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Instrument
Tubing- Technical Data
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.
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.
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.
When cutting tubing
do not use a hacksaw, the correct tool
is a tube cutter which has a sharp blade.
Using the correct
deburring tool (usually found on the cutter), deburr
both the inside and outside edges of tube ends.
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.
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.
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.
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.
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.
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.
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,
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.
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.
Ensure that extremely soft plastic tubing is
used with an insert. Never use metal ferrules
on plastic tubing.
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.
Links
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
Great
technical information from our sponsors PROCHEM
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Service
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Oil and Gas Speciality Valves - Pacson Valves
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Valving Solutions - BSM
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Selection
of Materials
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Steel Tube Chart - Tube weights and working pressures
Swage
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Teflon
Lined Braided Hoses
Useful
Facts About Stainless Steel
10,000psi
Fittings Specification
Other Links
The following technical link is available via Swagelok