Pipe and tube are two different words, people often use the pipe and tube interchangeably because they stand for the same thing, however, as a pipe and tube expert and engineer, it is necessary to know they are not the same, there are significant differences between pipe and tube.
Before listing the differences, we should know what is pipe and tube.
Pipe is a hollow section to transports gases or fluids in piping systems.
The term tube is mainly used for mechanical and structural applications, it also can be applied in heat exchanger and instrumentation systems.
Sizes
The key difference between pipe and tube is size expression, there is often confusion, let me try and explain as best I can what exactly is difference between them.
Difference in outside diameter (OD)
Nominal pipe size (NPS) defines the diameter of the pipe, it is a size standard established by the American National Standards Institute (ANSI), NPS specifies nominal OD, but not actual OD, the actual physical OD is larger than its nominal OD when pipe sizes not over 12”, NPS is more than 12” and above 12”, it is same as pipe actual size.
The standard combinations of pipe nominal diameter and wall thickness (schedule) are covered by the ASME B36.10 for carbon and alloy pipes, and ASME B36.19 specifications for stainless steel pipe respectively.
We give you some EXAMPLES to show pipe’s outside diameter.
- NPS 1 actual O.D. = 1.5/16″ (33.4 mm)
- NPS 2 actual O.D. = 2.3/8″ (60.3 mm)
- NPS 3 actual O.D. = 3.1/2″ (88.9 mm)
- NPS 4 actual O.D. = 4.1/2″ (114.3 mm)
- NPS 12 actual O.D. = 12.3/4″ (323.9 mm)
- NPS 14 ACTUAL O.D. = 14″ (355.6 MM)
The outside diameter of the tube is the actual value, not the nominal size, the inside diameter of a tube depends on the thickness of the tube, ID = OD – 2 × WT.
Difference in wall thickness (WT)
SCHEDULE specifies pipe wall thickness, it determines the inside diameter (ID) of a pipe together with NPS (OD), pipes are measured by inside diameter to allow a calculation for transportation speed, volumes and capacity, there are a few schedule numbers to specify wall thickness of the pipe, large schedule number represents heavier thickness.
- Sch 5S
- Sch 10
- Sch 10S
- Sch 20
- Sch 30
- Sch 40
- Sch 40S
- Sch 60
- Sch 80
- Sch 80S
- Sch 100
- Sch 120
- Sch 140
- Sch 180
The number with “S” is for stainless steel material pipes.
The schedule and the actual thickness of a pipe vary with the size of the pipe, for example, schedule 40S for wall thickness of pipe, there are the same schedule number, but different wall thickness values.
- NPS 1 – SCH 40S = O.D.33,4 mm – WT. 3,38 mm
- NPS 2 – SCH 40S = O.D.60.3 mm – WT. 3,91 mm
- NPS 3 – SCH 40S = O.D.88.9 mm – WT. 5,49 mm
- NPS 4 – SCH 40S = O.D.114.3 mm – WT. 6,02 mm
The wall thickness of the tube is expressed in inches or millimeters, it is often specified as BWG (Birmingham wire gauge.), and the actual physical OD of a tube is just the same as its nominal OD. The size of a tube will keep the same OD no matter what the wall thickness is.
For example for tube sizes:
- OD 3/4″ – WT 0.035″ (BWG 20 ) = OD 19,05 mm – WT 0,89 mm
- OD 1″ – WT 0.035″ (BWG 18 ) = OD 25,4 mm – WT 1,24 mm
- OD 1-1/2″ – WT 0.065″ (BWG 16 ) = OD 38,1 mm – WT 1,65 mm
In short: The tube is measured by outside diameter and wall thickness, and the pipe is measured by inside diameter, which indicates the rough (not the exact) fluid conveyance capacity of the tubular. The ID is expressed in “NPS” or “DN” (bore size), inside diameter of the pipe (ID) changes depending on the pipe schedule.
Learn more about pipe and tube sizes.
Size range
Pipe is typically available in larger sizes than the tube, outside diameter of the pipe can be from NPS 1/8 to NPS 40, even larger, it is the relatively narrower range for the tube, generally up to 5 inches, and this is determined by its applications.
Tolerance
In general, the tolerances are looser to pipes compared with tubes, on the one hand, there are very large dimensions for pipes, so it is not easy to control tolerance precisely, on the other hand, it is not necessary to make exact size to pipe use, on the contrary, tube size is usually controlled tight to match its applications.
When we calculate a pipe or a tube dimension including outside diameter, wall thickness and inside diameter, it should be noted that this calculation is just theoretical, pipes and tubes have tolerances in OD, WT and ID, therefore pipe and tube sizes are the theoretical value.
You can compare tolerances between ASTM A312 (for welded and seamless pipe) and ASTM A213 (seamless tube) specifications.
Shapes
A pipe is a round tubular shape, however, a tube can be a round, rectangular, square or oval form, even other shapes.
Strength
Tubes are stronger than pipes.
Price
Relatively lower price to pipe, some manufacturing processes are not necessary, like the annealing process, this reduces production cost, but to tube, the higher price will be calculated due to lower mills productivity per hour and the stricter requirements in terms of tolerances and inspections, tube requires more steps to ensure high quality.
Applications
Pipe is intended to be used to transport fluids or gas, tubes are used in structural applications, medical & food systems, also heat exchanger and instrumentation systems.
Conclusion
Pipe and tube differ in size measured, pipe is measured in inside diameter depending on NPS for outside diameter and Schedule for wall thickness, it is used for the transport of liquids and gases such as water, oil, natural gas, or propane. Tube has actual size value, more use situations, multiple shapes and higher price.