Structural tube vs. Fluid tube

Although both structural tubes and fluid conveyance tubes are steel pipes, they differ fundamentally in their design principles, standards, and quality control.

Key differences between structural tube and fluid conveyance tube:

Structural tubes: Their core function is to bear loads (support, bending, and tension, etc.). Like the "skeleton" of a building, they prioritize mechanical strength and stability.
Fluid Conveyance Tubes: Their core function is to safely and tightly transport fluids (water, gas, oil, etc.). Like the "blood vessels" of a building, they prioritize pressure-bearing capacity and sealing.

Absolutely Do Not Mix: Using structural tubes in pressurized fluid systems is strictly prohibited and extremely dangerous.

Structural tube:

Structural tubes are steel pipes used for load-bearing purposes in building structures, mechanical components, etc. They can be seamless steel tubes or welded steel tubes (such as ERW and LSAW). The most commonly used material is carbon steel, which can be divided into ordinary carbon structural steel and high-quality carbon structural steel. Structural pipes have a wide range of applications and are used in large quantities. They are mainly used in railways, bridges, building engineering, towers, mechanical frames, etc., to manufacture various metal structural components and mechanical parts that bear static and dynamic loads.

The main performance requirements for structural tubes are:
1. They must have good mechanical load-bearing capacity (tensile, bending, and compressive strength) and must not break or deform excessively under operating loads.
2. They must be easy to install; installation can be completed quickly according to general standards.
3. They must be durable and have a long service life under normal operating conditions.

Fluid tube:

Fluid tubes are seamless or welded pipes used to transport various liquids and gases (such as oil, natural gas, water, steam, etc.). Due to the special nature of the media they transport, fluid tubes have the following core requirements:

1. Good sealing performance: No leakage is allowed during transportation, especially for flammable, explosive, or toxic media.
2. Corrosion Resistance: Many transported media are corrosive, and it is necessary to ensure that the pipeline does not experience corrosion failure within its expected lifespan.
3. Smooth Inner Wall: Reduces fluid transport resistance and lowers energy consumption.

First, strictly speaking, they cannot be shared. Structural tubes require good pressure bearing capacity, while fluid pipes require good sealing performance. Therefore, the uses of the two are very different. Try not to use the wrong area.

Secondly, structural pipes have high requirements on cost, otherwise some steel tubes are not up to standard in terms of corrosion resistance or pressure bearing capacity, and are easily damaged. If water and food are transported by fluid pipelines, the hygienic requirements are stricter. In certain low-pressure, non-critical applications, substitution can be considered when specific conditions are met, but this must be calculated by professionals.

Structural tube vs. Fluid tube:

Below, we provide a detailed comparison from multiple perspectives:

1) Structural Tube

Core Function: Bearing Structural Loads (Support, Bending, and Torsion)
Common Standards: ASTM A500, ASTM A1085 (Higher Performance), GB/T 6728
Typical Applications: Building Frames, Mechanical Structures, Bridges, Vehicle Chassis, and Scaffolding. Hydrostatic Testing: Individual pipes are typically not subjected to hydrostatic testing.

Chemical Composition: The focus is on final mechanical properties, with relatively loose control over chemical composition. Carbon equivalent may be higher.
Key Performance Indicators: Yield strength, tensile strength, elongation, and cross-sectional geometry.

Production Process and Inspection: The production process primarily ensures mechanical properties and dimensions. Inspection focuses on mechanical property testing and dimensional measurement.
Appearance and Defect Permits: Certain surface defects that do not affect structural strength (such as minor scratches and scale) are permitted.

2) Fluid Tube

Core Function: Safely transport fluids under pressure (sealing and pressure resistance).
Common Standards: ASTM A106 (high temperature and high pressure), ASTM A53 (low and medium pressure), GB/T 8163.
Typical Applications: Oil/gas pipelines, steam pipelines, boiler piping, hydraulic systems, chemical equipment, and pipelines in the food and beverage industry.
Hydrostatic Testing: Hydrostatic testing is mandatory for each pipe, as it is the bottom line for ensuring the safety of pressure vessels.

Key performance indicators: Hydrostatic testing, non-destructive testing (NDT), chemical composition, and tolerance standards for defects.
Chemical elements: Chemical composition (such as the content of harmful elements such as S and P) is strictly controlled to ensure excellent weldability, toughness, and corrosion resistance.

Production process and inspection: The production process has higher requirements, and each pipe must undergo hydrostatic testing (testing to a pressure of at least 1.5-2 times the standard pressure). Non-destructive testing (such as eddy current ET and ultrasonic UT) is usually required to ensure the absence of defects.
Appearance and defect tolerance: Surface defects (such as cracks, scars, and folds) are subject to extremely strict limits, as these defects can become cracking points under pressure.

What are the risks of mixing structural pipe and fluid pipe?

Using structural pipe in fluid transportation systems is a significant safety hazard for the following reasons:

1. Unknown pressure capacity: Structural pipe has not been verified through hydrostatic testing. Tiny defects (such as cracks and inclusions) invisible to the naked eye may exist on the pipe wall. Under pressure, these defects can expand and eventually cause the pipe to burst.
2. Leakage Risk: Structural pipes do not require tight sealing. Even if they can withstand pressure, micro-leaks can still cause problems at the joints.
3. Material Compatibility: The strict control of the chemical composition of fluid pipes ensures their reliability during welding and corrosion resistance in specific media. Structural pipe materials may not possess these properties, leading to weld cracks or accelerated corrosion, further causing catastrophic failure.

Conversely, using fluid pipes as structural components is generally uneconomical. You pay extra for rigorous flaw detection and hydrostatic testing, but these features may be "overkill" for structural applications, and their mechanical properties may be inferior to those of structural pipes designed specifically for structural applications.

Read more: Carbon steel vs Stainless steel or Mechanical Tubing vs. Structural Tubing