Piping Basics for Beginners: Understanding Run, Set, Travel & Elbow Take-Off Calculations

In industrial piping design and fabrication, dimensional accuracy is critical. A small calculation error in spool length can cause misalignment, costly rework, and project delays.

This guide explains the fundamental piping geometry concept behind Run, Set, Travel, and how to calculate the correct Spool To Be Cut (STBC) when elbows are involved.

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1. The Geometric Foundation: Run – Set – Travel

When a pipe changes direction using elbows, the straight section between them forms an angled segment. This geometry can be represented as a right triangle:

• Run – Horizontal projection
• Set – Vertical projection
• Travel – True diagonal centerline length

Trigonometric Relationships

cos(θ) = Run / Travel

sin(θ) = Set / Travel

Travel = √(Run² + Set²)

Where θ is the elbow angle.

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2. What Is Travel?

Travel represents the true centerline distance between the tangent points of two elbows. It is not the pipe cut length because elbows consume part of that distance.

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3. Understanding Elbow Take-Off (TO)

The Take-Off (TO) is the distance from the elbow centerline to its tangent point. For Long Radius (LR) elbows:

TO = tan(Degree / 2) × NPS × 38.1

• Degree – Elbow angle (45°, 90°, etc.)
• NPS – Nominal Pipe Size (in inches)
• 38.1 mm – LR elbow radius factor per inch

Why 38.1 mm? Because Long Radius elbows have a radius equal to 1.5 × NPS, and 1.5 × 25.4 mm = 38.1 mm.

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4. Spool To Be Cut (STBC)

The correct pipe cutting length is calculated as:

STBC = Travel − (2 × TO) − (2 × WG)

• Travel – True centerline length
• TO – Take-off per elbow
• WG – Weld Gap

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5. What Is Weld Gap (WG)?

The Weld Gap is the clearance between the pipe end and the elbow to allow proper welding.

Typical values:

• 2 mm
• 3 mm
• Project-specific per WPS

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6. Practical Example

Given:

• NPS = 6"
• Elbows = 45°
• Run = 2000 mm
• Set = 2000 mm
• WG = 3 mm

Step 1 – Travel

Travel = √(2000² + 2000²)

Travel = 2828 mm

Step 2 – Take-Off

TO = tan(22.5°) × 6 × 38.1

TO ≈ 94.6 mm

Step 3 – STBC

STBC = 2828 − (2 × 94.6) − (2 × 3)

STBC ≈ 2633 mm

Final pipe cut length ≈ 2633 mm

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7. Why This Matters in BIM & 3D Modeling

Modern software such as AutoCAD Plant 3D, AVEVA E3D, and Revit calculates these values automatically. However, understanding the manual calculation ensures:

• Better fabrication control
• Accurate field adjustments
• Reduced rework
• Improved spool accuracy

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8. Common Beginner Mistakes

• Using center-to-center distance as cut length
• Ignoring weld gap
• Mixing units (inch/mm)
• Using short radius data instead of long radius
• Forgetting to divide angle by 2 inside the tangent function

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9. Final Takeaway

Piping geometry is simple trigonometry applied to real-world fabrication.

Mastering Run, Set, Travel, Take-Off, and Weld Gap gives you a strong foundation in piping fabrication and spool development.