Since 1985 numerous articles and government notices
have been released describing concerns about cylinders made from 6351
alloy aluminum. While the U.S. Government has determined no recall of
the nearly 25 million 6351 alloy cylinders is warranted, annual
inspection by trained inspectors is called for in the 1999
Department of Transportation (DOT) and
National Institute for Occupational Safety and
Health (NIOSH) bulletins.
A total of 15 SCUBA and fire fighter SCBA cylinders
have ruptured explosively in the US from sustained load cracking with an
additional 4 at international locations. Although small in numbers,
these ruptures caused property damage and personal injury.
Sustained load cracking (SLC), a metallurgical
anomaly, occasionally develops in 6351 aluminum alloy high-pressure
cylinders. As the name implies, SLC usually occurs in cylinders that
remained filled for long periods of time. US manufacturers that used
6351 alloy included Luxfer Gas Cylinders, Walter Kidde, Cliff Impact,
Norris Industries (SP6688) and Kaiser (SP6576). The permits for both
Norris and Kaiser have expired and those cylinders are no longer
approved for use in the US. Luxfer stopped using 6351 alloy by mid 1988
and now provides a complete listing on
its web site of dates when
various cylinder models were changed to a proprietary 6061 alloy.
Catalina Cylinders began making aluminum cylinders in 1986 using a 6061
alloy that is not susceptible to SLC.
SLC is a very slow progressing process.
Government
findings on a ruptured scuba cylinder showed two cracks had progressed,
one over 8.7 and the other 9.3 years. For this reason the DOT specifies
all cylinders made from 6351 alloy be carefully inspected by trained
inspectors (DOT recommends PSI, Inc inspector training) with special
attention given to the threaded neck area.
Quality inspections are the key to cylinder safety
whether for SLC or any of the other damage types that precipitate
cylinder ruptures. Several tools are now available to aid the inspector
when examining cylinder threads. Initially, the
DOT called for a small
magnifying mirror and light to assess threads. Now,
two excellent tools
provide a greatly improved view of the areas where SLC is likely to
propagate. One is called a Thread Inspection Pipe (TIP) and a less
expensive model called the Flare Optic Viewer (FOV).
In 1996 an eddy current instrument was designed
specifically for 6351 alloy aluminum cylinder SLC examination. The
Visual Plus instrument (sold by
AIT) was closely followed to market by
the Visual Eddy instrument (designed and sold by
Flare Technology).
Both instruments, when used correctly, will readily identify even very
small cracks. Unfortunately, these instruments are often improperly
used by operators causing many good cylinders to be incorrectly
condemned.
Formal, professional training is essential to ensure
technicians can properly operate the system and interpret the results.
A few dive businesses have required eddy current
instrument use on steel cylinders. These instruments were not designed
for use on steel cylinders and will not work. Further, there is no
history of cracking in steel cylinders. Because of improper instrument
use, Luxfer Gas Cylinders has issued a directive stating the proprietary
6061 alloy used on its cylinders made after May 1988 is not subject to
SLC and the eddy instruments currently in service will often produce
false positives, that is, report a crack where no crack exists.
Formally trained eddy current technicians learn how to avoid false
positives. That training is available at DEMA trade show seminars and
at numerous US and Canadian locations by
PSI affiliate instructors.
The US DOT has, in its proposed
HM220F rules that
all 6351 alloy cylinders be examined at least annually with an eddy
current instrument. If enacted as proposed then those millions of
cylinders, including scuba, found to be free of SLC will remain in
service. That same rule will require that those filling cylinders be
given some form of protection. Commonly used plastic garbage cans, sheet
metal tubs, etc. will not qualify as cylinder filling containers.
Some air stations no longer fill cylinders made
from 6351 alloy. While that is a personal decision, it may be a poor
business practice. A few dive business employees have told cylinder
owners that 6351 alloy cylinders have been recalled. NO, scuba cylinders
have been recalled. Others state that the cylinder must be condemned
outright. Such statements border on fraud and it is illegal to condemn
a cylinder without proof that it is damaged beyond allowable limits.
The Luxfer 2003 voluntary voucher program allowed owners of older
cylinders to replace their cylinders at a reduced cost. That program has
ended.
Several of the recent aluminum cylinder ruptures
have attracted considerable industry attention while the more than 24
steel cylinder ruptures over the years are forgotten. The prudent dive
industry professional should be very cognizant that ALL high-pressure
cylinders contain phenomenal energy and ALL should be given great care.
Every ruptured cylinder had obvious damage whether it was a crack in the
threads, damage from excessive heat, extensive corrosion or other
abuses.
The solution to safe cylinders is regular quality
inspections by
trained inspectors and diligent fill station
operators.
For more detailed information about cylinder safety
visit the PSI web site at
www.psicylinders.com. or contact the author at
psicylinders@msn.com.
Cylinder inspection tools are
available directly from PSI at:
PSI, Inc.
www.psicylinders.com
16932 Woodinville-Redmond Road, Suite A201
Woodinville, WA 98072
425.398.4300 email
psi@psicylinders.com
For Additional Reading:
INSPECTING CYLINDERS (PSI, Inc.)
SCUBA
REPRINT FILE (PSI, Inc.)
LUXFER
GUIDE FOR SCUBA CYLINDERS
LUXFER’S
SCBA CYLINDER VISUAL INSPECTOR GUIDE