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Compression
Fittings
ACHIEVING OPTIMUM RESULTS
-
Select fittings to match the tubing diameter and pipe
thread size.
-
Fittings must be handled correctly.
-
Assemble fittings correctly to manufacturer's
installation instructions and ensure all components are compatible (from the same
manufacturer).
PRESSURE RATINGS
ASSEMBLY
METRIC AND IMPERIAL FITTINGS
Compression fittings are supplied in either
Metric or Imperial tube sizes. It is very important that types are not mixed or used on
the incorrect tubing.
THREAD TYPES
There are several common thread configurations, in instrumentation design two types are commonally
utilised.
NPT
(National Pipe Tapered) threads
such as Pipe Threads General Purpose (inch) conforming to ASME B1.20.1
These
threads use the “interference” principle and the use of thread tape or other
thread sealant is essential. The thread tape
will stop leaks via the gaps in the thread and avoid galling of the threads by giving
lubrication.
Caution:
In ICEWEB opinion care should be taken on higher pressures to ensure the tolerance is
better than the requirements of ASME B1.20.1 as if the male and female threads are to the
outside of the tolerance then only 1.5 threads will be engaged or alternately the fittings
may “bottom out”. In both cases
there are potential for blowout. How many
times have you seen this scenario? Whilst the
1.5 threads in itself will contain most pressures without blowout it is quite possible
that the Instrument Fitter will gall the threads because of insufficient thread entry.
The recommendation is NPT threads are designed
to meet the dimensional requirements stated in ASME B1.20.1-1993 with the following
modifications, this will ensure that if both
the fitting and the mating instrument get a 4 to 6 thread engagement.
It is
important to specify the revised tolerance on the instrument as well, particularly on high pressure connections on
equipment which are designed for noxious or hydrocarbon fluids.
The L1
thread gauging requirement in sections 8.1 and 8.2 modified to:
Gauging External Taper
Threads with Ring Gauge (Male Threads)
When hand
tight the male fitting must engage into the ring gauge + 1/4 to + 1 turn more than the
start of the first scratch mark on the chamfer zone.
Gauging Internal
Threads with Plug Gauge (Female Threads)
When hand
tight the plug gauge must engage into the female fitting + 1/4 to + 1 turn below the last
thread scratch on the chamfer zone.
q BSP
(British Standard Pipe) (ISO228) Parallel
This type
of thread utilises a washer/gasket or O-ring to achieve a seal. It is important that any fitting also is fitted
with a pin lock arrangement so that the fitting cannot vibrate loose.
MIXING BSP/NPT AND ANY OTHER
THREAD STANDARD ON A FACILITY
It is highly recommended that no mixing of
thread types occur on a facility. If this
culture exists it is possible for example that a Parallel thread is installed into a
Tapered thread. Thus, potential for blowout
is large.
MAKING UP NPT CONNECTIONS
Engage
the threads handtight - minimum of 4 and a maximum of 6.
An additional 0.5-1.5 turns can be expected with
wrench tightness using a quality thread tape.
Hand
tight engagement should be checked before applying any thread tape.
USE OF THREAD TAPE
Thread
tape should always be utilised on NPT threads, however
never on the compression fittings themselves. Any tape should be high quality and NOT obtained
from the local hardware store. A good rule is to obtain thread tape that meets the
requirements of UL 340-M1979 (AGA) 3226, MIL-T-27730A or AG208. There are thread tapes that are specifically
designed for 316ss and other fittings.
There are
liquid type sealants available, these should
be selected based on their specification and any anti-galling features. These sealants
are particularly useful for hydraulic or clean services where small particles of tape
could contaminate valves, ports etc.
Making up
a fitting when utilising thread tape
q Initially ensure that the hand-tight joint assembly gives 4 to 6 full turns
of engagement.
q Unscrew the joint, cleaning any dirt on the threads and then apply tape to
the male thread. Begin with the first thread
in direction of the pitch ensuring the tape is wrapped with a slight overlap.
q Check that the tape does not overhang the first thread, as small portions of
tape may be cut off and enter the tubing system.
q Tension the tape but do not stretch or thin it out.
q Cut the tape and draw the free end around the threads tautly.
q Rub the tape onto the thread ensuring engagement with it. In the case of stainless steel fittings, apply a
second layer of tape.
q Re-assemble
hand-tight and make wrench tight with REASONABLE FORCE only to achieve another 0.5 to 1.5
turns.
Please note excessive force may damage the
fitting. Teflon provides lubrication which
will aid deformation or collapse of fittings if excessive force is used. Teflon can also exacerbate loosening of slack
joints.
q When it
is necessary to break and remake connections ensure that any old tape residues are removed
from the threads.
THREAD GAUGES – USE AND
HANDLING
q Use hand
tight pressure only when gauging fittings with precision thread gauges. Do not use gauges
where any rust, swarf or dirt is present in the thread. Threads must be clean and bright. If the
fitting cannot be cleaned, discard it.
q When
checking NPT threads with secondary gauges ensure that a hand tight engagement allows the
leading thread of the fitting to fall between the two gauging benchmarks.
q Do not
screw any thread gauges together. Thread
gauges are precision instruments. Engagement
will cause wear and possibly dry seizure in storage.
q When not
in use, gauges should be kept clean and in their protective case. It is good practice to apply a smear of Vaseline
(petroleum jelly) to gauges in storage.
Primary
Thread Gauges
Primary
gauges are intended to be used as a master set with which regular comparative checks of
secondary gauges can be made against a series of fittings.
Secondary
Thread Gauges (L1 Standard)
Secondary
gauges are intended for field use. They must
be regularly checked against primary gauges for wear and damage.
Testing
Frequency for Gauges
Frequency
of testing secondary gauges depends on the frequency of use. Testing is required after impact damage and
chipping. It is recommended that Primary
gauges and secondary gauges are checked every six months by profile graph inspection.
NPT
Standards
Primary
master gauges and secondary working gauges should be manufactured and supplied in
accordance with ASME B1.20.1-1993 specifications.
CORRECT MAKEUP GAUGES
These gauges are product specific and give
indication whether the make up is correct. It
is suggested that you contact your supplier to determine whether they are available for
the product.
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/
Again some great
information here.
http://www.swagelok.com
This Swagelok link has considerable technical information.
It is however difficult to navigate to, when
on the main page enter as a guest or register, go
to “fittings tube” and then click on “connects tubing to tubing” and
then click on any product and you will be into the technical information area.
PLEASE SUPPORT OUR SPONSORS WHO ARE EXPERTS IN THIS
AREA -
IT IS THEIR SUPPORT WHICH MAKES ICEWEB AVAILABLE:
INTERNATIONAL
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