Editor’s choice
HOW TO ENSURE YOU CHOOSE THE PROPER SMALL-
BORE FITTINGS By Sam McCulloch, associate product manager, Swagelok Company
small-bore fitting (Figure 1) plays an essential role in making sure the system performs to the highest standards – and therefore, should be chosen carefully and deliberately. There is no such thing as a universal fitting, so it is important to understand the different types of fittings and their roles in small-bore systems before making a choice. Unfortunately, it is not always an easy decision. To ensure you are choosing the right small-bore fitting for your application, technicians should be trained on what each fitting does and what role it plays in the overall system.
A THREADED FITTINGS
As the name implies, threaded fittings connect two threaded pieces. To ensure leak-proof performance, one fitting with threads on the outside (known as male threads) is inserted into a piece with threads on the inside (known as female threads) of the fitting and tightened to seal the interface. Two distinct varieties of threaded fittings exist. Threaded fittings either have straight (also known as parallel) threads or tapered threads (Figure 2). Straight-threaded fittings do not seal on the thread. It is necessary to include supplemental items like gaskets, O- rings, or metal-to-metal contact to ensure the connection is leak-proof. Typically, such fittings do not operate well in environments in which the system pressure exceeds 5,000 psi.
Figure 2. Tapered tube fittings may be used at pressures up to 15,000 psi, while fittings with straight threads must be used in applications that do not exceed 5,000 psi.
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mong the most critical parts of any small-bore fluid system are its connection points, which must be leak-free to prevent system failure and enhance overall safety. The
On the other hand, tapered threads seal as the separate pieces come together because the threads are angled. Thread sealants or tape must be used to fill in any gaps that exist, like those between crests and roots, to keep the connection leak-free. Proper sealants will also prevent galling, which occurs when two surfaces in contact with each other seize up through a process called cold welding. The construction of tapered threads allows them to be used in environments in which the pressures may reach up to 15,000 psi. Outside of their pressure limits, the decision to use straight or tapered threads is usually a matter of personal preference. Threaded fittings typically meet the commonly accepted standards. British Standard Pipe (BSP) or National Pipe Thread (NPT) are the two most common standards threaded connections are measured by. Once you have decided which threading system to use, it should be standardised throughout the entire system. This will prevent technicians from getting confused or misapplying the connections.
TUBE FITTINGS
Compression tube fittings (Figure 3) differ from threaded connections because they use ferrules instead of threads to prevent leaks. In such instances, a tightened nut compresses the ferrule between the nut and its receiving fitting. The ferrule bites into the tube and creates a strong grip and tight seal. If the ferrule is properly manufactured, it should grip well enough to be reliable. In addition, compression fittings are easier to assemble and disassemble than their threaded counterparts.
Not all compression fittings are designed the same way. Bite-type ferrules can either be manufactured as single- or double-ferrule designs and bow when assembled, which causes them to grip or indent the tube surface. No matter how well a ferrule is attached, vibration, pulsation, thermal shock, or side load can weaken its grip on the tube. In a dynamic system, this weakness can damage the tube or cause it to loosen, allowing leaks to occur. Double-ferrule mechanical grip designs often improve grip strength and seal performance
Figure 1. To enhance your fluid system’s safety and operation, choosing the right tube fitting for specific applications is critical.
Figure 3.
Compression tube fittings employ ferrules instead of threads to ensure leak-tight
performance.
thanks to a gas seal between the front ferrule and the outside diameter of the tubing. Additionally, this design creates a gas seal between the seal surface of the body (Figure 4). In a two-ferrule design, the back ferrule is the primary method of securing the tubing in the fitting. When it is installed, it axially advances the front ferrule and grips the tube radially. To create an even more robust grip and seal, some two-ferrule systems provide hinging- colleting action, which offers more material for the back ferrule to grip. The result is direct and axial support to the tube grip, which prevents the fitting from loosening when vibration occurs (Figure 5). Double-ferrule fittings create long lines of sealing contact between the ferrules and the tube and allow for a particularly strong gas
March 2024 Instrumentation Monthly
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