High pressure cylinder threads require careful
examination on a regular basis to ensure their integrity has not been
impaired by corrosion, damage, or cracking. While inspection is
required for both steel and aluminum cylinders
threads, aluminum cylinders are more likely to sustain thread damage.
The increased potential for 3AL cylinder thread
damage exists because the metal is relatively soft, is susceptible to
galvanic corrosion and, in some cases, subject to sustained load cracking.
Threads on steel SCUBA and SCBA may corrode from the presence of moisture,
particularly salt water, but are seldom impacted by dissimilar metal
corrosion or cross-threading by the valve during insertion.
Sustained Load Cracking occasionally occurs in the
threaded area of older aluminum cylinders made from 6351 alloy. Stress
Corrosion Cracking, although rare, might be found in cylinders made from
either 6351 alloy or the 6061 alloy now commonly used for making
cylinders. The discovery of cracking potential prompted the
Department of Transportation (DOT) to determine suitable methods for
examining threads during the visual inspection of 6351cylinders. In
addition, industry, working in cooperation with cylinder manufacturers and
PSI, Inc., the inspector training agency, developed tools to increase the
precision of thread inspection. These tools have been integrated
into the well- established protocol of annual cylinder visual inspections.
The DOT determined that a very small dental mirror
capable of reaching into both SCUBA and smaller SCBA necked cylinders was
the minimum necessary to adequately view thread integrity.
Fortunately, although those 7/16 inch diameter magnifying mirrors are no
longer used in the dental field, PSI, Inc. is able to special order those
mirrors from international sources.
Eddy current technology was also incorporated into
the aluminum thread assessment process. First developed by Flare
Technology, Inc., the Visual Plus device provided excellent detection of
otherwise difficult to see thread area cracks. It is sold by
Advanced Inspection Technology. Currently, eddy current machines are
also sold by Flare Technology under the brand name of Visual Eddy.
Various models of these two brands cost between $700.
and $1,000.
In 2000, Forest City Scuba began marketing a viewing
tube that illuminates and magnifies the cylinder thread area. Called
the Thread Inspection Pipe (TIP), the tool was well received by visual
inspectors because it is simple to operate and clearly shows each thread
during a careful evaluation. The device allows businesses with very
small cylinder inventories to have a device more effective than the
standard magnifying mirror and at less cost ($310.) than the eddy current
devices.
In the fall of 2002, Flare Technology introduced a
device similar to the TIP tool but is smaller in size. The Flare Optic
Viewer (FOV) develops 4.3 power of magnification in the cylinder thread
area. Its cost is $140.00.
The DOT requires its registered hydrostatic
requalifiers to have at least one of the tools
described here for enhancing the view of cylinder threads. The PSI,
Inc. visual inspection protocol likewise incorporates a magnifying mirror
or optionally, one of the more precise tools to examine the threads of all
cylinders being inspected.
Neither a cylinder hydrostatic
requalification nor formal visual inspection is complete without
examining threads, whether steel or aluminum. The minimum number of
complete, continuous threads varies among different cylinder designs and
pressure ratings. PSI, Inc. trained visual inspectors receive thread
standards during formal training. Also, cylinder manufacturers may
be contacted for minimum thread requirements. Remember, the thread
count must begin at the exterior (cylinder top) and proceed interiorly.
Only those threads that will be in contact with the valve may be counted.
For further information or to purchase magnifying
inspection mirrors, TIP or FOV illuminating thread viewing tools, contact
PSI, Inc. at email:
psi@psicylinders.com. or phone PSI in the
US at 425.398.4300.
Contact AIT for Visual Plus
Eddy Current device at 909-776-9994 or Flare Technology for the Visual
Eddy device at 847-439-7465