Tube fittings connect runs of tubing (for in-line, offset, multi-port, and mounting configurations) to other tubing sections, pipe, hose, or other components. Systems are typically configured with straight sections connected by fittings or specially formed connections and joints.

Tubing vs. Piping and Hose

To begin, it may be important to distinguish some terminology. In industry, "tube" and "pipe" are nearly interchangeable terms. Both describe long hollow cylinders of uniform material with a certain amount of rigidity and permanence. This is in contrast to a "hose", which is typically more portable and flexible, and is made up of multiple layers of different materials which vary based on the application. 

The difference between tubes and pipes is in how they are specified. Generally, tubes and their fittings are defined with exact rather than nominal dimensions. In other words, a tube's outer diameter will often measure exactly 1.5 inches if rated with a 1.5" OD. This is not the case with pipes, where the actual dimensions are usually not the nominal (specified) dimensions.

In addition, tubes are commonly associated with structural applications where the defining sizing specification is outside diameter (OD). Pipes are more often specifically vessels for fluid transport, where the defining specification for sizing is inside diameter (ID).

Applications

Fitting construction and material specifications are application-dependent; consulting the fitting supplier is typically a wise route for optimizing component selection. However, most tube fittings correspond to either hydraulic or pneumatic systems. The basic system type is the first step to determining the type of fittings right for the application.

• Hydraulic applications involve the transfer of liquid fluids such as water and other chemical solvents. Hydraulic fittings must have seals which prevent the leakage of liquid and often be resistant to rust or other possible chemical corrosion.
• Pneumatic applications involve the transfer of gases. Pneumatic fittings must have very tight seals to prevent gas leakage and must be resistant to chemical corrosion.
• Other applications such as structural design also incorporate tube fittings. These fittings must have strong physical integrity but typically do not require sealing because they do not carry fluids.

Types of Fittings

Tube fittings are distinguished based on their connection types and the functions they perform.

Connection

Fittings are attached to tubes via a number of different connection methods, each with its own conveniences and advantages.

Compression fittings are fittings which connect tubes using compression on a gasket, ring, or ferrule. The compression is typically made by tightening a nut onto the fitting over the tubing and ferrule, compressing and securing the tubing inside. Standard compression fittings do not require tools to assemble, making them convenient for quick, simple installation. However, they cannot withstand very high pressures and do not have as much flexibility as soldered fittings, making them less functional in systems with vibration, thermal cycling, and other dynamic forces. They also come in a limited number of materials (most commonly brass) and are best suited for metal on metal connections. 

Image Credit: DIYadvice.com | Hose & Fittings, Etc.  

This video shows how compression fittings are installed on plastic and copper tubing:

Video Credit: Youtube - heliatosSolar / CC BY-SA 4.0

Bite-type fittings are compressive fittings with a sharpened ferrule that "bites" the tube when compressed and provides the seal. Bite-type fittings, like standard compressive fittings, require no special tools to assemble but provide a stronger, higher pressure connection.

Mechanical grip fittings are two-ferrule assemblies. The back ferrule grips the tubing while pressing up against the front ferrule, which spring-loads the front ferrule and creates a seal between the tubing and fitting body. These fittings can be reassembled multiple times without damaging components or tubing. They have good resistance to mechanical vibration. 

Image Credit: Valvefittingtube.com 

Flare fittings consist of a body with a flared or coned end. Special flaring tools are used to install the tube to fit inside the flared end, providing a deep seal. Flare fittings can handle higher pressures and a wider range of operating parameters than standard compression fittings.

Image Credit: ChiCompany

End fittings provide specific surfaces for connecting tubes.

• Clamp ends are fittings which allow tubes to be clamped to the connection.
• Plain ends are fittings which allow tubes to be connected by adhesive, solder, or other forms.

Flange fittings are ports with flush surfaces perpendicular to the attached tube. These surfaces are joined and sealed via clamps, bolts, and/or welding. For more information on flanges, visit the Pipe Flanges Selection Guide on IEEE GlobalSpec.

Image Credit: Kee Safety Inc.

Luer locks are sleeve fittings that afford simple, effective connections (generally for short, single use) by a quick twisting action. It is most commonly used in medical and laboratory applications.

Image Credit: Ark-Plas Products, Inc.

Push-to-connect or quick coupler fittings have ends that are designed to accept the tubing by pushing it into the end. These fittings typically disconnect via some type of collar retraction. Quick connections are convenient for sections of the system requiring frequent disconnection and reconnection.

Image Credit: WaterTechOnline.com

Threaded fittings have screw threads (built-in grooves) on their inner (female) or outer (male) surfaces designed to accept tubing with matching threads. Threads which provide a simple connection but no seal are called straight threads. Tapered threads are designed to provide a tight seal for gases or fluids under pressure. Seal reliability can be improved by adding a coating or seal tape (Teflon). Especially precise threads are called "dry fit", meaning they seal without the need for an additional sealant, which is important in applications where sealant addition could cause contamination or corrosion.

The thread size is measured and based on the inside of the pipe or tube. Standard thread size standards include NPT (National Pipe Thread) and BSP (British Standard Pipe), though other standards exist and usage varies by country. Each standard corresponds to a particular number of threads per inch (TPI). Plumbingsupply.com provides an excellent overview on pipe thread sizing. Below is a chart depicting nominal thread sizes and their corresponding values.

Function

There are a vast number of types of fittings installed in tubing systems which perform different functions. Most fittings can be grouped into one of four categories:

Specifications

Sizing

Once the type of fitting required has been determined, the most fundamental aspect of tube fittings can be addressed: proper sizing. Proper sizing is essential to successful fitting selection, as oversized or undersized parts will either be completely incompatible or will seal or connect inadequately.

Sizes for fittings are typically given in millimeters (mm) or inches (" or in) depending on the system of measurement. Proper sizing requires the user to select a fitting which is sized using a system of measurement (Metric or English) that is compatible with the equipment or assemby being fitted.

The sizing of a fitting is defined by the inside diameter (ID) and outside diameter (OD) of its corresponding connections, measured in inches (in) or millimeters (mm). In other words, a fitting designed to connect to a tube with a 2" OD is rated as a 2" OD fitting. Inside diameter measures the diameter of the empty portion of the cylinder, while outside diameter includes the thickness of the tubing wall, as pictured below:

ID and OD measurements. Image Credit: Engineering Toolbox

Operation

The operating specifications that are most important for fittings include pressure and temperature.

Operating pressure range is the working range of pressures or the pressure ratings at which the fitting was designed to operate, typically measured in pounds per square inch (psi). Operating above or below this rating could cause the fitting to fail (i.e. break, leak, lose its seal).

Operating temperature range is the working range of temperatures or the temperature ratings at which the fitting was designed to operate, measured in degrees Fahrenheit (°F) or degrees Celsius (°C). Operating above or below this rating could cause the fitting to fail.

Materials

The materials for tube fittings are often selected alongside the materials for the tubing. Selection depends on factors such as cost, flexibility, media, environmental conditions, and required pressure ratings. Material choices include different types of plastic or metal.

Common metals include:

Aluminum - lightweight and corrosion resistant. Aluminum is commonly used for plumbing and is the preferred fitting material for aluminum tubing. By itself, aluminum has low tensile strength and is used when high corrosion resistance is needed. It is alloyed with zinc, copper, silicon, manganese, and/or other metals to improve its strength and hardness.

Brass - strong, durable, and corrosion resistant, with high temperature ductility and good conductivity. Brass is an alloy of copper and zinc, and is the most common tube fitting metal used in industry because of its machinability and its excellent performance properties. Brass fittings can have various protective or decorative finishes which should match the finish of the tubing.

Cast iron - strong and highly abrasion resistant. Cast iron fittings and tubes are used primarily in building construction for sanitary, storm drain, waste, and vent tubing applications because of their resistance to abrasive materials like sand, gravel, solid wastes, and debris.

Copper- extremely corrosion resistant with excellent conductivity. Copper fittings are important for many plumbing and heating applications, and are commonly used for residential water supply lines. Copper fittings are usually used in conjunction with copper tubing, and come in either soft or rigid forms. Soft or ductile copper is easily bent and maneuvered, and is the only type suitable for flare connections. Rigid copper is not bendable and requires directional fittings to go around corners and obstacles.

Steel - durable and strong, with a high resistance to heat. Steel is an alloy of iron and carbon - it is commonly alloyed with other metals to improve its corrosion resistance and durability. It is used in both commercial and industrial applications for carrying water, flammable gases, and other fluids. Galvanized steel is coated with zinc for rust and chemical corrosion resistance. Carbon steel is alloyed with higher levels of carbon for increased durability and strength.

Stainless steel- relatively strong with excellent chemical and corrosion resistance. Stainless steel is an alloy of steel that contains over 10.5% chromium, providing excellent corrosion resistance for sanitary applications and those dealing with aggressive fluids and materials.

Common plastics include:

PVC (poly-vinyl-chloride) - the most common plastic for tube fittings. PVC is a rigid plastic which can come in a variety of pressure ratings. Fittings are connected via threads or solvent welding (glue). Standard applications include cold water supply and drainage.

ABS (acrylonitrile butadiene styrene) - black and rigid plastic with similar properties and applications to PVC, but typically more economical.

Polyethylene (PE) - the most preferred plastic for hot and cold temperature applications. PE is grey or black, semi-flexible plastic. Fittings are used commonly for supply lines to sprinkler systems and underground geothermal heating loops.

Nylon - plastic with high chemical and corrosion resistance. It is the preferred plastic for applications involving flammable fluids, chemical solvents, and potable water.

Selection Tip: It is essential to check the compatibility of the system fluid with any proposed fitting materials. Incompatibility could result in corrosion causing leakage or system damage.

References

Engineering Toolbox - Pipes & Tubes

Pipe Fittings & Flanges - Pipe Fitting Selection Guide

Image Credit:

DK-LOK U.S.A. | Ark-Plas® Products, Inc. | Jergens, Inc.