Why is it necessary to distinguish between Tube and Pipe?
At a petrochemical project site, an engineer who did not know the difference between tube and pipe ordered Sch 40 steel pipes as heat exchanger pipes, which eventually resulted in 1.6 million yuan of materials being scrapped. This article will help you understand the difference between the two from various dimensions to avoid confusion.
Basic concept comparison
Differences in standard definitions (ASTM/ISO)
In the ASTM B36.10M standard, pipe is defined as “a fluid transport carrier based on the nominal pipe diameter (NPS), which proves that pipe is mainly based on its fluid transport capacity and its compatibility with other piping systems. In ASTM A249, tube emphasizes “a structural hollow body with precise size control”. The definition clearly highlights the accuracy required for the application.
Measurement method
Pipe: marked with NPS, which is an expression of many standard sizes, making it easier to match and connect.
Tube: marked with outer diameter (OD) and wall thickness (Wall Thickness), marking specific size information.
Tolerance grade
Pipe: does not need to be particularly accurate, with a tolerance range of ±0.3mm.
Tube: requires very high accuracy, with a tolerance range of ±0.1mm.
Historical Origin of Naming
“Pipe” originated from Latin, meaning a container for liquids, and is related to early steam pipes. With the development of the Industrial Revolution, steam power became a means of transportation, so pipe became a term for flow transportation.
Tube originated from the Latin word “tubulus” (small tube) in medical devices, which also indirectly reflects the precision requirements in the medical and biological fields. In medical gas pipeline systems, it is crucial to use Tube instead of pipe.
Core technical parameters
Size specification system comparison
parameter | Pipe | Tube |
Standard system | NPS/Schedule | OD+WT exact value |
Typical dimensions | 1/8″~80″ | 1mm~500mm |
Wall thickness calculation | Based on Schedule Level | Directly specify millimeters |
▶️ Key point: DN50 pipe corresponds to NPS 2″, while the same size tube is directly marked as 60.3mm×2.9mm
Differences in materials and performance
Mainstream pipe materials
Carbon steel (ASTM A106): the main material for oil and gas pipelines, accounting for 87% of the market share
Cast iron (EN 545): the main material used in water drainage systems, with excellent corrosion resistance and load-bearing capacity.
Tube special materials
Copper-nickel alloy (C70600): mainly used in seawater cooling systems, with excellent corrosion resistance and resistance to seawater corrosion.
316L stainless steel (ASTM A269): a material required for use in the food and pharmaceutical industries, with excellent pollution resistance.
Pressure test data: Under the same outer diameter, the bursting pressure of tube is 22%-35% higher than that of pipe (due to stricter wall thickness tolerance)
Application scenario analysis
Five core scenarios of Pipe
Long-distance fluid transportation (API 5L standard for oil and gas pipelines)
Building structure support (GB/T 3091 welded pipe for scaffolding)
Municipal water supply and drainage system (EN 545 certification for ductile iron pipes)
Low-pressure ventilation system (galvanized spiral duct)
Agricultural irrigation (ISO 1452 standard for PVC-U pipes)
Three areas where Tube cannot be replaced
Heat exchange equipment: shell and tube heat exchanger (ASME Section VIII requirements)
Precision fluid control: chromatograph pipeline (inner surface roughness Ra≤0.8μm)
Ultra-high hygiene standards: pharmaceutical water system (3D inner wall polishing + electropolishing treatment)
Selection decision
Media characteristics judgment:
Corrosive fluid → Select tube (better material tolerance)
Non-corrosive gas → Optional pipe (cost reduction of more than 40%)
Pressure level evaluation:
＜150psi → Interchangeable
＞300psi → Mandatory use of tube (avoid pipe burst risk)
Connection method confirmation:
Welding/flange connection → Suitable for pipe
Compression/flaring connection → Must use tube
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