Introduction on How to Read Isometric Drawings

Hey there, QA and QC inspectors! Today we are breaking down how to read an ISO drawing for your inspection work. No fancy jargon—just straight talk on what you need to know in the field.

What is an Isometric or ISO drawing ?

An ISO drawing is your 3D view of the piping. Unlike flat drawings, ISOs show pipes from an angle so you can see how everything fits together in the real world. This matters when you’re trying to figure out where welds go and how components connect.

Step 1: The Title Box

Start at the bottom-right corner. This tells you:

• Project name: Make sure you’re looking at the right job
• Drawing number: Each drawing has its own number
• Revision number: Check this carefully! Rev 2 might have different welds than Rev 1
• Date: When it was last updated

As a welding inspector, always verify you’re using the same revision that’s in your ITP (Inspection Test Plan). The wrong drawing means rejected welds and rework.

Step 2: The Grid System

ISO drawings use a system like this:

• Flat across the page (like looking East/West)
• Angled up-right at 30° (like looking North/South)
• Straight up (like looking at height)

This helps you locate where pipes and welds are positioned. When your foreman says “check the welds at coordinates E-5,” you’ll know where to go.

Step 3: Reading Pipe Lines

Pipes are shown as simple lines:

• Solid lines: Pipes you can see
• Dashed lines: Buried or hidden pipes
• Line thickness sometimes shows pipe size

For welders, follow these lines to see where your joints will be. For inspectors, these lines show your inspection path.

Step 4: Component Symbols

Here’s what you’ll commonly see:

• Valves: Various symbols depending on type (gate, globe, check)
• Elbows: Shown as angles in the pipe
• Flanges: Short lines across the pipe
• Welds: Small ticks or dots where pipes join

As a welder or inspector, pay special attention to where these symbols appear—they’re often critical connection points requiring specific weld procedures.

Step 5: Making Sense of Dimensions

The numbers tell you:

• How long each pipe run is (usually center to center)
• Pipe diameter (like 2″, 4″, 6″)
• How high or low pipes run compared to ground level
• Angles and slopes of pipes

Welders: These dimensions determine your fit-up. Inspectors: Use these to verify proper installation before approving welds.

Step 6: The Pipe Code

Each pipe has an ID code like: 6″-CS-PS-1201

Breaking this down:

• 6″ = Pipe size (diameter)
• CS = Carbon Steel (material)
• PS = Process Steam (what’s flowing through it)
• 1201 = Line number

This tells you what welding procedure to use and what inspection requirements apply. Different materials need different rod types and welding parameters.

Step 7: Weld Information – This Is Your Money Maker

For welders and inspectors, this is the critical part:

• Weld numbers: Each weld has a unique ID (like W-101)
• Weld symbols: Tell you what type of weld (butt, fillet, socket)
• Weld notes: Special requirements (100% RT, PWHT required, etc.)

Your ITP will reference these weld numbers when calling out inspection points. When documenting, always use these exact numbers to avoid confusion.

Step 8: Test Points

Look for marked spots that show:

• Where pressure tests connect
• NDT testing locations (RT, UT, MT, PT)
• PMI check points

As an inspector, these tell you exactly where to perform your tests. Make sure these points are accessible before welding is complete.

Step 9: The Parts List (BoM)

This is your inventory checklist:

• Item numbers match what’s on the drawing
• Descriptions of each part
• Material grades (A106-B, SS316L, etc.)
• Quantities needed

Before welding, check that you have the right materials. During inspection, verify what was installed matches what was specified.

Step 10: Comparing with P&IDs

Always check your ISO against the P&ID (the system flow diagram):

• ISO shows you how to build it
• P&ID shows how it’s supposed to function

If they don’t match, stop and ask questions. Better to delay than to weld something wrong.

Field Tips for Welders and Inspectors

When you’re on the job:

1. Orient yourself first—figure out which direction on the drawing matches reality
2. Mark up your drawing with a highlighter as you go
3. Take photos of completed welds with the drawing visible for documentation
4. Measure twice before confirming dimensions
5. When in doubt about a symbol or note, ask rather than guess

Remember: Every weld you make or inspect needs to match what’s on this drawing. The ISO is your roadmap to getting paid and avoiding costly rework.

By understanding these drawings, you’ll work faster, smarter, and with fewer mistakes. Whether you’re laying down beads or checking them, these skills make you more valuable on any job site.

Want to take your skills to the next level? Contact us about our specialized training for welders and inspectors!

Read it right, weld it right, inspect it right!