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How Many Types of Pipe and Tube Fittings

Pipe and tube

The improper connection of components most commonly causes system leakage. Selecting, installing, and maintaining the correct fittings for a system’s connections is essential for efficient plant operation. Professional attention must be paid to ensure that links are secure and any potential leaks are prevented.

Despite system parameter requirements, no ideal fitting connection guarantees leak-free operation.

Despite the availability of various fitting designs and technologies, all fitting types are still prone to leakage under certain conditions, especially when mechanical vibration is present.  However, quality fittings, when installed properly, can reduce leakage.

Leak prevention is an essential aspect of any professional fluid-handling system. Considerations for leak prevention include selecting the appropriate connecting devices for process piping and having personnel with the proper experience level for installation and maintenance.  In addition, learning different fittings’ design, installation, and maintenance aspects can help minimize potential problems and maximize system performance.

Tube Fittings

Compression Fitting

A compression fitting is the first tube fitting to be developed and is characterized by a three-part system consisting of a nut, body, and gasket ring or ferrule. These components utilize a friction grip on the tube, meaning no special tools are required for assembly compared to pipe connections which need thread chasers and a die.  This type of fitting can produce a highly effective metal-to-metal seal through the line-type seal. However, they can only withstand minimal pressure due to the friction grip. In addition, they are only available in a few materials, such as brass, with a tendency to be unreliable in systems with vibration, thermal cycling, and other dynamic forces.

Flare Fitting

Flare fittings provide a reliable seal for higher pressure and more extensive systems than traditional compression fittings. With a larger seal area and availability of various materials, they offer a convenient, remarkable solution in maintenance applications. However, special flaring tools are necessary to prepare the tubing for installation, and flaring can cause stress risers and axial cracks in thin or brittle tubing. To ensure a successful seal, uneven tube cuts should be avoided.

Bite-Type Fitting

Bite-type fittings are designed for professional applications requiring high-pressure ratings and no special tools for assembly. Our design is composed of a fitting with a nut, body, and ferrule(s) having a sharp leading edge that bites into the skin of the tubing to achieve holding ability. An additional seal is made between the ferrule and internal body taper on the long, deep surface.

The single ferrule design of our bite-type fitting requires the nose of the ferrule to perform two functions: biting into the tube provides a means of holding it and sealing the coupling body.  This can easily compromise one or both functions. Our two-ferrule separation of operations (the first to seal, the second to hold the tube) solves this problem, as it permits each element to be designed specifically for the task.

Mechanical Grip-Kind Fitting

Mechanical grip-type fittings are a reliable choice for professionals needing a two-ferrule design. This design features a live-loaded seal characteristic, which uses the spring action of the ferrules for a secure seal. When the fitting is pulled up, the front ferrule is spring-loaded for a tight fit, creating a fast join between the surfaces of the tubing and coupling body. To further enhance vibration resistance, a radial collecting or holding action of the back ferrule tube is grasped just outside the ferrule’s nose at a distance.

Additionally, a break and remake of the fitting are possible after installation without damaging the fitting or tubing. Some manufacturers offer a gauge to ensure proper and sufficient pull-up on initial installation for optimal performance. Under-tightening tube fittings, especially in more complex materials such as stainless steel, can cause leakage if not done correctly.

Pipe Fittings

 

Pipe Butt Weld

A pipe butt weld is a highly resistant pipe-fitting connection. However, it does have some disadvantages. Specifically, the welding equipment and specialized training needed to make the connection can be costly, and the installation time is more significant than other fitting options.  Furthermore, proper training is essential to ensure quality weld connections are achieved. Lastly, it limits accessibility for maintenance, as the maintenance person needs to be prepared to carry a torch or hacksaw to cut into a system line.

For professional purposes, pipe butt weld is an ideal choice for applications that require a highly resistant connection. Still, it is vital to consider the associated costs and the level of training necessary to ensure a quality weld connection. Additionally, the lack of accessibility for maintenance should be taken into account when determining the right fitting option.

 

National Pipe Thread Fitting

NPT fittings are some of the most common connections in process fluid-handling systems. With both the male and female ends featuring a tapered thread, the seal is created through a “crush seal” between the metal surfaces. As metal has a natural affinity to itself, especially when joining carbon steel or stainless steel, galling and tearing of the metal can occur during installation. To prevent damage, applying a lubricant or sealant with a lubricating agent on the male threads is essential. A popular choice is polytetrafluoroethylene (PTFE) tape. Oil or a sealant to the male threads is necessary for creating a successful connection for professional applications.

 

Society of Automotive Engineers Straight-Thread Fitting

The SAE (Society of Automotive Engineers) straight-thread fitting is a mechanical type designed to hold only the fitting in place. Unlike other thread types, the SAE does not provide a seal; instead, the sealing function is provided by an elastomer, typically located at the base of the male thread. This threaded seal offers the advantages of an NPT connection in that maintenance, accessibility, and the remaking of the fitting are significantly easier for the professional installer.  With its easy installation and remaking, the SAE straight-thread fitting is an increasingly popular choice among professionals.

International Standards Organization Parallel and Tapered-Thread Fitting

ISO Parallel and Tapered-Thread Fittings are a reliable and cost-effective alternative to NPT Tapered Thread Fittings and SAE Straight-Thread Fittings. They rely on threads to form the primary seal and feature a backup seal in the form of an elastomer, bonded metal washer, or gasket. This provides increased sealing integrity and assurance for professional users.

National Pipe Thread Fine Tapered Dry Seal Fitting

NPTF (National Pipe Thread Fine) tapered dry seal fittings are designed to provide a secure, leak-proof seal without lubrication. The dry seal threads feature more truncated roots than the crests, creating an interference fit that causes the roots to crush the crests of the mating threads. This ensures that there is always contact between the threads’ crest, root, and flank, forming a reliable seal.

Unfortunately, galling can occur in metals – such as carbon steel and stainless steel – when using this type of seal without lubrication, making initial installation and remake impossible. Professional engineers should ensure the fitting is correctly lubricated to prevent galling and provide a secure, leak-proof seal.

37-degree Army/Navy Flare Fitting

The 37-degree AN (Army/Navy) flare fitting is designed for professional use and features a straight mechanical thread, similar to the SAE and ISO straight- or parallel-thread design. This fitting holds components together and is used with a 37-degree male flared end, machined onto the end of the fitting, which mates with a female flared surface at the base of the female threaded port. This connection is predominantly used in hydraulic applications, providing a secure and reliable connection.

Your maintenance team will be better equipped to identify, diagnose and rectify system issues, maintain optimal performance and reduce costs associated with leakage when they have a thorough understanding of the characteristics of well-fitting designs. A professional tone is paramount to ensure accuracy and reliability when diagnosing and resolving system problems.

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