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Tube and Tube Fittings

Tube, like pipe, is a hollow structure designed to provide an enclosed pathway for fluids to flow. In the case of tubing, it is usually manufactured from rolled or extruded metal (although plastic is a common tube material for many industrial applications). This section discusses some of the more common methods for joining tubes together (and joining tube ends to equipment such as pressure instruments).

One of the fundamental 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 an 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.

Tubes are generally favored over pipe for small-diameter applications. The ability for 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 the connecting units between an instrument and a process pipe or vessel, the tube is commonly referred to as an impulse tube or impulse line1.

1Impulse lines are alternatively called gauge lines or sensing lines.

Tube, like pipe, is a hollow structure designed to provide an enclosed pathway for fluids to flow. In the case of tubing, it is usually manufactured from rolled or extruded metal (although plastic is a common tube material for many industrial applications). This section discusses some of the more common methods for joining tubes together (and joining tube ends to equipment such as pressure instruments).

Compression tube fittings

By far the most common type of tube fitting for instrument impulse lines is the compression-style fitting, which 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 this tube fitting shown here2 (prior to full assembly):

 

Tube_Fittings_Fig_015.JPG

 

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

Tube_Fittings_Fig_016.JPG

 

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

Tube_Fittings_Fig_017.JPG

 

2This happens to be a Swagelok brass instrument tube fitting being installed on a 3/8 inch copper tube.

When assembling compression-style tube fittings, you should always precisely follow the manufacturer’s instructions to ensure correct compression. For Swagelok-brand instrument tube fittings 1 inch in size and smaller, the general procedure is to tighten the nut 1-1/4 turns past fingertight. 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. Swagelok also provides special gauges which may be used to measure proper ferrule compression during the assembly process. Parker is another major manufacturer4 of instrument tube fittings, and their product line uses a single-piece ferrule instead of the two-piece design preferred by Swagelok. Like Swagelok fittings, Parker instrument fitting sized 1/4 inch to 1 inch require 1-1/4 turns past hand tight to properly compress the ferrule around the circumference of the tube. Parker also sells gauges which may be used to precisely determine when the proper amount of ferrule compression is achieved. Regardless of the brand, compression-style instrument tube fittings are incredibly strong and versatile. Unlike pipe fittings, tube fittings may be disconnected and reconnected with ease. No special procedures are required to “re-make” a disassembled instrument fitting connection: merely tighten the nut “snug” to maintain adequate 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. A very graphic illustration of the strength of a typical instrument tube fitting is shown in the following photograph, where a short section of 3/8 inch stainless steel instrument tube was exposed to high liquid pressure until it ruptured. Neither compression fitting on either side of the tube leaked during the test, despite the liquid pressure reaching a peak of 23,000 PSI before rupturing the tube3:

Tube_Fittings_Fig_018.JPG

 

3So is Gyrolok, Hoke, and a host of others. It is not my intent to advertise for different manufacturers in this textbook, but merely to point out some of the more common brands an industrial instrument technician might encounter on the job. 5It should be noted that the fitting nuts became seized onto the tube due to the tube’s swelling. The tube fittings may not have leaked during the test, but their constituent components should never be placed into service again!

Common tube fitting types and 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:

Tube_Fittings_Fig_019.JPG

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

A variation on the theme of tube connectors and unions is the bulkhead fitting. Bulkhead fittings are 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. In essence, the only difference between a bulkhead fitting and a normal fitting is the additional length of the fitting “barrel” and a special nut used to lock the fitting into place in the hole. The following illustration shows three types of bulkhead fittings:

Tube_Fittings_Fig_020.JPG

Tubing elbows are tube connectors with a bend. These are useful for making turns in tube runs without having to bend the tubing itself. Like standard connectors, they may terminate in male pipe thread, female pipe threads, or in another tube end:

Tube_Fittings_Fig_021.JPG

These elbows shown in the above illustration are all 90o, but this is not the only angle available 45o elbows are also common.

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). The only difference between a branch tee and a run tee is the orientation of the pipe end with regard to the two tube ends:

Tube_Fittings_Fig_022.JPG

Of course, branch and run tee fittings also come in female pipe thread versions as well. A variation of the theme of union tees is the cross, joining four tubes together:

Tube_Fittings_Fig_023.JPG

Special tube fittings are made to terminate tube connections, so they are sealed up instead of 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:

Tube_Fittings_Fig_024.JPG

Bending instrument tubing

Tube bending is something of an art, especially when done with stainless steel tubing. It is truly magnificent to see a professionally-crafted array of stainless steel instrument tubes, all bends perfectly made, all terminations square, all tubes parallel when laid side by side and perfectly perpendicular when crossing.

If possible, a goal in tube bending is to eliminate as many connections as possible. Connections invite leaks, and leaks are problematic. Long runs of instrument tubing made from standard 20 foot tube sections, however, require junctions be made somewhere, usually in the form of tube unions. When multiple tube unions must be placed in parallel tube runs, it is advisable to offset the unions so it is easier to get a wrench around the tube nuts to turn them. The philosophy here, as always, is to build the tubing system with future work in mind. A photograph of several tube junctions shows one way to do this:

 

Tube_Fittings_Fig_025.JPG

 

 

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Comments (7)Add Comment
0
Plastic Pipe Fittings
written by Plastic Pipe Fittings, December 03, 2012
Nice Blog!! Great Post !! Like your Blog and your ideas and i think its benificial for us and i keep to visit your blog regularly because i got lot of information through you blog
0
Instrumentation Tubes-
written by Mukesh Jain, December 10, 2012
Dear Sir,
We are please to inform you that we are Stockholders and Suppliers of Stainless Steel,Carbon Steel, Alloy Steel in the form of Pipes, Tubes, Tube-Fittings etc.
Our Aim is to Redefining Instrumentation Tubes.
We hope to build a long term business relationship with you,Our Website is http://www.neweagle.co.in
Please kindly send your specification so that we can send a detailed Price-list for your reference, we have attached basic Stock-List.
Hope to hear from you soon. Thanks!

Instrumentation Tubes-
SS Seamless Tube ASTM A269/A213 TP 304-304L / 316-316L / Monel / Inconel / Sanicro -28/Hastalloy -
1/8"OD
6mm OD x 0.049" W.T / 0.035"W.T
1/4"OD x 0.065"W.T/ 0.049" W.T / 0.035"W.T
3/8"OD x 0.065"W.T/ 0.049" W.T / 0.035"W.T
1/2"OD x 0.065"W.T/ 0.049" W.T / 0.035"W.T
3/4"OD x 0.065"W.T/ 0.049" W.T / 0.035"W.T
Tubes available in Fixed and Random Lengths.
Coiled tubing in length of 15-100 Mtrs.
Bright Annealed Tubes.
0
Plastic Pipe Fittings
written by Plastic Pipe Fittings, December 11, 2012
Great tips. very well-written, keyword-oriented and incredibly useful. its really interesting to many readers. I really appreciate this, thanks
0
Plastic Pipe Fittings
written by Plastic Pipe Fittings, February 18, 2013
Thank you again for another” feel good, uplifting, that there are good people in this world story”. We read so many stories that are negavite about people and what they do to others. A random act of kindness is always such a great thing to hear about. Peace and blessings.
0
Plastic Pipe Fittings
written by Plastic Pipe Fittings, February 18, 2013
Thank you again for another” feel good, uplifting, that there are good people in this world story”. We read so many stories that are negavite about people and what they do to others. A random act of kindness is always such a great thing to hear about. Peace and blessings.
0
below ground drainage
written by thomsonmatt33, September 25, 2013
Dear Sir,

We are inform you that Tubing,is usually manufactured from rolled or extruded metal (although plastic is a common tube material for many
industrial applications).

our Aim is some of the more common methods for joining tubes together ends
to equipment such as pressure instruments.
Tube fitting for instrument impulse lines is the compression-style fitting, which uses a compressible ferrule to perform the task of sealing fluid pressure.Tube fitting are the body, the ferrule, and the nut.

We hope to build a long term business relationship with you,more information please [visit here][/http://www.polypipe.com/building-products/products/below-ground-drainage]



0
...
written by Kiara Woodsland, February 23, 2016
I had a pipe burst in my boiler room to one of my water pipes. In order to fix it, I need a new pipe, along with some fittings. Since I haven't done too much work on my pipes before I haven't been quite sure on where to look for parts. Knowing the types and names of the different fittings might be helpful as I search. Thanks for sharing.

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