The terms “pipe” and “tube” are used interchangeably, but in engineering and industrial applications, they are different products. If you are ordering materials for a project, you need to know which ones you actually need. Pipe is mostly for moving liquids or gases, while a tube is often used for structural work or specific mechanical parts.
- 1. What is a Pipe?
- 2. What is a Tube?
- 3. Key Differences Between Pipe and Tube
- 4. Sizing and Measurement
- 5. Manufacturing Tolerances
- 6. Manufacturing Methods
- 7. Applicable Standards
- 8. Applications: When to Use Pipe vs Tube
- 9. Pipe and Tube in Nickel Alloys and Stainless Steel
- 10. Frequently Asked Questions
- 11. Conclusion
What is a Pipe?
A pipe is a round cylinder used to carry fluids or gases under pressure. Pipes are sized using Nominal Pipe Size (NPS).
For sizes between 1/8 inch and 12 inches, the NPS number is just the nominal size, which represents an approximate internal diameter. It doesn’t exactly match the inside or outside diameter. For example, a 2-inch pipe doesn’t actually measure 2 inches on the outside. For 14 inches and larger, the NPS does match the outside diameter.
Wall thickness for pipe is called a schedule (like Schedule 40 or Schedule 80). If you increase the schedule, the wall gets thicker, the inside diameter gets smaller, but the outside diameter stays the same.
Pipe Standards:
- ASME B36.10M (Steel)
- ASME B36.19M (Stainless Steel)
What is a Tube?
Tubes are measured by their actual outside diameter (OD) and their wall thickness. If you order a 1-inch tube, the outside will measure exactly 1 inch. Tubes can be square, rectangular, or oval.
Tubes are made with much tighter tolerances than pipes. This means the measurements are more exact, which is why they often cost more. They are used for things like heat exchangers, hydraulic lines, or frames where parts need to fit together perfectly.
Key Differences Between Pipe and Tube
| Feature | Pipe | Tube |
|---|---|---|
| Sizing method | Nominal Pipe Size (NPS) | Actual OD and wall thickness |
| Dimensions | Nominal (not actual for small sizes) | Actual measured values |
| Wall thickness designation | Schedule number | Specific wall thickness (mm/inches) |
| Shape | Round only | Round, square, rectangular, oval |
| Tolerances | Wider | Tighter |
| Primary purpose | Fluid/gas transport | Structural, mechanical, heat transfer |
| Cost | Generally lower | Generally higher |
| Ordering specification | NPS + Schedule | OD + Wall thickness (or gauge) |
Sizing and Measurement
This is usually where pipes and tubes differ the most.
Pipe Sizing
For NPS 1/8 through NPS 12, the outside diameter (OD) is fixed, and the wall thickness changes based on the schedule.
| NPS | Actual OD (inches) | Schedule 40 Wall (inches) | Schedule 80 Wall (inches) |
|---|---|---|---|
| 1/2 | 0.840 | 0.109 | 0.147 |
| 1 | 1.315 | 0.133 | 0.179 |
| 2 | 2.375 | 0.154 | 0.218 |
| 4 | 4.500 | 0.237 | 0.337 |
| 6 | 6.625 | 0.280 | 0.432 |
| 8 | 8.625 | 0.322 | 0.500 |
Tube Sizing
These are straightforward. The OD is exactly as what is stated.
| OD (inches) | Wall Thickness (inches) | ID (inches) |
|---|---|---|
| 0.500 | 0.035 | 0.430 |
| 1.000 | 0.049 | 0.902 |
| 1.500 | 0.065 | 1.370 |
| 2.000 | 0.083 | 1.834 |
Manufacturing Tolerances
Tubes are manufactured to tighter tolerances than pipes. This is the main reason for the cost difference.
- Pipe: OD tolerance typically +/- 1% for larger sizes, with wall thickness tolerance of -12.5%
- Tube: OD tolerance as tight as +/- 0.005 inches, with wall thickness tolerance of +/- 10%
This precision makes tubing important in applications where exact fit, smooth flow, or precise heat transfer calculations are required.
Manufacturing Methods
Both pipes and tubes can be manufactured through seamless or welded processes:
Seamless
The process starts with a solid metal billet that is heated and pierced to form a hollow shell. This shell is then rolled or drawn until it achieves the needed size. As seamless products don’t have a weld seam, they are preferred for high-pressure applications.
Welded (ERW, EFW, SAW)
A flat strip or plate is formed into a cylinder and welded along the seam. Electric Resistance Welding (ERW) is common for smaller sizes. Submerged Arc Welding (SAW) is used for larger diameters. Welded products are more cost-effective and readily available.
Applicable Standards
Pipe Standards
| Standard | Coverage |
|---|---|
| ASTM A106 | Seamless carbon steel pipe for high-temperature service |
| ASTM A312 | Seamless and welded austenitic stainless steel pipe |
| ASTM A333 | Seamless and welded steel pipe for low-temperature service |
| ASTM B167 | Nickel-chromium-iron alloy seamless pipe and tube |
| ASTM B619 | Welded nickel-chromium-molybdenum alloy pipe |
| ASME B36.10M | Welded and seamless wrought steel pipe dimensions |
| ASME B36.19M | Stainless steel pipe dimensions |
Tube Standards
| Standard | Coverage |
|---|---|
| ASTM A213 | Seamless ferritic and austenitic alloy steel boiler tubes |
| ASTM A249 | Welded austenitic steel boiler tubes |
| ASTM A269 | Seamless and welded austenitic stainless steel tubing |
| ASTM B163 | Seamless nickel alloy condenser and heat exchanger tubes |
| ASTM B516 | Welded nickel-chromium alloy tubes |
Applications: When to Use Pipe vs Tube
Pipe Applications
- Moving oil, gas, or water.
- Steam lines in power plants.
- Fire sprinkler systems.
- Chemical processing (often using nickel alloys like Hastelloy C276 for corrosion).
Tube Applications
- Heat exchangers and condensers.
- Hydraulic systems.
- Building frames, handrails, or car roll cages.
- Instrument lines that need precise flow.
Pipe and Tube in Nickel Alloys and Stainless Steel
In tough environments like chemical plants or offshore rigs, standard steel isn’t enough. You’ll often see pipes and tubes made from specific alloys to handle heat and corrosion.
- Inconel 600/625: Used for high-temperature service and marine environments.
- Incoloy 825: Good for processing acids.
- Duplex S31803: Common on offshore platforms because it’s very strong and resists salt water.
You can get fittings for all of these to match the specific pipe or tube size you are using.
Frequently Asked Questions
Can pipe and tube be used interchangeably?
In most cases, no. Pipe and tube have different sizing conventions, tolerances, and design purposes. Using one in place of the other can result in poor fit, incorrect pressure ratings, or structural inadequacy.
Why is a tube more expensive than a pipe?
Tubes need more precise dimensions and undergo extra steps like cold drawing and bright annealing. These extra steps make them more expensive to produce, but create a more accurate, high‑precision product.
How do I specify a pipe correctly?
Specify pipe by NPS (nominal pipe size), schedule (wall thickness), material grade, and applicable ASTM standard. For example: “NPS 4, Schedule 40, ASTM A312 TP316L.”
How do I specify a tube correctly?
Specify tube by OD, wall thickness, material grade, and applicable standard. For example: “1.5 inch OD x 0.065 inch wall, ASTM A269 TP304L.”
What does “Schedule 40” mean?
Schedule is a standardised designation for pipe wall thickness. Schedule 40 is a medium-weight wall commonly used for general-purpose applications. Higher schedule numbers like 80 or 160 indicate thicker walls for higher pressures.
Conclusion
The difference between pipe and tube is more than just the terminology. Pipe is for moving fluids and is sized by NPS and schedule. Tube is for precision and structural work and is sized by its actual outside diameter. Using the right one ensures your project fits together correctly and handles the pressure it needs to. Always double-check your sizing standards before ordering.
Need nickel alloy or stainless steel pipe and tube for your project? Get in touch with us now.