In industry, "tube" and "pipe" are nearly interchangeable terms. Both typically describe long hollow cylinders of uniform material with a certain amount of rigidity and permanence. This is in contrast to "hose", which is typically more portable and flexible, and is made up of multiple layers of different materials which vary based on the application. 

The difference between tubes and pipes is in how they are specified. Many tubes are defined with exact rather than nominal dimensions. In other words, a tube's outer diameter will often measure exactly 1.5 inches if rated with a 1.5" OD. This is not the case with pipes, where the actual dimensions are usually not the nominal (specified) dimensions.

In addition, tube dimensions and sizes are typically based on the outside diameter or width of the tube, likely because tubes are associated with structural applications more often than pipes. Pipes, which are more exclusively for fluid transport, are commonly sized based on the inside diameter (ID).

Types of Tubing

Tubing can be distinguished based on the materials used and its application. Both are paramount to proper tubing selection.

Materials

Material type is the most important characteristic in tube selection. Tubing material defines most of the structural and chemical properties of the tube. Materials include metals, plastics, and various other types.

Metals

Aluminum is a lightweight, corrosion resistant metal commonly used as a tube material for both plumbing and construction applications. By itself, aluminum has low tensile strength and is used when high corrosion resistance is needed. It is alloyed with zinc, copper, silicon, manganese, and/or other metals to improve its strength and hardness.

Copper is a corrosion resistant metal with excellent conductivity. Copper tubing is used frequently for plumbing and heating applications, especially for residential water supply lines. Copper tubing can come in either soft or rigid forms. Soft or ductile copper is easily bent and maneuvered. Rigid copper is not bendable and requires directional fittings to go around corners and obstacles.

Steel is durable and strong metal with a high resistance to heat. Steel is an alloy of iron and carbon - it is commonly alloyed with other metals to improve its corrosion resistance and durability. It is used as a tubing material for both commercial and industrial applications for both fluid transport and structural applications. Galvanized steel is coated with zinc for rust and chemical corrosion resistance. Carbon steel is alloyed with higher levels of carbon for increased durability and strength.

Stainless Steel is a relatively strong metal with excellent chemical and corrosion resistance. Stainless steel is an alloy of steel that contains over 10.5% chromium, providing excellent corrosion resistance for sanitary applications and those dealing with aggressive fluids and materials.

Plastics

Nylon is a plastic with high chemical and corrosion resistance. It is the preferred plastic for applications involving flammable fluids, chemical solvents, and potable water.

Polyethylene (PE) is the most preferred plastic for hot and cold temperature applications. PE is grey or black, semi-flexible plastic. Fittings are used commonly for supply lines to sprinkler systems and underground geothermal heating loops.

Polypropylene is a thermoplastic material that exhibits excellent hot and cold flow (heat resistance) and durability. It is recommended for vacuum systems, and also exhibits good chemical resistance to certain chemicals such as acids and caustics. It can be used in exposed applications because of its resistance to UV, weathering, and ozone.

Polyurethane is a plastic with good abrasion resistance, high tensile strength, and low-temperature flexibility.

PVC is a rigid plastic which can come in a variety of pressure ratings. Standard tubing applications include cold water supply and drainage.

Vinyl plastic tubing is a flexible material that offers high performance and has a broad range of chemical resistance, but is not recommended for use with food, beverages, or drugs.

Other Materials

Rubber tubing is used for flow lines for fluids and gases in pneumatic, hydraulic, process, medical and many other specialized applications

Fiberglass & composites are strong, durable materials which are impervious to many caustics and extreme temperatures.

Glass is a hard, brittle material consisting of a mixture of silicates that is usually transparent or translucent. It is considered to be a cooled liquid rather than a true solid. Glass is used in laboratories and other applications which require rigid, transparent tubing.

Selection tip: It may also be important to note the method of construction of the tubing. Hard materials like metals are typically rolled and welded together to form the cross section, while others are constructed seamlessly via extrusion.

Application

Tubing can be broken up into three separate categories based on application. Selecting the right tubing first requires distinguishing between these different types, since the properties of the tubing are unique to each.

Electrical sheathing - Tubes are designed around electrical cables, wires, or hose to provide protection from abrasion, ESD, and environmental effects. These tubes are typically have good electrical insulating properties.

Fluid transport - Tubes are designed to carry fluids (gas or liquid) from one point to another. These tubes must be chemical and corrosion resistant based on the fluids being handled.

Structural applications - Tubes are designed for use in construction as supports in buildings and structures. These tubes must have good mechanical properties.

Sizing

Second to the type of the tubing in the selection process is the size of the tubing, giving in either English units (inches) or Metric units (millimeters). Dimensions for tube sizing include outside diameter, inside diameter, and wall thickness. Nominal sizes for tubes are based on the outside diameter.  Manufacturers will often provide tolerances alongside size tube dimensions, which define the possible measurement variation to +/- a certain amount (e.g. 4" +/- .1"). 

Miniature-sized aluminum tubes. Image Credit: K&S Engineering

Shape

The shape of the tubing cross section is also important to consider. Although most tubes for fluid flow have circular cross sections, there are a number of other shapes available for different applications. The standard shapes are listed below, though there are many additional custom shapes.

• Circular

Image Credit: United Aluminum

• Oval

Image Credit: Morse Industries

• Rectangular

Image Credit: United Aluminum

• Square

Image Credit: United Aluminum

Performance Specifications

There are a number of performance specifications that can accompany tubes, depending on the application they are used for.

Pressure rating defines the minimum or maximum pressures the tube is designed to withstand, measured in pounds per square inch (psi). This rating is defined for tubes in fluid flow applications.

Maximum vacuum defines the maximum vacuum pressure that can be created in the tube, most frequently given in inches or mm of mercury, referenced below one standard atmosphere.

Minimum bend radius is a rating based on the acceptable amount of deformation the tubing cross-section can undergo, typically given in inches or millimeters. This is particularly important for structural applications where the tubing is subject to loading.

Temperature range defines the full range of ambient temperatures that the tubing is designed to operate within. This is particularly important in applications where the transported fluid or outside environment could be subject to extreme temperatures.

Finish & Temper

It may be important for the inner or outer surfaces of the tube to be tempered and finished in a certain way.

Temper Options

Annealed - used to produce a definite microstructure, usually one designed for best machinability, and/or to remove stresses, induce softness and alter ductility, toughness or other mechanical properties.

Half hard - produced to a Rockwell hardness range of 70 to 85 on the B scale for steel. Products of this temper are intended for limited cold forming.

Full hard - produced to a Rockwell hardness of 84 and higher on the B scale for steel.  Product is very stiff; it is intended for work where deformation is very minimal.

Finishes

Bright finish - tube is secondarily finished by polishing or pickling, leaving a bright finish.

Galvanized - tube is coated with a layer of zinc. The zinc coating provides the tube with greater corrosion resistance.

Painted - tube is painted or powder-coated for corrosion resistance and/or identification

Plated - tube is plated for corrosion resistance or identification.

Features

Depending on the application, certain tubing features may be important to consider. Important features include:

Anti-static - tubing material helps prevent the buildup of static electricity, a feature typically important for tubing which sheaths and protects electrical wires or components.

Explosion-proof - tubing is designed not to burst or explode in the event of an interior explosion. Tubing may also be constructed to withstand internal explosions of a specified gas, vapor, or dust.

Flame resistance - tubing is constructed to maintain integrity and prevent ignition in the presence of an open flame.

Spark resistance - tubing is constructed to maintain integrity in the presence of electrical discharge or sparking.

Reinforcement - tubing material is reinforced with fibers or banding for added strength.

Sterilizability - tubing has the ability to be sterilized for sanitary applications: methods include an autoclave, dry heat, gamma irradiation, and chemical disinfectant.

References

Hoseandfittings.com - Tubing

United Aluminum - Tubing

Image Credit:

ICO Rally | OMEGA Engineering, Inc. | Bobco Metals | Automationdirect.com  | Grainger Industrial Supply, Inc.