A recurring question is “what is the basic difference between pipe and tube”?
People use the words pipe and tube interchangeably, and they think that both are the same. However, there are significant differences between pipe and tube.
A recurring question is “what is the basic difference between pipe and tube”? The short answer is: A PIPE is a round tubular to distribute fluids and gases, designated by a nominal pipe size (NPS or DN) that represents a rough indication of the pipe conveyance capacity; a TUBE is a round, rectangular, squared or oval hollow section measured by outside diameter (“OD”) and wall thickness (“WT”), expressed in inches or millimeters.
There are a couple of key differences between tubes and pipes:
A pipe is a vessel – a tube is structural
A pipe is measured in terms of its ID (inside diameter)
A tube is measured in terms of its OD (outside diameter).
A hollow cylinder has 3 important dimensions. These dimensions are:
The Outside Diameter (OD)
The Inside Diameter (ID), and
The wall thickness (wt)
These three dimensions are related by a very simple equation:
OD = ID + 2*wt
One can completely specify a piece of pipe or tube by supplying any two of these numbers.
Tubing is typically used in structures so the OD (or Outside Diameter) is the important number. The strength of a steel tube depends on its wall thickness. So tubing is specified by the outside diameter as well as its wall thickness. Steel tubes are also not only supplied in round sections but can be formed into square and rectangular tubes.
Pipes are normally used to transport gases or fluids so it is important to know the capacity of the pipe. Here the internal cross-sectional area defined by the ID (or Inside Diameter) is important. It is common to identify pipes in inches by using NPS or “Nominal Pipe Size”. The metric equivalent is called DN or “diameter nominal”. The metric designations conform to International Standards Organization (ISO) usage and apply to all plumbing, natural gas, heating oil, and miscellaneous piping used in buildings. A plumber always knows that the id on the pipe label is only a *nominal* id.
As an example, a (nominal) 1/8″ wrought iron pipe will typically have a *measured* id of 0.269″ (schedule 40) or 0.215″ (schedule 80). The key in the difference is the application where both tube and pipe are used for. For instance: a (nominal) 1/8″ schedule 40 pipe will have a wall thickness of 0.068 (id=0.269) while a 1/8″ schedule 80 pipe will have a wall thickness of 0.095 (id=0.215). And these schedule numbers do not reflect a constant wall thickness. For instance, a (nominal) 1/4 schedule 40 pipe has a wt=0.088 while the same pipe in schedule 80 has wt=0.119
Generally speaking, a tube will have a consistent OD and its ID will change. Steel tubes used in structural applications would most likely be seam welded while pipes are normally a seamless steel product. Some steel tubes are also used in the transport of fluids, even though they are seam welded. These include steel tubes for water pipes and welded tubes are commonly used in the agricultural industry for manufacturing spindles. Such tubes will undergo a process called pressure testing were the tube is sealed at both ends and water is pumped through the tube up to a certain level of pressure. This will quickly indicate if there is a lead or a bad spot in the weld of the circular hollow section tested.
What is Tube?
The name TUBE refers to round, square, rectangular and oval hollow sections that are used for pressure equipment, for mechanical applications, and for instrumentation systems.
The word “tube” refers to round, square, rectangular, and oval hollow sections used for pressure equipment, for mechanical applications, and for instrumentation systems.
Tubes are designated by their outside diameter and wall thickness, which are exact measures in inches or millimeters. For tubes, the difference between the outside diameter and the wall thickness, multiplied by two, defines the inside diameter of the tube.
The most important physical properties of steel tubes are the hardness, the tensile strength, and low manufacturing tolerances.
Tubes are indicated with outer diameter and wall thickness, in inches or in millimeters.
How do you typically order steel pipe?
Steel pipe is ordered, or called out in their “NPS” size, or “Nominal Pipe Size” and “Schedule.” The nominal pipe size was established in 1927 by the American Standards Association to replace “Iron Pipe Size.” This system designates wall thicknesses based on smaller steps between sizes and the “Schedule” to specify the nominal wall thickness. On pipe sizes, 12” and under, the size loosely refers to the inside diameter (ID) of the pipe itself. So, while you may think a 1” pipe is actually a 1” outside diameter (OD), that 1” is closer to the ID of the pipe. In fact, schedule sizes designate wall thicknesses, so a schedule 5 will have a thinner wall thickness than a schedule 40.
Things get a little “gray” when you get into the larger pipe sizes such as 10” and above. As an example, for the larger sizes (anything above 12”), the NPS – which was closer to the inside diameter of the pipe before as we just learned – is now equal to the outside diameter of the pipe. Why is this? When the standard was created, the NPS was tied to the same inside diameter based on the standard wall thicknesses at the time. As those have evolved, based on processes and applications, the inside diameters developed to a point where the ID and OD were only indirectly related.
What is a Pipe Schedule?
Depending on size you are looking for, steel pipe is called out with schedules. Also, it can be called out as a standard wall (STD), extra strong or extra heavy (XS or XH), and double extra strong or double extra heavy (XXS or XXH). These terms, that were in the original Iron Pipe Sizes (IPS), are still a part of the steel pipe language that is used today.
Frankly, the schedule designations were introduced in the hopes of phasing out that terminology. Because they are still in use today, some of the schedules do not always align with the former “standard” and “XS”. As an example, Nominal Pipe Size 12” sch 40 is a 12.75” OD round pipe with a .406” wall thickness. But, a standard (STD) schedule, or 40s, has a wall thickness of .375”. When in doubt, it is always best to refer to a pipe size chart to ensure you are ordering and receiving the size you need.
Common Steel Pipe Sizes
Some of the most common pipe sizes you can order are:
|Nominal Pipe Size||OD (Inches)||SCH 40||SCH 80s|
Pipe is typically stocked in two lengths – 21’ material and 42’ material.