DIFFERENCES BETWEEN PIPES AND TUBES
The differences between pipes and tubes are explained in this article. These terms are sometimes used as alternatives, however, they refer to two different types of tubular, with different meanings, uses and, requirements.
WHAT ARE STEEL PIPES?
The word “steel pipe” refers to round hollow sections used for transmission and distribution pipelines and piping systems that convey fluids and gases – such oil & gas, propane, steam, acids, and water. These are products that find wide application in the petrochemical, oil & gas, marine and process industries.
The most important dimension for a steel pipe is the inside diameter (called “pipe ID”) which is expressed by a nominal size (“NPS” or “DN” in Europe, i.e. nominal diameter). The NPS is a rough indication of the fluid conveyance capacity of the hollow section (the diameter is also called bore size); the actual external diameter does not always match the nominal size (a 2 inch pipe, for instance, has an internal flow capacity of approximately 2 inches, but has an actual external diameter of 2.375 inches). For a specific NPS, the outside diameter is constant whereas the wall thickness increases at incremental schedules (so, the thicker the pipe, the smaller its inside diameter at any specific NPS value). For pipes, the most important mechanical parameters are the pressure rating, the yield strength, and the ductility.
The dimensional range for steel US steel pipes is set by ASME B36.10 (carbon and alloy grades) and ASME B36.19 (stainless steel and nickel alloy pipes); for European pipes, the reference standards are EN 10220 and EN 1127 (respectively, carbon/alloy and stainless/nickel-alloys).
Seamless and welded pipes
WHAT ARE STEEL TUBES?
The word “steel tube” is generally associated with round/square / rectangular hollow sections used for pressure equipment (boilers, heaters, and superheaters), and for instrumentation systems. For all these applications, the exact size of the outer diameter of the hollow section is critical (whereas the inner bore is more important for steel pipes) and the mechanical properties (yield, tensile strength, and elongation) are key. Steel tubes for pressure equipment are available in relatively short outside diameters ranges (up to 5 inches). The reference norm for steel tubes is ASTM A450, with the BWG/SWG alternative measurements.
The most important dimension for a steel tube is the outside diameter (“tube OD”), which is expressed in inches or millimeters and the wall thickness; the wall thickness (WT) of the steel tube can be expressed either in BWG, SWG or inches/millimeters (these are all alternative indications of the wall thickness of steel tubes). The difference between the outside diameter and the wall thickness (multiplied by two) defines the inside diameter of the tube.
The term tube is also used in association with tubular for mechanical applications (construction projects), which are available in a wider dimensional range than tubes for pressure equipment and piping instrumentation.
In terms of pricing, steel tubes are generally more expensive than steel pipe due to their stricter manufacturing tolerances and mills productivity (tons produced by the hour). The most important physical properties of steel tubes are the hardness, the tensile strength, and highly precise dimensions.
Steel tubes for heat exchangers, superheaters, and boilers
SUMMARY OF DIFFERENCES BETWEEN PIPES AND TUBES
To summarize the difference between pipe and tube and the pipe meaning vs. tube meaning
|Diameter||Is related to the internal size, which expresses the fluid conveyance capacity. Diameter is defined in NPS (nominal pipe size) or DN (nominal diameter). NPS doesn’t match with the true internal diameter in inches, the index is a loose and approximate indication of the actual measure.||Refers to the outside diameter, i.e. the OD (outside diameter). The measure represents the actual, or true, the outer sizes of the tube. The actual physical diameter matches exactly the OD size.|
|WT||Thickness of pipes is expressed in schedules (most common are Sch 40, Sch STD., Sch. XS/XH, Sch. XXS – but other schedules are possible, such as 5, 10, 20, 30, 60, 100, 120, 140, 160). The actual thickness of a given schedule varies by NPS.||Wall thickness (mm or inch or gauges). Gauge measures (BWG or SWG) are used for thinner wall thicknesses, whereas larger measures are expressed with fractional or inch indexes.|
|Section||Round only||Round, rectangular, squared|
|Production range||Extensive (up to 80 inches and above)||Rather narrow both in terms of OD and thickness|
|Tolerances (straightness, dimensions, roundness, etc)||Set but not too restrictive.||The very strict production process is generally lengthy and involves many quality checks around key dimensional parameters such as straightness, roundness, wall thickness, surface, etc.|
|Mills productivity||High yield (tons/hour) and large lots. Pipes are produced on large scale according to highly productive processes.||Low yield and small lots (due to relatively small dimensions and cumbersome inspection and quality control procedures throughout the manufacturing process)|
|Delivery time||Can be short||Generally longer|
|Market price||Relatively lower price than steel tubes||Higher due to productivity, strict tolerances, inspection requirements, …|
|Materials||Most common is carbon (API 5L, ASTM A106 / A53) and low alloy steels (ASTM A335)||From alloy steel upwards (ASTM A209, A213)|
|Joint connections||Laborious to ensure duration||Quick (threaded ends, couplings, etc).|