Tube & Tube Fittings – Instrument Tubing Connections

Tube is a hollow structure that is designed to provide an enclosed pathway for fluids to flow just in the same way as the pipe. Tubes are manufactured from rolled or extruded metal however plastic is a common tube material for many industrial applications. In this article we discuss some of the common methods for joining tubes together and joining tube ends to equipment such as process measurement instruments.

One of the basic differences between tube and pipe is that tube is never threaded at the end to form a connection. Instead, a device called a tube fitting must be used to couple a section of tube to another tube, or to a section of pipe, or to a piece of equipment such as process instrument. Unlike pipes which are thick-walled by nature, tubes are thin-walled structures. The wall thickness of a typical tube is simply too thin to support threads.

Generally, tubes are favored over pipe for small diameter applications. The ability of skilled workers to readily cut and bend tube with simple hand tools makes it the preferred choice for connecting instruments to process piping. When used as connecting units between an instrument and a process pipe or vessel, the tube is commonly referred to as an impulse tube or impulse line.

Compression Tube Fittings

Compression-style fitting is the most common type of tube fitting for impulse lines. It uses a compressible ferrule to perform the task of sealing fluid pressure. The essential components of a compression tube fitting are the body, the ferrule and the nut. The ferrule and body parts have matching conical profiles designed to tightly fit together, forming a pressure-tight metal-to-metal seal. Some compression fitting designs use a two-piece ferrule assembly, such as the tube fitting illustrated below:

Just before the assembly, we can see how the nut will cover the ferrule components and push them into the conical entrance of the fitting body:

After properly tightening the nut, the ferrule (s) will compress onto the outside of circumference of the tube, slightly crimping the tube in the process and thereby locking the ferrules in place as shown below:

Note that it is important always to precisely follow the manufacturer’s instructions to ensure accurate compression. Insufficient turning of the nut will fail to properly compress the ferrule around the tube, and excessive turning will over-compress the ferrule resulting in leakage. Special gauges are used to measure correct ferrule compression during the assembly process. The design of the gauge is such that its thickness will fit between the nut and the fitting shoulder if the nut is insufficiently tightened, but will not fit if it is sufficiently tightened. Thus the gauge has the ability to reveal an under-tightened fitting, but not an over-tightened fitting. Such gauges are called as ‘no-go gap gauges’ in the industry because their inability to fit between the nut and body shoulder of a tube fitting indicates a properly tightened fitting.

Unlike pipe fittings, tube fittings may be disconnected and reconnected with ease. No special procedures are required to re-do a dissembled instrument fitting connection: you simply tighten the nut to maintain enough force holding the ferrule to the fitting body, but not so tight that the ferrule compresses further around the tube than it did during initial assembly.

Common Tube Fitting Types & Names

Tube fittings designed to connect a tube to pipe threads are called connectors.

Tube fittings designed to connect one tube to another are called unions:

If a tube union joins together different tube sizes rather than tubes of the same size, it is referred to as a reducing union.

Bulkhead fitting is a variation of tube connectors and unions that is designed to fit through holes drilled in panels or enclosures to provide a way for a fluid line to pass through the wall of the panel or enclosure. Basically, the only difference between a bulkhead fitting and normal fitting is the additional length of the fitting ‘barrel’ and a special nut used to lock the fitting into place in the hole. Bulkhead fittings are illustrated below:

Tubing elbows are tube connectors with a bend (at 90°, 45°, etc.). They are useful for making turns in tube runs without having to bend the tube itself. Like standard connectors, they may terminate in male pipe thread, female pipe threads or in another tube end as demonstrated below:

Tee fittings join three fluid lines together. Tees may have one pipe end and two tube ends (branch tees and run tees), or three tube ends (union tees).

Special tube fittings are made to terminate tube connections, so they are sealed up instead of being open. A piece designed to seal off the open end of a tube fitting is called a plug, while a piece designed to seal off the end of an open tube is called a cap.

Related articles:

• Standard Pipe Sizes
• Instrument Piping Systems Installation Guidelines
• Pipe & Pipe Fittings: Common Connection Types & Features
• Instrument Installation – Mounting & Accessibility Guidelines
• Instrument Installation – Cabling Guidelines
• Testing, Pre-commissioning & Plant Commissioning Requirements