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Tube and pipe fitting considerations for reliable, leak-tight process lines

Unreliable process lines create excessive costs for industry in the form of higher operating expenditures, increased downtime and potentially negative effects on product quality.

John C Cox, Swagelok Company
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Article Summary
In energy costs alone, a few small leaks in a facility using air at 100 pounds per square inch gauge, with an electric consumption cost of approximately 6 cents/kilowatt-hour, can waste more than $22 000 annually. Those figures are much higher when the work to repair the leaks and the resultant downtime are taken into consideration. Ultimately, the intangibles, ranging from environmental pollution to on-the-job injuries, can come into play if the leaks go undetected or unattended for considerable periods of time.

Leakage most often occurs at the connections in a system, where fittings under stress can fail. Steps, however, can be taken to develop and improve fluid handling systems in a manner that minimises potential problems and enhances overall performance. Two of the most critical considerations are:
• The types of connecting devices used in joining process piping throughout the system
• The level of knowledge and practical experience of those installing and maintaining the application.

Any type of tube or pipe fitting is prone to leakage under certain conditions, especially if mechanical vibration is present. Although there is no “ideal” connection — one that offers 100%, leak-free operation under every system parameter requirement — certain fitting designs and technologies offer greater reliability than others. And if properly installed, quality fittings can correct leakage to less than 3%.

Tube fitting designs
Although pipe fittings traditionally had been used on a widespread basis, tube fittings have proven to be a reliable connection and gained their place in the market. They come in various designs, the benefits of which are dependent, to a large degree, on the demands of the application.

The compression fitting, the first tube fitting to be developed, is made up of three components: nut, body and gasket ring or ferrule. This design uses a friction grip on the tube. An advantage of the compression fitting is that no special tools are required in assembly, unlike pipe connections, which require thread chasers and dies to make up the threads. Furthermore, the seals can be, but are not always, line type, creating a dominant force in one small area and providing one of the most effective metal-to-metal seals available. However, this type of connection can withstand only minimal pressure as a result of the friction grip, is available in just a few materials (primarily brass), and often does not function well in systems having vibration, thermal cycling and other dynamic forces.

The flare fitting is made up of three components: nut, sleeve, and body with a flare or coned end. In some instances, the sleeve is used as a self-flaring option, usually on thinner wall or softer tubing materials. Compared to the original compression fitting, the flare fitting can handle higher pressures and wider system parameters, is available in a larger variety of materials, and has a larger seal area, which provides remake capabilities in maintenance applications. However, special flaring tools are required to prepare the tubing for installation. Additionally, flaring of the tubing can cause stress risers at the base of the flare or cause axial cracks on thin or brittle tubing. Uneven tube cuts with poorly designed rotational tube cutters or ineffective hacksaws will create an uneven sealing surface.

Bite-type tube fittings need no special tools for assembly and accommodate higher-pressure ratings than those of the original compression design. This design is composed of a fitting with a nut, body, and ferrule(s) having a sharp leading edge, which bites into the skin of the tubing to achieve holding ability. A second seal is made on the long, deep surface between the ferrule and internal body taper.

Bite-type fittings are typically single ferrule in design. This requires the nose of the ferrule to perform two functions: to bite into the tube to hold it and to provide a sealing element for the coupling body, an action that too easily can compromise one or both functions. A two-ferrule separation of functions (the first to seal, the second to hold the tube) would solve this problem, as the separation would permit each of the elements to be designed specifically for the task it is required to address.

Mechanical grip-type fittings usually are two-ferrule in design. These fittings also might use a live-loaded seal characteristic, which pertains to the spring action of the ferrules during sealing. Fitting pull-up spring loads the front ferrule as it seals by coining the surfaces of the tubing and coupling body. A radial colleting or holding action of the back ferrule grips the tube for a distance just outside the tube holding point of the ferrule nose to enhance vibration resistance.

Break and remake of the fitting after installation can be accomplished successfully without damaging either the fitting components or the tubing. In addition, some manufacturers offer a gauge to ensure proper and sufficient pull-up on initial installation. Under-tightening of tube fittings, especially in harder materials such as stainless steel, is considered a major cause of leakage.

Pipe fitting options
The fitting connection most resistant to both vibration and fatigue is a pipe butt-weld fitting. Its ability to resist vibration and fatigue is determined by the strength and integrity of the connection made.

However, pipe butt-weld fitting connections do have disadvantages. The welding equipment and specialised training required to make the connection can be costly. Additionally, the amount of time required to install pipe butt-weld fittings into a system is greater than that required for other fitting installation options. The degree of knowledge required by the installer should be factored into the equation as well. Thorough training is essential to ensure that quality weld connections are achieved. Finally, accessibility for future maintenance of the fluid system piping is limited, unless maintenance people are prepared to carry a torch or hacksaw to cut their way into a system line.

One of the most common types of connections found in process fluid handling systems is the threaded or screwed pipe fitting connections, of which there are several types. National Pipe Thread (NPT) fittings have a tapered thread on both the male and female ends. The seal is actually a “crush seal” between the joining metal surfaces, and occurs on the flank, crest and root of the tapered thread.
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