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Steel pipe is generally specified by Nominal Pipe Size (NPS) and Schedule and is commonly used in piping systems. Steel tube is usually specified by actual outside dimensions and wall thickness and is widely used in structural and mechanical applications.
However, the name alone cannot determine suitability. Buyers comparing welded steel pipes with tubing must also check the standard, dimensions, pressure or load, tolerances and connection method.
What Is the Difference Between Steel Pipe and Steel Tube?
The main differences concern sizing, shape and intended use.
| Comparison | Steel Pipe | Steel Tube |
|---|---|---|
| Size designation | NPS/DN and Schedule | Actual OD and wall thickness |
| Typical shapes | Round | Round, square, rectangular or oval |
| Main consideration | Flow, pressure, temperature and piping code | Dimensions, fit, load and fabrication |
| Common standards | ASTM A53, A106, API 5L | ASTM A500, A513 |
| Tolerances | Defined by the pipe standard | Often tighter for precision applications |
| Connections | Threaded, welded, flanged or grooved | Welded, formed, bolted or tube fittings |
| Typical uses | Water, gas, oil, steam and fire protection | Frames, machinery, vehicles and structures |
These are general distinctions rather than absolute rules. Some pipes are used structurally, while tubes are also found in boilers, heat exchangers and instrumentation systems.
How Are Steel Pipe and Steel Tube Sizes Specified?
Sizing is a common cause of ordering errors. A 2-inch pipe and a 2-inch tube do not normally have the same outside diameter.
How NPS and Schedule Define Pipe Size
NPS is a nominal designation, not a direct measurement of the inside or outside diameter. Each NPS corresponds to a standardized OD, while Schedule identifies a wall-thickness series.
Within the same NPS, a thicker wall reduces the inside diameter because the OD remains unchanged:
ID = OD − (2 × WT)
For example:
| Product | Actual OD | Wall Thickness | Approximate ID |
|---|---|---|---|
| NPS 2 Schedule 40 pipe | 2.375 in | 0.154 in | 2.067 in |
| 2-inch tube | 2.000 in | As specified | Depends on WT |
For NPS 12 and smaller, the actual OD is numerically larger than the NPS designation. From NPS 14 upward, NPS and OD are generally numerically equal.
How Actual Dimensions Define Tube Size
Tube is normally ordered by actual OD and wall thickness. Square and rectangular tubing is specified by its outside dimensions and wall thickness, such as 100 × 50 × 5 mm.
The stated dimension is still subject to the tolerance permitted by the applicable standard. For telescoping parts, buyers should specify OD, ID, wall thickness, straightness, ovality and internal weld-bead limits.
Refer to the steel pipe size chart before matching NPS pipe with fittings or existing systems.
How Do Shape, Tolerance and Manufacturing Differ?
Products governed by conventional piping standards are round. Tubing can be round, square, rectangular or specially shaped for structural, mechanical and architectural requirements.
Mechanical tubing often requires closer control of OD, ID, wall thickness, straightness and concentricity. Structural tubing follows the tolerances of its shape and product standard. This means tube is not automatically more precise; the required standard and purchasing specification determine the tolerance.
Both products can be welded or seamless. Pipe may be produced by ERW, LSAW, SSAW or seamless processes, while tubing may be welded, seamless, cold drawn or additionally sized. Manufacturing method alone does not define whether a product is a pipe or tube.
Surface finish also varies. Pipe can be black, galvanized, painted, lined or externally coated. Tube may be supplied mill-finished, cold-drawn, galvanized, painted or prepared for further fabrication. For structural frames, square and rectangular tubes provide flat surfaces that simplify welding and bolted connections.
Which Standards Apply to Steel Pipe and Steel Tube?
The product standard controls the material, manufacturing process, dimensions, tolerances, testing and marking requirements.
| Standard | Product Type | Typical Application |
|---|---|---|
| ASTM A53 | Welded or seamless pipe | Mechanical and pressure applications |
| ASTM A106 | Seamless pipe | High-temperature service |
| API 5L | Welded or seamless line pipe | Oil and gas pipelines |
| ASTM A500 | Structural tubing/HSS | Buildings, bridges and structures |
| ASTM A513 | ERW mechanical tubing | Machinery and precision components |
| EN 10210 | Hot-finished hollow sections | Structural applications |
| EN 10219 | Cold-formed hollow sections | Construction and general structures |
ASTM A53 covers pipe used in mechanical and pressure applications and ordinary steam, water, gas and air lines. ASTM A500 covers round, square, rectangular and special structural tubular shapes.
Buyers sourcing ASTM A500 structural tubing must still state the grade, shape, dimensions, wall thickness and length. A structural standard should not be used for pressure service unless the complete system design and applicable code permit it.
Is Steel Pipe or Steel Tube Stronger and More Expensive?
Neither product is inherently stronger. Strength depends on the material grade, dimensions, wall thickness, manufacturing condition and type of load.
For pressure service, engineers consider OD, wall thickness, material stress, operating temperature, corrosion allowance, joints and the piping code. A round section performs efficiently under internal pressure, but that does not make every steel pipe suitable for every pressure system.
For structural service, performance depends on cross-sectional area, moment of inertia, section modulus and load direction. Square and rectangular tubes can be efficient for bending and provide convenient connection surfaces. Round hollow sections often perform well under torsion and multidirectional loading.
Cost also depends on the complete specification. Precision tubing may cost more when it requires tight tolerances, cold drawing, internal bead removal or special finishing. Pipe may cost more when it requires seamless manufacture, a heavy wall, special alloy, coating, pressure testing or additional inspection.
A fair comparison must use equivalent material, dimensions, processing, testing and quantity.
How Should Buyers Choose Between Pipe and Tube?
Begin with the function, then identify the standard and dimensions required by the design.
| Requirement | Usually Select |
|---|---|
| Transport water, gas, oil or steam | Pipe made to the applicable piping standard |
| Use standard threaded, flanged or grooved fittings | NPS pipe |
| Form a square or rectangular frame | Structural tubing or HSS |
| Produce a precision mechanical part | Mechanical tubing |
| Create a telescoping assembly | Tubing with controlled OD, ID and weld bead |
| Support a structural load | Product specified by the structural design |
| Operate under internal pressure | Product approved by the piping code |
Round products require particular care because pipe and round tube can look similar. ASTM A53 may be used structurally when the design permits, while ASTM A500 round HSS is manufactured specifically for structural applications.
A complete RFQ should state:
- Product standard and grade
- NPS/DN or actual outside dimensions
- Schedule or wall thickness
- Shape and manufacturing method
- Length and dimensional tolerances
- End preparation and surface finish
- Testing and inspection documents
- Quantity and delivery destination
An inquiry that states only “2-inch steel tube” does not provide enough information for accurate production or pricing.
Confirm Your Pipe or Tube Specification with Baolai
Baolai supplies welded steel pipes, structural hollow sections and related steel products for piping, construction and industrial projects. Available services include customized dimensions and lengths, threading, grooving, coating, Mill Test Certificates and support for third-party inspection.
If a drawing or bill of materials uses pipe and tube inconsistently, send your steel pipe or tube requirements to Baolai. Include the application, standard, grade, dimensions, quantity and connection method so the team can identify specification or sizing conflicts before production.
Conclusion
The steel pipe vs steel tube decision begins with sizing and service. NPS pipe is generally suitable when a system relies on standardized piping dimensions, fittings and pressure requirements. Structural or mechanical tubing is usually preferable when actual dimensions, shape, fit or load performance are more important.
The final choice should be based on the applicable standard, grade, dimensions, tolerances, testing and project design rather than the product name alone.
FAQs
Can steel pipe and round tube have the same outside diameter?
Yes. Certain sizes may share the same numerical OD, but their wall thickness, tolerances, standards and permitted uses can differ. Confirm the actual dimensions and product standard before treating them as interchangeable.
Can standard pipe fittings be used with steel tubing?
Not automatically. Standard fittings are normally designed for specific pipe ODs, threads or groove dimensions. A tube should only be connected when its actual OD, connection system, pressure requirements and applicable code are compatible.
What tolerances should be specified for telescoping steel tubes?
Specify the maximum and minimum OD of the inner tube, the ID of the outer tube, wall-thickness tolerance, ovality, straightness and internal weld-bead height. Coating thickness should also be considered because it can reduce the available clearance.
Does Schedule 80 always have a higher pressure rating than Schedule 40?
Schedule 80 normally has a thicker wall than Schedule 40 for the same NPS, but Schedule is not a pressure rating. Allowable pressure also depends on material, temperature, pipe size, corrosion allowance, connections and the governing piping code.
