Metal tube (tubing) is made of metallic elements or alloys and is designed for specific industrial applications. Compared to other types of tubing, metal tube is stronger and most robust, making it better suited for structural applications and high-pressure fluid flow. Common metals used to construct metal tube include stainless steel, copper, steel, brass, and aluminum. Metal tube made of proprietary materials such as Hastelloy (Haynes International, Inc.) is also available.

Materials

Proper metal selection is the most important characteristic in tube selection. In addition to cost, other properties such as strength, chemical and corrosion resistance, thermal stability, and rigidity will differ based on the type of metal. There are a number of different metals to consider.

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.

Brass is an alloy of copper where the main alloying element is zinc. Brass has decent strength, corrosion resistance, and electrical conductivity, and can be polished easily.

Bronze is an alloy of copper where (typically) the main alloying element is tin. Other types of bronze consist of phosphorus, manganese, aluminum, or silicon as the main alloying element. Bronze exhibits good toughness, strength, corrosion resistance, and electrical and thermal conductivity.

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.

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.

Steel is a 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 fluid transport and structural applications in both commercial and industrial sectors. 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.

Titanium is a high-strength, low-weight material with high corrosion resistance. Titanium alloy tubing is more expensive than steel alloy or aluminum, but provides the highest strength-to-weight ratio of any structural metal, making it an excellent choice for a diverse range of applications.

Sizing

Sizing is the next most important consideration when selecting tubing. Dimensions for tubing include outside diameter, inside diameter, and wall thickness. These sizes are given in either English units (inches) or Metric units (millimeters), and are all based on the outside size, diameter, or width of the tubing. 

Construction

Metal tubes and pipes can be distinguished based on the method of construction; either welded, welded & drawn, or seamless.

• Seamless - tubing is seamless in construction. Tubes which are seamless are typically constructed via extrusion.
• Welded - tubing is constructed by rolling and welding metal sheets to form the cross-section. Welds are continuous along the length of the pipe.
• Welded and drawn (W&D) - tubing is constructed by welding and then precisely sized by drawing it through a die.

Tubing construction method comparison. Table Data Credit: RathGibson, Inc.

Shape

Because metal tubes are rigid, they hold to a specific shape. Most tubes designed for fluid flow have circular cross sections, but there are a number of other shapes available for different applications. The standard shapes include circular, oval, rectangular, and square, though there are many additional custom shapes. Circular tubes are the most common because they provide even strength distribution in all directions through the tube.

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 millimeters 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

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

• 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 tubes are covered with a thin layer of another metal which acts either as an indicator or as means to enhance exterior physical properties such as corrosion resistance or conductivity.

Features

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

• Coiled / Spiral - tubing is flexible enough to coil long continuous runs in either a wrapped reel or spring format.
• Finned / Fluted - tubing has fins or flutes formed on the exterior surface which greatly increase the surface area for heat dissipation. This is particularly important for tubing used in heat exchanger, refrigeration, and air conditioning applications.
• Multi-layered - tubing is constructed of multiple concentric layers.
• Sterilizability - tubing can be sterilized by one or more methods, including autoclaving, dry heat, gamma irradiation, and chemical disinfectant.

References

Metal Product Guides - OnlineMetals.com

Tubing - Hoseandfittings.com

Tubing - United Aluminum

Image Credit:

Grainger Industrial Supply | RK | Parker Hannifin