Piping One-Line Drawing

Written by Matt Milbury on . Posted in Mechanical Engineering

A piping single line drawing (or piping one line drawing) is a piping drawing that shows the size and location of pipes, fittings and valves. It is drawn to scale so the relationships of the aforementioned are correctly shown. Single line drawings take less time to create and show all the information required to layout and fabricate a piping system. 

Single line drawings in piping design

This article contains information on single line drawings as part of the layout process and in final design

Conceptual dEsign - Piping Layout

These drawings are the first step in converting the information on a P&ID to what tradespeople will use to fabricate the installation. These drawings are often hand drawn and are used as a tool for the designer to begin to layout pipe on a pipe rack. The designer should use the equipment location plan, site survey data, P&ID and other relevant drawings to produce an optimal piping layout. There may be several iterations of a single line piping drawing.  

Before the widespread acceptance of 3D software, a piping one line was one of the most critical steps in the design process. However, with the advent of 3D piping software, this changed as the 3D operator can layout and arrange the piping in many different configurations with ease.  Some companies will be structured so a senior piping designer creates a hand drawn single line which will be given to a 3D CAD operator or drafter to be drawn up. This approach keeps piping designers designing pipe while training the drafters how to layout pipe.

Front End Engineering Design (FEED)/ Detailed Design

On piping drawings that are issued for construction or fabrication, piping 12” and under is usually shown as a single line. The reason for this is at smaller diameters, it can be difficult to see details when the drawings are scaled with larger piping.

For simple pipe systems, single line piping drawings may be all that is needed. For more complicated systems or for instances where three dimensional piping software is being utilized, additional drawings may be created.

There are several differences between this drawing and an isometric drawing: First, isometric drawings are not drawn to scale. Secondly, isometric drawings are used for single pipe spool fabrication and do not show the relationship of these spools to other pieces of pipe. Finally, a single line piping drawing is an orthographic drawing (front, side, top views) and is a type of a piping arrangement drawing.

Single Line Piping Drawing Tips

  • The pipe centerline is used to represent the pipe location relative to other pieces of pipe or on a pipe rack. Dimensions between pipe should be centerline to centerline. Note: this is the same as dimensioning double line pipe.
  • Use standard pipe symbols as opposed to equipment specific outline drawings. Standard drawings should match the dimensions of the equipment and fittings. E.g. face-to-face dimensions should match, elbow and branch dimensions should be accurate. Etc.
  • Permanent connections, such as weld connections are shown with a heavy dot.
  • Pipe, flanges and fittings are drawn as heavy lines. Pipe supports and large pieces of equipment are shown as thin lines.
  • Adding line numbers is an easy way to cross reference that all piping on the P&ID is shown on the piping arrangement drawing.
  • Avoid unnecessary dimensioning whenever possible.
  • If possible, equipment, instrument locations & numbers, relevant pipe supports and other items that affect the piping design should be shown on these drawings.