ABC’s of Stainless Steel Tube Fittings

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By JeffreyThurber

Stainless Steel Tube Fittings are used extensively in fluid systems, and particularly in the Sanitary industries. Normally they are made with 304 and 316L Stainless Steel construction, which minimizes the likelihood of stress corrosion cracking. Reputable companies who manufacture Stainless Steel Tube fittings will test their products extensively to check the pressure of the hydraulic capacity, and ensure that the Fittings do not leak under pressure. Stainless Steel is valuable for its mechanical properties. The tensile strength of Stainless Steel is extremely strong, making for Fittings which are safe, reliable and durable. Stainless Steel Fittings are highly resistant to corrosion, and the leak rate is relatively low, provided the they are well-manufactured to industry standards.

Stainless Steel Tube Fittings should be manufactured to address the four most costly and critical issues of fluid systems: improper installation, vibration, leakage, thermal shock, providing you with a reliable, gaugeable product with excellent durability, and simplified maintenance. Installation should be accompanied by a guide to provide assurance that a fitting has been sufficiently tightened on installation. It should preferably be possible to install the Fittings with hand tools, and they should also be easy to disconnect and retighten for maintenance purposes. The ferrule, which is a conical piece of metal in the case of Stainless Steel Tube Fittings (but can be made of plastic), which compresses the tube as it is forced into a tapered seat, should be manufactured so that it can be used on a range of thick- or thin-walled tubing, as well as hard and soft tubing. It is essential that the ferrule is able to resist the effects of vibration, pressure and thermal cycling. Good quality Tubing is tested for pressure, performance at high and low temperatures, and how well it resists vibration which will affect durability tremendously. Hygienic Fittings in particular should lend themselves well to disconnection and retightening for maintenance purposes.

All Fittings should have a robust tube grip, which is a key element in reliable performance. There are numerous types of Fittings, and the system requirements must be taken into account in deciding which to use. Compression Fittings, which was the first type of Tube fitting, is made up of three components – a nut, a body, and a gasket ring, or ferrule. This type has significant disadvantages – the most important being that it can only withstand minimal pressure, and remake is extremely difficult because of the construction of the sealing ring. Screwed fittings are largely used to join schedule 40 pipe. Hygienic fittings are usually available in both 304 and 316 Stainless Steel, with male parts and liners suitable for butt-welding to tube. These include RJT fittings, IDF fittings and Clamp fittings. Most process lines have to be frequently dismantled for cleaning, and ranges of hygienic fitting should be suitable for this purpose and maintenance friendly.

Stainless Steel Fittings have elastomeric O-rings located near each open end to make a seal onto tubing. The tubing is first pushed onto the fitting: – the O ring is designed to be a loose push fit on the tubing. A permanent seal is then made using a special clamp which presses three projections onto the fitting, thus permanently deforming the end of the fitting. After pressing the rubber O-ring is clamped tightly between the tube and the fitting making a permanent leak-free joint.

Other types of Tube Fittings include Butt-weld Fittings, Pipe Couplings and Flanges. Although there are many materials suitable for Tube Fittings, Stainless Steel Fittings are particularly desirable in certain systems, and in certain environments – for instance a marine environment, and Hygienic systems. Provided you use Fittings from a reputable manufacturer, and they are appropriate for the system which requires them, you should benefit from excellent durability.

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