What Is a Piping Isometric Drawing? Symbols and Bill of Materials Guide

A piping isometric drawing is a single-lined, three-dimensional drawing of one pipe line, prepared on an isometric grid and not to scale. This drawing includes pipe line, dimensions, fittings, welds, and bill of materials (BOM). This guide will include the components of a piping isometric drawing, meanings of the commonly used symbols, reading line numbers and spool numbers, and preparation of bill of material with an example of bill of material. The notes on shop-floor errors come from fabricating pipe spools against customer isometrics at Kalpataru Piping. 

What is Piping Isometric Drawings?

A piping isometric drawing or isometric piping drawing is a drawing that shows one pipeline between one terminal point and another. It is drawn isometrically – isometric means that the pipe is drawn as if viewed from a corner, showing the length, width and height all in the same drawing. The drawing is not drawn to scale. This is an eye-catcher for beginners, because there is a reason: If there are many fittings in a congested area then it would not be readable if it is drawn on scale. The drafter puts the parts apart so that they can be read and the written dimensions represent the actual lengths. Read the numbers – do not use a ruler to measure the line. Most of the isometrics for large projects are normally created automatically from the 3D plant model. Hand-drawn or simple CAD isometrics are still frequently used for small jobs, repairs and field changes.

Isometric vs P&ID vs General Arrangement Drawing

These three drawings are often confused. Each one answers a different question for a different team.

In short, the P&ID tells you what the process needs, the GA tells you where things sit in the plant, and the isometric tells you exactly how to build one line.

Why Piping Isometrics Matter for Fabrication and Procurement

The isometric is the document that connects design to the workshop and the purchase desk. Four teams depend on it.

• Material take-off: every pipe length, fitting, flange, bolt and gasket is counted from the isometric and entered in the BOM.
• Shop fabrication: spools are cut, fitted and welded using the isometric dimensions and weld marks.
• Inspection: weld numbers and NDT requirements trace back to the drawing, which makes the inspection record auditable.
• Site erection: tie-in points, field welds and routing notes guide installation crews.

A BOM line that conflicts with the pipe class is a typical source of rework. A flange ordered one rating below the line’s pipe class is only caught when the spool reaches inspection, after cutting and welding are done.

Anatomy of an Isometric: Title Block, Drawing Body and BOM Area

Most isometrics follow a similar layout, even though companies format them differently.

• Title block: drawing number, line number, pipe class, design pressure and temperature, from and to points, revision and approvals.
• Drawing body: the pipe route, components, dimensions, elevations and notes.
• BOM area: usually at the bottom or on the right side, listing every item by item number.
• Orientation: a north arrow or direction symbol, normally at the top, so the pipe can be matched to the plot plan.

Understanding the Isometric Grid, Offsets and Direction

On an isometric grid, one axis is vertical and the other two run at 30 degrees from the horizontal. That places the three axes 120 degrees apart. You will sometimes see this described as a “60 degree” angle, which is the angle between each sloping axis and the vertical line measured the other way round. Both descriptions point to the same grid. Vertical pipe runs straight up and down, and horizontal pipe in the two plan directions follows the two sloping lines.

Types of Offsets

Not all pipe runs along the three main axes. A pipe that shifts position is called an offset, and isometrics show three kinds:

• Vertical offset: the pipe moves up or down while staying in one vertical plane.
• Horizontal offset: the pipe moves sideways in the horizontal plane.
• Combined (rolling) offset: the pipe moves in more than one direction at once, and a small right angle triangle on the drawing shows the travel in each direction.

Elevations and slope

Elevation callouts, written as EL followed by a height, tell you the level of the pipe centreline above a datum. Slope notes show where the pipe must fall in a given direction, for example for drainage.

Piping Isometric Symbols and What They Mean

Symbols are the shorthand of the isometric. The table below covers the ones you will meet most often. Symbol conventions vary by company and project, so always check the legend on the drawing you are working from.

A few of these deserve a closer look. Fittings such as elbows, tees and reducers are normally buttweld fittings on larger lines, and the drawing shows only their position, while the BOM carries their size, schedule and grade. Flanges work the same way: the drawing shows where the flange sits, and the BOM tells you whether it is a weld neck flange, a slip-on or a blind, along with its face and rating.

Weld symbols matter just as much. The difference between a shop weld and a field weld decides what is done in the workshop and what is left for site, and it affects spool length and transport. If you want to understand how the weld type changes the joint itself, our comparison of socket weld vs butt weld connections is a useful companion read.

Reading Line Numbers, Spool Numbers and Pipe Class

The line number

Every pipeline has a unique line number, and it usually packs several facts into one string. Here is an example we made up for illustration: 6″-PW-1024-A1A

• 6″ is the nominal pipe size.
• PW is the service code, here process water.
• 1024 is the sequence number of the line.
• A1A is the pipe class.

Companies write line numbers in different formats, so confirm the format in the project’s line list.

The spool number

A long line is rarely fabricated or shipped as one piece. It is divided into spools, which are sections sized for fabrication and transport, and then joined on site at the field welds. Each spool gets its own number, for example 1024-SP01, 1024-SP02, and so on. The spool number lets the shop, the inspector and the site crew refer to exactly the same piece. For the material side of this, see our ranges of stainless steel pipe spools and alloy steel pipe spools.

The pipe class

The pipe class is the key to the material. It links to a piping specification that fixes the pipe material and grade, wall thickness, flange rating, fitting types, gasket and bolting, and corrosion allowance. If you know the pipe class, you know what every component on the line must be made of. This is also how errors are caught, because anything in the BOM that conflicts with the pipe class is wrong.

What Is the Bill of Materials on a Piping Isometric?

The bill of materials is the list of every component needed to build the line shown on the isometric. It is produced from the drawing during material take-off, and it is what the buyer uses to order and what the store uses to issue material to the shop.

Typical BOM columns

• Item number: matches the balloon numbers on the drawing.
• Quantity or length: number of pieces, or metres of pipe.
• Description: what the item is, for example “90 degree long radius elbow, BW”.
• Size: nominal pipe size.
• Material and grade: for example ASTM A106 Gr B.
• Schedule or rating: wall thickness schedule, or flange class.
• End connection: buttweld, socket weld, flanged or threaded.
• Standard: the governing dimensional or material standard.

Item numbers are the link between drawing and list. Each balloon on the isometric points to one row in the BOM, so you can always go from a fitting on the drawing to its full description.

Related lists

Some isometrics also include a cut pipe list (the length of each pipe piece), a weld list (every weld with its number and type) and a bend table where bends are used. These sit next to the BOM and are used by the shop and the inspector.

BOM vs MTO

The BOM belongs to one isometric. The material take-off (MTO) adds up the BOMs of many isometrics into a total quantity for purchasing. Buyers work from the MTO, while the fabricator works from the BOM of each drawing.

Sample BOM

The table below is an illustrative example for a 6 inch carbon steel line. Always confirm every value against the project specification and the relevant ASME tables before ordering.

Item 1 is the pipe, which in this case is ASTM A106 Gr B pipe. Its size and wall thickness can be checked against a pipe size chart and a pipe schedule chart. For item 3, the Class 150 rating is explained in our guide on how flange ratings work. Item 6 uses nuts to the standard covered in ASTM A194 nuts and bolting. If the line has branches, the branch fittings also appear in the BOM, for example weldolets.

How Piping Isometrics Are Created

There are two methods of creating isometrics. They are created using design software like AutoCAD Plant 3D, AVEVA E3D or Hexagon Smart 3D, which is then fed by a Piping Component File (PCF) into the isometric software in order to maintain consistency in drawings and BOMs for large projects. When making isometric drawings or making changes in the field or repair work on small jobs, drafters use isometric grid paper or CAD to create the BOM from the finished drawing. In either case, each illustration must be reviewed to ensure proper dimensions and component details and be complete before approval and release.

How to Read a Piping Isometric Drawing, Step by Step

1. Read the title block first: line number, pipe class, design conditions, from and to points and the revision letter. Stop if the revision is not the latest issued.

2. Find the north arrow and match the drawing to the plot plan.

3. Trace the line from the start point to the end along the flow arrow, noting each change of direction, elevation (EL) and spec break.

4. Check each symbol against the legend, then read the dimensions rather than measuring the lines.

5. Tie each balloon number to its BOM row and check that material, rating and end connection agree with the pipe class.

6. Check shop and field weld marks against the spool breakdown before cutting.

Common Mistakes When Reading or Preparing Isometrics

• Reading dimensions as scaled lengths. The drawing is not to scale.
• Ignoring revision letters, so work proceeds on an old drawing.
• Skipping notes and the legend, where special requirements are often hidden.
• Confusing shop and field welds, which changes what is built in the workshop.

Conclusion

Three checks catch most isometric problems: read the dimensions instead of measuring the lines, match every symbol to the legend, and match every balloon number to a BOM row that agrees with the pipe class. Understanding piping isometric drawings helps reduce fabrication errors, avoid material mix-ups, and improve installation accuracy. If you are preparing spools from an existing isometric, you can send the drawing and BOM to Kalpataru Piping for a review before fabrication planning.

Frequently Asked Questions