While the terms tubes and pipes are often used interchangeably, technical differences exist between them. While all stainless-steel piping can be considered tubing, the reverse is not true; not all stainless-steel tubing can be considered piping. A tube can be defined as a long, hollow enclosure primarily made for some structural function. Broadly, a pipe can be considered a particular type of cylindrical tube. (All rounded tubes are not necessarily pipes.) However, tubes and pipes are most clearly distinguished by their applications rather than by specific structural features.
Generally speaking, tubes are distinguished by characteristics including more precise engineering requirements or tighter manufacturing tolerances than pipes, they are often square or rectangular, they are typically measured according to their external diameter, they can be produced by both hot working and cold working methods, and they are generally used for some type of structural support or application.
In contrast, pipes are generally distinguished in general by a different set of characteristics including almost always being rounded or circular, they are typically measured according to their internal diameter, they are usually made from hot rolled, black steel, and are almost always chosen over tubes for applications where transmission of a substance is required (e.g. transmission of gas or water in the pipe fitting and plumbing industries).
The contrast between tubes and pipes can be sharpened by juxtaposing round tubes and pipes. A 2-inch round tube will have an external diameter of 2 inches and most likely be used in a structural application. However, a 2-inch pipe will have an internal diameter of 2 inches and most likely be used to enable liquid flow.
Even though tubes and pipes technically differ, piping will be discussed in this article alongside steel tubing since piping comprises a large portion of general steel tubing usage. The close relationship between tubing and piping can be illustrated by a brief sketch of the overall history of steel tubing.
From the earliest times, civilizations have created tubing systems to transport necessary materials. The ancient Egyptians, Chinese, and Greeks all invented rudimentary forms of piping to transport resources like water. (In China, piping was used to transport natural gas for lighting purposes.) Materials used for these early forms of piping included copper (Egypt), bamboo (China), clay, stone, and bronze (Greece). In the first century, some European pipes began to be formed from lead. While not true pipes, the aqueducts built by the Romans and Persians are more well-known examples of early tubing/piping systems. In the early modern era, Boston witnessed an early example of waterworks (1652) which used hollow logs for “piping.” Later in American history, redwood trees were used for similar purposes in the West. Ironically, redwood outperformed metal pipes in terms of resistance qualities (e.g. to fungi, corrosion, etc.). Although piping for fluid transport was the main use of pre-industrial metal tubing, it was not the sole use. From the start of the modern era, metal “tubes” played a critical role in the firearms industries for muskets and cannons.
Metal tubing began to mature with the advent of the Industrial Revolution. The origin of steel tubing as we know it today can be back traced to Scottish engineer William Murdock, who used discarded musket barrels to form a coal lamp system for London in 1815. The success of his system spurred the need for improved metal tubing. In the middle of the nineteenth century, the steel industry received a boost from the appearance of the Bessemer process; by the 1870s, industrial rolling mills commonly formed pre-cut iron and steel into tubes. Since most of these century metal tubes were welded together, however, they suffered from coming apart after prolonged stress.
Since 1840, seamless tubes were produced – albeit inefficiently – by a process that involved drilling a hole in round billet and drawing the billet through some dies. In 1888, the production of seamless steel tubes was improved by casting the billet around a fireproof core. The invention and growth of stainless steel in the following century (1912 and onward) was another important development for steel tubing overall. Today, metal tubing and piping continues to play important structural and transport roles throughout industrial civilizations.
As alluded to earlier, tubing and piping can often be distinguished by their respective materials of composition. Piping is often made from carbon steel or low alloy steel, while tubing is often made from stainless or mild steel, aluminum, copper, and brass.
Stainless steel is particularly valued for tubing because of its corrosion resistance (its most important quality), temperature resistance, strength, and smooth, crack-free surface. When making stainless steel tubing, manufacturers have a number of stainless steel alloys at their disposal from which to choose. The most commonly used ones include 304 stainless steel and 316 stainless steel. These types are popular because they are low-maintenance, easy to clean, and resistant to corrosion. Although the initial costs of stainless steel tubing can be higher than those associated with other metals (aside from titanium), such tubing often pays off in the long term because its replacement and maintenance costs are so low.
Stainless steel tubing and piping can be fabricated in a number of ways. A steel tube’s production process depends mostly on whether it is a welded tube or seamless tube. Welded steel tubes are formed by forcing and sealing edges of processed raw steel together; seamless steel tubes are formed by stretching or drawing methods.
To weld stainless steel into a pipe, a manufacturer begins with a set of hot-rolled or cold-rolled steel tube coils, or strips. These coils are then passed through sets of grooved rollers in order to assume a rounded, tubular shape. Next, welding electrodes heat the processed coils and fuse them together along a seam through electrical resistance welding. Typically, welding residue must be removed (both internally and externally) before the tube is reduced in size by two more (semicircular) rollers and assumes it final dimensions. Two of the main advantages of the welding process are that it accomplishes high degrees of precision and that it does not require secondary processes.
For seamless tubing and pipes, manufacturers turn to a process much like extrusion called drawing. Basic hot metal extrusion employs high heat and a die through which molten metal is forced. Drawing, on the other hand, involves manufacturers taking a solid stainless-steel billet and using tensile force to stretch, or draw, the billet over a bullet-shaped piercing rod. In this way, manufacturers are able to create a hollow, regularly-shaped shell free of seams. (Drawing is usually performed at room temperature, but it can be done at elevated temperatures in order to reduce stress on hollow sections.) The seamless tube production process is completed at a reducing mill, where the tube is temporarily modified to an oval shape (for finishing purposes) before being properly reshaped by a rotary sizer. Perhaps the largest advantage of seamless stainless-steel tubing and piping is the fact that it is much less likely to split and can therefore withstand much higher amounts of pressure.
This method (also known as metal turning or spin forming) involves rotating a flat round disc of sheet metal on a spinning CNC lathe. Metal spinning offers low production costs and reduced material waste. Unfortunately, though, it is a process that is limited to the production of concentric shapes.
Types and Customizations
The success of stainless steel tubing has led to the creation of many variations. One highly variable characteristic of stainless steel tubing and piping is its size. Generally speaking, pipes are larger than tubes (since size is often more important in transport applications than structural ones). Tubes can be found in very small diameter applications; among the smallest such applications are hypodermic tubing (needles and syringes), which usually range from .002” to .239” on the inside and .005” to .259” on the outside. (Tubes of this size have their own gauge sizes, which are based on the inside and outside diameter of the tubes, rather than their wall thickness and outside diameter.)
The fundamental categories of welded stainless-steel pipe and seamless stainless-steel pipe have already been noted. (Welded tubing/piping can also be called electric resistance welded, or ERW, tubing.) Other types of notable types of stainless steel tubing include stainless steel square tubing (a pressure resistant alternative to traditionally round-shaped tubing) and corrugated stainless steel tubing (which is flexible and features a PVC outer layer).
Although not strictly a type of tubing, pipe fittings are attachments that allow for a variety of connections, such as those between standard tubes, threaded nipple tubes, elbows, tees, and reducers. To keep their contents contained, pipe fittings are topped with plugs and caps.
Tubes are used for incredibly diverse applications. Industries which rely on stainless steel tubing and piping include the automotive, pulp/papermaking, dental, medical, solar, construction, industrial, petrochemical, and semiconductor industries. Applications within these various industries include engine conduits and fuel lines, hydraulic systems, stainless steel hoses, dental implants, braces, medical instruments (like surgical tools and stainless steel hypodermic tubing, which is used to transfer medicine from a bottle into the bloodstream from under the skin), square tube usage (for bar foot railing, display racks, and stepladder assembly), gas heating systems (which use corrugated tubing, composed of an interior layer of stainless steel and an outer layer of PVC), internal factory transport (e.g. of gaseous material from one production area to another), and temporary structural support.
Arguably the most important consideration when searching for a product supplier is the level of customization they are able and willing to provide for your specific needs. Stainless steel tubing has many important characteristics such as length, straightness, cleanliness, surface finish, etc. that a qualified steel tubing supplier should be able to adjust as needed. When selecting a tubing supplier, consider any special options they might offer customers (e.g. flash rolling or special packaging/delivery services).
In addition, factors and choices that should be taken under consideration when ordering custom steel tubing and installing it for your specific application include which stainless steel is best for your application, if you need welded or seamless tubing, and whether tubes or pipes are better in case of connection needs.