No system integrity is considered complete without the critical link which is tubing. Tubing and fittings have been developed to work with different industrial applications that demand the utmost in product performance. Selection of tubing however, is based on compatibility of the tubing material with the media to be contained. High pressure tubing is particularly crucial in oil and gas extraction projects including commercial and military aerospace programs. High pressure tubing helps to extract oil and gas in some of the most hostile and unsafe environments which requires the strength and corrosion resistance of stainless steel.

General selection criteria for tubing

• Tubing material – stainless steel is the ideal material for applications with high pressure and high temperature with corrosive media. Carbon steel works well with oil, air and other specialty materials while copper can be considered for low pressure water and low temperature applications. However, it is important to take note that stainless steel fittings should only be used with stainless steel tubing since dissimilar materials in contact may result into galvanic corrosion. Furthermore, different materials have varying levels of hardness which can adversely affect the fittings ability to seal on the tubing.  

• Special care must be considered when selecting tubing for gas service in order to achieve gas-tight seals. Ferrules in instrument fittings must seal any surface imperfections. This can be accomplished with the ferrules penetrating the surface of the tubing. If the tubing provides radial resistance and if the tubing material is softer than the ferrules, penetration can be achieved.

• System pressure – system operating pressure is another factor in determining the type and size of tubing that will be used. In general high pressure tubing will require steel or stainless steel material. Heavy walled softer tubing material such as copper may be used if chemical compatibility exists with the media. However, due to higher strength of steel or stainless steel, it permits the use of thinner tubes without reducing the ultimate rate of the system. Tube fitting assemblies should never be pressurized beyond the recommended working pressure.

• Systems temperature – selection of proper tubing material also depends upon operating temperature. For low temperature media, copper and aluminum tubing can be used while stainless steel and carbon steel tubing are ideal for higher temperature media. For extremely high temperatures, special alloys such as Alloy 600 are strongly recommended. For operating temperatures above 800^oF (425^oC), consideration should be given to media. Stainless steel is susceptible to carbide precipitation which can lead to inter-granular corrosion at elevated temperatures. 

• Media – ensuring that the tubing will resist any chemicals to which it is exposed to is essential for reliable performance and longer life. For pneumatic applications, the tubing interior can be exposed to corrosives whether solid, liquid or gas. With industrial fluid-handling applications, it is usually the material of the tubing that is of primary concern. A good starting point would be the chemical resistance chart provided by manufacturers however, it should only be used as a reference since temperature and environmental conditions as well as the presence of other chemicals can affect tubing material selection.