Types of Tube Fittings: Learn the Key Differences

Your Guide to Choosing the Right Small-Bore Fittings

Reliable small-bore fluid systems should be designed and constructed with high-quality components—and one of the most important is the small-bore fitting. Used at critical connection points, fittings should provide leak-tight performance to help you maintain plant safety and operational efficiency.

There are a variety of types of fittings to choose from, however, and it may not always be clear which is the best choice when you’re performing maintenance on an existing system or building a new one.

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How should you choose the right fitting for your needs? Let’s investigate several common fitting types along with their specific characteristics and advantages:

Threaded Fittings

Threaded fittings make a connection by attaching two threaded pieces, one male (where threads project on the outside of the fitting) and the other female (where threads are found inside the fitting). Male threads are inserted into female threads.

Threaded fittings come in two distinct varieties: those with straight or parallel threads, and those with tapered threads. Straight threads are designed to hold a nut onto the body of a tube fitting and rely on supplemental items, including gaskets, O-rings, or metal-to-metal contact to create a leak-tight seal. For this reason, straight threads are typically used in applications where system pressure will not exceed 5000 psi.

Tapered threads, meanwhile, are designed to seal as the male and female threads are drawn together. They are constructed at an angle in relation to the centerline, whereas straight threads are parallel to the centerline. A thread sealant or thread tape is required to fill in the gaps between the crests and the roots to prevent system fluids from leaking at the connection. A good sealant will also help prevent galling (a phenomenon where two surfaces in contact seize up as a result of cold welding) during installation. Tapered threads will be effective for system pressures up to 15000 psi.

Straight threads are typically used when system pressure will not exceed 5000 psi. Tapered threads are effective for system pressures up to 15000 psi.

Learn more about thread sizes and pitch

Aside from the noted system pressure limits, the choice between straight or tapered threaded connections is often guided by user preference. Generally, available threaded fittings adhere to common standards; British Standard Pipe (BSP) and the National Pipe Thread (NPT) are two of the most common. It can be beneficial, though, to standardize the use of one type versus the other throughout a facility to reduce potential confusion or misapplication.

Compression Tube Fittings

In contrast to threaded fittings, compression tube fittings commonly utilize ferrules to create a leak-tight seal. When the fitting’s nut is tightened, the ferrule is compressed between the nut and the receiving fitting. As a result of this compression, the ferrule effectively bites into the tube, thus creating a strong grip and a tight seal. The reliability of the tube grip is related to how well the gripping ferrule performs this function. Compression fittings also offer the benefit of being simple to assemble and disassemble.

Compression fittings that feature a two-ferrule design can improve grip strength and seal performance.

There exist some key design differences among the variety of compression tube fittings commonly available today. A bite-type ferrule, as illustrated here in both a single- and double-ferrule design, bows when assembly occurs. This bowing action drives the leading edge of the biting ferrule into the tube to grip or indent the tube surface. The leading edge of these bite-type ferrules is intended to bite for proper tube grip. If there is vibration, pulsation, thermal shock, or side load exerted on the fitting, the minimal contact of the gripping ferrule offers little support behind the bite. This means in a dynamic system, the potential for either damage to the tube or pullout may exist.

Compression fittings that feature a two-ferrule design can improve grip strength and seal performance. Here, a gas seal is created between the front ferrule and the outside diameter of the tubing, as well as between the seal surface of the body. The back ferrule also acts as a primary mechanism to secure the tubing in the fitting. During installation, the back ferrule axially advances the front ferrule and radially applies an effective tube grip.

A two-ferrule, mechanical grip design that delivers hinging-colleting action can create an even more robust grip and seal. Hinging and colleting action provides more material of the back ferrule in close contact with the tube adjacent to the tube grip. This material provides direct and axial support to the tube gripping function. Additionally, this design reduces the likelihood of fittings backing off in applications where vibration is a concern and allows for a very small amount of movement in the fitting while maintaining the appropriate levels of grip and force. This movement, called “spring back,” creates robust vibration resistance. Additionally, by creating long lines of sealing contact between the ferrules and the tube, this mechanical design creates a highly reliable gas seal. They are a good choice for applications where leak-tight performance cannot be compromised.

Two-ferrule, mechanical grip fittings provide high vibration resistance while maintaining appropriate grip and force.

Medium-Pressure Tube Fittings

Medium-pressure mechanical grip fittings—an evolution of two-ferrule compression fittings—also deliver outstanding performance along with enhanced installation efficiency.

Swagelok^® FK series fittings provide a good example of how mechanical grip technology differs from compression-style fittings. Using similar hinging-colleting action to a standard Swagelok^® two-ferrule tube fitting shown here, the FK series differs by utilizing a female fitting body, a male nut, and pre-oriented ferrules. This design creates a unique dynamic wedge. The dynamic wedge allows for remakeable pullups by torque and provides gaugeability at initial assembly and remake of the tube fitting. The two-ferrule design creates a more robust bite into the tubing, creating a leak-tight gas seal upon initial installation and every remake.

FK series fittings additionally feature a pre-assembled cartridge that holds the two ferrules and nut. This helps to ensure proper ferrule orientation and installation. With these features, FK series fittings can help reduce installation time and lower assembly and maintenance costs while providing a more reliable connection, resulting in increased uptime in a variety of applications.

Cone and Thread Fittings

Finally, cone and thread fittings are available in a wide variety of alloys and have several design features that make them suitable for medium- to high-pressure applications. Their reliable performance has made them a standard high-pressure fitting choice in a variety of demanding applications for many years.

Cone- and thread-specific tubing, featuring thicker walls and a higher pressure rating, is typically required when cone and thread fittings will be used. A high-performance cone and thread fitting will feature a gland, a collar, a female port, and a weep hole that enables you to detect leaks and verify proper installation.

Special care must be taken when preparing and installing cone and thread fittings. Coning and threading tools are required to make the connection, along with lubricant to reduce friction during the cutting process. The tube must be properly coned and threaded before being joined with the fitting. It’s critical to ensure that no burrs, gouges, or scratches have occurred during the process. Once the tubing prep is complete, a collar is threaded onto the tubing and a gland nut is inserted into the fitting body for final tightening. For systems that experience regular shock or vibration, it is also recommended to use anti-vibration components to help extend the life of the tubing connection. If assembled properly, cone and thread fittings can provide reliable long-term performance in some of the most strenuous fluid system applications.

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Having learned some of the key differentiators among the variety of fitting technologies available today, you will be better equipped to make the right choice for your fluid system applications. However, this article only covers the basics of small-bore fittings—there are many other complexities involved with tube fitting selection and installation.

If you’re interested in further developing your understanding of the world of small-bore tube fittings, check out Swagelok's in-depth training opportunities. Our training programs provide valuable and practical tools for meeting day-to-day challenges related to fluid system design, operation, and maintenance. Through these in-depth, hands-on courses taught by experienced instructors, both new team members and industry veterans can find the information needed to master tube fitting selection, installation, inspection, and troubleshooting—among other fluid system topics. Learn more about available offerings at the link below.

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