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INSTRUMENT TUBING-
TECHNICAL DATA
Tubing selection
There is a huge selection of materials to
choose from when selecting instrument tubing. The
selection is dependent on the service, installation
and how long the tubing is required. It is
important that these are considered before making a choice.
One has to be careful when selecting polished tubing as the manufacturers
polishing can work harden it and increase the surface hardness to unacceptable
levels. Thus when specifying tubing it is best to select dull annealed
type, then one can be assured that no untoward hardening has occurred.
Tube Preparation
q Tube Handling
The tubing surface is
ESSENTIAL to sealing when using a compression fitting, hence good handling practices
should be used to reduce scratches and protect surface finish. For example dragging tubing across any surface
that could scratch its surface can cause real corrosion and sealing problems. On offshore facilities, scratches may lead to
corrosion of 316SS tubing from salt water pitting.
q Visual Inspection of Tubing
It is a good idea to
visually inspect instrument tubing to ensure it is free from scratches and other damage. Severe scratches or damage to tubing could affect
the safe installation of the compression fitting. It
is recommended that tubing in poor condition should be disposed of. Finished tubes should be scratch free, reasonably
straight and have smooth ends free of burrs.
q Cutting
When cutting tubing
do not use a hacksaw, the correct tool
is a tube cutter which has a sharp blade.
q Deburring
Using the correct
deburring tool (usually found on the cutter), deburr
both the inside and outside edges of tube ends.
q Cleaning
It is recommended
that instrument tubing is cleaned with dry instrument air, however where the service
requires a high level of cleanliness a proprietary cleaning agent should be used.
TUBING
FOR HAZARDOUS GAS SERVICE
On hazardous gas
service take care to ensure that the correct wall thickness has been chosen. Gas molecules are small and can migrate through
thinner wall tubing. Contact your tubing
supplier for details.
Tube Bending
q Tube Benders
On applications where
bending is required, a tube bender must be used. Follow
the tube bender manufacturer’s instructions to assure good bends. Either compression or draw type hand benders
should be used for tubing up to 1/2 inches. Above this size hydraulic tools are recommended.
MINIMUM STRAIGHT LENGTHS
To ensure proper
insertion depth into compression fittings minimum straight lengths of tubing are needed. The following information may be used as a
reference for the bending radius and for the minimum straight length before the bend
for proper installation.
· If the tubing O.D. is 1/2 inches or less, provide
at least 25mm straight length.
· If the tubing O.D. is greater than 1/2 inches
provide at least 50mm straight length.
If the straight length is not possible, an
elbow should be used.
Fitting Preparations
Compression fittings
are supplied assembled. The fitting, pipe
thread and general appearance should be inspected visually before use. It is recommended that fittings are not
disassembled. Always ensure that the tubing
material is softer than the fitting.
Tubing Installation
q If practical, it is recommended that instrument process impulse lines
are installed in the vertical plane. Horizontal
impulse lines are generally run with a slope of not less than one in twelve between the
process connection and the instrument. Slope
should be down from the tapping points for liquids and up for gases.
q For reasons of accessibility offset and stagger tube unions wherever
practicable.
q Tube fittings must not be used to support instruments, which should be
independently mounted.
q It is recommended that any Weld splatter on stainless tubing be
removed. In severe cases, the tubing must be
replaced.
SUPPORTING
TUBING
q It is a good idea to ensure all instrument impulse lines are as short
as possible. For lengths up to approximately
1m tubing is self-supporting, however for longer lengths it is recommended that the tubing
be supported at approximately 1m intervals.
q The number of joints in the impulse line should be kept to a minimum. Process and utility piping must not be used for
supporting instrumentation piping or tubing.
q Tubing should be adequately supported and braced. It is suggested that support fixing distances not
exceed the following:
| Outside Diameter
|
Distance
|
| 1/4 inch OD
|
Continuously Supported
|
| 1/2 inch OD
|
1.0m
|
| 3/4 inch OD
|
1.5m
|
| 1 inch OD
|
2.0m
|
q It is recommended that three or more tubes run in parallel are
supported in steel angle or channel, or on cable tray or ladder with appropriate
insulation.
q To avoid the possibility of corrosion, it is recommended that stainless
steel tube and/or fittings (316, duplex or otherwise) is NOT be in direct contact or
located directly under Zinc coated or galvanised components.
q Electrically non-conductive tubing clamps are available. These isolate
tubing from its supports,
Beware of Tubing Vibration!
It is suggested that
impulse lines for instruments shall be arranged to avoid:-
· Movement through thermal expansion.
· Mechanical damage from impact.
When tube vibration
is a problem, due to the closeness of machinery, process pressure changes or other
excitation, it is recommended:
· Tubing is clamped to the monoflange, process take-off or process line
using a suitable clamp with the minimum span between the compression fitting and the
clamp.
· A vibration loop (pigtail) is installed in the tube span between the
process line and any independent structure used to support the tube. The orientation of the loop should be chosen with
due regard to the probable relative motion between the connections.
· The vibration loop crossover has a minimum clearance of 2 x tube OD to
prevent fretting of the tube.
TUBE WELDING
There are several
instrument tube welders on the market, they
are excellent for hazardous gas applications. These
units provide consistent high quality welds when operated correctly.
PLASTIC
TUBING
Ensure that extremely soft plastic tubing is
used with an insert. Never use metal ferrules
on plastic tubing.
Training
It is strongly recommended that
any personnel associated with the installation of tube fittings and tubing attend the
excellent vendor courses that are Internationally available. Tubing and fittings may look easy, however they
are complex, precision items which require fully competent personnel to install them- we
suggest that individuals and companies take iceweb advice and attend a course.
MORE
INFORMATION?
Iceweb
suggests that you visit these links which have considerable information
associated with fittings.
Great
technical information from our sponsors PROCHEM
PIPELINE PRODUCTS
Axiswage
Ball Valves
Branch
Fittings
BSP
Fittings
Bu
Tech High pressure components
Buttweld
Fittings
Conversion
Chart
Duplex
Flanges
JIC
Fittings
Gyrolok
Twin Ferrule Tube Fittings
Heat
Exchanger, Condensor & Boiling Tubes
Hygienic
and Tube
Indent
Service
Manual
Sample Systems
NPT
Fittings
O'Brien
Tracepak and O'Brien Analytical
Offshore
Oil and Gas Speciality Valves
Pipe
and Tube
Prochem
Customised Manufacturing
Pyplok
Saflok
Couplers, Y-type Strainer and Swing Check Valves
Selection
of Materials
Socketweld
Fittings
Stainless
Steel Tube
Stainless
Steel Tube, Saddles and Leak Check
Stainless
Steel Tube Chart - Tube weights and working pressures
Swage
and Pipe Nipples and S/S Shim
Teflon
Lined Braided Hoses
Useful
Facts About Stainless Steel
10,000psi
Fittings Specification
http://www.hoke.com/
- some great
information here.
Hoke
GyrolokŪ Tubing Data Charts - These
charts are very useful and contain information on design, type of tubing, tubing
preparation, calculating Yield, Burst,
and Maximum Allowable Working Pressures, Maximum Allowable Stress Values for
Material at Various Temperatures, Stainless Steel Calculation Factor Tables,
Maximum Working/Burst Pressures and Tubing Tolerances
The
following technical link is available via Swagelok
Tubing
data
PLEASE
SUPPORT OUR SPONSORS WHO ARE EXPERTS IN THIS AREA- IT IS THEIR SUPPORT WHICH MAKES ICEWEB
AVAILABLE:
INTERNATIONAL
q http://www.hoke.com/