an External Supplier

Figure 9: Equipment from an External Supplier

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