Posted on December 05, 2017

Four Types of Instrumentation Tube Fittings

Kelly Forrest
Written by

Kelly Forrest

Posted in
Products, Instrumentation

Process measurement and control instrumentation is a vital part of most primary industries, including aerospace, defense, power generation, chemical processing, oil & gas, petrochemical, alternative fuels, shipbuilding and medical equipment.

AdobeStock_69730102.jpgPM&C instrumentation covers devices and components from control valves, fittings and tubing to flow meters, pressure sensors and level gauges.  They are used to transfer, measure and control system flow, pressure, temperature and provide system calibration. 

Instrumentation components are critical to the operations of the plant or a product grouping. Fittings, valves, and tubing are the veins and arteries of the instrumentation system.

Instrumentation Fittings

Tube Fittings

There are four primary styles of high to low pressure tube fittings.  They are differentiated by their end connections and applications, each having its own advantages and disadvantages. 

  • Compression Fittings: Slide over the tube and use a ferrule design to coin and seal on the OD of the tube.
  • Cone and Thread Fittings (C&T): Tube ends are coned (chamfered) and threaded. The threads allow for positive lock and sealing of the end face of tube to the fitting body.
  • Butt Weld Fittings: Permanently joins to the tube end by welding the surrounding metal at the tube end with the fitting.
  • Push-To-Connect Pneumatic Fittings: Instant connections just by pushing the tube into the fitting.

For most pressure applications (up to 15,000 PSI) fitting sizes are specified by the tubing OD (outside diameter); i.e. 1/4, 3/8, 1/2, 3/4 inch (6, 10, 12, 20 mm).  Each style of fitting is available with the specified size end connections and in different configurations, such as straight, elbow, tee and cross.

Sometimes they are used as reducing fittings; for instance from a larger size connection such as 3/8 to a smaller, 1/4 tube.  Many are also available with the designated style (i.e. compression) on one port to an alternative connection on another port, such as NPT or SAE thread, or male NPT to C&T and many other combinations.

1. Compression Fittings

The most common and easiest to install are compression fittings.   This design uses a sequential grip on the tube.  Compression fittings are made up of three components: nut, body and ferrule (single or double).  The ferrule coins (compresses) the tube and provides a tight grip on the tube surface.  Compression fittings have superior vibration resistance over C&T fittings, but not over butt weld fittings (see below).   This style of fitting does not require any special equipment or tools. Simply slide the tubing into the end connection, and tighten to the required torque with a box wrench (or torque wrench for precision applications).  Because of their ease of installation, the use of compression fittings is usually the least expensive overall assembly.

Compression fittings also perform well with thinner walled tubing, which allows for higher flow rates and can be more easily bent and handled.  Thinner walled stainless steel tubing used with compression fittings can allow up to 33% more flow through the tubing than the comparable size C&T. Thinner wall seamless tubing can bend with only 70% of the force required to bend a similar heavy  wall tube used with C&T fittings.  This allows for more flexibility in the application, particularly in field assembly where tube bending equipment is not assessable. However thinner walled tubing may not be able to withstand higher pressures, even in the “medium pressure” range.

2. Cone and Thread Fittings

Cone and Thread (C&T) connections are sometimes used in very high pressure applications (up to 150,000 PSI), but are most frequently used for pressures of up to 15,000 PSI.  They provide a solid and tight connection, where the tube is chamfered (“coned”) and countersunk into the fitting.  To insure a tight seal, the tube ends must be precision formed with a fine surface finish and then threaded so the tubing is assembled firmly against the joint.  All of this is typically performed in the field, significantly increasing the installation time and overall cost of using C&T fittings. However, like compression fittings C&T fittings can be broken down and re-assembled for maintenance.

These processes are not always repeatable or consistent. They can also introduce metal shavings and cutting oil into the tubing ID, creating the requirement for an additional cleaning operation. Special training is required to use the tools necessary for installation of C&T fittings.  And because the joints are threaded in uncontrolled environments, the fittings can become loose when subject to vibration.

3. Butt Weld Fittings

Though C&T fittings still provide a very solid connection when installed properly, some studies have found that for every three corrosion failures in the field, there is at least one vibration failure.  The assembly that provides the best protection against vibration is a butt weld fitting.  Since the metals are being permanently joined together, a well formed butt weld has the best resistance to vibration and fatigue.  However, a butt weld fitting connection still has some disadvantages, including cost.  The cost of welding equipment, a specialized welder and the extra labor time can make a butt weld connection the most expensive of the three primary types of instrumentation fitting assemblies.

In addition, field maintenance of fluid systems can be difficult if not impossible with butt weld fittings due to the potential inaccessibility of the joint and/or the maintenance crew not having the tools or skills available to cut and re-weld a connection.

4. Push-To-Connect Pneumatic Fittings

Push-to-connect fittings use an internal collet and external push button to connect tubing.  These composite fittings are either poly or constructed with nickel plated brass, resin, and polyoxymethylene plastic components.  Suitable for polyurethane or nylon tubing, push-to-connect air fittings guarantee leak-free and secure connections for all pneumatic applications.  Push-to-connect pneumatic fittings allow instant connections just by pushing the tube into the fitting, without the need for tools, and disconnections are just as easy.

These universally adapted fittings are ideal for both mobile and in-plant applications involving compressed air.  They are also offered for metric tubes with BSP threads in taper, parallel and metric form.  Push-to-connect fittings are available in a wide range of body designs and configurations — for tube OD of 1/8” to 1/2”.

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