Pipes Tubing & Wiring 

Why Use Tubing Over Pipe?

1 - Bending Quality - Tubing has strong but relatively thinner walls; is easy to bend. Tube fabrication is simple.
2 - Greater Strength - Tubing is stronger. No weakened sections from reduction of wall thickness by threading.
3 - Smooth bends result in streamlined flow and less pressure drop.
4- Less Space and Weight - With its better bending qualities and a smaller ;outside diameter, tubing saves space and permits working in close quarters.
Tube fittings are smaller and also weigh less.
5 -Flexibility -Tubing is less rigid, has less tendency to transmit vibration from one connection to another.
6- Fewer Fittings -Tubing bends substitute for elbows. Fewer fittings mean fewer joints, fewer leak paths.
7 - Tighter Joints- Quality tube fittings, correctly assembled, give better assurance of leak-free systems.
8- Better Appearance - Tubing permits smoother contours with fewer fittings.
9 - Cleaner Fabrication - No sealing compounds on tube connections. Again no threading; minimum chance of scale, metal chips, foreign particles in system.
10 - Easier Assembly and Disassembly - Every tube connection serves as a union. Tube connections can be reassembled repeatedly with easy wrench action.
11 - Less Maintenance - Advantages of tubing and tube fittings add up to dependable, trouble-free installations.

Compression tube fittings

Used to connect tubing to the instrument or process lines.

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

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:


When assembling compression-style tube fittings, you should always precisely follow the manufacturers 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 finger tight. 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.

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: just tighten the nut to maintain adequate force holding the ferrule to the fitting body, but not so tight that the ferrule compresses further around the tube more than it did during initial assembly. A very graphic illustration of the strength of a typical instrument tube fitting 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 tube

Bending the Tubing

As already noted, tube bendability is one of the outstanding advantages of using tubing. Careful measurement and accurate bending are essential to achieving desired installation requirements including the achievement of a correct tube and fitting connection.
1. Use a tube bender when bending tubing.

Tube Bender

In order to prevent the problems of flattening, kinking, or wrinkling, use a tube bender and ensure tubing is tightly locked in the bender.
                                                  Bending Problems

2. Provide for key bending dimensions.

R = Minimum Bend Radius
L = Minimum Length of Straight Tube required to fully bottom tubing in fitting body. It is important not to bend too small a radius which will cause excessive ovality and may lead to weakening of the tubing. Use of a proper tube bender will avoid this problem.
Minimum Straight Length of Tube Before Bend
A minimum straight length of tubing before a bend is required to:
•assure full insertion of tubing into fitting, necessary for proper installation
•assure that ferrules are not trying to seal and grip on out-of round tubing in area bend
•assure ferrules are contacting area of tubing is not work hardened.

Fractional, inches


Tube OD


Bend Radius



9/16 & 3/4









1 1/2

1 3/16

Metric, mm


 Tube OD


Bend Radius














3. Tubing Layout

Before marking the tubing for bending, it is important that a complete layout be identified including consideration, where appropriate of the use of expansion loops, offsets, staggered union locations, and vertical ganging. Always allow sufficient access to utilities and other equipment requiring maintenance. A goal in tube routing is to eliminate as many connections as possible. Connections invite leaks, and leaks are problematic.

4. Marking the Tubing for Bending
Mark the tubing with a pencil using a ferrule as a guide to make a straight line

Example: Assume the following layout is required for tubing:

Note: The 2˝ dimensions at each end do not violate the 1 17/32˝ minimum straight length dimension requiredbefore the first mark from the end.

For accurate bending, do not mark the tubing with the dimensions shown above. The tubing runs will be too long and the resultant piece will be asymmetrical. This occurs because when tubing is bent it does not exactly make right angle turns but, in effect, takes short cuts at each bend, as shown at right.

Required Length of Tubing

Using the layout on the previous page, the actual tubing length required is 7.52˝, calculated as follows:
Side 1 length (2˝)
[Side 2 length (4˝) - Gain for first 90° bend (0.24˝)]
[Side 3 length (2˝) - Gain for second 90° bend (0.24˝)]
2˝ + [4˝ - 0.24˝] + [2˝ - 0.24˝] = 7.52˝

Marking the Tubing

Mark the tubing based on the brackets [ ] shown above:

First mark: 2˝
Second mark: 4˝ - 0.24˝ = 3.76˝
Third mark for cutting: 2˝ - 0.24˝ = 1.76˝

To ensure best fit, do not cut until bending is complete.
Bending the Tubing
Best bends are made by using tube benders specific to the tubing size.

Locate the mark in the bender so that it is tangent to the 90° on the bender. Lock the tubing in place to avoid problem bends.

Bend the tubing by smoothly swinging the upper arm down.
Align the on the upper arm with the "90" on the dial. Allow for about 3° of springback.

After completing the bend, swing the short handle up and away from the tube.

Cutting the Tube End
To insure a good joint, tube must be cut off square. This can be accomplished with either a tube cutter or hacksaw.

Deburring the Tube End
The burrs formed by either the tube cutter or hacksaw must be removed prior to assembly to prevent those burrs from eventually damaging the system. O.D. burrs can prevent tubing from seating properly in a fitting body. I.D. burrs can restrict flow, as well as possibly break loose and damage fine filtration elements.


Burrs forms after using Tube Cutter                 Deburring Tool

Types of Connectors

If a tube union joins together different tube sizes rather than tubes
it is called a reducing union.

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.

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.

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

Routing of Bend
;Routing of lines is probably the most difficult yet most significant of these system design considerations. Proper routing involves connecting one point to another through the most logical path. 

Avoid excessive strain on joints - A strained joint will eventually leak.

Allow for expansion and contraction - Use a “U” bend to allow for expansion and contraction.

U-Bend Allowing for Expansion and Contraction

Keep tube lines away from components that require regular maintenance.

Have a neat appearance and allow for easy trouble shooting, maintenance and repair.

Offset Bends and Stagger Union Locations


Offset bends are used to increase accessibility to tube fitting unions for maintenance purposes. When offsetting in a ganged run, stagger the union locations to further ensure ease of access.

Vertically Gang Tubing
To the maximum extent possible, tubing should be ganged vertically rather than horizontally. Vertical ganging prevents the collection of dirt or any potentially corrosive medium. Vertical ganging additionally increases system safety, since, for example, floor-level horizontally ganged tubing may be stepped on.

Tube Clamping
When tubing is left unsupported, shock and vibration will cause the tubing to shake, and in turn, cause the fitting to loosen and leak or even allow tube to fail through fatigue. Tubing can be clamped individually, in sets, and can also be stacked.    

Tube Clamp material: Polypropylene   

Below you will find a chart of recommended spacing between clamps.
Clamp as close to each bend of the tube as possible; and you must clamp each side.
This eliminates thrust in all directions.

Threaded Joints

Tapered thread pipe fittings

For smaller pipe sizes, threaded fittings are more commonly used to create connections between pipes and between pipes and equipment (including some instruments). The intent of a tapered thread is to allow the pipe and fitting to “wedge” together when engaged, creating a joint that is both mechanically rugged and leak-free. When male and female tapered pie threads are first engaged, they form a loose junction:

NPT -National Pipe Taper is a U.S. standard for tapered threads used to join pipes and fittings. The taper on NPT threads allows them to form a seal when torqued as the flanks of the threads compress against each other.

NPT fittings must be made leak free with the aid of thread seal tape or
a thread sealant compound.

Thread Tape
Thread tape acts as a lubricant allowing more thread engagement,
preventing galling, and filling the gap between the crests and roots
of mating taper threads in order to prevent formation of a spiral leak path.
Wrap the tape in the direction (clockwise) of the thread. 
Draw the tape tightly around the thread, ensuring, at a minimum, one complete
wrap of the tape, (1¼ turns is recommended)overlapping slightly.
Be sure the tape does not overhang the first thread otherwise the tape could
deteriorate and contaminate the fluid system.
On stainless steel a double wrap is recommended to minimize
any possible galling, while providing a good seal.

Parallel thread pipe fittings

An alternative to tapered threads in pipe joints is the parallel thread.
In the United States, a common design of parallel-thread pipe fitting
is the SAE straight thread, after the Society of Automotive Engineers

Sealing is accomplished as the O-ring is compressed against the shoulder of the female fitting.
The threads serve only to provide force (not fluid sealing).

Another parallel-thread pipe standard is the BSPP, or British Standard Pipe Parallel.
Like the BSPT (tapered) standard, the thread angle of BSPP is 55o.
Like the SAE parallel-thread standard, sealing is accomplished by means of an O-ring
;which is compressed against the shoulder of the matching female fitting.

Flanged pipe fittings

Large industrial pipes are joined together by flanges.
A pipe “flange” is a ring of metal, usually welded to the end of a pipe,
with holes drilled in it parallel to the pipe centerline to accept several bolts.

Flange joints are made pressure-tight with a gasket between the
flange pairs prior to tightening the bolts.
A method of installing such a flange gasket is to first install only
half of the bolts (in the holes lower than the centerline of the pipe)
drop the gasket between the flanges, then insert the rest of the bolts:

It is very important to evenly distribute the bolt pressure.
This is done by tightening the bolt in a criss-cross sequence.
An illustrative torque sequence is shown in the following diagram
(the numbers indicate the order in which the bolts should be tightened):

Torque wrenches are used for measuring torque during the tightening process.
Another important procedure to observe when working with flanged pipe connections
is to loosen the bolts on the far side of the flange before loosening the bolts on the side
;of the flange nearest you. This is a precautionary measure against the spraying
of process fluid toward your face or body in the event of stored pressure inside of a flanged pipe.
reaching over the pipe to first loosen flange bolts on the far side, if any pressure happens
to be inside the pipe, it should leak there first, venting the pressure in a direction away from you.
Electrical Signal and Control Wiring

The neatness of assembly in electrical signal wiring is very important.
Neat installations are easier to trace and troubleshoot.
Here we see 120 volt AC power distribution wiring. Note how the hoop-shaped “jumper”
wires are all cut to the same length, and how each of the wire labels is oriented
such that the printing is easy to read:

This next photograph shows a great way to terminate multi-conductor signal cable to terminal blocks.
Each of the pairs was twisted together. The end of the cable is wrapped in a short section of
heat-shrink tubing for a neat appearance.

Connections and wire terminations

Many different techniques exist for connecting electrical conductors together:
screw, screwless, soldering, plug-in, and wire-wrap are some examples.
Most industrial field wiring connections utilize a combination of compression-style
crimp “ferrules” and screw terminals to attach wires to instruments and to other wires.

The following picture shows a typical terminal strip or terminal block array
where twisted pair signal cables connect to other twisted-pair signal cables:

If you look closely at this photograph, you can see the bases of crimp-style compression
ferrules at the ends of the wires, just where they insert into the terminal block modules.
These terminal blocks use screws to hold the wires in close electrical contact
with a metal bar inside each block, but straight ferrules have been crimped on the
end of each wire to provide a more rugged tip for the terminal block screw to hold to.
A close-up view shows what one of these straight ferrules looks like:

The insulation is removed 1/4" from the tip, inserted into the ferrule and then crimped.

A close-up photograph of a single terminal block module shows how
screw-clamp system works. Into the right-hand side of this block
single wire (tipped with a straight compression lug) is clamped securely.
No wire is inserted into the left-hand side:

Some terminal blocks are screwless, using a spring clip to make firm
mechanical and electrical contact with the wire’s end.

In order to extract or insert a wire end from or two a “screwless” terminal block,
insert a narrow screwdriver into a hole in the block near the insertion point,
then pivot the screwdriver (like a lever) to exert force on the spring clip.
This next picture shows a pair of “isothermal” terminals designed
to terminate thermocouple wires . Here you can see how the bare
tip of the screw applies pressure to the wire inserted into the block

Rotary force is applied to each wire by screws and should only be used for solid wire.
Stranded wire would become frayed with this setup.
Many field instruments, however, do not possess “block” style connection points.
Instead, they are equipped with pan-head machine screws designed to compress
the wire directly between the head of the screw and metal.
Solid wires may be terminated to such a screw-head connection point by partially
wrapping the bare wire end around the screw’s head then tightening it.

The problem with directly compressing a wire tip beneath the head of a screw
is that the tip is subjected to both compression and shear forces.
As a result, the wire’s tip tends to become mangled with repeated connections.
Also, tension on the wire will turn the screw, loosening it after sometime.
This termination technique is not suitable for stranded wire,
because the shearing forces caused by the screw head’s rotation
will wear out the individual strands.
The best way to attach a stranded wire tip directly to a screw-style
connection point is to first crimp a compression-style terminal to the wire.
The flat metal “lug” portion of the terminal is then inserted underneath the screw head where it can easily handle the shearing and compression forces exerted by the head.
An exception is when the screw is equipped with a square washer underneath the head,
designed to compress the end of a stranded wire with no shear forces.
Many industrial instruments have termination points like this, for the express purpose of convenient termination to either solid or stranded wire ends.
This next photograph shows five such stranded-copper wires connected to screw-style
connection points on a field instrument using compression-style terminals:

Compression-style terminals come in two basic varieties: fork and ring.
An illustration of each type is shown here:

Fork terminals are easier to install and remove, since they only require
loosening of the connector screw rather than removal of the screw.
Ring terminals are more secure, since they cannot “fall off” the
connection point if the screw accidentally loosens.
Compression-style terminals are wholly unsuitable for solid wire.
Although the initial crimp may feel secure, compression terminals
lose their tension rapidly on solid wire, especially when there is
any motion or vibration stressing the connection.
Compression wire terminals should only be used on stranded wire!
Properly installing a compression-type terminal on a wire end requires the use
of a special crimping tool. The next picture shows one of these tools in use:

Note the different grooves on the crimping tool, for different wire sizes (gauges).
One groove is used for 16 gauge to 10 gauge wire, while the groove being used
in the picture is for wire gauges 22 through 18 (the wire inside of the crimped terminal
happens to be 18 gauge).This crimping tool does most of the compression on the
underside of the terminal barrel, leaving the top portion undisturbed.
The final crimped terminal looks like this when viewed from the top.

An industry-standard for attaching terminal blocks and small
electrical components to metal panels uses DIN rails.
This is a narrow channel of metal made of bent sheet steel
with edges designed for plastic components to “clip” on.
The picture shows terminal blocks, relay sockets, fuses mounted to
a horizontal length of DIN rail in a control system enclosure:

Pictures of a terminal block cluster clipped onto a length of DIN rail shows how specially-formed arms on each terminal block module edges of the DIN rail for a secure mounting:
The DIN rail itself mounts on to any flat surface by means of screws inserted through the slots in its base.      In most cases, the metal subpanel of an electrical enclosure to which all electrical components in that enclosure are attached.
An advantage of using DIN rail to secure electrical components versus individually mounting those components to a subpanel is convenience: much less labor is required to mount and unmount                         a DIN rail-mounted component than a component attached with its own set of dedicated screws.
This feature eases the task of altering a panel’s configuration.
Many different devices are manufactured for DIN rail mounting.
It is easy to upgrade or alter a panel layout simply by unclipping components,
sliding them to new locations on the rail, or replacing them with other types or styles of components.

This picture shows some of the types available in DIN rail mount components.
From left to right we see four relays, a power supply, and three HART protocol converters,
all clipped to the same extruded aluminum DIN rail
DIN rail is available in both stamped sheet-steel and extruded aluminum forms.
Two materials is shown here, sheet steel and aluminum.

The form of DIN rail shown in all photographs so far is known as “top hat” DIN rail.
A variation in DIN rail design is the so-called “G” rail, with a different shape:

Many modular terminal blocks are formed with the ability to clip
to either style of DIN rail, such as these two specialty blocks,
the left-hand example being a terminal block with a built-in disconnect switch,
and the right-hand example being a “grounding” terminal block whose
termination points are electrically common to the DIN rail itself:

If you examine the bottom structure of each block, you will see formations designed
to clip either to the edges of a standard (“top hat”) DIN rail or to a “G” shaped DIN rail.
Smaller DIN rail standards also exist, although they are far less common than the standard 35mm size:
Pre-printed terminal numbers can be attached to DIN rail type terminal blocks.
This makes documentation much easier, with each terminal connection
having its own unique identification number: